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Stereodivergent asymmetric catalysis is an important technology that can allow efficient access to various stereoisomers of a given product with multiple stereocenters from the same set of starting materials, but its application to the synthesis of a highly strained cyclopropane compound has remained unexplored to date. We report here the first diastereodivergent enantioselective synthesis of bicyclic aminocyclopropanes by lanthanum-catalyzed asymmetric carboamination/annu-lation of cyclopropenes with aminoalkenes. This protocol features 100% atom efficiency, good yield (up to 90%), and high chemo- (up to >20:1) and stereoselectivity (up to >20:1 dr and 99% ee), constituting a unique route for the efficient synthesis of two different diastereoisomers of a given chiral bicyclic aminocyclopropane compound. [ABSTRACT FROM AUTHOR]

We report realization of the first enantioselective Cope-type hydroamination of oximes for asymmetric nitrone synthesis. The ligand promoted asymmetric cyclopropene "hydronitronylation" process employs a Cu-based catalytic system and readily available starting materials, operates under mild conditions and displays broad scope and exceptionally high enantio- and diastereocontrol. Preliminary mechanistic studies corroborate a CuI-catalytic profile featuring an olefin metallaretro- Cope aminocupration process as the key C-N bond forming event. This conceptually novel reactivity enables the first example of highly enantioselective catalytic nitrone formation process and will likely spur further developments that may significantly expedite chiral nitrone synthesis. [ABSTRACT FROM AUTHOR]

Visible-light-promoted addition of α-bromoacetophenones onto the cyclopropene π-system in the presence of the fac-Ir(ppy)3 catalyst was shown to afford the corresponding 1(4H)-naphthalenones. The syn-carboarylation of the cyclopropene is followed by a cyclopropane ring opening under the basic conditions, allowing the formation of two C-C bonds and the generation of 1(4H)-naphthalenones bearing an all-carbon benzylic quaternary stereocenter. [ABSTRACT FROM AUTHOR]

The reaction between La atoms and 1,3-butadiene is carried out in a laser-vaporization molecular beam source. Metal-hydrocarbon species with formulas La(CnHn) (n = 2, 4, and 6) and La(CmHm+2) (m = 4 and 6) are observed with time-of-flight mass spectrometry and characterized with mass-analyzed threshold ionization spectroscopy. A lanthanum-benzene complex [La(C6H6)] is formed by 1,3-butadiene addition to lanthanacyclopropene [La(C2H2)] followed by molecular hydrogen elimination. Lanthanacyclopropene is an intermediate generated by the primary reaction between La and 1,3-butadiene. Two other intermediates produced by the La + 1,3-butadiene reaction are La[η4-(1-buten-3-yne)] [La(C4H4)] and 1-lanthanacyclopent-3-ene [La(C4H6)]. The La(benzene) complex exhibits distinctive metal-ligand bonding from that of the three intermediates as shown by the adiabatic ionization energies and ground electron configurations. [ABSTRACT FROM AUTHOR]

The factors controlling the reactivities and stereoselectivities in the Diels–Alder reactions of substituted cyclopropenes with butadiene were explored with M06-2X density functional theory. Differences in reactivities result from differences in the hyperconjugative aromaticities and antiaromaticities of the cyclopropenes. When the 3-substituent is a σ-donor, the ground state is destabilized, and the reactivity is enhanced. Acceptors have the opposite effect. Electrostatic, secondary orbital, and steric effects are all found to influence stereoselectivities. [ABSTRACT FROM AUTHOR]

An efficient approach toward enantioenriched eight-membered heterocycles via the intramolecular formal substitution of bromocyclopropanes with oxygen-based nucleophiles has been developed. The reaction proceeds via a reactive cyclopropene intermediate, which undergoes a rapid 8-endo-trig cyclization affording cis-fused [6.1.0] bicyclic products exclusively. The quaternary chiral center in the cyclopropene governs the configuration of the other two stereocenters in the final product. [ABSTRACT FROM AUTHOR]

An efficient generation of functionalized allyl copper(I) species via cyclopropene ring-opening coupling reaction is reported, which enables stereoselective access to allylphosphonates. Mechanistic studies uncovered stereochemistry to be controlled by both ligand and substrate electronics, with the latter likely arising from pronounced arene-Cu(I) interaction in electron-deficient substrates. The study unravels a novel approach to access functionalized nucleophilic allylcopper species upon which three-component coupling reactions might be developed. [ABSTRACT FROM AUTHOR]

A Ce atom reaction with ethylene was carried out in a laser-vaporization metal cluster beam source. Ce(C2H2) formed by hydrogen elimination from ethylene was investigated by mass-analyzed threshold ionization (MATI) spectroscopy, isotopic substitutions, and relativistic quantum chemical computations. The theoretical calculations include a scalar relativistic correction, dynamic electron correlation, and spin-orbit coupling. The MATI spectrum exhibits two nearly identical band systems separated by 128 cm-1. The separation is not affected by deuteration. The two-band systems are attributed to spin-orbit splitting and the vibrational bands to the symmetric metal-ligand stretching and in-plane carbon-hydrogen bending excitations. The spin-orbit splitting arises from interactions of a pair of nearly degenerate triplets and a pair of nearly degenerate singlets. The organolanthanide complex is a metallacyclopropene in C2v symmetry. The low-energy valence electron configurations of the neutral and ion species are Ce 4f¹6s¹ and Ce 4f¹, respectively. The remaining two electrons that are associated with the isolated Ce atom or ion are spin paired in a molecular orbital that is a bonding combination between a 5d Ce orbital and a π* antibonding orbital of acetylene. [ABSTRACT FROM AUTHOR]

The formation of thorium metallacyclopentadiene and metallacyclopropene complexes is significantly influenced by the steric and electronic properties of the internal alkyne employed during their syntheses. The reduction of (η5-C5Me5)2ThCl2 (2) with potassium graphite (KC8) or lithium in the presence of internal phenyl(alkyl)acetylenes (PhC≡CR) selectively yields the corresponding Cs-symmetric thorium metallacyclopentadienes (η5-C5Me5)2Th[η²-C(Ph)=C(R)C(Ph)=C(R)] (R = Me (4), iPr (5), C6H11 (6)), while the phenyl(silyl)acetylene PhC≡CSiHMe2 gives the C2v-symmetric metallacyclopentadiene (η5-C5Me5)2Th[η²-C(SiHMe2)=C(Ph)C(Ph)=C(SiHMe2)] (7). However, the sterically more encumbered acetylene derivative PhC≡CSiMe3 affords the chloro metallacyclopropene complex [(η5-C5Me5)2Th(η²-C2Ph(SiMe3))(Cl)][Li(EDM)2] (8), whereas Me3SiC≡CSiMe3 forms the chloro alkenyl complex [(η5-C5Me5)2Th[C(SiMe3)=CHSiMe3](Cl) (9), in which the chloro metallacyclopropene intermolecularly activates a C-H bond of the solvent (C7H8). Density functional theory (DFT) studies complement the experimental findings and rationalize the selectivity observed in the C-C bond formation. [ABSTRACT FROM AUTHOR]

This computational study uncovered the origin of the contradicting results in two recent DFT studies of the Rh(III)-catalyzed C-H activation/cycloaddition reactions between N-phenoxyacetamide and cyclopropenes. It was found that the β-carbon elimination of the tricyclic intermediate occurs very faciely via a conformer in which the opening of the three-membered ring is trans to the Cp* ligand so that the steric repulsion between the two moieties is avoided. Thus, the conclusions of our previous study were reconfirmed. [ABSTRACT FROM AUTHOR]

The synthesis and X-ray crystal structure of a novel Au(I)-precatalyst applied to intermolecular alkyne hydroamination is reported. Density functional theory (DFT) calculations revealed the cyclopropenium counterion of this Au(I)-precatalyst imparts stability through H-bonding and other noncovalent interactions. [ABSTRACT FROM AUTHOR]

Stereodivergent metallacycle-mediated cross-coupling reactions are described for the synthesis of densely functionalized vinylcyclopropanes from the union of alkynes with cyclopropenes. Strategies explored include hydroxyl-directed and nondirected processes, with the latter of these delivering vinylcyclopropanes with exquisite levels of regio- and stereoselectivity. Challenges inherent to these coupling reactions include diastereoselectivity (with respect to the cyclopropene) and regioselectivity (with respect to both coupling partners). [ABSTRACT FROM AUTHOR]

The crystal structures of tris(ethylmethylamino)-cyclopropenium chloride and tris(diethylamino)-cyclopropenium iodide reveal the presence of closely bound dicationic dimers formed from two closed-shell monomer units. The distances between the C3 centroids of the staggered monomers are at the short end of those normally found in p-stacked neutral arenes, let alone charged aromatic rings. Computational analysis reveals that short-range interactions are dominated by strong dispersion forces, enabling metastable dicationic dimers to form without covalent intermolecular bonding. Surrounding counterions then provide a background source of charge balance, imparting strong thermodynamic stability to the system. Additionally, these counterions form a weak but attractive electrostatic bridge between the monomer units, contributing to the surprisingly short observed intermolecular C3-C3 centroid distance. [ABSTRACT FROM AUTHOR]

A density functional theory (DFT) study has been conducted to elucidate the mechanism of the rhodium(III)-catalyzed C-H activation of N-phenoxyacetamide, where the amido component of an internal oxidant serves as a leaving group. The impact of different substrates (alkynes versus cyclopropenes) on the reaction mechanism has been discussed in detail. The pathway for cyclopropene substrate proceeded via a Rh(V) nitrene, while Rh(III) remained unchanged throughout the pathway for alkyne substrate. The C-O bond-forming reductive elimination and O-N bond cleavage steps simultaneously occurred for the alkyne substrate. However, the C-O bond was formed by an electrocyclization from a Rh(III) intermediate for the cyclopropene substrate. The energy profiles for the cyclopropene substrate were accompanied by a change in spin-state because the triplet spin state of a Rh(V) nitrene complex is lower than that of the singlet spin state. [ABSTRACT FROM AUTHOR]

This study challenges the problem of the activation of a CH bond of methane by soluble transition metal complexes. High pressure solution NMR, isotopic labeling studies, and kinetic analyses of the degenerate exchange of methane in the methyl complex [TpMe2NbCH3(c-C3H5)(MeCCMe)] (1) are reported. Stoichiometric methane activation by the mesitylene complex [TpMe2Nb(CH2-3,5-C6H3Me2)(c-C3H5) (MeCCMe)] (2) giving 1 is also realized. Evidence is provided that these reactions proceed via an intramolecular abstraction of a β-H of the cyclopropyl group to form either methane or mesitylene from 1 or 2, respectively, yielding the transient unsaturated η²-cyclopropene/metallabicyclobutane intermediate [TpMe2Nb(η²-c-C3H4) (MeCCMe)] A. This is followed by its mechanistic reverse 1,3-CH bond addition of methane yielding the product. [ABSTRACT FROM AUTHOR]

A hydrazone-based carbene/alkyne cascade produced a variety of bridged and fused polycyclic products. NaOSiMe3 is a superior base for conversion of hydrazones to diazoalkanes. A key mechanistic intermediate, a ring-fused cyclopropene, has been isolated and characterized. [ABSTRACT FROM AUTHOR]

A Hg(OTf)2catalyzed intramolecular arene 1,4-additionreaction of N-benzyl donor–acceptor cyclopropenecarboxamideswas developed to synthesize a series of [3.2.2]nonatriene derivatives.This novel reaction is also observed with silver(I) catalysts knownto form metal carbene intermediates in competition with the Buchnerreaction. [ABSTRACT FROM AUTHOR]

Density functional theory calculations were conducted to develop a mechanistic understanding of the Rh(III)-catalyzed C-H activation/cycloaddition reactions of N-phenoxyacetamide and N-pivaloxybenzamide with cyclopropenes, and insights into the substrate-dependent chemoselectivity were provided. The results showed that the divergence originated from the different reactivity of the seven-membered rhodacycles from the insertion of cyclopropene into the Rh-C bond. In reactions of N-pivaloxybenzamide, such an intermediate undergoes the pivalate migration to form a cyclic Rh(V)-nitrenoid intermediate in a reaction that is easier than the opening of the three-membered ring by β-carbon elimination, leading finally to a tricyclic product with retention of the cyclopropane moiety by facile reductive elimination. While similar Rh(V)-nitrenoid species could also be possibly formed in Cp*Rh(III)-catalyzed reactions of N-phenoxyacetamide, the β-carbon elimination occurs more easily from the corresponding seven-membered rhodacycle intermediate and the subsequent O-N bond cleavage gives rise to an unexpected dearomatized (E)-6-alkenylcyclohexa-2,4-dienone intermediate. The E/Z isomerization of this intermediate is required for the final cyclization to 2H-chromene, and interesting metal-ligand cooperative catalysis with Rh(III) carboxylate was disclosed in the C=C double bond rotation process. [ABSTRACT FROM AUTHOR]

By judiciously modulating the ring strain and sterics, we developed a class of disubstituted cyclopropenes that selectively underwent single monomer addition in ring-opening metathesis but readily underwent alternating ring-opening metathesis polymerization with low-strain cyclic olefins in a living fashion. The substituents on cyclopropenes effectively inhibited homoaddition and prevented secondary metathesis on the polymer backbone. The resulting polymers had controllable molecular weights and end groups, very low dispersities, and high regularity in microstructure under optimized conditions. 1H and 13C NMR spectroscopy and MALDI-TOF MS showed a rigorously alternating sequence, Interestingly, disubstituted cyclopropenes were found to present zero-order kinetics, indicating their rapid single addition and the rate-determining ring opening of the low-strain olefin. [ABSTRACT FROM AUTHOR]

A newtype of Rh2(OAc)4-catalyzed [2,3]-sigmatropicrearrangement of sulfur ylides is reported. A series of cyclopropeneswere successfully employed for [2,3]-sigmatropic rearrangement bya reaction with either allylic or propargylic sulfides. Under theoptimized conditions, the reaction afforded the products in moderateto excellent yields. In these transformations, the vinyl metal carbenesgenerated in situ from the cyclopropenes were effectively trappedby sulfides, resulting in the formation of corresponding productsupon [2,3]-sigmatropic rearrangements. [ABSTRACT FROM AUTHOR]

The catalytic addition of terminalalkynes to 3,3-diarylcyclopropenesin the presence of a Rh(I)/binap complex proceeded to give the cycloadditionproducts in good yields, where a 1,4-Rh shift is involved as a keystep. [ABSTRACT FROM AUTHOR]

Conspectus: Activation of unsaturated carbon-carbon bonds by means of transition metal catalysts is an exceptionally active research field in organic synthesis. In this context, due to their high ring strain, cyclopropenes constitute an interesting class of substrates that displays a versatile reactivity in the presence of transition metal catalysts. Metal complexes of vinyl carbenes are involved as key intermediates in a wide variety of transition metal-catalyzed ring-opening reactions of cyclopropenes. Most of the reported transformations rely on intermolecular or intramolecular addition of nucleophiles to these latter reactive species. This Account focuses specifically on the reactivity of carbenoids resulting from the ring-opening of cyclopropenes in cyclopropanation and C-H insertion reactions, which are arguably two of the most representative transformations of metal complexes of carbenes. Compared with the more conventional α-diazo carbonyl compounds, the use of cyclopropenes as precursors of metal carbenoids in intramolecular cyclopropanation or C-H insertion reactions has been largely underexploited. One of the challenges is to devise appropriately substituted and readily available cyclopropenes that would not only undergo regioselective ring-opening under mild conditions but also trigger the subsequent desired transformations with a high level of chemoselectivity and stereoselectivity. These goals were met by considering several substrates derived from the readily available 3,3-dimethylcyclopropenylcarbinols or 3,3-dimethylcyclopropenylcarbinyl amines. In the case of 1,6-cyclopropene-enes, highly efficient and diastereoselective gold(I)-catalyzed ring-opening/intramolecular cyclopropanations were developed as a route to diversely substituted heterocycles and carbocycles possessing a bicyclo[4.1.0]heptane framework. The use of rhodium(II) catalysts enabled us to widen the scope of this transformation for the synthesis of medium-sized heterocyclic scaffolds incorporating an eight-membered ring. The reactivity of rhodium(II) carbenoids generated from 3,3-dimethylcyclopropenylcarbinols was also investigated in intramolecular C(sp³)-H insertions. Despite their low electrophilic character, these purely donor rhodium(II) carbenoids underwent remarkably efficient diastereoselective 1,5- or 1,6-C-H insertions allowing access to a wide variety of substituted cyclopentanols, cyclohexanols, bicycloalkanols, and tetrahydropyrans with high level of diastereoselectivity and with complete tolerance of a free hydroxyl group. The products arising from the gold(I)- or rhodium(II)-catalyzed ring-opening/intramolecular cyclopropanation or C-H insertion of 3,3-dimethylcyclopropenylcarbinols or 3,3-dimethylcyclopropenylcarbinyl amines always incorporate an isopropylidene moiety, which can potentially undergo subsequent oxidative cleavage into a carbonyl group without epimerization. By virtue of this operation, the 3,3-dimethylcyclopropenyl group formally behaves as a valuable surrogate for an α-diazoketone, with obvious advantages considering the ease of access to the corresponding substrates and that no hazardous reagents are involved in their preparation. These studies have set a useful basis for the development of other reaction pathways involving metal carbenoids generated from these readily available families of substituted cyclopropenes, including the investigation of the yet underexploited synthetic potential of purely donor rhodium(II) carbenoids. [ABSTRACT FROM AUTHOR]

The cycloadditions of tetrazines with cyclopropenes and other strained alkenes have become among the most valuable bioorthogonal reactions. These reactions lead to bicyclic Diels-Alder adducts that spontaneously lose N2. We report quantum mechanical (QM) and quasiclassical trajectory simulations on a number of these reactions, with special attention to stereoelectronic and dynamic effects on spontaneous N2 loss from these adducts. QM calculations show that the barrier to N2 loss is low, and molecular dynamics calculations show that the intermediate is frequently bypassed dynamically. There is a large preference for N2 loss anti to the cyclopropane moiety rather than syn from adducts formed from reactions with cyclopropenes. This is explained by the interactions of the Walsh orbitals of the cyclopropane group with the breaking C-N bonds in N2 loss. Dynamical effects opposing the QM preferences have also been discovered involving the coupling of vibrations associated with the formation of the new C-C bonds in the cycloaddition step, and those of the breaking C-N bonds during subsequent N2 loss. This dynamic matching leads to pronounced nonstatistical effects on the lifetimes of Diels-Alder intermediates. An unusual oscillatory behavior of the intermediate decay rate has been identified and attributed to specific vibrational coupling. [ABSTRACT FROM AUTHOR]

The synthesis, structure, and reactivity of an actinide metallacyclopropene were comprehensively studied. The reduction of [η5-1,2,4-(Me3C)3C5H2]2ThCl2 (1) with potassium graphite (KC8) in the presence of diphenylacetylene (PhC ≡ CPh) yields the first stable actinide metallacyclopropene [η5-1,2,4-(Me3C)3C5H2]2Th (η²-C2Ph2) (2). The magnetic susceptibility data show that 2 is indeed a diamagnetic Th(IV) complex, and density functional theory (DFT) studies suggest that the 5f orbitals contribute to the bonding of the metallacyclopropene Th--(η²-C=C) moiety. Complex 2 shows no reactivity toward alkynes, but it reacts with a variety of heterounsaturated molecules such as aldehyde, ketone, carbodiimide, nitrile, organic azide, and diazoalkane derivatives. DFT studies complement the experimental observations and provide additional insights. Furthermore, a comparison between Th and group 4 metals reveals that Th4+ shows unique reactivity patterns. [ABSTRACT FROM AUTHOR]

A novel Cu-catalyzed diastereo- and enantioselective desymmetrization of cyclopropenes to afford nonracemic cyclopropylboronates is described. Trapping the cyclopropylcopper intermediate with electrophilic amines allows for the synthesis of cyclopropylaminoboronic esters and demonstrates the potential of the approach for the synthesis of functionalized cyclopropanes. [ABSTRACT FROM AUTHOR]

A new class of cyclopropenes containinga chroman-4-one motif weresynthesized using 3-diazochroman-4-one and phenylacetylene with rhodium(II)catalyst and followed by cycloisomerization to give 2-substitutedor 3-substituted 4H-furo[3,2-c]chromene,respectively. Using BF3·Et2O as catalyst,2-substituted 4H-furo[3,2-c]chromenewas exclusively obtained in 70% yield. Using Cu(OTf)2ascatalyst, 3-substituted 4H-furo[3,2-c]chromene was obtained in 95% yield with 98:2 regioselectivity. Aone-pot cascade addition–cycloisomerization process was alsodeveloped with no need to isolate cyclopropenes of chroman-4-one intermediates. [ABSTRACT FROM AUTHOR]

Target identification of bioactive compounds within the native cellular environment is important in biomedical research and drug discovery, but it has traditionally been carried out in vitro. Information about how such molecules interact with their endogenous targets (on and off) is currently highly limited. An ideal strategy would be one that recapitulates protein–small molecule interactions in situ (e.g., in living cells) and at the same time enables enrichment of these complexes for subsequent proteome-wide target identification. Similarly, small molecule-based imaging approaches are becoming increasingly available for in situ monitoring of a variety of proteins including enzymes. Chemical proteomic strategies for simultaneous bioimaging and target identification of noncovalent bioactive compounds in live mammalian cells, however, are currently not available. This is due to a lack of photoaffinity labels that are minimally modified from their parental compounds, yet chemically tractable using copper-free bioorthogonal chemistry. We have herein developed novel minimalist linkers containing both an alkyl diazirine and a cyclopropene. We have shown chemical probes (e.g., BD-2) made from such linkers could be used for simultaneous in situ imaging and covalent labeling of endogenous BRD-4 (an important epigenetic protein) via a rapid, copper-free, tetrazine-cyclopropene ligation reaction (k2 > 5 M–1 s–1). The key features of our cyclopropenes, with their unique C-1 linkage to BRD-4-targeting moiety, are their tunable reactivity and solubility, relative stability, and synthetic accessibility. BD-2, which is a linker-modified analogue of (+)-JQ1 (a recently discovered nanomolar protein–protein-interaction inhibitor of BRD-4), was subsequently used in a cell-based proteome profiling experiment for large-scale identification of potential off-targets of (+)-JQ1. Several newly identified targets were subsequently confirmed by preliminary validation experiments. [ABSTRACT FROM AUTHOR]

The rearrangement reactions of silylated alcohols bearing the highly strained structures of cyclopropene and cyclopropanol connected in adjacent positions have been studied under ZnI2- and Rh(I)-catalyzed conditions. The results show intriguing carbon skeletal reorganizations of such system under these conditions. The ZnI2-catalyzed reaction proceeds with C-O cleavage and the rearrangement of the resultant carbon cation, leading to the breaking of the C-C single bond that connects two three-memebered rings. In contrast, the Rh(I)-catalyzed reaction involves σ-bond oxidative addition of the cyclopropene moiety, followed by β-carbon elimination of the cyclopropane moiety. [ABSTRACT FROM AUTHOR]

The asymmetric total syntheses of the natural products (+)- and (-)-frondosin B and (+)-frondosin A are reported based on a diastereoselective cycloaddition between tetrabromocyclopropene and an annulated furan to provide a highly functionalized common building block. The bridged bicyclic intermediate could be stereo- and chemoselectively manipulated to produce the two structurally distinct members of the frondosins. Both syntheses feature regioselective palladium-coupling reactions and an unprecedented phosphine-mediated ether bridge cleavage. Surprisingly, the planned enantiose-lective synthesis of frondosin B led to the opposite epimer of the natural product, suggesting an unusual late stage stereoinversion at C8. Frondosin A, but not frondosin B, was shown to have selective antiproliferative activity against several B-cell lines. [ABSTRACT FROM AUTHOR]

Thetandem allylation of aldehydes or an aldimine with allylzincintermediates derived from organozinc reagents and cyclopropenes isdescribed. The present three-component reaction involves carbozincationof cyclopropene and sequential cleavage of a cyclopropylzinc intermediatein situ without a transition-metal catalyst. The allylzinc intermediatesgenerated in situ, which is an α,β-unsaturated acylanionequivalent, gave the corresponding homoallylic alcohols or amine ingood yields. [ABSTRACT FROM AUTHOR]

For quite a long time, nonclassical carbocations have only been regarded as special intermediates with limited cases in solvolysis reactions. However, the present work shows that in common reaction, typical nonclassical carbocations may be involved in and have significant effects on the reaction mechanisms. In this work, DFT studies have been performed on the mechanism of gold(I)-catalyzed cycloisomerization of 1,5-enynes containing cyclopropene moiety at PBE1PBE/6-31+G**/SDD level. An unprecedented pathway containing two consecutive 1,3-cationic alkylidene migrations of nonclassical carbocation intermediates derived from norbornenyl cation, rather than the generally considered Wagner-Meerwein 1,2-alkyl migrations, was found. Detailed structural analysis shows the nature of this 1,3-cationic alkylidene migration: it is promoted by strong cation-π interaction between the cationic center and the double bond. Topological analysis shows that for certain nonclassical carbocation intermediates (1c'-A and 1c'-F), there do exist bond critical point and bond path between the cationic center and the double bond. On the basis of the mechanisms proposed, the product selectivity controlled by the substituent effects was also rationalized. [ABSTRACT FROM AUTHOR]

The Diels-Alder reactions of the cycloalkenes, cyclohexene through cyclopropene, with a series of dienes?1,3-dimethoxybutadiene, cyclopentadiene, 3,6-dimethyltetrazine, and 3,6-bis(trifluoromethyl)- tetrazine?were studied with quantum mechanical calculations and compared with experimental values when available. The reactivities of cycloalkenes as dienophiles were found by a distortion/interaction analysis to be distortion controlled. The energies required for cycloalkenes to be distorted into the Diels-Alder transition states increase as the ring size of cycloalkenes increases from cyclopropene to cyclohexene, resulting in an increase in activation barriers. The reactivities of the dienes are controlled by both distortion and interaction energies. In normal Diels-Alder reactions with cycloalkenes, the electron-rich 1,3-dimethoxybutadiene exhibits stronger interaction energies than cyclopentadiene, but the high distortion energies required for 1,3-dimethoxybutadiene to achieve transition-state geometries overtake the favorable interaction, resulting in higher activation barriers. In inverse-electrondemand Diels-Alder reactions of 3,6-dimethyltetrazine and 3,6-bis(trifluoromethyl)tetrazine, the reactivities are mainly controlled by interaction energies. [ABSTRACT FROM AUTHOR]

DFT calculations have been carried out in order to rationalize and predict the ring-opening regioselectivity of substituted cyclopropenes in the presence of gold(I) catalysts. It has been shown that the regioselectivity of these ring-opening processes is driven by the relative π-donor ability of the substituents on the cyclopropene double bond (C1 and C2). A stronger π-donor substituent at C2 favors Au(I)-induced polarization of the double bond toward C1, resulting in preferential breaking of the C1-C3 bond. An excellent correlation between ΔE and the difference in the C1-C2 p(π) orbital population was observed for a broad range of substituents, providing a useful predictive model for gold-induced cyclopropene ring-opening. Furthermore, it was found that the stability of the resulting gold-stabilized allyl-cation intermediates do not follow the same trend as the ring-opening reaction energies. Generally, the more facile ring-opening process led to the less thermodynamically stable intermediate, which lacked stabilization of the carbocation by a π-donor in the α-position. [ABSTRACT FROM AUTHOR]

Bioorthogonal chemistries have provided tremendous insight into biomolecule structure and function. However, many popular bioorthogonal transformations are incompatible with one another, limiting their utility for studies of multiple biomolecules in tandem. We identified two reactions that can be used concurrently to tag biomolecules in complex environments: the inverse electron-demand Diels-Alder reaction of tetrazines with 1,3-disubstituted cyclopropenes, and the 1,3-dipolar cycloaddition of nitrile imines with 3,3-disubstituted cyclopropenes. Remarkably, the cyclopropenes used in these transformations differ by the placement of a single methyl group. Such orthogonally reactive scaffolds will bolster efforts to monitor multicomponent processes in cells and organisms. [ABSTRACT FROM AUTHOR]

Convenient access to highly enantioenriched substituted quinolizidines has been achieved by chiral dirhodium(II) carboxylate-catalyzed dearomatizing formal [3 + 3]-cycloaddition of isoquinolinium/pyridinium methylides and enol diazoacetates. Coordination of Lewis basic methylides to dirhodium(II) prompts the rearrangement of the enol-carbene that is bound to dirhodium to produce a donor-acceptor cyclopropene. The donor-acceptor cyclopropene is in equilibrium with the dirhodium-bound enol-carbene and undergoes both enantioselective [3 + 3]-cycloaddition from the dirhodium-bound enol-carbene and diastereoselective [3 + 2]-cycloaddition by uncatalyzed reaction of the cyclopropene with isoquinolinium or pyridinium methylides. Increasing the mol % of catalyst loading suppresses the [3 + 2]-cycloaddition pathway. [ABSTRACT FROM AUTHOR]

Reduction of the first known halogen-containing substrate by nitrogenase (N2ase), 3,3-difluorocyclopropene (DFCP), was investigated. Reduction requires both N2ase proteins (MoFe and Fe protein), ATP, and an exogenous reductant (dithionite, DT), as with N2 and known alternative substrates of the enzyme. Two major products providing evidence for reductive C–F bond cleavage were confirmed, propene (P1, requiring 6e–/6H+) and 2-fluoropropene (P2, 4e–/4H+). Both were identified by GC-MS and NMR spectroscopy, and had the same Km constants (0.022 atm, 5.4 mM). Reduction of 1,2-dideuterated DFCP (d2-DFCP) further revealed that (i) in both P1 and P2, two deuterium atoms are retained, one on carbon-1 and one on carbon-3, indicating that C═C bond cleavage rather than C–C bond cleavage is involved during DFCP reduction at least to P2 (assuming no F migration); (ii) no selectivity was observed in formation of cis and trans isomers of 1,3-d2-2-fluoropropene, whereas cis-1,3-d2-propene is the predominant 1,3-d2-propene product, indicating that one of the bound reduction intermediates on the pathway to propene is constrained geometrically. A reduction mechanism, consistent with hydride transfer as a key step, is discussed. Reductive C–F bond cleavage is an ability of N2ase that further demonstrates the unique and remarkable scope of its catalytic prowess. [ABSTRACT FROM AUTHOR]

With tris(4-methoxyphenyl)phosphine as the nucleophilic reagent, the readily available cyclopropene-1,1-dicarboxylates undergo a ring-opening reaction to generate a Wittig-type intermediate, which would react with aromatic aldehydes to yield (E)-5-aryl-2-(methoxycarbonyl)-2,4-pentadienoates. It is interesting to observe that the regioselectivity of the ring-opening reaction is switched. [ABSTRACT FROM AUTHOR]

Regio- and diastereoselective Lewis acid catalyzed cycloaddition reactions between imines and donor–acceptor cyclopropenes generated from silyl-protected enoldiazoacetates provide direct access to stable cyclopropane-fused tetrahydroquinolines and, with cyclopropane ring opening under mild conditions, to 1H-benzazipine derivatives. [ABSTRACT FROM AUTHOR]

A synthetic method to prepare 2,4a-dihydro-1H-fluorenes efficiently from gold(I)-catalyzed 1,2-acyloxy migration/cyclopropenation/Nazarov cyclization of 1,6-diyne carbonates and esters is described. The suggested reaction pathway provides rare examples of [2,3]-sigmatropic rearrangement in this class of compounds as well as the involvement of an in situ formed cyclopropene intermediate in gold catalysis. Experimental and ONIOM(QM:QM') [our own n-layered integrated molecular orbital and molecular mechanics(quantum mechanics:quantum mechanics')] computational studies based on the proposed Au carbenoid species provide insight into this unique selectivity. [ABSTRACT FROM AUTHOR]

Incorporation of a trifluoromethyl group into the structure of 4-(alkoxycarbonyl)vinyldiazocarbonyl compounds greatly decreases the tendency of the carbenoid intermediates formed during Rh(II)-catalyzed reactions to undergo intermolecular processes. Instead, they are prone to experience intramolecular [1,5]- and [1,3]-electrocyclizations to produce reactive cyclopropenes and furans, and these are capable of further transformations. [ABSTRACT FROM AUTHOR]

Allylic cyclopropenecarboxylates undergo ring expansion reactions to give 2-allyloxyfuran intermediates, which subsequently rearrange to Δβ,γbutenolides via a Claisen rearrangement or to the corresponding Δα,βbutenolides via further Cope rearrangement. Also described are methods for chirality transfer in the rearrangement of nonracemic allylic esters. [ABSTRACT FROM AUTHOR]

DFT/M06 calculations were carried out to better understand the mechanism and regio- and chemo-selectivities of our previously discovered formal C-H amination of silyl cyclopropenes by azodicarboxylates (Chem. Commun. 2012, 48, 10990). The results revealed that the initial Alder-ene reaction between the two reactants follows a stepwise mechanism and the subsequent allylic transposition proceeds via a concerted [1,3]-migration of hydrazodicarboxylate. For the Alder-ene process of 1-silyl-2-methylcyclopropene, different electronic effects of the substituents make the C1 much more negatively charged and thus more reactive than C2 in the regiochemistry-determining electrophilic azodicarboxylate addition step. In addition, the poor regioselectivity caused by a C2 ether linkage and the lower reactivity of ene donors other than cyclopropenes with azodicarboxylate were well explained by the computational results. Furthermore, the divergent allylic transposition of the Alder-ene intermediates was rationalized, and the steric repulsion between the silyl group and the hydrazodicarboxylate moiety was suggested as the driving force in promoting the allylic transposition. The barrier for the rate-controlling [1,3]-migration of hydrazodicarboxylate from an intermediate containing the C1-N bond is 21.1 kcal/mol, whereas a higher barrier of 27.5 kcal/mol is required for the similar rearrangement of the thermodynamically more stable intermediate containing the C2-N bond. [ABSTRACT FROM AUTHOR]

C–C σ-bond activation of cyclopropene byRuCl2(PPh3)2and IrCl(CO)(PMe3)2was theoretically investigated. The activationbarrierand the reaction energy calculated here indicate that the reactionoccurs more easily by the ruthenium complex than by the iridium complex,which is consistent with the experimental results that the Ru–vinylcarbenespecies is formed but the Ir–vinylcarbene is not. The valencebond analysis of the CASSCF wave function disclosed that the Ir–vinylcarbenebond is significantly weaker than that of the Ru complex because thed6square pyramidal complex of Ru is more favorable thanthe d8complex of Ir for this bonding interaction. In boththe Ru and Ir systems, a precursor complex is formed by coordinationof the C1C2double bond of cyclopropenewith the metal center, where C1and C2are sp2carbons and C3is an sp3carbon. Inthe transition state, one C–C single bond (named C1–C3) is almost broken, but the M–C1bond (M = Ru or Ir) and another C–C single bond (named C2–C3) are becoming stronger. When movingfrom the transition state to a metal–vinylcarbene product,the C1C2double bond changes to theC1–C2single bond with the concomitantchange of the C2–C3single bond to theC2C3double bond. To induce these bondformation and bond breaking processes, the valence state of the metalcenter must change in the reaction. The promotion energy to the valencestate becomes smaller in the ruthenium reaction system when goingfrom the reactant to the product but becomes considerably larger inthe iridium reaction system. This is the reason that the C–Cσ-bond cleavage of cyclopropene occurs more easily in the rutheniumcomplex than in the iridium complex. The difference in promotion energybetween the ruthenium and iridium systems is reasonably interpretedin terms of d–d orbital splitting by ligand-field and d electronnumber. [ABSTRACT FROM AUTHOR]

The development of a catalytic carbonyl-olefin metathesis strategy is reported, in the context of the ring-opening metathesis of cyclopropenes with aldehydes using a simple hydrazine catalyst. The key to this reaction is a conceptual blueprint for metathesis chemistry that forgoes the traditional reliance on [2 + 2] cycloaddition modes in favor of a [3 + 2] paradigm. [ABSTRACT FROM AUTHOR]

Described is a Cu-catalyzed directed carbozincation of cyclopropenes with organozinc reagents prepared by I/Mg/Zn exchange. This protocol broadens the scope with respect to functional group tolerance and enables use of aryl iodide precursors, rather than purified diorganozinc precursors. Critical to diastereoselectiviry of the carbozincation step is the removal of magnesium halide salts after transmetalation with ZnCl2. [ABSTRACT FROM AUTHOR]

Gold(I)-catalyzed reactions of thiols, thiophenols, and thioacids with 3,3-disubstituted cyclopropenes occur in a regioselective and chemoselective manner to produce either vinyl thioethers or primary allylic thioesters in good yields. A survey of commonly used gold(I) catalysts shows Echavarren's cationic gold(I) catalyst to be most tolerant of deactivation by sulfur. A novel digold with bridging thiolate complex is characterized by X-ray crystallography, shedding light on a possible deactivation pathway. [ABSTRACT FROM AUTHOR]

To understand the mechanism of allene formation throughthe rearrangement of cyclopropenes catalyzed by PtCl2,we have performed a detailed density functional theory calculationstudy on a representative substrate, 1-(trimethylsilyl)-2-(phenylethyl)cyclopropene.Three reaction pathways proposed in the original study have been examined;however the calculated results seem not to completely rationalizethe experimental findings. Alternatively, by performing an exhaustivesearch on the potential energy surface, we present a novel mechanismof PtCl2, which is fixed appropriately on the cyclopropene/alleneto form the linear Cl–Pt–Cl disposition, a vital configurationfor catalyzing the rearrangement of cyclopropene. The newly proposedmechanism involves an SN2-type C–C bond activationof the cyclopropene by PtCl2fixed on a cyclopropene moleculevia the d−π interaction between the metal center andthe substrate to form the product precursor PtCl2-allenewith the metal center coordinated to the external CC bondin the allene framework. Once formed, the PtCl2-alleneimmediately serves as a new active center to catalyze the rearrangementreaction rather than directly dissociating into the allene productand the PtCl2catalyst due to its high stability. Duringthe catalytic cycle, an allene-PtCl2-allene sandwich compoundis identified as the most stable structure on the potential energysurface, and its direct dissociation results in the formation of theproduct allene and the regeneration of the catalytically active centerPtCl2-allene with an energy demand of 24.4 kcal/mol. Thisprocess is found to be the rate-determining step of the catalyticcycle. In addition, to understand the experimental finding that theH-substituted cyclopropenes do not provide any allenes, we have alsoperformed calculations on the H-substituted cyclopropene system andfound that the highest barrier to be overcome during the catalyticcycle amounts to 35.2 kcal/mol. This high energy barrier can be attributedto the fact that the C–H bond activation is more difficultthan the C–Si bond activation. The theoretical results notonly rationalize well the experimental observations but provide newinsight into the mechanism of the important rearrangement reaction. [ABSTRACT FROM AUTHOR]

The mechanisms of PtCl2-catalyzed rearrangementof cyclopropenesto allenes have been investigated using density functional theorycalculations carried out at the B3LYP/6-31G(d,p) (LANL2TZ(f) for Pt)level of theory. The solvent effect was taken into account by B3LYP/6-311(d,p)(LANL2TZ(f) for Pt) single-point calculations with the integral equationformalism polarizable continuum model (IEFPCM) in dichloromethane.The radii and nonelectrostatic terms were taken from Truhlar and co-workers’universal solvation model (SMD). Three pathways which lead to theformation of allene via a [1,2]-C–C bond migration (regioselectivity)and C–Si bond migration were performed. Our calculations suggestthe following. (1) The major pathway of the cycle involves an initial[1,2]-silyl shift leading to a platinum intermediate. A subsequent[1,2]-C–C bond shift of this intermediate then provides thecorresponding allenes. (2) Due to the consequence of an interactionbetween the Lewis acidic platinum and the allene, the rearrangementof α-alkoxy-substituted cyclopropenes does not give the alleneas the final product. (3) The calculations suggest that the electroniceffect of a silyl substituent on the cyclopropene is essential forthis reaction. Furthermore, the silyl groups provide not only theβ-cation-stabilizing effect but also a facile migration groupfor the whole reaction. [ABSTRACT FROM AUTHOR]

Cyclopropenimines are shown to be a highly effective new dass of enantioselective Brønsted base catalysts. A chiral 2,3-bis(dialkylamino)cyclopropenimine catalyzes the rapid Michael reaction of a glycine imine substrate with high levels of enantioselectivity. A preparative scale reaction to deliver 25 g of product is demonstrated, and a trivial large scale synthesis of the optimal catalyst is shown. In addition, the basicity of a 2,3-bis(diallcylamino)cydopropenimine is measured for the first time and shown to be approximately equivalent to the P1-tBu phosphazene base. An X-ray crystal structure of the protonated catalyst is shown along with a proposed mechanistic and stereochemical rationale. Cyclopropenimines are shown to be a highly effective new class of enantioselective Brønsted base catalysts. A chiral 2,3-bis(dialkylamino)cyclopropenimine catalyzes the rapid Michael reaction of a glycine imine substrate with high levels of enantioselectivity. A preparative scale reaction to deliver 25 g of product is demonstrated, and a trivial large scale synthesis of the optimal catalyst is shown. In addition, the basicity of a 2,3-bis(diallcylamino)cydopropenimine is measured for the first time and shown to be approximately equivalent to the P1-tBu phosphazene base. An X-ray crystal structure of the protonated catalyst is shown along with a proposed mechanistic and stereochemical rationale. [ABSTRACT FROM AUTHOR]