Search

This review summarizes double functionalization strategies for diaryliodonium(iii) salts involving the C–IIII and ortho C–H bonds, which provide short-step access to highly functionalized molecules.

Boron-boron multiple bonds, such as those found in diborenes and diborynes, are typically stabilized by σ-donor ligands that furnish electron density to these otherwise electron-deficient species. These compounds are not only of fundamental importance in the study of chemical bonding, but can also activate small molecules in a chemistry reminiscent of that carried out by transition metals. In the pursuit of designing new and improved σ-donor ligands to stabilize diborenes and diborynes suitable to activate small molecules, we performed density functional calculations to evaluate the Lewis basicity of a series of σ-donor ligands. For this evaluation, we analysed the interaction between the boranes and the σ-donor ligands in model systems L→BX3 (X=F and Me) using energy decomposition analyses. We found that electronic bond energies of the L→BX3 adducts correlate well with the ionization energies of the ligands and that ligands with high or medium basicity stabilize diborynes better than ligands with low basicity. We also learnt that beryllium-based ligands are promising since they are able to stabilize L→B≡B←L diborynes without significantly reducing the triple bond character of the B≡B bond.

Carbene‐stabilized diborynes of the form LBBL (L=N‐heterocyclic carbene (NHC) or cyclic alkyl(amino)carbene (CAAC)) induce rapid, high yielding, intermolecular ortho‐C−H borylation at N‐heterocycles at room temperature. A simple pyridyldiborene is formed when an NHC‐stabilized diboryne is combined with pyridine, while a CAAC‐stabilized diboryne leads to activation of two pyridine molecules to give a tricyclic alkylideneborane, which can be forced to undergo a further H‐shift resulting in a zwitterionic, doubly benzo‐fused 1,3,2,5‐diazadiborinine by heating. Use of the extended N‐heteroaromatic quinoline leads to a borylmethyleneborane under mild conditions via an unprecedented boron‐carbon exchange process.

Meridional tridentate N-heterocyclic carbene (NHC)-based pincer ligands contribute to a substantial growth in modern organometallic chemistry in both homogeneous catalysis and luminescence materials. Among all NHC-based pincer ligands, the dianionic LX2-type CCC-pincer ones constitute the smallest subcategory owing to their limited ligand frameworks suitable for complexation. This work reports a one-pot, high-yield synthesis of a homoleptic anionic all-carbon bis-pincer iridium(III) complex (4) directly from a bis(aryl)-substituted dipyrido-annulated (dpaAr2) imidazolium salt and [Ir(COD)Cl]2 via a cascade of deprotonation/C-H activation processes. Both experimental complexation chemistry and computational mechanistic investigation suggest that the large bite angle and π-rich character of the dpaAr2 NHC are responsible for its facile complexation as a dianionic LX2-type CCC-pincer ligand precursor. The all-carbon ligated iridium(III) complex (4) bearing a π-conjugated ligand scaffold showed remarkably low oxidation potentials, which allows future investigations in its redox chemistry and photophysical properties.

A cyclic alkyl(amino)carbene‐stabilized 1,4‐diborabenzene (DBB) ligand enables the isolation of 18‐electron two‐legged parent piano‐stool Fe\(^{0}\) and Ru\(^{0}\) complexes, [(η\(^{6}\)‐DBB)M(CO)\(_{2}\)], the ruthenium complex being the first of its kind to be structurally characterized. [(η\(^{6}\)‐DBB)Fe(CO)\(_{2}\)] reacts with E\(_{4}\) (E=P, As) to yield mixed DBB‐cyclo‐E\(_{4}\) sandwich complexes with planar E\(_{4}\)\(^{2-}\) ligands. Computational analyses confirm the strong electron‐donating capacity of the DBB ligand and show that the E\(_{4}\) ligand is bound by four equivalent Fe−P σ bonds.

Iridabicycles [Ir{κ3 -N,C,O-(pyC(H)=C(C(O)Me)2 }(Cl)(L-L)](L-L=cod (cod=1,5-cyclooctadiene), 1 a; bipy (bipy=2,2'-bipyridine), 1 b) have been obtained by oxidative coordination of 3-(pyridine-2-yl-methylene)pentane-2,4-dione L1, to the complexes [{Ir(μ-Cl)(cod)}2 ] and [{Ir(μ-Cl)(coe)2 }2 ] (coe=cis-cyclooctene), the latter in the presence of bipy. Remarkably, cleavage of the C3 -C(O)Me bond of L1 has instead been achieved in the reaction with [Ir(Cl)(dmb)2 ] (dmb=2,3-dimethylbutadiene), yielding a compound formulated as [Ir{κ2 -N,C-(pyC(H)C(C(O)Me))}(CO)(μ-Cl)(Me)]2 , 2. Treatment of dimer 2 with DMSO or PMe3 produced the complexes[Ir{κ2 -N,C-(pyC(H)C(C(O)Me)}(CO)Cl(Me)L] (L=DMSO, 3 a; PMe3 , 3 b). Plausible mechanisms for the reactions leading to complexes 1 and 2 are proposed by means of DFT calculations.

The reactions of terminal acetylenes with doubly Lewis base-stabilised diborenes resulted in different outcomes depending on the nature of the ligands at boron and the conformation of the diborene (cyclic versus acyclic). N-heterocyclic carbene (NHC)-stabilised diborenes tended to undergo anti-selective hydroalkynylation at room temperature, whereas [2 + 2] cycloaddition was observed at higher temperatures, invariably followed by a C–N bond activation at one NHC ligand, leading to the ring-expansion of the initially formed BCBC ring and formation of novel boron-containing heterocycles. For phosphine-stabilised diborenes only [2 + 2] cycloaddition was observed, followed by a rearrangement of the resulting 1,2-dihydro-1,2-diborete to the corresponding 1,3-isomer, which amounts to complete scission of both the BB double and CC triple bonds of the reactants. The elusive 1,2-isomer was finally trapped by using a cyclic phosphine-stabilised diborene, which prevented rearrangement to the 1,3-isomer. Extensive density functional theory (DFT) calculations provide a rationale for the selectivity observed.

The reaction of Power’s digalladeltacyclane with Au(PPh3)Cl led to formation of a digallium gold complex (2) with both chloride and gold bridging between two gallium atoms. The bonding parameters f...

The reductive coupling of a N-heterocyclic carbene (NHC)-stabilized aryldibromoborane yields a mixture of trans- and cis-diborenes in which the aryl groups are coplanar with the diborene core. Under dilute reduction conditions two diastereomers of a borirane–borane intermediate are isolated, which upon further reduction give rise to the aforementioned diborene mixture. DFT calculations suggest a mechanism proceeding via nucleophilic attack of a dicoordinate borylene intermediate on the aryl ring and subsequent intramolecular B–B bond formation.

Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H–OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH−) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations., A (bisborane)triarylphosphine was developed to spontaneously generate H2 from water under ambient conditions, revealing an unprecedented multi-centered electron relay mechanism for a metal-free umpolung of proton to hydride.

The first catalytic procedure for the electrophilic nitration of phenols was developed using iodosylbenzene as an organocatalyst based on iodine(III) and aluminum nitrate as a nitro group source. This atom-economic protocol occurs under mild, non-Bronsted acidic and open-flask reaction conditions with a broad functional-group tolerance including several heterocycles. Density functional theory (DFT) calculations at the (SMD:MeCN)Mo8-HX/(LANLo8+f,6-311+G*) level indicated that the reaction proceeds through a cationic pathway that efficiently generates the NO2+ ion, which is the nitrating species under neutral conditions.

Herein, three novel Pt(II) complexes with formula [trans-Pt(Br-PyBenz-X)(Cl)2(DMSO)] (1–3) having Br-pyridylbenz-(imida, oxa or othia)-zole (L1–3) derivatives as potential bidentate ligands, under an unusual κ1-N-coordination mode are reported. All compounds were obtained straightforwardly via reaction of corresponding LPB1–3 and [Pt(Cl)2(DMSO)2] (DMSO = dimethyl sulfoxide), at 100 °C in acetonitrile, respectively. 1–3 complexes were characterized by analytical and spectroscopic data: melting point, FT-IR, Raman, UV/Vis and NMR experiments. Cyclic voltammetry studies show an irreversible two-electron process at −0.50 and −0.51 V, which was ascribed to the Pt(II)/Pt(IV) couple, for complexes 2 and 3. The crystal structure of complex 2 was elucidated by single-crystal X-ray diffraction, where the platinum atom exhibits a square plane geometry, where LPB2 adopts an unusual mono-coordinated mode via an N-κ1-benzoxazole ring. According to DFT calculations the first N-coordination exchanging one DMSO molecule is favourable, while the second N-coordination is highly impeded.

A series of twenty segmented poly(ester-urethane)s (PEUs) were synthesized, by the reaction of four different macrodiols derived from e-caprolactone (CL) and 1,8-octanediol (Oct), 1,6-hexamethylene diisocyanate (HDI), and three different carotenoids, (3R,3′R,6′R)-lutein (1) obtained from a renewable source such as marigold (Tagetes erecta L.), (3R,3′S)-zeaxanthin (2) and (3R,3′S)-astaxanthin (3) obtained by partial synthesis from 1, as chain-extender agents. The mechanical properties of the PEUs showed evident plastic behavior. Some PEUs samples exhibited double segregation phases attributed to PCL (soft segment) and 1, 2, and 3/HDI (hard segment) detected by DSC. This work represents the first report of the use of carotenoids as chain extenders in the synthesis of PEUs.

The chemoselective reaction of the C- followed by the O-centered naphthyl radicals with the more electron-deficient hypervalent bond of the diaryliodonium(III) salts is described. This discovered reactivity constitutes a new activation mode of the diaryliodonium(III) salts which enabled a one-pot doubly arylation of naphthols through the sequential Csp2 - Csp2 /O- Csp2 bond formation. The naphthyl radicals were generated in the reaction by the tetramethylpiperidinyl radical (TMP·) which resulted from the homolytic fragmentation of the precursor TMP2O. Experimental and DFT calculations provided a complete panorama of the reaction mechanism.

The ring-opening polymerization (ROP) of L-lactide (L-LA) was induced by the catalytic action of bismuth subsalicylate (BiSS) using linear aliphatic diols [HO(CH2)nOH, where n = 2, 3, 4, 5, 6, and 8] as initiators and chain transfer agents. The theoretical and experimental degree of polymerization (DP) in all samples of α,ω-hydroxy telechelic poly(L-lactide) (HOPLLAOH) had a good agreement in all samples, an effect attributed to the interaction of BiSS with HO(CH2)nOH inducing a transfer reaction. HOPLLAOH was synthesized and characterized by a range of analytical techniques, confirming the insertion of methylene groups from the initiator into the main chain of the polyester. The glass-transition temperature (Tg) of HOPLLAOH was found to be proportional to the number of methylene groups present in the diol. Various parameters regarding the ROP of L-LA were studied, such as temperature, time of reaction, amount of catalyst, and the nature of the diols. A kinetic study of the reaction allowed the determination of the rate constants (k) and activation energy (Ea). A mechanism of initiation is proposed based on a computational study using density functional theory (DFT), evidencing the role of the alkyl diol as an initiator, producing an alkoxide (Bi–OROH). This species then acts as a nucleophile, attacking the carbonyl group, inducing its insertion, and ultimately completing the ring-opening of L-LA.

Density functional theory (DFT) was applied to mimic active species of ethylbenzene dehydrogenase (EBDH), a molybdoenzyme. Considering the impact of several aspects including transition metals, Bronsted-Lowry acid/base conjugate pairs, the trans effect, and supporting ligands upon the relevant activation barriers for methane and toluene led to conclusions that, on average, the transition states for methane activation/functionalization are roughly 13 kcal/mol higher than those for toluene, which reflects the difference in homolytic C-H bond enthalpies for methane and toluene (Δ BDE ∼ 16 kcal/mol). For the seven different methane functionalization catalysts, the average free energies for the H-atom abstraction (HAA) and radical rebound transition states are 36 ± 15 and 41 ± 17 kcal/mol, respectively, with the Cr complexes being toward the lower end of these ranges, and the heavier Mo and W oxo complexes being closer to the upper end. The analysis of the free energies indicates that the energies of the two transition states and the intermediate that lies between them are all positively correlated, thus synthetic strategies to reduce one barrier should positively impact to the other. Among the various chemical parameters, the impact of metal and organic substrate was seen to be more significant than the impact of acid/base conjugate pair, trans ligand and supporting ligand substituents.

CO\(_2\) is found to undergo room-temperature, ambient- pressure reactions with two species containing boron-boron multiple bonds, leading to incorporation of either one or two CO\(_2\) molecules. In one case, a thermally-unstable intermediate was structurally characterized, indicating the operation of an initial 2+2 cycloaddition mechanism in the reaction.

A practical electrophilic bromination procedure for phenols and phenol–ethers was developed under efficient and very mild reaction conditions. A broad scope of arenes was investigated, including the benzimidazole and carbazole core as well as analgesics such as naproxen and paracetamol. The new I(III)-based brominating reagent PhIOAcBr is operationally easy to prepare by mixing PIDA and AlBr3. Our DFT calculations suggest that this is likely the brominating active species, which is prepared in situ or isolated after centrifugation. Its stability at 4 °C after preparation was confirmed over a period of one month and no significant loss of its reactivity was observed. Additionally, the gram-scale bromination of 2-naphthol proceeds with excellent yields. Even for sterically hindered substrates, a moderately good reactivity is observed.

Recently, the hydrophenoxylation of alkynes mediated by a gold catalyst has been shown to occur through a bimetallic process where the active species are [Au(OPh)(IPr)] and [Au(η2-alkyne)(IPr)]+. On the other hand, Cazin and co-workers have reported an improvement of the hydrophenoxylation reaction by using a combination of [Cu(OPh)(IPr)] and [Au(η2-alkyne)(IPr)]+. Herein, we performed DFT calculations to rationalize the performance of the Cu/Au bimetallic system. We have studied the rate determining step (rds) proposed by Poater et al. for the potential initial reagents PhO–[M] and alkyne–[M] species. Silver was also included in this study in order to understand the effect of the metal. Moreover, the effect of the steric hindrance on the rds was investigated with less sterically demanding ligands such as IMes, SIMes and IMe. Overall, the [Cu]/[Au] system was computed to be superior to the [Au]/[Au] system, but the [Ag]/[Au] couple exhibited the lowest energy barrier for the rds providing an enhancement of the heterometallic catalysis. We show that the rds is quite sensitive to the ligand steric hindrance.

A new bis(cyclopentadienyl) dialane is prepared, which shows controlled, selective dialumination reactions with a conventional alkyne, an electron-rich alkyne, and an azide. The reactions provide structurally diverse products, featuring a range of aluminium coordination numbers, cyclopentadienyl binding modes, and cyclic motifs. The variable nature of the bonding in the Cp*Al units allows a range of binding modes depending on the electronic requirements of the Al atom and provides new possibilities to the chemistry of dialanes, as demonstrated by the isolation of a double internal Lewis adduct with "ring-slipped" Cp* rings in this work.

A catalytic, practical and high-yielding procedure for the synthesis of indenes by direct Csp3–H activation under gold(I) catalysis was developed. The scope of the protocol was determined by synthesizing some electron-neutral, electron-poor as well as electron-rich derivatives including the dibenzofurane and carbazole heterocycles. The mechanism of this reaction was elucidated by theoretical calculations using a ONIOM(M08-HX/mixed-basis:PM6) hybrid scheme. Thereby we found a pericyclic transformation involving a [1,5]-H shift generating a gold(I)–carbene that evolves to the indene derivative. In comparison with several reports, our protocol presents a direct activation of the Csp3–H bond.

The reaction of tetraiododiborane (B2I4) with trans- [Pt(BI2)I(PCy3)2] led to isolation of the diplatinum(II) complex [{(Cy3P)(I2B)Pt}2(μ2:h3:h3-B2I4)], supported by the bridging diboranyl dianion ligand [B2I4]2-. The complex is the first transition metal complex of a diboranyl dianion, as well as the first example of intact coordination of a B2X4 (X = halide) unit of any type to a metal center.

The combination of electron-rich diaminoalkynes and ditopic Lewis acids diboranes(4) leads to unusual uncatalysed diboration reactions involving internal Lewis adduct, zwitterion, and C-C bond formation. The products are novel multicyclic, charge-separated compounds with intramolecular dative bonds.

A series of sixteen new 2-julolidin-imidazo[1,2-a]pyridine bound-type bis-heterocycles were synthesized in good to excellent yields (61–98%) via an MW-assisted Groebke–Blackburn–Bienayme (GBB) reaction. Then, experimental studies were conducted to determine the luminescence properties of these compounds. Various high quantum yields were found. In fact, one product exhibited a quantum yield (86.6%) comparable to that of the reference compound rhodamine (94.8%). Finally, further computational studies were performed by means of TD-DFT to calculate their HOMO–LUMO distributions and theoretical absorption spectra. A good agreement between the experimental and computational results was obtained, which provides a rationalized explanation for the observed structure/property relationships.

The first catalytic procedure for the electrophilic nitration of phenols was developed using iodosylbenzene as an organocatalyst based on iodine(III) and aluminum nitrate as a nitro group source. This atom-economic protocol occurs under mild, non-Brønsted acidic and open-flask reaction conditions with a broad functional-group tolerance including several heterocycles. Density functional theory (DFT) calculations at the (SMD:MeCN)Mo8-HX/(LANLo8+f,6-311+G*) level indicated that the reaction proceeds through a cationic pathway that efficiently generates the NO

A series of fifteen 2-tetrazolylmethyl-isoindolin-1-one linked-type bis-heterocycles were synthesized in 10–76% yields under mild conditions via a one-pot Ugi-azide/(N-acylation/exo-Diels–Alder)/dehydration process from furan-2-ylmethanamine, aldehydes, isocyanides, azidotrimethylsilane, and maleic anhydride. Density functional theory calculations were performed using the polarizable continuum model (toluene)-M06-2X-D3/6-311+G(d)//M06-2X-D3/6-31G(d) level of theory to obtain the full energy profile when investigating over eight possible pathways. An anthracene-containing analogue displayed a distribution of its highest occupied molecular orbital–lowest unoccupied molecular orbital throughout both cyclic moieties.

Density Functional Theory (DFT) based calculations were performed at the PCM (toluene)-M06-2X/6-311+G(d)//M06-2X/6-31G(d) level of theory to rationalize the construction of the pyrrolo[3,4-b]pyridin-5-one core from a variety of 5-aminooxazoles and maleic anhydride because there are two possible reaction pathways; A = N-acylation/aza Diels–Alder/aromatization and B = aza Diels–Alder/N-acylation/aromatization. After analyzing the nineteen energy profiles, we concluded that the reaction route depends on the stereoelectronic nature of substituents of the 5-aminooxazole. Two examples followed Path A whereas the others seventeen preferred Path B.

Boryl, borylene, and base-stabilized borylene complexes of manganese and iron undergo a range of different reactions when treated with isonitriles including single, double, and partial isonitrile insertions into metal-boron bonds, ring formation, isonitrile coupling, and the liberation of new monovalent boron species. Two of the resulting cyclic species have also been found to react selectively with anhydrous HCl to form ring-opened products. The diverse isonitrile-promoted reactivity of transition-metal-boron compounds has been explored computationally.

The mechanism of methylation of Vaska's complex trans-[ClIr(CO)(PPh3)2] by trimethylgallium was studied and the identification of the spectroscopically detected intermediates was achieved with the aid of computational methods. The reaction pathway, computed by means of density functional theory (M05-2X-D3/def2-SVP), involves the initial formation of a chloride-bridged adduct trans-[(Cl·GaMe3)Ir(CO)(PPh3)2] to then proceeds to a transition state [(μ2-Cl,C-ClMeGaMe2)Ir(CO)(PPh3)2]. This transition state subsequently evolves to the methylated adduct [MeIr(CO)(PPh3)2·(GaMe2Cl)] to finally release the alkylated product trans-[MeIr(CO)(PPh3)2] together with GaMe2Cl.

A catalytic, practical and high-yielding procedure for the synthesis of indenes by direct Csp3-H activation under gold(i) catalysis was developed. The scope of the protocol was determined by synthesizing some electron-neutral, electron-poor as well as electron-rich derivatives including the dibenzofurane and carbazole heterocycles. The mechanism of this reaction was elucidated by theoretical calculations using a ONIOM(M08-HX/mixed-basis:PM6) hybrid scheme. Thereby we found a pericyclic transformation involving a [1,5]-H shift generating a gold(i)-carbene that evolves to the indene derivative. In comparison with several reports, our protocol presents a direct activation of the Csp3-H bond.

The reaction of the tetrahalodiboranes(4) B2F4, B2Cl4, and B2Br4 with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(Cy3P)2Pt(BF2)2] (1) and the novel borylborato complexes trans-[(Cy3P)2Pt{B(X)–BX3}] (2, X = Cl; 3, X = Br), respectively. The trans influence of the borylborato group was found to be one of the strongest ever observed experimentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) F2B–BMes2 and the new derivative F2B–BAn2 (An = 9-anthryl) toward a range of platinum(0) complexes was investigated. Reactions with relatively nonbulky platinum(0) complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R3P)2Pt(BF2)(BMes2)] (6, R = Me; 7, R = Et) as well as the first example of a fourfold-unsymmetrical bis(boryl) complex, [(Me3P)(Cy3P)Pt(BF2)(BMes2)] (12). The use of a more bulky Pt complex provided access to the unprecedented dinuclear bis(boryl) complexes [{(iPr3P)Pt}2(μ-BF2)(μ-BAr2)] (8, Ar = Me...

The coordination chemistry and stability of aminoborylene ligands bearing different N-heterocyclic carbene (NHC) stabilizing groups has been investigated with Group VI and VIII metals. NHC-aminoborylene complexes have been accessed via reduction of NHC-dihaloaminoborane adducts with Na2[M(CO)x] species (M = Fe, Ru, Cr, W). Imidazol-2-ylidene-stabilized aminoborylene ligands were found to afford thermally robust metal-borylene complexes, which are inert to oxidation, hydrolysis, and insertion of unsaturated substrates. Such ligands have additionally been demonstrated to be significantly more electron releasing than NHCs and other carbon-based ligands by infrared spectroscopy, and can be regarded as unique examples of highly nucleophilic borylene ligands isolobal to classical NHCs. In contrast, cyclic alkylaminocarbene (CAAC)-bound dihaloaminoboranes were found to be reduced by one or two electrons upon reaction with Na2[M(CO)x] species to form either a stable borane-centered radical, or the free CAAC-amino...

We report two rare instances of an insertion into the strong (ca. 170 kcal mol-1) BN triple bond of iminoboranes. In the first, a silylene inserts into di-tert-butyliminoborane to form an iminosilane. In the second, the highly crowded iminoborane Ter-NB-TMP (TMP = 2,2,6,6-tetramethylpiperidyl, Ter = 2,6-(diphenylmethyl)-4-tert-butylphenyl) can be forced to react with Pip-CC-Pip (Pip = piperidyl) at 60 °C. The reaction product is the apparent result of Pip-CC insertion into the iminoborane BN bond.