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A ruthenium complex bearing an “anti-Bredt” N-heterocyclic carbene was synthesized, characterized and evaluated as a catalyst for olefin metathesis. Good conversions were observed at room temperature for the formation of di- and tri-substituted olefins by ring-closing metathesis. It also allowed for the ring-opening metathesis polymerization of cyclooctadiene, as well as for the cross-metathesis of cis-1,4-diacetoxy-2-butene with allylbenzene, with enhanced Z/E kinetic selectivity over classical NHC-based catalysts.

The synthesis and characterization of abnormal N-heterocyclic carbene, cyclic (alkyl)(amino)carbene, and mesoionic carbene copper(I) complexes are reported. These organometallic species are obtained via a versatile and inexpensive synthetic pathway using readily available reactants, namely copper oxide and iminium salts. The catalytic activity of this series of complexes was evaluated in the [3+2] cycloaddition of alkynes with azides (CuAAC). Outstanding catalytic properties were observed for the abnormal NHC- and triazolylidene-based copper(I) complexes.

A novel synthetic route gives access to mesoionic carbene and cyclopropenylidene supported gold chloride complexes. The corresponding cationic MIC-gold complex obtained by chloride abstraction allows for the first transition metal-catalyzed functionalization of both nitrogens of parent hydrazine.

ConspectusCarbenes are compounds that feature a divalent carbon atom with only six electrons in its valence shell. In the singlet state, they possess a lone pair of electrons and a vacant orbital and therefore exhibit Lewis acidic and Lewis basic properties, which explains their very high reactivity. Following the preparation by our group in 1988 of the first representative, a variety of stable carbenes are now available, the most popular being the cyclic diaminocarbenes.In this Account, we discuss another class of stable cyclic carbenes, namely, cyclic (alkyl)(amino)carbenes (CAACs), in which one of the electronegative and π-donor amino substituents of diaminocarbenes is replaced by a σ-donating but not π-donating alkyl group. As a consequence, CAACs are more nucleophilic (σ-donating) but also more electrophilic (π-accepting) than diaminocarbenes. Additionally, the presence of a quaternary carbon in the position α to the carbene center provides steric environments that differentiate CAACs dramatically fr...

Anti-Bredt di(amino)carbene supported gold(I) chloride complexes in combination with KB(C6F5)4 as chloride scavenger promote the hydroarylation of styrenes, norbornene, and enones with dialkylanilines in fair to high yields with good para-selectivity.

Shortly after their discovery at the end of the 80s, stable singlet carbenes have been recognized as excellent ligands for transition metal based catalysts, and as organo-catalysts in their own right. At the end of the 2000s, it has been shown that they can coordinate main group elements in their zero oxidation state, and even activate small molecules. This review covers examples in the literature dealing with the most recent application of stable singlet carbenes, namely their use to stabilize radicals and radical ions that are otherwise unstable.

The superior catalytic activity of the newly synthesized NHC-catalyst is attributed to steric factors.

An Au—hydrazine complex, generated in situ, efficiently catalyzes the hydroamination of alkynes with hydrazine.

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

Tetrachloro-o-benzoquinone reacts with bis(diisopropylamino)thioxophosphoranenitrile N-[(phenyl)(2,4,6-trifluoromethylphenyl)phosphanyl]imide 1b affording the corresponding σ5λ5-phosphorus-substituted nitrilimine 2b; addition of HCl to 2b leads to hydrazonoyl chloride 4b which gives back nitrilimine 2b by simple filtration on silica gel.

The synthesis of original (phosphanyl)diazo derivatives possessing either stannyl or phosphonio groups at the diazo carbon is described. The reactivity of transient (phosphanyl)(stannyl)carbenes as well as preliminary X-ray data concerning a (phosphanyl)(phosphonio)carbene are presented.

The reaction of a diazo lithium salt with an electrophile is a quite general method for the synthesis of nitrilimines. A variety of stable 1,3-dipoles of this type can be prepared providing there is at least one electron-withdrawing substituent present. Most of the nitrilimines thermally rearrange into the isomeric diazo derivatives. Under irradiation, these nitrilium betaines can either rearrange to the corresponding carbodiimides or undergo a nitrogennitrogen cleavage. The regioselectivity observed for the 2+3 cycloadditions fits nicely with the predictions based on frontier molecular orbital theory

Palladium(0) catalyzed gem-dimethylation of hexachlorocyclotriphosphazene with tetramethylstannane is described as well as the high yield monoallylation of halogenophosphorus or -boron compounds by allyltrialkylstannanes under photolytic conditions.

Photolysis of [bis(dicyclohexylamino)phosphanyl](trimethylsilyl)diazomethane leads to the corresponding stable carbene1b, which reacts with triethylborane affording a borane–carbene adduct2b, characterized by spectroscopy in solution before rearrangement and fragmentation into P-dicyclohexylamino-C-(ethyl)(trimethylsilyl)phosphaalkene3b; trimethylsilyltrifluoromethanesulfonate adds to 1b giving the methylenephosphonium4b.

The synthesis and reactivity of diazomethylenephosphoranes 2, bis(phosphanyl)carbodiimide 5, and N-phosphanyl nitrilimine 10 are presented. The synthesis of transient N-phosphanyl isodiazirine 19 and its rearrangement into 1,3,2λ5-diazaphosphete 20 are discussed.

Aluminum chloride-amine complexes promote transamidation reactions of primary, secondary, and tertiary amides. Combining this method with the known activating effect of an electron-withdrawing group on the amide nitrogen allows transamidation reactions with hindered and aromatic amines

Aluminum trichloride promotes the nucleophilic ring-opening reaction of cyclic imides and oxazolidinones with amines.

The synthesis and reactivity of 4-π-electron 4-membered heterocycles possessing a phosphazene moiety is described.

1,2λ5-azaphosphete 1 reacts with cyclohexylamine leading to five-membered phosphazenes 7 and 8. This result has been rationalized by the protonation of the ring nitrogen atom followed by nucleophilic attack of the amide at the β-carbon of the ring. Addition of p-cresol and p-thiocresol to 1 affords the linear phosphazenes 12 and 13, resulting from protonation of the α-carbon atom of the ring followed by nucleophilic attack at phosphorus. Compounds 7 and 12 have been characterized by single crystal X-ray diffraction studies.

Overridden: Phosphorus heterocycles are playing tricks with the well-established concept of Huckel antiaromaticity. The long interval between the discoveries of benzene and cyclobutadiene is mirrored by the gap of more than a century between the initial synthesis of hexachlorocyclotriphosphazene (1) and heterocycles of type 2. However, the analogy stops here. Although derivatives of 2 feature a four π electron four-membered ring, they are not antiaromatic.

Publisher Summary Carbenes are electron-deficient, two-coordinate carbon compounds that have two nonbonding electrons at one carbon. This chapter reviews the theoretical aspect, synthesis, structural features, reactivity, and ligand properties of phosphinocarbenes. The (phosphino) (silyl)- and (phosphino) (phosphonio) carbenes are the only stable carbenes that feature a π-donor and a π-type-withdrawing substituent. They have a singlet ground state with a planar environment at phosphorus and a short phosphorus–carbon bond distance. However, because of the reluctance of phosphorus to keep this planar geometry, these “push-pull” carbenes behave in a manner very close to that of the most of the transient carbenes. Their lack of reactivity toward transition-metal centers is probably due to the excessive steric hindrance about the carbene center. They are excellent ligands for metal centers, and some of the complexes prepared show promising catalytic activity. Multinuclear nuclear magnetic resonance (NMR) spectroscopy is the most informative technique for analyzing the structure and bonding of phosphinocarbenes. The reactivity of phosphinocarbenes is strongly dependent on the nature of the other carbene substituent. A few complexes featuring a phosphinocarbene ligand in a number of different coordination modes (A, B, and C) are described.

A (phosphanyl)(trifluoromethyl)carbene is shown to be stable for weeks in solution at temperatures up to −30°C. Its chemical behavior exactly matches that of its transient congeners, even to the extent that subtle effects, such as the recently discovered weak π interaction with aromatics, are observed. The influence of substituents on the structure of push-pull carbenes is demonstrated by a single-crystal x-ray diffraction study of a (phosphanyl)[2,6-bis(trifluoromethyl)phenyl]carbene. The lifetime of these molecules makes them accessible to a wider range of standard techniques, allowing their chemical and physical behaviors to be studied in more detail than was previously possible.

The transition-metal-catalyzed hydroamination reaction, that is, the addition of an N—H bond across a carbon-carbon multiple bond, has been widely studied.[1,2] We have recently reported that cationic gold(I) complexes,[3,4] supported by cyclic (alkyl)(amino)carbene (CAAC) ligands,[5] readily catalyze the intermolecular hydroamination of alkynes with a variety of amines,[6] including ammonia.[6c] Based on preliminary mechanistic studies, we postulated that the key step of the catalytic cycle was the formation of a tricoordinate gold complex (I), which was followed by inner-sphere C—N bond formation, as first postulated by Tanaka et al.,[7a] and Nishina and Yamamoto[7b,c] (Scheme 1). However, for other gold catalysts,[8] Che et al.[9a] and Li et al.[9b] hypothesized an outer-sphere nucleophilic attack to the alkyne complex II, a mechanism widely accepted for palladium[10] and platinum complexes.[11] Herein, our attempts to isolate a gold(I) complex of type I have led to the structural characterization of two (CAAC)(η1-alkene)AuI complexes, and to the discovery of two catalytic reactions: the intramolecular hydroammoniumation using tertiary ammonium salts, and the aminomethylation of carbon–carbon triple bonds. Scheme 1 Possible intermediates I and II in the gold-catalyzed hydroamination of alkynes, and the gold complexes A and B used in this study. For obvious entropic reasons, intramolecular hydroamination reactions occur under much milder conditions than their intermolecular analogues, and therefore are better suited for characterizing reaction intermediates. We first chose N-methyl-2-(2-phenylethynyl)aniline (1), as the rigidity of the phenyl spacer group places both the amino and the alkyne moieties in the ideal positions to coordinate to the metal center. The reaction of 1 with a stoichiometric amount of cationic gold(I) complex B at room temperature was monitored by multinuclear NMR spectroscopy. The instantaneous disappearance of both 1 and B was observed, along with the formation of a new complex, which was isolated in near quantitative yield. However, the spectroscopic data suggested that it was not the desired tricoordinate gold(I) complex of type I, but rather complex 2, presumably formed from the coordination of the hydroamination product 3 to the metal (Scheme 2). Even by monitoring the reaction at −70°C, no trace of starting material 1 could be observed, the conversion into 2 being complete in a few seconds. The high catalytic activity was confirmed using 5 mol% of B; at room temperature, heterocycle 3 formed instantaneously in almost quantitative yield. As expected, addition of one equivalent of B to heterocycle 3 led cleanly to complex 2. Scheme 2 Stoichiometric and catalytic gold(I)-catalyzed intramolecular hydroamination of 1. To prevent the hydroamination process, and thereby possibly characterize a putative tricoordinate gold(I) complex, we prepared the corresponding tertiary amine 4a. A stoichiometric amount of cationic gold(I) complex B was added at room temperature to a solution of 4a in CDCl3, and the reaction was monitored by NMR spectroscopy (Scheme 3). Once again, we observed the instantaneous disappearance of both 4a and B, and the clean formation of a new complex 5, which was isolated in 98% yield. Surprisingly, single-crystal X-ray diffraction[12] revealed that 5 was a gold(I) η1-alkene complex resulting from the addition of the tertiary amino group to the coordinated alkyne (Figure 1, left). Complex 5 is structurally reminiscent of complexes recently isolated by Hammond et al,[13] and Gagne et al.,[14] from the gold-promoted cyclization of allenoates, and the intramolecular hydroarylation of allenes, respectively.[15] Interestingly, the formation of gold(I) η1-alkene complexes of type 5 is not limited to the rigid 2-alkynyl-benzenamine 4a. A mixture of (CAAC)AuCl(A)/AgOTf (1:1) instantaneously reacted stoichiometrically at room temperature with aliphatic N,N-diethylamino alkyne 6 to afford complex 7, which was isolated in 95% yield and fully characterized (Figure 1, right). Figure 1 ORTEP of gold complexes 5 (left) and 7 (right); ellipsoids set at 50% probability. Hydrogen atoms, solvent molecules, and the corresponding counterions are omitted for clarity. Scheme 3 Synthesis of complexes 5 and 7. Not surprisingly, treatment of complex 5 with one equivalent of trifluoromethanesulfonic acid induced immediate proto-deauration,[2,16] affording the corresponding gold-free cyclic ammonium salt 8a (Scheme 4). The stoichiometric two-step transformation of 4a into 8a (through 5) led to the question of whether this process could be catalytic for gold, or if the presence of triflic acid would preferentially protonate the basic tertiary amine,[7a,17] preventing the cyclization process. Addition of one equivalent of triflic acid to solution of alkynyl amine 4a in chloroform readily gave rise to the corresponding acyclic ammonium salt 9. In the absence of the gold catalyst, no cyclization occurred, even under heating a CDCl3 solution of 9 in a sealed tube at 120 °C for three days[18] (Table 1, entry 1). In contrast, we were pleased to observe that in the presence of 5 mol% of a 1:1 mixture of (CAAC)AuCl(A)/AgOTf, derivative 9 underwent the desired cyclization to form 8a in 98% yield after only 3 h at 70°C (Table 1, entry 2). Scheme 4 Stoichiometric and catalytic syntheses of 8a. X=TfO. Table 1 Gold(I)-catalyzed intramolecular hydroammoniumation of 4 with HOTf.[a] Following a brief investigation into the scope of this hydroammoniumation reaction, we found that aryl or alkyl substituents were tolerated on the alkyne, and that the cyclization process occurred under milder conditions when a more weakly basic amine was used. For example, the hydroammoniumation of 4 f readily occurred in a few minutes at room temperature (Table 1, entry 7). Examples in the literature of the direct carboamination of alkynes (the addition of a carbon–nitrogen bond to a carbon–carbon triple bond) are very rare. Yamamoto et al.[19] have reported the platinum- and palladium-catalyzed intramolecular C—N bond addition of amides and N,O-acetals, and Cacchi et al.[20] reported the palladium-catalyzed cyclization of 2-alkynyl-N-allyl-N′-trifluoroacetyl benzenamine. Although the cleavage of a relatively weak carbon–nitrogen bond was involved in both cases, these results prompted us to investigate the related methylamination reaction: in the presence of 10 mol% of a 1:1 mixture of (CAAC)AuCl(A)/KB(C6F5)4, 2-alkynyl-N,N′-dimethyl-benzenamines 4a–d was transformed into 2,3-disubstituted indoles 10a–d in good to excellent yields after 20 h at 160°C (Scheme 5). This rearrangement tolerates aryl or alkyl substituents on the alkyne. However, in the case of the N-ethyl-N′-methyl-benzenamine 4e, loss of ethylene was observed, and the 3-unsubstituted indole 3 was cleanly formed. Scheme 5 First examples of catalytic methylamination of alkyne. These results suggest that, at least for the intramolecular variant, the gold-catalyzed hydroamination of alkynes does not involve a tricoordinate gold complex of type I. Importantly, cationic gold(I) complexes supported by CAAC ligands promote the intramolecular hydroammoniumation and methylamination reactions of alkynes. The scope of these catalytic reactions is under investigation.

The reaction between various bis(dialkylamino)chlorophosphines and N-isopropyl-substituted lithium amides afforded bis(α,α′-phosphino)imines in a ‘one-pot’ procedure. These compounds react with sulfur to generate intramolecularly hydrogen-bonded ylide derivatives. The molecular structure of (Pr2iN)2PCH2C(NBut)CH2P(NPri)2 has been determined by X-ray crystallography.

A cationic (CAAC)gold(I) complex promotes the addition of all types of nontertiary amines to a variety of allenes, affording allylic amines in good to excellent yields; the amino fragment always adds to the less substituted terminus of the CCC skeleton.