Carbene acceptors

Further work by Trost established the involvement of metallacarbenoid species.292,293 A dimer product 307, incorporating a cyclopropyl group, was observed in the reaction of 304 in the presence of the highly electron-deficient palladole catalyst 305 (Scheme 78). This transformation is the signature of an intermediate of type 306. This chemistry could be rendered useful by playing with other unsaturated bonds as the carbene acceptor, and a variety of polycyclic adducts such as 308 could be synthesized. 

Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptor-substituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,1072-1074,1076,1085,1086],... 

In the absence of a carbene acceptor during thermal decomposition of diarzirines, cyclopropanes can still be formed (3) as illustrated in Eq. 3 ... 

If a diazoalkane (2), made from diazomethane and an acid chloride, is heated or photolysed in the absence of a carbene acceptor, the carbene (3) rearranges to an electrophilic ketene" (4) which captures a nucleophilic solvent, e.g. to give ester (5). The result is that the chain length of the original acid (1) has been increased by one CH2 group. 

The dediazoniation of 3//-diazirines can be effected thermally or photochemically [22]. In the absence of carbene acceptors, the initially formed carbenes isomerize to give olefins, e.g. ... 

The common overall reaction in carbene chemistry is methylene transfer. A (substituted) methylene fragment is eliminated from the "carbene precursor and inserted into the carbene acceptor . Kinetic methods have been widely employed to determine whether divalent carbon intermediates are involved in these reactions. Two major lines of investigation may be distinguished, (/) kinetic studies of the decomposition of carbene precursors, and ( ) determination of the relative rates of product formation. The mechanistic possibilities range from the intervention of free carbenes to fully concerted methylene transfer reactions. 

Two other methods which have been widely used for the preparation of cyclopentadienylides involve the carbenic decomposition of diazo-cyclopentadiene in the presence of a suitable carbene acceptor, and the condensation of cyclopentadienes with sulphoxides or arsine oxides, e.g. 

The reaction of acyl chlorides with diazomethane is fast and selective in the presence of a C=C bond, but not in the presence of other carbene acceptors. Knowledge of the chemoselective reactivity of carbenes is of prime importance for planning the retro-Amdt-Eistert disconnection of the target molecule. 

Only c -dimethylcyclopropanes were observed. The trimethylstannyl carbene, similarly generated, yields the anri-7-stannylnorcaraae with cyclohexene, but unlike the case of the silylcarbene which dimerizes, no trace of distannylethylenes was found, even in the absence of carbene acceptors. The rearrangement of the carbenes p-Me3MC6H4CH (M=C, Si, Ge, or Sn) in the vapour phase is thought to be step-wise (Scheme 1). With M=C, no dimethylindane was found, in contrast... 

Wanzlick showed that the stability of carbenes is increased by a special substitution pattern of the disubstituted carbon atom [12-16]. Substituents in the vicinal position, which provide n-donor/a-acceptor character (Scheme 2, X), stabilize the lone pair by filling the p-orbital of the carbene carbon. The negative inductive effect reduces the electrophilicity and therefore also the reactivity of the singlet carbene. 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Electron-deficient 1,3-dienes are known to react when heated with metho-xy(aryl)- or methoxy(alkyl)carbene complexes to afford vinylcyclopropane derivatives with high regioselectivity and diastereoselectivity [8a, 24]. Cyclo-propanation of the double bond not bearing the acceptor functional group and... 

The 1,3-dipolar cycloadditions are a powerful kind of reaction for the preparation of functionalised five-membered heterocycles [42]. In the field of Fischer carbene complexes, the a,/ -unsaturated derivatives have been scarcely used in cyclo additions with 1,3-dipoles in contrast with other types of cyclo additions [43]. These complexes have low energy LUMOs, due to the electron-acceptor character of the pentacarbonyl metal fragment, and hence, they react with electron-rich dipoles with high energy HOMOs. 

The Diels-Alder reaction of activated olefins is considered as one of the most useful and predictable reactions in organic synthesis. The electron-acceptor character of the pentacarbonylmetal fragment makes a,/J-unsaturated carbene complexes ideal substrates for the [4S+2C] cycloaddition reaction with dienes. 

Chiral diaminocarbene complexes of copper were used in asymmetric conjugate addition of diethylzinc to Michael acceptors. Achiral copper carbene complexes derived from imidazolium salts were synthesized and characterized for the first time by Arduengo in 1993 [43]. In 2001, Woodward reported the use of such Arduengo-type carbene in copper-catalyzed conjugate addition and showed their strong accelerating effect [44]. The same year, Alex-... 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

Additional evidence for the contention that metathesis carbenes are nucleophilic was offered by Gassman in an interesting series of trapping experiments utilizing Michael acceptors as carbene traps (15, 17). Thus, an ethylidene carbene generated from 2-butene was trapped by ethyl acrylate to yield the expected ethylcyclopropyl ester, although yields were quite low. 

---