Carbene complexes analogy with olefins

Both W(CO)5[C(C6Hs)2] and the analogous di-p-tolylmethylene complex have been used in model studies of the olefin metathesis reaction.2 3 In contrast to heteroatom-stabilized carbene complexes such as W(CO)s [C(OCH3)(C6Hs)], pentacarbonyl(diphenylmethylene)tungsten(0) reacts with alkenes to give cyclopropanes and 1,1-diphenylalkenes.2 The compound W(CO)5 [C(C6H5)2] is the best reported catalyst for the metathetical polymerization of 1-methylcyclo-butene.4... 

Mechanistic studies of rhodium porphyrins as cyclopropanation catalysts have resulted in the spectroscopic identification of several potential intermediates in the reaction of ethyl diazoacetate with olefins, including a diazoniumfethoxy-carbonyl)methyl-rhodium complex formed by electrophilic addition of the rhodium center to the a-C atom of ethyl diazoacetate [29]. It is not known if analogous intermediates are also formed in analogous reactions of copper catalysts. However, the initial part of the catalytic cycle leading to the metal carbene intermediate is not of primary concern for the enantioselective reactions described herein. It is the second part, the reaction of the metal-carbene complex with the substrate, that is the enantioselective step. 

Rh(TTP) reacts with alkyl halides, acyl halides, aroyl halides, and sulfonyl halides, but it shows no evidence of reaction with molecular hydrogen. These observations further emphasize the fact that Rh(TTP) is essentially a nucleophile and it therefore reacts with those reagents RX that can oxidatively add by nucleophilic attack (34). Rh(TTP) does not react with H2, and H2 seems always to add to (P complexes via a concerted mechanism (35). It appears that Rh(TTP) has very little diradical character, i.e. it is not a good analog of a carbene (35). It is possible that this unreactivity may be associated with the stereochemistry of chelation by the macrocyclic ligand. Earlier studies on the oxidative addition reactions of Rh(I) complex with a tetraaza macrocycle revealed that the Rh(I) had strong nucleophilic properties but the activation of molecular H2 was not reported (36, 37). This possibility is supported by reports that dialkyl sulfide complexes of rhodium chloride catalyze the hydrogenation of olefins (38). 

---