And carbene insertion

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

Photoexcitation of cycloalkenes introduces additional features because the ring limits the extent to which the double bond can twist. Cyclohexene, cycloheptene, and cyclooctene give rise to ring contraction and carbene insertion products. 

Scheme 3-6 Bis-cyclopentene formation via double Michael addition and carbene insertions in the reaction of nucleophiles with bis-iodonium diynes.

The mononuclear complex 13 was also employed for carbene transfer reactions from ethyl diazoacetate (EDA) in a range of reactions that led to the alkenation of aldehydes, cyclopropanation, and carbene insertion into N—H and O—H bonds [32]. The complex proved particularly adept at the last process, especially aromatic amines and aliphatic alcohols. Addition of the PIN ligand (l-isopropyl-3-(5,7-dimethyl-l,8-naphthyrid-2-yl)imidazol-2-ylidene) to [Ru2(CO)4(OAc)2], foUowed by treatment with Na[BAr 4] gave the dinuclear complex [Ru2(PIN)2(CO)4][[BAr 4] 2, which showed some improved reactivity compared to 13, particularly in the transfer of CH(C02Et) to aldehydes [109]. 

Cycloheptenes and cyclooctenes on irradiation, in addition to cis-trans-iso-merization, undergo ring contraction and carbene insertion reactions [6, 7]. 

The analogous methoxy carbene complexes could not be isolated by following this carbene transfer route, and instead products of carbene insertion into the palladium-aryl bond 54 are observed, for example. Equation (20). These reactions are directly analogous with those observed for alkyl- and arylpalladium heterocyclic carbene complexes, in which both reductive elimination and carbene insertion reactions have been observed (Section 8.04.2.3.4). 

Since carbene ligands =CH2 and =CR2 have electronic structures related to that of CO, it is expected that methylene and carbene insertions are possible. There are only very few studies, however, because carbene ligands are more fragile than CO. 

Olefin AND carbene insertion into metal hydrides... 

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