Methylene homologations

D. 1-Methoxy Group as a Eormaldehyde Synthon Methylene Homologation. 115... 

SCHEME 30. Allylic and allenic zinc reagents via methylene homologation... 

Methylene homologations of organometallic reagents using organozincs... 

The first practical approach applying this strategy consisted of a carbocupration of an alkynyl sulfoxide 47 followed by methylene homologation of 48 with a zinc carbenoid and /3-elimination (equation 20)25. Yields of 1,1-disubstituted allenes 50 are good to high as outlined in Table 4. 

In this strategy a copper-catalysed carbozincation32 is employed, leading to organozinc 64 which, upon reaction with diiodomethane, undergoes a methylene homologation. Note the excellent diastereoselectivity even for two groups as similar as methyl and methyl-D3 ... 

A notable double insertion of methylacetylene into the methylene bridge has been observed with the classical -y-lactone-type cobalt complexes 1. The heterocyclic methylene ligand (CRR ) originally attached to both metal centers is displaced by the new alkylidene ligand ju.-C(CH3)— C(H)=C(CH3)—C(H)=CRR (116, 122) (Scheme 28a other ligands omitted for clarity). This example also points to the possibility of methylene homologation in the coordination core of a catalyst (see Section IX). 

Methylene homologation, with zinc carbenoids, 9, 127 a-Methylene-y-lactone, via carbonylation, 11, 549 Methylenes... 

Hydrogenation catalysts. A number of new optically active phosphine ligands have been developed for selective enantioselective reduction of N=C bonds of amino acid precursors, including DPCB(l)1, the methylene homolog (2)2 of dipamp, and the novel ligand 3, in which the source of chirality is a Re atom.5 A ligand 4 has been used as a Rh(I) complex for hydrosilylation.4... 

Spi-zinc carbenoids such as (iodomethyl)zinc iodide or bis(iodomethyl)zinc have also been used for the selective creation of carbon-carbon bonds through a methylene homologation.295... 

Methylene homologation of vinylcoppers. Allylmetal species are formed, which undergo transpositional alkylation. 

Zinc carbenoids such as (iodomethyl)zinc iodide or 6t (iodomethyI)zinc have been successfully used for the formation of new carbon-carbon bonds allowing selective methylene homologations. The synthesis of homoallylic amines has been reported by a three-component coupling of aldimines, diiodomethane, and alkenylzirconocenes, in the presence of dimethylzinc (Scheme 2-157). The formation of homoallylic products and the a t/-relationship of the final product were rationalized by the intermediacy of a six-membered cyclic chair-like transition state (Scheme 2-158). 

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