Nickel catalysis methylenecyclopropanes

A remarkable product selectivity is also observed in the case of methylenecyclopropanes with geminal diphenyl substitution. Whereas under nickel catalysis [Ni(cod)2 at SO-TO C] 18 is selectively dimerized to tra7w-l,l,6,6-tetraphenyldispiro[2.1.2.1]octane, which can be obtained in 28% yield at a conversion of 40%, substrate 18 reacts in a completely different manner with palladium(O) catalysts derived from (t/ -allyl)( -cyclopentadienyl)palladium(II) and triisopropylphosphane. Besides isomerization to 19, proceeding at temperatures above 85 °C, the monospiro derivative 20 is formed as the major product. Additionally, minor amounts of a formal [3 + 3] dimer 21 can be isolated. The latter probably arises from a palladium-mediated, stoichiometric reaction as the yield of 21 could not be improved under any conditions in catalytic runs. On prolonged heating, thermal isomerization of the methylenecyclopropanes to form... 

A suitable approach to the synthesis of spiro[2.3]hexanes is the [2-1-2] cycloaddition of alkenes to the double bond of methylenecyclopropanes. This reaction is often described as a codimerization and usually requires catalysis by a nickel(O) complex such as bis(cycloocta-l,5-diene)nickel. l,l-Dimethyl-2-methylenecyclopropane reacts with alkyl acrylates to give a mixture of alkyl cis- and tranj-l,l-dimethylspiro[2.3]hexane-5-carboxylates 1 (19-40%) and alkyl 3,3-dimethyl-4-methylenecyclopentanecarboxylate 2 (60-81 %). The proportion of spiro [2.3]hexane derivative was highest when rer/-butyl acrylate was used as the activated alkene. 

When geminally disubstituted methylenecyclopropanes 9 are employed, however, the formation of pyrroles is impossible due to the presence of a tetrasubstituted ring carbon. These reactions therefore selectively yield a-methylene-d -pyrrolines. °° It is remarkable that palladium(O), as well as nickel(O), catalysis gives rise to the same products. The nickel(0)-catalyzed reactions require only mild reaction conditions and are thus especially convenient for preparative purposes. 

The nickel(0)-mediated codimerization of methylenecyclopropane, as well as 2,2-disubstituted derivatives with electron-deficient olefins, has proven to be an excellent system for auxiliary controlled selectivity. As described in Section 1.6.1.2.3.1, the combination of methylenecyclopropane with alkyl acrylates and Ni(cod)2 catalysis results in the clean formation of typeB cycloadducts under mild conditions. These systems allow facile chiral modification in the form of acrylate esters or amides involving nonracemic residues and employment of the chiral camphorsultam7n as the auxiliary leads to impressive diastereoselectivity 1. The stereoselectiv-... 

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