Triarylphosphine Ligands with Hemilabile Alkoxy Groups: Ligands for Nickel(II)‐Catalyzed Olefin Dimerization Reactions. Hydrovinylation of Vinylarenes, 1,3‐Dienes, and Cycloisomerization of 1,6‐Dienes

Abstract

Substitution of one of the phenyl groups of triphenylphosphine with a 2-benzyloxy-, 2-benzyloxymethyl- or 2-benzyloxyethyl-phenyl moiety results in a set of simple ligands, which exhibit strikingly different behaviour in various nickel(II)-catalyzed olefin dimerization reactions. Complexes of ligands with 2-benzyloxyphenyl-, 2-benzyloxymethylphenyl-diphenylphosphine (<b>L5</b> and <b>L6</b> respectively) are most active for hydrovinylation (HV) of vinylarenes, with the former leading to extensive isomerization of the primary 3-aryl-1-butenes into the conjugated 2-aryl-2-butenes even at -55 °C. However, 2-benzyloxymethyl-substituted ligand <b>L6</b> is slightly less active, leading up to quantitative yields of the primary products of HV at ambient temperature with no trace of isomerization, thus providing the best option for a practical synthesis of these compounds. In sharp contrast, hydrovinylation of a variety of 1,3-dienes is best catalyzed by nickel(II)-complexes of 2-benzyloxyphenyldiphenylphosphine, <b>L5</b>. The other two ligands, 2-benzyloxymethyl-(<b>L6</b>) and 2-benzyloxyethyl-diphenylphosphine (<b>L7</b>) are much less effective in the HV of 1,3-dienes. Nickel(II)-catalyzed cycloisomerization of 1,6-dienes into methylenecyclopentanes, a reaction mechanistically related to the other hydrovinylation reactions, is also uniquely effected by nickel(II)-complexes of <b>L5</b>, but not of <b>L6</b> or <b>L7</b>. In an attempt to prepare authentic samples of the methylencyclohexane products, nickel(II)-complexes of <i>N</i>-heterocyclic carbene-ligands were examined. In sharp contrast to the phosphines, which give the methylenecyclopentanes, methylenecyclohexanes are the major products in the (<i>N</i>-heterocyclic carbene)nickel(II)-mediated reactions.