Triarylphosphine Ligands with Hemilabile Alkoxy Groups. Ligands for Nickel(II)-Catalyzed Olefin Dimerization Reactions. Hydrovinylation of Vi-nylarenes, 1,3-Dienes, and Cycloisomerization of 1,6-Dienes.
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 (L5andL6respectively) 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 ligandL6is 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,L5. The other two ligands, 2-benzyloxymethyl-(L6) and 2-benzyloxyethyl-diphenylphosphine (L7) 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 ofL5, but not ofL6orL7. In an attempt to prepare authentic samples of the methylencyclohexane products, nickel(II)-complexes ofN-heterocyclic carbene-ligands were examined. In sharp contrast to the phosphines, which give the methylenecyclopentanes, methylenecyclohexanes are the major products in the (N-heterocyclic carbene)nickel(II)-mediated reactions.
Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18 Avenue, Columbus, OHIO 43210, USA ; phone: (001)-614-688-3543.
Souvagya
S
Biswas
Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18 Avenue, Columbus, OHIO 43210, USA ; phone: (001)-614-688-3543.
Aibin
A
Zhang
Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18 Avenue, Columbus, OHIO 43210, USA ; phone: (001)-614-688-3543.
Balaram
B
Raya
Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18 Avenue, Columbus, OHIO 43210, USA ; phone: (001)-614-688-3543.
T V
TV
RajanBabu
cycloisomerization; diene; hemilabile ligands; hydrovinylation; ligand effects; nickel