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 (L5 and L6 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 L6 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, 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 of L5, but not of L6 or L7. In an attempt to prepare authentic samples of the methylencyclohexane products, nickel(II)-complexes of N-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.

NHC/Nickel(II)‐Catalyzed [3+2] Cross‐Dimerization of Unactivated Olefins and Methylenecyclopropanes

“…[8] The intermediates, such as cyclopropylmethyl-Ni/Pd II , often undergo either b-carbon eliminations or insertion of other partners, providing a general route to build hydro-functionalized products. Recently, [NHC-Ni(allyl)]BAr F (NHC = N-heterocyclic carbene) was identified as a cross-hydroalkenylation catalyst [9] by using cyclopropenes and alkynes, and it involves a cyclopropane ring-opening (Scheme 1 c). [9j] Thus, we become interested in [NHC-Ni(allyl)]BAr F for MCP activation.…”

Broadly Applicable Stereoselective Syntheses of Serrulatane, Amphilectane Diterpenes, and Their Diastereoisomeric Congeners Using Asymmetric Hydrovinylation for Absolute Stereochemical Control

“…The diene substrate was significantly less reactive vis-à-vis the vinylarenes, requiring temperatures > − 10 °C as compared to − 70 °C for the vinylarenes. Ligand L7 , which we found 41 to be an exceptional ligand for the nickel-catalyzed hydrovinylation of vinylarenes, was ineffective for the hydrovinylation of the 1,3-diene. In sharp contrast, nickel complex of ligand L6 gave quantitative yield of the product 50 in a diastereomeric ratio of 59:41 [C 11 ( S ):C 11 ( R )].…”

NHC/Nickel(II)‐Catalyzed [3+2] Cross‐Dimerization of Unactivated Olefins and Methylenecyclopropanes

“…Our study commenced with a simple MCP, 1 a , and the terminal alkene 2 a by using the [IPr‐Ni(allyl)]BAr F catalyst reported for cycloisomerization and hydroalkenylation– (Table , entry 2). To our surprise, the postulated cross‐hydroalkenylation or β‐carbon‐elimination products were not observed, but a new [3+2] product ( 3 aa ) was observed for the first time.…”