Ruthenium-Catalyzed Hydroboration and Dehydrogenative Borylation of Linear and Cyclic Alkenes with Pinacolborane

Abstract

The bis(dihydrogen)ruthenium complex RuH2(H2)2(PCy3)2 (1) catalyzes efficiently the borylation of linear and cyclic alkenes with pinacolborane. Similar results are obtained by using RuH[(μ-H)2Bpin](σ-HBpin)(PCy3)2 (2) as catalytic precursor. Selective hydroboration into the corresponding linear pinacol boronate is achieved in the case of 1-hexene, 1-octene, styrene, and allylbenzene. In the case of styrene, phenethyl pinacolboronate is isolated in 87% yield. Faster conversions of HBpin are obtained by increasing the alkene:borane ratio, with less than 10% of alkene hydrogenation. Isomerization into trans-β-methylstyrene is observed in the case of allylbenzene. Dehydrogenative borylation is competitive with ethylene and cyclic alkenes with large rings. Hydroboration of cyclohexene is highly favored, whereas for cyclodecene, vinylboronate is produced with only traces of allylboronate. Cyclooctene provides the two unsaturated boron-attached products, vinyl- and allylboronate in a 1:3 ratio, whereas only allylboronate is obtained from cycloheptene. The coupling of cyclodecenyl pinacolboronate with the aryl bromide (CF3)2C6H3Br, using standard catalytic Suzuki−Miyaura conditions, gives the corresponding product (CF3)2C6H3(C10H17) in 85% yield. Mechanistic investigations allow the characterization of the hydrido(boryl)(ethylene) complex RuH(Bpin)(C2H4)(PCy3)2 (3) as a key intermediate in the catalytic cycle.