Hydroboration. 67. Cyclic Hydroboration of Acyclic α,ω-Dienes with 9-Borabicyclononane/Borane-Dimethyl Sulfide

Article abstract of DOI:10.1021/jo00180a023

Hydroboration of acyclic α,ω-dienes, 1,3-butadiene, 1,4-pentadiene, 1,5-hexadiene, 1,7-octadiene, 1,8-nonadiene, 1,9-decadiene, 1,11-dodecadiene, and 1,13-tetradecadiene, with 2 molar equiv of 9-borabicyclo<3.3.1>nonane (9-BBN), followed by redistribution of the resulting dumbbell-shaped trialkylboranes with 1 molar equiv of borane-methyl Sulfide complex (BMS), has been investigated.With 1,3-butadiene, the initial redistribution product, the five-membered boracyclane, borolane, underwent a rapid ring-opening reaction to give the known 1,6-diboracyclodecane. 1,4-Pentadiene and 1,5-hexadiene afforded the corresponding boracyclanes, borinane and borepane, in quantitative yields.With all other dienes, the initial redistribution products were polymeric.They were converted to the methyl esters by treatment with methanol, and these products were depolymerized into cyclic derivatives in 88-98percent yield by vacuum distillation at 175-200 deg C.In every case the cyclization was accompanied by varying amounts of isomerization so that the major products were both the parent B-methoxyboracyclane and the corresponding B-methoxy-2-n-alkylborinane.Thus, 1,6-heptadiene afforded B-methoxyborocane and B-methoxy-2-ethylborinane in 3:1 ratio and a total yield of 95percent.With 1,7-octadiene, 1,8-nonadiene, 1,9-decadiene and 1,11-dodecadiene, the isomerized product constituted the major constituent of the distillate. 1,13-Tetradecadiene afforded the parent boracyclane and the isomer in a 47:53 ratio and a combined yield of 82percent.The B-methoxyboracyclanes were converted to the corresponding cyclic ketones by the DCME reaction in 55-65percent yield.