Hydroboration of Sterically Hindered Olefins under High Pressure

Article abstract of DOI:10.1021/jo00143a003

Hydroboration of sterically hindered olefins such as tetramethylethylene, tetraisopropylethylene, trans-1,2-di-tert-butylethylene, and 1,2-dimethyldi-tert-butylethylene has been investigated at high pressure (6000 atm).The previously unknown trithexylborane was prepared.The boron of trithexylborane was found to be isomerized much faster to a terminal carbon than that of dithexyl- and thexylboranes. trans-1,2-Di-tert-butylethylene yields a monoalkylborane at both 1 atm and high pressure (5000-6000 atm).Evidence is presented that is consistent with the hypothesis thata multialkylborane is formed at high pressure that dissociates without an isomerization upon releasing the pressure to 1 atm.Tetraisopropylethylene was found to give no hydroborated products at 1 atm or at high pressure (3500-6000 atm). (E)- and (Z)-1,2-Dimethyldi-tert-butylethylene was prepared, characterized, and hydroborated.A monoalkylborane is formed at 1 atm.At high pressure a trialkylborane is formed, which after returning to 1 atm, yields mainly the isomerized borane. 5β-Chol-9(11)-ene was synthesized from deoxycholic acid.This was found to yield a monoalkylborane after hydroboration at 1 atm or high pressure, in contrast to a literature report for a structurally similar steroid.An isomerized olefin was isolated after oxidation of the steroidal borane.