Thexylborane Explained

Thexylborane is a borane with the formula [Me₂CHCMe₂BH₂]₂ (Me = methyl). The name derives from "t-hexylborane" (although the group is not the standard tert-hexyl group), and the formula is often abbreviated ThxBH₂. A colorless liquid, it is a rare, easily accessed monoalkylborane. It is produced by the hydroboration of tetramethylethylene:^[1]

B₂H₆ + 2 Me₂C=CMe₂ → [Me₂CHCMe₂BH₂]₂

Reactions

Thexylborane is generated in situ. In solution, it isomerizes over the course several days to the 2,3-dimethyl-1-butyl derivative, shown as the monomer here:

Me₂CHCMe₂BH₂ → Me₂CHCH(Me)CH₂BH₂

Thexylborane allows the synthesis of ketones by coupling a pair of alkenes with carbon monoxide, which serves as a carbonyl linchpin:

Me₂CHCMe₂BH₂ + 2 RCH=CH₂ → Me₂CHCH(Me)CH₂B(CH₂CH₂R)₂

Me₂CHCH(Me)CH₂B(CH₂CH₂R)₂ + CO + H₂O → O=C(CH₂CH₂R)₂ + ...An important feature of this reagent is that the thexyl group almost never undergoes anionotropic 1,2-migration from boron to a neighboring atom.^[2]

References

1. Thexylborane-A Highly Versatile Reagent for Organic Synthesis via Hydroboration. Synthesis. 1974. 2. 77–89. 1974. Negishi. Ei-Ichi. Brown. Herbert C.. 10.1055/s-1974-23248. 96012955 .
2. Aggarwal . Varinder K. . Fang . Guang Yu . Ginesta . Xavier . Howells . Dean M. . Zaja . Mirko . 2006-01-01 . Toward an understanding of the factors responsible for the 1,2-migration of alkyl groups in borate complexes . Pure and Applied Chemistry . en . 78 . 2 . 215–229 . 10.1351/pac200678020215 . 13833993 . 1365-3075. free .