Directive effects in the monohydroboration of alkynes and enynes

Abstract

Monohydroboration of terminal alkynes such as 1-octyne and phenylacetylene with dicyclohexylborane or thexylborane (2,3-dimethyl-2-butylborane) places the boron nearly exclusively at the terminal position of the triple bond. 2-Methyl-1 -buten-3-yne and 1 -ethynylcyclohexene react similarly with these mono-and dialkylboranes to afford high yields of the corresponding terminal dienylboranes. The direction of addition of dicyclohexylborane or disiamylborane (bis(3-methyl-2-butyl)borane) to unsymmetrically disubstituted alkynes is markedly affected by the size of the substituents attached to the triple bond. Thus, the /er/-butyl group directs the boron nearly exclusively to the less hindered position of the triple bond of /eri-butylmethylacetylene, /eri-butyl-«-butylacetylene, and rerí-butylisopropylacetylene. Also, secondary alkyl groups direct the boron at least 85 % toward the «-alkyl-substituted end of a triple bond. This observed regiospecificity in the hydroboration of disubstituted alkynes thus provides a convenient route for the selective synthesis of highly substituted ketones via the hydroboration-oxidation reaction. Thexylborane displays a high selectivity for the less hindered carbon of the triple bond in rm-butyl-n-alkylacetylenes. However, replacing the tert-butyl by a sec-alkyl group results in significant loss of selectivity for the attack at the carbon carrying the «-alkyl substituent. Finally, borane is essentially nonselective in its addition to disubstituted alkynes.

Hydroboration of Enynes and Mechanistic Insights

“…Later, they extended their studies to hydroboration of substituted enynes, 2-methyl-1-buten-3-yne, 1-ethynylcyclohexene and 1-hexen-5-yne, with Cy 2 BH or thexylborane (2,3-dimethyl-2butylborane). [17] Monohydroboration of 2-methyl-1-buten-3-yne gave trans-3-methyl-l,3-butadiene-l-d, after the deuterolysis of the intermediate dienylborane. The result showed that BÀ H bond addition takes place exclusively at the terminal carbon of triple bond.…”

“…This result showed the directing effect of the boryl moiety in the hydroboration reaction. Later, they extended their studies to hydroboration of substituted enynes, 2‐methyl‐1‐buten‐3‐yne, 1‐ethynylcyclohexene and 1‐hexen‐5‐yne, with Cy 2 BH or thexylborane (2,3‐dimethyl‐2‐butylborane) . Monohydroboration of 2‐methyl‐1‐buten‐3‐yne gave trans ‐3‐methyl‐l,3‐butadiene‐l‐ d , after the deuterolysis of the intermediate dienylborane.…”

Alkyne Reductions

“…The equivalent reaction with internal alkynes is more controlled, and reasonable yields of the corresponding cis -trialkenylboranes can be obtained (Scheme 15.1 ) [37] . However, unsymmetrical internal alkynes react with only modest regioselectivity [38] . Both steric and electronic factors affect the regioselectivity of monohydroboration ( Figure 15.3 ).…”

Formation and thermal rearrangements of some dimers of butadiene and piperylene. Tests of the validity of thermochemical-kinetic arguments for identification of common biradical intermediates

“…Optically active 3a was prepared from (lS,2S)-(+)-dimethyI trans-1,2-cyclobutanedicarboxylate (34), 56.2 (c 20.3, CCl4), 38.2% optically pure, by reduction to the dialdehyde, which then was treated with methylidenetriphenylphosphorane. The water was removed under reduced pressure and the contents of the flask were taken up in ether.…”