HYDROBORATION .51. HYDROBORATION OF REPRESENTATIVE ALLENES WITH 9-BORABICYCLO[3.3.1]NONANE - EXCEPTIONAL DIRECTIVE EFFECT PROVIDING A DIRECT SYNTHESIS OF B-ALLYL-9-BORABICYCLO[3.3.1]NONANE DERIVATIVES

The exceptional sensitivity toward structure exhibited by 9-borabicyclo[3.3.1 jnonane (9-BBN) in the hydroboration of simple olefins, conjugated and nonconjugated dienes, and acetylenes carries over to the hydroboration of alienes. With few exceptions, the 9-BBN hydroboration of alienes results in the attack of boron at the end carbons of the aliene moiety giving the S-allyl-9-BBN derivative. Hydroboration of an unsymmetrical aliene could lead to two allylboranes. However, only the one with boron bound to the less substituted position is produced. For example, hydroboration of 2-methyl-2, 3-pentadiene with 9-BBN affords only the 2-bora-4-methyI-3-pentenyl adduct. This may not reflect solely the initial position of boron attack on the aliene since such allylboranes possess a permanent allylic rearrangement whereby the boron can easily migrate to the less substituted site. Simple control of the stoichiometry of these hydroborations appears to lead primarily to monohydroboration products. Only propadiene exhibits exclusive dihydroboration. In sharp contrast to the behavior of 9-BBN, hydroboration of internal alienes with other dialkylboranes, such as disiamylborane or dicyclohexylborane, reveals preferential attack of the boron at the central carbon of the allenic chain, forming vinylboranes. For example, 2, 4-dimethyl-2, 3-pentadiene (tetramethylallene) is converted essentially quantitatively to the vinylborane with dicyclohexyl- and disiamylborane, but exclusively to the allylborane with 9-BBN. Allylboranes formed via hydroboration of alienes with 9-BBN may be used for the allylboration of carbonyl derivatives (transferring the allyl group in the common organometallic manner from boron to the carbonyl carbon). The sequence aliene hydroboration/carbonyl allylboration provides an attractive alternative to the familiar Grignard allylation scheme. © 1979, American Chemical Society. All rights reserved.