Hydroboration-oxidation of alkynes

Aldehydes are prepared by the hydroboration-oxidation of alkynes (see Section 5.3.1) or selective oxidation of primary alcohols (see Section 5.7.9), and partial reduction of acid chlorides (see Section 5.7.21) and esters (see Section 5.7.22) or nitriles (see Section 5.7.23) with lithium tri-terr-butox-yaluminium hydride [LiAlH(0- Bu)3] and diisobutylaluminium hydride (DIBAH), respectively. 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Hydroboration-Oxidation of Alkynes Hydroboration-oxidation of an alkyne gives anti-Markovnikov addition of water across the triple bond. Di(secondary isoamyl)borane, called disiamylborane, is used, since this bulky borane cannot add twice across the triple bond. On oxidation of the borane, the unstable enol quickly tautomerizes to an aldehyde. (See Section 9-9F.)... 

Aldehydes and ketones have a central role in organic synthesis, and efficient procedures for their preparation are of great importance. Such compounds are synthesized in a number of ways, including hydration or hydroboration-oxidation of alkynes (Eqs. 16.10 and 16.11, respectively, and Chap. 11) and reaction of carboxylic acids or their derivatives with organometallic reagents or reducing agents... 

Hydroboration-oxidation of alkynes is believed to proceed via a mechanism that is similar to the mechanism invoked for hydroboration-oxidation of alkenes (Mechanism 9.3). Specifically, borane adds to the alkyne in a concerted process that gives an is ft-Markovnikov addition. There is, however, one critical difference. Unlike an alkene, which only possesses one it bond, an alkyne possesses two it bonds. As a result, two molecules of BH3 can add across the alkyne. To prevent the second addition, a dialkyl borane (R2BH) is employed instead of BH3. The two alkyl groups provide steric hindrance that prevents the second addition. Two commonly used dialkyl boranes are disiamylborane and 9-BBN ... 

The addition of borane to alkenes is stereospecifically cis and leads to the formation of tri-alkylboranes. These may be oxidized to alcohols using the anion of hydrogen peroxide. Overall addition of water is achieved, in a c/s-stereospecific, anti-Markovnikov manner. Hydroboration/oxidation of alkynes gives ketones, after tautomerization of the enol formed. c/s-Dihydroxylation of alkenes is accomplished with catalytic OSO4 plus an oxidant such as NMO or K3[Fe(CN)g]. This contrasts with the formation of frans-diols by epoxidation of alkenes followed by the opening of the epoxide with hydroxyl ion. 

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