Synthesis of B-R-9-BBN Not Available via Hydroboration

The B-alkyl, alkenyl, alkynyl, or aryl-9-BBN derivatives not available via hydroboration are obtained by alkylation of 9-BBN or B-C1-9-BBN or B-MeO-9-BBN with organolithium, Grignard reagent, organocadmium, lithium dialkylcuprates, (phenylthio)alkylcuprates, andhydroboration-carbonylation-reduction. 

In THF or hydrocarbon solvents, 9-BBN reacts rapidly with 1 equiv of organolithium reagents at -78 C. The intermediate formed is oxidized with methyl iodide [1] or protonolyzed with methanesulfonic acid [2] to afford excellent yields ofB-R-9-BBN (Chart 23.1 Table 23.1) [1]. 

B-Chloro-9-BBN, prepared readily from 9-BBN by treatment with dry hydrogen chloride in ether, reacts rapidly with organolithiums at -78 °C in n-pentane. The intermediate ate complex is not stable and breaks down to organoborane and lithium chloride even at -78 C (Chart 23.2) [1]. 

The lithium salt precipitates out quantitatively from pentane solution. Consequently, the pure B-R-9-BBN is isolated by decantation from the salt and removal of the solvent. It is necessary that the stoichiometry of the reaction is closely controlled in order to achieve high yields. The excess organolithium reagent reacts to give the insoluble ate complex. 

The results of alkylation of B-C1-9-BBN with organolithiums are summarized in Table 23.2 [1]. 

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