Dialkylboron halides

Alkynyltriethylborates react with dialkylboron halides to form a new boron-carbon... 

A rationale similar to this may be used to explain the selective formation of fO)-enolates of tert-heptylthio- and rer/-butylthiopropionates (Scheme 5.15 and ref. [101]) the Et3CS- replaces the Me3CO- in the Scheme 5.18 rationale) and the selective formation of ketone Z(0)-enolates with dialkyl boron triflates and (0)-enolates with dialkylboron halides (Scheme 5.16 and 5.17 the triflates are more likely to ionize than the halides, thus favoring ionization over direct deprotonation of the zwitterion). 

Reaction of boron trihalides with trialkylboranes in the presence of catalytic amounts of sodium borohydride or diborane induces a redistribution to generate dialkylboron halides in good yields."" Generally, the reaction requires high temperature. The importance of this redistribution reaction... 

Organoboron halides also can be converted to organoboranes via controlled hydridation reactions using lithium aluminum hydride. A stepwise hydroboration to prepare mixed dialkylboron halides or trialkylboranes has been developed. The process successfully solves the dihydrobora-tion problem noted in early hydroboration reactions that employed borane-methyl sulfide. The... 

SCHEME 23.30 Syntheses of mixed dialkylboron halides via stepwise hydroboration. 

A modified stepwise hydroboration avoiding the use of lithium aluminum hydride has also been developed (Schane 23.30). It was discovered that trialkylsilane reacts rapidly with boron trihalide to form unsolvated dihaloborane. Subsequent reaction with alkenes in the presence of sufficient boron trihalide occurs rapidly at -78°C to afford alkylboron dihalides without detectable contamination by dialkylboron halides. If additional equivalents of both trialkylsilane and alkene are added, dialkylboron halides are obtained. The methodology has been successfully applied to the synthesis of l,l,I-tris(dichloroboryl)alkanes, important precursors for dicarbapentaboranes. ... 

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