Preparation and Reactions of Organoboron Compounds

Convenient procedures for the generation of borane, mono and dialkylboranes from lithium borobydride have been reported and a highly efficient synthesis of triorganylboranes via a modified standard organometallic route developed. This last procedure Involves direct reaction of magnesiiun, organic halide and 

BF3 etherate in diethyl ether. If the solvent is changed to THF then the reaction results in quantitative formation of the tetra-organoborate in the one case studied. Triethylborane can be made in 90% yield by a one-pot synthesis involving triethylaluminium.  

A stereospecific synthesis of B-( )-l-alkenyl-9-BBN derivatives that are not available by hydroboration reactions has been developed. The 9-BBN dimer has featured in thermal isomerization and thermolysis studies and similar studies on dimesityl-3-hexyl-borane show that, in comparison to the diphenyl compound, it is quite resistant to thermal isomerization and an explanation is offered.  

Sequential oxidation of trialkylboranes takes place with trimethylamine N-oxide and oxidation of a chiral boronic acid by flavoenzyme cyclohexanone oxygenase proceeds with retention of configuration at the migrating centre in an analogous manner to peroxide oxidation. One electron oxidation of alkyltriphenyl borate anions leads to carbon-boron bond cleavage and the formation of free alkyl radicals.  

Potassium trisiamylborohydride can now be made in a convenient manner and the reaction of cyclic boronic esters possessing a wide variety of sterlc requirements with potassium hydride gives rise to the corresponding borohydrides. These represent a new class of reducing agents whose stability and reactivity have been explored. Copper(I) alkylborohydrldes are obtained from 1 1 mixtures of cuprous chloride and sodium borohydride on reaction with alkenes in THF. 

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