Boron compounds reactions, borane reagents

Boron trifluoride is used for the preparation of boranes (see Boron compounds). Diborane is obtained from reaction with alkafl metal hydrides organoboranes are obtained with a suitable Grignard reagent. 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

Any of these BH3 compounds adds readily to most alkenes at room temperature or lower temperatures. The reactions usually are carried out in ether solvents, although hydrocarbon solvents can be used with the borane-dimethyl sulfide complex. When diborane is the reagent, it can be generated either in situ or externally through the reaction of boron trifluoride with sodium borohydride ... 

The addition compound of trimethylamine and monobromo-borane has been prepared by reaction of trimethylamine-borane with either bromine or boron tribromide,4 but an excess of either reagent gives further bromination. Hydrogen bromide, however, gives only the monobromo derivative, even when added in excess. The synthesis is similar to that of trimethylamine-... 

The Focus On box on page 433 described a hydroboration reaction that produces a single enantiomer of a chiral alcohol as the product. The chirality of one enantiomer of the boron hydride reagent is used to control the formation of a single enantiomer of the product. As discussed in that Focus On box, the drawback to this reaction is that it requires one mole of the chiral borane for each mole of chiral alcohol that is produced. The chiral reagent is rather expensive because it must be resolved or prepared from another enantiomerically pure compound. A more desirable process would use the expensive chiral reagent as a catalyst so that a much smaller amount could be employed to produce a larger amount of the chiral product. 

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