Boron-hydrogen bonds, exchange

The exchange equilibrium of Eq. (75) is also catalyzed by compounds containing boron-hydrogen bonds, such as (CH3)4B2H2 (140,141). 

Dicarbaclovopentaborane(5) is stable up to about 150°C, at which temperature it slowly decomposes affording tan solids and hydrogen 315). When allowed to come into contact with acetone, trimethylamine, carbon dioxide, air, or water at room temperature no noticeable reaction ensues, which is in contrast to the reactivity of most boron hydrides with these reagents. The deuterium in deuteriodiborane exchanges with the three hydrogens bonded to the boron atoms, but not with the hydrogens bonded to the carbon atoms 314). 

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. 

At low temperatures in a variety of solvents, hexaborane(lO) undergoes exchange of a maximum of five hydrogens with deuterodiborane(6) 209>. The product is shown by boron-11 and proton nmr to be substituted only at basal terminal sites. There is no precedent for an exchange of this type at low temperature for a boron hydride without BH2 groups and it has been proposed 209> that the unique short boron-boron bond in the base of the pentagonal pyramidal framework of hexaborane(lO) is involved in the reaction intermediate. 

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