Reactions of Diboron Tetrahalides with Unsaturated Compounds

REACTIONS OF DIBORON TETRAHALIDES WITH UNSATURATED COMPOUNDS 

One of the most interesting and characteristic reactions of the diboron tetrahalides is their ready addition across carbon-carbon double and triple bonds. This process, which has been called diboration 101) by analogy to the well-known hydroboration reaction, provides a convenient route to vicinal bis(dihaloboryl)organic derivatives  

The resulting compounds may be converted to other bis(boryl) derivatives by a variety of metathetical reactions appropriate to organoboron dihalides, by oxidative hydrolysis to wfc-diols, or, in certain cases, to hydrocarbons by protonolysis. 

The addition reaction has been observed with a variety of olefins, acetylenes, and with cyclopropane 23). Both B2CI4 and B2F4 can add to suitable unsaturated molecules. Unsuccessful attempts to achieve similar reactions with tetraamino and tetraalkoxydiboron compounds and with 1,2-bis-(dimethylamino)-l,2-diethyldiborane(4) have been noted 13, 68). Evidence for formation of bis(boryl) derivatives from the reaction of mixtures of B2[N(CH3)2]4 and BCI3 with olefins has been mentioned (115) the reaction was presumed to involve a mixed dimethylaminochlorodiboron derivative. Lewis base complexes of B2CI4 are unreactive toward olefins (56). 

Most workers have presumed that the addition reaction proceeds via TT-donation from the olefin into the vacant/) orbitals of the diboron compound (55). Stereochemical evidence (discussed more fully below. Section III,B) is in general agreement with such a four-center mechanism, and there is as yet no convincing evidence for an alternative. The involvement of the boron p orbitals is clearly indicated by the lack of reactivity of B2Cl4-base complexes as well as by the failure to obtain addition with compounds such as the aminodiborons in which the coordinative saturation at the boron atoms can be removed or reduced by pir-pn bonding. 

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III. Reactions of Diboron Tetrahalides with Unsaturated Compounds 251... 

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