Boron-halogen bond

Prior to 1958 only nitrile-adducts were reported to result from the reaction of nitriles with trihaloboranes (c.f. 36)). The first reported example of the 1,2-addition of boron-halogen bonds to a C=N group involves the reaction of tri-fluoroacetonitrile with trichloroborane or tribromoborane 4) and leads to dimeric derivatives. A 1 1 ratio of the cis-trans isomers is obtained as indicated on the basis of 19F n.m.r. studies. 

Exchange involving boron-halogen bonds and boron-oxygen bonds has been observed in the synthetically important reactions for the preparation of alkyldihaloboranes from alkylboroxines and boron trihalides (165, 167, 169). 

Because both donor acceptor and boron-halogen bonds are labile in many adducts, donor-for-halogen exchange is possible giving ionic adducts of 1 1 stoichiometry ... 

The fact that BF3 forms a wider range of complexes than the other tri-halides is probably due to a greater difficulty of heterolysis with the B—F bond than with the B—Cl and B—Br bonds. Thus molecules of the alcohols, aldehydes and ketones, which form addition compounds with BF3, break the boron-halogen bonds in the other halides ... 

G. Reactions Leading to Formation of Boron-Halogen Bonds.304... 

Negishi reported the regioselective and stereoselective addition of boron-halogen bonds across terminal alkynes (Scheme 6.39) [81]. This work utilized propyne as a prototypical terminal alkyne and BBr as the boron source. The chemistry took place at low temperatures and generated the 1,2-adduct with Z-stereochemistry. Trapping of this compound through the addition of pinacol afforded the vinylboronate. 

Brown postulated that increasing the Lewis acidity of boron would improve the reaction rate of the reduction thus optimising the ee s observed. This was achieved by introducing a boron-halogen bond, with chlorine proving the best choice, and so DIP-Cl was developed. 

In conclusion we turn to work involving compounds with boron-halogen bonds. The thermal decomposition of (5) above 1200 °C involves the expul-... 

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