ABSTRACT
Due to the empty p-orbital on boron, boron halides (BX3:X = F, Cl, Br, I) and organoboron halides (RBX2, R2BX) are Lewis acids. The observed Lewis acidity of boron trihalides is in the order BI3 > BBr3 > BCl3 > BF3 which is exactly the reverse of that expected on the basis of the relative σ-donor strengths of the halide anions but is easily rationalized by considering back bonding through the halogen p-orbitals.1 One major application of BX3 in organic syntheses is C-O bond cleavage in ethers.2 The notable features of boron halide ether cleavage are mild reaction conditions and high regio-and chemoselectivities. As haloboration reagents, boron trihalides are used to synthesize (Z)-2-halo-1-alkenylboron halides,3 highly functionalized intermediates that play an important role in the syntheses of numerous vinyl halides, alkadienes, alkenynes, and ole™nic products. In addition, boron trihalides are used as halogenation reagents in conversions of alcohols into the corresponding halides.4 Halogen exchange between alkyl halides and boron trihalides has also been reported.5 The reaction of boron trihalides with N,O-based chiral ligands is also an active research area because the chiral boron Lewis acids thus obtained possess the capacity to induce chirality in other molecules. The concept has been successfully used for enantioselective Diels-Alder reactions.6
