Application of arylboron difluoride Lewis acid catalysts to the Diels- Alder reaction: Convenient, non-volatile alternatives to boron trifluoride

Article abstract of DOI:10.1016/S0040-4020(99)00472-X

Comparative studies were carded out on boron trifluoride etherate, phenylboron difluoride and meta-nitrophenylboron difluoride for the Lewis acid catalysed Diels-Alder reaction of cyclopentadiene and a range of standard dienophiles in tetrahydrofuran solution. Phenylboron difluoride showed remarkably similar reactivity in terms of yield and endo to exo selectivity to boron trifluoride, whereas meta-nitrophenylboron difluoride was more reactive than either boron trifluoride or phenylboron difluoride and showed more marked differences in endo: exo ratios. These results contrast to some extent with gas-phase semi-empirical calculations (PM3), which suggest that boron trifluoride and meta-nitrophenylboron difluoride should have similar reactivity; phenylboron difluoride being less reactive. However, since arylboron difluorides are easily prepared, these Lewis acids represent a group of potentially highly tuneable catalysts for Diels-Alder reactions.