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

Comparative studies were carried out on boron trifluoride etherate, phenylb oron difluoride and meta-nitrophenyboron difluoride for the Lewis acid cata lysed Diels-Alder reaction of cyclopentadiene and a range of standard dieno philes in tetrahydrofuran solution. Phenylboron difluoride showed remarkabl y similar reactivity in terms of yield and endo to exo selectivity to boron trifluoride, whereas meta-nitrophenylboron difluoride was more reactive th an either boron trifluoride or phenylboron difluoride and showed more marke d differences in endo: exo ratios. These results contrast to some extent wi th gas-phase semi-empirical calculations (PM3), which suggest that boron tr ifluoride and meta-nitrophenylboron difluoride should have similar reactivi ty; phenylboron difluoride being less reactive. However, since arylboron di fluorides are easily prepared, these Lewis acids represent a group of poten tially highly tuneable catalysts for Diels-Alder reactions. (C) 1999 Elsevi er Science Ltd. All rights reserved.