COMPLEXES CONTAINING A LEWIS-ACID AND BRONSTED ACID FOR THE CATALYTICASYMMETRIC DIELS-ALDER REACTION

Complexes containing a Lewis acid and a Bronsted acid have been prepar ed by the reaction of an amino alcohol and a trihaloborane. The struct ures of the complexes were determined by B-11, H-1, and C-13 analysis and shown to be the acyclic structures 5 rather than the possible cycl ic structures 6 which only contain the single Lewis acid. A variety of amino alcohols were combined with BBr3 and BCl3 and tested in Diels A lder reactions between methacrolein and cyclopentadiene. Whilst very h igh exo selectivity was observed, enantioselectivity was variable depe nding on the amino alcohol used, Prolinol 4 gave the highest ee (97%) but quinidine gave the highest ee (64%) for a naturally occurring amin o alcohol.