Vk. Aggarwal et al., COMPLEXES CONTAINING A LEWIS-ACID AND BRONSTED ACID FOR THE CATALYTICASYMMETRIC DIELS-ALDER REACTION, Tetrahedron : asymmetry, 6(6), 1995, pp. 1301-1306
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.