The presence of biphenylenediol8 accelerates the rate of some Die&Alder reactions. Catalysis via a complex involving two hydrogen bona5 (see 6) is proposed.
The development of catalysts for the asymmetric induction of Diels-Alder reactions is a subject of considerable current interest. To date, attention has focused almost exclusively on devising substances which function as chiral Lewis acids, but no general solutions have yet emerged.1
A putative alternate strategy for promoting asymmetric Diels-Alder reactions involves the use of hydrogen bonding to position (and activate) the dienophile within a chiral environment. The ability of hydrogen-bond-donating solvents to accelerate the rate of Diels-Alder reactions was recognized several decades ago,* but the possibility of using hydrogen bonding to control the outcome of Diels-Alder reactions has gone largely unexamined.3~4~
In 1984, Hine and colleagues6 reported that biphenylenediol 1 forms doubly hydrogen-bonded complexes with oxygen-bearing partners such as pyrone 2 (43). Ln 1987, the mom acidic dinitrobiphenylenediol4 was shown to exceed 1 as a hydrogen bond donor.78 X-Bay crystallographic studies& have established that the pyrone ring in complex 3 is essentially cophmar~Jo with the biphenylenediol ring system, and that the two hydrogen bonding protons of 1 are positioned at the "ends" of the two sp* lone pairs projecting from the carbonyl oxygen of the pyrone.