<i>Pyro</i>-Borates, <i>Spiro</i>-Borates, and Boroxinates of BINOLAssembly, Structures, and Reactivity

VANOL and VAPOL ligands
are known to react with three equivalents
of B­(OPh)3 to form a catalytic species that contains a
boroxinate core with three boron atoms, and these have proven to be
effective catalysts for a number of reactions. However, it was not
known whether the closely related BINOL ligand will likewise form
a boroxinate species. It had simply been observed that mixtures of
BINOL and B­(OPh)3 were very poor catalysts compared to
the same mixtures with VANOL or VAPOL. Borate esters of BINOL have
been investigated as chiral catalysts, and these include meso-borates, spiro-borates, and diborabicyclo-borate esters. Borate esters are often in equilibrium, and their
structures can be determined by stoichiometry and/or thermodynamics,
especially in the presence of a base. The present study examines the
structures of borate esters of BINOL that are produced with different
stoichiometric combinations of BINOL with B­(OPh)3 in the
presence and absence of a base. Depending on conditions, pyro-borates, spiro-borates, and boroxinate species
can be generated and their effectiveness in a catalytic asymmetric
aziridination was evaluated. The finding is that BINOL borate species
are not necessarily inferior catalysts to those of VANOL and VAPOL
but that, under the conditions, BINOL forms two different catalytic
species (a boroxinate and a spiro-borate) that give
opposite asymmetric inductions. However, many BINOL derivatives with
substitutents in the 3- and 3′-positions gave only the boroxinate
species and the 3,3′-Ph2BINOL ligand gave a boroxinate
catalyst that gives excellent inductions in the aziridination reaction.
BINOL derivatives with larger groups in the 3,3′-position will
not form either spiro-borates or boroxinate species
and thus are not effective catalysts at all.