In order for hybrid catalysis to be feasible, it is critical that the
Lewis acid and Lewis base moieties do not poison each other. Our catalysts have been designed with this objective in mind, but
structural studies could potentially confirm this. To shed light on
the specific interactions between the precatalysts and Lewis acids,
we have initiated NMR, MS, and X-ray studies of a variety of systems.
Preliminary NMR studies of Zn(II) containing catalysts demonstrate
that complexation is indeed occurring, but the highly
broad signals have thus far precluded characterization of discrete
complexes. Mass spectrometry with electrospray ionization of 18c
mixed with Zn(OTf)2 shows signals corresponding to two precatalyst
molecules bound to zinc (see Supplementary data). Though
it is possible that such 2:1 complexation observed in the mass
spectrometer is not reflective of the major species present under
reaction conditions, at this stage we cannot rule out the presence of
higher order complexes with precatalysts such as 18c that may
involve the reversible coordination of the amine nitrogen to zinc.
Thus far, we have obtained one crystal structure to indicate that
our general catalyst design with a central five-membered heterocycle
can facilitate the formation of metal complexes without selfquenching
of the Lewis acid and Lewis base sites. A single crystal
was obtained by mixing 12a and NiI2 in 1:1 MeCN:benzene (Fig. 2).
Though this catalyst gave only trace reaction in the model reaction,
it does illustrate that the amine and Lewis acid moieties can be
situated close together without self-quenching. Two independent
octahedral complexes, each with a 2:1 precatalyst to metal ratio,
were present in the unit cell. It is clear that the pyrrolidine nitrogens
in this structure are uncomplexed to metals, and thus would
be available for activation of a donor reactant. Efforts are ongoing to
obtain crystal structures of active aldol catalysts.