Stoichiometric and catalytic reactivity of tris(oxazolinyl)borate main group metal compounds

摘要

Scorpionate ligands, exemplified by the tris(pyrazolyl)borate (Tp) ligand, have been widely used to form metal catalysts across the periodic table. This class of monoanionic ligands commonly binds in either a bi- or tridentate fashion, and is capable of donating up to six electrons to the metal center. However, the B-N bond is susceptible to isomerization resulting in a loss of steric protection around the metal center. We set out to address this problem by developing a new class of scorpionate ligands based on oxazoline rings in which the B-N bond is replaced by a more robust B-C bond.The first example of this new oxazoline-based scorpionate ligand, tris(4,4-dimethyl-2-oxazolinyl)phenyl borate, ToM, is prepared by reaction of 2-lithio-4,4-dimethyl-2-oxazolide and 0.30 equiv of dichlorophenylborane. The resulting lithium salt is found to be competent as a transfer agent to Group IV transition metals via salt metathesis. Both the protonated form of ToM and the thallium salt, HToM and TlToM, are also successfully prepared, and are found to transfer ToM to transition metals in higher yield than the lithium analog. The steric bulk of this ligand is greater than that of tris(3,5-Me2-pyrazolyl)borate (Tp*), as quantified by solid angles of crystallographically characterized zirconium(IV) complexes.The unique steric pocket created by the ToM ligand led us to examine the reactivity of ToM main group metal compounds. When reacted with [AlMe3]2, ToM binds in a bidentate fashion to give κ2-ToMAlMe2. The reactivity of this compound is explored and compared to the bis(oxazolinyl)borate, {κ2-PhMeB(OxMe2)}AlMe2, which is synthesized via a methide abstraction/oxazoline coordination reaction of the parent borane PhB(OxMe2)2 with[AlMe3]2. These compounds were found to be surprisingly robust, and were capable catalysts for the ring opening polymerization of lactide at high temperature.The aluminum metal center proved to be too small for tridentate coordination of ToM, and so we began investigating the reactivity of the larger metal, magnesium. ToMMgMe is coordinatively saturated as the tridentate coordination of ToM prevents the binding of ethereal solvents to the magnesium center. The steric bulk of ToM results in a well-defined, monomeric species with no evidence of homoleptic species resulting from Schlenk equilibrium. ToMMgMe is an active precatalyst for intramolecular hydroamination/cyclization at 50 yC. The catalytic system displays Michaelis-Menten-type kinetics which is consistent with a mechanism involving reversible catalyst-substrate association prior to cyclization. The isolated magnesium amidoalkene intermediate does not undergo cyclization, however addition of trace amounts of substrate allows cyclization to occur. Therefore, we propose a two-substrate, six-center transition state involving concerted C-N bond formation and N-H bond cleavage as the turnover limiting step of the catalytic cycle.ToMMgMe is also an effective pre-catalyst for the cross-dehydrocoupling of Si-H in organosilanes and N-H bonds in amines to give Si-N bonds and H2. Using this catalyst system, a range of silazanes can be prepared in high conversion and high yields. In reactions for which multiple dehydrocoupling steps could give mixtures and oligomers, carefully controlled ratios of silane to amine allow isolation of a single product. Interestingly, both hydrazine and ammonia can be functionalized via coupling with tertiary silanes to selectively give the corresponding aminosilanes R3SiNHNH2 and R3SiNH2.