I. Synthesis of alpha-amino acid esters. Dynamic kinetic resolution of zirconaaziridines with an optically active carbon dioxide synthon. II. Mechanism of alpha-amino amidine formation from zirconaaziridines and carbodiimides

I. Chiral non-racemic cyclic carbonates insert into the Zr-C bonds of zirconaaziridines, Cp2Zr[eta2-N(R)CH(R)](THF). The maximum diastereoselectivity of such insertions can be determined from insertion of racemic cyclic carbonates into the racemic zirconaaziridine. 1,2-diphenylethylene carbonate (DPEC) insertion into alpha-C-aryl and benzyl zirconaaziridines generally exhibits good diastereoselectivity. Because the alpha-position of zirconaaziridines is stereochemically labile, the selectivity obtained in insertions of optically active DPEC into zirconaaziridines is determined by the relative rates of carbonate insertion and zirconaaziridine racemization. Curtin-Hammett kinetic evaluation shows that the maximum selectivity will be obtained when racemization of the zirconaaziridine is much faster than insertion of DPEC. This is the case when a solution of DPEC is added slowly to the zirconaaziridine in order to keep the rate of insertion slow. Doing so results in the dynamic kinetic resolution (DKR) of zirconaaziridines using DPEC as an optically active source of CO2. The organic fragment can be cleaved from the metal and transesterified to give optically active alpha-amino acid esters in high ee.;Attempted synthesis of zirconaaziridines containing beta-hydrogens results in formation of eta3-1-azaallyl zirconocene hydrides Cp2Zr[eta3-N(R)CHCH(R)]H via beta-hydride elimination. These complexes are in equilibrium with small amounts of the zirconaaziridine isomer. The zirconaaziridine isomer is more reactive, thus treatment with DPEC results in insertion of the carbonate into the zirconaaziridine Zr-C bond. Furthermore, the azaallyl zirconocene hydrides are rapidly racemizing (105 s-1) at room temperature, also resulting in the racemization of the zirconaaziridine isomer. The azaallyl olefin binding strength in these complexes is ca 10 kcal/mol and the activation energy for zirconaaziridine formation is 20 kcal/mol; therefore zirconaaziridine formation is much slower than the racemization of azaallyl zirconocene hydrides.;II. Carbodiimides insert into the Zr-C bonds of zirconaaziridines, Cp2Zr[eta2-N(R)CH(R)](THF), giving alpha-amino amidines after protonolysis. Whereas the insertion of 1,3-di-p-tolyl carbodiimide is irreversible, the insertion of bis(trimethylsilyl)carbodiimide is reversible. This reversibility is significant at high [THF] with the zirconaaziridine being the exclusive zirconocene in 12 M THF. Kinetic data show that the THF ligand must dissociate from the zirconaaziridine before the carbodiimide can insert. Further kinetic investigation showed that the THF ligated zirconaaziridine is rapidly equilibrating with small concentrations of the ligand free zirconaaziridine on the time-scale of carbodiimide insertion. Our investigations are consistent with the hypothesis that heterocumulene coordination is a prerequisite for insertion.