INSERTION OF ISOCYANATES, CO2, AND ETHYLENE CARBONATE INTO THE ZR-C AND ZR-N BONDS OF IMINE COMPLEXES - CONSTRUCTION OF CHIRAL CENTERS LIKE THOSE IN ALPHA-AMINO-ACIDS

In some cases zirconocene-imine complexes insert CO2; more generally they insert isocyanates and cyclic carbonates. Isocyanates can insert into either the Zr-C or the Zr-N bond; protonolysis of the zirconacycle resulting from Zr-C insertion gives an amide, whereas protonolysis of the zirconacycle resulting from Zr-N insertion gives a urea. Steric hindrance on the imine nitrogen or the isocyanate discourages insertion into the Zr-N bond and gives clean Zr-C insertion. The molecular structure of an N-phenyl imine complex (5a) has been determined by single-crystal X-ray diffraction. A coordinated THF in 5a exchanges with free THF by a dissociative mechanism. Coordination of isocyanates to the Zr of 5a has not been observed before their insertion. The isocyanate insertion reactions of imine complexes such as 5 are irreversible. A chelating o-methoxy substituent on the N-phenyl of an imine complex (18) also prevents insertion into the Zr-N bond and gives clean Zr-C insertion. The treatment of 18 with ethylene carbonate gives a spirocyclic complex (23); methanolysis of 23 in benzene gives the methyl ester of phenylglycine. A crossover experiment suggests that the free beta-hydroxyethyl ester is an intermediate in the benzene methanolysis of 23.