Aryloxycarbonylcarbene Complexes of Bis(oxazolinyl)pyridineruthenium as Active Intermediates in Asymmetric Catalytic Cyclopropanations

The reaction of [RuCl2(pybox)‐(C2H4)] (1) (pybox = 2,6‐bis[4′S)‐isopropyloxazolin‐2′‐yl]pyridine) and 2,6‐di‐tert‐butyltolyl diazoacetate (4) (DBT‐DA) in benzene at 50°C gave a stable 2,6‐di‐tert‐butyltolylcarbonylcarbene‐ruthenium complex 5 in 94% yield. The structure of 5 was characterized by NMR spectroscopy. 2,6‐Diisopropylphenyl diazoacetate (6) and 2,4,6‐trimethylphenyl diazoacetate (7) also gave the corresponding carbene complexes 9 and 10, respectively. Asymmetric carbene transfer from the carbene complexes to styrene resulted in formation of the trans isomer of phenylcyclopropanecarboxylates 2 with high enantioselectivity: 2 was obtained as the sole product in 80% yield (55% ee) from 5 at 80 °C and in 82% yield (97% ee) from 9 at 60°C; from 10 at 40°C, a mixture of 2 and 3 in a ratio of 97:3 was formed in 91% yield (97% ee for 2 and 99% ee for 3). After the carbene transfer reaction, the ethylene complex 1 could be regenerated and isolated by treatment of the reaction mixture under an ethylene atmosphere. The carbene complexes 9 and 10 (2 mol%) exhibited catalytic activity in the asymmetric cyclopropanation of styrene with the corresponding diazoacetates.