Asymmetric intramolecular cyclopropanation of diazo compounds with metallosalen complexes as catalyst: structural tuning of salen ligand
摘要:
Intramolecular cyclopropanation of alkenyl alpha-diazoacetates and alkenyl diazomethyl ketones was examined by using optically active (ON+)Ru(11)(salen) and Co(II)(salen) complexes as catalysts. For the cyclization of 2-alkenyl alpha-diazoacetates, Co(II)(salen) complexes 9 and 10 were found to be superior catalysts to the corresponding (ON+)Ru(11)(salen) complexes 4 and 5. On the other hand, (ON+)Ru(II)(salen) complex 2 was found to be the catalyst of choice for the cyclization of 3-alkenyl diazomethyl ketones, and complex 4 was found to be a good catalyst for the cyclization of (E)-4-alkenyl diazomethyl ketones. The present study demonstrates that metallosalen complexes, especially optically active (ON+)Ru(II)(salen) and Co(11)(salen) complexes, can serve as efficient catalysts for the cyclization of alkenyl diazocarbonyl compounds, if a suitable salen ligand is used as the chiral auxiliary. (C) 2003 Elsevier Science Ltd. All rights reserved.
Optically active Co(II)–salencomplexes (5 and 7) were found to catalyze highlyenantioselective intramolecular cyclopropanation of 2-alkenyl α-diazoacetates in the presence of N-methylimidazole under high substrate concentration.