Highly efficient cyclization of o-iodobenzoates with aldehydes catalyzed by cobalt bidentate phosphine complexes:: A novel entry to chiral phthalides

Methyl 2-iodobenzoates 1a-c undergo cyclization reactions with various aromatic aldehydes 2 a-m (RC6H4CHO: R=H 2a, 4-CH3 2b, 4-tBu 2 c, 4-OMe 2 d, 3-OMe 2 e, 4-Cl 2 f, 4-CF3 2g, 4-CN 2h, 4-Ph 2i; benzo[d][1,3]dioxole-5-carbaldehyde (2j), -napthaldehyde (2k), benzofuran-2-carbaldehyde (21), and isonicotinaldehyde (2m)) in the presence of [CoI2(dppe)] (dppe = 1,2-bis(diphenylphosphino)ethane) and Zn powder in dry THF at 75 degrees C for 24 h to give the corresponding phthalide derivatives 3a-m and 3q-t in good to excellent yields. Under similar reaction conditions, less reactive aliphatic aldehydes, heptanal (2n), butyraldehyde (2o), and 2-phenylacetaldehyde (2p) also underwent cyclization reactions with la to provide 3n-p, respectively, in fair to good yields. The catalytic reaction can be further extended to cinnamyl aldehyde (2q) with la to give the corresponding phthalide derivative 3u. This synthetic method is compatible with a variety of functional groups on the aryl ring of 2. The high efficiency of the cobalt catalyst containing a dppe (dppe=1,2-bis(diphenylphosphino)ethane) ligand encouraged us to investigate the asymmetric version of the present catalytic reaction by employing bidentate chiral ligands. Thus, aromatic aldehydes 2a-c, 2f, and 2g undergo cyclization with 2-iodobenzoate (1a) smoothly in the presence of [CoI2(S,S)-dipamp}] ((SS)-dipamp(1S,2S)-(+)-bis[2-methoxyphenyl]phenylphosphino)ethane) and zinc powder in THF at 75 degrees C for 24 h, giving the corresponding (S)-phthalides 4a-e in 81-89% yields with 70-98% ee. A possible mechanism for the present catalytic reaction is proposed.