Regio‐ and Stereoselective Reductive Coupling of Bicyclic Alkenes with Propiolates Catalyzed by Nickel Complexes: A Novel Route to Functionalized 1,2‐Dihydroarenes and γ‐Lactones

Abstract 7‐Oxabenzonorbornadienes derivatives 1 a – d underwent reductive coupling with alkyl propiolates CH 3 CCCO 2 CH 3 ( 2 a ), PhCCCO 2 Et ( 2 b ), CH 3 (CH 2 ) 3 CCCO 2 CH 3 ( 2 c ), CH 3 (CH 2 ) 4 CCCO 2 CH 3 ( 2 d ), TMSCCCO 2 Et ( 2 e ), (CH 3 ) 3 CCCO 2 CH 3 ( 2 f ) and HCCCO 2 Et ( 2 g ) in the presence of [NiBr 2 (dppe)] (dppe=Ph 2 PCH 2 CH 2 PPh 2 ), H 2 O and zinc powder in acetonitrile at room temperature to afford the corresponding 2alkenyl‐1,2‐dihydronapthalen‐1‐ol derivatives 3 a – n with remarkable regio‐ and diastereoselectivity in good to excellent yields. Similarly, the reaction of 7azabenzonorbornadienes derivative 1 e with propiolates 2 a, b and d proceeded smoothly to afford reductive coupling products 2alkenyl‐1,2‐dihydronapthalene carbamates 3 o – p in good yields with high regio‐ and stereoselectivity. This nickel‐catalyzed reductive coupling can be further extended to the reaction of 7oxabenzonorbornene derivatives. Thus, 5,6‐di(methoxymethyl)‐7‐oxabicyclo[2.2.1]hept‐2‐ene ( 4 ) reacted with 2 a and 2 d to furnish cyclohexenol derivatives bearing four cis substituents 5 a and b in 81 and 84 % yield, respectively. In contrast to the results of 4 with 2 , the reaction of dimethyl 7oxabicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylate ( 6 ) with propiolates 2 a – d afforded the corresponding reductive coupling/cyclization products, bicyclo[3.2.1] γ ‐lactones 7 a – d in good yields. The reaction provides a convenient one‐pot synthesis of γ ‐lactones with remarkably high regio‐ and stereoselectivity.