Nickel-catalyzed highly stereoselective ring opening of 7-oxa- and azanorbornenes with organic halides

Nickel-catalyzed ring-opening reactions of 7-heteroatom norbornadienes and norbornenes with various organic halides to give products with multiple stereocenters are described. Treatment of 7-oxabenzonorbornadiene (la) and 7-carbomethoxy-7-azabenzonorbornadiene (Ib) with aryl iodides (ArI) in the presence of NiCl2(PPh3)(2) and Zn powder gave the corresponding ring-opening addition products cis-1,2-dihydro-2-aryl-l-naphthol (2a-m) and methyl N-[cis-1,2-dihydro-2-aryl-l-naphthyl]-carbamate (3a-e) completely stereoselectively in 40-99% yields. The nickel system also catalyzes the reaction of highly substituted oxabicyclic [2.2.1] compounds (Ic-e) with organic halides (PhI, PhCH2Br, PhCHCHBr, and PhCBrCH2) to give the corresponding ring-opening products (4a-d, 5, 6a,b) that consist of four fixed stereocenters. Studies on the effect of solvent on the reaction of la with PhI show that CH3CN gives the highest yield of product 2a; no product 2a is observed when toluene, dichloromethane, methanol, DMF, or DMSO is used as solvent. Addition of extra PPh3 to the reaction mixture reduced the yield of 2a. A mechanism is proposed to account for the formation of these nickel-catalyzed ring-opening addition products.