TaCl5-catalyzed carbomagnesiation of some norbornenes with ethyl Grignard reagents

The stereoselective carbomagnesiation reaction of bicyclo[2.2.1]hept-2-ene, endo-tricyclo[5.2.1.02,6]deca-3,8-diene, and exo–endo-5-methylbicyclo[2.1.1]hept-2-ene with EtMgX, where X = Cl or Et, occurs in the presence of catalytic amounts of TaCl5 to give chloro- and ethyl[2-exo-(2′-norbornylethyl)]magnesiums in high yields. The multistep reaction mechanism involving norbornene and ethylene through tantalacyclopentanes as key intermediates is also postulated. DFT calculations confirm the thermodynamic preference for the resulting exo product.