TaCl5-catalyzed carbomagnesiation of some norbornenes with ethyl Grignard reagents

The stereoselective carbomagnesiation reaction of bicyclo[2.2.1]hept-2-ene, endo -tricyclo[5.2.1.0 2,6 ]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 TaCl 5 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. The TaCl 5 -catalyzed reaction between norbornenes and EtMgX, where X = Cl or Et, was studied for the first time. The reaction proceeds stereoselectively resulting in the formation of chloro- and ethyl[2- exo -(2′-norbornylethyl)]magnesiums in high yields. The multistep mechanistic scheme through tantalacyclopentanes is proposed. High product yields are obtained in tetrahydrofuran. • TaCl 5 catalyzes carbomagnesiation of norbornenes with ethyl Grignard reagents. • Chloro- and ethyl[2- exo -(2′-norbornylethyl)]magnesiums are the final products. • Tantalacyclopentanes are postulated as key intermediates. • High product yields (90%) are obtained in tetrahydrofuran. • DFT studies confirm the thermodynamic preference for the resulting exo product.