Unusual pathway of the tantalum-catalyzed carboalumination reaction of alkenes with triethylaluminum

Carboalumination of 1-alkenes (1-hexene, 1-octene, 1-decene) with Et3Al in the presence of catalytic amounts of TaCl5 results in a mixture of 2-(R-substituted)- and 3-(R-substituted)-n-butylaluminums (1:1 ratio) in total yields of 75–85%. The TaCl5-catalyzed 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 Et3Al leads to the formation of diethyl[2-exo-(2′-norbornylethyl)]aluminums in high yields. DFT calculations confirm the thermodynamic preference of the final exo product. The multistep reaction mechanisms for the formation of the resultant organoaluminums through tantalacyclopentanes as key intermediates are also discussed.