Multiple Mechanistic Pathways for Zirconium-Catalyzed Carboalumination of Alkynes. Requirements for Cyclic Carbometalation Processes Involving C−H Activation

The reactions of internal and terminal alkynes with organoalanes containing Et, n-Pr, and i-Bu groups in the presence of Cp2ZrCl2 and MeZrCp2Cl were investigated with the goal of clarifying mechanistic details of some representative cases. Three fundamentally different processes, i.e., (i) C−M bond addition without C−H activation in the alkyl group, (ii) cyclic C−M bond addition via C−H activation, and (iii) hydrometalation, have been observed, and the courses of these reactions significantly depend on (i) the nature and number of alkyl groups in organoalanes, (ii) their amounts, and (iii) solvents. The reaction of alkynes with Et3Al in the presence of 0.1 equiv of Cp2ZrCl2 in nonpolar solvents, e.g., hexanes, proceeds via C−H activation to give the corresponding aluminacyclopentenes. Investigation of the reaction of 5-decyne with 1−3 equiv of Et3Al and 1 equiv of Cp2ZrCl2, which gave mono-, di-, or trideuterated (Z)-5-ethyl-5-decene as shown in Scheme 10, together with the previously reported structural ...