Hydroalumination of Alkenes and Alkynes by Primary Aluminum Hydrides under Mild Conditions

The
reactions of the sterically crowded primary alane (ArPri8AlH2)2 (ArPri8 = C6H-2,6­(C6H2-2,4,6-Pri3)2-3,5-Pri2) with
alkynes and alkenes are described. It is shown that hydroalumination
of the terminal alkynes HCCSiMe3 and HCCPh readily occurs
under mild conditions via the cis-addition of the
Al–H moiety across the CC triple bond with no evidence of hydrogen
elimination. Hydroalumination was observed also with a range of terminal
olefins, but no reactivity was observed with internal alkenes or alkynes.
The relatively high reactivity of (ArPri8AlH2)2 was attributed to the steric
crowding of the large terphenyl substituent, which favors dissociation
of the alane and increases the availability of the more reactive three-coordinate
aluminum site in the monomer. In keeping with this view, studies of
the reactions of the three primary alanes (ArPri8AlH2)2, (ArPri4AlH2)2 (ArPri4 = C6H3-2,6­(C6H3-2,6-Pri2)2), and (ArMe6AlH2)2 (ArMe6 = C6H3-2,6­(C6H2-2,4,6-Me3)2) with alkenes
showed that the reaction rates are inversely proportional to the size
of the terphenyl substituent, consistent with higher reactivity of
the aluminum monomer. The structures of the alkenyl insertion products,
ArPri8Al­(CHCHPh)2 and ArPri8Al­(CHCHSiMe3)2, the alkylated derivative, ArPri8Al­(CH2CH2SiMe3)2, and the precursor aluminates {Li­(OEt2)­H3AlArPri8·Li­(OEt2)2H3AlArPri8}, (LiH3AlArPri8)2, and alanes (ArPri8AlH2)2, and (ArPri4AlH2)2 were determined
by X-ray crystallography.