Quest for Active Species in Al/B-Catalyzed CO 2 Hydrosilylation.

We demonstrate the catalytic role of aluminum and boron centers in aluminum borohydride [(2-Me 2 CH 2 C 6 H 4 )(C 6 H 5 )Al(μ-H) 2 B(C 6 H 5 ) 2 ] ( 6 ) during carbon dioxide (CO 2 ) hydrosilylation. Preliminary investigations into CO 2 reduction using [(2-Me 2 NCH 2 C 6 H 4 )(H)Al(μ-H)] 2 ( 1 ) and [Ph 3 C][B(3,5-C 6 H 3 Cl 2 ) 4 ] ( 2 ) in the presence of Et 3 SiH and PhSiH 3 resulted in CH 2 (OSiR 3 ) 2 and CH 3 OSiR 3 , which serve as formaldehyde and methanol surrogates, respectively. In pursuit of identifying the active catalytic species, three compounds, B(3,5-C 6 H 3 Cl 2 ) 3 ( 3 ), [(2-Me 2 NCH 2 C 6 H 4 )(3,5-C 6 H 3 Cl 2 )Al(μ-H) 2 B(3,5-C 6 H 3 Cl 2 ) 2 ] ( 4 ), and [(2-Me 2 NCH 2 C 6 H 4 ) 2 Al(THF)][B(3,5-C 6 H 3 Cl 2 ) 4 ] ( 5 ), were isolated. Among compounds 2 - 5 , the highest catalytic conversion was achieved by 4 . Further, 4 and 6 were prepared in a straightforward method by treating 1 with 3 and BPh 3 , respectively. 6 was found to be in equilibrium with 1 and BPh 3 , thus making the catalytic process of 6 more efficient than that of 4 . Computational investigations inferred that CO 2 reduction occurs across the Al-H bond, while Si-H activation occurs through a concerted mechanism involving an in situ generated aluminum formate species and BPh 3 .