PhPCHCHB(CH) and Its Formaldehyde Adduct as Catalysts for the Reduction of CO with Hydroboranes.

We study two metal-free catalysts for the reduction of CO with four different hydroboranes and try to identify mechanistically relevant intermediate species. The catalysts are the phosphinoborane PhP(CH)BBN (), easily accessible in a one-step synthesis from diphenyl(vinyl)phosphine and 9-borabicyclo[3.3.1]nonane (H-BBN), and its formaldehyde adduct PhP(CH)BBN(CHO) (), detected in the catalytic reduction of CO with as the catalyst but properly prepared from compound and -formaldehyde. Reduction of CO with H-BBN gave mixtures of CH(OBBN) () and CHOBBN () using both catalysts. Stoichiometric and kinetic studies allowed us to unveil the key role played in this reaction by the formaldehyde adduct and other formaldehyde-formate species, such as the polymeric BBN(CH)(PhP)(CHO)BBN(HCO) () and the bisformate macrocycle BBN(CH)(PhP)(CHO)BBN(HCO)BBN(HCO) (), whose structures were confirmed by diffractometric analysis. Reduction of CO with catecholborane (HBcat) led to MeOBcat () exclusively. Another key intermediate was identified in the reaction of with the borane and CO, this being the bisformaldehyde-formate macrocycle (HCO){BBN(CH)(PhP)(CHO)}Bcat (), which was also structurally characterized by X-ray analysis. In contrast, using pinacolborane (HBpin) as the reductant with catalysts and usually led to mixtures of mono-, di-, and trihydroboration products HCOBpin (), CH(OBpin) (), and CHOBpin (). Stoichiometric studies allowed us to detect another formaldehyde-formate species, (HCO)BBN(CH)(PhP)(CHO)Bpin (), which may play an important role in the catalytic reaction. Finally, only the formaldehyde adduct turned out to be active in the catalytic hydroboration of CO using BH·SMe as the reductant, yielding a mixture of two methanol-level products, [(OMe)BO] (, major product) and B(OMe) (, minor product). In this transformation, the Lewis adduct (BH)PhP(CH)BBN was identified as the resting state of the catalyst, whereas an intermediate tentatively formulated as the Lewis adduct of compound and BH was detected in solution in a stoichiometric experiment and is likely to be mechanistically relevant for the catalytic reaction.