Zinc-Catalyzed Hydroboration of Carbon Dioxide Amplified by Borane-Tethered Heteroscorpionate Bis(Pyrazolyl)methane Ligands.

The borane-functionalized (BR) bis(3,5-dimethylpyrazolyl)methane () ligands (BR: 9-borabicyclo[3.3.1]nonane or 9-BBN), (BR: BCy), and (BR: B(CF)) were synthesized by the allylation-hydroboration of . Metalation of with ZnCl yielded the heteroscorpionate dichloride complexes . The reaction of with ZnEt led to the formation of the zwitterionic complex . The reaction of complex with two equivalents of KHBEt under a carbon dioxide (CO) atmosphere gave rise to the formation of the dimeric bis(formate) complex , in which its borane moieties intermolecularly stabilize the formate ligands of opposite metal centers. The allylated precursor and its zinc dichloride, diethyl and bis(formate) complexes , , and were also isolated. The catalyst systems composed of 1 mol % of or and two equivalents of KHBEt hydroborated CO at 1 bar with pinacolborane (HBPin) to the methanol-level product HCOBPin (and PinBOBPin) in yields of 42 or 86%, respectively. The catalyst systems using the unfunctionalized complex and KHBEt or KHBEt/OctBR (BR: 9-BBN or BCy) hydroborated CO to HCOBPin but in 2.5- to 6-fold lower activities than those exhibited by /KHBEt. The hydroboration of CO using as a catalyst led to yields of 39-43%, comparable to those obtained with /KHBEt. The results confirmed that the catalytic intermediates benefit from the incorporated boranes' intra- or intermolecular stabilizations.