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

Abstract

The borane-functionalized (BR2) bis(3,5-dimethylpyrazolyl)methane (LH) ligands 1a (BR2: 9-borabicyclo[3.3.1]nonane or 9-BBN), 1b (BR2: BCy2), and 1c (BR2: B(C6F5)2) were synthesized by the allylation-hydroboration of LH. Metalation of 1a,b with ZnCl2 yielded the heteroscorpionate dichloride complexes [(1a,b)ZnCl2] 3a,b. The reaction of 1a with ZnEt2 led to the formation of the zwitterionic complex [Et(1a)ZnEt(THF)] 5. The reaction of complex 3a with two equivalents of KHBEt3 under a carbon dioxide (CO2) atmosphere gave rise to the formation of the dimeric bis(formate) complex [(1a)Zn(OCHO)2]2 8, in which its borane moieties intermolecularly stabilize the formate ligands of opposite metal centers. The allylated precursor Lallyl and its zinc dichloride, diethyl and bis(formate) complexes [(Lallyl)ZnCl2] 2, [(Lallyl)ZnEt2] 4, and [(Lallyl)Zn(OCHO)2] 7 were also isolated. The catalyst systems composed of 1 mol % of 3a or 3b and two equivalents of KHBEt3 hydroborated CO2 at 1 bar with pinacolborane (HBPin) to the methanol-level product H3COBPin (and PinBOBPin) in yields of 42 or 86%, respectively. The catalyst systems using the unfunctionalized complex [(LH)ZnCl2] 6 and KHBEt3 or KHBEt3/nOctBR2 (BR2: 9-BBN or BCy2) hydroborated CO2 to H3COBPin but in 2.5- to 6-fold lower activities than those exhibited by 3a,b/KHBEt3. The hydroboration of CO2 using 8 as a catalyst led to yields of 39-43%, comparable to those obtained with 3a/KHBEt3. The results confirmed that the catalytic intermediates benefit from the incorporated boranes’ intra- or intermolecular stabilizations.