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

Abstract

The borane-functionalized (BR 2 ) bis(3,5-dimethylpyrazolyl)methane (L H ) ligands 1a (BR 2 : 9-borabicyclo[3.3.1]nonane or 9-BBN), 1b (BR 2 : BCy 2 ), and 1c (BR 2 : B(C 6 F 5 ) 2 ) were synthesized by the allylation− hydroboration of L H . Metalation of 1a,b with ZnCl 2 yielded the heteroscorpionate dichloride complexes [(1a,b)ZnCl 2 ] 3a,b. The reaction of 1a with ZnEt 2 led to the formation of the zwitterionic complex [Et(1a)ZnEt(THF)] 5. The reaction of complex 3a with two equivalents of KHBEt 3 under a carbon dioxide (CO 2 ) 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 L allyl and its zinc dichloride, diethyl and bis(formate) complexes [(L allyl )ZnCl 2 ] 2, [(L allyl )ZnEt 2 ] 4, and [(L allyl )Zn(OCHO) 2 ] 7 were also isolated. The catalyst systems composed of 1 mol % of 3a or 3b and two equivalents of KHBEt 3 hydroborated CO 2 at 1 bar with pinacolborane (HBPin) to the methanol-level product H 3 COBPin (and PinBOBPin) in yields of 42 or 86%, respectively. The catalyst systems using the unfunctionalized complex [(L H )ZnCl 2 ] 6 and KHBEt 3 or KHBEt 3 /nOctBR 2 (BR 2 : 9-BBN or BCy 2 ) hydroborated CO 2 to H 3 COBPin but in 2.5-to 6-fold lower activities than those exhibited by 3a,b/KHBEt 3 . The hydroboration of CO 2 using 8 as a catalyst led to yields of 39−43%, comparable to those obtained with 3a/KHBEt 3 . The results confirmed that the catalytic intermediates benefit from the incorporated boranes' intra-or intermolecular stabilizations.