Theoretical Study on Bismuth(III) Catalysts for Synthesis of Phenylsulfonyl Fluoride: Reasons of Their Catalysis

Bismuth­(III) complex with diarylsulfone ligand (diAr-SO2) is a non-transition metal catalyst reported recently for the synthesis of arylsulfonyl fluorides. We investigated this catalytic reaction using DFT and SCS-MP2 calculations for geometries and energies, respectively. This catalytic reaction occurs through transmetalation between (BF4)­Bi­(diAr-SO2) and phenylboronic acid (PhB­(OH)2), SO2 insertion into the Bi–Ph bond of (Ph)­Bi­(diAr-SO2), and fluorination of the PhOSO group of (PhOSO)­Bi­(diAr-SO2) by Selectfluor. The rate-determining step is the transmetalation for diAr-SO2 with (CH3, CH3) and (CF3, CF3) but either the transmetalation or fluorination for diAr-SO2 with (CH3, CF3), where (R1, R2) means diAr-SO2 has R1 and R2 substituents on its aryl groups. The activation energy (ΔG°‡) of the rate-determining step increases in the order (CH3, CF3) < (CH3, CH3) < (CF3, CF3). This increasing order is consistent with the experimentally observed substituent effects on catalytic activity. The transmetalation is difficult to occur in the absence of potassium phosphate (K3PO4) but occurs with moderate activation energy in the presence of K3PO4 because K3PO4 activates the B–Ph σ-bond of phenylboronic acid and stabilizes the dissociating B­(OH)2 moiety through electrostatic interaction. The substituents on diAr-SO2 play an important role in the transmetalation; when diAr-SO2 has (CF3, CF3), K3PO4 strongly interacts with the Bi­(diAr-SO2) species to form an overly stable adduct to enlarge considerably the ΔG°‡ value. When diAr-SO2 has either (CH3, CF3) or (CH3, CH3), the stabilization energy of the adduct is similar to each other, but the energy destabilization occurs more largely upon going to the asymmetric transition state from the adduct in the (CH3, CH3) case than in the (CH3, CF3) case. Thus, the use of diAr-SO2 with (CH3, CF3) is favorable for the transmetalation. The SO2 insertion into the Bi–Ph bond of (Ph)­Bi­(diAr-SO2) occurs with a moderate ΔG°‡ value, whereas the SO2 insertion is difficult to occur when the sulfone (SO2) group of diAr-SO2 is replaced with a CH2 group. The SO2 insertion occurs via a nucleophilic attack of the Ph group to SO2. However, (Ph)­Bi­(diAr-SO2) with (CH3, CH3) is not the most reactive because not only the HOMO energy of (Ph)­Bi­(diAr-SO2) but also factors such as the Biδ+–(C6H3R)δ− (R = CH3 or CF3) bond dipole moment and the Bi–C6H3R bond strength participate in determining the reactivity of (Ph)­Bi­(diAr-SO2) for the SO2