Deoxygenative CO₂ conversions with triphenylborane and phenylsilane in the presence of secondary amines or nitrogen-containing aromatics

Abstract

BPh3 catalyzed the N-methylation of secondary amines and the C-methylenation (methylene-bridge formation between aromatic rings) of N,N-dimethylanilines or 1-methylindoles in the presence of CO2 (1 atm) and PhSiH3 without solvent...

Mechanism of BPh3‐Catalyzed N‐Methylation of Amines with CO2 and Phenylsilane: Cooperative Activation of Hydrosilane

“…The Lewis acidity of the catalyst needs to be well tuned to make all the steps feasible. This hypothesis accounts for the experimental results that B(C 6 F 5 ) 3 was much less active than BPh 3 , [8] which is also consistent with the previous report by Okuda that [HB(C 6 F 5 ) 3 ] − was much less active for CO 2 hydroboration than [HBPh 3 ] − [19] . In the rate‐determining step, TS3‐B , the oxyanion leaves the boron atom, forming formaldehyde, and a silyloxyanion is simultaneously liberated to attack the silicon atom of the counter cation, delivering a siloxane (Figure 3f).…”

“…The other involves the reaction of 1 with silyl formate to deliver N ‐methylformanilide. Once N ‐methylformanilide is formed, 2 can be produced by using only BPh 3 and PhSiH 3 as confirmed previously by a control experiment [8,14] . N ‐Methylformanilide and BPh 3 act as a nucleophile and the Lewis acid, respectively, to activate PhSiH 3 (Scheme 3), generating an ion pair consisting of a siloxyiminium species and [HBPh 3 ] − .…”

“…These results suggest that 1 participates in the activation of PhSiH 3 and/or CO 2 under the solvent‐free conditions. Based on a series of experiments, [8] we assumed two pathways from 1 to 2 (Scheme 2a). One involves the reaction of 1 with silyl formate to give N ‐methylformanilide, which is further reduced to 2 [14] .…”

“…Recently, we reported that triphenylborane smoothly catalyzed the N‐methylation of N ‐methylaniline ( 1 ) with CO 2 (1 atm) and PhSiH 3 without solvent at 30 °C (Scheme 1c) [8] . In addition, methylene‐bridge formation between aromatic rings (C‐methylenation) of N , N ‐dimethylaniline ( 2 ) or 1‐methylindole ( 4 ) also proceeded at 30–40 °C with this catalytic system (Schemes 1d,e) [8] . In sharp contrast, B(C 6 F 5 ) 3 showed little or no activity under otherwise the same conditions.…”

“…In sharp contrast, B(C 6 F 5 ) 3 showed little or no activity under otherwise the same conditions. Although several methods for successive C−H/C−C bond formation with CO 2 have been reported, [8,9] the above deoxygenative CO 2 conversions at 30–40 °C are quite unique [10–12] . In fact, other catalysts reported so far usually require much higher temperature to achieve the N‐methylation of amines with CO 2 and hydrosilane.…”