BPh
3 catalyzes 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 CO
2 and PhSiH
3; these reactions proceed at 30–40 °C under solvent-free conditions. In contrast, B(C
6F
5)
3 shows little or no activity.
11B NMR spectra suggested the generation of [HBPh
3]
−. The detailed mechanism of the BPh
3-catalyzed N-methylation of
N-methylaniline ( 1 ) with CO
2 and PhSiH
3 was studied by using DFT calculations. BPh
3 promotes the conversion of two substrates (
N-methylaniline and CO
2) into a zwitterionic carbamate to give three-component species [Ph(Me)(H)N
+CO
2−⋅⋅⋅BPh
3]. The carbamate and BPh
3 act as the nucleophile and Lewis acid, respectively, for the activation of PhSiH
3 to generate [HBPh
3]
−, which is used to produce key CO
2-derived species, such as silyl formate and bis(silyl)acetal, essential for the N-methylation of 1 . DFT calculations also suggested other mechanisms involving water for the generation of [HBPh
3]
− species.
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