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DFT computations have been performed to gain insight into the mechanisms of formylation/methylation of amines ( e.g. methylaniline ( 1a )/2,2,4,4-tetramethylpiperidine ( 2a )) with CO 2 and hydrosilane ([Si]H 2 , [Si] = Ph 2 Si), catalyzed by 1,3,2-diazaphospholene ( [NHP] H). Different from the generally proposed sequential mechanism for the methylation of amine with CO 2 , i.e. methylation proceeds via formylation, followed by further reduction of formamide to give an N -methylated amine, the study characterized a competition mechanism between formylation and methylation. The chemoselectivity originates from the competition between the amine and [NHP] H hydride to attack the formyloxy carbon of [Si](OCHO) 2 (the insertion product of CO 2 into [Si]H 2 ). When the attack of an amine ( e.g.  1a ) wins, the transformation affords formamide ( 1b ) but would otherwise ( e.g.  2a ) result in an N -methylated amine ( 2c ). The reduction of formamide by [Si]H 2 or [NHP] H is highly unfavorable kinetically, thus we call attention to the sequential mechanism for understanding the methylation of amine with CO 2 . In addition, the study has the following key mechanistic findings. The activation of CO 2 by [NHP] H establishes an equilibrium: [NHP] H + CO 2 ⇄ [NHP] OCHO ⇄ [NHP] +  + HCO 2  - . The ions play catalytic roles to promote formylation via HCO 2  - or methylation via  [NHP] +  . In 1a formylation, HCO 2  - initiates the reaction, giving 1b and silanol byproducts. However, after the initiation, the silanol byproducts acting as hydrogen transfer shuttles are more effective than HCO 2  - to promote formylation. In 2a methylation, [NHP] +  promotes the generation of the key species, formaldehyde and a carbocation species ( IM17 +  ). Our experimental study corroborates our computed mechanisms.

chemical science

Formylation or methylation: what determines the chemoselectivity of the reaction of amine, CO<sub>2</sub>, and hydrosilane catalyzed by 1,3,2-diazaphospholene?

Formylation or methylation: what determines the chemoselectivity of the reaction of amine, CO2, and hydrosilane catalyzed by 1,3,2-diazaphospholene?

Formylation or methylation: what determines the chemoselectivity of the reaction of amine, CO₂, and hydrosilane catalyzed by 1,3,2-diazaphospholene?