A theoretical verification of the Lewis acid-catalyzed transformation of diazo compounds via a borane carbene species

As versatile carbene precursors, diazo compounds have been widely used for carbene transfer reactions in organic synthesis. Different from the well-studied transition-metal catalyzed activation of diazo compounds, the activation modes of the Lewis acid-catalyzed activation of diazo compounds are still unknown. Herein, using a representative Lewis acid, the mechanisms of the borane-catalyzed activation/transformation of diazo compounds are extensively investigated. Three possible diazo compound activation modes are suggested and compared. The computational results show that the formation of a borane carbene species is more favorable than the other two activation modes, which is different from the previously accepted activation mechanism, where a borane-ligated free carbene is the key intermediate. Moreover, the stereoselectivity of the reaction is also disclosed by using noncovalent interaction (NCI) and atom-in-molecules (AIM) analyses, while the role of the Lewis acid (LA) is uncovered through a projection of orbital coefficient vector (POCV) analysis.