Copper-catalysed electrophilic carboamination of cyclopropenes with arylboron reagents – a computational mechanistic probe

Abstract

A comprehensive computational exploration of the copper-catalysed umpolung-enabled three-component-coupling type electrophilic carboamination of cyclopropenes with arylboronic esters and a prototype aminating electrophile is reported. By examining plausible mechanistic scenarios advanced before for crucial elementary steps together with scrutinising performance-degrading pathways, we were able to replace previous hypotheses with a computationally verified mechanistic proposal. It comprises stepwise transmetalation of {P^P}Cu^I butoxide with Ph-Bnpg arylboronate to deliver the phenylcopper nucleophile. The distinct reactivity of cyclopropene's strained CC linkage renders arylcupration particularly rapid and irreversible, thereby almost completely disabling the rival performance-degrading formation of undesirable arylamines. The aminating electrophile then approaches the thus generated cyclopropylcopper, which triggers electrophilic amination to afford {P^P}Cu^I benzoate with the release of the 2-arylcyclopropylamine product. Umpolung-enabled amination favourably evolves through a two-step inner-sphere S_N2-type oxidative displacement/N–C bond generating a reductive elimination sequence via an intervening formal {P^P}Cu^III alkyl amido carboxylate intermediate. Conversion of {P^P}Cu^I benzoate, which represents the catalyst resting state, into the catalytically competent phenylcopper favours a multi-step salt metathesis/transmetalation sequence to involve the {P^P}Cu^I butoxide. The DFT-derived defining barrier for the turnover-limiting transmetalation aligns well with reported catalyst performance data. Spatial demands of the cyclopropene's substituents are found to be crucial for achieving a high degree of diastereoselectivity through diastereoselectivity-directing phenylcupration. While suitable electronic and steric modulation of the cyclopropene leaves the catalytic performance virtually unchanged, an electron-poor arylboronate is likely to enhance the catalytic outcome.