Cyclic (alkyl)(amino)carbenes (CAACs) are receiving remarkable attention because of their ambiphilic properties. The formal replacement of one nitrogen atom (π–electron donating, σ–withdrawing) by an alkyl group (σ–donating) makes these carbenes more electrophilic while retaining the nucleophilic character. Thanks to the ambiphilicity, CAAC ligands have been applied in diverse fields such as light–emitting materials as CAAC–Cu complexes and novel transition metal catalysis.<br/>Following the pioneering work on CAACs, other classes of ambiphilic NHCs have been developed such as cyclic (amino)(aryl)carbenes (CAArCs). Ambiphilicity of CAArCs could be increased by replacing the amino group with an aryl group. In the same vein, The indole scaffold is a promising candidate for ambiphilic NHCs due to its cyclic (alkyl)(amino) fragment and the presence of an aryl group, in close resemblance to known ambiphilic NHCs. Moreover, because the indole motif is one of the most common N–heterocycles found in drug molecules and natural products numerous synthetic methods for the indole moiety have been reported to date.<br/>Herein, the synthesis of indole derived ambiphilic N–heterocyclic carbene ligand, indol–2–ylidene (IdY), is described. A series of indolenium precursors were prepared on a gram scale in good yields. Trapping experiments with elemental selenium, [RhCl(cod)]<sub>2</sub> and CuCl provided the expected carbene adducts. Further computational and spectroscopic studies supported the ambiphilicity of IdY, which lies between cyclic (alkyl)(amino)carbenes (CAAC–5) and cyclic (amino)(aryl)carbene (CAArC). The copper complexes show high percent buried volume and allow for carboboration of terminal alkynes within 30 minutes in a demonstration of synthetic utility with good yields and high regioselectivity. Further development of the IdY framework for free carbene isolation is currently underway in our laboratory and will be reported in due course.

Indol–2–ylidene (IdY): Ambiphilic N–heterocyclic carbene derived from indole

ACS Spring Meeting 2021

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