Controlling (E/Z)‐Stereoselectivity of −NHC=O Chlorination: Mechanism Principles for Wide Scope Applications

Abstract

Organic halogen compounds are cornerstones of applied chemical sciences. Halogen substitution is a smart molecular design strategy adopted to influence reactivity, membrane permeability and receptor interaction. Chiral bioreceptors may restrict the stereochemical requirements in the halo‐ligand design. Straightforward (but expensive) catalyzed stereospecific halogenation has been reported. Historically, PCl5 served access to uncatalyzed stereoselective chlorination although the stereochemical outcomes were influenced by steric parameters. Nonetheless, stereochemical investigation of PCl5 reaction mechanism with carbamoyl (RCONHX) compounds has never been addressed. Herein, we provide the first comprehensive stereochemical mechanistic explanation outlining halogenation of carbamoyl compounds with PCl5; the key regioselectivity‐limiting nitrilimine intermediate (8‐Z.HCl); how substitution pattern influences regioselectivity; why oxadiazole byproduct (P1) is encountered; stereo‐electronic factors influencing the hydrazonoyl chloride (P2) production; and discovery of two stereoselectivity‐limiting parallel mechanisms (stepwise and concerted) of elimination of HCl and POCl3. DFT calculations, synthetic methodology optimization, X‐ray evidence and experimental reaction kinetics study evidence all supported the suggested mechanism proposal (Scheme 2). Finally, we provide mechanism‐inspired future recommendations for directing the reaction stereoselectivity toward elusive and stereochemically inaccessible (E)‐bis‐hydrazonoyl chlorides along with potentially pivotal applications of both (E/Z)‐stereoisomers especially in medicinal chemistry and protein modification.