Possible Mechanisms and Origin of Selectivities for Phosphine‐Catalyzed [2+n] (n=3, 4) Annulations of Saturated Amines and δ‐Acetoxy Allenoates

The density functional theory (DFT) calculations were performed to gain insights into possible reaction mechanisms and origin of selectivities on phosphine catalyzed [2+3] and [2+4] annulations of saturated amine with allenoate for the first time. The computed results reveal that allenoate prefers to serve as a C3 synthon rather than C4 synthon under phosphine catalysis, and the whole catalytic cycle generally undergoes through eight processes, i. e. (1) adsorption of catalyst, (2) dissociation of AcO− group, (3) deprotonation of saturated amine, (4) nucleophilic attack by the deprotonated amine anion to the electrophilic phosphonium diene intermediate, (5) [1,6]‐proton shift, (6) ring closure of five‐membered heterocycle, (7) [1,2]‐proton shift, and (8) desorption of catalyst and the formation of 3‐pyrroline. The fourth step was identified as both chemo‐ and stereo‐selectivities determining step, and the pathway associated with R‐isomer is energetically favorable pathway. Further non‐covalent interaction (NCI) and Bader's atoms‐in‐molecules (AIM) analysis reveal that the C−H⋅⋅⋅O interaction has a significant contribution to the stability of R‐configured transition state. The obtained insights should be valuable for understanding the general principle for the detailed mechanism and predicting the origin of the chemo‐ and stereo‐selectivities on the phosphine catalyzed [2+3] and [2+4] annulations of saturated amine with allenoate.