p-Block Initiator/Ylide Systems for the Polyhomologation Reaction: Computational Benchmarking of Group III Organoinitiators and Ylides Versus Trihexylborane/Corey’s Ylide

We computationally investigate the initiation step in polyhomologation (PH) and evaluate the reaction mechanisms of multiple initiator/ylide combinations with the aim of expanding the scope of traditional PH while preserving its key advantages. All energy profiles were benchmarked against the well-established trihexylborane/dimethylsulfoxonium methylide (Corey’s ylide) system. Our model set comprises tripropylborane, -alane, and -galane initiators paired with nitrogen- and phosphorus-based methylides. The tripropylborane/trimethylammonium methylide mechanism closely corresponds to the reference pathway, and diazonium methylide in combination with tripropylborane, tripropylalane, or tripropylgalane also reproduces the characteristic features of the established PH reaction pathway, with 1,2-migration barriers ranging from 16.9 to 24.8 kcal  mol⁻¹. These results indicate that such initiator/ylide pairs constitute experimentally viable candidates for extending PH chemistry. Across all systems, nitrogen-derived ylides consistently promote PH initiation more efficiently than their phosphorus analogs, primarily due to the lower strain associated with the structural deformation of the ylide fragment and the different rigidity imposed by its substituents. Together, these mechanistic trends provide a coherent picture of initiation in PH and, supported by complementary computational analyses, offer an initial framework that can guide future development of new p-block-based initiator/ylide systems.