Intermolecular Allene–Alkyne Coupling: A Significantly Useful Synthetic Transformation

Because of its wide applicability in the synthesis of diverse unsaturated building blocks, allene–alkyne (AA) coupling has emerged as a powerful methodology to drive useful chemical transformations in the past two decades. Considering recent discoveries in this active research area, in this Review, we aim to support chemists in the selection of suitable methods, particularly intermolecular approaches, according to the synthesis needs. However, inherent similarities between the reactivities of π-systems create considerable obstacles while predicting selectivity in an intermolecular platform. Therefore, herein, intermolecular couplings for the chemoselective, stereoselective, and regioselective syntheses of acyclic and cyclic C-skeletons are reviewed. However, considering the lack of a well-organized review in this field, we initially provide a brief historical overview of the types of thermally induced reactions that are mostly feasible for electronically biased coupling components and are nonselective. Subsequently, emerging selective strategies are described, which include activator-controlled and neighboring functional group-assisted regio-, stereo-, and π-skeleton-divergent processes. Overall, this Review is divided into two parts. In the first part, strategies for the fabrication of acyclic C-skeletons are explained according to the reaction types. In the second part, synthetic transformations affording cyclic structures are reviewed based on the substrate class (that is, nonassisted and assisted substrates). Key reaction intermediates of each subset of coupling and factors affecting product-selective discrimination of the active species with the highlight of rationale are systemically summarized. Understanding of these substrate structure- and catalyst-dependent factors provides insights into the regulation of the chemoselectivities of title AA cross-couplings and, therefore, the design of alternative catalytic routes with distinct mechanistic features.