Phosphine-Catalyzed Unsymmetric [2 + 2] Annulation of Allenyl Phosphonates for Highly Substituted Cyclobutenes.

Allenes, as versatile intermediates with unique cumulative double bond systems, have been widely used in cycloaddition reactions for constructing various cyclic skeletons, yet the selective synthesis of cyclobutenes via double allenes [2 + 2] annulation still remains a long-standing challenge. Herein, we report a phosphine-catalyzed unsymmetric [2 + 2] annulation of allenyl phosphonates for the efficient construction of highly substituted cyclobutenes. The reaction exhibits broad substrate scope, delivering the target cyclobutenes in moderate to good yields with exceptional Z/E selectivities and diastereoselectivities. Mechanistic studies reveal that the reaction proceeds via phosphine-induced zwitterion formation and deprotonation of allene. The Lewis acid enables precise control of diastereoselectivity, while silane effectively suppresses allene isomerization to conjugated dienes and prevents catalyst deactivation, thus ensuring reaction efficiency. Furthermore, the synthesized cyclobutene derivatives demonstrate good structural modifiability through diverse transformations. This work enriches the field of phosphine-catalyzed allene chemistry, fills a gap in allene-based [2 + 2] cyclization modes, expands the methodological toolbox for cyclobutene synthesis, and provides a versatile platform for accessing complex cyclobutene-containing molecules.