One Lump or Two? A Plurality of Pathways in Gold(III)-Catalyzed Cyclization Transforming Propargyl Acetates to a Carene-like Bicyclo[4.1.0]heptane
The bicyclo[4.1.0]heptane substructure, featured in a number of natural products, is economically formed via gold(III)-mediated cycloisomerization of a 5-acetoxy-1,6-enyne. This Ohloff-Rautenstrauch rearrangement takes place with high regio- and stereocontrol and purportedly proceeds through either of two pathways that differ in the order of major events: cyclization followed by ester migration ("cyclization first") or its transpose ("migration first"). Implicit solvent-phase (dichloroethane) electronic structure calculations [IEFPCM-B2PLYP-D3/def2-TZVP//IEFPCM-B2PLYP/6-31G(d)-LANL2DZ] aimed at elucidation of the minimum energy pathway corresponding to the "cyclization first" and "migration first" pathways are presented herein. Both pathways feature multiple steps and are characterized by low-energy barriers, indicating that facile interconversion of structures on the surface is possible. In addition, the highest-energy structures for each of the two pathways are very close in energy (ΔΔE⧧ < 3.0 kcal/mol). Relative turnover frequency (TOF) and degree of TOF control (XTOF) calculations indicate that although a cyclization first pathway may dominate, both cyclization and acyl migration processes influence the rate of this reaction
Hines, Jeremy M., Jesse J. Eason, and Matthew R. Siebert. "One Lump or Two? A Plurality of Pathways in Gold (III)-Catalyzed Cyclization Transforming Propargyl Acetates to a Carene-like Bicyclo [4.1. 0] heptane." Organometallics 36, no. 4 (2017): 920-926.