Divergent Gold‐Catalyzed Rearrangement of 1‐Alkenyl‐2‐alkynylcyclopropanes: Enyne Transformation Controlled by a Silicon Moiety

Abstract

Here we report a gold(I)‐catalyzed rearrangement for a special type of alkynylcyclopropanes, such as (6‐ethynylbicyclo[3.1.0]hex‐2‐en‐6‐yl)silanes. These enynes evolved through an isomerization reaction towards the formation of isomeric alkynylcyclopropanes instead of the expected alkynylcyclohexadienes. A computational study on the reaction mechanism revealed the participation of a polycyclic gold(I) carbene complex as intermediate instead of a σ‐allylic gold cation which is in agreement with the expected influence in the energy of those intermediates due to the presence of the silyl group. Finally, this gold(I) carbene intermediate could be experimentally intercepted through intramolecular cyclopropanations.

Gold-catalysed rearrangement of unconventional cyclopropane-tethered 1,5-enynes

“…Starting from the 4-Me-[Au] structure (0.0 kcal·mol −1 ), alkyne coordination to gold facilitates a 6- endo-dig π C–C attack by the exocyclic double bond to obtain transition structure TS1 (+11.3 kcal·mol −1 ). This step differs from our previously reported reaction of cyclopentadiene-derived compounds A , 12,13 which reacted also via the 6- endo-dig π C–C attack but through the only available cyclic alkene (see Scheme 1). From TS1 , cleavage of the cyclopropane leads to a less strained structure I (−11.9 kcal·mol −1 ).…”

“…In this case, treatment of 4m with the in situ generated [(JohnPhos)Au + ] catalyst, under otherwise identical reaction conditions, led to the regioselective formation of indene 7 (52%), which was also reduced to indane 8 to provide additional confirmation of the structure after the rearrangement. As earlier observed from compounds A , 12,13 this result can be explained by the 6- endo-dig π C–C attack on the internal alkene and cyclopropane ring-opening to generate intermediate VIII . Then, a regioselective rearrangement leads to cationic species IX , which evolves to alkyne intermediate X , analogous to compounds D (see Scheme 1).…”

“…Then, a rare double simultaneous 1,3-alkyl shift generates species C , which evolves to alkyne D can explain the reaction outcome. 13 Driven by these results, we considered studying the reactivity of structurally related 6-alkynyl-4-alkylidenebicyclo[3.1.0]hex-2-enes E , which might be eventually achieved by cyclopropanation of fulvenes F , 14 with a suitable carbene equivalent G (Scheme 1C). The exocyclic alkene of compounds E provides an alternative 1,5-enyne moiety which might impact the reactivity when compared to compounds A .…”