Rhodium-catalyzed 1,3-acyloxy migration and subsequent intramolecular [4+2] cycloaddition of vinylallene and unactivated alkynew

The Diels–Alder cycloaddition is arguably the most powerful reaction for the construction of substituted six-membered rings from various 2p and 4p reactants. However, unactivated 2p and 4p systems either require very harsh conditions or fail to undergo cycloadditions. In many cases, transition metal catalysts facilitated chemical transformations that were inaccessible under thermal conditions and they have proven to be of great value in cycloaddition reactions. Vinylallenes are unique 4p-components that have been used in [4+2] cycloadditions with activated alkenes and alkynes under thermal conditions. The cycloaddition of vinylallenes with unactivated alkynes required transition metal catalysts such as platinum or rhodium complexes. In both cases, polysubstituted benzene derivatives were obtained. We recently found that [Rh(CO)2Cl]2 could catalyze 1,3acyloxy migration of propargyl ester 1 to form allene 3, which was previously realized mainly by p-acidic metal-based catalysts such as silver, copper, platinum, and gold in various tandem transformations. We envisioned that the combination of this novel reactivity of Rh(I) catalyst and its ability to promote cycloadditions might allow the tandem transformation from enyne 4 to product 6: a net 1,3-acyloxy migration of propargyl ester 4, a subsequent [4+2] cycloaddition of the resulting vinylallene with a tethered alkyne, and a final isomerization of isotoluene 5 to polysubstituted aromatic compound 6. In a related report, Liang and co-workers developed a PtCl2-catalyzed tandem 1,3-acyloxy migration followed by [4+2] cycloaddition of vinylallene and alkyne to form similar aromatic products. The unactivated 2p alkyne partners, however, were limited to terminal alkynes in the PtCl2-catalyzed reaction. We thought that the scope of this tandem process could be expanded to include internal alkynes using rhodium catalyst via a different mechanism. Surprisingly, isotoluene derivative 5 was observed as the major product from rhodiumcatalyzed isomerization of enyne 4. A stable bicyclic compound 7 containing a cyclohexenone substructure was isolated upon hydrolysis (Scheme 1). When we treated compound 4a with a catalytic amount of [Rh(CO)2Cl]2, an isotoluene derivative 5a was isolated in 88% yield (Table 1, entry 1). The expected aromatic compound was not detected at all. When the reaction time was extended to approximately 1 h, compound 5 was almost completely decomposed into unidentified byproducts. Other Rh(I) catalysts did not promote the tandem acyloxy migration and cycloaddition process (entries 2–4), suggesting that the electron-withdrawing CO ligand was critical. Certain gold and platinum catalysts facilitated the 1,3-acyloxy migration; no cycloaddition product, however, was observed in any case (entries 5–10). The reaction worked equally well when the acetate was changed to pivolate (eqn (1)). Product 5b could be isolated in high yield when the reaction was run for 5 min. Isotoluene derivatives 5a and 5b were stable enough to be isolated and characterized by NMR. We tried to expand the scope of this tandem reaction, yet we Scheme 1 1,3-Acyloxy migration and [4+2] cycloaddition of vinylallene and alkyne.