Computational Study on Mechanisms and Origins of Selectivities in Rh(I)-Catalyzed Cycloisomerizations of 1,6-Allenynes with Tethered Unsaturated Carbon–Carbon Bonds

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Abstract

Rhodium(I)-catalyzed cycloisomerization reactions of 1,6-allenynes with a tethered alkene (homoallylallene–alkyne substrate) or alkyne (homopropargylallene–alkyne substrate) have recently shown great potential in the construction of polycyclic skeletons. To understand the influence of the tethered unsaturated carbon–carbon bond on cycloisomerization mechanisms, density functional theory (DFT) calculations have been performed in this work. Our calculations indicate that both cycloisomerizations involve oxidative cyclization and migratory insertion but the distinct regioselectivity between alkene and alkyne insertions and the contrasting reactivity of rhodium-alkyl and rhodium-alkenyl intermediates contribute to the divergent cycloisomerization mechanisms. In the case of alkene-tethered 1,6-allenyne, both 1,2- and 2,1-insertions of alkene into the Rh–C(sp2) bond of the five-membered rhodacyclic intermediate are plausible, and the resulting rhodium-alkyl intermediates will afford cycloisomerization products through reductive elimination. In contrast, only 1,2-alkyne insertion is practical in the reaction of alkyne-tethered 1,6-allenyne, and the formed rhodium-alkenyl intermediate cannot undergo reductive elimination but rather rearranges into a reactive rhodacyclopropene, thereby releasing cyclic products through the 1,2-migration of rhodium carbene. Further distortion/interaction analysis on insertion transition states suggests that the different regioselectivity arises from the distinct features of configurational rearrangement for alkene and alkyne fragments during 1,2-migratory insertion. The computations also highlight the effects of the alkyne substituent and carbon chain length of the 1,6-allenyne reactant on the product selectivity.

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