Origins of Endo Selectivity in Diels–Alder Reactions of Cyclic Allene Dienophiles

Abstract

Strained cyclic allenes, first discovered in 1966 by Wittig and co‐workers, have recently emerged as valuable synthetic building blocks. Previous experimental investigations, and computations reported here, demonstrate that the Diels–Alder reactions of furans and pyrroles with 1,2‐cyclohexadiene and oxa‐ and azaheterocyclic analogs proceed with endo selectivity. This endo selectivity gives the adduct with the allylic saturated carbon of the cyclic allene endo to the diene carbons. The selectivity is very general and useful in synthetic applications. Our computational study establishes the origins of this endo selectivity. We analyze the helical frontier molecular orbitals of strained cyclic allenes and show how secondary orbital and electrostatic effects influence stereoselectivity. The LUMO of carbon‐3 of the allene (C‐3 is not involved in primary orbital interactions) interacts in a stabilizing fashion with the HOMO of the diene in such a way that the carbon of the cyclic allene attached to C‐1 favors the endo position in the transition state. The furan LUMO, allene HOMO interaction reinforces this preference. These mechanistic studies are expected to prompt the further use of long‐avoided strained cyclic allenes in chemical synthesis.

Reactivity and Selectivity of the Diels–Alder Reaction of Anthracene in [Pd6L4]12+ Supramolecular Cages: A Computational Study

“…To better understand the origins of selectivity, we have performed the distortion/interaction analysis − on the selectivity-determining transition states. We partitioned each transition state into two parts: 1a and 1b (Figure ).…”

Understanding Current Density In Molecules Using Molecular Orbitals

“…While we focus on even [n]cumulenes in this paper, the helical MOs are also seen in appropriately substituted polyynes, 28,29 odd [n]cumulenes, 29 metallacumulenes orpolyynes, [30][31][32][33] spiro-compounds, 34 and cyclo-allenes. 35,36 We choose to focus on the even [n]cumulenes as these have helical MOs in their ground state, and long molecules result in long helical MOs, which aids visualization.…”