Understanding the Torquoselectivity in 8π-Electrocyclic Cascade Reactions: Synthesis of Fenestradienes versus Cyclooctatrienes

Abstract

Unusual and novel polycyclic cyclooctatrienes, fenestradienes, and fenestrenes form readily from trienynes depending on the structure of the starting trienynes and the reaction conditions. The experimentally observed high torquoselectivities and complete diastereoselectivities of the 8pi-electrocyclization products have been thoroughly studied using density functional computations at B3PW91/6-31G(d,p). The different P- and M-helical topologies for the Mobius aromatic transition structures are the origin of the observed torquoselectivities in the cyclooctatrienes. The P-helical topologies direct the newly formed single bonds into a favorable equatorial position of the neighboring cycloalkane moieties (X = ring size) that retain their most stable conformation. The M-helical transition structures lead to an axial connection for the smaller rings (X = 4-6) and an equatorial connection for the seven- and eight-membered cycloalkanes. This leads to unfavorable conformations for the larger cycloalkane moieties. Experiments and computations show that for trienynes involving small neighboring cycloalkane groups (X = 4-6) M-helical topology is preferred toward cyclooctatrienes and in the following the corresponding fenestradienes can be formed as kinetic or even thermodynamic products; they convert to their more stable cyclooctatriene valence isomers derived from P-helical transition structures at higher temperatures. For larger cycloalkane moieties with more conformational flexibility only cyclooctatrienes with torquoselectivities derived from P-helical transition structures form.

Enhanced photoswitching of bridged azobenzene studied by nonadiabatic ab initio simulation

“…67,68 The ROKS vertical excitation energies for the structures determined in this work (Z: 3.40 eV, E: 2.59 eV) are in reasonable agreement with the CASPT2 results for the same structure (Z: 3.12 eV, E: 2.52 eV) and the experimental data 47 (Z: 3.07 eV, E: 2.53 eV). The agreement between ROKS and CASPT2 is excellent for the other two sets of Z and E ground state structures obtained from CASSCF (10,8), 66 while the CASSCF energies are consistently too high compared to CASPT2 by about 0.3 eV.…”

Asymmetric electrocyclic reactions

“…184 8p-6p Cascades have the potential to form structurally remarkable frameworks including fenestrenes, as demonstrated by Suffert et al 189,190 Cis-reduction of the alkyne using nickel boride at room temperature yielded the corresponding fenestrene as a single isomer in up to 93% yield. Subsequent mechanistic investigations have determined that the fenestrene formed is the kinetic product; exposure of the alkyne to the same reducing conditions at 70 1C, or the fenestrene to microwave irradiation led to the thermodynamic anti-cyclooctatriene (Scheme 37).…”

An Expeditious and Atom‐Economical Synthesis of a New Generation of Substituted [4.6.4.6]Fenestradienes

“…[10] Although, fenestradienes 4 were obtained in excellent yields, the method required the use of tin compounds, a sensitive nickel(0) catalyst, and hydrogen, and the intermediate trienyne species 3 was prone to polymerization if not handled with special care (Scheme 1). [10] Although, fenestradienes 4 were obtained in excellent yields, the method required the use of tin compounds, a sensitive nickel(0) catalyst, and hydrogen, and the intermediate trienyne species 3 was prone to polymerization if not handled with special care (Scheme 1).…”