Mechanism of the Forbidden [3s,5s]-Sigmatropic Shift: Orbital Symmetry Influences Stepwise Mechanisms Involving Diradical Intermediates

Abstract

The mechanisms of [3s,5s]‐sigmatropic shifts of octa‐1,3,7‐triene and 7‐methylenenona‐1,3,8‐triene have been elaborated using B3LYP and BPW91 density functional theory and CASPT2 methods. These orbital symmetry forbidden rearrangements are stepwise, involving diradical intermediates. A comparison with several [3,3]‐sigmatropic shifts of substituted hexadienes and of [5,5]‐sigmatropic shifts that are allowed, but nevertheless follow stepwise paths, shows that the activation barrier for the disallowed [3,5] shift is significantly larger than that for the stepwise reactions that are orbital symmetry allowed. Cyclic diradicals that have an aromatic circuit of electrons including the two radical centers and conjugated π or σ bonds are stabilized as compared to cyclic diradicals with an antiaromatic circuit of electrons. This applies to the transition states leading to and from the diradicals and influences the activation energies of stepwise sigmatropic shifts. The magnitudes of these effects are small but will have a significant influence on the rates of competing processes. This series of calculations has been used to assess the relative capabilities of the two functionals. We find that BPW91 underestimates the endothermicity of diradical formation and the barrier to diradical formation whereas B3LYP overestimates these quantities.

Potential Rearrangements in the Reaction Catalyzed by the Indole Prenyltransferase FtmPT1

“…This rearrangement is technically a [3,5]‐sigmatropic shift, and is therefore unlikely to be a concerted process as it is symmetry forbidden 32. 33 Instead it could proceed in a stepwise fashion by a secondary cyclopentyl carbocation (not shown in the scheme). The prenyl‐group methyls would accelerate the cyclization that leads to this intermediate by the gem‐dimethyl effect 34.…”

Rearrangements in the mechanisms of the indole alkaloid prenyltransferases

“…The first-formed iminium ion must rearrange to give the C-2 normal prenylated benzylic cation. This is unlikely to be a concerted process as it would constitute a [3s,5s] sigmatropic shift which is forbidden by selection rules [55,56]. Instead, it could occur in a stepwise process via a secondary cyclopentyl cation (not shown).…”

Reversible Cyclopropane Ring‐Cleavage Reactions within Etheno‐Bridged [4.3.1]Propelladiene Frameworks Leading to Aza‐ and Oxa‐[5.6.5.6]fenestratetraenes

“…This result is unexpected because the [3,5]-sigmatropic rearrangement is thermally disallowed and when it does occur, it is thought to proceed through a stepwise pathway involving biradical intermediates. [6] Inspection of the frontier molecular orbitals (Figure 4) appears to provide an explanation in that the molecular geometry facilitates overlap between the HOMO (the p orbital of the diene) and the p* orbital of the carbonyl group (CO p*). This overlap results in an activation barrier of approximately 100 kJ mol À1 .…”

“…This is relatively low in comparison to the values that are typical for thermally allowed hydrocarbon-based pericyclic reactions, which have activation barriers of approximately 140 kJ mol À1 . [6] Under the "reaction conditions" used in the computational work (À40 8C), the calculated barriers associated with direct conversion of both 9 into 6 and 9 into 11 translate into half-lives of greater than 260 years. By comparison, the corresponding barriers associated with the Me 2 HAl-complexed compounds confer half-lives that are consistent with the time-scale of the actual experiments.…”

“…Another possible pathway to compounds 5 and 6 involves the direct [3,5]-sigmatropic rearrangement of imine 8 and aldehyde 9 to give tetraenes 12 and 6, respectively. Theoretical studies by Houk and co-workers [6] on the [3,5]-sigmatropic reaction of the parent all-carbon system suggest that diradical intermediates are likely to be involved. Studies by Kohmoto et al [7] also indicate that this type of reaction can occur within the norcaradiene framework.…”