Computational study on the reduction and solvolysis of triplet chlorobenzenes

Abstract

In this work we explored the excited state reactivity of triplet chlorobenzenes with density functional theory and a cluster‐continuum approach. We modeled two competing reactions: a direct abstraction of hydrogen from a solvent molecule and solvolysis via photo‐SN2Ar. Electron donating (–OMe, –CH2SiMe3, –SiMe3) and withdrawing (–CN) substituents not only have distinct effects on the triplet geometries, inducing structural distortions due to the relief of excited state antiaromaticity, but also affect the reactivity of the system. Therefore, electron‐rich chlorobenzenes favor the radical reduction, while electron deficiency opens the possibility for solvolysis. Both reactions are energetically comparable to or more favorable then the dissociation of the C‐Cl bond to form triplet or singlet aryl cations, the intermediates considered responsible for these reactivities. Our findings can be correlated with experimental results on similar systems available from the literature, deeming the proposed pathways as viable alternatives to established mechanisms involving aryl cations.

Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief

“…Very recently, Baird presented a qualitative theoretical rationalization of this observation, [177] based on the results from Markert et al [175] Triplet state reactivity of substituted chlorobenzene was recently explored computationally by Nitu and Crespi. [178] The excited-state antiaromaticity was found to be relieved by puckering (Figure 20), whereupon one unpaired triplet spin localizes on the carbon attached to the chlorine atom, in line with what has been observed earlier by Baranac-Stojanovic. [126] The resultant species have varied reactivity depending on their further substitution; electron-rich chlorobenzenes favor the radical reduction (Figure 20A), while electron deficient species undergo solvolysis through a photo-S N 2Ar reaction (Figure 20B).…”

“…Triplet state reactivity of substituted chlorobenzene was recently explored computationally by Nitu and Crespi [178] . The excited‐state antiaromaticity was found to be relieved by puckering (Figure 20), whereupon one unpaired triplet spin localizes on the carbon attached to the chlorine atom, in line with what has been observed earlier by Baranac‐Stojanovic [126] .…”

Meta‐Ortho Effect on the Excited State Pathways of Chloroanilines

“…For the optimisation, state average over two states was used (SA2‐CASSCF), while three or twelve states were used for the XMS treatment (XMS‐CASPT2). For the latter case, a larger active space was chosen, including the two n orbitals of Cl, the n orbital of N and the 3 s Rydberg orbital of N. In all cases, def2‐TZVP, which was previously benchmarked for similar compounds, was chosen as basis set [15] . The Resolution of identity with auxiliary basis set (RICD) was used to approximate the two‐electron integrals [41] .…”

“…[5,9] Even recently, this side of the excited state of benzene continues to offer surprising facets. [7,11,12] Indeed, the synthetic possibilities that the excited state of small aromatics offer (e. g. for dearomatisation reactions [5] or photo-S N 1 arylations [13][14][15] ) are bountiful and not yet fully explored. Direct excitation of aromatics can indeed be used to generate synthetically appealing intermediates.…”

Excited‐state aromaticity and antiaromaticity special issue

“…Halder et al [ 11 ] study the photoenolization of aromatic ketones and aldehydes with an ortho ‐alkyl group which proceeds on the triplet‐state surface. In a mechanistic study, Nitu and Crespi [ 12 ] explore two competing triplet‐state pathways for the reduction and solvolysis of variously substituted chlorobenzenes, where the substituents help alleviate the excited‐state antiaromaticity.…”