Mechanism of Triplet Photosensitized Diels−Alder Reaction between Indoles and Cyclohexadienes:  Theoretical Support for an Adiabatic Pathway

Abstract

Diels-Alder reactions between indoles (InHs) and 1,3-cyclohexadienes (CHDs) were achieved by using aromatic ketones as photosensitizers. For instance, irradiation of deaerated dichloromethane solutions containing benzoylthiophene (BT, 1 mM), indole (20 mM), and phellandrene (40 mM), in the presence of an acylating agent, led to the N-acetylated Diels-Alder cycloadduct in 46% yield (endo:exo ratio of 1.8:1). Energy transfer from the BT triplet to the CHD gave rise to diene dimers as byproducts. Several combinations of CHDs, InHs, and aromatic ketones were tested; the Diels-Alder reaction was found to be a general process, except when the indole nucleus was substituted at position 2 or 7 and when aromatization of the CHD was favored. Theoretical calculations support a stepwise mechanism involving a triplet ternary complex TC(T1), arising from a nearly barrierless reaction between CHD and the 3(BT...HIn) exciplex. All subsequent steps proceed downhill in the triplet excited state, leading to a triplet cycloadduct-sensitizer CA-BT(T1) radical pair. Attempts to detect this species, which is basically an aminyl/BT ketyl radical pair, were performed by laser excitation of a solution containing BT, phellandrene, and indole. The observed transient absorption spectra could be compatible with the added spectra of the expected components of the radical pair.

Mechanistic Investigation of Visible-Light-Induced Intermolecular [2 + 2] Photocycloaddition Catalyzed with Chiral Thioxanthone

“…The 6-31G* basis set was used for all atoms. 41,42 MeCN was used to model the experimentally used PhCF 3 solvent due to the absence of the solvation parameters for PhCF 3 in Gaussian software, employing the polarizable continuum model (PCM) solvation model. 48,49 Frequency calculations were carried out at the same level of theory to provide the thermochemistry analysis and to confirm that the number of imaginary frequency of intermediate and transition states is 0 and 1, respectively.…”

“…Following the recent mechanistic studies on photosensitization reactions, − we used the hybrid Becke3LYP (B3LYP) functional , to optimize the geometry of all species. The 6-31G* basis set was used for all atoms. , MeCN was used to model the experimentally used PhCF 3 solvent due to the absence of the solvation parameters for PhCF 3 in Gaussian software, employing the polarizable continuum model (PCM) solvation model. , Frequency calculations were carried out at the same level of theory to provide the thermochemistry analysis and to confirm that the number of imaginary frequency of intermediate and transition states is 0 and 1, respectively. Intrinsic reaction coordinates (IRCs) − were used to confirm the relationship between the transition-state structures and the related intermediates on potential energy surface.…”

Selectivity in Radical Cation Cycloadditions

“…Furthermore, the symmetry preserving pathway is subject to Jahn-Teller distortions that increase the activation energy even more. 28,36 The overall result is an acceleration of slow or symmetry forbidden cycloadditions, [37][38][39] electrocyclic reactions, [40][41][42][43][44][45] sigmatropic shifts, [46][47][48] An explanation for the increased reaction rates observed for radical cation Diels-Alder reactions has been proposed by Bauld and is shown in Scheme 4.1. 51 The same principle is applicable for other radical cation pericyclic reactions.…”

Formation of Six‐Membered (and Larger) Rings

“…Recently, it has been reported that pp à triplet ketones, in particular benzoylthiophenes, can also photosensitize the cycloaddition between indoles and cyclohexadienes [50,51].…”