Exploring Two-State Reaction Pathways in the Photodimerization of Cyclohexadiene

Transition states for the dimerization of 1,3-cyclohexadiene: a DFT, CASPT2, and CBS-QB3 quantum mechanical investigation

Quantum mechanical calculations using restricted and unrestricted B3LYP density functional theory, CASPT2, and CBS-QB3 methods for the dimerization of 1,3-cyclohexadiene (1) reveal several highly competitive concerted and stepwise reaction pathways leading to [4 + 2] and [2 + 2] cycloadducts, as well as a novel [6 + 4] ene product. The transition state for endo-[4 + 2] cycloaddition (endo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 28.7 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 19.0 kcal/mol) is not bis-pericyclic, leading to nondegenerate primary and secondary orbital interactions. However, the C(s) symmetric second-order saddle point on the B3LYP energy surface is only 0.3 kcal/mol above endo-2TS. The activation enthalpy for the concerted exo-[4 + 2] cycloaddition (exo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 30.1 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 21.1 kcal/mol) is 1.4 kcal/mol higher than that of the endo transition state. Stepwise pathways involving diallyl radicals are formed via two different C-C forming transition states (rac-5TS and meso-5TS) and are predicted to be competitive with the concerted cycloaddition. Transition states were located for cyclization from intermediate rac-5 leading to the endo-[4 + 2] (endo-2) and exo-[2 + 2] (anti-3) cycloadducts. Only the endo-[2 + 2] (syn-3) transition state was located for cyclization of intermediate meso-5. The novel [6 + 4] "concerted" ene transition state (threo-4TS, DeltaH(double dagger)(UB3LYP(0K)) = 28.3 kcal/mol) is found to be unstable with respect to an unrestricted calculation. This diradicaloid transition state closely resembles the cyclohexadiallyl radical rather than the linked cyclohexadienyl radical. Several [3,3] sigmatropic rearrangement transition states were also located and have activation enthalpies between 27 and 31 kcal/mol.

1,3-Dipolar Cycloadditions of Electrophilically Activated Benzonitrile N-Oxides. Polar Cycloaddition versus Oxime Formation

The reactions of electrophilically activated benzonitrile N-oxides (BNOs) toward 3-methylenephthalimidines (MPIs) have been studied using density functional theory (DFT) at the B3LYP/6-31G* level. For these reactions, two different channels allowing the formation of the [3 + 2] cycloadducts and two isomeric (E)- and (Z)-oximes have been characterized. The 1,3-dipolar cycloadditions take place along concerted but highly asynchronous transition states, while formation of the oximes is achieved through a stepwise mechanism involving zwitterionic intermediates. Both reactions are initiated by the nucleophilic attack of the methylene carbon of the MPIs to the carbon atom of the electrophilically activated BNOs. The analysis based on the natural bond orbital (NBO) and the topological analysis of the electron localization function (ELF) at the transition structures and intermediates explains correctly the polar nature of these reactions. Solvent effects considered by the PCM model allow explaining the low incidence of the solvent polarity on the rate and composition of the reactions.

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

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.