A DFT and CASSCF Study of Photocycloaddition Reactions of Biradicals from 6-Amino-2-(3-thienoyl)-1,4-benzoquinone Gitanjali Sharma, 1 Ignatious Abraham, 1 Ram T. Pardasani,* 1 Prasad V. Bharatam, 2 and Tulsi Mukherjee 3 1 Department of Chemistry, University of Rajasthan, Jaipur-302 055, India 2 Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER), Sector-67, Mohali-160 062, India 3 Chemistry Group, Bhabha Atomic Research Centre, Mumbai-400 085, India Received April 1, 2009; E-mail: rtpardasani@gmail.com DFT-B3LYP and CASSCF calculations have been performed using a 6-31G* basis set to study photocycloaddition reactions of biradicals, generated by irradiation of 6-amino-2-(3-thienoyl)-1,4-benzoquinone, with ethylene. The calculated parameters of biradicals and transition states have also been compared with ground state parameters to completelyelucidate the reaction mechanism of [2 + 2] and [3 + 2] photocycloaddition reactions. Preference for a particular cycloaddition pathway is ascertained by the relative stability between initially formed triplet biradical and another triplet biradicalformed by hydrogen shift. Biradicals or biradicaloid species derived from quinones present fascinating chemistry. 1 Photochemical [2 + 2]- and [3 + 2]-cycloaddition reactions of quinones present very useful synthetic methods for preparing four-membered and five- membered rings respectively. 2,3 Diverse medicinal and bio- logical properties 4,5 of 1,4-benzoquinones make them highly suitable substrates for such cycloadditions. A number of [2 + 2] photocycloaddition reactions which lead to oxetane species have been reported. 6,7 Recently Marminon et al. 8 have reported one-pot [3 + 2]-cycloaddition reaction of 1,4-benzo- quinone and benzyl azide whereas Murphy and Neville 9 have carried out comparative study of [2 + 2] and [3 + 2] cyclo- addition reaction of 1,4-benzoquinone with allylsilane. Density functional methods provide detailed electronic structure of biradical generating species like 1,4-benzoqui- nones. 10-14 Most of these studies are limited to understanding the electronic structure of species rather than understanding the mechanism of the reaction. In previous papers we have reported exhaustive theoretical studies on a wide variety of thermal and photochemical cycloadditions which principally include the stereochemical course of [3 + 2]-cycloaddition reactions of azomethine ylides 15-17 and theoretical aspects of [4 + 2] cycloadditions 18,19 of hetero-1,4-benzoquinones. Encouraged by these results we have focused our attention on a comparative study of [2 + 2] and [3 + 2] photocycloaddition reactions of 6-amino-2-(3-thienoyl)-1,4-benzoquinone. The results are pre- sented herein. Computational Details All the calculations have been carried out using Gaussian98 20 and MOPAC6 21 suites of programs. Harmonicvibration frequen- cies of all stationary points have been computed to characterize them as energy minima (all frequencies are real) or transition states (one and only one imaginary frequency). The molecular and transition state geometries for the reaction of 6-amino-2- (3-thienoyl)-1,4-benzoquinone, through various possible path- ways, [3 + 2]/[2 + 2], with ethylene have been located using CASSCF and B3LYP methods, with a 6-31G* basis set. The active space used in the CASSCF calculations isdiscussed in detail at appropriate places in the succeeding section. Intrinsic reaction coordinate calculations have been carried out using B3LYP/ 6-31G* level on one of the transition states to confirm the reaction path. In the B3LYP calculations of an open shell system, spin unrestricted formalization has been used. For all the minima on the singlet potential energy surface, the calculated (s 2 ) values are close to 1.0, so they are pure singlet biradicals, the triplet stationary points have (s 2 ) values close to 2.0 as expected. Results and Discussion The first task was to choose a model quinone. Literature survey revealed that aminobenzoquinone moiety is a compo- nent of the molecular framework of several natural products, e.g., kinamycine, and streptovaricine. 22,23 Further, the [3 + 2] cycloadducts of such quinones may act as precursors toward mitomycin antibiotics. 24 It has also been reported 25 that the thienyl group is a pharmacologically active moiety and enhances antimicrobial and antibacterial activity of quinones. Keeping these features inmind the synthetically feasible model quinone 1 (Scheme 1) was selected for the present study. To beginwith we have examined the relative stability of the three products that may be formed by [3 + 2]/[2 + 2] photo- cycloaddition of quinone 1 in their ground state (Scheme 1). The results have been summarized in Table 1. A carefullook at Table 1 reveals that [3 + 2] photocyclo- adduct has highest stabilization energy as compared to the other two or in other words photocycloadduct 2 formed by [3 + 2] © 2009 The Chemical Society of Japan Published on the web December 10, 2009; doi:10.1246/bcsj.82.1477 Bull. Chem. Soc. Jpn. Vol. 82, No. 12, 1477–1484 (2009) 1477