No abstract

No abstract

Abstract

is in accord with Barltrop's experimental and Jug's SIN-DO1 conclusions concerning cyanopyrrole. Photolysis of l,&dimethylpyrazole was also carried out at 77 K in frozen acetonitrile resulting in the formation of imidazoles that are consistent with only the P4 and P6 permutation patterns. No P7 imidazole was detected even after 33% of the reactant had disappeared. These observations are consistent with our calculated results, which suggest that Iz forms on a relatively flat potential energy surface, such as S1, and that subsequent reactions occur on the ground-state surface. ConclusionMNDO calculations and experimental product distributions are used to provide a qualitative explanation of the phototransposition of pyrazole to imidazole via [ 1,3]-sigmatropic shift of nitrogen. Initial electrocyclic ring closure of pyrazole and walk of nitrogen occur on the S1 potential energy surface. The return to the ground state is via a radiationless transition from I&) to TS2(So). The second [1,3]-shift of nitrogen may be a biradical process; however, it is difficult to clearly distinguish between concerted and biradical mechanisms a t the MNDO level.Kinetic and thermodynamic arguments are in qualitative agreement with the experimentally observed relative product distributions of the P6 and P, imidazoles and support the assignment of the second walk to a groundstate process. Photolysis at decreasing temperature results in increasing P6 to P, product ratios. This is consistent with a ground-state thermal reaction and a smaller barrier for rearomatization than walking as suggested by the theoretical calculations.Acknowledgment. The authors thank A. E. Johannesen and the staff of the College Computing Center for their assistance during the early stages of this project. We are grateful to Professor M. C. Zerner of the University of Florida for providing a copy of his INDO program. DECstation 3100 and Encore Multimax 520 computers at Worcester Polytechnic Institute were used for the computations.Registry No. R, 288-13-1; PB,P,, 288-32-4; IJ,, 74613-28-8; I&, 74613-34-6.

. Can. J. Chem. 69, 1171 (1991).The stereochemistries of the 2+2 cycloaddition products obtained from the photochemical addition reaction between N-benzoylindole or N-carboethoxyindole and the alkenes cyclopentene, cyclohexene, cycloheptene, cis-and trans-2-butene, and cis-and trans-4-octene are examined. The structures of the products are shown to be consistent with a photo-cycloaddition mechanism involving the intermediacy of triplet 1,4-biradical species. The quantum yields of adduct formation between N-benzoylindole and both cis-and trans-octene were measured as a function of alkene concentration. The results suggest that cis-octene reacts with the indole derivative's triplet excited state with a rate constant of (1.7 + 0.3) X lo7 M-' s-' . The results are also consistent with the immediate products of this reaction being 1,4-biradicals, 98% of which revert to the ground state indole derivative and alkene, and only 2% of which proceed to cycloadduct. In marked contrast, the same treatment suggests that trans-octene reacts with the triplet excited state of N-benzoylindole with a rate constant estimated to be in the range of 1 X lo6 and 6 X 105M-I s-', and it appears that the 1,4-biradicals formed revert much less efficiently to the starting materials; it is estimated that between 67 and 100% of the 1,4-biradicals proceed to cycloadducts. In the reaction with cis-octene biradical reversion leads to the formation of trar~s-octene ("Schenk isomerization"); the quantum yield of this process is determined to be 0.074 * 0.004, which may imply that approximately 75% of the biradicals collapse to cis-alkene and 25% collapse to the trans isomer.Key words: indole, photocycloaddition, 1,4-biradicals.DAVID J. HASTINGS and ALAN C. WEEDON. Can. J. Chem. 69, 1171 (1991).On a ttudiC les stCrCochimies des produits de cycloaddition 2 + 2 obtenus par rCaction d'addition photochimique entre le N-benzoylindole ou le N-carboCthoxyindole et les alcknes suivants : cyclopentene, cyclohexkne, cycloheptkne, cis-et transbut-2-Cne et cis-et trar~s-oct-4-kne. On dkmontre que les structures des produits correspondent B celles attendues sur la base d'un mecanisme de photo-cycloaddition impliquant des espkces 1,4-biradicalaires triplets comme intermkdiaries. On a dCterminC les rendements quantiques pour la formation des adduits entre le N-benzoylindole et les cis-et trans-octknes en fonction de la concentration d'alckne. Les rksultats suggkrent que le cis-octkne rtagit avec I'Ctat excitC triplet du dCrivt de l'indole avec une constante de vitesse de (1,7 * 0,3) X lo7 M-' s-'. Les rCsultats sont aussi en accord avec I'existence de 1,4-biradicaux comme produits immkdiats de cette rCaction dont 98% retourneraient aux Ctats fondamentaux de l'alckne et du dCrivC de l'indole alors que seulement 2% conduiraient au produit de cycloadduction. Par ailleurs, un traitement similaire suggkre que le trans-octkne rCagit avec l'ttat excite triplet du N-benzoylindole avec une vitesse de rCaction qui se situerait entre 1 x lo6 et 6 X lo5 M -I s-I alors que le...