Time-resolved IR studies of cyclic enone triplet excited states and their reactions with alkenes

Abstract

The pp* triplet excited states of 3-phenyl-2-cyclohexenone (1) and 3-phenyl-2-cyclopentenone (2) have been observed by time-resolved infrared (TRIR) spectroscopy. The reaction of various alkenes with the triplet excited state of 2 has also been studied by TRIR spectroscopy, and the partitioning between fragmentation and ring closure of the biradical intermediates involved has been estimated. Within a series of structurally related biradicals, more stable ones undergo ring closure more efficiently. Scheme 1 Representative enone-alkene [2 + 2] reactions. y Dedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays. z Electronic supplementary information (ESI) available: Optimized geometries and ab initio-calculated IR frequencies and intensities for cyclic enones. See

Internal conversion and intersystem crossing in α,β-enones: a combination of electronic structure calculations and dynamics simulations

“…Whereas, this type of conformational change is not so large in CPO due to the geometrical constraints of the ring, thus the two triplet states are seen to be nearly isoenergetic, which is consistent with the spectroscopic observations. 25,28 The computed S 0 -S 1 (np*) vertical excitation energy of AC is 91.3 kcal mol À1 by the CASPT2//CAS(6,5)/6-31G* calculation, which reproduces the measured value of 331.5 nm (86.3 kcal mol À1 ). 34 For CPO, the S 0 -S 1 (np*) vertical excitation energy was computed to be 89.5 kcal mol À1 at the CASPT2// CAS(8,7)/6-31G* level, which is in reasonable agreement with the experimental observation.…”

“…In the solution-phase environment, [1,3] H-migration was observed to be the major channel with a product yield of 0.78 and was suggested to take place in the T 1 (pp*) state of AC. 22 For CPO and related cyclic a,b-enones, their typical photoinduced reactions include cycloaddition, hydrogen abstraction, and rearrangements, and have been investigated in many experiments [23][24][25][26][27][28][29] and theoretical calculations. [30][31][32][33] These studies provided important insights into the multi-channel photodissociation and photoisomerization mechanisms of a,b-enones, and emphasized the significant role played by the internal conversion (IC) and intersystem crossing (ISC) in these reactions.…”