Molecular Simulations of Photoaddition Selectivity and Chirality in Challenging Photochemical Reactions

Abstract

Abstract The origins of cycloaddition selectivities and chirality controls in particular and challenging photochemical reactions were examined. The profiles of the energies and stereochemical changes of the photoreactions were characterized by molecular simulations using dynamic molecular orbital methods. The photoreactions possess particular factors which are not explained by the frontier molecular orbital (FMO) theory. Particular photoreactions are examined by following the addition selectivities and two chiral controls by use of chiral hosts. (1) Species-(singlet or triplet), peri-, site-, regio-, and stereo-selectivities in the [2 + 2]cycloadditions and [4 + 4]cycloadditions of 2-pyridone (1). (2) Inversion of regioselectivity (hh/ht ratio) by the linkage-length in intramolecular [2 + 2]cycloadditions of α,β-unsaturated furanones 7 and by the alkene ring-size in intermolecular [2 + 2]cycloadditions of cycloalkenecarboxylates 11–13 with 2-cyclohexenone (10). (3) The occurrence of the hydrogen-shift reaction in the [2 + 2]photocycloaddition system between 4-hydoxycoumarin (17) and 3,4-dihydro-2H-pyran (18o). (4) Chiral control of selective [4π]-electrocyclic isomerizations of 1 to photopyridone 20 by hydrogen-bonding of chiral amide hosts 21. The singlet excited state, 1* was inferred to be enantiomeric conformers, 1*(+) and 1*(−), and the complex 1·(l)-21 proceeds to 1*(+)·(l)-21, followed by quenching to (R)-20. (5) Sensational chiral isomerization of (Z)-cyclooctene (28Z) to chiral (E)-cyclooctene (29E) by chiral benzenepolycarboxylates (30). 28Z was inferred to accompany asymmetric 28Z and the diastereomeric exciplexes Ex1 (28Z·30*) and proceeds with chiral isomerization via the one-sided rotation to Ex2 (29E*·30), followed by quenching to chiral 29E. The molecular simulations for the photoreactions by MOPAC-PM5, (PM3 for hydrogen-bonding), UCIS (for singlets), and UB3LYP (for triplets) are found to be successful and show that the origin of the photoreaction selectivities is essentially determined by the first step (TS1) energies and the stereochemistries are dependent on the excited species presented in the Concluding Remarks. We also propose some applications of the molecular simulations.