Expanding the Hammett Correlations for the Vinylheptafulvene Ring-Closure Reaction

TY - JOUR

T1 - Expanding the Hammett Correlations for the Vinylheptafulvene Ring-Closure Reaction

AU - Kilde, Martin Drøhse

AU - Hansen, Mia Harring

AU - Broman, Søren Lindbæk

AU - Mikkelsen, Kurt Valentin

AU - Nielsen, Mogens Brøndsted

PY - 2017/2/10

Y1 - 2017/2/10

N2 - A selection of 2,3-diarylated photochromic dihydroazulenes (DHAs) was prepared by following two different protocols. The first protocol relies on the synthesis of a 3-bromo-substituted dihydroazulene, which is subjected to a Suzuki cross-coupling reaction in the final step. In the second protocol, the two aryl substituents are introduced early in the synthesis. The DHAs are photoactive and undergo light-induced ring-opening reactions to form vinylheptafulvene (VHF) isomers. These VHFs are quickly converted back to DHAs at room temperature in thermal ring-closure reactions. The kinetics of these reactions was studied at –20 °C for two series for which the substituent at the C-2 position (DHA numbering) was kept constant, and a linear free-energy relationship (Hammett correlation) was revealed in both cases. The relationships show that an electron-donating substituent at C-3 (the exocyclic VHF carbon atom) enhances the ring-closure reaction, in contrast to its influence when placed at C-2. In a computational study (density functional theory), three different methods were benchmarked against the experimental results. Overall, this work shows for the first time how the VHF to DHA switch is influenced by donor–acceptor substitution at the exocyclic heptafulvene carbon atom

AB - A selection of 2,3-diarylated photochromic dihydroazulenes (DHAs) was prepared by following two different protocols. The first protocol relies on the synthesis of a 3-bromo-substituted dihydroazulene, which is subjected to a Suzuki cross-coupling reaction in the final step. In the second protocol, the two aryl substituents are introduced early in the synthesis. The DHAs are photoactive and undergo light-induced ring-opening reactions to form vinylheptafulvene (VHF) isomers. These VHFs are quickly converted back to DHAs at room temperature in thermal ring-closure reactions. The kinetics of these reactions was studied at –20 °C for two series for which the substituent at the C-2 position (DHA numbering) was kept constant, and a linear free-energy relationship (Hammett correlation) was revealed in both cases. The relationships show that an electron-donating substituent at C-3 (the exocyclic VHF carbon atom) enhances the ring-closure reaction, in contrast to its influence when placed at C-2. In a computational study (density functional theory), three different methods were benchmarked against the experimental results. Overall, this work shows for the first time how the VHF to DHA switch is influenced by donor–acceptor substitution at the exocyclic heptafulvene carbon atom

KW - Cyclization

KW - Donor-acceptor systems

KW - Fused-ring systems

KW - Kinetics

KW - Photochromism

U2 - 10.1002/ejoc.201601435

DO - 10.1002/ejoc.201601435

M3 - Journal article

SN - 1434-193X

SP - 1052

EP - 1062

JO - European Journal of Organic Chemistry

JF - European Journal of Organic Chemistry

IS - 6

ER -