Solvolytic behavior of bicyclo [3.1.0]hex-3-en-2-yl derivatives. Mechanism of photolysis of benzene in hydroxylic media

Abstract

sented implies that I-R is antihomoaromatic, i.e., that it is less stable than the isomeric radical II-R in which the conjugated system is not cyclic. Extension of the available orbital argument to the anions suggests that II-Ris preferred to I-R-, but experimental confirmation is lacking.Breslow10 has demonstrated the utility of electrochemical methods in obtaining otherwise unavailable information on very unstable antiaromatic species, such as triphenylcyclopropenyl anion. We suggest that this method is even more valuable in the study of antihomoaromatic species because these may be generated by electron-transfer reactions of the corresponding homoaromatic species, before they revert to the more stable linearly conjugated isomer. Attempts to prepare antihomoaromatic species by chemical reactions such as protonation11 should yield preferentially the linearly conjugated species and not the desired intermediate.(8) If the isomerization of I-R+ to II-R+ precedes electron transfer to II-R+, then the reduction potential reflects the free-energy changes for the isomerization and the electron transfer, as in the reduction of cyclooctatetraene to its planar radical anion: T.

Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes

“…The MeOH 2 + approaches from the endo face, leading to incorporation of the proton in the 6- endo position, in accord with the reported photolysis experiments using deuteriophosphoric acid. 32 The activation free energies for the subsequent methanol addition are 10.9 kcal/mol ( exo ) and 15.5 kcal/mol ( endo ), respectively.…”

“…Both mechanistic pathways involve relief of the S 1 -state antiaromatic character of benzene, but in different ways. Many pieces of evidence have been acquired for pathway A 31 , 32 and pathway B, 24 , 33 but no previous study has unambiguously confirmed one mechanism over the other. With regard to ESAA relief, any pathway that allows for disruption of the 6π-electron cycle which is antiaromatic in S 1 will provide for such relief.…”

“…This was explored by Berson et al, who found the main observed photoproduct of irradiation of benzene in acidic D 2 O to be exo-8, which was found to be formed by epimerization of the primary photoproduct endo-8 (Scheme 8a). 32 He also observed selective endo incorporation of deuterium in position 6 and no addition of deuterium from the exo site leading to 9. We now confirm this through a study of the photoreaction of benzene-d 6 in aqueous solvent under acidic conditions (Scheme 8b).…”

“…We first re-examined the synthetic potential of the reaction. The earliest studies reported low reaction yields (few percent) and long irradiation times (weeks). , Since then, several substrates (benzene, , toluene, t Bu-benzene, , o -xylene, tri- t Bu-benzene, benzonitriles, − and trifluoro­methylbenzene and toluenes) have been reported to undergo the photoinduced rearrangement and addition of methanol, ,, acetic acid, , water, , and 2,2,2-trifluoro­ethanol ,,,− under acidic conditions (Figure ). The quantum yields for several of the substrates were low (∼0.05). , Also, the published reactions have never been used before for synthetic applications.…”

“…The photochemical reaction of benzene in nucleophilic media leading to substituted bicyclo[3.1.0]­hexenes previously gained considerable attention, and several different mechanisms were proposed. ,,− Our hypothesis is that the ESAA character of benzene impacts its photoreactivity in one of two ways: either through increased basicity (proton affinity) facilitating protonation of benzene in its S 1 and T 1 states followed by structural rearrangements (mechanism A, Scheme ), or through the unimolecular rearrangement of S 1 -state anti­aromatic benzene to the prefulvene conical intersection which leads to benzvalene (mechanism B). A third possible mechanism in which benzene is protonated in the S 0 state, followed by excitation of the formed benzenium ion, occurs only in an extremely strong superacid, FSO 3 H-SbF 5 -SO 2 CIF .…”