Abstract
Benzene exhibits a rich photochemistry which can provide access to complex molecular scaffolds that are difficult to access with reactions in the electronic ground state. While benzene is aromatic in its ground state, it is antiaromatic in its lowest ππ* excited states. Herein, we clarify to what extent relief of excited-state antiaromaticity (ESAA) triggers a fundamental benzene photoreaction: the photoinitiated nucleophilic addition of solvent to benzene in acidic media leading to substituted bicyclo[3.1.0]hex-2-enes. The reaction scope was probed experimentally, and it was found that silyl-substituted benzenes provide the most rapid access to bicyclo[3.1.0]hexene derivatives, formed as single isomers with three stereogenic centers in yields up to 75% in one step. Two major mechanism hypotheses, both involving ESAA relief, were explored through quantum chemical calculations and experiments. The first mechanism involves protonation of excited-state benzene and subsequent rearrangement to bicyclo[3.1.0]hexenium cation, trapped by a nucleophile, while the second involves photorearrangement of benzene to benzvalene followed by protonation and nucleophilic addition. Our studies reveal that the second mechanism is operative. We also clarify that similar ESAA relief leads to puckering of S1-state silabenzene and pyridinium ion, where the photorearrangement of the latter is of established synthetic utility. Finally, we identified causes for the limitations of the reaction, information that should be valuable in explorations of similar photoreactions. Taken together, we reveal how the ESAA in benzene and 6π-electron heterocycles trigger photochemical distortions that provide access to complex three-dimensional molecular scaffolds from simple reactants.
Highlights
Benzene is the archetype aromatic molecule.[1−3] Yet, while its reactivity in the electronic ground state (S0) is well-established, there are many aspects that remain to be explored and exploited related to its photophysics and photochemistry
It is known that benzene becomes antiaromatic in states,[11,14−16] and that relief of this destabilizing its S1 and T1 antiaromatic character is a driving force for photochemical reactions of various benzene derivatives in their T1 states.[20,22]
We show that S1-state antiaromaticity relief of the benzene molecule itself is strongly involved in the photochemical process that leads up to formation of substituted bicyclo[3.1.0]hex-2-enes in nucleophilic media under acidic conditions
Summary
Benzene is the archetype aromatic molecule.[1−3] Yet, while its reactivity in the electronic ground state (S0) is well-established, there are many aspects that remain to be explored and exploited related to its photophysics and photochemistry. Followed by excitation of the formed benzenium ion, occurs only in an extremely strong superacid, FSO3H-SbF5-SO2CIF.[34] benzene in the S0 state remains unprotonated even in very strong acids such as triflic acid.[35] Two other mechanistic proposals earlier turned out to be invalid mechanisms that involved either prefulvene as a reactive intermediate[4,31] or a cis,cis,trans-cyclohexa-1,3,5-triene with Möbius topology.[36] With this foundation we used a combined computational and experimental approach to reinvestigate the photochemical rearrangement of benzenes to bicyclo[3.1.0]hexenes in nucleophilic media under acidic conditions. Focus in our investigation is given to the impact of ESAA on the photoreactivity of simple substituted benzenes, and it is revealed that relief of S1-state antiaromaticity is a strongly contributing factor to the observed photochemical deformation of the benzene ring, which leads to the substituted bicyclo[3.1.0]hexenes
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