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

• 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. 

Ämnesord

NATURVETENSKAP  -- Kemi -- Organisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Organic Chemistry (hsv//eng)

Nyckelord

Addition reactions

Benzene

Ground state

Isomers

Photoreactivity

Protonation

Quantum chemistry

Scaffolds

Electronic ground state

Molecular scaffolds

Nucleophilic additions

Photorearrangement

Quantum chemical calculations

Stereogenic centers

Substituted benzenes

Synthetic utility

Excited states

bicyclo[3.1.0]hexene derivative

cyclohexene derivative

heterocyclic compound

molecular scaffold

pyridinium derivative

silabenzene

solvent

unclassified drug

Article

chemical phenomena

controlled study

electron

excited state antiaromaticity

isomer

nucleophilicity

photochemistry

synthesis

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