Laser-Induced Carbene–Carbene Rearrangement in Solution: The Diphenylcarbene–Fluorene Rearrangement

Abstract

Diphenylcarbene (DPC) generated by high-intensity laser photolysis of diphenyldiazomethane rearranges to fluorene (FL) by two distinct mechanisms as revealed by methyl-group labeling. Thus, excimer laser irradiation of <i>p</i>,<i>p</i>′-dimethyldiphenyldiazomethane generates 3,6-dimethylfluorene (3,6-DMF) and 2,7-dimethylfluorene (2,7-DMF), which were identified by fluorescence measurements as well as GC-MS and comparison with authentic materials. 3,6-DMF corresponds to direct bond formation between <i>ortho</i> positions in DPC, referred to as <i>ortho</i>,<i>ortho</i>′ coupling. 2,7-DMF corresponds to a carbene–carbene rearrangement, whereby DPC undergoes ring expansion to phenylcycloheptatetraene (PhCHT) followed by ring contraction to <i>o</i>-biphenylylcarbene (<i>o</i>-BPC), which then cyclizes to FL. The carbene–carbene rearrangement dominates over the <i>ortho</i>,<i>ortho</i>′ coupling under all conditions employed. The <i>ortho</i>,<i>ortho</i>′ coupling must take place in a higher excited state (most likely S₂ or T₁) of DPC, because it is not observed at all under thermolysis conditions, where only S₁ and T₀ are populated. The carbene–carbene rearrangement may take place either in a hot S₁ state or more likely in a higher excited state (S₂ or T₁)