Competitive hydrogen and carbomethoxy migration in the photochemistry of 7-carbomethoxy-3,4-benzotropilidene

Abstract

The preparative photolysis of 7-carbomethoxy-3,4-benzotropilidene ( 7) in cyclohexane at 300–380 nm yields 6-carbomethoxy-2,3-benzonorcaradiene ( 11) 51%; 7-carbomethoxy-2,3-benzonorcaradiene ( 5) 35%; and 6-carbomethoxy-2,3-benzobicyclo[3.2.0]hepta-2,6-diene ( 6) 14%; as photoproducts. Deuterium labeling has established that the major product ( 11) arises from a formal 1,7-hydrogen shift, while 5 is formed by a formal 1,7-carbomethoxy migration. A substantial isotope effect was noted in the photolysis of the 7-deutereo-7-carbomethoxy-3,4-benzotropilidene. Mechanisms are discussed for the direct production of 6 from 7. Multiplicity studies indicate that these reactions of 7 occur from the excited singlet state and quantum yield determinations demonstrate that the overall reactive process is highly efficient.