Copper(I)-Catalyzed Inter- and Intramolecular [2+2]-Photocycloaddition Reactions of Alkenes

Photoinduced [2 + 2]-cycloaddition of alkenes to form cyclobutanes is one of the most well-studied and synthetically useful reactions among all the photochemical processes. The utility of this reaction in organic synthesis stems from its ability to create in a single step a complex multicyclic carbon network

with up to four asymmetric centers on the cyclobutane ring in a regio-and stereoselective fashion. The scope of this photoreaction has been widened further by the proclivity of the cyclobutane ring to undergo facile ring expansion and carbon-carbon bond fragmentation. A sequence of [2 + 2]-photocycloaddition and ring expansion or C-C bond fragmentation has been extensively employed for the construction of five-

to eight

-membered rings. The generally accepted mechanism

that accounts for the majority of the 2

π

+ 2

π

photocycloaddition reactions involves excitation of the alkene on UV irradiation either directly or sometimes with sensitizer to the triplet-excited state. Quenching of the triplet-excited state by a ground-state alkene leads to an excited-state complex or exciplex that either directly or through biradical intermediates gives rise to cyclobutanes. The actual mechanism involved in a cycloaddition process depends on the substrates and the reaction conditions employed. Alkenes conjugated to a

π

-system, such as enones having UV absorption maxima around 300 nm, easily lead to triplet-excited states on direct irradiation, and these add to a ground state alkene to give cyclobutanes. A classic example of enone-olefin cycloaddition

is the addition of isobutylene to cyclohexenone

to produce the adduct

(Scheme 1).