Photoinduced Cyclization of (o‐Alkylbenzoyl)phosphonates to Benzocyclobutenols

Abstract

(o-Alkylbenzoyl)phosphonates readily cyclize to highly strained benzocyclobutenols simply upon irradiation with UV light. The remarkable efficiency is ascribed to the electron-accepting character of the phosphonate substituent, which facilitates thermal ring closing of the o-quinodimethane intermediate and suppresses reversion to the starting carbonyl compound.

Synthetic Approach to Benzocyclobutenones Using Visible Light and a Phosphonate Auxiliary

“…Here, we report a new, two-step procedure to synthesize benzocyclobutenones through visible light induced cyclization of ( o -alkylbenzoyl)­phosphonates that we have recently developed . An H-phosphonate assumes a key role as the recyclable auxiliary; the H-phosphonate is readily introduced onto aroyl chlorides, facilitates visible light induced construction of the four-membered ring skeletons, and is removed for regeneration of a carbonyl group, recovered, and reused.…”

“…We recently reported that the photoinduced cyclization of ( o -alkylbenzoyl)­phosphonates 1 forming phosphonate-substituted benzocyclobutenols 4 is far more efficient and reproducible than that of ordinary ketones (Scheme ). The origin of the efficiency is mainly ascribed to the electron-accepting nature of the phosphonate substituent. The key intermediate to form a four-membered ring by 4π-electron cyclization is o -quinodimethane 3 .…”

“…Thus, we examined an elimination reaction of an H-phosphonate from phosphonate-substituted benzocyclobutenols 4 , which were prepared by photoinduced cyclization of ( o -alkylbenzoyl)­phosphonates (Table ). When dimethyl phosphonate derivative 4a was treated with triethylamine in toluene at 70 °C for 2 h, a base-induced ring-opening reaction selectively took place to afford 1a in 13% yield, and 75% of 4a remained (entry 1). Benzocyclobutenone 5 was not formed at all.…”