An elusive thermal [2 + 2] cycloaddition driven by visible light photocatalysis: tapping into strain to access C2-symmetric tricyclic rings
A mild and operationally simple methodology is reported for the synthesis of cyclobutane rings imbedded within a C2-symmetric tricyclic framework. The method uses visible light and an iridium-based photocatalyst to drive the oft-stated “forbidden” thermal [2 + 2] cycloaddition of cycloheptenes and analogs. Importantly, it generates cyclobutane with four new stereocenters with excellent stereoselectivity, and perfect regioselectivity. The reaction is propelled forward when the photocatalyst absorbs a visible light photon, which transfers this energy to the cycloheptene. Key to success is, upon excitation to the triplet via sensitization from the photocatalyst, the double bond isomerizes to give the transient, highly strained, trans-cycloheptene. The trans-cycloheptene undergoes a strain relieving thermal, intermolecular [π2s + π2a] cycloaddition with another cis-cycloheptene. X-ray analysis reveals that the major product is the head-to-head, C2-symmetric all trans-cyclobutane. Additionally, a dramatic display structural complexity enhancement is observed with the use of chiral cycloheptenols possessing one stereocenter, which results in the formation of cyclobutanes with six contiguous stereocenters with good to excellent diastereocontrol, and can be used to isolate single stereoisomers of stereochemically complex cyclobutanes in good yield.
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