Formation of an exceptionally stable ketene during phototransformations of bicyclo[2.2.2]oct-5-en-2-ones having mixed chromophores

Abstract

Photochemical reactions of bicyclo[2.2.2]oct-5-en-2-ones having mixed chromophores like a 5,6 dibenzoyl moiety and bulky electron-deficient substituents like phenyl or isopropenyl at the bridgehead position were analyzed for the first time in different solvents and upon irradiation with different wavelengths. In all cases, a regioselective photoinduced 1,5-phenyl migration leading to vinyl ketenes from the more congested site of the molecule to the less congested one has been observed. The ketenes were exceptionally stable both in air and solution. Its stability studies in acetonitrile through time-dependent UV absorption spectra revealed that it remained almost unchanged at least for a couple of weeks.

Photochemical Access to Substituted β-Lactams and β-Lactones via the Zimmerman–O’Connell–Griffin Rearrangement

“…This limits the structure of the ketenes and the reaction conditions available for the synthesis of β-lactams. An alternative method of producing ketenes in situ in a highly efficient manner is the photoinduced rearrangement of the dibenzoyl ethylene scaffold ( 1 in Scheme A), which was first discovered by Zimmerman, O’Connell, and Griffin in the 1960s (Scheme A). − Despite thorough mechanistic investigations, this rearrangement has only been utilized in the synthesis of 4-oxo-butanoic esters and acids ( 3 in Scheme A) to date. − Therefore, using the Zimmerman–O’Connell–Griffin (ZOG) rearrangement in combination with imines for the synthesis of substituted β-lactams is a novel and attractive strategy but potentially challenging. Typically, high-energy UV light is employed to carry out the ZOG rearrangement, − and under these reaction conditions, several potential side reactions can occur, such as imine E/Z -isomerization and imine cycloadditions. , A method relying on visible light could circumvent this problem, thereby making the ZOG rearrangement a powerful tool for the construction of four-membered rings under mild conditions.…”

“…The mechanism of the formation of the intermediate ketene has been thoroughly investigated in previous reports of this reaction and follows the pathway presented in Scheme . The E -1,2-dibenzoyl ethylene 4 is first isomerized to the Z -1,2-dibenzoyl ethylene 1 , which upon further excitation undergoes an intramolecular ipso attack of one of the oxygens to one of the phenyl rings. ,,,− ,, The formed spiro radical I collapses to form the ketene intermediate 2 , which is intercepted by an imine to form the final product via the Staudinger synthesis. , To further support the formation of the ketene 2 , ethanol was used as a nucleophile, thereby resulting in the formation of the butanoate ester (for discussion, see the Supporting Information).…”

“… 5 − 7 Despite thorough mechanistic investigations, this rearrangement has only been utilized in the synthesis of 4-oxo-butanoic esters and acids ( 3 in Scheme 1 A) to date. 8 − 12 Therefore, using the Zimmerman–O’Connell–Griffin (ZOG) rearrangement in combination with imines for the synthesis of substituted β-lactams is a novel and attractive strategy but potentially challenging. Typically, high-energy UV light is employed to carry out the ZOG rearrangement, 5 − 7 and under these reaction conditions, several potential side reactions can occur, such as imine E/Z -isomerization and imine cycloadditions.…”