Enantiodifferentiating photocyclodimerization of cyclohexa-1,3-diene sensitized by chiral arenecarboxylates

Abstract

The enantiodifferentiating photosensitized cyclodimerization of cyclohexa-1,3-diene 1 was performed over a range of temperatures in the presence of chiral arene(poly)carboxylates, giving endo-and exo-[4 ϩ 2] cyclodimers (2a, 2b) and anti-and syn-[2 ϩ 2] cyclodimers (3a, 3b). Among the three chiral cyclodimers (2a, 2b, 3a), only 2b was obtained as an optically active species with an enantiomeric excess (ee) of up to 8.2%. The detailed reaction mechanism and the origin of the enantiodifferentiation have been elucidated, and the crucial role played by the 'microenvironmental polarity' around the chromophore in determining the photoreactivity and the ee of the product is also discussed.

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…797,798 Inoue and coworkers studied the enantioselectivity of the [2 + 2] photodimerization of 1,3-cyclohexadiene upon sensitization with chiral arene(poly)carboxylates (up to 8% ee). 799 The extended chromophore of benzocarborane allowed for its direct excitation, which led in the absence of oxygen and hydrogen donors to a clean [2 + 2] photodimerization. 800 An intramolecular variant of the [2 + 2] photocycloaddition between a diene and an olefin was investigated by the Pulido group.…”

“…Direct excitation of 1,3-dienes is normally not feasible, and photocycloaddition of this compound class is preferentially performed in the presence of a sensitizer. For studies on the thermal fragmentation of various cyclobutanes, the group of von E. Doering utilized the benzophenone-mediated [2 + 2] photodimerization of various dienes. , Inoue and co-workers studied the enantioselectivity of the [2 + 2] photodimerization of 1,3-cyclohexadiene upon sensitization with chiral arene­(poly)­carboxylates (up to 8% ee ) . The extended chromophore of benzocarborane allowed for its direct excitation, which led in the absence of oxygen and hydrogen donors to a clean [2 + 2] photodimerization …”

Microenvironmental Polarity Control of Electron-Transfer Photochirogenesis. Enantiodifferentiating Polar Addition of 1,1-Diphenyl-1-alkenes Photosensitized by Saccharide Naphthalenecarboxylates

“…In sharp contrast to the rapidly growing interest and insight, as well as obtained ee, in enantiodifferentiating photosensitization of unimolecular isomerization processes, the investigation and knowledge of bimolecular photoaddition processes are lacking in both depth and variety, which renders our understanding and control of such photoenantiodifferentiation insufficient. In light of this, the enantiodifferentiating [2 + 2] photocyclodimerizations of aryl vinyl ethers and 4-methoxystyrene were examined in the presence of chiral naphthalene(di)carboxylates, giving the corresponding cyclodimers in extremely low ee's (<1%) 14a. However, Kim and Schuster reported product ee's of up to 15% for the [4 + 2] photocycloaddition of trans -β-methylstyrene to 1,3-cyclohexadiene sensitized by (−)-1,1‘-bis(2,4-dicyanonaphthalene) in toluene at −65 °C …”

Positive Photocatalysis of a Diels−Alder Reaction by Quenching of Excited Naphthalene−Indole Charge-Transfer Complex with Cyclohexadiene

“…In this context, naphthalene (NP) has rarely been used as photocatalyst in organic transformations . However, substituted naphthalenes, such as cyano-, carboxy-, and alkoxynaphthalenes, photocatalyze a variety of reactions, such as cyclizations, addition of alcohols to alkenes, dimerizations, cross-cycloadditions, and cycloreversions …”