Cation−π Interaction Controlled Selective Geometric Photoisomerization of Diphenylcyclopropane

Abstract

Geometric isomerizations of alkenes and cyclopropanes have played a key role in the development of mechanistic organic photochemistry. 1 Although novel strategies toward obtaining cis geometric isomers have been developed for alkene photoisomerization 2 no method is currently available for selective geometric isomerization of trans-diarylcyclopropanes. We now present a strategy based on differential cation-π interaction that has enabled us to selectively convert trans-diphenylcyclopropane (DPC) to the corresponding cis isomer.The ability of cations to interact with π-systems, especially phenyl rings, has been implicated in a number of important processes. 3 The enhanced stability resulting from binding of a potassium ion to more than one benzene ring has been quantified in the gas phase, 4 while crystal structures are available in which sodium ions form sandwich structures. 5 Hence the "bowl"-shaped cis-DPC with optimally poised phenyl rings may be expected to bind to a cation more strongly than the trans isomer ( Figure 1). Ab initio calculations on alkali cation complexes with cis-DPC (vide infra) reveal that the binding energies are comparable to those of sandwich structures with free benzene ligands, and also are cation dependent. These insights have allowed us to selectively convert trans-DPC to the cis isomer. Since Y zeolite contains a large number of cations that are free to interact with the guest molecules, we have employed it as the medium for the above selective trans to cis photoisomerization. 6 As reported in the literature, triplet sensitization of DPC in acetonitrile solution gave a photostationary mixture consisting of both cis and trans isomers (45% cis and 55% trans; p-methoxyacetophenone as sensitizer in acetonitrile). 1b,7 For zeolite irradiations, both triplet sensitizer (xanthone, p-methoxyacetophenone, and benzophenone) and DPC were included within alkali cation (Li + , Na + , K + , Rb + , and Cs + ) exchanged Y zeolites. 8 All photostationary state measurements were made from both transand cis-DPC. The loading levels of the sensitizer and DPC were maintained at one molecule per 10 supercages. The above samples were dried on a vacuum line (10 -4 Torr at 65°C for 12 h), transferred to dry hexane (10 mL) and following bubbling with nitrogen irradiated as hexane slurries. The products were extracted with methylene chloride-diethyl ether and analyzed by GC. The geometric isomers accounted for over 90% of the extracted materials (GC, calibration method). The photoreaction was clean and the photostationary state was reached within 48 h. The photostationary state composition was cation dependent (percent cis at the photostationary state: Li + Y, 91%; Na + Y, 92%; K + Y, 88%; Rb + Y, 85% and Cs + Y, 65%; the error limit was (4% based on at least six independent runs).A slightly modified conventional potential energy surface for the photoreaction helps one understand the unexpected selective isomerization observed in this study ( Figure 1). The photostationary state composition is generally cont...

The Cation–π Interaction in Small‐Molecule Catalysis

“…They subsequently demonstrated that free alkali metal ions introduced into the reaction medium via a dry, metal-exchanged zeolite exert significant influence over the diastereomeric composition of the photostationary state (Scheme 14, Table 2). [54] While all of the alkali metal ions (Li + , Na + , K + , Rb + , and Cs + ) favor the formation of cis isomer 50a , the smaller ions (Li + and Na + ), which generally engage in stronger cation–π interactions, afford the highest diastereomeric ratios.…”

Controlling Photoreactions Through Noncovalent Interactions Within Zeolite Nanocages

“…Ab initio calculations on alkali metal ion complexes with cis -DPC reveal that the BAs are comparable to those of sandwich structures with free benzene ligands and are also alkali metal ion dependent (Table 10.2 ). 32 In contrast to cis -DPC, in the trans isomer, the alkali metal ion would bind only to a single benzene ring. This insight allowed us to selectively convert trans -DPC 35a to the cis isomer 34a (Scheme 10.9 ).…”

Reaction Control by Molecular Recognition – A Survey from the Photochemical Perspective

“…It was found that the photoisomerization of trans -diphenylcyclopropane 121a occurred in the cavity of alkali cation -exchanged Y zeolite to give cis -120a in high hv + 113 114 effi ciency, while the photolysis of cis -120a failed to give any trans -isomer. 134 This is ascribed to the formation of cation -π complex in which two phenyl groups of cis -121a sandwich an alkali metal ion. The cis -isomer is thus stabilized and the isomerization reaction is inhibited.…”

“…The dramatic improvement of de is believed to occur as a result of cationcarbonyl and cation -nitrogen dipolar interactions that fi x the conformation of reactants, with asymmetric centres of the chiral amide moiety being located closer to the chirogenic center. 114,132,134,138,143 Chiral photochemistry in zeolite is an intriguing topic that has attracted intensive attention. Although theoretically zeolites can be chiral, until now no chiral zeolite has been separated in an enantiomerically pure form.…”