Enantioselective Photochromic Reaction of a [2.2]Metacyclophan-1-ene We are grateful for financial support from The Association for the Progress of New Chemistry (ASPRONC).

Abstract

Photochromic compounds have been extensively studied owing to their potential as photomemory and photoswitching materials. [1] Photochromic compounds reversibly change their absorption spectra (color) upon photoirradiation. However, reading the photomemory by changes in the absorption spectrum causes the destruction of the memory, since photoirradiation results in excitation of photochromic compounds. [1] Therefore, various attempts at nondestructive readout have been reported, [1] for example, by locking one of the photoisomers by the addition of chemical substances [2] or by electrooxidation (gated photochromic reaction). [3] Reading the change in chiral information originated in photochromic reactions is one of the most promising systems, since chiral information can be read without excitation of photochromic compounds. [4,5] In photocyclization systems, for example, diarylethenes, [5a] fulgides, [5b] and spiropyrans, [5c] the enantiomers of the photochromic compounds undergo diastereoselective photocyclization. In these systems, one should be aware of the diastereoselectivity of the cyclization, since such selectivity is altered by the conditions (e.g. temperature, medium, etc). This fact encouraged us to study enantiomeric photocyclization, that is, a photochromic reaction between two enantiomers in which no racemization takes place.We chose 1,2-disubstituted [2.2]metacyclophan-1-enes (MCP-1-enes), [6] which we recently reported, [7] as the enantiomeric photocyclization system (Scheme 1). It is well known that ring inversion of the phenyl groups in the anti 8,16dimethyl[2.2]metacyclophane system is forbidden because of the steric hindrance between the inner methyl groups and the opposite phenyl groups. [8] For similar reasons, ring inversion of MCP-1-enes does not occur, and MCP-1-enes are chiral molecules because no mirror symmetry exists in the molecule. UV UV Vis Vis tBu Me Me tBu N O O p-tolyl Me Me tBu tBu N O O p-tolyl tBu Me Me tBu N O O p-tolyl Me Me tBu tBu N O O p-tolyl 1a 1b enantiomers enantiomers Scheme 1. Enantioselective photochromic reaction of [2.2]MCP-1-ene 1 a and THP 1 b.Therefore, one can expect that racemization of the enantiomers of MCP-1-enes does not take place. MCP-1-enes show photochromism, [6,7] and tetrahydropyrenes (THP) formed from MCP-1-enes by UV irradiation are also chiral. It is expected that no racemization occurs, even during the photochromic reaction, and that the photochromic reaction takes place with both enantiomers. We have designed and synthesized some MCP-1-enes and found that the enantiomers of maleimide-type MCP-1-ene 1 a are easily separated. [9] MCP-1-ene 1 a was synthesized from 1,2-dicyano[2.2]MCP-1-ene [7] in three steps. [10] Figure 1 shows Figure 1. Changes in the absorption spectrum of 1 a (0.16 mm in hexane) upon irradiation with 313-nm light. Numbers indicate the irradiation time (s).the changes in the absorption spectrum of a solution of 1 a in hexane upon irradiation with 313-nm light. The pale yellow solution of 1 a changed to violet upon UV irrad...