Stereospecific [2+2] photocycloaddition in a pseudodimeric complex between N-ammoniopropylstyrylpyridine and 18-crown-6-containing styrylpyridine

Abstract

A quaternary 4 styrylpyridinium salt having the N ammoniopropyl substituent forms a pseudodimeric head to tail complex with neutral 18 crown 6 containing 4 styrylpyridine in MeCN through H bonding. This complex undergoes stereospecific [2+2] photocycloaddition due to preorganization of the ethylene bonds in the syn arranged chromophoric fragments of the components. The structure of the rctt isomer of the cyclobutane derivative obtained was established by NMR spectroscopy and X ray diffraction analysis.The self assembly of organic and bioorganic molecules into supramolecular assemblies in solutions through rela tive weak H bonds is widely known. 1,2 At the same time, the self assembly of the photosensitive compounds in volving hydrogen bonding is poorly studied, while it is of great interest due to possible photochemical reactions within the supramolecular assemblies formed that are not observed under other conditions. Crown containing un saturated compounds undergo self assembly in the pres ence of different cations. 3 These photosensitive compounds containing the C=C bond are subject to E-Z isomeriza tion, electrocyclization, and [2+2] photocycloaddition (PCA). We suggested that a versatile approach to the self assembly of crown containing unsaturated compounds can make use of the primary ammonium groups incor porated into the unsaturated compound itself or a photo insensitive substrate. Such type of self assembly may be illustrated by the example of the following supramolecu lar structures. Bis(18 crown 6)stilbene forms extremely stable donor-acceptor complexes with the N,N´ di(am monioalkyl) derivatives of viologen analogs that can serve as fluorescent molecular sensors for the alkaline earth metal cations. 4,5 18 Crown 6 containing bis styryl dyes behave as selective optical molecular sensors for diam monioalkane ions; 6 the stereospecific intramolecular PCA reaction can proceed in the pseudosandwich head to head complexes of dyes that formed to afford a single isomer of a cyclobutane derivative. 7 Due to the H bond ing between the ammonium groups and the crown ether fragments, the crown containing styryl dyes having the N ammonioalkyl substituent in the heterocyclic residue form stable head to tail dimeric complexes wherein efficient stereospecific autophotocycloaddition 8 occurs. Alternatively, on exposure to light, the formation of a relatively stable cation "capped" complex occurs in the Z form of a dye with intramolecular coordination of the ammonium group by the crown ether fragment. 9 The above mentioned phototransformations result in consid erable hypsochromic shifts in the absorption spectra of the unsaturated compounds, which can be used for the design of the systems for optical recording of information.Recently, we have shown 10 that two yellow styryl dyes of the 4 pyridyl series, one of which contained the N ammoniopropyl substituent ((E) 1) and the second one contained the fragment of 18 crown 6 ((E) 2), formed a sufficiently stable pseudodimeric complex (logK = 3.52) in MeCN. Due to cooperative ...

Synthesis, structure, and stereospecific cross-[2+2] photocycloaddition of pseudodimeric complexes based on ammonioalkyl derivatives of styryl dyes

“…Apart from the signals of minor E and Z isomers of the initial compounds 1a-d,f,h and 2a,b, the 1 H NMR spectra of photolysis products exhibit a new set of signals at about 5 ppm, which is characteristic of cyclobutane derivatives of a similar structure. 8,14,16,18,22 While comparing these signals with analogous signals of the previously studied isomer rctt-3a, 9 we concluded that all of the obtained cyclobutanes have a rctt stereochemistry and, hence, they are formed from pseudodimeric complexes 1Á2 in which the dyes were arranged in the syn-head-to-tail fashion (see Schemes 1 and 5). The signals of all cyclobutane protons of rctt-3a overlap little with each other; therefore, they all are doublets of doublets (ABCD type spin system).…”

“…These values are typical of 1,2,3,4-tetrasubstituted cyclobutanes that we studied earlier. 8,14,16,18,22…”