Electrocyclization, photochromism

Matsuda, K. and Irie, M. (2006) Recent developments of 67t-electrocyclic photochromic systems. Chemistry Letters, 35, 1204-1209. 

Recent development of 67r-electrocyclic photochromic systems 06CL1204. 

Photochromism Based on Electrocyclic Reactions. The most common general class of photochromic systems involves reversible electrocychc reactions. Within this general class, the most weU-studied compounds are the indolino spiropyrans and indolino spiroxa2ines. 

Uchida and Irie have reported a photochromic system based on ESIPT to an alkene carbon.82 They observed that vinylnaphthol 121 isomerizes to the ring-closed 123 when irradiated with 334 nm light ( = 0.20, Eq. 1.34). The reaction is photoreversible since irradiation of 123 (at400 nm) regenerates the starting vinylnaphthol. The authors proposed a mechanism in which ESIPT from the naphthol OH to the [3-alkenyl carbon gives intermediate o-quinone methide 122, which undergoes subsequent electrocyclic... 

Pyrans and napthopyrans (chromenes) are photochromic compounds that undergo photochemically induced electrocyclic ring opening to give colored ortho-quinone methides.95-98 For example, chromene 153 opens on irradiation to give 154 (Eq. 1.41). 

A number of photochromic systems have been extensively investigated that undergo cis-trans isomerization (indigos, azo compounds) cleavage (spiropyrans), electrocyclic processes (fulgides, 1,2-diarylethenes) [8.229, 8.244, 8.245], For instance, cis-trans isomerization of a thio-indigo derivative allows the reading of pyrene excimer or monomer fluorescence [8.246]. The 1,2-dithienylethene system presents particularly attractive interconversion properties by photoreversible cyc-litation [8.245],... 

New photochromic dyes with electrocyclic reactions have been proposed on the basis of 1,5-electrocyclization of heterogenous pentadienyl— anions (124). Still newer are investigations into the photocyclization of 2,4,6-tri-isopropylbenzophenones for vinyl polymers in the glassy state (133). 

Synthesis of the dibenzofuran (27) by irradiation of grisa-3, 5 -diene-2, 3 -dione (28) is believed to involve electrocyclic ring opening followed by intramolecular cycloaddition to the ketene and elimination of carbon dioxide, as shown in Scheme 2.24 Analogous photocyclizations are responsible for the photochromism exhibited by heterocyclic fulgides such as ( )-a-3-furyl-ethy idene(isopropylidene)succinic anhydride (29), which on irradiation... 

The presence of functional groups in the heterocycle frequently determines the nature of the photoreaction observed. The major product of irradiation of 2-methoxyfuran (191) in the gas phase or in solution is the lactone (192).157 Similarly, 2-nitrofuran (193) undergoes a photoreaction typical of a,f -unsaturated nitro compounds to give the oxime (194) by the pathway shown in Scheme 13.158 A different process is observed, however, on irradiation of thenitrovinylbenzo[6]furan(195) to give the 6-hydroxy-l,2-oxazine(196)159 this transformation is viewed as proceeding via an electrocyclization pathway (Scheme 14) for which there is a precedent in the known photochromism of nitrostyrenes. 

Photochromism is a reversible transformation of a single chemical species between two states, the absorption spectra of which are clearly different, the transition in at least one direction being induced by electromagnetic radiation [1], The widest and most important group of the photochromic system is based on electrocyclic reactions [2,3] a few have been commercially successful (polymer-based photochromic eyewear, novelty items and security printing inks). Several other photochromic systems based on E,Z-isomerization, cycloaddition reaction, electron or proton transfer have potential industrial applications [4],... 

This review surveys our studies devoted to the photoswitchable molecular receptors based on photochromic crown ethers. Photochromic systems described in the review may be classified into three groups according to the reaction types E,Z-isomerization, [2+2]-photocycloaddition reactions and electrocyclic reaction. It has proved the groups to be an especially suitable basis for photochromic systems, and promising for the industrial applications. 

It is known from literature that several reversible photochemical reactions, such as geometric isomerism of azobenzene [7], electrocyclic reaction of dihydroindolizines, fulgides and diarylethylenes with heterocyclic groups [8-10], dimerization of anthracene [11], and photochromic reaction of spirocompounds [12] have been also employed to provide photocontrol over metal-ion binding ability of crown ethers. 

Macrocyclic Systems whose Photochromism is Based on Electrocyclic Reaction... 

Thus, the study of the crown containing systems whose photochromism is based on the electrocyclic transformation demonstrates that the complex formation process can be successfully applied for controlling of its spectral and photochemical properties. 

The photochromic properties of 2/7-chromene derivatives has generated much interest in recent years. Under UV irradiation these molecules can undergo reversible electrocyclic opening of the pyran ring to afford colored ortho-quinone methides <2005T11730, 2005T1681>. 

Valence tautomerism refers to the interconversion of isomers without any accompanying rearrangement including proton transfer. Heterocyclic examples are essentially electrocyclic reactions (Scheme 15). If one of the isomers is colored and the position of the equilibrium can be changed by heat or light, the heterocycles can have useful thermochromic or photochromic applications. 

Volume 1 describes the important developments in the synthesis and study of photochromic compounds and related devices over the past 10 years in industrial and academic laboratories. A brief survey of the contents, chapter by chapter, is useful. The first six chapters cover photochromic systems which are based on pericyclic electrocyclic reactions ... 

Chapter 6 (Photochromism of dihydroindolizines and related systems). The dihydro-, tetrahydro- and hexahydroindolizine compounds involving one, two or three nitrogen atoms make up another family of compounds that exhibits a generally thermoreversible photochromic system based on 1,5-electrocyclization reactions (Scheme 6). Photobleaching is sometimes superimposed on the thermal back reaction. 

P. J. Darcy, H. G. Heller, P. J. Strydom, and J. Whittall, Photochromic heterocyclic fulgides. Part 2. Electrocyclic reactions of (E)-a-2,5-dimethyl-3-furylethylidene (alkyl-substituted methylene) succinic anhydrides, J. Chem. Soc., Perkin. Trans. 1, 1981, 202. 

SYNTHESES OF PHOTOCHROMIC MOLECULES BASED ON A 1,5-ELECTROCYCLIC REACTION... 

Viewing photochromic systems on the basis of a 1,5-electrocyclization and its reverse, molecules must be envisaged in which the open ring as well as the cyclic structure have a similar energy content. This is the challenge in synthesizing molecules with tailor-made photochromic properties in connection with easy access to these molecules. 

Table 6.1. Categories of 1,5-Electrocyclizations in New Photochromic Systems Discussed...

In the new photochromic systems (7/9) based on the ring opening of an azacyclopentene ring and 1,5-electrocyclization, the basic process is the breaking or reforming of a 0 bond (Scheme 13). 

Irradiation of the photochromic DHI 7 gives rise to the colored forms 8 and 9. The thermal back or fade reaction 9 —> 7 (i.e., the 1,5-electrocyclization), can be easily studied kinetically using UV spectroscopy. The rate constant k thus determined can be converted by the equation ... 

The use of dibenzoannellated 1,2-diazines such as benzo[c]cinnoline in the cyclopropene route (a) affords interesting 1,2 systems of Type 37.48 49 The photochromism is due to the 1,5-electrocyclic reaction, as shown in Scheme 19. 

Electrocyclization is an efficient reaction that has been employed in creating new photochromic systems (see also Ref. 3). The main challenge in the field is to prepare colorless and colored forms having almost equal energy content. 1,5-Electrocyclization of the colored betaine to the colorless cyclic structure can be controlled by the substitution pattern of the molecules. Tailor-made molecules with appropriate properties can thus be prepared. This basic concept has been extended from monoaza- (DHI), to diaza-(DHPP) and finally to triaza-5-ring heterocycles. Partially hydrogenated systems are possible as well. 

The characteristics of the photochromic systems based on 1,5-electrocyclization are (1) a neutral molecule as educt gives a zwitterionic species as primary product, and (2) reversibility is possible only if secondary reactions leading to heteroatomic compounds are suppressed. Typical structural elements are therefore spiro(or gem-) substituted to avoid rearomatization. 

Volume 1 includes nine chapters, the first six of these deal with the main established families of organic photochromes which have a few real and many potential applications. Their photochemical processes are based on pericyclic electrocyclic reactions. The three other chapters concern hydrogen or group rearrangement, and electron transfer. Seven out of the nine main authors, selected from all over the world, have not written chapters for previous books and importantly, three are from companies. Four chapters cover families not reviewed... 

Electrocyclization is a photochemical reaction capable of generating photochromic species. An example of a 1,3-electrocyclization reaction involving a monocyclic aryloxirane is the interconversion of m-stilbene oxide and the corresponding carbonyl ylide, shown in Scheme 2 [1]. 

Another class of three-membered heterocyclic compounds that exhibit photochromic activity via 1,3-electrocyclization reaction is the aziridines. For the most part, similar to the oxiranes, these materials require low temperatures to demonstrate photochromic behavior [6]. Also, some of the photochromic transformations are not completely reversible. However, several room temperature stable ylides have been reported for aziridines possessing the general form of Scheme 5 [I], where X and Y may be a mixed combination of nitro, methoxy, and hydrogen groups. The... 

The next subclass of photochromic compounds are those based on 1,5-electrocyclization. An example of An + 2 1,5-electrocyclization is given in Scheme 6. 

There are several categories of photochromic reactions electrocyclization of conjugated polyenes [3], E-Z isomerization of a double bond [4], radical forma-... 

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