Substituted Spiropyrans

The V1/2 values obtained for dioxane solutions (10 4M) of spirobenzopyrans substituted on the chromene moiety are presented in Table 2. In the nitro-substituted spiropyran series (6, 28, and 29), the 6-nitro derivative is usually more stable. OCH3 and CHO substituents are also more stabilizing when on C6 (cf. 6 and 32 30 and 32). Indeed, the electron-donor efficiency of the methoxy group is higher when at C6 as indicated by the Hammett ct constant values, which are -0.76 for the C6 (para) position and -0.39 for the C8 (ortho) position. For the nitro-substituted spiropyrans, the electron-withdrawing character of N02 depends on the substitution position, with Hammett constants of 0.66, 0.78, and 0.80, for N02 at C7, C6, and C8, respectively. 

Having observed that C6 substitution had a major effect on photodegradation, Gautron prepared a series of C6-substituted derivatives and studied their behavior with respect to photoexcitation. The results presented in Table 3 show that indeed electron-donor groups at C6 efficiently stabilize the photochrome against photodegradation (cf. 33, 36, and 3 3 and 33 vs. 34 and 6). 

The results presented in Table 3 show that among the spiropyrans with a nitro substituent at C6, Cl, and C8, the C6-substituted compound is the most stable. OCH3 and CHO have the same effect. Here again, electron-donor substituents, e.g., CH3 and OCH3, stabilize the photochrome against photodegradation. 

The analysis of the results presented in Table 4 indicates that EDGs on the chromene moiety decreases photodegradation (37 vs. 40), whereas EWGs on C8 have the opposite effect (see 41). 

A CH3 group on C3 greatly decreases stability whereas QH5 substitution yields stabilization (Table 5, 47 vs. 48). It was noted that, in general, the rate constant k of the thermal bleaching reaction was not affected by the ct Hammett constants of substituents on the indoline moiety whereas for substituents on the chromene moiety EDGs produced an increase of k. 

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C. Moustrou, A. Samat, R. Guglielmetti, R. Dubest, and F. Gamier, Synthesis of thiophene-substituted spiropyrans and spirooxazines precursors of photochromic polymers, Helv. Chim. Acta 78, 1887-1893 (1995). 

Futami Y, Chin MLS, Kudoh S, Takayanagi M, Nakata M (2003) Conformations of nitro-substituted spiropyran and merocyanine studied by low-temperature matrix-isolation infrared spectroscopy and density-functional-theory calculation. Chem Phys Lett 370 460 68... 

Cottone, G., Noto, R., La Manna, G., and Fornili, S.L., Ab initio study on the photoisomers of a nitro-substituted spiropyran, Chem. Phys. Lett., 319, 51, 2000. 

The spiroindolinobenzopyran 2 is a classical example of spiropyran and is easily prepared by the condensation of l,3,3-trimethyl-2-methyleneindo-line (Fischer s base) and salicylaldehyde in anhydrous ethanol or benzene (Scheme 2).ia The nucleophilic attack of Fischer s base on the carbonyl group (like an enamine) gives an aldol product, which undergoes ring closure followed by dehydration. This condensation is reversible therefore, an exchange of the salicylaldehyde component of spiropyran with a different salicylaldehyde is possible. For example, when a solution of spiropyran 2 (Scheme 2) was refluxed with 3,5-dinitro-substituted salicylaldehyde, the open form of 6,8-dinitro-BIPS was obtained.2... 

Indolines, benzoxazole, and benzothiazole are possible as 2-methylene heterocycles. The number of known spirooxazine derivatives is much less than for the spiropyrans. This may be partly due to lack of many substituted o-nitrosonaphthols and partly due to lack of sufficient stability of spiro-oxazines. The structures of parent spirooxazines and the Xmax of their photomerocyanine forms are listed in Table 5. The Xmax of the colored forms of compounds 41-43 are not described in the literature. 

Unlike spiropyran, 2 -substitution in spirooxazine has no effect on Alkoxy, chloro, and nitro substituents at 5-position, and alkyl substituent at 1-position in the indoline component have small effects on the Xmix of the colored form. 

Nitro-substitution especially at the 6-position of BIPS opens up a triplet pathway for photo-isomerization. This pathway runs in parallel to the singlet manifold. This increases the yield and, in turn, may lead to photo-aggregation that is observed for these compounds. Photochemical ring closure to the spiropy-ran form is more efficient for these 6-nitro-substituted compounds. The photochemistry of 6-nitro-BIPS merocyanine is similar to that of unsubstituted BIPS(s) however, the 6,8-dinitro compound efficiently cyclizes upon excitation to form the spiropyran closed form via a singlet manifold. 

Spiropyrans refers in general to a (substituted) 27/-pyran having a second ring system, usually (but not necessarily) heterocyclic, attached to the 2-carbon atom of the pyran in a spiro manner as shown in structure 1 i.e., a carbon atom is common to both rings. Compounds in which the second ring system is merely a saturated carbocycle such as cyclohexyl or adamantyl are better considered simply as 2,2-dialkylpyrans, and are discussed in Chapter 3. 

The third approach is useful when the spiropyran itself (or, more correctly, its intermediates) is easily available, the necessary substituted intermediates are not, and the spiropyran tolerates the chemical transformations involved. Halogenation and nitration of BIPS and spiro(dinaphthopyrans) can be carried out by standard methods bromo substituents replaced by cyano or lithium, nitro substituents reduced to amino, etc. For example, the aldehyde group in 6-formylBIPS can be reduced, oxidized, caused to react with organometallics, condensed with active methylene groups, converted to cyano, and derivatized to give a wide variety of 6-substituted BIPS. These reactions cannot be usefully carried out directly on 5-formylsalicylal-... 

Salicylaldehydes give spirobenzo pyrans, often referred to simply as spiropyr-ans. In principle, true monocyclic spiropyrans would be prepared from 3-hydro-xyacrylaldehyde, which is the enol form of malonaldehyde. A reaction of Fischer s base with malonaldehyde or its chloro derivative gave complex mixtures containing the open form, but nitromalonaldehyde sodium salt gave with Fischer s base hydroiodide a 24% yield of beautiful orange crystals of the open form (3) (A. = 483 mm)7 Recently a series of 2-substituted malonaldehydes (4) became... 

The heterocyclic alkyl quaternary salts used for preparing spiropyrans are usually obtained by N-alkylating the heterocyclic bases by standard methods. The common bases having an active methyl group of the indolenine, benzothiazole, benzoxazole, and quinoline series some of their substitution products, and a few of their quaternary salts are commercially available. The pyrylium, thiopyrylium, 2-azaazulenium and bcnz[c,d]indolenium salts needed for potentially infrared-absorbing spiropyrans are not (as of the time of writing). 

The Fischer s bases, 2-alkylidene-l,3,3-trisubstituted indolines, which may be further substituted in the indoline ring, are the most versatile and useful bases for preparing spiropyrans. For convenience, the name Fischer s base will be used hereafter in the general sense indicated above, as well as for the specific compound 2-methylene-l,3,3-trimethylindoline. Substituents in the 1- and 3-positions are methyl unless otherwise specified thus 3-phenylFischer s base denotes the 1,3-dimethyl-3-phenyl compound. 

Reaction between butanesultone and 2,3,3-trimethylindolenine gave a 74% yield of the water-soluble N-(n-butylsulfonate) inner salt of Fischer s base this salt and others from substituted indolenines were used as intermediates for cyanine and merocyanine dyes, but not specifically for spiropyrans. 21... 

Formyl-l,2,3,3-tetramethylindoline (29) was converted to various 5-([3-heterocyclylvinyl) indolines (30) via condensation with heterocyclic quaternary salts and subsequent N-dealkylation by triphenylphosphine. The resultant indolines were oxidized with mercuric acetate to the 5-substituted Fischer s bases (31, R = benzoxazol-2-yl quinolin-2-yl quinolin-4-yl pyridin-4-yl 1,3,3-trimethylindo-len-2-yl). These were used as intermediates for cyanine dyes their conversion to spiropyrans was not recorded. 41... 

Sulfonation of Fischer s base in 30% oleum gives a 97% yield of the 5- and not the 6-substituted derivative, according to NMR spectral evidence. The compound was used to prepare cyanine and styryl dyes, but no spiropyrans were reported.44... 

Upon treatment with acid (or on contact with an acidic surface), many spiropyrans give the salt of the open form or the open form itself, depending upon the relative base strengths of the spiro and open forms. Thus, treatment of several BIPS (7-diethylamino, 6-nitro, and 5 -nitro) with trifluoroacetic acid in the nonprotic solvents acetonitrile and chloroform gave the protonated merocyanine form, which upon neutralization with base gave the open colored form.180 This sequence of operations causes coloration by a non-thermal, non-photochemical route the adsorption coloration was utilized in the early applications of spiropyrans in carbonless (pressure-sensitive) copy papers. In this application, dialkylamino-substituted spirodi(benzopyrans) were preferred paper containing BIPS compounds turned pink on storage. 

M. A. GaTbershtam, N. N. Artamonova, and N. P. Samoilova, Synthesis of 3 -acyl-substituted indoline spiropyrans, Chem. Heterocycl. Cpds., 1975, 167-172. 

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