Spiro-oxazines

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. 

In their closed forms, spiropyran, spiro-oxazine, and some naphthopyrans are chiral about the sp carbon atom that joins their orthogonal or out-of-plane ring... 

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

Aramaki and Atkinson were also active in work on the spiro-oxazines [65]. They noted that for NOSH in many polar and nonpolar solvents the picosecond time-resolved resonance Raman spectra simply built up over 50 psec with no shape evolution. The same finding was concluded from transient absorption measurements over the same time scale. The spectra/absorbances were then constant for 1.5 nsec. These authors suggest that only two isomers can be expected to contribute to the merocyanine spectra because those trans about the y-methene bridge bond attached to the naphthalene ring are sterically crowded due to short interproton distances. There was no evidence for the X transient in their study however, the 50-psec convoluted pulse profile may be expected to mask this sortlifetime species even if it were present. 

To finalize this section, we will comment on the isoindoline-derived spiro-oxazine NOSI5. This compound also photo-converts efficiently and very rapidly into their open merocyanine. In this case, the reaction is over within 30 psec and has a biexponetial rise with 2- and 15-psec lifetimes. The merocyanine spectrum does not evolve in time after this time. A broad absorption occurs around time zero, which is either the excited singlet state or the often hypothesized transient X. The data for NOSI5 are shown in Fig. 6 The biexponential absorption rise... 

The naphthopyran ring-opening reactions have not been as well studied as they have for the spiropyrans and spiro-oxazines. Aubard et al. [75] recently reported that in acetonitrile and hexane, irradiation of CHRl resulted in a broad transient spectrum after 0.8 psec, having three maxima at 360-370 nm, 500 nm, and 650 nm. At 1.8 psec, a well-defined band forms at 425 nm. From 10 to 100 psec, there is little further evolution except for a continued growth in the peak at 425 nm of about 15%. There is also a decrease in the overall bandwidth. The mechanism in Scheme 9 has been proposed, where B2 and B, are isomers of the mero-form. Three isobestic points were identified in the transient spectra at different times, suggesting four transient states. Forming between 0.8 and 1.6 psec, the Bi state was assigned as the cis isomer. This had a spectrum similar to that obtained for Tamai s X transient of the spiro-oxazine NOSH, which was obtained at subpicosecond time scales [26]. 

The opposite direction of spectral shift with solvent polarity is observed for the spiro-oxazine [4] and naphthopyran mero-forms and this is generally accepted to infer a quinoidal HOMO and a zwitterionic LUMO state in their cases. This shift is shown also in Fig. 7b for a naphthopyran TT isomer (CHR2). Of course, H-bonding and other specific interactions will also affect the position of the mero-form spectral maximum. In the case of the substituted BIPS, spiro-oxazine, and CHR2, there is evidence to support their respective assignments to a zwitterionic and quinoidal forms, including x-ray [36,80-85] and NMR [36,55,86-88] data. 

Clearly, the TC isomer has its spectrum red-shifted compared to the TT isomeric form. Although spectra for individual isomeric forms of spiro-oxazine and spiropyran merocyanines are not available, it has been shown that the different isomers have very different spectra. In fact, Abe and co-workers [89] have shown that the merocyanine of spiro-oxazine can be converted photochemically between two states having different absorption spectra. Even earlier work carried out at low temperature and or with visible-light irradiation suggested that the spiropyran merocyanine isomers also exhibit significant differences between their absorption spectra [6-8]. 

Two permanent merocyanines have been reported for the spiro-oxazines [85]. These were NOSH heteroanellated by imidazo [l,2-a]pyridine and imidazo [l,2-a]pyrimadine. Several tests have been conducted to determine the nature of these species. H -NMR data show that the indoline nitrogen is not highly charged and the crystal structure indicates that the ground state is essentially the quinoidal form. The most stable form was found to be the TTC isomeric form by x-ray analysis. The dipole moment of these permanent spiro-oxazine merocyanines was around 3.84 D, which is much lower than the values reported for spiropyran merocyanines. 

Spiro-oxazine ring closure takes the opposite trend with respect to solvent polarity compared to the spiropyrans [4,25,68]. Again, we can rationalize this with the... 

Abe and co-workers [89] worked on the spiro-oxazine system NOSI6 shown in Scheme 13 and found markedly different ring-closure kinetics. Note that in the case of Abe and co-workers compound NOSI6, there are no unfavorable interactions between the proton on the naphthalene and the methine proton in either the TTT and CTT forms. In fact, TTT and CTT should be the most planar forms. Compare that to the case of the NOSH merocyanine, which was studied using H-NOE and found to be an equilibrium mixture of TTC and CTC [88]. Ring closure would clearly be a more favorable series of rotations to form the closed-form geometry for CTC and TTC than it would be for CTT and TTT. 

B. Spiro-oxazine Photochemical Ring Closure and Merocyanine Photochemistry... 

Schneider et al. [63] investigated the photochemistry of the spiro-oxazine merocyanines pumping and probing at 570 nm in acetonitrile. The found that the solution bleached within the <5-psec pulse duration. The bleached state recovered with at least a biexponential behavior, and from their fluorescence decay measurements, three exponentials were required to fit the decay. They attribute these findings to the possibility of three merocyanine isomers that are in equilibrium. Their compounds feature geminal ethyl groups on the indoline moieties and this may influence the system as compared to NOSIl. 

Spiro-oxazine (NOSH) photo-induced ring closure reactions were first described by Bohne et al., who used two-laser two-color excitation in the UV and visible regions [71,72]. In this work, they found that photoexciting the merocyanine in cyclohexane leads to a bleached product which recovers quantitatively to the merocyanine form over 30 xsec. The transient bleach state had an absorbance... 

In the case of the single-photon transformation, this is not a general phenomenon and few examples exist. Recently, the crystalline-state photo-transformation of spiropyran [107], naphthopyran [77], and spiro-oxazine [106] chemical systems have been reported. None of these studies, however, give information about transient states except that the study on naphthopyran system described a longer-... 

Unsubstimted spiropyrans similarly exhibit rapid ring-opening photochemistry to the spiro-oxazines also via a singlet state. One transient state has been identified also in polar solvents. This was assigned as 5i, but by analogy with the spiro-oxazines, this may be similar to X that was proposed by many workers. Photochemical ring closure appears to be inefficient and we often observe non-... 

Oxidation of A-acyltyramines 264 with IBTA proceeds with intramolecular participation of amido group oxygen in nonnucleophic solvents to give spiro-oxazines 265 (Scheme 66, route a ). In nucleophilic solvents such as alcohol or acetic acid, addition of solvent at the para position occurs to give quinone 266 (Scheme 66, route b ) (91JOC435). 

Flash vacuum pyrolysis (FVP) of the tetrahydro-l,3-oxazine derivative 171, derived from the condensation of spiro-oxazine 170 and methoxymethylene-substituted Meldmm acid, resulted in formation of pyrrolo[l,2-r ][l,3]oxazine 172, which in (CD3)2CO at —20°C proved to be a 47 53 mixture of the keto (A) and enol (B) tautomers (Scheme 28) <2002J(P1)548>. 

Spironaphthoxazine with a crown ether, 8, also shows photochromic behavior that is sensitive to the type of alkali and alkaline earth metal ions [14]. Other derivatives of spiropyran and spiro oxazine, 9-12, which can chelate metal ions were synthesized, and the effects of complexation on their photochromic behavior have been investigated [15-17]. 

V. Malatesta, R. Millini, and L. Montanari, Key intermediate product of oxidative degradation of photochromic spiro-oxazines. X-ray crystal structure and electron spin resonance analysis of its 7,7,8,8-tetracyanoquinodimethane ion-radical salt, J. Am. Chem. Soc., 117, 6258-6264 (1995). 

V. Malatesta, F. Renzi, M. L. Wis, L. Montanari, M. Milosa, and D. Scotti, Reductive degradation of photochromic spiro-oxazines. Reaction of the merocyanine forms with free radicals, J. Org. Chem. 60, 5446-5448(1995). 

An increased interest in the synthesis and study of new photochromic spiro-oxazines (SOs) is due to their improved properties compared to SPs, i.e,. then-stability to photodegradation.64 This has stimulated research on the influence of specific structural features of SOs on their properties. The SOs that have been studied by X-ray diffraction analysis to date can be divided into two groups derivatives ofbenzoxazines and derivatives of annellated benzoxazines. The latter... 

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