Tautomerization, salicylic acid

The photophysics of solid salicylic acid (SA) has been studied by using steady-state and time-resolved spectroscopic techniques [207,208], Dimers of SA form in two possible structures (59 and 60) due to fast ground-state double proton transfer. Dual fluorescence is observed at 380 nm and 440 nm, which are ascribable to the excited-state double proton transfer between different dimeric structures of SA. The enol form is more stable in the ground state. However, in the excited singlet state, the keto form has a lower potential energy [207], This excited enol-keto tautomerism has a barrier height of -1250 cm as is calculated from the dependence of dual fluorescence on excitation wavelength in the... 

Sulfasalazine (5-[4-[(2pyridylideneamino)sulfonyl]phenyldiazenyl]salicylic acid) (40) was first isolated in triclinic form and it was shown by X-ray analysis that the molecule is present in the amide tautomeric form 40c (2001ACX435). Later it was isolated in monoclinic form, and this form was shown by X-ray analysis to exist in the imino form 40d (Scheme 31) (2005AXCo226). 

In the Kolbe reaction, phenoxide ion reacts with CO2 to produce salicylic acid through a mechanism in which the phenoxide ion reacts like an enolate anion attacking the electrophilic carbon of CO2, followed by loss of a proton and keto-enol tautomerization to give the salicylate anion. 

The vast majority of papers devoted to tautomerization dynamics deal with ESIPT reactions. Since Weller s suggestion that the large Stokes shift he measured for salicyhc acid fluorescence was caused by rapid proton transfer in the excited state [62], and the development of techniques to study this on a femtosecond timescale, the field has blossomed. Most of the 2000 papers on tautomerization dynamics is on ESIPT, from both an experimental and a theoretical point of view. The number of compounds exhibiting ESIPT is far too large to discuss here. It ranges from molecules as simple as malonaldehyde to systems as complicated as 3-hydroxyflavone or 2-(2 -hydroxyphenyl)benzothiazole. In particular, substituted salicylic acids and ortho-hydroxybenzaldehydes have attracted much attention from both experimentalists and theoreticians. Weller s idea is depicted in Figure 1.10. 

The dashed arrow indicates the excited-state proton transfer. There are a few cases of dual emission from both the normal (E) and tautomeric (T) forms. Salicylic acid is not one of them in most solvents. 

Photochemically Liduced Tautomerism of Salicylic Acid and Its Related Derivatives... 

Bisht, P.B., Petek, H., Yoshihara, K., and Nagashima, U., Excited state enol-keto tautomerization in salicylic acid a supersonic free jet study, J. Chem. Phys., 103, 5290,1995. 

Sulfasalazine,5- 4- [(2-pyridylideneamino)sulfonyl] -phenyldiazenyl salicylic acid, a derivative of 5-aminosalicylic acid and sulfapyridine, is a pharmaceutical compound used in the treatment of inflammatory bowel disorders and rheumatoid arthritis [90]. It was shown that the sulfasalazine can be obtained in two crystal forms, as a monoclinic or as a triclinic form. These forms differ not only in the spatial arrangement of the molecules but also in their tautomeric form (Scheme... 

Similar azomethine ylides 48 are involved as key intermediates in the racemi-zation of optically active a-amino acids in the presence of a catalytic amount of aldehyde (83JOC843). Grigg confirmed this mechanism (83TL4457) and carried out the reactions of optically pure a-amino acids with salicyl- or o-methoxybenzaldehyde and maleic anhydride or N-phenylmaleimide in acetic acid. Cycloadducts of N-unsubstituted azomethine ylides 48 with dipolaropbiles were obtained only as racemates. These results indicate that the a-amino acids first produced the corresponding imine carboxylic acids with the retention of optical activity but these then undergo a tautomeric equilibration with N-protonated azomethine ylides 48 losing their optical purity. The ylides 48 were captured by several dipolaropbiles as racemic cycloadducts. As the imines of a-amino acids usually suffer spontaneous decarboxylation (as will be discussed later in Section (II,E), the formation of... 

---