Ring thermal activation

As in the case of benzothiazoles and benzimidazoles, the excited-state proton transfer in 2-(2 -hydroxyphenyl)benzoxazole was studied both experimentally and computationally. The results closely resemble the observations for the other species The cw-enol form is preferred in the Sq ground state and the cw-keto form in the 5i excited state. Moreover, the proton transfer appears to be due to vibrational relaxation rather than thermal activation, suggesting that the aromatic ring has an impact on the transfer reaction of these systems [95JPC12456, 99JST255]. 

Hexatrienes undergo disrotatory ring closure by thermal activation to afford cyclohex-adienes in agreement with the Woodward-Hoffmann rule (delocalization band in Scheme 8) [41 3]. Photo-irradiation of hexatrienes is known to give bicylic products in a stereospecific [4n +2nJ manner (delocalization band in Scheme 8) [40] in contrast to this rule. 

This section describes reactions in which elimination to form a double bond or a new ring occurs as a result of thermal activation. There are several such thermal elimination reactions that are used syntheses, some of which are concerted processes. The... 

Aromatic and heteroaromatic rings can participate effectively in the 8ir electrocyclization of nitrones. Upon thermal activation, the benzo derivative 130a did not produce the expected products (92TL61).The conversion of the other derivatives 130b-f at 420°C did, however, give the annulated pyrrole 132 and pyridone 133 derivatives, via the intermediates 131, along with some minor by-products (Scheme 38) (94CB247). 

Thermal activation of alkyne-substituted clusters frequently results in the loss of one or more carbon monoxide ligands (418, 445, 446). Concomitant with this loss is an alteration in the bonding mode of the organic ligand in order to retain the electron balance within the molecule (107). Such a reaction is shown in Fig. 41, where an osmacyclopentadiene ring is transformed into a trisubstituted-f/5-cyclopentadienyl system. Metal-metal bond formation may take place in some examples (446, 447). 

The Srn 1 reaction has been applied to heterocycles. Among five-membered ring compounds, halothiophenes have been the most studied they have been shown to be susceptible to both electron-stimulated and photostimulated reactions, and have been converted to the corresponding acetonitriles [150], acetones [151], and phenyl-sulfides [ 152] in low to medium yields. In the study with the benzenethiolate anion it has been shown that the yield is low because of fragmentation of the adduct radical anion it can be increased by adding an electron acceptor, e.g. benzonitrile which prevents decomposition. Further applications include the thermally activated SrnI reaction between 3-iodobenzothiophene and enolates [153] and the photo-stimulated reaction of 3-halo-2-aminobenzothiophenes [154]. 

The reason that esters frequently give better yields of oxetane is not known, but it seems likely that reaction of hydroxide ion would u> primarily on the carbonyl carbon atom, to form an Intermediate of i fa> usual type- This might reasonably decompose either (a) with con carted attack on the halogeii-subetituted carbon atom, or (b) inii. -i 3-halogenoalkoxide ion which is thermally activated by tlw he i.l decomposition of the intermediate, thus fadlitating ring cloeure. 

There are several examples of catenanes where ring movements can be induced by external stimulations like simple chemical reactions or homogeneous or heterogeneous electron transfer processes [91-93], but only very few cases are reported in which the stimulus employed is light. It has been shown that in azobenzene-containing [2]catenanes like 31 + (Fig. 29) it is possible to control the rate of thermally activated rotation of the macrocyclic components by photoisomerization of the azobenzene moiety [119, 120]. Such systems can be viewed as molecular-level brakes operated by light. 

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