Thermal ring-opening reaction

A thermal ring opening reaction of an imine-substituted cyclopropene led to a mixture of 2,3,4-trisubstituted and 3,4,5-trisubstituted pyrroles <06TL5793>. 

Whereas the thermal ring-opening reaction of oxrranes and aziridines is frequently used for generation of carbonyl ylides and azomethine ylides, the analogous procedure starting with thiiranes does not produce the expected thiocarbonyl ylides (8). However, in the case of tetraaryl-substituted thiiranes, the photolytically mediated reaction with tetracyanoethylene (TCNE) is believed to occur via a single electron transfer (SET) mechanism, also involving a thiocarbonyl ylide as a likely intermediate (75,76) (Scheme 5.14). 

A similar thermal ring opening reaction has been postulated for the conversion of AT-vinyl-1,2-dihydropyridine (46) to styrene (47) and triphenylpyridine. Ring opening to an acyclic polyene was also proposed (equation 16). This appears to be a general reaction of AT-vinyl-1,2-dih dro ridines (82JOC492). 

An alternative way of analyzing the thermal ring-opening reaction is to look at the phases of the molecular orbitals involved. These can be visualized as follows ... 

Figure 9. The thermal ring-opening reaction of 2-aryl-2-methylnaphthopyrans proceeds via a transition state, represented by only two of the possible resonance formulas. (The sign means that the first resonance formula is viewed from two different directions in order to put different molecular fragments into the plane of the paper. Therefore, the fragment Z is not situated in the plane of the paper.) R/R means H/H or a benzo ring.

Figure 10. Thermal ring-opening reactions of 2-bvenzyloxybenzopyran 22 (top) and spirobibenzo-pyran 23 (bottom) proceed via transition states, represented by only one of the possible resonance formulas in both cases. 

The finding of a AG value of 81.5 kJ mol" for 56 (Table 12) means that its half-life,/0.5, before thermal ring opening at room temperature is of the order of only 10 s. Similarly low barriers were seen for several spirobipyrans in Tables 6 and 8. Therefore, the thermal ring-opening reactions of spiropyrans represent further examples of fast isomerizations. 

Cycloaddition reactions of the indole 2,3-double bond are not limited to alkenes as partners. Acetylenic compounds have also been used in photochemical cycloaddition reactions with indoles to produce cyclobutenone derivatives. There have been extensive studies on the reaction of indoles with dimethyl acetylenedi-carboxylate (61, DMAD), which produce a number of structurally distinct products [31]. By devising a photosensitized cycloaddition reaction of DMAD to activated indoles 60 in the presence of acetophenone, Neckers and Davis were able to produce the cyclobutene derivatives 62 in good yields (Scheme 14) [32]. The resulting cyclobutenes can then be converted to the corresponding benzazepines 63 via thermal ring-opening reactions. 

Analogous results were secured in the thermal ring-opening reaction of the corresponding diphenyl-substituted azirine 346/ 2-Vinyl-substituted 2H-azirines have also been suggested as intermediates in the thermolysis of a number of substituted butadienyl azide systems. ... 

Horii, T., Abe, Y., and Nakao, R., Theoretical quantum chemical study of spironaphthoxazines and their merocyanines. Thermal ring-opening reaction and geometrical isomerization, /. Photochem. Photobiol. A Chem., 144, 119, 2001. 

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