Intramolecular Thermal and Photochemical Reactions

Under flash vacuum thermolysis (FVT) the 1,2,3-triazine (32) was readily thermolyzed to an alkyne, a nitrile, and nitrogen in high yields. For unsymmetrically substituted 1,2,3-triazines as FVT substrates, fragmentation proceeded selectively a bulky substituent at C-4(6) made the adjacent C—N bond break more easily than the opposite C—N bond. The FVT method was applied to the synthesis of the fluorinated alkynes, perfluoro-3-methyl-l-butyne and difluoroethyne [(33), R = (CF3)2CF, F] (Equation (1)) (89CC1657, 91CC456). It has been claimed, however, that tris(dimethylamino)-1,2,3-triazine forms the monocyclic azete. 

Thermally above 600°C, and also photochemically, 1,3,5-triazine decomposes to three molecules of hydrogen cyanide. Most 1,2,4-triazines are thermally very stable. 

Certain polysubstituted pyridines yield isolable Dewar pyridines, as illustrated in equation (2) (see CHEC 2.05). 

Irradiation of pyridine itself gives Dewar pyridine, observable spectroscopically, which in water is hydrolytically ring-opened to form H2N(CH = CH)2CHO, but in a matrix fragments to cyclobutadiene and HCN. 

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Reactivity of Nonconjugated Rings 14.10.5.1 Intramolecular Thermal and Photochemical Reactions... 

The proposed mechanism for the photochemical cleavage of nBn ethers (see Scheme 3) involves a n tt transition of the nitrogroup and an intramolecular, benzylic hydrogen abstraction by the excited nitro group. Rearrangement leads to a hemiacetal that decomposes to a free alcohol and to 2-nitrosobenzaldehyde that undergoes further thermal and photochemical reactions. 

Nitramine rearrangements occur in quinoline , pyridine , and thiazole derivatives, and although migration into both rings is reported for the first type, studies in the pyridine series indicate an intramolecular pathway . Thermal and photochemical reactions of iV-nitroanilines and naphthylamines give 30 to 50% yields of... 

The fundamental types of thermally and photochemically induced intramolecular transformations are summarized in Scheme 1. All reactions of this class involve intermediates in which aromaticity is lost hence they are most common in the classes of less aromaticity, i.e. polyhetero rings, cationic rings, rings containing carbonyl groups. However, polysubstitution, especially by bulky groups, can also induce reactions by strain relief in transition states. Most of the reactions known are photochemical. 

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

C-C bond dissociation energies are not the only molecular properties affected by electron transfer. In addition, radical cations exhibit a number of unique characteristics flexible structures [23], a low sensitivity towards steric effects [24], low activation barriers for inter- and intramolecular reactions [25-28], high acidities [29-32] and the inversion of the thermochemical stability order for certain tautomeric systems. Examples in the recent literature demonstrate that it is worthwhile thinking about how the changed molecular properties can be used for the design of new reactions that complement the thermal and photochemical reactivity patterns [26,33-35]. 

When two methylenecyclopropane fragments were linked by a Cj-chain to form a 1,5-hexa-diene, intramolecular reactions can be initiated both thermally and photochemically. Heating in toluene gave rise to a Cope rearrangement with formation of l,r-divinylbicyclopropyl. ° When the starting material was irradiated in benzene, it underwent, among other reactions, an intramolecular [2 +2] cycloaddition. The reaction mixture comprised three different products in an overall yield of 73%. 

Electron-Transfer Reactions. It is well known that thermal and photochemical electron-transfer reactions exhibit characteristic pressure dependences and associated volumes of activation (see Sections II, III, and VI). It is therefore realistic to expect that photoinduced thermal electron-transfer reactions will also exhibit a characteristic pressure dependence that should reveal mechanistic information on the nature of the reaction. Recent interest in the mechanistic understanding of long-distance electron-transfer reactions prompted an investigation of the effect of pressure on intramolecular electron transfer in ruthenium-modified cytochrome c [151] (a typical example of a closely related intermolecular electron-transfer reaction was... 

Four different types of (2 + 2)-cycloaddition and -cycloreversion reactions of heterocyclic compounds are known intermolecular and intramolecular reactions, both thermal and photochemical. Three of these have already been discussed in the previous sections on (2 + 2)-cycloadditions, and as far as the mechanism is concerned both the forward and the reverse reaction suffer from the same ambiguity Do they proceed via a concerted or a nonconcerted mechanism do they involve an ionic or a diradical intermediate are they symmetry-allowed or forbidden So far only one reaction type is known to be limited to the reverse reaction, viz., the thermal intramolecular reaction [Eq. (10)], in which one o-bond is broken and a conjugated -electron system is... 

The intramolecular diyl trapping reaction was studied in a variety of solvents (THF, MeOH, acetonitrile), and the diyl was generated both thermally and photochemically [14]. The photo-induced deazetation of 41 in methanol at -6 °C afforded the desired tricyclopentanoid 40 in an excellent 84% yield. The transition state model portrayed by 49 nicely rationalizes the stereochemical outcome. The solvent study revealed that its choice had essentially no effect upon the product ratio at any given temperature. However, we did discover that methanol, a solvent which had not been utilized previously in intramolecular 1,3-diyl trapping reactions, was very useful for low temperature studies. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Muller et al. have also examined the enantioselectivity and the stereochemical course of copper-catalyzed intramolecular CH insertions of phenyl-iodonium ylides [34]. The decomposition of diazo compounds in the presence of transition metals leads to typical reactions for metal-carbenoid intermediates, such as cyclopropanations, insertions into X - H bonds, and formation of ylides with heteroatoms that have available lone pairs. Since diazo compounds are potentially explosive, toxic, and carcinogenic, the number of industrial applications is limited. Phenyliodonium ylides are potential substitutes for diazo compounds in metal-carbenoid reactions. Their photochemical, thermal, and transition-metal-catalyzed decompositions exhibit some similarities to those of diazo compounds. 

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