Thermal aromatic systems

Aromatic character in isoxazoles has been studied from a number of viewpoints, and these studies indicate that although isoxazole may be formally considered an aromatic system, the disposition of the ring heteroatoms modifies this character to an appreciable extent. From a qualitative viewpoint, thermal stability and electrophilic attack at the 4-position may be considered consistent with an aromatic character. Furthermore, NMR chemical shifts of the ring protons are consistent with those of an aromatic compound. References related to these studies may be found in Section 4.16.2.3.4. 

THERMAL CLEAVAGE OF BENZYLIC BONDS IN AROMATIC SYSTEMS... 

Secondly, the carbon framework holding the exocyclic double bonds could be extended. This is demonstrated by naphtharadialene 5, a highly reactive intermediate which has been generated by thermal dehydrochlorination from either the tetrachloride 178 or its isomer 179106. Radialene 5 has not been detected as such in these eliminations rather, its temporary formation was inferred from the isolation of the thermolysis product 180 which was isolated in 15% yield (equation 25). Formally, 5 may also be regarded as an [8]radialene into whose center an ethylene unit has been inserted. In principle, other center units—cyclobutadiene, suitable aromatic systems—may be introduced in this manner, thus generating a plethora of novel radialene structures. 

Applying these rules in pericyclic reactions it has been shown and a generalization given that thermal reactiom occur via aromatic transition states while photochemical reactions proceed via antiaromatic transition state. A cyclic transition state is considered to be aromatic or isoconjugate with the corresponding aromatic system if the member of conjugated atoms and that of the n... 

It should be emphasized that the wide scope of nucleophilic aromatic photosubstitution does not imply that it will work indiscriminately with any combination of aromatic compound and nucleophile. On the contrary, there are pronounced selectivities. The general picture now arising shows a field with certainly as much variability and diversification as chemists, in the course of growing experience, have learned to appreciate in the area of classical (thermal) aromatic substitution. It is one of the aims of this article to contribute to a description and understanding of the various reaction paths and mechanisms of nucleophilic aromatic photosubstitution, hopefully to the extent that valuable predictions on the outcome of the reaction in novel systems will become feasible. 

The thermal cycloaddition of azides to acetylenes is the most versatile route to 1,2,3-triazoles, because of the wide range of substituents that can be incorporated into the acetylene and azide components. The accepted mechanism for the reaction is a concerted 1,3-dipolar cycloaddition. The rates of addition of phenyl azide to several acetylenes have been measured the rates of formation of the aromatic triazoles are not appreciably different from the rates of cycloaddition to the corresponding olefins, indicating that the transition-state energy is not lowered significantly by the incipient generation of an aromatic system. 

Diazonium salts can also be used to form ethers and phenols. Reaction of diazonium salt with an alcohol generates an ether, while thermal hydrolysis of the diazonium salt yields a phenol. Figure 13-30 illustrates both formations. As seen in Figure 13-31, this process also works on substituted aromatic systems. 

Polynitro derivatives of monocychc aromatic systems (trinitrobenzene, trinitrotoluene, tetranitro-iV-methylaniline, trinitrophenol, etc.) have long been used as explosives [1]. It has been found that a series of polynitroderivatives of biphenyl, diphenylmethane and 1,2-diphenylethylene (stilbene) are explosives liable to detonate on grinding or impact [2]. The same may be true of other polynitro derivatives of polycyclic systems not normally used as explosives (e.g. polynitro-fluorenones, -carbazoles, etc. Penta- and hexa-nitrobenzophenones are also high-energy explosives [3]. The thermal stability of 33 polynitroaromatics was studied by DTA [4]. Two empirical equations relating the heat of decomposition to the heat of detonation have been developed and used to calculate the heats of detonation for 47 polynitroaryl compoimds [5]. 

Another approach to synthesize stable Diels-Alder adducts of Cjq was introduced by Mullen and co-workers [41—43], The use of o-quinodimefhane derivatives as dienes, prepared in situ, leads to the formation of thermally stable cycloadducts (Scheme 4.5). As with the isobenzofuran addition product [13], a cycloreversion of these adducts would need to overcome the stabilization provided by the aromatic system and would also give the unstable o-quinodimefhane intermediate. A fast ring inversion, at elevated temperatures, of the cyclohexane moiety causes a 2 -symmetry of the cycloadduct, leading to 17 lines for the fuUerenyl carbons in the NMR spectra [41]. 

Formal replacement of one double bond in thiepin (51) by a sulfur atom leads to 1,2- (53) and 1,4-dithiin (54). The latter compound has been synthesized,70 and it is a non-planar, thermally stable molecule with a reactivity widely different from that of aromatic systems. A refined HMO method (Model B)71 has been used successfully to explain known properties of 54, and remarkably good agreement was obtained between the calculated and experimental C—S—C and C—C—S bond angles. For the parameters used (8S =0, pcc = 1.06, pcs = 0.77), the highest occupied orbital is anti-bonding, in agreement with the high... 

Aromatic compounds have a special place in ground-state chemistry because of their enhanced thermodynamic stability, which is associated with the presence of a closed she of (4n + 2) pi-electrons. The thermal chemistry of benzene and related compounds is dominated by substitution reactions, especially electrophilic substitutions, in which the aromatic system is preserved in the overall process. In the photochemistry of aromatic compounds such thermodynamic factors are of secondary importance the electronically excited state is sufficiently energetic, and sufficiently different in electron distribution and electron donor-acceptor properties, ior pathways to be accessible that lead to products which are not characteristic of ground-state processes. Often these products are thermodynamically unstable (though kinetically stable) with respect to the substrates from which they are formed, or they represent an orientational preference different from the one that predominates thermally. 

The reaction of carbenoids with aromatic systems was first reported by Buchner and coworkers in the 1890s.6 The reaction offers a direct entry to cycloheptatrienes and has been used to synthesize tropones, tropolones and azulenes.6 Neither the thermal nor copper-catalyzed reactions, however, proceed in good yield. The problems associated with these transformations were clearly demonstrated in a recent reexamination of die thermal decomposition of ethyl diazoacetate in excess anisole (137).129 A careful analysis of the reaction mixture revealed the presence of seven components (138-144) in 34% overall yield (Scheme 29). The cycloheptatrienes (138M142) were considered to be formed by cyclopropanation followed by electrocyclic ring opening of the resulting norcaradienes. A mixture of products arose because the cyclopropanation was not regioselective and, also, the initially formed cycloheptatrienes were labile under the reaction conditions. 

X-ray diffractometry is the most powerful method to determine atomic coordinates of molecules in the solid state. X-ray crystal structure analysis was, however, rarely applied in the early years of development of persistent, long-lived alkyl carbocations and studies were only performed to investigate structures of carbocations of aryl derivatives and aromatic systems.65 This is due to the low thermal stability of alkyl carbocations and to the difficulties in obtaining single crystals of carbocations suitable for analysis. Since then, however, methods and instrumentation have improved significantly and X-ray crystal structure analysis has become a powerful tool to solve structural problems of carbocations.65,66... 

The proposed reaction schemes (Figures 4-17 and 4-18) show the molecular breakdown commencing at the extremities of both molecules to leave a polar core that eventually separates from the liquid reaction medium. In the case of the decomposition of the amphoteric molecule, thermal degradation could just as easily commence at the aliphatic carbon sulfur bonds followed by thermal scission of the alkyl moieties from the aromatic systems. If the removal of the... 

If LVMO and HOMO are not compatible, a forbidden or high-energy process would normally be necessary to effect cyclization otherwise, anti cycloaddition (Fig. 10c) might be possible, as will be discussed shortly. It may be even simpler to imagine the reactants as they would appear in the transition state and inquire whether one has an incipient aromatic system, i.e. Htickel (4% + 2)77 cycle (Dewar, 1966 Fukui, 1965, 1966). If so, the cycloaddition is thermally allowed otherwise, forbidden. The nature of these predicted closures would normally be reversed for reactions of the first excited states. 

The thermal stability of the endoperoxides of aromatic compounds enhances going from the, only spectroscopically detectable, endoperoxides of benzene derivatives to those from acenes, which are stable enough to be stored for several days. The main rearrangement is reversion to singlet oxygen and the starting materials and depends on the aromatic system and on the type of substituents in the meso positions when they carry the... 

Oxadiazole (1) is a thermally stable neutral aromatic molecule (65JA5800). Other aromatic systems are 1,3,4-oxadiazolium cations (2) and the exocyclic-conjugated mesoionic 1,3,4-oxadiazoles (3) and 1,3,4-oxadiazolines (4). Also known are derivatives of the non-aromatic reduced systems, 2,3-dihydro-l,3,4-oxadiazole (A2-l,3,4-oxadiazoline 5),... 

Trithiadiazepine 5 and 1,3,5,2,4,6-trithiazepine 6 have remarkable thermal stability for molecules with such a high proportion of heteroatoms they do not decompose even on prolonged heating at 180 C. Such unusual stability indicates that these heterocycles are delocalized 10-aromatic systems. 

Kawasaki [113] (Scheme 57). The enyne intermediate 150, which is formed by Sonogashira coupling, undergoes a thermal benzannulation in the sense of a [4 + 2]-cycloaddition followed by rearrangement to the aromatic system. 

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