Thermal valence isomerization

In principle, thermal valence isomerization of the 3,4-benzo-2-thiabicyclo[3.2.0]hepta-3,6-diene skeleton 1 should lead to the 1-benzothiepin ring system. 

Depending upon the substitution pattern, a thermal valence isomerization of 1,4-dioxocins 4 to the tricyclic jyn-benzene dioxides (xyn-3,8-dioxatricyclo[5.1,0.02-4]oct-5-enes) 3 can be detected. On the other hand, the valence isomerization of sin-benzene dioxides (anti-benzene dioxides do not undergo such rearrangements) provides a general approach to 1,4-dioxocins 4. 

The second example refers to oxomolybdenum dithiolate complex [Md O(qdt)2], where qdt stands for quinoxaline-2,3-dithiolate. This complex also manifests thermal valence isomerism. Helton et al. (2001) also explain it with the change of the molybdenum oxidation state ... 

When 4-isoxazolines with an alkyl substituent carrying an acidic a-hydrogen at the 3-position undergo a thermal valence isomerization into azomethine ylides 160, the acidic a hydrogen migrates to the anionic center to form enolated bisenamine intermediates 161 (70CB3196). A process similar to the cyclization of 161 providing pyrroles was discussed in Section II,A. 

The mechanism of this thermal valence isomerization was formulated as proceeding by an intermediate diradical. Extended Hiickel calculations have shown that the opening of the endo compounds into diradicals is a symmetry-forbidden process, in contrast to the ring opening of the exo derivatives. ... 

Soon after the first synthesis, a notable landmark, the second route to la, was achieved by Jones and Bickelhaupt they reported that the thermal valence isomerization of Dewar benzene type isomer of la, [6.2.2]propelladiene (8a), proceeded smoothly, giving la quantitatively (Scheme 2) [4]. This reaction provided an entirely new general synthetic method for [n]paracyclophanes. However, the overall sequence of reactions was not enough to give a large quantity of la at that time, because the precursor 8a was prepared as a minor product of silver-(I)-catalyzed isomerization of the bicyclopropenyl derivative 7. 

Anastassiou has summarized in two reviews the knowledge about IH-azonine (41a) [72ACR281 78AHC(23)55]. Compound 41a as well as its salts (N M" ) are aromatic compounds which exist as such and not as imine polyenic forms. Tliis compound demonstrates a valence isomerism 41a/41b similar to that of l//-azepine (14a/14c see Section II,A,1) the transformation 41a 41b occurs upon irradiation. 9-Azabicyclo[6.1.0]nona-2,4,6-triene 41b displays no tendency to thermal isomerization to 41a at ambient temperature (72ACR281). 

The facile, photoinduced valence isomerization of ethyl 1//-azepine-l-carboxylate to ethyl 2-azabicyclo[3.2.0]hepta-3,6-diene-2-carboxylatehas been studied as a potential solar energy storage system.101102 Unfortunately, the system proved to be inefficient due to build up of polymeric material during the thermally induced, exothermic retro-reaction. 

Attempts to induce valence isomerization of 5W-dibcnz[c,e,]azepine (3) to dihydrophenanthro-[9,10-6]azirine under thermal conditions have failed.85 However, the aziridine 5 is formed, albeit in low yield (3 %), by irradiating the dibenzazepinc 3 in dichloromethane solution. Isomerization can also be achieved by deprotonation of SH-dibenzIr.eJazepine with lithium diiso-propylamide at — 78 "C, and then allowing the resulting anion 4 to reprotonate by heating the reaction mixture at 50°C.85... 

In an attempt to prepare the parent system via the valence isomerization of the. ryn-benzene-bisepisulfide 2, which in turn was generated from the syw-benzenebisepoxide 3 via diol 4, no valence isomerization to 1,4-dithiocin (1) was detected.3 The bisepisulfide 2 is a thermally rather unstable compound and decomposes in solution even at 20 C, leading to benzene and sulfur as the only isolated products. 

Besides the parent system oxonin (l),5,6 which is thermally unstable and undergoes a valence isomerization to 3a,7a-dihydrobenzofuran (2) above 35 C, some annulated derivatives are known. All reported systems are nonaromatic and exist in nonplanar conformations. 

In connection with the chemistry of the reactive transient species, nitrene, the chemistry of azepines is well documented u. Also, the chemistry of oxepins has been widely developed due to the recent interest in the valence isomerization between benzene oxide and oxepin and in the metabolism of aromatic hydrocarbons 2). In sharp contrast to these two heteropins, the chemistry of thiepins still remains an unexplored field because of the pronounced thermal instability of the thiepin ring due to ready sulfur extrusion. Although several thiepin derivatives annelated with aromatic ring(s) have been synthesized, the parent thiepin has never been characterized even as a transient species3). 

Valence Isomerization of the 2-Thiabicyclo[3.2.0]heptadiene Moiety In principle, a valence isomerization of thiabicyclo[3.2.0]heptadiene skeleton would lead to a thiepin ring system. Wynberg et al. 23) reported that the photochemical adduct (28) from benzo[6]thiophene and dimethyl acetylenedicarboxylate was not thermally stable. When heated in diglyme, it loses sulfur to give dimethyl 1,2-naphthalenedicarboxylate. This reaction presumably proceeds via ring opening of 28 to 2,3-dimethoxycarbonylbenzo[6]thiepin (29) which readily eliminates sulfur. This synthetic route was successfully applied to the reaction of electron-deficient acetylenes with enamines of 2,3-dihydrobenzo[fe]thiophen-3-ones in which the enamine moiety constitutes part of a thiophene system. When 3-pyrrolidin-l-yl-benzo[6]thiophene (30) was allowed to react with dimethyl acetylenedicarboxylate... 

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

The chemistry of unsaturated azepines is dominated by their polyene character. The absence of ir-delocalization confers instability on the ring system as witnessed by the many and various ring transformations undergone in acid and base solution, and under thermal and photolytic conditions. Most of the major reactions of azepines involve the neutral molecule e.g. cycloadditions (Section 5.16.3.8.1), metal carbonyl complexation (Section 5.16.3.8.2), dimerizations (Section 5.16.3.2.3) and photo- and thermo-induced valence isomerizations (Section 5.16.3.2.1). 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

Recently, a new reactivity index has been proposed (80H(14)1717> which predicts accurately the site selectivity of photocyclization of substituted cycloheptatrienes to their bicyclic valence tautomers. Unfortunately, application of the method to substituted lH-azepines is far less successful. For example, for 2-methyl-l-methoxycarbonyl-lH-azepine (37 R = 2-Me) AGrs values for C-2—C-5 and C-4—C-7 cyclization are calculated as 0.093 and 0.040 kJ mol-1, respectively, i.e. predicting the 1-methyl isomer (39) as the major product. Experimentally, however, the reverse is true, the yields being 93.5% for 3-methyl (38 R = Me) and 6.5% for 1-methyl (39 R = Me). The corresponding photoinduced valence isomerizations of 1-benzazepines to 3,4-benz-2-azabicyclo[3.2.0]hepta-3,6-dienes (38a) have been recorded (80JOC462). These isomerizations have also been achieved thermally in the presence of silver ion (80TL3403). 

As stated in Section 5.17.1.4, simple thiepins (e.g. 44) are generally too reactive to be isolable under ambient conditions. Thiepins (49), (50) and (51) are among the relatively few stable monocyclic thiepins to have been reported and the majority of reactivity studies on thiepins have been carried out on polycyclic thiepins. The chemical reactivity of thiepins can be considered separately from the reactivity of the valence tautomeric thianorcaradienes more readily than was the case for oxepins-arene oxides. A spontaneous thermal extrusion of sulfur appears to occur from the episulfide tautomer of thiepins and the stable thiepins (49)-(51) would thus appear to exist exclusively in this valence isomeric form. 

Many thermal and photochemical valence isomerizations leading to cyclobutanes have been investigated with regard to mechanistic aspects. However, preparative applications also exist, especially when eight-membered unsaturated compounds are used as substrates. 

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