Thermal cycloreversion

The comparable strain between a cyclobutyl and cyclopropyl ring suggests the feasibility of a vinylcyclobutane to cyclohexane rearrangement. Nevertheless, such a process appears to proceed much less facilely and is complicated by [2 + 2] cycloreversion. Thermal rearrangements of vinylcyclobutanones have been reported (Eq. 92)118). However, the synthetically most useful reactions appear to be the base... 

Interestingly, in the inverse-electron-demand Diels-Alder reactions of oxepin with various enophiles such as cyclopentadienones and tetrazines the oxepin form, rather than the benzene oxide, undergoes the cycloaddition.234 236 Usually, the central C-C double bond acts as dienophile. Oxepin reacts with 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienone to give the cycloadduct 6 across the 4,5-C-C double bond of the heterocycle.234 The adduct resists thermal carbon monoxide elimination but undergoes cycloreversion to oxepin and the cyclopenta-dienone.234... 

The reaction of oxepin with dimethyl 5-oxo-2,3-diphenylcyclopenta-l,3-diene-l,4-dicarboxy-late takes a different course. Two products 7 and 8 can be isolated, 7 is the [4 + 2] adduct of the cyclopentadienone across the central C-C double bond of the oxepin, the other, 8, is thought to be a [4+6] cycloadduct across the triene system of the oxepin.237 In boiling benzene, the [4 + 2] adduct 7 undergoes no cycloreversion, but rearranges to the tricyclo[5.3.02,4]deca-5,8-dien-10-one system.237 The [4+6] adduct, however, is stable under thermal conditions. 

In yet another approach towards the synthesis of cyclocarbons by cycloreversion, Adamson and Rees [71] prepared the 1,2,3-triazole-fused dehydroannulenes 42 - 44, as mixtures of regioisomers in ca. 30 % overall yield, by oxidative Hay coupling of the protected 4,5-diethynyl-l,2,3-triazole 41 (Scheme 7). No investigations have yet been reported on the thermal or mass spectrometric [3-1-2] cycloreversions of 42-44, with loss of the triazole moieties and ultimate formation of the cyclocarbons Cis, C24, and C30, respectively. 

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

A potential source for generating monomeric 23 is found in the l,2Xs-oxa-phosphetanes 21 and 22 18-20>, Their mass spectra contain peaks at M+ — 140, corresponding to [2 + 2]-cycloreversion to olefin and 23 however, the latter fragment (m/e = 140) was not found in the mass spectra. Although it cannot be explicitly stated whether this fragmentation is induced by electron-impact or thermally, a thermal reaction in the mass spectrometer certainly appears plausible. Such a reaction can indeed be accomplished on a preparative scale under milder conditions, as previously reported for 21 (R = C6HS) (Sect. 2.1). 

The A5-phosphorin 164 and the bicyclic compound 165 are precursors of isopropyl metaphosphate, 16). Thermal fragmentation of 165 leads via [2 -I- 2]cycloreversion to triphenyltoluene 166 and isopropyl metaphosphate 167. The latter is identified as isopropyl phosphate after reaction with water. The mass spectrum of 165 is also dominated by this fragmentation picture (m/e 442 (16%) = M + m/e 320 (100%) = M + — 167). 

Pyrazoles can be synthesized by thermal cycloreversion of adducts formed in the 1,3-dipolar cycloaddition of alkyldiazoacetates with norbornadiene. The rate of the primary process of cycloaddition is accelerated by iron pentacarbonyl (Scheme 88)155 a similar catalytic effect has been observed during the formation of ethyl 5-phenyl-A2-pyrazoline-3-carboxylate from cycloaddition of ethyl diazoacetate and styrene.155 Reactions of this type are catalyzed presumably because of coordination of one or both reactants to the transition metal, and a wider study of the effect of a variety of complexes on 1,3-dipolar cycloaddition processes would be valuable. 

The thermal cycloreversion of imines 268, i.e. formation of isocyanide and alkyne, which would be expected by analogy with cyclopropenone decarbonylation and in reversal of cyclopropenone imine formation (see p. 25), was found to be only a minor side-reaction203). 

In contrast to cyclopropenones, most methylene cyclopropene derivatives do not undergo clearly defined thermal transformations neither cycloreversion analogous... 

The scope of the microwave technique in the preparation of fullerene derivatives was determined in the well known Diels-Alder reaction of C6o with anthracene (1) [71], which has been reported to occur under thermal conditions (13% [71a], reflux, toluene, three days 25% [71b], reflux, benzene, 12 h) (Scheme 9.22). In addition to 76, multiply-substituted adducts that undergo cycloreversion to the starting materials were formed. 

In a total synthesis of coriandrin 68, the isocoumarin unit is generated by the thermal rearrangement of an indenone epoxide 67, the first application of a cycloreversion route to isocoumarin synthesis (Scheme 46) <00TL3677>. 

A photoextrusion of a nitrogen molecule from a partially saturated tetrazolo[l,5- ]pyridine derivatives has been described by Quast et al. <1998EJ0317> (Scheme 11). The starting bicyclic compound 39 when irradiated at low temperature (at -60 °C) afforded annulated iminoaziridine 40 as a mixture of (E)- and (Z)-isomers. These two geometric isomers equilibrated at higher temperature (20 °C). Upon heating of the mixture of ( )-40 and (Z)-40, a thermal cycloreversion took place with methyl isocyanide elimination to afford the dihydropyrrole 41. 

Triazolines can decompose under various conditions. Compound 93 leads to the corresponding pyrrole 94 under acidic conditions (Equation 4) <2003TL1477>, whereas thermal treatment of bicycle 95 leads to 96 by a cycloreversion pathway (Equation 5) < 1996JP11617>. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Cycloreversions of a 1,2-disilacyclobutane (42) and a 1,2-digermacyclobutane (43) have been induced in solution both thermally and photochemically fragmentation of sterically congested (42) follows Scheme 5 paths a and b, respectively fragmentation of (43) yields (46) (which photodissociates to 48) in each case. " ... 

The isorniinchnone cyclization/isocyanate cycloreversion process for substituted furan synthesis has been well studied, as exemplified by the conversion of 104 to 106 (Scheme 19.19). In a solid-phase adaptation of this transformation, two groups independently utilized this reaction to estabhsh a traceless self-cleaving method for the synthesis of substituted furans [176, 177]. Further investigation of the thermal requirements of this cycloreversion led to its application in the split-pool synthesis of a small library of amides [178]. 

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]. 

Furthermore, a cycloreversion has been observed in the case of the thermally labile azoniaallene salts (61) (Z = O, S). When Z = S, this proceeds within an hour at room temperature to yield the aryl isothiocyanate and the charged cumulene (66) which is isolated as the acyliminium salt (67) <9ICB2537>. When Z = O and R = Ph, cycloreversion occurs at 83°C (refluxing 1,2-dichloroethane) to give the aryl isocyanate and (66), which is further cyclized to the quinazolinium salt (68) (Scheme 4) <89T5825>. 

A new approach to the cyclodeca-1,5-diene systems [cf. (281) and (282)] characteristic of germacranolides involves a photochemical-cycloaddition and thermal-cycloreversion sequence shown in Scheme 27. ... 

Grigg and co-workers (310) recently examined the 1,3-APT reaction of various aldoximes (270) (R or R = H) with divinyl ketone (Scheme 1.56). While ketoximes 270 (R = R) form a mixture of adducts, 271 and 272 via nitrone 273, the aldoximes selectively afford 272 (as a mixture of endo and exo diastereoisomers). Under the thermal reaction conditions, the oxime starting materials can undergo ( /Z) isomerization, while the nitrone intermediate was expected to be unaffected and the isolated cycloadducts showed no interconversion via cycloreversion. Thus, the increasing selectivity for endo-212 [via ( )-273, R = H] over exo-212 [via (Z)-273, R = H] with the increasing size of the aldoxime substituent was attributed primarily to the inhibition of oxime isomerization by steric clash between R or R and the oxime OH. In contrast, Lewis acid catalysis, in particular by hafnium (iv) chloride, of the cycloaddition of various aldoximes with this dipolarophile gave exo-271 exclusively (216). 

A special case involves the thermal decomposition of 3,4-dinitrofuroxan (104). The cycloreversion is already observed at room temperature and the nitroformo-nitrile oxide could be trapped with electron-deficient nitriles. The cycloadditions with styrene, phenylacetylene, frani-stilbene, and cyclohexene, however, led to complex mixtures of products that could not be separated (104). In the related case of a furoxan with an a-hydrogen adjacent to the sulfonyl group, the reaction was proposed to proceed according to course (b) (Scheme 6.7). 

Noteworthy is a thermal cycloreversion of a tricydic tosylate carboxylate that translates chirality of five carbon centers into three double bonds (2 C=C and 1C=O)... 

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