Electrocyclic equilibrium

The chemistry of cyclooctatetraene is interesting and unusual. Particularly noteworthy is the way in which it undergoes addition reactions to form products that appear to be derived from the bicyclic isomer, bicyclo[4.2.0]2,4,7-octa-triene, 8. In fact, there is an electrocyclic equilibrium between cyclooctatetraene and 8 (Section 21-10D) and, although the position of equilibrium lies... 

Cyclohexadienes are in electrocyclic equilibrium with 1,3,5-hexatrienes. Neither compound is strained, and the cyclohexadiene has one more cr bond than the hexatriene, so the cyclohexadiene is lower in energy. The hexatriene is more conjugated than the cyclohexadiene, so it is more reactive photochemically. Under both thermal and photochemical conditions, then, the cyclohexadiene is favored over the hexatriene. 

Cyclopropanones are in electrocyclic equilibrium with oxyallyl cations. The cyclopropanone is generally lower in energy, but the oxyallyl cation is not so much higher in energy that it is kinetically inaccessible. Oxyallyl cations can undergo cycloadditions, as will be discussed later. 

Cyclobutenes are in electrocyclic equilibrium with 1,3-butadienes. 1,3-Butadienes are lower in energy, because cyclobutenes are very strained, and it is therefore possible to convert a cyclobutene to a butadiene thermally. The reverse reaction, the conversion of a 1,3-butadiene to a cyclobutene, does not usually proceed under thermal conditions, as it involves an uphill climb in energy. However, the more conjugated 1,3-butadienes absorb light at longer wavelengths than cyclobutenes, so it is possible to convert a 1,3-butadiene to a cyclobutene... 

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

Azabicyclo[3.2.0]hepta-3,6-dienes are the 4 5 bicyclic valence isomers of 1//-azepines. In some cases there is an equilibrium between the bicycle and the azepine, whereas with other 1//-azepines photolysis yields an azabicycloheptadiene that can be isolated and characterized. For example, ethyl 2-azabicyclo[3.2.0]hepta-3,6-diene-2-carboxylate (1), the photoinduced valence isomer of ethyl l//-azepine-l-carboxylate (2), undergoes a clean, exothermic, first order, electrocyclic ring opening (AG = 20 kJ mol-1) to the parent 1//-azepine 2 on heating at 113-143C in an inert solvent (e.g., hexadecane).101... 

The thermally induced electrocyclic ring opening of 2-alkyl- and 2-acyl-3.4-benzo-2-azabicyclo-[3.2.0]hepta-3,6-dienes 2 to l//-l-benzazepines 1 (see Section 3.2.1.4.1.1.) are photorever-sible.23,37 38 Also, l-acyl-1//-benzazepines 1 (R = acyl), in refluxing xylene in the presence of silver(I) tetrafluoroborate, are in thermal equilibrium with their valence isomers the 2-acyl-3,4-benzo-2-azabicyclo[3.2.Oj nepia-3,6-uienes 2 (R = acyl).23-38... 

Surprisingly little is known about isomerization reactions of [4]radialenes. 5,6,7,8-Tetravinyl[4]radialene 127 is certainly a unique case since it appears to be in equilibrium with its precursors in synthesis, 125 and 126, by electrocyclic reactions (equation 8). As 127 has not been isolated in pure form and is unstable with respect to polymerization, no details on this possible equilibrium are known79. 

As described in this chapter, vinylidene complexes of Group 6 metals have been utilized for the preparation of various synthetically useful compounds through electrophilic activation or electrocyclization of terminal alkyne derivatives. These intermediates are quite easily generated from terminal alkynes and M(CO)6, mostly by photo-irradiation and will have abundant possibilities for the catalytic activation of terminal alkynes. Furthermore, it should be emphasized that one of the most notable characteristic features of the vinylidene complexes of Group 6 metals is their dynamic equilibrium with the it-alkyne complex. Control of such an equilibrium would bring about new possibilities for unique metal catalysis in synthetic reactions. 

The bicyclic 3,4-dihydropyridines (51) and (53) illustrate that the electrocyclic ring opening is sensitive to substituents (80TL599). The 3,4-dihydropyridine (51) has been observed to be in equilibrium with the azepine (50). Consistent with the analogous carbocyclic system, norcaradiene-cycloheptatriene, the monocycle (50) is the predominant species present in this equilibrium. In contrast, the dimethyl derivatives (52) and (53) exist almost exclusively in the 3,4-dihydropyridine form. A greater steric interaction between these methyl substituents in (52) was given in explanation of these observations. 

The thermal ring closure reaction of a 1,3,5-triene to a 1,3-cyclohexadiene occurs by a concerted disrotatory electrocyclic mechanism. An example of the latter is the oxepin-benzene oxide equilibrium (7) which favors the oxepin tautomer at higher temperatures (Section 5.17.1.2). Oxepin (7) was found to rearrange to phenol during attempted distillation at normal pressure (67AG(E)385>. This aromatization reaction may be considered as a spontaneous rearrangement of the oxirane ring to the dienone isomer followed by enolization (equation 7). 

The bicyclic isomers 109 are clearly formed by electrocyclic ring closure of their initially formed monocyclic valence isomers and not from the nucleophilic attack of the transition metal anion on traces of 52 in equilibrium with OFCOT, as shown by the low-temperature reaction of [Fe(f/5-C5H5)-(CO)2] . At -78°C, the l9F NMR spectrum of the crude reaction mixture indicates exclusive formation of the monocyclic complex 108b, but on warming the solution to room temperature the resonances of the bicyclic valence isomer 109b grow in (184). 

In this example the ring system is compatible with the allowed stereochemistry the disrotatory equilibrium between 4.55 and 4.56 has no problems. On the other hand, rings can constrain or even prevent allowed electrocyclic reactions. In the cyclobutene 4.57, for example, the ring fusion... 

A novel route to the azepinone system in 93 and 94, based on an intramolecular nitrone-eneallene cycloaddition (in 91 and 92, which were accessed in turn from 88 via 89 and 90) and subsequent rearrangement via N-O bond homolysis and an electrocyclic recyclization step) has been described (Scheme 11) <2005EJ02715>. On heating 93 in toluene, equilibrium with the isomeric azepinone 95 was established, although comprising less than 3% of 95. The general synthetic approach was applied to the synthesis of an analogue of the alkaloid astrocasine. 

Stereoselectivity of some methyl substitution on the alkenyl group was examined by the series 228 [277] (Scheme 66). Irradiation of methanol solutions of 228 at >290 nm resulted in the clean conversion of each reactant into a mixture of two diastereomers of 231. The 231 are then converted thermally (25-100°C) to equilibrium mixtures with 229 and 230. High diastereomeric excesses (R2 is trans to R1 or R3) in 229-231 are always observed. This fact indicates that the photoin-duced electrocyclization to a cyclobutene 231 of one diene unit of the bicy-clo[6.3.0]undecatriene intermediate 230 undergoes disrotatory photoclosure in only one of two possible directions. 

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