By Cycloreversion

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. 

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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... 

Initial [2 + 2] cycloaddition of the alkyne to the saturated chromium carbene complex followed by [2 + 2] cycloreversion to yield a l-chroma-l,3,5-hexatriene may be an energetically more favorable reaction pathway (Figure 2.25). However, no energy minima could be located on the path from the starting carbene complex to the chromahexatriene. Hence, the metallacyclobutene shown in Figure 2.25 does not represent an intermediate but only a transition state. 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

Azetes are known to undergo [2+2] cycloreversion to provide acetylene and hydrogen cyanide <1996JPC1569>. The reaction of chloroacetophenone oxime 659 with LDA yielded another oxime 660 (Equation 252). The formation of this product has been explained by [2+2] cycloreversion of the azetes 661 formed in the reaction (Scheme 84) <2003AGE5613>. 

A photochemical method for the generation of odd-membered linear polyynes occurs by [2+2]cycloreversion of [4.3.2]propellatrienes under extrusion of indane (Scheme 13-13) [75]. Although the cycloreversion step is high-yielding, the preparation of the propellane starting material is much more tedious than the preparation of the 3,4-dialkynyl-3-cyclobutene-l,2-diones for SS-FVP. 

By [2 + 2 + 2] cycloreversion it is possible to generate the mixed systems l7 and 2s starting from the appropriate precursors. 

Triazine (38) is ideal for inverse electron-demand Diels-Alder cycloadditions, for example, with azulene to give a l,4-bis(CF3)phthalazine (89CB711). A rare example of the synthesis of a five-membered heterocycle originating from [4 + 1] cycloaddition followed by [4 + 2] cycloreversion was reported using (38). The intermediate tetraazanorbomadienimine (39) is highly strained and eliminates N2 [82AG(E)284]. 

Scheme 6. Photochemical [2 + 2] cycloreversion of the dodecadehydro[18]annulene in furan as the solvent, followed by [4 + 2] Diels-Alder cycloaddition, gives the oxanorbornadiene adducts 38-40 [66]...

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). 

With 0,-y-unsaturated a -diazo ketones, the resulting [2.1.0]-bicyclic systems (40) were quite unstable and underwent a [2 + 2] cycloreversion to generate ketenes (41), which were then trapped by nucleophiles (Scheme 7). The overall scheme has been named a vinylogous Wolff rearrangement and offers a novel entry to products usually derived from a Claisen rearrangement.102 A recent report describes its application for functionalized angular alkylation in fused ring systems.103 In contrast, the intramolecular re-... 

The irradiation of a l,2-disila-3-thietane gives products from a [2 + 2] cycloreversion, which may be trapped by ethanol53. 

The thus formed heterocycles 412a decompose or isomerize thermally the required reaction temperature depends on the substituents. The isomerization leads to diazomethane derivatives 413a, whereas the decomposition by [2 + 3] cycloreversion reaction gives bis(trimethylsilyl)diazomethane and short-lived silanimines 414a, which dimerize in most cases. The ratio isomerization/cycloreversion depends on R, the solvent and the temperature and more cycloreversion is observed at higher temperature. An unfavourable side reaction is the insertion of Me2Si=NR into Si—N bonds (formation of 415a and 686 see below). 

Apeloig and coworkers have trapped (Me3Si)2Si from the photoinduced [2 + 2] cycloreversion to (Me3Si)2Si=Si(SiMe3)2 and 2,2/-biadamantylidene of the corresponding 1,2-disilacyclobutane186. Since laser flash photolysis experiments indicated that the disilene was the sole primary photoproduct, the silylene was believed to be formed by photodissociation of the disilene. 

Studies in the benzene series have been reported by Bock and Rittmeyer365 by carrying out the thermal fragmentations in the gas phase. Finally, few examples of the application of [2 + 2] cycloreversions in the preparation of thioketenes have been reported321. Irradiation in an argon-matrix of 2,3-thiophenedicarboxylic acid anhydride afforded, among other products, thioketene (76) in 20% yield. 

Tobe s group also succeeded in applying the [2+2]cycloreversion process to the formation of smaller carbon cages, notably C36 [40]. Macroscopic quantities of C35 have been produced before [41, 42] and were shown to contain carbon cages that are covalently connected to form polymeric clusters of overall D6h-symmetry. In their efforts to obtain C36 from acetylenic precursors, Tobe et al. prepared cyclophynes 19 and 20 [40]. LD time-of-flight mass spectra of 19 depict a signal for the anion of cyclophyne C3(3H8, generated from 19 by four-fold... 

The 1,3-diazetidine derivatives are most commonly synthesized by the dimerization of heterocumulenes. At elevated temperature they undergo dissociation and are in equilibrium with their monomers. For example, 1,3-diazetidine 108, formed by the reaction of a-naphthyl isocyanate 106 with dicyclohexylcarbodiimide 107, upon thermal dissociation furnished products of its [2+2] cycloreversion reaction, that is, cyclohexyl isocyanate 110 and a-naphthylcyclohexylcarbodiimide 109 (Scheme 10) <1962AGE621>. 

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