Cyclobutene rearrangements

A similar sequence of reactions takes place with the enamlnes of cyclic ketones (55-57) the initially formed unstable cyclobutene rearranges with insertion of two carbon atoms into the ring. A wide variety of cyclic ketones have been allowed to react in this way. For instance, the enamine (75) gave 76 on reaction with dimethyl acetylenedicarboxylate in refluxing toluene (55) and the heterocyclic enamine (77) obtained from dihydro-3-(2H)-... 

The intramolecular Diels-Alder reaction has become one of the most promising methods for the construction of complex cyclohexene moieties 104). In view of the fact that cyclobutenes rearrange thermally and conrotatary to butadienes (Eq. (9))8l l05) and their willingness to undergo Diels-Alder reactions with dienophiles (Eq. (10))... 

R. Wallace, Chem. Phys., 37, 285 (1979). Vibronic State Symmetry, Selection Rules and Transition Probabilities for a Molecular Rearrangement Process. The Butadiene-Cyclobutene Rearrangement. 

The reaction of enamines with acetylenic compounds has been the subject of intensive studies. In general, alkyl acetylenedicarboxylates and propiolates undergo initial cycloaddition (Chapter 18) to form cyclobutene derivatives. These cyclobutenes rearrange, in most cases spontaneously, to form products whose structures depend on the starting enamine and on the reaction conditions126-128 (Scheme 45). 

Most cyclopropylmethyl to cyclobutyl rearrangements are cationic in nature and these reactions are covered in Section 7.A.3.2. For cyclopropylcarbene to cyclobutene rearrangements see Section 8.A.3. 

Anion-accelerated divinylcyclobutane rearrangements 8333 Cyclobutene Rearrangements... 

FIGURE 20.5 The thermal opening of a cyclobutene to a 1,3-butadiene. The cis disubstituted cyclobutene rearranges to the cis,trans butadiene. 

The classic example is the butadiene system, which can rearrange photochemi-cally to either cyclobutene or bicyclobutane. The spin pairing diagrams are shown in Figure 13. The stereochemical properties of this reaction were discussed in Section III (see Fig. 8). A related reaction is the addition of two ethylene derivatives to form cyclobutanes. In this system, there are also three possible spin pairing options. 

Such reductive ring contractions of sulfones are formally similar to two other methods capable of supplanting a sulfur atom by a carbon-carbon double bond the Ramberg-Backlundand Stevens rearrangements. The distinguishing feature of this novel approach to cyclobutenes consists in the resultant higher level of alkyl substitution at the sp -hybridized centers. 

An unusual seven-membered ring-forming rearrangement was observed m the addition of diazomethane to a perfluorinated cyclobutene [5] (equation 3)... 

Cesium fluonde in dimethylformamide catalyzes the isomerization offluori-nated cyclobutenes, perfluorobipyntmdines, and their oligomers to products with expanded rings [30, 31, 32] The product distribution in cobalt tnfluonde fluorina-tion depends strongly on the temperature of the reaction [33] Fluorinated 1-dimethylamino-5,6,7,8-tetrafluoro-l,4-dihydro-l,4-ethenonaphthalene rearranges in protic media to a biphenyl derivative [34] (equation 8)... 

The yields of thermal rearrangements of some perfluorinated olefins are very low The fact that perfluorocyclobutene yields perfluoro-l,3-butadiene at 650 °C only in a 12% yield [7] is due to the higher thermodynamic stability of perfluoro cyclobutene compared with the-open chain product [72 ]... 

A more conventional cycloaddition occurs with activated acetylenes, however, the intermediate cyclobutene adducts undergo rearrangement to give insertion of two carbon atoms into the enamine chain (55). Thus the enamine (16) reacted with methyl propiolate to give the dienamino ester (73), presumably via the cycloaddition product (65a). 

Enamino ketones and esters also react with dimethyl acetylenedicarboxylate (67). Again cycloaddition appears to occur and the unstable cyclobutene intermediates rearrange to give insertion of two carbon atoms. 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Scheme 1. Schreiber s oxy-Cope rearrangement/cyclobutene ring opening strategy for the synthesis of cyclodecadienone 8.

The cationic pathway allows the conversion of carboxylic acids into ethers, acetals or amides. From a-aminoacids versatile chiral building blocks are accessible. The eliminative decarboxylation of vicinal diacids or P-silyl carboxylic acids, combined with cycloaddition reactions, allows the efficient construction of cyclobutenes or cyclohexadienes. The induction of cationic rearrangements or fragmentations is a potent way to specifically substituted cyclopentanoids and ring extensions by one-or four carbons. In view of these favorable qualities of Kolbe electrolysis, numerous useful applications of this old reaction can be expected in the future. 

In some cases, double bonds add to triple bonds to give cyclobutenes, apparently at about the same rate that they add to double bonds. The addition of triple bonds to triple bonds would give cyclobutadienes, and this has not been observed, except where these rearrange before they can be isolated (see 15-63) or in the presence of a suitable coordination compound, so that the cyclobutadiene is produced in the form of a complex (p. 60). 

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