Cyclobutanes elimination reactions

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyclobutene derivatives tend to open to give extremely reactive dienes, namely ortho-c]uin(xlimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G, Aalbersberg, 1975 R.P. Thummel, 1980). 

Cyclobutane derivatives are involved in all known syntheses of the parent [4]radialene (Scheme 8). The first approaches used /1-elimination reactions on compounds 5836, 5936 and 6036- 38 to introduce all four exocyclic C=C bonds in one operation. 

Bicyclo[1.1.0]butanes 63 can also be prepared by elimination reactions of cyclobutane derivatives 62. As a result of the nonplanarity of a cyclobutane ring, Cl and C3 are only separated by a distance of approximately 2.1 A. This unique structural feature of cyclobutanes explains the remarkable ease with which they can be transformed into bicyclo[1.1.0]butanes. 

A summary of most of the available routes to cyclobutenes and cyclobutenones is given in equations (4H7). These tqiproaches can be divided into two major strategies, (a) Cycloaddition between an alkyne and an alkene to give directly a cyclobutene. In the case where the alkene partner is a ketene, cyclobutenones are obtained (see Section 6.1.5). (b) Introduction of a double bond into a preformed cyclobutane derivative, typically via an elimination reaction. 

The occurrence of methylene elimination by reaction (7) is suggested by the presence of traces of cyclopropane in the products, as well as allene which may result from C3H6 decomposition. The decrease of propene yield with increasing pressure is attributed to a pressure-dependence of reaction (8). The direct decomposition of cyclobutane by reaction (9) is thought not to occur, but is equivalent to (2)-I- (5) or to (6) if C4H7 decomposes to the appropriate fragments. 

Aldehydes show an elimination reaction (loss of carbon monoxide, CO), that is not possible with ketones. Butanal, for example, photodissociates to propane and carbon monoxide. Cyclic ketones dissociate to a diradical (41 from cyclopentanone), which then reacts in any of several ways including elimination to ethene or 42 and coupling to cyclobutane. Formation of cyclobutane and ethene is accompanied by expulsion of CO prior... 

Elimination Reactions. A Cu(OTf)2/Hunig base combination provides a useful method for converting l,l-bis(phenylthio)-cyclobutanes to l-(phenylthio)cyclobutenes (eq 12) ... 

All known routes to the parent [4] radialene (3) use elimination reactions of appropriately functionalized cyclobutane derivatives (Scheme 4.12), namely, fourfold Cope elimination or Hofmann degradation of tetra(aminoxide)... 

By using freshly preeipitated copper powder at 200 °C, perfluroallenes can be prepared by the elimination of bromine fluoride or iodine fluoride from the corresponding starting matenals [70] The reaction is carried out at low pressure, 1-2 mm of Hg, and the products quickly dimenze into cyclobutane derivatives (equation 39)... 

The photolysis of cyclic diazo ketones in hydroxylic solvents leads to ring contracted carboxylic acid derivatives via this ketocarbene -> ketene rearrangement. Examples of such reactions are given in (2.24)239) and (2.25) 240). In this last example a photoequilibrium between the diazo ketone and its valence isomer, a diazirine, has been observed, both products then eliminating nitrogen to afford the cyclobutane carboxylic acid. 

Aluminum-free titanocene-methylidene can be generated by thermolysis of titana-cyclobutanes 6, which are prepared by reaction of the Tebbe reagent with appropriate olefins in the presence of pyridine bases [9]. Alternatively, the titanacyclobutanes are accessible from titanocene dichloride and bis-Grignard reagents [10] or from 71-allyl titanocene precursors [11]. The a-elimination of methane from dimethyltitanocene 7 provides a convenient means of preparing titanocene-methylidene under almost neutral conditions [12] (Scheme 14.5). 

The product of the reaction may be still further broken down by eliminating the second carboxyl group. (Preparation of cyclobutane from the dicarboxylic ester formulated above.)... 

Penta-alkylphosphorane intermediates are also inferred from the products of the reaction of these ylides with silacyclobutane, from which hydrogen is eliminated.44 However, the cyclobutane ring is left intact when the reaction is carried out with more bulky ylides (Scheme 10).46 Silicon- and germanium-substituted allenes have been prepared by the reaction of monometallated ketens with stable ylides, e.g. (43).46... 

The very low value of the energy of activation for this isomerization is of considerable interest. Comparison with the decomposition of cyclobutane shows a reduction of 30 kcal mole caused by the presence of the double bond. If a similar transition state were involved in both reactions, then this difference would be a measure of the extra strain energy of the cyclobutene. This is quite unrealistically high. Thus we eliminate the possibility that the reaction path is as shown below ... 

There are several reactions of heterocyclic or carbocyclic six-membered rings which yield cyclobutanes by elimination of two ring heteroatoms, by extrusion of one heteroatom together with one ring carbon or by rearrangement, so that two ring carbons are bound as a substituent at the cyclobutane ring after the reaction. 

The substrate for the synthesis of cyclobutanes by elimination of molecular nitrogen is the 3,4,5,6-tetrahydropyridazine skeleton 1. The most important methods for obtaining these compounds are the hetero Diels-Alder reaction of suitable dienes with azo dienophiles followed by oxidative hydrolysis of the adducts, and the oxidation of six-membered cyclic hydrazines (see Houben-Weyl, Vol. E16d, pp 1-111 and ref 1). 

Despite the fact that /-eliminations are frequent reaction modes for cyclobutanes, because Cl and C3 are only separated by approximately 2.1 A, only -elimination of hydrogen fluoride is found for the reaction between 3,3-difluorocyclobutane-l-carbonitrile (27) and a variety of bases such as ferf-butoxide in diethyl ether, ferf-butoxide in /er/-butyl alcohol and sodium hydride in tetrahydrofuran.10... 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

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