Cyclobutenes photolysis

In molecules containing two cyclobutenes, addition of organic azides 26 yielded two adducts, e.g. reaction with 50 with benzyl azide 56 produced the cr-isomer 57a in which the (V-benzyl substituents were sy -aligned and the C2-isomer 57b in which they were anti-orientated (Scheme 7). The structure of syn-isomer 57a was confirmed by X-ray (Figure 2). The fact that both isomers yielded the same fns-aziridine 58 upon photolysis made separation of the individual triazoline isomers unnecessary. 

Cyclobutenes have been applied as latent functionality of 1,4-dicarbonyl systems. Photolysis of (466) gave a 1.5 1 mixture (60%) of (467a) and (467b), which were cleaved by ozone and subsequent reduction of the ozonides yielded the epimeric (468). Cyclization and dehydration process converted (468) to the furan (469) 160). Furan (469) was converted to hibiscone C in a few steps, 60). 

The procedure described is a modification of that developed by Diels and Criegee. Bicyclo[2.1.0]pentane has been prepared by the pyrolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene, the photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene, the pyrolysis of N-phenyl-2-oxo-3-azabicyclo[2.2.1]heptane, and the addition of methylene to cyclobutene. ... 

A mechanistic scheme involves intramolecular single-electron transfer (SET) from the enamino moiety toward carbon 5 of the ring, and subsequent transannular interaction in zwitter-ionic biradical 259 with formation of stabilized zwitter-ion 260. Opening of the oxetane ring in 260 leads to cyclobutene 256, while secondary photolysis of the latter gives olefin 257 and alkyne 258 (Scheme 100). 

The double )5-scission pathway becomes dominant in bicyclic systems (Equations (7)-(9) and Scheme 13). Thus, cyclopentene ozonide (69) gives cyclopropane (Equation (7)) <68TL329l>. Photolysis of the ozonide derived from 1,4-benzodioxins (70) provides a method for the preparation of labile o-benzoquinones (71) (Scheme 13) <87JOC56l6>. Photolysis can also provide a route to unstable compounds and transient species such as the aziridine-2,3-dione (72) (Equation (8)), identified at 77 K using infrared spectroscopy <80JA6902>. Relatively unstable azacarbapenems (73) have been prepared by photolysis of tricyclic compounds containing a cyclobutene ozonide (Equation (9)). On silica, the 1,2,4-trithiolane (74) underwent photo-equilibration (Equation (10)) with the 1,3-dithetane (75) and sulfur. 

In a reaction similar to 7-47, certain sulfones, both cyclic and acylic,461 extrude S02 on heating or photolysis to give ring-contracted products.462 An example is the preparation of naphtho(f )cyclobutene shown above.463 In a different kind of reaction, five-membered cyclic sulfones can be converted to cyclobutenes by treatment with butyllithium followed by LiAIH4,464 e.g.,... 

Cyclohexadienes are of course 1,3-dienes, and in certain cases it is possible to convert them to cyclobutenes instead of to 1,3,5-trienes.377 An interesting example is found in the pyrocalciferols. Photolysis of the syn isomer 89 (or of the other syn isomer, not shown)... 

The 1,3-diene—cyclobutene interconversion can even be applied to benzene rings. For example,379 photolysis of l,2,4-tri-/-butylbenzene (92) gives l,2,5-tri-/-butyl[2.2.0]hexadiene (93, a Dewar benzene).380 The reaction owes its success to the fact that once 93 is formed,... 

The light-induced reaction of o-quinone diazides finds occasional use in the synthesis of a large number of compounds which are used in photography.273,277 Photolysis of diazoindauoues (CCVIII and CCX) provides a new synthetic route to cyclobutene carboxylic acid derivatives CCIX and CCXI, respectively.117 A good demonstration that ketenes are actually involved in the photodecomposition of o-quinone... 

In some cases the nature of the products resulting from photolysis of dienes and trienes depends on whether the reaction occurs from the excited singlet state or the triplet state. The triplet states can be populated exclusively by suitable sensitizers. For example, 3-methylene-1, 5-hexadiene gives a cyclobutene derivative (A) from the singlet state and tricyclo ring compound (B) from triplet state ... 

Kinetic analysis of the rearrangement of benzylfluorocarbene, generated by laser flash photolysis of the corresponding diazirine, gave a rate constant of 9.2 x 106 s 1 at 26 °C with activation entropy —17.2 eu and activation energy 3.25 0.34 kcal mol-1, very similar to the values for the chlorocarbene.80 A product analysis study of the thermolysis and photolysis of the diazirine (73) in the presence of tetramethylethylene showed tiiat die ring-expanded cyclobutene and the cyclopropanation products do not arise via a common intermediate.81 The ring expansion was proposed to occur by loss of N2 from the diradical intermediate (74). 

It is well known that the thermal decomposition of a bicyclo[1.1.0]butane gives the corresponding 1,3-butadiene probably via the Woodward-Hoffmann allowed concerted pathway, while photolysis of cyclobutene also provides 1,3-butadiene as a major product. A remarkable difference in the reaction modes between the silicon and carbon analogs of the bicyclo[1.1.0]butane/cyclobutene system would partially be a consequence of the fact that the formation of the silicon analog of a conjugated diene is highly undesirable. 

Cyclobutene has been reported to undergo 1,3 cycloaddition to benzene under photolysis conditions.355 Although the endo stereochemistry of the cyclobutane ring in this adduct is assumed (see 371), pyrolysis of the hydrocarbon at ca 250 °C gave dihydrotriquinacene 372 as the major product. Using cis-3,4-dimethylcyclo-butene, 373 was obtained and similarly transformed to 374 without loss of stereochemistry by thermolysis.376 ... 

Examples of transformations of a cyclobutene compound 30 into organic products include formation of an ester 31 by oxidation of 30b with DMSO,58 formation of a cyclohexadienone 32 by reaction of 30c with an alkyne,58,60 and formation of a naphthole 33 by photolysis of the phenyl derivative 30d (Scheme 12).61... 

As in other photochemical reactions, alkenes seem to react in preference to alkynes, and in the sensitized photolysis of 2-methylbutenyne the product isolated is a dialkynylcyclobutane (equation 56) rather than a cyclobutene. ... 

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