Prismane synthesis, photochem

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

In contrast to piece-by-piece consecutive or linear syntheses like these, other things being equal a convergent synthesis, in which two at least ronghly equally-sized halves of amolecule are joined, is more efficient [71]. Avery specnlative illustration of this is the assembling of the two cubane units of 65 to give [4,4]prismane (Fig. 12.25). Here a tetraester, in principle preparable from a tetrol and a tetracar-boxylic acid, is decarboxylated, perhaps photochemically and at low temperature, to... 

The Photochemical synthesis of alkyl substituted benzene takes advantage of various pathways to provide one or more of the valence bond isomers or a positional isomer of the starting material is illustrated for 1,2,4-tiibutyl benzene, shown below. The formation of the 1,3,5-isomer results from cycloreversion of the prismane depicted or arises via the benzvalene to deld the least sterically congested product. By an analogus sequence o-xylene can gives nv and p-isomer. 

Extrusion of Nitrogen.— Two important photochemical nitrogen extrusion reactions deserve mention. The first was in the synthesis of the highly strained prismane (188) in low yield by irradiation of the azo-precursor (187) in CD2CI2 at temperatures above 30 The second was in the conversion of the pyrazoline... 

In this chapter, prototypes of the photochemical reactions useful for the synthesis of cage compounds are outhned first, and then photochemical approaches towards the synthesis of [n]prismanes as representative cage compounds are reviewed. 

Scheme 1 illustrates the synthesis of prismane ([3]prismane) 1 by Katz and Acton Benzvalene 5 reacts with the powerful dienophile, 4-phenyltriazohnedione 6, in diethyl ether-dioxane to give the 1 1 adduct 7 (60-70%). Refluxing 7 with KOH in MeOH/HjO, followed by treatments with acidic CuCljand aqueous NaOH, gives azo compound 8. Irradiation of 8 through Pyrex in isobutane at 35°C affords prismane 1 (1.8%) as an explosive colorless liquid. Prismane is stable at room temperature. One of the key steps of this synthesis is a photochemical nitrogen extrusion (Scheme 1). 

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