Anthracene dimerisation

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

Cyclopenta[fc]dioxanes (44) are accessible from the reaction of the dioxenylmolybdenum carbene complex (43) with enynes <96JOC159>, whilst an intramolecular and stereoselective cyclisation of (Ti5-dienyl)tricarbonyliron(l+) cations affords chiral frans-2,3-disubstituted 1,4-dioxanes <96JOC1914>. 2,3-Dimethylidene-2,3-dihydro-1,4-benzodioxin is a precursor of the 3,8-dioxa-lff-cyclopropa[i]anthracene, which readily dimerises to dihydrotetraoxaheptacene (45) and the analogous heptaphene <96AJC533>. 

High resolution NMR has been used to investigate the photo-dimerisation of 9-methylanthracene. In the solid state, only the trans dimer is formed whereas in benzene solution both the trans and cis dimers are produced. Studies show that the maximum domain size of the minor component is about 0.3 pm, and that the reaction occurs at crystal defects in the monomer. Photodimerisation of the 9-substituted anthracenes An-CHzN- MejBr- (166), AnCH2C02 Na-" (167), AnCH20H (168), AnCOMe... 

Several other compounds which are easily reduced in a comparably explicable manner are exemplified by butadiene 7.173,159 already discussed on p. 108, diphenylethene 7.174,1076 naphthalene 7.175,1077 anthracene 7.1761078 and phenanthrene 7.177.1079 In reductions in liquid ammonia without alcohol present, two electrons must be added to the LUMO of the starting material before it is basic enough to abstract a proton from ammonia. The addition of the second electron to the radical anion is sometimes so slow that dimerisation occurs to some extent, as we can see in the case of diphenylethene and have already seen in earlier reactions. 

Stacks in which the neighbouring molecules are equidistant are termed regular, while those with dimerisation are termed alternating. The different stackings are of vital importance for determining the character of these materials as insulators, semiconductors, or metallic conductors. Fig. 2.17 indicates schematically that the TTF-TCNQ crystal is built up from separate stacks. An example of mixed stacks is the insulator anthracene-PMDA (Fig. 6.14). 

Cycloaddition reaction between two aromatic substrates yields dimerised products except benzene. For example, 2-substituted naphthalene and anthracene give dimerised product on photolysis. 

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