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Container Systems 1: An Experimental and Theoretical Study of the High-Pressure Intramolecular Cycloaddition of Tethered Furans and Anthracenes onto Cyclobutene-1,2-diesters
* 1 , 2
1  Laboratory for Physical Organic Chemistry, Department of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička c. 54, 10000 Zagreb, Croatia
2  Intelligent Polymer Research Institute, University of Wollongong, Northfields Avenue, Wollongong, NSW, 2522, Australia

Abstract: A new class of container molecules is described and the first steps in producing protypes are reported. Central to the approach is the formation of molracs with cyclobutene-1,2-difurfuryl esters at the terminus or similar functionality at the bridgehead of a central norbornane subunit. The synthesis of the furfuryl starting materials is described as well as their anthracenyl counterparts. Conversion to the container systems involved the intermolecular linking of the furfuryl or anthracene by treatment with dimethyl acetylene dicarboxylate (DMAD) in a Diels-Alder protocol under thermal or high-pressure conditions. In practice, no intermolecular linking occurred between the molrac substrates and only products from Diels-Alder 1:1-addition with DMAD were produced. Intramolecular addition of one of the furfuryl units onto the cyclobutene p-bond was detected under high-pressure conditions and this intramolecular product was capable of isolation and characterisation by working at room temperature or below, but reverted to starting material above room temperature (t0.5= 30 min at 40 oC). When conducted in the presence of DMAD a single 1:1-adduct was obtained in which one furfuryl moiety was intramolecularly cyclised and the other present as the DMAD adduct; again this product underwent retro-Diels-Alder cycloaddition at 40 oC. Similar intramolecular cyclisation was observed with the bis-anthracenyl esters. Treatment of the bridgehead bis-furfuryl molracs with DMAD under high-pressure afforded three diasteriomeric bis-adducts in which DMAD had added at both furan sites, but no evidence for higher order intermolecular products was observed. Photochemical dimerisation was detected on exposure to solutions of the anthracenyl derivative at the 9,10-position of the anthracene but no higher order linking was observed. The stereoselectivity of the intramolecular attack of the furfuryl diene with the dienophilic cyclobutene gave a single adduct by underface attack in which the oxa-bridge is endo-positioned. AM1 activation energies indicate that this result is predicted on thermodynamic grounds where the observed isomer is 52.8 kJ/mol more stable than its exo-isomer. Activation energies, however favour formation of the exo-isomer by 35.2 kJ/mol, indicating that the cycloaddition is reversible under the high-pressure conditions. Comparison with calculations for adducts formed with the cyclobutene-1,2-dimethylester and furan (endo=136.2 kJ/mol; exo=188 kJ/mol) indicate that relief of ring strain enhances the rate of retro-Diels-Alder in the tethered system.
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