Synthesis of a Variety of Bichromophoric “Ball-and-Chain” Systems Based on Buckminsterfullerene (C<sub>60</sub>) for the Study of Intramolecular Electron and Energy Transfer Processes
作者:James M. Lawson、Anna M. Oliver、Daniel F. Rothenfluh、Yi-Zhong An、George A. Ellis、Millagahamada G. Ranasinghe、Saeed I. Khan、Andreas G. Franz、Padma S. Ganapathi、Michael J. Shephard、Michael N. Paddon-Row、Yves Rubin
DOI:10.1021/jo960512p
日期:1996.1.1
Diels-Alder reaction of C-60 with the 1,3-dienes 7e-h, 8a, 8b, and 8d-h affords the ''ball-and-chain'' systems 2e-h, 3a, 3b, and 3d-h bearing two chromophores linked via a rigid, hybrid saturated polynorbornane-bicyclo[2.2.0]hexane (''norbornylogous'') hydrocarbon bridge. Analogous reaction with the bis(diene) 9 affords the soluble dumbbell system 4 bearing two C-60, chromophores. The norbornylogous bridge is a strong mediator of electron and energy transfer via a through-bond coupling mechanism. The norbornylogous donor-bridge-diene units 7d-h, 8a, 8b, and 8d-h were prepared in a straightforward manner from bicyclo[2.2.2]octane precursors by extending the bridges with linearly fused norbornane-bicyclo[2.2.0]hexane moieties through execution of the tandem Mitsudo-Smith series of reactions. The X-ray structure of the dimethoxybenzene-bridge-C-60 system 3a reveals favorable self-complementarity manifested by the unusual packing structure of 3a in the crystal. Molecular mechanics, semiempirical, and ab initio conformational analyses of compounds 2e, 3a, 3b, 3e, 3f, 3h, 68, and 70 (MM2, Sybyl, CVFF, AM1, HF/3-21G) were performed to quantify their ability to adopt two nondegenerate boat conformations, i.e,, extended and folded conformers, as well as their kinetic barrier of interconversion. A similar treatment of the C-60-bridge-C-60 system 4 revealed unusual preference for the folded-folded conformer (18.9 kcal/mol at CVFF level), which was not reproduced by the AM1 method (0.11 kcal/mol). The reduction potentials of the systems 2e, 3a, and 3e were about 0.1-0.5 V more negative than C-60, and the third reduction potential (E(3)) Of the 6-bond system 2e was 0.14 V more negative than the corresponding wave for the 10-bond system 3e. This shift was attributed to the closer proximity of the dimethylaniline donor group to the C-60 surface for 2e vs 3e.