No abstract

A systematic determination of electronic coupling matrix elements in U-shaped molecules is demonstrated. The unique architecture of these systems allows for the determination of the electronic coupling through a pendant molecular moiety that resides between the donor and acceptor groups; this moiety quantifies the efficiency of electron tunneling through nonbonded contacts. Experimental electron-transfer rate constants and reaction free energies are used to calibrate a molecular-based model that describes the solvation energy. This approach makes it possible to experimentally determine electronic couplings and compare them with computational values.

A series of 5-substituted (X) 2,2-dimethoxyadamantanes (4, X ) F, Cl, Br, COOCH 3 , OCH 3 , CH 3 , C 6 H 5 , p-NO 2 C 6 H 4 , p-BrC 6 H 4 , SiMe 3 , and SnMe 3 ) as well as 2-methylene-5-tert-butyladamantane (6, Y ) CH 2 ; X ) C(CH 3 ) 3 ) have been synthesized and characterized. Reduction of the ketals 4 under ionic conditions with triethylsilane and phenylsilane provide π-facial diastereoselectivities for hydride trapping of 5-substituted (X) 2-methoxyadamant-2-yl cations (1, R ) OCH 3 T 5). A comparison of this data with known diastereoselectivities for nucleophilic capture of tertiary and secondary 5-substituted (X) 2-adamantyl cations highlights that stereoselectivity in these systems is a function of electron demand. Diastereoselectivities for the hydrochlorination of 6 (Y ) CH 2 , X ) C(CH 3 ) 3 ) in CH 2 Cl 2 and NO 2 CH 3 have been determined and compared with the corresponding data for the silicon analogue (6, Y ) CH 2 ; X ) SiMe 3 ). Low electron demand coupled with the stereoelectronic requirement of double hyperconjugation appears the most likely explanation, rather than a long-range steric factor, for the total lack of stereoselectivity in the Clcapture of the 2-methyl-5-(trimethylsilyl)adamant-2-yl cation (1, Y ) CH 2 ; X ) SiMe 3 ) in NO 2 CH 3 as solvent.