Multicenter Organic Redox Systems Composed of Bis(1,4-dihydronicotinamides): Optimal Conformation for Intramolecular Electronic Interaction

Article abstract of DOI:10.1021/ja00383a033

Bis(1,4-dihydronicotinamides) were prepared as dimers of 1-benzyl-3-(N-ethylcarbamoyl)-1,4-dihydropyridine <(N-Et)BNAH>, which are structurally classified into three categories: flexible n,Bzl)>, partially rigid , and doubly linked, rigid, and cyclic dimers n,p-Xyl) and cBisNAH(C₄,o-Xyl)>.Bis(1,4-dihydronicotinamides) of another family regarded as singly 6,Pr) and BisNAH(Pr,C₆)> and doubly linked 6,C₆)> dimers of 1-propyl-3-(N-propylcarbamoyl)-1,4-dihydropyridine <(N-Pr)PNAH> were also synthesized.The reactivity of bis(dihydronicotinamides) in dichloromethane was subjected to change to a wide extent depending upon their molecular structures in the reduction of hexachloroacetone.Among bis(dihydronicotinamides) regarded as dimers of (N-Et)BNAH, cBisNAH(C₄,o-Xyl) was the most reactive.The overall reactivity order with respect to this family followed the sequence: cBisNAH(C₄,o-Xyl) > BisNAH(C_n,Bzl) (n = 4,6) > BisNAH(Et,o-Xyl) > cBisNAH(C₄,p-Xyl) ca.BisNAH(C₈,Bzl) ca.BisNAH(Et,m-Xyl) > cBisNAH(C₆,p-Xyl) ca.BisNAH(C₁₀,Bzl) > BisNAH(Et,p-Xyl) = (N-Et)BNAH.An effective charge-transfer interaction, which emerges from the favorable face-to-face arrangement of the two dihydronicotinamide rings in the transition state of reduction, is responsible for the kinetic enhancement.Both cBisNAH(C₄,o-Xyl) and BisNAH(C_n,Bzl) (n = 4,6) may assume a close face-to-face geometry without inducing significant strain in the transition state.The prevailing reactivity of the former over the latter was attributed to an entropy effect, since the two nicotinamides in the former attain such a favorable conformation already in the ground state while those in the latter may rather take an extended geometry without mutual interaction, as confirmed by spectroscopic measurements.As for the other family of bis(dihydronicotinamides), the reactivity sequence was as follows: BisNAH(C₆,Pr) > cBisNAH(C₆,C₆) ca.BisNAH(Pr,C₆) > (N-Pr)PNAH.A lower reactivity of the doubly linked bis(dihydronicotinamide) cBisNAH(C₆,C₆) was attributed to a much less favorable conformation of the nicotinamide rings involved.Molecular geometries of the present bis(dihydronicotinamides) as well as their dehydrogenated counterparts were examined by electronic and NMR spectroscopy.