Abstract
60Co γ-irradiation of both lithium trans-2-butenoate (11) and lithium trans-2-butenoate·formamide (12) affords the same dimer, dilithium trans-5-methyl-2-heptenedioate (13). However, stereochemical analysis of products 22 and 24 from the analogous trans-2-butenoate-2-d salts 21 and 23 established that C–C and C–H bond formation occur stereospecifically by syn addition to the double bond in lithiumtrans-2-butenoates 11 and 21 and anti addition to the double bond in lithium trans-2-butenoate·formamide complexes 12 and 23. These reactions, which provide an unprecedented, chemospecific one step synthesis of dicarboxylate 13, add significantly to the synthetic scope of the γ-ray induced reactions of crystalline metal trans-2-butenoates that lead to cyclic and acyclic dimers and acyclic trimers by γ-ray initiated radical chain reactions. The stereochemistry of products 22 and 24 is that predicted by analysis of crystal packing, consistent with least-motion principles of the topochemical postulate as shown in Fig. 12 and 13. Analysis of the crystal structures, with respect to nearest neighbors, is consistent with the hypothesis that formation of carbon–carbon bonds in propagation step 1 and hydrogen atom transfer in propagation step 2 are topochemical and controlled by the crystal lattice. Analysis of the packing diagrams provides a pathway for chain propagation throughout the crystal that consumes all the molecules in the unit cell. The dimerization of 12 is much more rapid and proceeds in much higher yield than that of 11, probably as a result of significantly shorter C⋯C contacts and a more robust pathway for hydrogen transfer.