Pyridines linkage isomers

A variety of linkage isomer pairs have been produced from somewhat more complex ligands, such as substituted pyridines and benzoic acids, for example (5a) and (5b).77,78 These complexes have been employed in detailed studies of inner-sphere electron transfer reactions in order to assess the importance of the nature and orientation of the bridge between redox centres on intramolecular electron transfer rates.77-80... 

Purpureocobalt — see Cobalt, chloropentaammine-Purpureocobaltic chloride —see Cobalt, chloropentaammine-, chloride Pyridine, 2-hydroxy-5-methyl-dodecacobalt complexes, 168 Pyridine complexes linkage isomers, 186 Pyrimidine complexes... 

Momenteau s route to doubly-strapped porphyrins was easily adapted to produce compounds in which an axial base was incorporated into one of the straps Condensation of tetra(o-hydroxyphenyl)porphyrin (250) (mbcture of four isomers) with one equivalent of 1,12-dibromododecane gave a mixture of two singly-linked porphyrins, depending on whether adjacent (255), or opposite (254) meso-phenyl groups were linked. This mixture was reacted with 3,5-bis(3-bromopropyl)pyridine 256 and the desired cross trans-linked isomer 257 isolated by preparative tic (5% overall yield) (Scheme 77). A similar porphyrin 259 was prepared from a,p,a,p-tetra(o-aminophenyl)porphyrin (142) in this case the straps were tied to the porphyrin skeleton by amide linkages (Scheme 78). Following iron insertion and reduction, visible absorption and H-NMR spectra of both compounds were consistent with a five-coordinate high spin (S = 2) iron(II) complex. 

Most of these catal3rsts are relativdy nonspecific and will attack other pyrophosphate-containing compounds. Snake venom 17J ) and potatoes 175) have been foimd to be extremely good sources of enzymes which will act on both the oxidized and reduced forms of DPN and TPN. Jacobson and Kaplan (176) have purified an enzyme from pigeon liver acetone powder which cleaves the pyridine coenzymes at the pyrophosphate linkage. It is of interest that this catalyst reacts only with the reduced and not the oxidized forms of DPN and TPN. However, the enzyme cleaves the oxidized a-isomer of DPN 177), whereas it does not cleave the enzymically active 9-isomer 176). The specificity of the enzyme appears to be due to the structure of the nucleotides rather than to the positive chaige of the pyridine nitrogen. 

At high temperatures, alkyllithiums have been observed to alkylate aromatic rings by addition to the r-system and subsequent loss of LiH. Of the butyllithiums, r-BuLi has been found to be more reactive than either of its isomers in the alkylation of naphthalene and other condensed aromatics. Addition of r-BuLi, as well as other organolithiums, across the azomethine linkage of pyridine and similar nitrogen heterocycles is a facile process. ... 

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