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Preparation of heteroaryl † phenylmethanes and a C and N NMR spectroscopic study of their conjugate carbanions. Rotational isomerism and charge maps of the anions and ranking of the charge demands of the heterocycles

Journal of the Chemical Society, Perkin Transactions 2, ISSN: 1472-779X, Issue: 4, Page: 481-488
1991
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  • Citations
    31
    • Citation Indexes
      31
  • Captures
    1

Article Description

2-Benzylpyridazine, 4-benzylpyrimidine, 2-benzylpyrimidine and 2-benzylpyrazine, (5-8), have been prepared in order to study their C and N spectra and those of their conjugate carbanions (1-4). These systems are aza-homologues of the previously reported benzylpyridines and have been considered to evaluate the effect of aza-substitution upon rotational isomerism and charge maps in the anions. Two synthetic approaches have been followed: (i) decarboxylation of α-(heteroaryl)phenylacetic acids, in turn obtained by nucleophilic substitution of phenylacetonitrile anion on the pertinent halogenoazine (or a correspondingly available derivative); (ii) by nucleophilic substitution of benzyl(tributylphosphonium)ylide on the pertinent halogenoazine. The C and N NMR data for 1-4 indicate that, at room temperature, there is slow rotation about the bond between the carbanionic carbon and the carbon atom of the heterocycle: this generates geometrical isomerism in the anions 1-4. The NMR data are treated with the π-charge-shift equations (1)-(4) to obtain the local variations of the π-electron density. By evaluating, in anions 1-4, the fraction of π-charge transferred to the heterocycle from the carbanionic carbon it is possible to obtain the charge demands, c of the heterocycles, and thus rank them on the same scale as primary organic functionalities. It is found that the 4-pyrimidyl is the strongest electron-withdrawing heterocyclic residue, comparable with the acetyl group.

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