Preparation of heteroaryl phenylmethanes and a 13C and 15N NMR spectroscopic study of their conjugate carbanions. Rotational isomerism and charge maps of the anions and ranking of the charge demands of the heterocycles

Abstract

2-Benzylpyridazine, 4-benzylpyrimidine, 2-benzylpyrimidine and 2-benzylpyrazine, (5–8), have been prepared in order to study their ¹³C and ¹⁵N 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 (tributylphos-phonium)ylide on the pertinent halogenoazine. The ¹³C and ¹⁵N 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_x 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.