A ¹³C‐NMR study on the phenyllithium‐diazine adducts and the corresponding dihydrodiazines

Pyrimidine, 4‐phenylpyrimidine, pyridazine, 3‐phenylpyridazine and pyrazine were treated with phenyllithium in ether, yielding adducts resulting from attack of phenyllithium on the various positions of the heteroaromatic ring. With pyrimidine addition took place mainly at C(4), with pyridazine at C(3). By using TMEDA, addition at C(2) in 4‐phenylpyrimidine and at C(4) in pyridazine was strongly promoted. The structure of the adducts was studied by NMR spectroscopy. The charge distribution pattern in the C(4)‐adduct of pyrimidine (2a) and in both the C(3)‐adduct (10a) and C(4)‐adduct (12a) of pyridazine was determined by comparing the carbon chemical shifts of these compounds with those of the corresponding dihydrodiazines obtained by hydrolysis of the adducts. C(5) in 2a was found to carry little negative charge. C(4) in 10a was shown to have a considerable amount of charge while the electron density at C(6) in 10a and both C(3) and C(5) in 12a was moderate.