Studies in pyrimidine chemistry.

The synthesis of a series of 2-chloro-5-(hetero)arylpyrimidines and 5-(hetero)arylpyrimidines has been investigated. The aryl substituted pyrimidines are readily obtainable from [3-(dimethylamino)-2-aryl allylidene] dimethylammonium perchlorates whilst the heteroaryl substituted pyrimidines are best prepared by the photolysis of the appropriate 5-iodopyrimidine in the presence of a heteroarene.The photolysis of a series of 5-idopyiraidines in heteroarene solutions has been investigated, the reaction giving high yields except for the case of 4-chloro-5-iodopyrimidine, this result being apparently due to the high reactivity of the 4-chloro substituent.The kinetics of the reaction between piperidine and a series of 2-chloro-5-(hetero)arylpyrimidines in aqueous dioxan has been investigated. The order of electron-withdrawing ability of the heteroaryl substituents was found to be 1-methylpyrrol-2-yl < phenyl < 2-thienyl 2-furyl. This result is shown to be consistent with current ideas on the nature of these substituents.The kinetics of the reaction between phenacylbromide and a series of 5-(hetero)-arylpyrimidines in acetonitrile was investigated. The order of electron-donating ability of the heteroaryl substituents was found to be 1-methylpyrrol-2-yl > 2-furyl> 2-thienyl > phenyl. An explanation is proposed for this order. The molecular structure of 2,4-diazido-5-iodopyrimidine was determined by single crystal X-ray crystallographic studies. The molecule was found to exist in the diazido form rather than one of the several possible tretrazolo tautomers. 2,4-Diazidopyrimidines are usually found to exist in one of the tetrazolo forms, the 5-iodo substituent is shown to stabilize the diazido tautomer. An attempt was made to determine the molecular structure of 2,4-diazido-6-methylpyrimidine which has been the subject of some controversy but due to crystal, twinning an X-ray crystallographic study could not be completed.