13C NMR SPECTROSCOPIC EVALUATION OF NUCLEOPHILICITY/BASICITY AND CAFFEINE-DERIVED N-HETEROCYCLIC CARBENE COMPLEXES

Abstract

REACTION OF CAFFEINE WITH ALKYL HALIDES IS ONLY SUCCESSFUL WITH IODOMETHANE YIELDING (1-H)+I?. THE LOW REACTIVITY OF SUCH XANTHINES WAS EVALUATED BY 13C NMR SPECTROSCOPY USING COMPLEXES OF THE TYPE TRANS-[PDBR2(IPR2-BIMY)(L)] (IPR2-BIMY = 1,3-DIISOPROPYLBENZIMIDAZOLIN-2-YLIDENE; 2, L = CAFFEINE; 3, L = BN-XANT; 4, L = IPR-XANT). IT WAS FOUND THAT CAFFEINE AND THE BENZYL-SUBSTITUTED DERIVATIVE HAVE SIMILAR NUCLEOPHILICITY, WHILE THE ISOPROPYL ANALOGUE IS SLIGHTLY MORE NUCLEOPHILIC IN ACCORDANCE WITH THE INDUCTIVE EFFECTS OF THE SUBSTITUENTS. USING (1-H)+I? BIS- AND MONO-NHC COMPLEXES TRANS-[PDX2(IPR2-BIMY)(1)] (5) AND TRANS-[PDI2(CH3CN)(1)] (6) WERE PREPARED. HALIDE SCRAMBLING DUE TO DIFFERENT ANIONS IN THE ONE-POT REACTION MIXTURE WAS ENCOUNTERED AND STUDIED FOR THE FORMER. IN RELATION TO THE NUCLEOPHILICITY STUDY, A LINEAR CORRELATION (R2 = 0.8079) WAS FOUND TO EXIST BETWEEN THE 13CCARBENE NMR SHIFT OF IPR2-BIMY AND PKB VALUES OF COORDINATED AMINES IN TRANS-[PDBR2(IPR2-BIMY)(AMINE)] C