Influence of CH3 substituents on tetrahydropyrimidin-2-ylidene: σ-donating properties and in situ catalytic activities of precursor salts/Pd(OAc)2 system for C-C coupling reactions

In this study, symmetrical and non-symmetrical 1,3-dialkyltetrahydropyrimidinium salts (1 center dot HCl and 2 center dot HBr, respectively), bearing two methyl groups at C-5 position of the pyrimidine ring were synthesized and characterized. One of the new salts 1,3-bis(2,4,6-trimethylbenzyl)-5,5-dimethyl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate (1a center dot HBF4, 1a = 6-Bn*-Me-2) was converted to cis-[RhCl(6-Bn*-Me-2)(CO)(2)] to determine the sigma-donating properties in comparison to those of their 5-membered NHC complexes. Furthermore, the in situ catalytic activities of all salts, in combination with Pd(OAc)(2), were tested for the HeckeMizoraki and SuzukieMiyaura type C-C coupling reactions. In contrast to the expectation, only the substituents on N,N'-atoms were found to play an important role in the catalytic activity. (C) 2011 Elsevier B.V. All rights reserved.