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·HCl and 2·HBr, respectively), bearing two methyl groups at C5 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·HBF4, 1a = 6-Bn∗-Me2) was converted to cis-[RhCl(6-Bn∗-Me2)(CO)2] to determine the σ-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 Heck–Mizoraki and Suzuki–Miyaura 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.

Graphical abstractThe in situ catalytic activities of tetrahydropyrimidinium salts (1 and 2) in combination with Pd(OAc)2 were tested for the Heck–Mizoraki (H–M) and Suzuki–Miyaura (S–M) type C–C coupling reactions. 1a·HBF4 was converted to cis-[RhCl(1a)(CO)2] (5) to determine the σ-donating properties with those of their 5- and 6-membered NHC complexes. IR data obtained from [RhCl(6-NHC)(CO)2] studies showed that 6-Bn∗ and 6-Bn∗-Me2 are similar in σ-donor properties.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The type of substituents on the nitrogen atoms and/or NHC ring have a crucial role in catalysis. ► The majority of studies have focused on the substituent effect of 5-membered NHCs. ► The properties of 6-membered rings have not been investigated in great detail. ► This study aims to fill the gap.