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1,3‐Dialkyl‐ and 1,3‐Diaryl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene Rhodium( i) and Palladium( II ) Complexes: Synthesis, Structure, and Reactivity
Author(s) -
Mayr Monika,
Wurst Klaus,
Ongania KarlHans,
Buchmeiser Michael R.
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305437
Subject(s) - rhodium , chemistry , tetrafluoroborate , medicinal chemistry , bromide , palladium , monoclinic crystal system , reactivity (psychology) , organic chemistry , catalysis , ionic liquid , crystal structure , medicine , alternative medicine , pathology
The synthesis of novel 1,3‐diaryl‐ and 1,3‐dialkylpyrimidin‐2‐ylidene‐based N‐heterocyclic carbenes (NHCs) and their rhodium( i) and palladium( II ) complexes is described. The rhodium compounds bromo(cod)[1,3‐bis(2‐propyl)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene]rhodium ( 7 ), bromo(cod)(1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene)rhodium ( 8 ) (cod=η 4 ‐1,5‐cyclooctadiene, mesityl=2,4,6‐trimethylphenyl), chloro(cod)(1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene)rhodium ( 9 ), and chloro(cod)[1,3‐bis(2‐propyl)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene]rhodium ( 10 ) were prepared by reaction of [{Rh(cod)Cl} 2 ] with lithium tert ‐butoxide followed by addition of 1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidinium bromide ( 3 ), 1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidinium tetrafluoroborate ( 4 ), 1,3‐di‐2‐propyl‐3,4,5,6‐tetrahydropyrimidinium bromide ( 6 ), and 1,3‐di‐2‐propyl‐3,4,5,6‐tetrahydropyrimidinium tetrafluoroborate, respectively. Complex 7 crystallizes in the monoclinic space group P 2 1 / n , and 8 in the monoclinic space group P 2 1 . Complexes 9 and 10 were used for the synthesis of the corresponding dicarbonyl complexes dicarbonylchloro(1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene)rhodium ( 11 ), and dicarbonylchloro[1,3‐bis(2‐propyl)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene]rhodium ( 12 ). The wavenumbers ν(CO I)/ν(CO II) for 11 and 12 were used as a quantitative measure for the basicity of the NHC ligand. The values of 2062/1976 and 2063/1982 cm −1 , respectively, indicate that the new NHCs are among the most basic cyclic ligands reported so far. Compounds 3 and 6 were additionally converted to the corresponding cationic silver( i) bis‐NHC complexes [Ag(1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene) 2 ]AgBr 2 ( 13 ) and [Ag{1,3‐bis(2‐propyl)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene} 2 ]AgBr 2 ( 14 ), which were subsequently used in transmetalation reactions for the synthesis of the corresponding palladium( II ) complexes Pd(1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene) 2 2+ (Ag 2 Br 2 Cl 4 4− ) 1/2 ( 15 ) and Pd[1,3‐bis(2‐propyl)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene) 2 ]Cl 2 ( 16 ). Complex 15 crystallizes in the monoclinic space group P 2 1 / c , and 16 in the monoclinic space group C 2 / c . The catalytic activity of 15 and 16 in Heck‐type reactions was studied in detail. Both compounds are highly active in the coupling of aliphatic and aromatic vinyl compounds with aryl bromides and chlorides with turnover numbers (TONs) up to 2 000 000. Stabilities of 15 and 16 under Heck‐couplings conditions were correlated with their molecular structure. Finally, selected kinetic data for these couplings are presented.