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摘要
The allylic amination reaction catalyzed by [(NHC)Pd(allyl)Cl] complexes has been studied, and the presence of PPh3 found to be essential for the catalytic system to be active. [(1-Mesityl-3-methylimidazol-2-ylidene)Pd(eta(3)-C3H5)Cl] has been used to optimize the conditions of the reaction with (E)1,3-diphenylprop-3-enyl acetate and benzylamine under biphasic conditions (aqueous base/CH2Cl2). The best yields of isolated product (98%) were obtained using aqueous 1 m K2CO3. The influence of the NHC ligand and the allyl fragment on the pre-catalyst was also examined. Two new neutral [(NHC)Pd(eta(3)-1-RC3H4)Cl] complexes [NHC = 1-mesityl-3-methylimidazol-2-ylidene or 1-(2,6-diisopropylphenyl)-3-methylimidazol-2-ylidene; R = H or Ph] have been prepared and a decrease of the reaction time observed with the former. NMR studies have shown that this pre-catalyst is more easily activated than its eta(3)-allyl analogue and that the predominant activation pathway involves attack of the amine at the allyl fragment. This reaction occurs exclusively in the presence of PPh3, thus suggesting that cationic [(NHC)Pd(allyl)(PPh3)](+) complexes, which are more electrophilic, are formed in situ and allow the amine to react with the allyl fragment. A tetrafluoroborate cationic complex has therefore been prepared from [(NHC)Pd(allyl)Cl], PPh3, and AgBF4 and fully characterized. This complex is an active pre catalyst in the allylic amination reaction. The scope of the reaction was examined under the optimized reaction conditions with several different nitrogen nucleophiles and allylic acetates. The amination products were obtained in yields ranging from 73 to > 98%, except for that from cyclohexenyl acetate and dibenzylamine. Finally, the reaction performed directly with the allylic alcohol and benzylamine led to a mixture of allylic amines in 9% yield.
- 出版日期2009-5
