Cyclometalated and Alkoxyphenyl-Substituted Palladium Imidazolin-2-ylidene Complexes. Synthetic, Structural, and Catalytic Studies

Some new N,N‘-bis(aryl)imidazolinium salts (3a−h), in which the N-aryl groups have one unsubstituted ortho position and alkyl (Me, Pri) or alkoxy (OMe, OPri) substitution at the other ortho and/or para positions, have been prepared. They were used for the synthesis of N-heterocyclic carbene complexes of palladium in which the N-heterocyclic carbene is attached to cyclometalated or electron-rich aromatic rings (4, 5, 11d,e). The reaction of Pd(tmed)(CH3)2 with the N-heterocyclic carbene generated from 3b and base gave the dimethyl complex 6, which quantitatively eliminated ethane to form the Pd(0) complex 7. The latter was converted to a new type of “pincer” complex (8) by facile cyclometalation of both aromatic groups of the N-heterocyclic carbene ligand. The activities of the new complexes 4 and 9d,e in the Heck reaction of aryl halides were compared. At higher temperatures the complexes 9d,e show low activity in the coupling of aryl chlorides.本研究制备了一系列新型N,N'-二芳基咪唑啉鎓盐（3a~h），该类化合物的N-芳基基团均带有一个未取代的邻位，且在另一邻位和/或对位带有烷基（甲基Me、异丙基Pri）或烷氧基（甲氧基OMe、异丙氧基OPri）取代基。该类盐可用于合成钯基氮杂环卡宾配合物，其中氮杂环卡宾与环金属化或富电子芳香环相连，对应配合物编号为4、5、11d、e。由3b与碱原位生成的氮杂环卡宾与四甲基乙二胺合二甲基钯[Pd(tmed)(CH3)2]发生反应，得到二甲基钯配合物6；该配合物可定量消除乙烷，生成零价钯（Pd(0)）配合物7。通过氮杂环卡宾配体两个芳香基团的快速环金属化反应，配合物7可转化为新型钳形配合物8。本研究对比了新型配合物4与9d、e在芳基卤代烃赫克反应（Heck reaction）中的催化活性。在较高温度下，配合物9d、e在氯代芳烃偶联反应中仅表现出较低的催化活性。