N-杂环卡宾钯催化剂

2010年，Michael G. Gardiner 和他的同事合成了新型的氮杂环卡宾化合物，A colorless solution of 1 over anhydrous Na2CO3 in dry MeOH was heated at 508C for two hours to give a red solution, from which red crystals of 2 were obtained in 84%yield after filtration and concentration (Scheme 1).[8] Complex 2 has high stability as a solid (more than one year) and in MeOH and THF (several months). The complex is tolerant to moisture, but reacts quickly with atmospheric oxygen in solution and the solid state.Peter D. W. Boyd, Alison J. Edwards, Michael G. Gardiner, Angew. Chem. Int. Ed. 2010, 49, 6315 –6318.2008年，Takeshi Makino和他的同事得到了具有二齿氮杂环配体的钯配合物，The bidentate NHC-palladium complexes 4a–e were prepared by the reaction of 1-(4-iodoaryl)-3-aryl-4,5-dihydroimidazolinium salt (1)[16] and xanthenediboronic acid (2)[17] in the presence of Pd(PPh3)4 and Ag2O followed by palladation[18] (Scheme 1). The bis(imidazolidene) derivative 6 was also synthesized in a similar way (Scheme 2).Takeshi Makino, Hyuma Masu, Kosuke Katagiri, Ryu Yamasaki, Isao Azumaya,and Shinichi Saito，Eur. J. Inorg. Chem. 2008, 4861–4865.There are many examples of monodentate NHCs, but only a few examples of alkane-bridged chelating biscarbene ligands. Chelated carbenes are expected to be more stable since one possible decomposition pathway, reductive elimination of the carbene, should be slower for this conformationally restricted case. A chelating coordination is one way to obtain highly stable catalysts capable of tolerating harsher reaction conditions than traditional phosphine catalysts. Sebastian Ahrens 和他的同事化合物长链烷烃桥联配体的钯配合物，Sebastian Ahrens, Alexander Zeller, Maria Taige, and Thomas Strassner Organometallics, Vol. 25, No. 22, 2006N-Heterocyclic carbenes (NHCs), first prepared independently by Wanzlick and Schnherr[1] and fele[2] in 1968, attracted little interest from the chemical community until 1991, when Arduengo et al. revealed the first stable, crystalline NHC(1, IAd).[3] The potential of this class of compounds to serve as spectator ligands in transition-metal complexes was recognized in 1995 by Herrmann et al.[4] Soon thereafter, the exploitation of the remarkable potential of NHC ligands in catalysis began. The above seminal works led to the development of a variety of other NHC platforms (see right column)[5] and their transition-metal complexes for catalytic applications. However, only NHCs derived from imidazolium or 4,5-dihydroimidazolium salts have found wide-spread use in homogeneous catalysis to date. The most important example is the ruthenium metathesis catalyst developedby Grubbs and co-workers, for which the Nobel Prize was awarded. Replacement of one of the two tricyclohexyl phosphane ligands in the generation I Grubbs catalyst with the bulky carbene SIMes (3) led to significant improvements in terms of catalyst stability, activity, and substrate range in subsequent generations.[6] Palladium is another transition metal capable of directing a wide range of useful transformations,[7] in particular C-C and C-heteroatom cross-coupling and carbopalladation reactions.[8] The use of bulky carbenes, in particular IPr (4) and SIPr (5), as ligands in these transformations has also resulted in significant improvements in catalyst performance compared to the more traditional phosphane ligands.[1] H.-W. Wanzlick, H.-J. Schnherr, Angew. Chem. 1968, 80, 154; Angew. Chem. Int. Ed. Engl. 1968, 7, 141– 142.[2] K. fele, J. Organomet. Chem. 1968, 12, P42– P43.[3] A. J. Arduengo III, R. L. Harlow, M. Kline, J. Am. Chem. Soc.1991, 113, 361– 363.[4] W. A. Herrmann, M. Elison, J. Fischer, C. Kchter, G. R. J. Arthus, Angew. Chem. 1995, 107, 2602– 2605; Angew. Chem. Int. Ed. Engl. 1995, 34, 2371– 2373.[5] F. E. Hahn, Angew. Chem. 2006, 118, 1374–1378; Angew. Chem. Int. Ed. 2006, 45, 1348–1352.[6] M. Scholl, S. Ding, C.-W. Lee, R. H. Grubbs, Org. Lett. 1999, 1,953–956.[7] Handbook of Organopalladium Chemistryfor Organic Synthesis (Ed.: E. Negishi), Wiley, New York, 2002.[8] Metal-catalyzed cross-coupling reactions, 2nd ed. (Eds.: A.de Meijere, F. Diederich), Wiley, New York, 2004.In terms of catalysis, the activity of these complexes has been scarcely examined, that is, only in the Heck, the Suzuki-Miyaura, and the Buchwald-Hartwig reactions. Compound 1 was found to catalyze the coupling of 4-bromoacetophenone and butyl acrylate at low catalyst loadings but was only studied for limited examples.22 On the other hand, 6 showed only poor activity in the Heck reaction, probably because of the lack of steric pressure from the thiazolydene ligand.17b In 2004, Glorius reported the outstanding activity of 2 and 3 in the Suzuki-Miyaura reaction.16c These complexes, possessing an NHC of the IBiox family, allowed for the formation of a tetra-ortho-substituted biphenyl in high yield. Tested as well in the Suzuki-Miyaura coupling, complex 7 was found to be efficient for the coupling of aryl bromidesand chlorides in water,16e while 8 coupled only bromides but with a larger scope, involving unactivated and sterically hindered substrates.16dIn 2002, we studied the activity of 5 in the N-aryl amination reaction.20 This complex was found to be highly efficient for the coupling of aryl bromides and chlorides. A variety of amines could be coupled with activated, unactivated, encumbered, and heteroaromatic halides in high yields and in short reaction times (Scheme 2). Interestingly, due to the robustness of 5, reactions could be carried out on the benchtop under aerobic conditions without loss of activity. Recently 5 has been shown as excellent precatalysts in the Suzuki-Miyaura reaction.20b22 McGuinness, D. S.; Cavell, K. J. Donor-Functionalized Heterocyclic Carbene Complexes of Palladium(II): Efficient Catalysts for C-C Coupling Reactions. Organometallics 2000, 19, 741–748.17(b) Yen,S. W.; Koh, L. L.; Hahn, F. E.; Huynh, H. V.; Hor, T. S. A. Convenient Entry to Mono- and Dinuclear Palladium(II) Benzothiazolin-2-ylidene Complexes and Their Activities Toward Heck Coupling. Organometallics 2006, 25, 5105–511216(c) Altenhoff, G.; Goddard, G.; Lehmann, C. W.;Glorius, F. Sterically Demanding Bioxazoline-Derived N-Heterocyclic Carbene Ligands with Restricted Flexibility for Catalysis. J. Am. Chem. Soc. 2004, 126,15195–15201.(d) Shi, M.; Qian, H.-X. A Stable Dimeric Mono-Coordinated NHCPd(II) Complex: Synthesis, Characterization, and Reactivity in Suzuki-Miyaura CrossCoupling Reaction. Appl. Organometal. Chem. 2005, 19, 1083–1089.(e) Huynh, H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K. Palladium(II) Complexes of Sterically Bulky, Organometallics 2006, 25,3267–3274.20(b) Diebolt, O.; Braunstein, P.; Nolan, S. P.; Cazin, C. S. J. Room temperature activation of arylchlorides in Suzuki-Miyaura coupling using a [PdCl2(NHC)]2 complex (NHC ) N-heterocyclic carbene). Chem. Commun., 2008,3190–3192.Typically, NHC-containing palladacycles are synthesized in high yields by addition of a nucleophilic carbene to an acetate- or halogen-bridged palladacycle dimer. In 2003, Iyer described the synthesis and applications of palladacyles 9-11.25 These precatalysts were tested in the Heck reaction where they displayed good to high activity. With aryl bromides, TONs between 40 000 and 90 000 were observed, whereas the use of chlorides was less successful. The activity of compound 10 was further studied in the Suzuki-Miyaura reaction where, as observed in the Heck, aryl bromides were easily coupled and aryl chlorides were found to be more reluctant partners. A large series of NHC-containing phosphapalladacycles, including 12-15, was reported by Herrmann.26 Their catalytic activity in the Heck reaction was investigated,showing promising results for further improvement. Notably, the use of 15 allowed for the coupling of aryl chlorides without the need for additives. Bedford and co-workers reported the formation of phosphite palladacycles 16-19 and studied their activity in the Suzuki-Miyaura reaction.27 Overall, these catalysts performed quite poorly (17 being the most efficient) and could only couple unhindered and activated aryl bromides.2825 Iyer, S.; Jayanthi, A. Saturated N-Heterocyclic Carbene Oxime and Amine Palladacycle Catalysis of the Mizoroki-Heck and the Suzuki Reactions. Synlett 2003, 1125–1128.26 Frey, G. D.; Schu ¨ tz, J.; Herdtweck, E.; Herrmann, W. A. Synthesis and Characterization of N-Heterocyclic Carbene Phospha-Palladacycles and Their Properties in Heck Catalysis. Organometallics 2005, 24, 4416–4426.27 Bedford, R. B.; Betham, M.; Coles, S. J.; Frost, R. M.; Hursthouse, M. B. An Evaluation of Phosphine and Carbene Adducts of Phosphite- and Phosphinite-Based Palladacycles in the Coupling of Alkyl Bromides With Aryl Boronic Acids. Tetrahedron 2005, 61, 9663–9669.28 Bedford, R. B.; Betham, M.; Blake, M. E.; Frost, R. M.; Horton, P. N.; Hursthouse, M. B.; Lo ´ pez-Nicola ´ s, R.-M. N-Heterocyclic Carbene Adducts of Orthopalladated Triarylphopshite Complexes. Dalton Trans. 2005, 2774–2779.在近期的研究中，钯基催化剂被大量的应用于有机合成反应体系，并且发挥了巨大的作用.钯基催化剂的最主要优势是在形成C-C键，C-O键，C-N键，甚至 C-S键的同时，不会影响反应物的其他官能团，并且反应条件温和$这样的催化过程不仅符合绿色环保的主题，又经济高效，在生态环境保护愈来愈重要的今天，钯基催化剂因为不会产生严重的环境污染问题，所以在工业上具有较高的经济价值[1]1.1钯催化偶联反应相对于传统的均相催化体系，钯催化体系应用于偶联反应可以解决催化剂重复使用性差,反应产物分离困难等问题.其中最具有代表性的就是是heck反应和suzuki反应[2]这两类反应在有机反应中对于构建碳碳键十分重要，自被发现以来，它们已经被广泛地应用于医药合成,新型材料合成以及天然产物的合成中.heck和 Mirozoki 最早于20世纪中后期发现了heck反应，并且由 heck通过的深入的研究逐渐发展起来.heck 反应的主要过程是在钯的催化下，使活化的不饱和烃和卤代烃发生反应，生成的主要产物为反式取代物,反应中较常使用的是芳基卤代烃，反应的常用温度在20-180o C[3].最初，heck 反应并没有得到化学研究者们足够的重视，但是随着化学工业的发展，heck 反应可以通过一步反应就得到碳碳键的特点更符合现代工业高效%环境友好的要求，因此近年来引起了一股研究热潮.Dr.Herrmann等[4]发明了最早的环钯催化剂，并将应用于heck 反应中，反应15h，可以达到90%的转化率，同时该催化剂对含有吸电子基团的氯代苯也有很好的活性。