Journal of Organometallic Chemistry 881 (2019) 79e129 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem Review Functionalized nitrogen ligands (CeN) for palladium catalyzed cross- coupling reactions (part II) Arjun Kumbhar Department of Chemistry, Padmabhushan Dr. Vasantraodada Patil College, Tasgaon, Affiliated to Shivaji University, Kolhapur, Maharashtra, 416312, India article info abstract Article history: In recent years, considerable effort has been focused in Pd catalyzed cross-coupling reactions, especially Received 6 April 2018 the use of less reactive and economically viable substrates like aryl chlorides. Unfortunately, Pd com- Received in revised form plexes containing the ligands having only N as a donor atom has some limitations, as it couples, mostly 22 September 2018 aryl iodides and bromides with different nucleophiles, and shows less activity towards aryl chlorides. Accepted 24 September 2018 This restriction can overwhelm by the use of Pd complexes containing N in combination with the C as a Available online 3 October 2018 donor atom such as palladacycles, pincers, PEPPSI and carbene ligands. The advantages of these ligands include high activity with enhanced selectivity, less toxicity, moisture, air as well as thermal stability. Keywords: Palladium Most importantly, such complexes have broad applications in catalysis under ambient conditions. This e Nitrogen ligands part of compressive review highlights the results of the highly active C N based Pd complexes and their NeC complexes applications in cross-coupling reactions. In the next part, we will cover all ligands and complexes con- Cross-couplings taining N in combination with P, O and S as a donor atoms (Pd catalysts based on CeP, CeO and CeS ligands). Though, the number of CeN based Pd complexes containing Ferrocene and Buchwald ligands were reported for Pd catalyzed cross-coupling reaction, these complexes will be covered in the next part of the article. © 2018 Published by Elsevier B.V. Contents 1. Introduction . ................................................. 80 2. Pd complexes having a ligands containing N and C atoms . .................................... 80 3. Palladacycles . .. ................................................. 80 3.1. Imine palladacycles . ........................80 3.2. Oxime palladacycles . ........................83 3.3. Amine palladacycles . ........................88 4. Pincer complexes . ................................................. 90 4.1. Symmetrical pincers (NCN) . ........................90 4.2. Unsymmetrical pincers . ........................99 5. Nstabilized NHC complexes . ................................................ 101 5.1. Amine stabilized NHC complexes . .. .......................102 5.2. PEPPSI themed analogues . .......................109 5.3. NHC-palladacycles . .......................119 5.4. NHC-pincers . .......................121 6. Conclusion . ................................................ 125 Acknowledgements . .......................126 References.................................................................................................. .......................126 E-mail address: [email protected]. https://doi.org/10.1016/j.jorganchem.2018.09.020 0022-328X/© 2018 Published by Elsevier B.V. 80 A. Kumbhar / Journal of Organometallic Chemistry 881 (2019) 79e129 1. Introduction applications possessing anionic four-electron (bidentate) or six- electron (tridentate) donor ligands, with five-membered N con- The constant discovery of ligands for Pd catalysts in the area of taining rings [3]. Most importantly, it is possible to regulate the cross coupling reactions has been focused by the advancement of electronic and steric properties of palladacycles solely by following novel ligands and complexes with improved activity and selectivity, ways; which stimulate the use of less reactive aryl chlorides. Unfortu- nately, Pd complexes containing the ligand having only N as a donor (1) Changing the size of the metallacyclic rings (5e7). atom has some limitations, as it couples, mostly aryl iodides and (2) The nature of the metalled carbon atoms (aliphatic, aromatic, bromides with different nucleophiles, and shows less activity about vinylic). aryl chlorides [1]. This restriction can overwhelm by the use of Pd (3) The type of the donor group having N, P, S, O atoms as well as complexes containing N in combination with C as a donor atom that its substituents (alkyl, aryl) and includes palladacycles, pincers, PEPPSI and carbene complexes. The (4) The nature of ancillary ligands (halide, triflate, acetate, advantage of these complexes includes high activity with enhanced phosphine, nitrile or solvent). selectivity, and less toxicity. They are moisture, air and thermally stable. Most importantly, such complexes have broad applications The palladacycle works in two different ways; firstly the PdeC in catalysis under ambient conditions. bond remains intact in reaction sequence, i.e. cyclopalladated unit In this perspective, this review focus on the Pd complexes with is used as an ancillary ligand, and secondly the palladated carbon ligands containing C and N atoms, and their applications in catal- atom can be functionalized by various groups. The application of ysis, especially in cross-coupling reactions (Fig. 1). palladacycles as reagents as well as catalysts for different reactions is recently reviewed by Beletskaya and Cheprakov [4a], Pfeffer [4b] 2. Pd complexes having a ligands containing N and C atoms and Dupont [4c]. The palladacycles are prepared by a number of methods such as CeH activation, oxidative addition, trans- The traditional Pd catalysts such as Pd(PPh3)4,Pd2(dba)3, metallation, or nucleophilic addition onto an unsaturated bond fi PdCl2(PPh3)2,(h3-allyl-PdCl)2, PdCl2(RCN)2 used for many cross- which often organize a ve or six membered chalets due to the coupling reactions has its own drawbacks and limitations, like establishment of a stable PdeC bond and helped with the coordi- they are air and moisture sensitive, require higher Pd loading. Now nation of the two-electron donor group. The first use of pallada- a day ‘Palladacycle, Pincer, N-Heterocyclic Carbene (NHC) and cycle as a precatalyst is reported in the mid-1980s [5]. In 1995 PEPPSI architectures’ have become popular alternatives for tradi- Herrmann et al. [6] applied palladacycle for CeC coupling reaction, tional catalysts (Fig. 2). Hence, these complexes are thoroughly hence called ‘Herrmann-Beller palladacycle’. Recently, there are investigated class of organopalladium compounds in metal catal- many outstanding reports which addressed the importance of ysis [2]. These catalysts have been emerging as a favorable option palladacycles for different cross coupling reactions. Consequently, for traditional catalysts, as they are more stable towards the air and most of studies showed that the mechanism of palladacycle in- moisture as well as easy to handle. volves Pd(0)/Pd(II) oxidation states [7]. Though the precise mech- anism is yet changeable, there are many groups suggested the 3. Palladacycles involvement of Pd(II)/Pd(IV) oxidation states [8]. The CeN palla- dacycle serves as a mere packaging for palladium that may call ‘Palladacycles are cyclic palladium complexes incorporating at ‘disposable wrapper’ [4a]. least one CePd bond in their molecular architecture’. The PdeC Herein, we wish to report applications of CeN palladacycles as a bond in most of the palladacycles is reactive towards a range of precatalyst in various cross-coupling reactions. For simplification, nucleophiles and electrophiles. Many of the palladacycles can be we grouped these palladacycles into three groups. easily preparable, recoverable and recyclable. Since, the 1960s, numerous common palladacycles have been reported for many 3.1. Imine palladacycles The imine palladacycles have been reported as highly stabile catalysts for many cross-coupling reactions. Additionally, these complexes can be easily modified to incorporate different func- tionalities, which will allow them to immobilize it on suitable supports so that the catalyst becomes easily recoverable and recyclable. These palladacycles are prepared by the complexation of Pd precursors with imine that are obtained from the corresponding aromatic aldehydes/ketones. Milstein and coworkers [9a] reported thermally stable and recoverable dimeric cyclopalladated imine palladacycles (C1a and C2a-b) for the Heck arylation reaction (Scheme 1). The complexes were prepared by treatment of Pd(OTf)2 or Pd(OAc)2 with the cor- responding imines in THF. All the catalysts À (0.35e0.70 Â 10 5 mmol) recorded excellent catalytic activity for the Heck coupling of different aryl halides (I and Br) and different olefins with TON 3000e1429000 in NMP using Na2CO3 or Et3Nasa base at 140 C. The palladacycle C1a was also active towards the Suzuki coupling reaction of non-activated aryl bromides with TONs 5 up to 10 using K2CO3 as a base in o-xylene at 130 C[9b]. To get insight into the reaction mechanism, they carried out a competitive reaction with the five aryl bromides under pseudo-first order Fig. 1. The main area of this review. conditions. A. Kumbhar / Journal of Organometallic Chemistry 881 (2019) 79e129 81 Fig. 2. Schematic representation of (a) Palladacycle, (b) Pincer, (c) Modified PdeNHC and PEPPSI architectures. A Milstein type homogeneous palladacycles C1b and C3a-b as palladacycle. Of all the catalysts, the palladacycle C6a (0.1e0.01