The synthesis of Pd-NHC complexes follows the methods used for the synthesis of transition metal NHC complexes. The synthesis of Pd-NHC complexes can also be achieved through substitution of a labile ligand L in a Pd-L complex. Labile ligands typically include cyclooctadiene, dibenzylideneacetone, bridging halides, or phosphines. This process can be used in conjunction with the in situ generation of free carbenes. Pd-NHC complexes can also be synthesized through transmetalation with silver-NHC complexes. The transmetallated NHCs can either be isolated for subsequent reaction with palladium in a two-step method, or generated in the presence of palladium in a one-pot reaction. However, generation of Pd-NHC complexes by Ag transmetallation is cost-prohibitive and hampered by Ag complexes’ light sensitivity.[4]
The utility of palladium-catalyzed cross-coupling reactions is enhanced by the use of N-heterocyclic carbene ligands. Indeed, Pd-NHC complexes have been proven effective in Suzuki-Miyaura, Negishi, Sonogashira, Kumada-Tamao-Corriu, Hiyama, and Stille cross-coupling. Compared to the corresponding Pd-phosphine catalysts, Pd-NHC catalysts can be faster, exhibit broader substrate scope, all with higher turnover numbers.[4]
InSuzuki-Miyaura cross-couplings, the traditional coupling partners are organobromides and organoboron compounds. While Suzuki-Miyaura cross-couplings typically employ organobromides as coupling partners, organochlorides are more desirable electrophiles for cross-coupling due to their lower cost. The sluggish reactivity of the C-Cl bond is often a problem. With the advent of Pd-NHC complexes, organochlorides have emerged as viable partners in Suzuki-Miyaura cross coupling.[4][6]
The use of NHC-Pd-PEPPSI complexes in Negishi cross-coupling has resulted in high turnover numbers and turnover frequencies.[7] Additionally, NHC-Pd complexes can be used to couple sp3 centers to sp3 centers in higher yield than their non-NHC Pd analogs.[8] However, studies of Pd-NHC complexes and their utility in Negishi coupling are currently lacking despite these promising results.[4]
Pd-NHC complexes used in Sonogashira cross-coupling effect temperature stability in the complex.[9] As in other Pd-NHC mediated cross-coupling reactions, the use of Pd-NHC complexes also allow higher turnover numbers than their NHC-free counterparts.[10] NHC-palladacycles permit copper-free Sonogashira reactions to be carried out.[11][12]
The use of Pd-NHC complexes in Heck-Mizoroki cross-coupling permits the use of cheaper, ample supplies of aryl chloride substrates.[4] Additionally, the activity and stability of the catalyst in Heck-Mizoroki coupling can be enhanced by adjusting the 1,3 substituents on the imidazole ring.[13]
^ abMarion, Nicolas; Navarro, Oscar; Mei, Jianguo; Stevens, Edwin D.; Scott, Natalie M.; Nolan, Steven P. (March 2006). "Modified (NHC)Pd(allyl)Cl (NHC = N-Heterocyclic Carbene) Complexes for Room-Temperature Suzuki−Miyaura and Buchwald−Hartwig Reactions". Journal of the American Chemical Society. 128 (12): 4101–4111. doi:10.1021/ja057704z. PMID16551119.
^Viciu, Mihai S.; Navarro, Oscar; Germaneau, Romain F.; Kelly, Roy A.; Sommer, William; Marion, Nicolas; Stevens, Edwin D.; Cavallo, Luigi; Nolan, Steven P. (March 2004). "Synthetic and Structural Studies of (NHC)Pd(allyl)Cl Complexes (NHC = N-heterocyclic carbene)". Organometallics. 23 (7): 1629–1635. doi:10.1021/om034319e.
^ abcdefgFortman, George; Nolan, Steven (2011). "N-Heterocyclic carbene (NHC) ligands and palladium in homogeneous cross-coupling catalysis: a perfect union". Chemical Society Reviews. 40 (10): 5151–5169. doi:10.1039/C1CS15088J. PMID21731956.
^Gstöttmayr, Christian; Volker, Böhm; Eberhardt, Herdtweck; Grosche, Manja; Herrmann, Wolfgang (2002). "A Defined N-Heterocyclic Carbene Complex for the Palladium-Catalyzed Suzuki Cross- Coupling of Aryl Chlorides at Ambient Temperatures". Angewandte Chemie International Edition. 41 (8): 1363–1365. doi:10.1002/1521-3773(20020415)41:8<1363::aid-anie1363>3.0.co;2-g.
^Nasielski, J.; Hadei, N.; Achonduh, G; E. A. B., Kantchev; O'Brien, C.J. (2010). "Structure–Activity Relationship Analysis of Pd–PEPPSI Complexes in Cross-Couplings: A Close Inspection of the Catalytic Cycle and the Precatalyst Activation Model". Chemistry: A European Journal. 16 (35): 10844–10853. doi:10.1002/chem.201000138. PMID20665575.
^Niloufar, Hadei; Kantchev, Eric; O'Brien, Christopher (2005). "The First Negishi Cross-Coupling Reaction of Two Alkyl Centers Utilizing aPd − N-Heterocyclic Carbene (NHC) Catalyst". Organic Letters. 7 (17): 3805–3807. doi:10.1021/ol0514909. PMID16092880.
^Batey, Robert; Shen, Ming; Lough, Alan (2002). "Carbamoyl-Substituted N-Heterocyclic Carbene Complexes of Palladium(II): Application to Sonogashira Cross-Coupling Reactions". Organic Letters. 14 (9): 1411–1414. doi:10.1021/ol017245g.
^Chinchilla, Rafael; Nàjera, Carmen (2007). "The Sonogashira Reaction: A Booming Methodology in Synthetic Organic Chemistry". Chemical Reviews. 107 (3): 874–922. doi:10.1021/cr050992x. PMID17305399.
^McGuinness, David; Cavell, Kingsley (2000). "Donor-Functionalized Heterocyclic Carbene Complexes of Palladium(II): Efficient Catalysts for C - C Coupling Reactions". Organometallics. 19 (5): 741–748. doi:10.1021/om990776c.
^Taige, Maria; Zeller, Alexander; Ahrens, Sebastian; Goutal, Sigrid; Hardtweck, Eberhardt (2006). "New Pd–NHC-complexes for the Mizoroki–Heck reaction". Journal of Organometallic Chemistry. 692 (7): 1519–1529. doi:10.1016/j.jorganchem.2006.11.050.
