NHC-Palladium Complexes

[(NHC)Pd(allyl)Cl] complex.^[1]

In organometallic chemistry, palladium-NHC complexes are a family of organopalladium compounds that have been investigated for applications in homogeneous catalysis.^[2] N-Heterocyclic carbenes (NHCs) are a family of carbene-like ligands, which bind tightly to transition metals, including palladium. Traditionally, palladium phosphine complexes have had a significant role in homogeneous catalysis, e.g. alkoxycarbonylation and cross-coupling reactions, so it is logical that related reactivity would be exhibited by the corresponding NHC-Pd complexes.^[2][3]

Synthesis

The synthesis of Pd-NHC complexes follow 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.^[2]

Pd-NHC complexes in catalytic cross-coupling

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.^[2]

Suzuki-Miyaura cross-coupling

In Suzuki-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 desirous 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.^[2][4]

Negishi coupling

The use of NHC-Pd-PEPPSI complexes in Negishi cross-coupling has resulted in high turnover numbers and turnover frequencies.^[5] Additionally, NHC-Pd complexes can be used to couple sp3 centers to sp3 centers in higher yield than their non-NHC Pd analogs.^[6] However, studies of Pd-NHC complexes and their utility in Negishi coupling are currently lacking despite these promising results.^[2]

Sonogashira coupling

Pd-NHC complexes used in Sonogashira cross-coupling effect temperature stability in the complex.^[7] 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.^[8] NHC-palladacycles permit copper-free Sonogashira reactions to be carried out.^[9][10]

Heck-Mizoroki coupling

The use of Pd-NHC complexes in Heck-Mizoroki cross-coupling permits the use of cheaper, ample supplies of aryl chloride substrates.^[2] 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.^[11]

References

1. www.sigmaaldrich.com. Sigma-Aldrich http://www.sigmaaldrich.com/technical-documents/articles/chemfiles/nhc-based-palladium.html. Missing or empty |title= (help)
2. Fortman, George; Nolan, Steven (2011). "N-Heterocyclic carbene (NHC) ligands and palladium in homogeneous cross-coupling catalysis: a perfect union". Chemical Society Reviews. 40: 5151–5169. doi:10.1039/C1CS15088J.
3. Eastman, K. "N-Heterocyclic Carbenes (NHCs)" (PDF). www.scripps.edu.
4. 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.
5. 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: 10844–10853. doi:10.1002/chem.201000138.
6. 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.
7. 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: 1411–1414. doi:10.1021/ol017245g.
8. Kim, Jong-Ho; Lee, Dong-Ho; Jun, Bong-Hyun; Lee, Yoon-Sik (2007). "Copper-free Sonogashira cross-coupling reaction catalyzed by polymer-supported N-heterocyclic carbene palladium complex". Tetrahedron Letters. 48 (40): 7079–7084. doi:10.1016/j.tetlet.2007.08.015.
9. Chinchilla, Rafael; Nàjera, Carmen (2007). "The Sonogashira Reaction: A Booming Methodology in Synthetic Organic Chemistry". Chemical Reviews. 107: 874–922. doi:10.1021/cr050992x.
10. McGuinness, David; Cavell, Kingsley (2000). "Donor-Functionalized Heterocyclic Carbene Complexes of Palladium(II): Efficient Catalysts for C - C Coupling Reactions". Organometallics. 19: 741–748. doi:10.1021/om990776c.
11. 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: 1519–1529. doi:10.1016/j.jorganchem.2006.11.050.