Palladium–NHC complex

{{main article|Transition metal NHC complex}}

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.

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.

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 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.{{cite journal|last1=Gstöttmayr|first1=Christian|last2=Volker|first2=Böhm|last3=Eberhardt|first3=Herdtweck|last4=Grosche|first4=Manja|last5=Herrmann|first5=Wolfgang|title=A Defined N-Heterocyclic Carbene Complex for the Palladium-Catalyzed Suzuki Cross- Coupling of Aryl Chlorides at Ambient Temperatures|journal=Angewandte Chemie International Edition|date=2002|volume=41|issue=8|pages=1363–1365|doi=10.1002/1521-3773(20020415)41:8<1363::aid-anie1363>3.0.co;2-g|pmid=19750765 }}

The use of NHC-Pd-PEPPSI complexes in Negishi cross-coupling has resulted in high turnover numbers and turnover frequencies.{{cite journal|last1=Nasielski|first1=J.|last2=Hadei|first2=N.|last3=Achonduh|first3=G|last4=E. A. B.|first4=Kantchev|last5=O'Brien|first5=C.J.|title=Structure–Activity Relationship Analysis of Pd–PEPPSI Complexes in Cross-Couplings: A Close Inspection of the Catalytic Cycle and the Precatalyst Activation Model|journal=Chemistry: A European Journal|date=2010|volume=16|issue=35|pages=10844–10853|doi=10.1002/chem.201000138|pmid=20665575}} Additionally, NHC-Pd complexes can be used to couple sp3 centers to sp3 centers in higher yield than their non-NHC Pd analogs.{{cite journal|last1=Niloufar|first1=Hadei|last2=Kantchev|first2=Eric|last3=O'Brien|first3=Christopher|title=The First Negishi Cross-Coupling Reaction of Two Alkyl Centers Utilizing aPd − N-Heterocyclic Carbene (NHC) Catalyst|journal=Organic Letters|date=2005|volume=7|issue=17|pages=3805–3807|doi=10.1021/ol0514909|pmid=16092880}} However, studies of Pd-NHC complexes and their utility in Negishi coupling are currently lacking despite these promising results.

Pd-NHC complexes used in Sonogashira cross-coupling effect temperature stability in the complex.{{cite journal|last1=Batey|first1=Robert|last2=Shen|first2=Ming|last3=Lough|first3=Alan|title=Carbamoyl-Substituted N-Heterocyclic Carbene Complexes of Palladium(II): Application to Sonogashira Cross-Coupling Reactions|journal=Organic Letters|date=2002|volume=14|issue=9|pages=1411–1414|doi=10.1021/ol017245g|pmid=11975591 }} 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.{{cite journal|last1=Kim|first1=Jong-Ho|last2=Lee|first2=Dong-Ho|last3=Jun|first3=Bong-Hyun|last4=Lee|first4=Yoon-Sik|title=Copper-free Sonogashira cross-coupling reaction catalyzed by polymer-supported N-heterocyclic carbene palladium complex|journal=Tetrahedron Letters|date=2007|volume=48|issue=40|pages=7079–7084|doi=10.1016/j.tetlet.2007.08.015}} NHC-palladacycles permit copper-free Sonogashira reactions to be carried out.{{cite journal|last1=Chinchilla|first1=Rafael|last2=Nàjera|first2=Carmen|title=The Sonogashira Reaction: A Booming Methodology in Synthetic Organic Chemistry|journal=Chemical Reviews|date=2007|volume=107|issue=3|pages=874–922|doi=10.1021/cr050992x|pmid=17305399}}{{cite journal|last1=McGuinness|first1=David|last2=Cavell|first2=Kingsley|title=Donor-Functionalized Heterocyclic Carbene Complexes of Palladium(II): Efficient Catalysts for C - C Coupling Reactions|journal=Organometallics|date=2000|volume=19|issue=5|pages=741–748|doi=10.1021/om990776c}}

The use of Pd-NHC complexes in Heck-Mizoroki cross-coupling permits the use of cheaper, ample supplies of aryl chloride substrates. 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.{{cite journal|last1=Taige|first1=Maria|last2=Zeller|first2=Alexander|last3=Ahrens|first3=Sebastian|last4=Goutal|first4=Sigrid|last5=Hardtweck|first5=Eberhardt|title=New Pd–NHC-complexes for the Mizoroki–Heck reaction|journal=Journal of Organometallic Chemistry|date=2006|volume=692|issue=7|pages=1519–1529|doi=10.1016/j.jorganchem.2006.11.050}}

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Category:Organopalladium compounds

Category:Organometallic chemistry

Category:Carbon-carbon bond forming reactions

Category:Catalysis