N-Heterocyclic Carbenes in Late Transition Metal Catalysis

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

2.3.5. Multi-NHCs Linked by Spacers 3568 2.4. The Ag2O Route 3570 2.4.1. Feasibility 3570 2.4.2. Complications 3571 2.4.3. Theoretical Consideration 3572 2.5. Applications 3572 2.5.1. Ag(I)-NHCs in NHC Transfer 3572 2.5.2. Ag(I)-NHCs in Catalysis 3572 2.5.3. Ag(I)-NHCs in Medicine 3572 2.5.4. Ag(I)-NHCs in Nanomaterials 3573 3. Au(I)and Au(III)-NHCs 3573 3.1. Historical Background 3573 3.2. General Synthetic Methods 3573 3.3. Formation of Au(I)and Au(III)-NHCs 3574 3.3.1. Neutral [Au(NHC)L] 3574 3.3.2. Ionic [Au(NHC)L][Anion] 3576 3.3.3. Multinuclear Au(I)-NHCs 3578 3.3.4. Other Classes of Au(I)-NHCs 3578 3.3.5. Au(III)-NHC Complexes 3579 3.4. Applications 3579 3.4.1. Au(I)and Au(III)-NHCs in Catalysis 3579 3.4.2. Au(I)-NHCs in Medicine 3580 4. Cu(I)and Cu(II)-NHCs 3581 4.1. Historical Background 3581 4.2. General Synthetic Methods 3582 4.3. Formation of Cu(I)and Cu(II)-NHCs 3583 4.3.1. Complexes Containing the Cu(NHC)2 Core 3583 4.3.2. [Cu(NHC)(Halide)] 3583 4.3.3. [Cu(NHC)(Ligand)] 3584 4.3.4. Multinuclear Cu(I)and Cu(II)-NHCs 3589 4.4. Catalysis 3591 4.4.1. Past Events 3591 4.4.2. Recent Advancements 3591 5. Photoluminescence 3592 6. Conclusions 3594 7. Abbreviations 3594 8. Acknowledgments 3595 9. References 3595

Coinage Metal−N-Heterocyclic Carbene Complexes

Heterocyclische Carbene – Synthese und Koordinationschemie

Transition metals are finding increasing use in organic synthesis on the borderline between “organic” and “inorganic” chemistry. Advantage is taken thereby of the fact that metal-induced CC bond formation often takes place with remarkable selectivity. The rapid development that has taken place in this area of chemistry is clearly demonstrated by the carbene complexes, examples of which are now known for almost all transition elements, and which have transformed from organometallic curiosities into synthetically useful reagents in less than two decades since the first studies of E. O. Fischer. They are not only suitable as carbene-transfer agents but also undergo interesting cycloadditions with other ligands in the co-ligand sphere. Their manipulation requires techniques no more complicated than those for Grignard reactions. Thus, carbene complexes can also be used in the synthesis of natural products such as vitamins or antibiotics.

Carbene Complexes in Organic Synthesis [New Synthetic Methods (47)]

Modes of Reaction of 2‐ and 3‐Hydroxyalkyl Isocyanides with Metal Compounds; Oxazolidinylidene and Perhydrooxazinylidene Complexes of Palladium, Platinum, and Gold

Die Metallverbindungen MX2 (X = Cl, Br, I, ClO4) und MX2L2 (L MeCN, PhCN, PPh3) der Nickeltriade reagieren mit den (2- und 3-Hydroxyalkyl)isocyaniden CNCHRCHR′OH (R R′ H;R Et, R′ H;R H,R′ Me) und CNCH2CH2CH2OH zu den stabilen wasserloslichen Tetracarben-Komplexen 3a – c, 4a – c, 5a – c. 15a – c, 22a, b) und 3d – 5d, 15d), deren Anionen z. B. gegen BF4−, BPh4− oder [PdI4]2- austauschbar sind. In einigen Fallen werden auch gemischte Komplexe (z. B. Pd{CN4- (7d), (20)) erhalten. Spektro-skopischen (13C-NMR, UV) wie Rontgenstrukturdaten (von (5a)) zufolge sind die Carbenkomplexe als C-metallierte, aber nicht metallstabilisierte Oxazolinium-Spezies aufzufassen. Die vier Liganden-Funfringe stehen auf der PdC4-Koordinationsebene angenahert senkrecht. Zwischen dem Komplex-Dikation und zwei Chlorid-Anionen bestehen starke NH…Cl-, zu weiteren Kationen schwachere CH…Cl- und schwache CH…O-Wasserstoff-bruckenbindungen.



Metal Complexes of Functional Isocyanides, XI. Homoleptic Carbene Complexes, II. Tetrakis(oxazolidin-2-ylidene)- and Tetrakis(perhydrooxazin-2-ylidene)nickel, -palladium, and -platinum



The compounds MX2 (X = Cl, Br, I, ClO4) and MX2L2 (L MeCN, PhCN, PPh3) of the nickel triad react with the (2- and 3-hydroxyalkyl)isocyanides CNCHRCHR′OH (R R′ H; R Et, R′ H; R H, R′ Me) and CNCH2CH2CH2OH to give the stable water-soluble tetracarbene complexes (3a – c, 4a – c, 5a – c, 15a – c, 22a, b) and (3d – 5d, 15d), the anions of which can be exchanged for e.g. BF4−, BPh4− or [PdI4]2-. In some cases, also mixed complexes (e.g. Pd{CN4 (7d), (20)) are obtained. According to spectroscopic (13C NMR, UV) and X-ray structural data (of (5a)), the carbene complexes are to be regarded as C-metalated but not metal-stabilized oxazolinium species. The four five-membered rings are almost perpendicular with respect to the PdC4 coordination plane. The complex dication and two chloride anions are strongly NH…Cl-bonded while further linkage occurs through weaker CH…Cl and weak CH…O hydrogen bridge bonding.

Metallkomplexe funktioneller Isocyanide, XI. Homoleptische Carbenkomplexe, II. Tetrakis(oxazolidin-2-yliden)- und Tetrakis(perhydrooxazin-2-yliden)nickel, -palladium und -platin

Die (Hydroxyalkyl)isocyanide CN[CH2]n OH (n = 2 (1a), 3 (1d), 5 (1e)), CNCH(Et)CH2OH (1b) und CNCH2CH(ME)OH (1c) werden durch Wasserabspaltung aus N-(Hydroxyalkyl)formamiden mit aquimolaren Mengen Phosgen in Gegenwart von Triethylamin hergestellt, spektroskopisch (IR, 1H-, 13C-NMR) vermessen und in Metallkomplexe eingebaut, die 1a – e in der „offenkettigen” Form stabilisieren. Die OH-Gruppe metallkoordinierter Hydroxyisocyanide last sich verestern oder gegen Cl austauschen. Umgekehrt konnte CNCH2OC(O)Ph in einer Reihe neu dargestellter Metallkomplexe nicht zum Isocyanhydrin(CNCH2OH)-Liganden verseift werden.



Metal Complexes of Functional Isocyanides, X. On Hydroxyalkyl Isocyanides and Their Stabilization in Metal Complexes



The hydroxyalkyl isocyanides CN[CH2]n OH (n = 2 (1a), 3 (1d), 5 (1e)), CNCH(Et)CH2OH (1b), and CNCH2CH(Me)OH (1c) have been synthesized from N-(hydroxyalkyl)formamides by dehydration with equimolar amounts of phosgene in the presence of triethylamine, spectroscopically (IR, 1H, 13C NMR) characterized and introduced into metal complexes, which stabilize 1a – e in the „open chain” form. The OH group of coordinated hydroxy isocyanides can be esterified or substituted for Cl. Conversely, hydrolysis of CNCH2OC(O)Ph in a series of newly prepared metal complexes of this ligand to give isocyanohydrin (CNCH2OH) complexes has not been accomplished.

Metallkomplexe funktioneller Isocyanide, X. Über (Hydroxyalkyl)isocyanide und ihre Stabilisierung in Metallkomplexen

Metallkomplexe mit mehrfunktionellen isocyaniden

Reaktionsweisen von 2‐ und 3‐Hydroxyalkylisocyaniden mit Metallverbindungen; Oxazolidin‐ und Perhydrooxazin‐ylidenkomplexe von Palladium, Platin und Gold

Klaus Bartel

Papers

Modes of Reaction of 2‐ and 3‐Hydroxyalkyl Isocyanides with Metal Compounds; Oxazolidinylidene and Perhydrooxazinylidene Complexes of Palladium, Platinum, and Gold

Metallkomplexe funktioneller Isocyanide, XI. Homoleptische Carbenkomplexe, II. Tetrakis(oxazolidin-2-yliden)- und Tetrakis(perhydrooxazin-2-yliden)nickel, -palladium und -platin

Metallkomplexe funktioneller Isocyanide, X. Über (Hydroxyalkyl)isocyanide und ihre Stabilisierung in Metallkomplexen

Metallkomplexe mit mehrfunktionellen isocyaniden

Reaktionsweisen von 2‐ und 3‐Hydroxyalkylisocyaniden mit Metallverbindungen; Oxazolidin‐ und Perhydrooxazin‐ylidenkomplexe von Palladium, Platin und Gold