Stable (amino)(phosphino)carbenes: difunctional molecules

(Amino)(phosphino)carbenes are stable due to the donation of the nitrogen lone pair, the phosphino group remains strongly pyramidalized. Reactions can be performed selectively at the carbene center, but also at the phosphorus center leading to new stable carbenes. These difunctional molecules can be considered as hybrid ligands.     

Stable (aryl)(phosphino)carbenes: new ligands for transition metals

The first direct complexation of a stable phosphino carbene to a transition metal center is described. Both eta1- and eta2-coordination modes have been characterized spectroscopically. The Fischer-type structure of the eta1-carbene rhodium complex was revealed by an X-ray diffraction study.     

Synthesis of a Stable Biphenyl‐Bis(carbene) and ‐Bis(silylene) [M{(NCH2But)2C6H3‐3,4}]2 (M = C or Si)

Treatment of the biphenyl derivative [S=C{(NCH₂Bu^t)₂C₆H₃‐3,4}]₂ or [Cl₂Si{(NCH₂Bu^t)₂C₆H₃‐3,4}]₂ with C₈K afforded the new bis(carbene) 1 or the first bis(silylene) 2 , respectively. The X‐ray structure of 2 is presented.     

Stable optically pure phosphino(silyl)carbenes: reagents for highly enantioselective cyclopropanation reactions

The stability of phosphino(trimethylsilyl)carbenes bearing cyclic diamino substituents on phosphorus is strongly dependent on the steric hindrance of the nitrogen substituents. Phosphinocarbenes 3 and 7, derived from the trans-N,N'-diisopropylcyclohexane-1,2-diamine and N,N'-diisopropyl-1,2-ethanediamine, are not observed; instead the 1,3-diphosphete 4 and a novel six-membered heterocycle 8, which results from the dimerization of 3 and the reaction of 7 with its diazo precursor 6, respectively, have been isolated. In contrast, the phosphino(silyl)carbene 14 derived from N,N'-di-tert-butyl-1,2-ethanediamine has been isolated in high yield. By using the enantiomerically pure (S,S)-, and (R,R)-N,N'-di-tert-butyl-1,2-diphenyl-1,2-ethanediamines, the first optically pure phosphino(sily)carbenes (S,S)-17 and (R,R)-17 have been prepared. They react with methyl acrylate to give the corresponding cyclopropanes (S,S,R,R)-19 and (R,R,S,S)-19 with a total syn diastereoselectivity and an excellent enantioselectivity (de>98 %).     

Stable bis(diisopropylamino)cyclopropenylidene (BAC) as ligand for transition metal complexes

Several nickel(0), palladium(II), and rhodium(I) complexes have been prepared using for the first time the stable bis(diisopropylamino)cyclopropenylidene (BAC). Based on single crystal X-ray diffraction studies and spectroscopic data, the structural and electronic properties of these complexes are discussed. Moreover, their similarities and differences with the analogous NHC complexes are emphasized.     

Employing Aryl‐Linked Bis‐mesoionic Carbenes as a Pincer‐Type Platform to Access Ambient‐Stable Palladium(IV) Complexes

The study of palladium(IV) species has great implications for Pd^II/Pd^IV‐mediated catalysis. However, most of the Pd^IV complexes rapidly decompose under ambient conditions, which makes the isolation, characterization and further reactivity study very challenging. The reported ancillary ligand platforms to stabilize Pd^IV species are dominated by chelating N‐donors such as bipyridines. In this work, we present two Pd^IV complexes with scarcely used C‐donors as the supporting platform. The anionic aryl donor and MIC (MIC=mesoionic carbene) are combined in a [CC′C]‐type pincer framework to access a series of ambient‐stable Pd^IV tris(halido) complexes. Their synthesis, solid‐state structures, stability, and reactivity are presented. To the best of our knowledge, the work presented herein reports the first isolated Pd^IV–MIC as well as the first Pd^IV carbene‐based aryl pincer.     

Co(III)/Zn(II)-catalyzed dearomatization of indoles and coupling with carbenes from ene-yne ketones via intramolecular cyclopropanation

A straightforward and efficient protocol for dearomatizing indoles is described. The reaction, catalyzed by an inexpensive Co(III)/Zn(II) catalyst, starts from easily accessible N-pyrimidinyl indoles and ene-yne ketones. Mild reaction conditions, high diastereoselectivity, a broad substrate scope, effective functional group tolerance, and reasonable to remarkable yields were observed.     

Heteroaromatic carbenes (review)

We correlate information on the chemistry of heteroaromatic carbenes. We consider methods for generation of electrophilic and nucleophilic heteroaromatic carbenes, their reaction pathways, and their role in organic synthesis.Translated from Khimiya Geterosiklicheskikh Soedinenii, No. 9, pp. 1155–1170, September, 1992.     

Silver(I) N-heterocyclic carbene halide complexes: A new bonding motif

Two new NHC silver halide salts have been synthesised and structurally characterised, and they show the two most common structural forms for such salts: the (NHC)AgX and the forms (both have a stoichiometry of 1 NHC:1 Ag:1 halide). A third new compound has been synthesised and structurally characterised: this compound has an unprecedented 2 NHC:1 Ag:1 halide stoichiometry and exhibits a planar arrangement of two coordinating NHCs, and a coordinating bromide about the central silver. The Ag-Br bond is long in the solid state and, together with solution NMR data, this suggests that a formulation of (NHC)₂Ag⁺Br⁻ might be more appropriate.     

Effect of solvent polarity on N-H insertion versus rearrangement of alkylidene carbenes

Alkylidene carbenes, when generated from o-aminobenzophenones and lithiated trimethylsilyldiazomethane, give a mixture of alkyne (by rearrangement) and indole (by N-H insertion). It has been found that this ratio of indole to acetylene shows a linear dependence on the polarity of the ethereal solvent.     

Rhodium and iridium complexes of N-heterocyclic carbenes: Structural investigations and their catalytic properties in the borylation reaction

Bridged and unbridged N-heterocyclic carbene (NHC) ligands are metalated with [Ir/Rh(COD)₂Cl]₂ to give rhodium(I/III) and iridium(I) mono- and biscarbene substituted complexes. All complexes were characterized by spectroscopy, in addition [Ir(COD)(NHC)₂][Cl,I] [COD = 1,5-cyclooctadiene, NHC =  1,3-dimethyl- or 1,3-dicyclohexylimidazolin-2-ylidene] (1, 4), and the biscarbene chelate complexes 12 [(η⁴-1,5-cyclooctadiene)(1,1′-di-n-butyl-3,3′-ethylene-diimidazolin-2,2′-diylidene)iridium(I) bromide] and 14 [(η⁴-1,5-cyclooctadiene)(1,1′-dimethyl-3,3′-o-xylylene-diimidazolin-2,2′-diylidene)iridium(I) bromide] were characterized by single crystal X-ray analysis. The relative σ-donor/π-acceptor qualities of various NHC ligands were examined and classified in monosubstituted NHC-Rh and NHC-Ir dicarbonyl complexes by means of IR spectroscopy. For the first time, bis(carbene) substituted iridium complexes were used as catalysts in the synthesis of arylboronic acids starting from pinacolborane and arene derivatives.     

Cyclic (Alkyl)(amino)carbenes (CAACs): Recent Developments

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) are among the most nucleophilic (σ donating) and also electrophilic (π‐accepting) stable carbenes known to date. These properties allow them to activate a variety of small molecules and enthalpically strong bonds, to stabilize highly reactive main‐group and transition‐metal diamagnetic and paramagnetic species, and to bind strongly to metal centers, which gives rise to very robust catalysts. The most important results published up to the end of 2013 are briefly summarized, while the majority of this Review focuses on findings reported within the last three years.     

Persistent (amino)(silyl)carbenes

(Amino)(silyl)carbenes, prepared by substitution reactions at a carbene center, can survive several days at 0 degrees C. These species are not push-pull carbenes as their phosphino analogues and therefore are excellent ligands for transition-metal centers.