Mohand Melaimi

Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

A Crystalline Phosphinyl Radical Cation

One-electron oxidation of a readily available phosphaalkene derived from a cyclic (alkyl)(amino)carbene affords a phosphorus-centered radical cation that is indefinitely stable both in solution and in the solid state, allowing a single X-ray diffraction study to be performed. This species can be regarded as a phosphinyl radical bearing a cationic substituent or, alternatively, as a carbene-stabilized phospheniumyl radical (carbene-RP(+*)).

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.

Deprotonation of a Borohydride: Synthesis of a Carbene-Stabilized Boryl Anion†

An acidic hydride! Thanks to the presence of a π-acceptor cyclic alkyl amino carbene and of two electron-withdrawing nitrile groups, a borohydride reacts with a base to give a carbene-stabilized boryl anion, which reacts with carbon and metal electrophiles at the boron center. Dipp = 2,6-diisopropylphenyl, KHMDS = potassium bis(trimethylsilyl)amide.

Isolation of bis(copper) key intermediates in Cu-catalyzed azide-alkyne “click reaction”

Isolation of bis(copper) complexes that are kinetically more active than their mononuclear counterparts in the most popular “click chemistry” reaction. The copper-catalyzed 1,3-dipolar cycloaddition of an azide to a terminal alkyne (CuAAC) is one of the most popular chemical transformations, with applications ranging from material to life sciences. However, despite many mechanistic studies, direct observation of key components of the catalytic cycle is still missing. Initially, mononuclear species were thought to be the active catalysts, but later on, dinuclear complexes came to the front. We report the isolation of both a previously postulated π,σ-bis(copper) acetylide and a hitherto never-mentioned bis(metallated) triazole complex. We also demonstrate that although mono- and bis-copper complexes promote the CuAAC reaction, the dinuclear species are involved in the kinetically favored pathway.

Cyclic alkyl amino carbene (CAAC) ruthenium complexes as remarkably active catalysts for ethenolysis.

An expanded family of ruthenium-based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed-oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100,000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340,000, at a catalyst loading of only 1 ppm. This is the first time a series of metathesis catalysts has exhibited such high performance in cross-metathesis reactions employing ethylene gas, with activities sufficient to render ethenolysis applicable to the industrial-scale production of linear α-olefins (LAOs) and other terminal-olefin products.

Gold-Catalyzed Hydroarylation of Alkenes with Dialkylanilines

Anti-Bredt di(amino)carbene supported gold(I) chloride complexes are readily prepared in two steps from the corresponding isocyanide complexes. In the presence of KB(C6F5)4 as chloride scavenger, they promote the unprecedented hydroarylation reaction of alkenes with N,N-dialkylanilines with high para-selectivity. The latter are challenging arenes for Friedel-Craft reactions, due to their high basicity.

Bis(diphosphaferrocene) palladium(II) dimer complexes as efficient catalysts in the synthesis of arylboronic esters

Abstract Bis(diphosphaferrocenes)PdCl 2 dimers efficiently catalyse cross-coupling reactions between aryl iodides and pinacol borane in dioxane at 80 °C to afford the corresponding aryl boronic esters derivatives in excellent yields.

Isolation of Neutral Mononuclear Copper Complexes Stabilized by Two Cyclic (Alkyl)(amino)carbenes

Two (cAAC)2Cu complexes, featuring a two-coordinate copper atom in the formal oxidation state zero, were prepared by reducing (Et2-cAAC)2Cu(+)I(-) with metallic sodium in THF, and by a one-pot synthesis using Me2-cAAC, Cu(II)Cl2, and KC8 in toluene in a molar ratio of 2:1:2, respectively. Both complexes are highly air and moisture sensitive but can be stored in the solid state for a month at room temperature. DFT calculations showed that in these complexes the copper center has a d(10) electronic configuration and the unpaired electron is delocalized over two carbene carbon atoms. This was further confirmed by the EPR spectra, which exhibit multiple hyperfine lines due to the coupling of the unpaired electron with (63,65)Cu isotopes, (14)N, and (1)H nuclei.

A σ4, λ5-phosphinine palladium complex: a new type of phosphorus ligand and catalyst. Application to the Pd-catalyzed formation of arylboronic estersElectronic supplementary information (RSI) available: experimental procedures for a detailed preparation of complex 4 and cross-coupling experiments. See http://www.rsc.org/suppdata/cc/b2/b203550b/

A palladium(II) complex of a S–P–S pincer ligand featuring a metallated ylidic P-atom efficiently catalyzes the coupling between pinacolborane and various iodoaryls to yield the corresponding arylboronic esters (TON between 5100 and 76500).