Carbones

Synthesis

Carbodiphosphoranes

One strategy for the synthesis of carbodiphosphoranes involves the use of a reducing agent on a carbon reagent in its +2 or +4 oxidation state. The first successful synthesis of a compound now recognised as a carbodiphosphorane was achieved by Ramirez et al. in 1961 with this method. By stirring methylidebis-(triphenylphosphonium) bromide with potassium metal suspended in a diglyme solution, the potassium reduced the starting material to form hexaphenylcarbodiphosphorane as a stable, yellow, crystalline solid.

Alternative methods to synthesise alkyl-substituted carbodiphosphoranes involve the deprotonation or elimination of carbon (IV) or carbon (II) starting materials. Reacting a carbon (IV) or carbon (II) diphosphine salt with a strong base such as sodium hydride or sodium amide can deprotonate the centre carbon atom to form the desired carbodiphosphorane. Alternatively, a halide-substituted phosphonium salt can undergo an elimination reaction in the presence of a strong base to form a carbodiphosphorane.

Synthetic methods have also been developed for more diverse carbodiphosphoranes. Methylenediphosphines will undergo a reaction with hexafluoroacetone or thioacetone to form O-substituted and S-substituted carbodiphosphoranes respectively. Cyclic carbodiphosphoranes have also been successfully synthesised through the reaction of bis(diisopropylamino)phosphino diazomethane with bis(dialkylamino)phosphenium triflate in excess benzonitrile followed by deprotonation with hexamethyldisilazide.

Carbodicarbenes

The first carbodicarbene synthesis was achieved much later than the first carbodiphosphorane synthesis, in 2008 by Dyker et al. The first step was the methylation of bis(N-methylbenzimidazol-2-yl)methane using methyl triflate and the second step was the deprotonation of the carbon (II) species using potassium bis(trimethylsilyl)amide (KHMDS) to yield the desired N-heterocyclic-carbene-substituted carbone.

Similar non-cyclic carbodicarbenes have also been successfully synthesised from iminium salts through the condensation of two equivalents of the starting material in dimethylacetamide (DMA), followed by nucleophilic substitution with dimethylamine, then deprotonation with n-butyllithium to form a tetraaminoallene which acts as a carbodicarbene. Additionally, a method of facile synthesis of asymmetric carbodicarbenes was developed by Chen et al. in 2015 by using a simple nucleophilic substitution reaction. Reacting an olefin substituted with a N-heterocyclic carbene scaffold with a thioether containing a different NHC moiety generates a product which can be readily deprotonated to afford a carbodicarbene with two different carbene substituents with improved functionality.

Reactivity

Basicity

The presence of two lone pairs on the central carbon atom make it possible for carbones to act as Brønsted-Lowry bases and accept two protons from an acid. The typical first proton affinity for a carbodiphosphorane ranges from 209.3 kcal mol⁻¹ (875.7 kJ mol⁻¹) for the weakest base to 287.6 kcal mol⁻¹ (1203 kJ mol⁻¹) for the strongest base and second proton affinities ranging from 70.6 kcal mol⁻¹ (295 kJ mol⁻¹) to 188.9 kcal mol⁻¹ (790.4 kJ mol⁻¹). For comparison, the proton affinity of potassium hydroxide is 1101.8 kJ mol⁻¹, indicating that carbodiphosphoranes can function as strong bases. Carbodicarbenes can act as even stronger bases than carbodiphosphoranes with first proton affinities reaching as high as 294.3 kcal mol⁻¹ (1231 kJ mol⁻¹). However, the second proton affinities for carbodicarbenes are comparable to those of carbodiphosphoranes and exhibit variability depending on the identity of the N-heterocyclic carbene substituent with a range of values from 155.3 kcal mol⁻¹ (649.8 kJ mol⁻¹) to 168.4 kcal mol⁻¹ (704.6 kJ mol⁻¹). This is due to the increased delocalisation of the π-symmetric lone pair over the carbon atoms of the N-heterocyclic carbene substituents which increases the dependence of the second proton affinity on the identity of the substituent.

Ligands

In addition to being strong Brønsted-Lowry bases, carbones are also nucleophilic and act as strong Lewis bases when coordinating with transition metals and main group elements. Several computational studies found that carbodiphosphoranes bound tightly to tungsten and nickel with metal-ligand bond dissociation energies that were greater than those of carbon monoxide metal-ligand bonds for certain compounds. Experimentally, a variety of metal-carbodiphosphorane complexes have been synthesised and characterised, including with metals such as tungsten, nickel, copper, silver, and gold. The gold complex is of particular note as it is the first geminal digold complex and provides experimental evidence supporting the structure of carbodiphosphoranes as a carbon (0) compound with two lone pairs on the central carbon atom donating to the gold atoms.

Carbodicarbenes have also been shown to form complexes with different transition metals such as rhodium and gold. In the former experiment, when a rhodium carbonyl complex was coordinated to a carbodicarbene, the carbon-oxygen stretching frequency was observed at 2014 cm⁻¹ which is significantly lower than the same carbon-oxygen stretching frequency when rhodium is coordinated to a N-heterocyclic carbene (between 2058 cm⁻¹ and 2036 cm⁻¹) which is indicative of a strong π-donating effect from the second carbon lone-pair of the carbone.

Reactivity in transition metal complexes

Transition metal complexes containing a carbone ligand also exhibit a wide range of reactivity. In 2015, Pranckevicius et al. synthesised a ruthenium (II) catalyst coordinated to two different carbodicarbene ligands that was able to catalytically reduce olefins with excellent diastereoselectivity and similar activity to Crabtree’s catalyst. Palladium(II) catalysts with bis(pyridine)carbodicarbene ligands have been shown to be successful catalysts for Suzuki-Miyaura and Heck-Mizoroki coupling reactions while rhodium (I) catalysts coordinated to carbodicarbene pincer ligands have been shown to hydroaminate and hydroarylate dienes.

Reactivity in main group complexes

Carbones can also form complexes with main group elements. The strong σ- and π-donating properties of carbones have made them optimal tools for stabilising reactive main-group-based species. Carbodicarbenes have been employed in the successful synthesis of novel boron-containing compounds such as borenium ions, which can exhibit useful optical properties, as well as a dicationic tricoordinate hydridoboron compound. Carbones have also been used in the first synthesis of stable carbon-bismuth species with π-bonding character. Carbodicarbenes have also seen significant utility in the field of beryllium chemistry with the synthesis of a five-membered beryllacycle through C-H activation as well as beryllacycle ring expansion.