Selective cleavages of carbon-carbon bonds catalyzed by transition-metal complexes have been shown to be increasingly versatile tools for organic synthesis allowing for complementary synthetic strategies Numerous examples of transition-metal catalyzed C-C bond activations of three- and four-membered ring systems have been reported These strained rings have been shown to engage in a variety of new ring-opening rearrangements and cycloaddition reactions leading to valuable structures Besides strain-driven transformations, other facilitating strategies to enforce the C-C bond activation of unstrained molecules have been developed as well While the variety of different transformations is less abundant, they concentrate on chelation-assisted reactions using appropriate permanent or transient directing groups In particular, the cleavage and subsequent functionalization of the C-CN bonds and decarbonylation processes operating by an excision of carbon monoxide from ketone derivatives have witnessed a large progress

Catalytic C-C Bond Activations via Oxidative Addition to Transition Metals.

N,N-Dimethylformamide: much more than a solvent

Cleavage of carbon-carbon bonds by transition metals under homogeneous conditions has recently received much scientific and technological interest. In this review, an overview of this field is presented. The first part deals with stoichiometric reactions involving carbon-carbon bond breaking. The second part features catalytic reactions, especially those related to organic synthesis.

Cleavage of Carbon—Carbon Single Bonds by Transition Metals

Conventional organic synthesis has been mainly based upon the reactivities of π-bonds and polar σ-bonds. Carbon–carbon single bonds are nonpolar and generally far less reactive. Although they remain intact under most reaction conditions, it is possible to activate and cleave them if suitable organometallic compounds or metal catalysts are applied. Such C–C single bond cleavage reactions are attracting increasing attention in the context of synthetic chemistry because they provide a unique and more straightforward route from readily available substances to targets, while requiring significantly fewer steps. The present Perspective aims to exemplify the potential of metal-catalyzed C–C single bond cleavage for organic synthesis.

Potential of Metal-Catalyzed C–C Single Bond Cleavage for Organic Synthesis

Dimethylformamide (DMF) acts as an efficient source of carbon monoxide and dimethylamine in the palladium-catalyzed aminocarbonylation (Heck carbonylation) of p-tolyl bromide to provide the dimethylamide. Addition of amines to the reaction mixture in excess delivers the corresponding aryl amides in good yields. The amines employed, benzylamine, morpholine, and aniline, all constitute good reaction partners. The reaction proceeds smoothly with bromobenzene and more electron-rich aryl bromides, but electron-deficient aryl bromides fail to undergo aminocarbonylation. The reactions are conducted at 180-190 degrees C for 15-20 min with microwave heating in a reaction mixture containing imidazole and potassium tert-butoxide: the latter is required to promote decomposition of the DMF solvent at a suitable rate. The beneficial effects of controlled microwave irradiation as an energy source for the rapid heating of the carbonylation reaction mixture are demonstrated. The carbonylation procedure reported herein, which relies on the in situ generation of carbon monoxide, serves as a convenient alternative to other carbonylation methods and is particularly applicable to small-scale reactions where short reaction times are desired and the direct use of carbon monoxide gas is impractical.

Dimethylformamide as a carbon monoxide source in fast palladium-catalyzed aminocarbonylations of aryl bromides

IT has been shown1,2 from an examination of the polarographic behaviour of some Rh(III)-arsine and phosphine complexes in dimethylformamide (DMF) as solvent that these complexes undergo reaction with the solvent. To elucidate this reaction and to identify the reaction product, an investigation has been undertaken, the results of which are reported here.

Formation of Rh(I)-Carbonyl Complex by the Reaction with Some Non-alcoholic, Oxygen-containing Solvents

POLAROGRAPHIC behaviour of the ions Co(dipy)3
3+ and Co(dipy)3
2+ shows that they are reversibly reduced at the dropping mercury electrode to a compound of univalent cobalt which does not decompose in neutral aqueous or ethanolic solutions.

A New Compound of Univalent Cobalt

IN previous communications1,2 the influence of the nature and concentration of anions on the rate of electroreduction of Co(NH3)5Xn+ complexes has been reported. The influence exerted depends on the nature of the ligand X. This phenomenon has been explained by the assumption that the complex particle does not behave as a conducting sphere with uniform charge distribution (as assumed usually) but that the non-homogeneous charge distribution contributes considerably to the electrostatic free energy of activation and has to be taken into account. The ligand X represents the region of maximum non-homogeneity which induces an opposite non-homogeneity in the ionic atmosphere, the cations of the supporting electrolyte being preferentially attracted into the parts of the ionic atmosphere near the negative pole of the central particle. This increased concentration of cations shields the negative pole of the complex particle or might lead to the formation of an ion-pair between the positively charged complex particle and the cation. Both these effects make the attainment of the favourable orientation (that is, with ligand X removed from the positively charged electrode surface2) of the particle in the activated complex depolarizer-electrode2 much easier and increase the rate of electro-reduction.

Influence of Polyvalent Cations on the Rate of Electro-reductio n and Aquation of Co(NH 3 ) 5 X n+ Complexes

THE rate of the reduction of the complexes [Co(NH3)5
X] at the dropping mercury electrode depends on the nature of the supporting electrolyte and increases in the order: SO4 = ≪ ClO4− <NO3− <Cl−<Br− as is manifested by the shift of the half-wave potential of the reduction wave with the change of the supporting electrolyte. The halides Cl−, Br− (and under suitable conditions also I−) cause a further, quite different effect: at a certain concentration of the halide ion a new reduction wave appears which starts from the potential of the dissolution of mercury. The relative height of this wave (that is, the ratio of the positive wave to the total diffusion current) increases non-linearly with increasing concentration of the anion until all the complex is reduced in this positive wave (Fig. 1).

Substitution Reactions of Cobaltic Complexes catalysed by Adsorption

A POLAROGRAPHIC depolarization effect due to carbon dioxide in aqueous solutions occurring at − 2.2 V. (from the normal calomel electrode) has been described by Van Rysselberghe1 and v. Stackelberg2. This effect is, however, indistinct. When using absolute (99.8 per cent) ethyl alcohol with 0.3 M tetramethyl ammonium chloride in electrolysis with the dropping mercury cathode, a well-defined wave with a maximum at − 2V. (Fig. 1) is formed on the current-voltage curve. The behaviour shows it to be due to the evolution of hydrogen from carbonic acid, the carbon dioxide molecule being regarded as not reducible. Here nitrogen containing 4 vol. per cent of carbon dioxide was bubbled through the solution for 3–4 min. at room temperature. The equilibrium concentration of carbon dioxide dissolved under its pressure in the gaseous phase is attained very quickly (in about 2 min.). The ensuing limiting current is strictly proportional to the partial pressure of carbon dioxide.

A. A. Vlček

Papers

Formation of Rh(I)-Carbonyl Complex by the Reaction with Some Non-alcoholic, Oxygen-containing Solvents

A New Compound of Univalent Cobalt

Influence of Polyvalent Cations on the Rate of Electro-reductio n and Aquation of Co(NH 3 ) 5 X n+ Complexes

Substitution Reactions of Cobaltic Complexes catalysed by Adsorption

Polarographic Effect of Carbon Dioxide in Solutions of Ethyl Alcohol