Volume 75, Issue 4

Investigations into olefin ring-closing metathesis (RCM) have led to a general method for the synthesis of coumarins. Catalysts with higher activity, such as the second-generation ruthenium catalyst, promote the intramolecular reaction between two-electron deficient olefins. This method allows for convenient access to a variety of coumarins substituted at both the 3- and 4-positions, as well as a tetrasubstituted example.

The idea of “bucky ferrocene” intrigues chemists owing to its aesthetic beauty and to its polarized, extended conjugated system, but the idea has remained hypothetical. The synthesis of close analogs of Fe(C60Me5)Cp and Fe(C70Me3)Cp was achieved in two steps from [60] and [70]fullerenes, relying on an oxidative C–H bond activation starting with an iron(I) complex and the corresponding cyclopentadienyl precursors, respectively. The molecules can be regarded as members of a new organometallic conjugated system connected to a graphitic system. Numerous synthetic transformations and self-organization procedures that are known for the parent ferrocenes and fullerenes are expected to be employable toward productive use of the ferrocene/fullerene hybrid molecules in chemistry and materials sciences.

Routes to pincer and chelate carbene complexes of Pd, Rh, and Ir include double geminal CH activation, metallation, and CH activation. Abnormal binding via imidazole C5 can occur, and ion pairing can strongly influence C2 vs. C5 binding. Prior ligand binding via pyridine N before metallation is not essential. Computational methods are used extensively, for example, to compare N vs. C binding in imidazole. The possibility of C binding by histidine in metalloenzymes is discussed.

Catalytic reactions resulting from our C–X (X = H, C, O, N, halide) bond activation studies are described. Aryl chlorides can react with aluminum alkyls in preference to bromides. Using PCP-type Pd catalysts, Heck reaction with aryl iodides and bromides can proceed without involvement of Pd(0). Ru-catalyzed oxidative coupling of arenes with alkenes using O 2 was accomplished. Using specifically designed systems, the scope and mechanisms of C–C activation in solution was studied and compared to C–H activation. C–C activation by Rh(I), Ir(I), Ni(II),Pt(II), Ru(II), and Os(II) was observed. Metal insertion into a strong C–C bond can be kinetically and thermodynamically more favorable than the competing C–H activation. Selective, single-step oxidative addition of a strong C–C bond to a metal was observed and kinetically evaluated. Catalytic C–C hydrogenolysis was demonstrated. A combination of C–C activation and C–R formation (R = aryl, silyl) resulted in unusual methylene transfer chemistry. Selective activation of aryl–O and Me–O bonds was observed. New types of interactions between metals and arenes and unusual quinonoid complexes, including quinone methides, xylylenes, methylene arenium, and a metallaquinone, were discovered. C–H and C–C agostic complexes of cationic metals, proposed as intermediates in bond activation, were isolated. Stabilization and controlled release of biologically relevant, extremely unstable, simple quinone methides, was accomplished.

An overview of the metal-mediated synthesis and use of nanosized metallodendrimers is given with emphasis on electron-transfer processes (molecular batteries consisting in dendrimers decorated with a large number of equivalent redox-active centers) and catalytic reactions (electron-transfer-chain catalytic synthesis of dendrimers decorated with ruthenium carbonyl clusters, redox catalysis of nitrate and nitrite electroreduction in water by star-shape hexanuclear catalysts).

Synthetic strategies in the cyclopentadienyl (Cp) chemistry of the groups 13–15 elements, are based on the following phenomena: (1) low barriers for haptotropic and dyotropic shifts, for sigmatropic rearrangements, and for other types of Cp migration; (2) stabilization of elements in their low-oxidation state by π-complexation; (3) tuning of steric andelectronic effects by suitable ring substituents; and (4) easy homolytic and heterolytic cleavage of El-C(Cp) bonds. These strategies are applied to: (a) the formation of a Si 2 Me 4 doubly bridged dicyclopentadienyl cobalt complex, (b) the chemistry of half-sandwich [Me 5 C 5 Ga] and sandwich [(Me 5 C 5 ) 2 Si] complexes, (c) the formation of P 6 , (d) the deposition of Al, Ga, Si, Ge, and As from respective Cp-El precursor molecules, and (d) the “dynamic covalent chemistry” in ferrocenylgallium(III) compounds.

Stability constants for different aliphatic 2-hydroxycarboxylic acid complexes in aqueous solutions with protons and metal ions published between 1960 and the end of 1994 have been critically evaluated.