Triphenylsilanolate ligands were found to impart excellent reactivity and outstanding functional group tolerance on molybdenum alkylidyne complexes, which catalyze alkyne metathesis reactions of all sorts. The active species either can be obtained in high yield by adaptation of the established synthesis routes leading to Schrock alkylidynes or can be generated in situ from the molybdenum nitride complex 11, which itself is readily accessible in large quantity from inexpensive sodium molybdate. Complexation of the active silanolate complexes 12 and 24 with 1,10-phenanthroline affords complexes 15 and 25, respectively, which are stable in air for extended periods of time. Although these phenathroline adducts are per se unreactive vis-a-vis alkynes, catalytic activity is conveniently restored upon exposure to MnCl(2). Therefore, the practitioner has the choice of different alkyne metathesis (pre)catalysts, which are easy to handle yet broadly applicable and exceedingly tolerant. A host of representative inter- as well as intramolecular alkyne metathesis reactions, including applications to a considerable number of bioactive and, in part, labile natural products, shows the remarkable scope of these new tools. Moreover, it was found that the addition of molecular sieves (5 A >or= 4 A >> 3 A) to the reaction mixture significantly improves the chemical yields while simultaneously increasing the reaction rates. This benefit is ascribed to effective binding of 2-butyne, which is released as the common byproduct in reactions of alkynes bearing a methyl end-cap. Thus, alkyne metatheses can now be performed at ambient temperature with neither the need to apply vacuum to drive the conversion nor recourse to tailor-made substrates. The structures of representative examples of this new generation of alkyne metathesis catalysts in the solid state were determined by X-ray analysis.

Nitride- and alkylidyne complexes of molybdenum endowed with triarylsilanolate ligands are excellent (pre)catalysts for alkyne-metathesis reactions of all sorts, since they combine high activity with an outstanding tolerance toward polar and/or sensitive functional groups. Structural and reactivity data suggest that this promising application profile results from a favorable match between the characteristics of the high-valent molybdenum center and the electronic and steric features of the chosen Ar(3)SiO groups. This interplay ensures a well-balanced level of Lewis acidity at the central atom, which is critical for high activity. Moreover, the bulky silanolates, while disfavoring bimolecular decomposition of the operative alkylidyne unit, do not obstruct substrate binding. In addition, Ar(3)SiO groups have the advantage that they are more stable within the coordination sphere of a high-valent molybdenum center than tert-alkoxides, which commonly served as ancillary ligands in previous generations of alkyne metathesis catalysts. From a practical point of view it is important to note that complexes of the general type [(Ar(3)SiO)(3)Mo≡X] (X = N, CR; R = aryl, alkyl, Ar = aryl) can be rendered air-stable with the aid of 1,10-phenanthroline, 2,2'-bipyridine or derivatives thereof. Although the resulting adducts are themselves catalytically inert, treatment with Lewis acidic additives such as ZnCl(2) or MnCl(2) removes the stabilizing N-donor ligand and gently releases the catalytically active template into the solution. This procedure gives excellent results in alkyne metathesis starting from air-stable and hence user-friendly precursor complexes. The thermal and hydrolytic stability of representative molybdenum alkylidyne and -nitride complexes of this series was investigated and the structure of several decomposition products elucidated.

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The molybdenum nitride complex [(Ph(3)SiO)(3)Mo[triple bond]N] (10), which can be conveniently prepared in situ from readily available [(Me(3)SiO)(2)((Me(3)Si)(2)N)Mo[triple bond]N] (9) and Ph(3)SiOH, and the pyridine adduct of 10, [(pyridine)(Ph(3)SiO)(3)Mo[triple bond]N] (11), exhibit excellent catalytic activity in alkyne metathesis reactions of all kinds. Adduct 11 is sufficiently stable to be weighed in air and is therefore much easier to use than any of the established, structurally defined alkyne metathesis precatalysts known to date. This new system is compatible with a host of functional groups, including esters, amides, alkenes, carbamates, ethers, silyl ethers, sulfonates, thioethers, THP acetals, glycosides, ketones, enoates, thiophenes, pyridines, thiazoles, and nitro groups, and even accepts protected propargyl alcohols as substrates. It has been used for the preparation of key intermediates for bioactive natural products, such as gallicynoic acid I, homoepilachnene, epothilone A, cruentaren A, and a fully synthetic latrunculin analogue. However, 9 and 11 react with aldehydes and acid chlorides to give the corresponding nitrile derivatives.

Photocontrol of a piperidine's Brønsted basicity was achieved by incorporation of a bulky azobenzene group and could be translated into pronounced reactivity differences between ON- and OFF-states in general base catalysis. This enabled successful photomodulation of the catalyst's activity in the nitroaldol reaction (Henry reaction). A modular synthetic route to the photoswitchable catalysts was developed and allowed for preparation and characterization of three azobenzene-derived bases as well as one stilbene-derived base. Solid-state structures obtained by X-ray crystal structure analysis confirmed efficient blocking of the active site in the E isomer representing the OFF-states, whereas a freely accessible active site was revealed for a representative Z isomer in the crystal. To correlate structure with reactivity of the catalysts, conformational dynamics were thoroughly studied in solution by NMR spectroscopy, taking advantage of residual dipolar couplings (RDCs), in combination with comprehensive DFT computational investigations of conformations and proton affinities.