Abstract
A new family of structurally well-defined molybdenum alkylidyne catalysts for alkyne metathesis, which is distinguished by a tripodal trisilanolate ligand architecture, is presented. Complexes of type 1 combine the virtues of previous generations of silanolate-based catalysts with a significantly improved functional group tolerance. They are easy to prepare on scale; the modularity of the ligand synthesis allows the steric and electronic properties to be fine-tuned and hence the application profile of the catalysts to be optimized. This opportunity is manifested in the development of catalyst 1f, which is as reactive as the best ancestors but exhibits an unrivaled scope. The new catalysts work well in the presence of unprotected alcohols and various other protic groups. The chelate effect entails even a certain stability toward water, which marks a big leap forward in metal alkylidyne chemistry in general. At the same time, they tolerate many donor sites, including basic nitrogen and numerous heterocycles. This aspect is substantiated by applications to polyfunctional (natural) products. A combined spectroscopic, crystallographic, and computational study provides insights into structure and electronic character of complexes of type 1. Particularly informative are a density functional theory (DFT)-based chemical shift tensor analysis of the alkylidyne carbon atom and 95Mo NMR spectroscopy; this analytical tool had been rarely used in organometallic chemistry before but turns out to be a sensitive probe that deserves more attention. The data show that the podand ligands render a Mo-alkylidyne a priori more electrophilic than analogous monodentate triarylsilanols; proper ligand tuning, however, allows the Lewis acidity as well as the steric demand about the central atom to be adjusted to the point that excellent performance of the catalyst is ensured.
Highlights
In a recent Communication, we disclosed complex 1a as the prototype of a new generation of molybdenum alkylidyne complexes, termed “canopy catalysts”, for alkyne metathesis because of their distinguishing tripodal silanolate ligand framework (Figure 1).[1]
Even though the focus of these two parallel investigations had been somewhat different, both reached the conclusion that such complexes are more than just a tethered variant of catalyst 2a carrying triphenylsilanolate ligands, which had set new standards in the field of alkyne metathesis when it was introduced by our group a decade ago.[3−6] Outlined below is a comprehensive study into this new family of canopy catalysts, including the optimization of their synthesis, a first ligandtuning exercise, structural investigations, a portrayal of their electronic nature, and new insights into the elementary steps of the catalytic cycle
For the longer O−Si and Si−C bonds, the bulk is sufficiently remote from the molybdenum center not to impede substrate binding or product dissociation; only for very hindered alkynes is the size of the silanolates critical.[16]
Summary
In a recent Communication, we disclosed complex 1a as the prototype of a new generation of molybdenum alkylidyne complexes, termed “canopy catalysts”, for alkyne metathesis because of their distinguishing tripodal silanolate ligand framework (Figure 1).[1]. The spectra do not provide any indication for protonation of the alkylidyne: ligand exchange rather than destruction of the operative Mo≡CR unit is (mainly) accountable for the loss of activity This observation is in line with recent experimental as well as computational data from the literature.[21,63] water remains detrimental, a half-life on the order of 1 h is a chemical virtue without precedent in metal alkylidyne chemistry in general; it is fully appreciated if one considers the extreme sensitivity of earlier generations of alkyne metathesis catalysts, not least the otherwise very powerful complex 3 and its precursors.[7−10,34,64] In this context, we reiterate that it has become common practice to supplement the reaction mixtures with molecular sieves.[3,4] this additive primarily serves as a sequestering agent for the released 2-butyne in reactions conducted at RT, it ensures a level of dryness and a lifespan of a canopy catalyst that allow certain applications to be carried out in technical-grade solvents that need not be rigorously dried and purified prior to use. This excellent profile may help to correct the common misperception that high-valent early transition metal catalysts in general and MoVI-based alkylidyne and alkylidene catalysts in particular provide only limited opportunities when working with polyfunctionalized compounds.[70]
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