“Canopy Catalysts” for Alkyne Metathesis: Molybdenum Alkylidyne Complexes with a Tripodal Ligand Framework

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 95 Mo 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.

Productive Alkyne Metathesis with “Canopy Catalysts” Mandates Pseudorotation

“… 7 − 9 A new generation of “canopy catalysts” of type 2 distinguished by a tripodal silanolate ligand framework shows an even better application profile. 10 − 13 …”

“…20 − 23 The exclusive formation of 4 from one of the best available catalysts is therefore nonintuitive. 10 , 11 A veritable conundrum accrues when the behavior of the tungsten analogue 3 (R = 2,6-Me 2 C 6 H 3 -) is also taken into consideration, which furnished the canonical metallacyclobutadiene 5 on reaction with 3-hexyne. It took 1 week for the latter to transform into 6 by what represents a single “turnover”; complex 3 is hence catalytically incompetent.…”

Lessons from Natural Product Total Synthesis: Macrocyclization and Postcyclization Strategies

“…This excellent application profile also proved to be enabling in many other advanced applications and has recently been improved even further. 49 …”

“…This example further illustrates the excellent application profile of the molybdenum alkylidyne catalysts at hand. 47 − 49 …”

“…The required substrate was prepared from diyne 70 , again relying on RCAM as the key step catalyzed by 63 / 64 (and, shortly thereafter, by new “canopy catalyst” 76 developed in parallel in our laboratory) ( Scheme 13 ). 49 The subsequent trans -hydrostannation of derived diol 72 worked as planned in a highly regio- and stereoselective manner, 67 , 68 furnishing 73 as virtually the only product that underwent a palladium-catalyzed methoxycarbonylation 70 to give enoate 74 . Unfortunately, however, the elaboration of 74 into sinulariadiolide met with failure: although C–C bond formation via Michael reaction did occur and the allylic −OH group was preserved, it was this latter substituent that thwarted the success by cleaving the incipient nine-membered ring via translactonization with the formation of 75 .…”