W-183 Nmr Spectroscopy Guides The Search For Tungsten Alkylidyne Catalysts For Alkyne Metathesis

Triarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for alkyne metathesis but lead to disappointing results and poor stability in the tungsten series.H-1,W-183 heteronuclear multiple bond correlation spectroscopy, exploiting a favorable(5)J-coupling between the(183)W center and the peripheral protons on the alkylidyne cap, revealed that these ligands upregulate the Lewis acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2] cycloaddition step is over-stabilized and the catalytic turnover brought to a halt. Guided by the(183)W NMR shifts as a proxy for the Lewis acidity of the central atom and by an accompanying chemical shift tensor analysis of the alkylidyne unit, the ligand design was revisited and a more strongly pi-donating all-alkoxide ligand prepared. The new expanded chelate complex has a tempered Lewis acidity and outperforms the classical Schrock catalyst, carrying monodentatetert-butoxy ligands, in terms of rate and functional-group compatibility.