Abstract
For numerous enabling features and strategic virtues, contemporary alkyne metathesis is increasingly recognized as a formidable synthetic tool. Central to this development was the remarkable evolution of the catalysts during the past decades. Molybdenum alkylidynes carrying (tripodal) silanolate ligands currently set the standards; their functional group compatibility is exceptional, even though they comprise an early transition metal in its highest oxidation state. Their performance is manifested in case studies in the realm of dynamic covalent chemistry, advanced applications to solid-phase synthesis, a revival of transannular reactions, and the assembly of complex target molecules at sites, which one may not intuitively trace back to an acetylenic ancestor. In parallel with these innovations in material science and organic synthesis, new insights into the mode of action of the most advanced catalysts were gained by computational means and the use of unconventional analytical tools such as 95Mo and 183W NMR spectroscopy. The remaining shortcomings, gaps, and desiderata in the field are also critically assessed.
Highlights
Barely younger than olefin metathesis, it took a long time for this transformation to step out of the shadow of its exceptionally impactful sibling in order to gain a sharp profile in its own right
It became very soon very clear that the proper choice of ancillary ligands about the active alkylidyne unit is a critical determinant of catalytic activity.[2,12,13]
Before the turn of the millennium, alkyne metathesis faced the paradox of being well understood but hardly relevant. This situation has changed since ; the reaction is increasingly recognized as truly enabling and relevant from the strategy point of view; it is clearly more than a subordinate relative of olefin metathesis
Summary
That is, the redistribution of the alkylidyne units of a pair of acetylene derivatives with the aid of a transitionmetal catalyst, has been known since the late 1960s.1 barely younger than olefin metathesis, it took a long time for this transformation to step out of the shadow of its exceptionally impactful sibling in order to gain a sharp profile in its own right. The derived “canopy” complexes of type 31 or 32 are privileged catalysts They combine the advantages of the parent silanolate-supported molybdenum alkylidynes 23 with a higher robustness against protic substituents including unprotected alcohols (for specific examples, see Schemes 16 and 17).[75] Even a certain stability toward moisture was noticed; there remains much room for improvement, the ability to perform alkyne metathesis reactions in technical grade solvents is may suffice to say that a single tertiary amine sufficed to quench the activity of first and second generation Grubbs-type catalysts in a very closely related RCM-setting en route to the sibling alkaloid ingenamine.[96]. One might hope that this transformation anticipates future cleavage of N2, e.g., when bound end-on to an appropriate metal center; if so, it would open a conceptually different foray toward N2 activation devoid of any redox steps
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