Impact of Ligands and Metals on the Formation of Metallacyclic Intermediates and a Nontraditional Mechanism for Group VI Alkyne Metathesis Catalysts

Abstract

The intermediacy of metallacyclobutadienes as part of a [2 + 2]/retro-[2 + 2] cycloaddition-based mechanism is a well-established paradigm in alkyne metathesis with alternative species viewed as off-cycle decomposition products that interfere with efficient product formation. Recent work has shown that the exclusive intermediate isolated from a siloxide podand-supported molybdenum-based catalyst was not the expected metallacyclobutadiene but instead a dynamic metallatetrahedrane. Despite their paucity in the chemical literature, theoretical work has shown these species to be thermodynamically more stable as well as having modest barriers for cycloaddition. Consequentially, we report the synthesis of a library of group VI alkylidynes as well as the roles metal identity, ligand flexibility, secondary coordination sphere, and substrate identity all have on isolable intermediates. Furthermore, we report the disparities in catalyst competency as a function of ligand sterics and metal choice. Dispersion-corrected DFT calculations are used to shed light on the mechanism and role of ligand and metal on the intermediacy of metallacyclobutadiene and metallatetrahedrane as well as their implications to alkyne metathesis.

The Ascent of Alkyne Metathesis to Strategy-Level Status

“…Hence, the ligand design was revisited with the hope of forming catalysts that retain these virtues yet are structurally well-defined. The platform shown in Scheme proved adequate: trisilanols 30 are sufficiently preorganized to ligate a single metal center but flexible enough to support the different intermediates passed through during a catalytic turnover. ,, The preparation of ligands of this type is straightforward, scalable, and modular; the same design was independently pursued by the group of Lee. , …”

“…75,77,93 The preparation of ligands of this type is straightforward, scalable, and modular; the same design was independently pursued by the group of Lee. 94,95 The derived "canopy" complexes of type 31 or 32 are indeed 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).…”

“…96 For this promising application profile, the canopy catalysts have already been subject to intense experimental and computational scrutiny. 69,75,77,95 Since a comprehensive discussion is beyond the scope of this Perspective, a portrait in "al fresco" style must suffice. Thus, the geometric constraints of the podand ligand framework lead to a slightly more Lewis-acidic Mo center, which favors substrate binding.…”

“…Pseudorotation about the adjacent Mo−O bond allows this handicap to be circumvented (Scheme 9): 77,95 it results in exchange of the R 1 and R 3 substituents on the metallacyclic ring while maintaining the original tautomeric form; in so doing, an unprecedented gateway to product formation is opened. The actual pseudorotation passes through a "bent" metallacyclic intermediate E that can also connect to a metallatetrahedrane G. 69,77 In line with early reasoning, however, G was shown to be an off-cycle intermediate, which could even be isolated in certain settings.…”

Classifying and understanding the reactivities of Mo based alkyne metathesis catalysts from 95Mo NMR chemical shift descriptors

“…[6][7][8][9][10] Alkyne metathesis in particular has seen major recent progress with the introduction of metal alkylidyne catalysts bearing tripodal silanolate ligands ('canopy catalysts'). [11][12][13][14][15][16] Despite significant synthetic advances, there is a growing need for and interest in spectroscopic descriptors that provide information about electronic structure and relate to catalytic performances.…”