Siloxide Podand Ligand as a Scaffold for Molybdenum-Catalyzed Alkyne Metathesis and Isolation of a Dynamic Metallatetrahedrane Intermediate

Abstract

Triphenylsiloxide is one of the most successful ancillary ligands for Mo­(VI)-alkyne metathesis catalysts. It was proposed that flexible siloxide ligands allow Mo–O–Si bond angles to modulate the electrophilicity of MoC and thereby promote the catalysis. Introduction of a siloxide podand ligand allowed elucidation of the effect of ligand flexibility and Mo–O–Si angles on the electrophilicity of MoC. It also allowed for the isolation of a rare metallatetrahedrane of Mo­(VI) which was found to be dynamic in solution.

183W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

“…Such hyper-stabilization of the metallacycle is well known to be the result of an overly Lewis-acidic metal fragment. [20,[37][38][39] In this context, it needs to be emphasized that treatment of an analogous molybdenum canopy complex with 3-hexyne unexpectedly furnished a metallatetrahedrane complex; [11,12] such disparate behavior of tungsten and molybdenum alkylidynes with the same ancillary ligand sphere is striking and unprecedented.…”

“…[1][2][3][4][5] Their functional group compatibility is largely unrivaled; [6][7][8][9][10] it has recently been further improved by the development of a second catalyst generation distinguished by a tripodal silanolate ligand framework. [11,12] Specifically, the "canopy complex" 3 and relatives maintain the virtues of the parent complex 1, yet allow the chelate effect to be harnessed in form of an improved stability towards protic sites; in conjunction with the well-balanced electrophilic character and proper steric protection of the operative Mo CR unit, this results in an excellent overall application profile. [11] From a historic perspective, however, tungsten alkylidyne complexes had taken the lead: [13] Complex 4 a developed by Schrock and co-workers was the first molecularly welldefined catalyst for alkyne metathesis; [14] it played a quintessential role in deciphering the mechanism of this transformation [15] and empowered early applications, [16] even though these examples also witnessed that the functional group tolerance is limited.…”

“…Molybdenum alkylidyne complexes endowed with triarylsilanolate ligands such as 1 , the corresponding ate‐complex 2 and the derived bench‐stable phenanthroline adduct [ 1 ⋅(phen)] set the standards in the field of alkyne metathesis (Figure 1). [1–5] Their functional group compatibility is largely unrivaled; [6–10] it has recently been further improved by the development of a second catalyst generation distinguished by a tripodal silanolate ligand framework [11, 12] . Specifically, the “canopy complex” 3 and relatives maintain the virtues of the parent complex 1 , yet allow the chelate effect to be harnessed in form of an improved stability towards protic sites; in conjunction with the well‐balanced electrophilic character and proper steric protection of the operative Mo≡CR unit, this results in an excellent overall application profile [11]…”

183W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

“…[1][2][3][4][5] Their functional group compatibility is largely unrivaled; [6][7][8][9][10] it has recently been further improved by the development of asecond catalyst generation distinguished by at ripodal silanolate ligand framework. [11,12] Specifically,t he "canopy complex" 3 and relatives maintain the virtues of the parent complex 1,yet allow the chelate effect to be harnessed in form of an improved stability towards protic sites;i n conjunction with the well-balanced electrophilic character and proper steric protection of the operative Mo CR unit, this results in an excellent overall application profile. [11] From ahistoric perspective,however,tungsten alkylidyne complexes had taken the lead: [13] Complex 4a developed by Schrock and co-workers was the first molecularly welldefined catalyst for alkyne metathesis; [14] it played aquintessential role in deciphering the mechanism of this transformation [15] and empowered early applications, [16] even though these examples also witnessed that the functional group tolerance is limited.…”

“…Such hyper-stabilization of the metallacycle is well known to be the result of an overly Lewis-acidic metal fragment. [20,[37][38][39] In this context, it needs to be emphasized that treatment of an analogous molybdenum canopy complex with 3-hexyne unexpectedly furnished ametallatetrahedrane complex; [11,12] such disparate behavior of tungsten and molybdenum alkylidynes with the same ancillary ligand sphere is striking and unprecedented.…”