Introducing A Podand Motif to Alkyne Metathesis Catalyst Design: A Highly Active Multidentate Molybdenum(VI) Catalyst that Resists Alkyne Polymerization

Abstract

There has been significantly growing interest in recent years in the transition-metal-catalyzed metathesis of alkenes and alkynes.[1] The synthetic potential of alkyne metathesis, however, is much less explored though they have shown enormous potential in the preparation arylene ethynylene polymers, [2] macrocycles, [3] and in natural product synthesis. [1c, 4] Typically, the metal alkylidyne catalysts for alkyne metathesis contain a tungsten or molybdenum-carbon triple bond and alkoxide/amide ligands, [1][2][3][4][5] and their catalytic activity can be tuned by judicious ligand design. [1][2][3][4][5][6][7][8][9] Coordination of small molecules, and in particular 2-butyne (a common metathesis byproduct), to the hexavalent molybdenum alkylidyne complex is known to be an interfering reaction and leads to undesired alkyne polymerization (through the ring-expansion mechanism, which requires two open substrate-binding sites) as well as nonproductive reaction pathways.[10] Polyhedral oligomeric silsesquioxane (POSS) and silica are the only reported ligands to date that can overcome this long-standing problem. [9a, 11] However, the siloxane-based approach lacks tunability in the catalyst structure, thus making it difficult to study the structure-activity relationship of the catalyst and tune its activity. Our present study is aimed at the design of a multidentate organic ligand that can block one substratebinding site of the molybdenum center to inhibit the undesired alkyne polymerization while also keeping the structural tunability for introducing customizable electronwithdrawing substituents to improve both the metathesis activity and functional group tolerance.Taking advantage of the favorable trigonal pyramid geometry of trisubstituted amines, [12] we designed the triphenolamine ligand L1 (Scheme 1) that would allow the effective coordination of the three phenol moieties to molybdenum, with the three methylene units blocking one substrate-binding site of the metal center. The synthesis of the multidentate triphenolamine ligand (L1) was achieved in good yield starting from the corresponding methyl-protected salicylaldehyde followed by reductive amination and deprotection (Scheme 1). A crystal of the complex 1 was obtained from a 1:1 mixture of the molybdenum(VI) propylidyne precursor and L1 using a solvent system comprising nitrobenzene and carbon tetrachloride.[13] The single-crystal X-ray structure analysis showed a phenoxide-bridged dimer of complex 1 with an octahedral coordination geometry around each metal center (Figure 1). Interestingly, the trigonal-pyramidal geometry of the triphenolamine ligand enables the coordination of the central nitrogen to molybdenum, thus efficiently blocking one open binding site of the complex. These interesting features are anticipated to make the catalyst 1 resistant to the interfering alkyne polymerization, and the strong chelating effect of the multidentate ligand should significantly enhance the catalyst stability and its activity.

Multicolor Live-Cell Chemical Imaging by Isotopically Edited Alkyne Vibrational Palette

“…33 Therefore, we switched to a newly engineered podand-supported molybdenum catalyst ( 8 ) with high catalytic activity and stability developed by Zhang and Jyothish. 34 Substrate 5 is still inert to metathesis with 6 in the presence of catalyst 8 at room temperature (entry 3), but at 80 °C the reaction yields an inseparable 2.2:1 mixture of metathesis product and unreacted starting material, along with uncharacterized byproducts (entry 4). Reducing the catalyst loading and the equivalence of 6 fails to yield 5 (entry 5), indicating that a high concentration of alkyne is required to drive the reaction of this relatively sterically encumbered substrate.…”

“…Impressively, EdU– 13 C was prepared via an alkyne cross-metathesis reaction using a podand-supported molybdenum(VI) catalyst. 34 Alkyne vibrational tags can now be employed in three mutually orthogonal versions as reporters for high-resolution, multicolor chemical imaging and subsequent studies on spatial colocalization and functional interactions. This work thus represents important progress toward chemical imaging of complex biological processes in live cells.…”

Synthesis of a conjugated porous Co(ii) porphyrinylene–ethynylene framework through alkyne metathesis and its catalytic activity study

“…[46][47][48] was used as the catalyst in the alkyne metathesis polymerization of M1. The monomer 5,10,15,20-tetrakis(4 0 -propynylphenyl)porphyrin Co(II) 1 (M1) was synthesized in one step from the commercially available 5,10,15,20-tetrakis(4 0 -bromophenyl)porphyrin Co(II) through the Negishi cross-coupling reaction (Scheme 1).…”

Phosphoraneiminato tungsten alkylidyne complexes as highly efficient alkyne metathesis catalysts

“…In particular, significant efforts were made to establish alkylidyne complexes as homogeneous catalysts for applications in alkyne metathesis and to develop catalyst systems that live up to expectations with regard to their activity, substrate compatibility, and required reaction temperature [8]. More recently [9], a number of new tungsten-and molybdenum-based catalysts were introduced that rely on the use of ancillary imidazolin-2-iminato [10,11], silanolate [12,13], or chelating polyphenolate ligands [14]; our own main contribution to this field comprises the preparation of imidazolin-2-iminato neopentylidyne and benzylidyne complexes [RC^M(NIm tBu …”