Highly Active Multidentate Ligand‐Based Alkyne Metathesis Catalysts

Abstract

Alkyne metathesis catalysts composed of molybdenum(VI) propylidyne and multidentate tris(2-hydroxylbenzyl)methane ligands have been developed, which exhibit excellent stability (remains active in solution for months at room temperature), high activity, and broad functional-group tolerance. The homodimerization and cyclooligomerization of monopropynyl or dipropynyl substrates, including challenging heterocycle substrates (e.g., pyridine), proceed efficiently at 40-55 °C in a closed system. The ligand structure and catalytic activity relationship has been investigated, which shows that the ortho groups of the multidentate phenol ligands are critical to the stability and activity of such a catalyst system.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…[84] More recently,t he Zhang group developed tris(2-hydroxybenzyl)methanel igandsb yr eplacing the silicon atom of the previously reported phenol with ac arbon atom. [85] Catalyst 31 with isopropyl groups in the ortho position of the ligand (R 1 = iPr) displayed excellent catalytic activity towards challenging propynylated benzaldehyde and afforded isolated yields of 98 %( 3mol %c atalyst, molecular sieves,C Cl 4 ,4 0 8C). Interestingly,c omplexes with only weakly electron-withdrawing substituents (R 2 = Br) afforded the highest conversions of o-propynylpyridine reported to date, whereas catalytic activity towards this substrate dropped significantly with catalystsb earing more electron-withdrawing groups (R 2 = NO 2 )o nt he ligands.…”

“…[10,86] Nonetheless,t he complexes were not able to convert terminal alkynes even at 40 8C. [85] Recently,F ürstner and his group synthesized two new multidentate trisilanol ligands (32,33), which they intended to coordinate to molybdenum alkylidyne complexes (Figure 3). [87] Initially,t hey attemptedaligand exchange reactionw ith the tribromo alkylidyne precursor 21.H owever,b ecause of low solubility of the ligand salts this proceduref ailed.…”

“…[153] Development of more robust molybdenuma lkylidyne complexes with multidentate ligands by Zhang and his group enabled access to ab roaderr ange of ethynylenebridged polymers. [83][84][85]154] For instance, complex 29 catalyzed the polymerization of ap orphyrin-based diyne affording a polymer with am olecular weighto fM n = 12 100 and aP DI of 1.5. [83] Additionally,t he Zhang group demonstrated that catalytic ADIMETr epresents ac onvenientt ool for generating polydiacetylenes (PDAs), whichf unction as quasi 1D semiconductors and thus, have been applied as organic conductors, [155] sensory materials [156] or in non-linear optics.…”

Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne Metathesis Catalysts

“…[20,22] To remedy the issue,t he ammonium-, silicon-and carbon-tethered variants of 6 (Z = (NMe) + ,SiMe, CH) were prepared, which do indeed entail higher activities. [23] Forthe lack of the anchoring N-atom, however, more than one active species is generated on reaction with 3; rigorous proof of formation of well-defined catalysts of podand topology is missing for any of these systems. [24][25][26] It is against this backdrop that the current study must be seen.…”

Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne Metathesis Catalysts

“…Although derived podand complexes were isolated and characterized, only the in situ generated mixture showed appreciable catalytic activity. [23] Forthe lack of the anchoring N-atom, however, more than one active species is generated on reaction with 3; rigorous proof of formation of well-defined catalysts of podand topology is missing for any of these systems. [20,22] To remedy the issue,t he ammonium-, silicon-and carbon-tethered variants of 6 (Z = (NMe) + ,SiMe, CH) were prepared, which do indeed entail higher activities.…”