Alkyne metathesis

As outlined above, another important branch of metathesis is alkyne metathesis, the coupling of two alkynes under the formation of a new alkyne. In analogy to olefin metathesis, alkyne metathesis can be divided into special reaction patterns. Alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM), ring-opening alkyne metathesis polymerization (ROAMP) and acyclic diyne metathesis polymerization (ADIMET) can be applied to form new carbon-carbon triple bonds (Scheme 22).

Scheme 22: Different types of alkyne metathesis reactions. Alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM), ring-opening alkyne metathesis polymerization (ROAMP) and acyclic diyne metathesis (ADIMET).^[6b,6d]

In contrast to olefin metathesis, alkyne metathesis usually is performed with internal alkynes, sometimes leading to non-volatile by-products, which must be removed selectively to ensure complete conversion.^[11e] The Fürstner group proposed the usage of 5 Å molecular sieves to remove small alkynes, such as butyne, from reaction mixtures.^[140] This allowed for higher yields and higher reaction rates.

The analogy between alkyne and olefin metathesis also holds in terms of mechanism. For one, the corresponding reactive species are metal alkylidynes. Furthermore, alkyne metathesis proceeds according to the Katz mechanism (Scheme 23) under formation of metallacyclobutadiene complexes by a [2+2]-cycloaddition.^[11e] Those metallacyclobutadienes then decompose in a [2+2]-cycloreversion step to form (in case of a productive metathesis event) a new metal alkylidyne and a new alkyne.

Scheme 23: Katz mechanism. Alkyne metathesis proceeds via [2+2]-cycloaddition, formation of a metallacyclobutadiene and subsequent [2+2]-cycloreversion. All reactions are reversible, arrows for back-reactions have been omitted for clarity.^[6b,6d]

Importantly, the metallacyclobutadiene must rearrange to release the desired new alkyne. One difference between alkene and alkyne metathesis is, that in alkene metathesis terminal olefins can be converted, whereas this often is a problem in alkyne metathesis. If terminal alkynes are used polymerization can be observed as competitive pathway after a few minutes.^[141]

Terminal alkyne metathesis (TAM) leads to metallacyclobutadienes (Scheme 24) with hydrogen substituents in α- and/or β-position. Elimination of one (basic) X-type ligand and the acidic β-proton in the metallacyclobutadiene under formation of HX results in the key intermediate for alkyne polymerization, a so-called deprotio-metallacycle (Scheme 24, (A)).

Coordination of another alkyne substrate to the deprotio-metallacycle then leads to reduction of W(VI) to W(IV) and to the formation of a metal alkylidene. The metal alkylidene then polymerizes alkynes under formation of unsaturated polymers via metallacyclobutene intermediates.

Scheme 24: Competitive pathways to alkyne metathesis: In the presence of terminal alkynes deprotio-metallacycles can form by elimination of HX (deprotonation) from the complex. Subsequently, under coordination of a further substrate, W(VI) is reduced to W(IV) under formation of an alkylidene. The alkylidene can then polymerize alkynes via metallacyclobutenes.^[141]

Also, cyclotrimerization^[142], resulting from metallacyclobutadiene-ring-expansion, was observed for (mono-)substituted alkynes, when chlorides instead of alkoxides were used as X-type ligands (Scheme 25). Upon cyclotrimerization, reduced cyclopentadienyl complexes of W(IV) are formed. Since, similar to olefin metathesis, undesired reaction pathways seem to lead to reduced W(IV) species, coordination of stabilizing ligands could lead to improved catalyst performance.

Scheme 25: Competitive pathways to alkyne metathesis: Cyclotrimerization under formation of a reduced, substituted cyclopentadienyl W(IV)-complex.^[142]

Comparable to metal alkylidenes, metal alkylidynes can be divided into Schrock and Fischer carbynes.^[143] In Schrock carbynes, the carbyne is in the quartet configuration (three unpaired electrons in the sp-, py- and pz-orbital), whereas in the Fischer carbynes, the carbyne is in the dublett configuration (two paired electrons in the sp- and one electron in an empty p-orbital).

Consequently, the bonding situation in Schrock carbynes can be described by three dative bonds. For Fischer carbynes, σ- donation from the carbyne sp- into empty metal orbitals plus one covalent bond, as well as back donation into the empty carbyne p-orbital must be considered.

Figure 25: (A) Dublett and quartet configuration of carbynes. (B) Bonding situation in Fischer- and Schrock- type carbynes.^[143]

Metathesis-active group 6 metal alkylidyne complexes can be divided into five sub-groups.

[6b-d] On the one hand, ill-defined catalysts based on silica-immobilized metal trioxides^[144] or mixtures of Mo(CO)6 and phenols have been applied. Huge efforts in this field have been made by Mortreux et al. On the other hand, the well-defined catalyst systems can be split into the Schrock-type catalysts and the more recently developed Fürstner- and Tamm-type complexes. The Cummins-Fürstner-Moore systems belong to both groups, the active species

Figure 26: Metathesis-active group 6 metal alkylidynes. Important catalyst systems developed by the Mortreux, Cummins, Fürstner, Schrock and Tamm groups.

As for alkene metathesis, the development of suitable catalyst systems was a process that started with ill-defined, supported transition metal salts. In 1968, Bailey et al. published on the first alkyne disproportionation catalyzed by silica-supported tungsten trioxide.^[144a] Later, in 1972 Mortreux et al. reported that the same was true for molybdenum trioxide supported on silica.^[144b] Then, in 1974, the Mortreux group introduced a catalyst system based on molybdenum hexacarbonyl and phenols (resorcinol, α-napthalin) that successfully catalyzed alkyne metathesis.^[142] Despite its intolerance towards functional groups and the ill-defined active species, this system has been extensively used and empirically optimized through additives by several groups. This demonstrates the lack of a decent library of alkylidyne catalysts in comparison to alkene metathesis catalysts in the early days of metathesis.

Scheme 26: High- and low-oxidation state routes to group 6 metal alkylidynes. (A) High-oxidation state routes including the synthesis of a) neopentylidyne complexes by α-hydrogen abstraction^[145], b) cleavage of alkynes or nitriles with binuclear tungsten hexa-tert-butoxide^[146] and c) exchange of nitride by alkyne ligands.^[147] (B) Low-oxidation state route starting from tungsten hexacarbonyl by oxidative transformation of Br(CO)4M≡R into Br3M≡R(DME) with bromine.^[148]

The well-defined Schrock type systems^[145] usually take up the form M≡R(X)3(DME) and can be prepared via different synthesis routes (Scheme 26). Complexes of the type W≡R(OtBu)3

W≡CtBu(CH2CMe3)3.^[145] Subsequently, the alkylidyne alkyl complex is protonated with HCl in DME to yield W≡CtBuCl3(DME). Different alkoxides can be introduced by reaction with lithium alkoxides.^[145] Disadvantages of this route are the restriction of the alkyne to substituents without β-hydrogen as well as the low yield of W≡CtBu(CH2CMe3)3. Another published route, although restricted to tert-butoxide ligands, is the cleavage of acetylenes by ditungsten hexa-tert-butoxide (Scheme 26, (A), b)).^[146] The same binuclear tungsten complex also enables cleavage of nitriles to form the desired tungsten alkylidyne complex and stoichiometric amounts of tungsten nitride complex (Scheme 26, (A), b)). An interesting expansion of this route by the Hopkins group, overcoming the limitation to the tert-butoxide ligand, is the possibility to convert the received W≡R(OtBu)3 species into W≡R(X)3 complexes by reacting them with BX3 in DME.^[149] This provides access to alkylidyne complexes with modularity in the alkylidyne and alkoxide ligand. In addition, Johnson et al. published on the possibility to introduce alkylidyne ligands to molybdenum nitride complexes by simple conversion with internal alkynes (Scheme 26, (A), c)).^[147] A further, highly modular route to high-oxidation state group 6 metal alkylidynes with respect to accessible alkylidyne ligands was developed by McDermott et al. starting from tungsten hexacarbonyl (Scheme 26, (B)).^[148] This so-called

“low-oxidation state route” relies on low-oxidation state X(CO)4M≡R complexes previously synthesized by Fischer. The key step is the oxidative transformation of X(CO)4M≡R with bromine in DME to afford Br3M≡R(DME) complexes, that can be converted to (OR)3M≡R(DME) catalysts by treatment with the corresponding lithium or potassium alkoxides.

The Fürstner-Cummins-Moore system is based on triamido ligands and profits from a stabilization of the metal center by nitrogen lone-pair donation. The first metathesis-active complexes of this kind were introduced by the Fürstner group.^[150] They cleaved CH2Cl2 with Mo(N(tBu)Ar)3 complexes (first synthesized by Cummins et al. ^[151]), leading to the methylidyne complex Mo≡CH((N(tBu)Ar)3 and the chloro complex MoCl(N(tBu)Ar)3 (Scheme 27).^[150]

Fürstner, counterintuitively, claimed the chloro complex to be the active species, since the methylidyne was shown to be inactive.^[6c] On the other hand, Schrock proposed that the formation of mixed species of the type Mo(CH)(NArR)3-xClx leads to the observed catalyst activity.^[6c] In addition, Moore et al. transferred the synthesis protocol to complexes bearing different alkynes (Mo≡CMe(N(tBu)Ar)3 and Mo≡CEt(N(tBu)Ar)3). Moore et al. significantly improved the synthesis with a newly developed reductive recycling strategy.^[152] The application of magnesium successively lead to complete conversion of Mo(N(tBu)Ar)3 to the desired alkylidyne species by converting the chloride-complex into the educt which then in turn could again be transformed into desired product and by-product (Scheme 27).

Scheme 27: Development of alkyne metathesis catalysts involving triamido ligated molybdenum alkylidynes by the groups Cummins, Fürstner and Moore.^[150-153]

Immobilization on silica resulted in efficient alkyne metathesis catalysts. Also, in situ protonation of amido ligands in Mo≡CEt(N(tBu)Ar)3 with various phenols, especially para-nitro-phenol (Cummins et al.) resulted in active alkyne metathesis catalysts.^[152] Several complexes bearing tridentate triphenolamines were described by Zhang et al.^[153a] Most probably, addition of phenols leads to the partial protonation of the amine ligands and active phenolate complexes. In support of this hypothesis, Cummins et al. published on the synthesis of tris-alkoxide molybdenum alkylidyne complexes from a metalaazaridine hydride complex (Scheme 27, bottom).^[153b] In the last synthesis step, the amido ligands were removed by

The Fürstner group synthesized a library of siloxide-based alkyne metathesis catalysts.^[140,154]

The complexes are either isolated as pentacoordinated neutral complexes of the type M≡R(OSiR´)3.(HOSiR´) (silanol adduct, Scheme 28), M≡R(OSiR´)3.MeCN or in the form of ate complexes of the type [M≡R(OSiR´)4]^-K⁺ (ate-DME and ate-Et2O, Scheme 28). However, in all cases the neutral tetracoordinated M≡R(OSiR´)3 complexes are believed to be the active species. If three equivalents of KOSiPh3 are used, mixtures of the above described species (and additional non-identified products) are formed, whereas with >4 equivalents, the ate complexes are formed exclusively.

Scheme 28: Siloxide-based catalysts developed by Fürstner et al. Synthesis route to and variable structures of siloxide-based catalysts.^[140,154]

Upon addition of bipyridine or phenanthroline the catalysts do not only loose one silanolate ligand but also become air stable (phenanthroline adduct, Scheme 28). Although, as to be expected, inactive in alkyne metathesis, those air stable adducts can easily be activated to form the active M≡R(OSiR´)3 species in situ by addition of ZnCl2. Siloxides are special ligands in terms of steric as well as electronic properties. Whereas the triphenylsiloxide ligand provides enough steric protection to prevent bimolecular decomposition, it does not hinder coordination of substrate (except maybe for extremely bulky ones).

Figure 27: Visualization of orbital interactions and hybridization for a bent (sp³) and a linear (sp) Si-O-Mo bond.^[154]

Siloxides in sum are weaker donors than alkoxides, since pπ→d-donation competes with back donation from the metal d-orbitals into the low-lying Si-C σ*-orbital. Furthermore, they are electronically flexible since donation into the metal d-orbitals is highly dependent on the Mo-O-Si angle Θ. Donation reaches its maximum for Θ = 180° (sp hybridization, Figure 27), since both orthogonal pπ-orbitals can engage in donation to metal d-orbitals. Fürstner postulated, that this angle is not static in solution, but might adapt to the requirements of each intermediate of the catalytic cycle, therefore explaining the high activity and stability of the siloxide based systems.^[154]

Figure 28: Left: Hypothetical alkyne adducts (top) and isolated neutral adducts of proposed active species (bottom).

Metallacyclobutadiene formation requires a cis orientation of the alkyne substrate to the alkylidyne ligand. Right, in

Since neutral adducts with square-pyramidal (MeCN adduct, Figure 28) and trigonal bipyramidal (HOSiPh3 adduct, Figure 28) geometries have been observed, it is still unclear, whether alkyne coordination takes place cis or trans to the alkylidyne in the Fürstner-type systems (Figure 28, left, bottom). Metallacyclobutadiene formation requires a cis orientation of alkyne to alkylidyne (cis alkyne complex, Figure 28). In one case a tetracoordinated, neutral complex with the above proposed structure for the active species with a p-methoxy-benzylidyne ligand was successfully isolated (Figure 28, in dashed frame). This complex is one of the most reactive and selective alkyne metathesis catalysts up to date. Fürstner et al.

furthermore developed a route to complexes of the type M≡N(OSiAr)3 starting from Na2MoO4

in a straightforward three-step procedure, which can be converted into alkylidynes according to the protocol introduced by Johnson et al. (vide supra, (Scheme 26, (A), c)). ^[147]

Figure 29: Design strategy for imidazoline-2-iminato coordinated alkyne metathesis catalysts developed by the Tamm group. Imidazolin-2-iminato ligands as monoanionic analogues to imido ligands in alkene metathesis catalysts and push-pull electronics on the metal center.^[155]

Tamm et al. synthesized alkylidyne complexes of imidazolin-2-iminato ligands (Figure 29).^[155]

They transferred the concept to stabilize high oxidation state metal centers with sterically demanding dianionic ligands from alkene metathesis catalysts (imido ligand) to alkynes (Figure 29). They proposed that the imidazolin-2-iminato ligand presents a monoanionic analogue of an imido ligand. Complexes were received from the conversion of imidazolin-2-iminato ligands with trisalkoxy alkylidyne molybdenum and tungsten complexes under replacement of one alkoxide. Installation of electron-withdrawing alkoxides at the metal center was essential to create a favorable push-pull system (Figure 29). The catalysts were

successfully used in various alkyne metathesis reactions, such as ACM^[156], RCAM^[157] and ROAMP^[158] of cyclooctyne at room temperature. An isolated metallacyclobutadiene complex confirmed that the Katz mechanism was operative.

Scheme 29: First effective catalyst for terminal alkyne metathesis (TAM) and donor ligand promoted formation of deprotio-metallacyclobutadienes.^[159]

The Tamm group also reported on the first catalysts that efficiently promote terminal alkyne metathesis (TAM, Scheme 29) and terminal ring-closing metathesis (TRAM).^[159] The complex is a common molybdenum-based Schrock-type alkylidyne with three hexafluoro-tert-butoxide ligands and a 2,4,6-trimethylbenzylidyne ligand (Scheme 29). The high acidity of the fluorinated alkoxide and its resulting reduced propensity to take up a proton account for the fact that no polymerization of terminal alkynes was observed for this catalyst.^[159]

Polymerization proceeds via formation of deprotio-metallacycles (vide infra, Scheme 24) which is in turn facilitated by basic donor ligands. Lately, they also published on derivates of the successful 2,4,6-benzylidyne complex with longer chain fluorinated alkoxides of molybdenum and tungsten.^[160]

Scheme 30: (A) Products derived from NCAM and (B) catalytic cycles of nitrile alkyne cross-metathesis (NCAM) and competing alkyne cross-metathesis (ACM).^[161]

In 2007 Johnson et al. published on the first catalytic nitrile-alkyne cross-metathesis (NCAM).^[161a] In NCAM, nitriles are converted with internal alkynes to yield all possible alkyne cross nitrile metathesis products. However, competing alkyne cross-metathesis results in low conversion to the desired products (Scheme 30, (A)). Metal nitrides as well as metal alkylidynes are proposed to be the active species (Scheme 30, (B)). Studies showed, that the initially formed unsymmetrical alkyne is not accumulated but instantly consumed, indicating that alkyne metathesis proceeds faster than NACM.^[161] Hence, although NACM presents an interesting new playground for chemists, a lot of progress must be made until it will become applicable for a wide range of substrates.

Figure 30: Stereoselective access to (E)- and (Z)-double bonds by combination of ARCM and stereoselective hydrogenation.^[9a,162]

In many instances, especially in drug synthesis, stereoselective synthesis of (E)- or (Z)-double bonds is crucial. Although (E)- and (Z)-double bonds are now in many cases accessible through stereoselective RCM (MAP-type catalysts, vide supra), an interesting alternative was highlighted by Fürstner and co-workers.^[9a,162] They successfully combined ARCM with stereoselective hydrogenation (Figure 30). Through careful choice of the hydrogenation catalyst both, (E)- and (Z)-isomer, can be gained from the ARCM product. Even though alkynes are often more expensive than alkenes, the described methodology offers the advantage that many alkyne metathesis catalysts don´t attack double bonds, thereby widening the scope to substrates with additional olefinic moieties.