Metathesis reactions of Re(V) carbyne complexes with functionalized terminal alkynes

Abstract

Alkyne metathesis is a cornerstone reaction in synthetic chemistry. However, metathesis of terminal alkynes remains a rare accomplishment, both catalytically and stoichiometrically. To overcome this challenge, we explored reactions of non-d⁰ carbyne complexes with terminal alkynes. It was found that d² Re(V) carbyne complexes, specifically Re(≡CR)Cl₂(PMePh₂)₃, can undergo stoichiometric metathesis with a range of terminal aryl and aliphatic alkynes (HC≡CR′), yielding substituted carbyne complexes Re(≡CR′)Cl₂(PMePh₂)₃ and HC≡CR. These stoichiometric metathesis reactions are compatible with functional groups such as aldehydes, alcohols, esters, and even unprotected carboxylic acids. Density Functional Theory (DFT) calculations indicate that the formation of substituted carbyne complexes is both thermodynamically and kinetically more favorable than that of methylidyne complexes.

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Introduction

Metathesis of transition metal carbyne (or alkylidyne) complexes with alkynes is a fundamentally important transformation that plays a key role in catalytic alkyne metathesis reactions.¹ The reactivity is now well-documented for internal alkynes.^2–4 On the basis of this reactivity, a library of catalysts have been invented for metathesis reactions of internal alkynes, including those based on well-defined or in situ generated high valent d⁰ W(VI)⁵ and Mo(VI)⁶ carbyne complexes, and d² Re(V)⁷ carbyne complexes. With these catalysts, alkyne metathesis is finding growing practical applications in areas such as organic synthesis,⁸ polymerization^3a,9,10 as well as dynamic covalent chemistry.¹¹

Terminal alkynes are highly attractive substrates for alkyne metathesis reactions due to their common use in organic synthesis and their accessibility compared to internal alkynes, such as methyl-capped alkynes.¹² Despite their potential, catalytic metathesis reactions involving terminal alkynes are rarely reported.¹³ Competent catalysts for these reactions are restricted to a few high-valent d⁰ Mo(VI) and W(VI) carbyne catalysts, for example, Mo(≡CC₆H₄-p-OMe)(OSiPh₃)₃,^{12,14,15,16,17,6l} W(≡CMes){OCMe₂(CF₃)}₃,¹⁸ Mo(≡CMes){OCMe(CF₃)₂}₃,^19–22 and [MesC≡Mo{OSi(OtBu)_3−nPh_n}₃] (n = 1, 2).²³ More often, terminal alkynes polymerize when exposed to a typical high-valent carbyne catalyst.^{1a,6b,24,25,26,27}

To advance the development of new catalysts for terminal alkyne metathesis, it is crucial to identify carbyne complexes that can readily undergo metathesis reactions with terminal alkynes including those with different functional groups. Additionally, understanding of the activity and selectivity of metathesis reactions of carbyne complexes with terminal alkynes is essential. However, these issues remain largely unaddressed. Notably, well-defined stoichiometric metathesis reactions of terminal alkynes are exceedingly scarce. They have only been described for reactions of Mo(≡CtBu){OCMe₂(R)}₃ (R = Me, CF₃) with simple alkynes HC≡CR′ (R′ = nPr, iPr, Ph) to give Mo(≡CR′){OCMe₂(R)}₃,²⁸ and for the slow (in two weeks) reaction of HC≡CtBu with W(≡CMe)Cl(PMe₃)₄ to give W(≡CtBu)(η²-HC≡CMe)Cl(PMe₃)₂.²⁹

We herein report our recent findings that d² Re(V) carbyne complexes, specifically Re(≡CR)Cl₂(PMePh₂)₃, can undergo stoichiometric metathesis with a range of terminal alkynes. For the first time, metathesis reactions of carbyne complexes have been demonstrated for terminal alkynes with functional groups including aldehydes, alcohols, esters, and unprotected carboxylic acids.

Results and discussion

Reactions of Re(≡CCH₂Ph)Cl₂(PMePh₂)₃ with terminal aryl alkynes HC≡CAr

Inspired by the recent discovery that d² Re(V) carbyne complexes can catalyse metathesis of internal alkynes,⁷ we decided to explore their potential in metathesis reactions with terminal alkynes. In this work, we focus on stoichiometric alkyne metathesis reactions of Re(V) carbyne complexes (see Schemes 1–3 below). We began by investigating the reaction of phenylacetylene with Re(≡CCH₂Ph)Cl₂(PMePh₂)₃ (1), a complex that can be easily prepared on a large scale.^4b

Scheme 1 Metathesis reactions of Re(≡CCH₂Ph)Cl₂(PMePh₂)₃ (1) with terminal aryl alkynes HC≡CAr (2a–e) in toluene. Reaction conditions: 4 equiv. of HC≡CAr, 100 °C, 2 h. The isolated yields are given in parenthesis. ^a3a could also be isolated in 83% yield from the reaction with 20 equiv. of HC≡CAr at r.t. for 30 h.

Scheme 2 Metathesis reactions of Re(≡CCH₂Ph)Cl₂(PMePh₂)₃ with terminal alkyl alkynes HC≡CR (4a–d) in toluene. Reaction conditions: 10–20 equiv. of HC≡CR, r.t., 24–30 h or 4 equiv. of HC≡CR, 45–90 °C, 2–5 h. The isolated yields are given in parenthesis for reactions at 45–90 °C, and in bracket for reactions at r.t.

Scheme 3 Metathesis reactions of Re(≡CR)Cl₂(PMePh₂)₃ (R = Ph, (CH₂)₄OH) with 4 equiv. of terminal alkyl alkynes HC≡CR in toluene at 100 °C for 2 h. The ratio refers to molar ratio estimated by in situ NMR.

As monitored by NMR spectroscopy (see Fig. S1†), the complex 1 can undergo metathesis reaction with phenylacetylene in toluene at room temperature. With 20 equivalents of HC≡CPh, the reaction produced the expected metathesis product, Re(≡CPh)Cl₂(PMePh₂)₃ (3a), in approximately 22% yield after 6 hours, 57% after 12 hours, and 86% after 24 hours (Scheme 1). In contrast, no metathesis product was observed for the reaction of 1 with 20 equivalents of the internal alkyne PhC≡CPh even after 48 hours. A higher temperature (e.g., 110 °C) is required for the metathesis reaction of PhC≡CPh to proceed.^4b The observations suggest that terminal alkynes are significantly more reactive than internal alkynes in alkyne metathesis.

As anticipated, the metathesis reaction of 1 with PhC≡CH proceeded at a higher rate at higher temperatures. When the reaction was carried out at 100 °C with four or less equivalents of PhC≡CH, the complex 1 was consumed completely within two hours to give the expected metathesis product Re(≡CPh)Cl₂(PMePh₂)₃ (3a) as the major product. When five or more equivalents of PhC≡CH was used, the reaction at 100 °C produced a mixture of unidentified side products.

More interestingly, complex 1 can also undergo metathesis reactions with functionalized terminal aryl alkynes (Scheme 1). For example, it reacted with the aryl terminal alkynes 2b bearing a CH₂OH group and 2c bearing an aldehyde group to give the corresponding metathesis products 3b and 3c, respectively. The formation of 3b and 3c indicate that the metathesis reaction can tolerate OH and CHO groups. The result is interesting as OH and CHO functional groups are often incompatible or reactive with typical high valent d⁰ carbyne complexes.

Metathesis products were also obtained by treating the complex 1 with terminal alkynes bearing polyaromatic rings. For example, the complexes 3d and 3e were formed from the metathesis reactions of the naphthalene derivative 2d and the pyrene derivative 2e respectively. Notably, complex 3e is a rare example of a metal carbyne complex with a large extended aromatic system.

The metathesis products 3a–e are air-stable solids. The complex 3a is a known compound and has been fully characterized by NMR as well as X-ray diffraction as described previously.^4b The structures of 3b–3e can be readily assigned on the basis of their spectroscopic data. For example, the ³¹P{¹H} NMR spectrum of 3c showed a triplet at −3.2 ppm and a doublet at −12.4 ppm. The ¹³C{¹H} NMR spectrum showed the carbyne signal at 258.8 ppm and that of CHO at 192.2 ppm.

The structure of the complex 3d has also been confirmed by X-ray diffraction. As shown in Fig. 1, it adopts an octahedral geometry with the carbyne ligand trans to one of the chloride ligands. The naphthalene group of the carbyne ligand lies almost in the same plane containing the rhenium metal center and the two chloride atoms. The Re≡C bond distance is 1.773(3) Å, and the Re–C(1)–C(2) angle is 171.2(3)°, which are typical for rhenium carbyne complexes.³⁰ The structural feature of the coordination sphere of 3d is similar to that of 3a.

Fig. 1 The molecular structure of Re(≡C–C₁₀H₆-6-OMe)Cl₂(PMePh₂)₃ (3d). The hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Re(1)–Cl(1) 2.4635(8), Re(1)–Cl(2) 2.5367(8), Re(1)–P(1) 2.4128(8), Re(1)–P(2) 2.4525(8), Re(1)–P(3) 2.4476(8), Re(1)–C(1) 1.773(3), C(1)–C(2) 1.444(5), Cl(1)–Re(1)–Cl(2) 81.55(3), P(3)–Re(1)–P(2) 168.59(3), Re(1)–C(1)–C(2) 171.2(3).

Reactions of Re(≡CCH₂Ph)Cl₂(PMePh₂)₃ with terminal alkyl alkynes HC≡CR

Encouraged by the successful metathesis reactions of 1 with terminal aryl alkynes, we expanded our exploration to include terminal aliphatic alkynes (Scheme 2). As indicated by the in situ ³¹P{¹H} NMR spectrum (see Fig. S2†), the complex Re(≡CCH₂Ph)Cl₂(PMePh₂)₃ (1) reacted with 10 equivalents of 1-hexyne, HC≡C(CH₂)₃Me (4a), in toluene at room temperature to give the expected metathesis product Re(≡C(CH₂)₃Me)Cl₂(PMePh₂)₃ (5a) in yields of ca. 40% in six hours and nearly 65% in 24 hours. The metathesis product Re(≡C(CH₂)₃Me)Cl₂(PMePh₂)₃ (5a) was isolated as a yellow solid. Similarly, the alkyne HC≡C(CH₂)₂CO₂Me (4c), bearing an ester group, reacted with complex 1 to yield the metathesis product Re(≡C(CH₂)₂CO₂Me)Cl₂(PMePh₂)₃ (5c), which was isolated as an orange solid (see Fig. S3†).

The alkynol HC≡C(CH₂)₃CH₂OH (4b) also undergoes a smooth metathesis reaction with the complex 1 to yield Re(≡C(CH₂)₃CH₂OH)Cl₂(PMePh₂)₃ (5b), indicating that the metathesis reaction is compatible with protic functional groups. Most impressively, complex 1 also reacted with the alkyne HC≡C(CH₂)₂CO₂H (4d), bearing a unprotected carboxylic acid group, to afford the metathesis product Re(≡C(CH₂)₂CO₂H)Cl₂(PMePh₂)₃ (5d), which can be isolated as an orange-yellow solid.

The metathesis products 5 are all stable in the solid state and can be stored under ambient condition for months without deterioration. Their structures can be readily assigned on the basis of their spectroscopic data. The structures of 5b–d have also been confirmed by X-ray diffraction. The molecular structure of 5d is presented in Fig. 2, and those of 5b and 5c are given in the ESI (see Fig. S15 and S16†). The structural features of 5b–d in the coordination sphere are similar to that of 1. Their NMR data are fully consistent with the solid-state structures.

Fig. 2 The molecular structure of Re(≡C(CH₂)₂CO₂H)Cl₂(PMePh₂)₃ (5d). The hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Re(1)–C(1) 1.746(2), Re(1)–Cl(1) 2.5074(5), Re(1)–Cl(2) 2.5369(5), Re(1)–P(1) 2.3804(5), Re(1)–P(2) 2.4443(5), Re(1)–P(3) 2.4765(6), C(1)–C(2) 1.488(3), Re(1)–C(1)–C(2) 174.72(17), P(2)–Re(1)–P(3) 161.319(19), Cl(1)–Re(1)–Cl(2) 87.170(17).

In general, the reactivity of aliphatic and aryl terminal alkynes towards complex 1 is similar. Both types of alkynes react to produce the substituted carbyne Re(≡CR′)Cl₂(PMePh₂)₃ as the major product, along with minor unidentified side products that exhibit ³¹P{¹H} signals around 24 ppm. However, in situ NMR experiments indicate that aliphatic terminal alkynes are slightly more reactive than aryl alkynes in the early stages of the reaction. For example, the reaction of PhC≡CH (20 equiv.) with complex 1 yielded 3a in 22% after 6 hours, while the reaction of n-CH₃(CH₂)₃C≡CH (10 equiv.) with complex 1 produced 5a in 38% yield over the same period.

Metathesis reactions of Re(≡CR)Cl₂(PMePh₂)₃ (R = Ph, (CH₂)₄OH)

In addition to complex 1, other complexes of the type Re(≡CR)Cl₂(PMePh₂)₃ could also undergo metathesis reactions with terminal alkynes (Scheme 3 and Fig. S4†). For example, the aryl carbyne complex 3a reacted with excess HC≡CCH₂Ph to give the complex 1, and with HC≡C(CH₂)₂CO₂Me to give the complex 5c (Scheme 3). The alkyl complex 5b reacted with HC≡C(CH₂)₂CO₂Me to give the complex 5c (Scheme 3 and Fig. S4†).

The metathesis reactions of 1 are noteworthy as well-defined stochiometric metathesis reactions of terminal alkynes are rare and have seldom been demonstrated with both high valent and non-d⁰ carbyne complexes. To the best of our knowledge, well-defined metathesis products have been only reported for the reactions of high valent d⁰ Mo(VI) complexes Mo(≡CtBu){OCMe₂(R)}₃ (R = Me, CF₃) with HC≡CR′ (R′ = nPr, iPr, Ph, TMS)²⁵ and the reaction of the d² W(IV) complex W(≡CMe)Cl(PMe₃)₄ with HC≡CtBu.²⁶ The reactions reported here represents the first example of stoichiometric metathesis reactions of carbyne complexes with terminal alkynes with functional groups such as alcohol, ester, aldehydes and unprotected carboxylic acid.

It is more common for carbyne complexes³¹ to react with terminal alkyne to give non-metathesis products. For example, high valent d⁰ carbyne complexes often react with terminal alkynes to give metallacyclobutadienes and deprotiometallacyclobutadienes,³² which can initiate the polymerization of terminal alkynes. Carbyne complexes of types L_nMR(≡CR′) can react with terminal alkynes R′′C≡CH to give carbene complexes L_nM(=CRR′)(η²-R′′C≡CH),³³ or L_nM(≡CR′)(CH=CRR′′).³⁴ Strained carbynes such as osmallapentalynes can undergo [2 + 2] cycloaddition reactions with terminal alkynes to afford metallacyclobutadienes derivatives.³⁵ Reactions of alkynes with CO-containing carbyne complexes can give products derived from coupling of the alkyne with carbyne and CO ligands.³⁶ The square-planar derivatives OsX(≡CPh)(IPr)(PiPr₃) (X = Cl, F) reacted with HC≡CR (R = Ph, CO₂Me) to give carbene complexes Os(C≡CR)Cl(=CHPh)(IPr)(PiPr₃).³⁷

The functional group compatibility of the present metathesis reactions is remarkable, considering the reported reactivity of high-valent d⁰ carbyne complexes. These complexes are known to undergo Wittig-like reactions with compounds bearing a C=O double bond, for example, ketones, aldehydes, esters and even CO₂.³⁸ Additionally, they react acidic HX substrates, such as water and phenols, to form alkylidene complexes or products derived from further protonation of the alkylidene intermediates.³⁹

Theoretical studies on the selectivity of terminal alkyne metathesis reactions

Scheme 4 show a plausible mechanism for the metathesis of complexes Re(≡CR)Cl₂(PMePh₂)₃ (6′) with alkynes HC≡CR′ to give Re(≡CR′)Cl₂(PMePh₂)₃. A complex Re(≡CR)Cl₂(PMePh₂)₃ (6′) could undergo a substitution reaction with HC≡CR′ to give the alkyne–carbyne complex Re(≡CR)(η²-HC≡CR′)Cl₂(PMePh₂)₂ (7′) which could evolve to the metallacyclobutadiene complex 8′(β). Subsequent cyclo-reversion of 8′(β) would produce the alkyne–carbyne complex 9′, which could react with PMePh₂ to give the substituted carbyne complex 6′′. In principle, the alkyne–carbyne complex Re(≡CR)(η²-HC≡CR′)Cl₂(PMePh₂)₂ (7′) could also undergo a cycloaddition reaction to give the isomeric metallacyclobutadiene complex 8′(α), which would evolve to the methylidyne complex Re(≡CH)Cl₂(PMePh₂)₃ (6′c) via the alkyne–carbyne complex intermediate Re(≡CH)(η²-RC≡CR′)Cl₂(PMePh₂)₂ (10′). However, the expected methylidyne complexes were never detected in our experiments.

Scheme 4 Two pathways for the metathesis reactions of Re(≡CR)Cl₂(PMePh₂)₃ with terminal alkynes HC≡CR′.

To understand the selectivity of the metathesis reactions, we have studied the reaction profiles for metathesis reactions of terminal alkynes with model carbyne complexes of the type Re(≡CR)Cl₂(PMe₃)₃. The ligand PMePh₂ was modeled by PMe₃ in the study in order to reduce the computational cost.⁴⁰

Fig. 3 shows profiles for the reaction of the model alkyl carbyne complex Re(≡CMe)Cl₂(PMe₃)₃ (6a) with the terminal aryl alkyne HC≡CPh.⁴¹ The profiles clearly indicate that the reaction leading to the phenylcarbyne complex Re(≡CPh)Cl₂(PMe₃)₃ (6b) and the terminal alkyne HC≡CMe is both thermodynamically (by 6.84 kcal mol⁻¹) and kinetically (by 4.4 kcal mol⁻¹) favored over that to the methylidyne complex Re(≡CH)Cl₂(PMe₃)₃ (6c) and the internal alkyne PhC≡CMe. The formation of phenylcarbyne complex Re(≡CPh)Cl₂(PMe₃)₃ (6b) is thermodynamically favored (by 3.56 kcal mol⁻¹), while that of the methylidyne complex Re(≡CH)Cl₂(PMe₃)₃ (6c) is thermodynamically unfavored by 3.28 kcal mol⁻¹. The reactions of Re(≡CPh)Cl₂(PMe₃)₃ (6b) with HC≡CPh, Re(≡CMe)Cl₂(PMe₃)₃ (6a) with HC≡CMe, and Re(≡CCH₂Ph)Cl₂(PMe₃)₃ (6d) with PhC≡CH show similar profiles (see Fig. S6, S8, and S12†). The computational results are aligning well with our experimental observations that the methylidyne complex Re(≡CH)Cl₂(PMePh₂)₃ was not detected in the reactions Re(≡CR)Cl₂(PMePh₂)₃ with terminal alkynes HC≡CR′.

Fig. 3 The calculated energy profile for the metathesis reactions of the complex Re(≡CMe)Cl₂(PMe₃)₃ (6a) and HC≡CPh. The relative free energies and electronic energies (in parentheses) are given in kcal mol⁻¹.

The thermodynamic preference for forming substituted carbyne complexes (e.g., 6b and 9a) over methylidyne complexes (e.g., 6c and 10a) may be due to the lower stability of methylidyne complexes. As indicated by hydrogenation enthalpy values, internal alkynes are generally more stable than terminal alkynes due to stabilization through hyperconjugation, inductive, and conjugation effects.⁴² The higher stability of substituted carbyne complexes relative to methylidyne complexes can be attributed to similar effects.

In general, the metallacyclobutadienes (the β-isomers, e.g. 8a(β)) that evolve to substituted carbyne complexes were found to be more stable than the isomeric metallacyclobutadienes (the α-isomers, e.g. 8a(α)) that evolve to methylidyne complexes. The relative stability of the isomeric metallacyclobutadiene intermediates can be partially attributed to steric effect. β-Isomers (e.g. 8a(β)) contain a β-H and two substituents on two α-carbons. α-Isomers contain an α-H and two substituents on two adjacent carbons. Thus, α-isomers are sterically less favorable due to the steric repulsion of the neighbouring substituents. In agreement with the hypothesis, difference in the stability between the isomeric metallacyclobutadienes derived from reactions of Re(≡CR)Cl₂(PMe₃)₃ with PhC≡CH is increased from 2.8 for R = Me to 5.3 kcal mol⁻¹ for R = CH₂Ph (see Fig. S10 and S12†).

Conclusion

In summary, we have successfully demonstrated that d² Re(V) carbyne complexes Re(≡CR)Cl₂(PMePh₂)₃ can undergo metathesis reactions with terminal aryl and aliphatic alkynes HC≡CR′ to selectively give substituted carbyne complexes Re(≡CR′)Cl₂(PMePh₂)₃ and HC≡CR. Remarkably, this is the first time that stoichiometric metathesis reactions have been shown to work with terminal alkynes bearing reactive functional groups, including aldehydes, alcohols, esters, and unprotected carboxylic acids. These findings highlight the potential for further exploration of non-d⁰ carbyne complexes in the development of catalysts for terminal alkyne metathesis.

Author contributions

G. J. conceived the project and supervised the findings of this work. B. P. and W. B. carried out the syntheses and characterizations. L. C. K. performed the computations. H. H. Y. S. and I. D. W. performed the XRD. G. J., P. B., W.B. wrote the manuscript and all authors contributed to the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

Crystallographic data for 3d (CCDC no. 2413226), 5b (CCDC no. 2413480), 5c (CCDC no. 2413479), and 5d (CCDC no. 2413227) have been deposited at The Cambridge Crystallographic Data Centre, and can be obtained from https://www.ccdc.cam.ac.uk/structures/.†

Acknowledgements

This work was supported by the Hong Kong Research Grants Council (Project No.: 16308721, 16302322, 16305724, 16300023). We acknowledge the support from HKUST Central High Performance Computing Cluster for providing computational resources.

References

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4. Examples of recent work with non-d⁰ carbyne complexes: (a) W. Bai, K. H. Lee, H. H. Y. Sung, I. D. Williams, Z. Lin and G. Jia, Alkyne Metathesis Reactions of Rhenium(V) Carbyne Complexes, Organometallics, 2016, 35, 3808–3815; (b) W. Bai, W. Wei, H. H. Y. Sung, I. D. Williams, Z. Lin and G. Jia, Syntheses of Re(V) Alkylidyne Complexes and Ligand Effect on the Reactivity of Re(V) Alkylidyne Complexes toward Alkynes, Organometallics, 2018, 37, 559–569; (c) L. Y. Tsang, L. C. Kong, H. H. Y. Sung, I. D. Williams and G. Jia, Synthesis and Alkyne Metathesis Activity of Pincer Rhenium Carbyne Complexes, Organometallics, 2024, 43, 849–858; (d) M. Tomasini, M. Gimferrer, L. Caporaso and A. Poater, Rhenium Alkyne Catalysis: Sterics Control the Reactivity, Inorg. Chem., 2024, 63, 5842–5851.
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6. Selected pioneering works: (a) L. G. McCullough and R. R. Schrock, Metathesis of Acetylenes by Molybdenum(VI) Alkylidyne Complexes, J. Am. Chem. Soc., 1984, 106, 4067–4068; (b) L. McCullough, R. Schrock, J. Dewan and J. Murdzek, Preparation of Trialkoxymolybdenum(VI) Alkylidyne Complexes, Their Reactions with Acetylenes, and the X-ray Structure of Mo[C₃(CMe₃)₂][OCH(CF₃)₂]₂(C₅H₅N)₂, J. Am. Chem. Soc., 1985, 107, 5987–5998 More recent examples: (c) S. Lysenko, J. Volbeda, P. G. Jones and M. Tamm, Catalytic Metathesis of Conjugated Diynes, Angew. Chem., Int. Ed., 2012, 51, 6757–6761; (d) J. Heppekausen, R. Stade, R. Goddard and A. Fürstner, Practical New Silyloxy-Based Alkyne Metathesis Catalysts with Optimized Activity and Selectivity Profiles, J. Am. Chem. Soc., 2010, 132, 11045–11057; (e) R. R. Thompson, M. E. Rotella, P. Du, X. Zhou, F. R. Fronczek, R. Kumar, O. Gutierrez and S. Lee, Siloxide Podand Ligand as a Scaffold for Molybdenum-Catalyzed Alkyne Metathesis and Isolation of a Dynamic Metallatetrahedrane Intermediate, Organometallics, 2019, 38, 4054–4059; (f) J. Hillenbrand, M. Leutzsch and A. Fürstner, Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne Metathesis Catalysts, Angew. Chem., Int. Ed., 2019, 58, 15690–15696; (g) J. Hillenbrand, M. Leutzsch, E. Yiannakas, C. P. Gordon, C. Wille, N. Nöthling, C. Copéret and A. Fürstner, “Canopy Catalysts” for Alkyne Metathesis: Molybdenum Alkylidyne Complexes with a Tripodal Ligand Framework, J. Am. Chem. Soc., 2020, 142, 11279–11294; (h) Y. Ge, S. Huang, Y. Hu, L. Zhang, L. He, S. Krajewski, M. Ortiz, Y. Jin and W. Zhang, Highly active alkyne metathesis catalysts operating under open air condition, Nat. Commun., 2021, 12, 1136; (i) J. Groos, P. M. Hauser, M. Koy, W. Frey and M. R. Buchmeiser, Highly Reactive Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Catalysts for Alkyne Metathesis, Organometallics, 2021, 40, 1178–1184; (j) Z. J. Berkson, L. Lätsch, J. Hillenbrand, A. Fürstner and C. Copéret, Classifying and Understanding the Reactivities of Mo-Based Alkyne Metathesis Catalysts from ⁹⁵Mo NMR Chemical Shift Descriptors, J. Am. Chem. Soc., 2022, 144, 15020–15025; (k) J. Hillenbrand, J. N. Korber, M. Leutzsch, N. Nöthling and A. Fürstner, Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap, Chem. – Eur. J., 2021, 27, 14025–14033; (l) J. Korber, C. Wille, M. Leutzsch and A. Fürstner, From the Glovebox to the Benchtop: Air-Stable High Performance Molybdenum Alkylidyne Catalysts for Alkyne Metathesis, J. Am. Chem. Soc., 2023, 145, 26993–27009.
7. (a) M. Cui, W. Bai, H. H. Y. Sung, I. D. Williams and G. Jia, Robust Alkyne Metathesis Catalyzed by Air Stable d² Re(V) Alkylidyne Complexes, J. Am. Chem. Soc., 2020, 142, 13339–13344; (b) M. Cui, H. H. Y. Sung, I. D. Williams and G. Jia, Alkyne Metathesis with d² Re(V) Alkylidyne Complexes Supported by Phosphino-Phenolates: Ligand Effect on Catalytic Activity and Applications in Ring-Closing Alkyne Metathesis, J. Am. Chem. Soc., 2022, 144, 6349–6360; (c) M. Cui, J. Huang, L. Y. Tsang, H. H. Y. Sung, I. D. Williams and G. Jia, Exploring efficient and air-stable d² Re(V) alkylidyne catalysts: toward room temperature alkyne metathesis, Chem. Sci., 2024, 15, 18318–18326.
8. (a) J. L. Sutro and A. Fürstner, Total Synthesis of the Allenic Macrolide (+)-Archangiumide, J. Am. Chem. Soc., 2024, 146, 2345–2350; (b) T. Varlet, S. Portmann and A. Fürstner, Total Synthesis of Njaoamine C by Concurrent Macrocycle Formation, J. Am. Chem. Soc., 2023, 145, 21197–21202; (c) J. Tang, W. Li, T. Y. Chiu, F. Martinez-Pena, Z. Luo, C. T. Chong, Q. Wei, N. Gazaniga, T. J. West, Y. Y. See, L. L. Lairson, C. G. Parker and P. S. Baran, Synthesis of portimines reveals the basis of their anti-cancer activity, Nature, 2023, 622, 507–513; (d) Z. Meng, S. M. Spohr, S. Tobegen, C. Farès and A. Fürstner, A Unified Approach to Polycyclic Alkaloids of the Ingenamine Estate: Total Syntheses of Keramaphidin B, Ingenamine, and Nominal Njaoamine I, J. Am. Chem. Soc., 2021, 143, 14402–14414; (e) E. Yiannakas, M. I. Grimes, J. T. Whitelegge, A. Fürstner and A. N. Hulme, An Alkyne-Metathesis-Based Approach to the Synthesis of the Anti-Malarial Macrodiolide Samroiyotmycin A, Angew. Chem., Int. Ed., 2021, 60, 18504–18508; (f) A. G. Dalling, G. Späth and A. Fürstner, Total Synthesis of the Tetracyclic Pyridinium Alkaloid epi-Tetradehydrohalicyclamine B, Angew. Chem., Int. Ed., 2022, 61, e202209651; (g) K. Yahata and A. Fürstner, Total Synthesis of the Guangnanmycin A Alcohol, Angew. Chem., Int. Ed., 2024, 63, e202319070; (h) D. Isak, L. A. Schwartz, S. Schulthoff, G. Perez-Moreno, C. Bosch-Navarrete, D. Gonzalez-Pacanowska and A. Fürstner, Collective and Diverted Total Synthesis of the Strasseriolides: A Family of Macrolides Endowed with Potent Antiplasmodial and Antitrypanosomal Activity, Angew. Chem., Int. Ed., 2024, 63, e202408725; (i) F. Schmidt, A. Viswanathan Ammanath, F. Gotz and M. E. Maier, Synthesis of Berkeleylactone A by Ring-Closing Alkyne Metathesis, Eur. J. Org. Chem., 2023, e202300615.
9. For reviews, see: (a) H. Yang, Y. Jin, Y. Du and W. Zhang, Application of alkyne metathesis in polymer synthesis, J. Mater. Chem. A, 2014, 2, 5986–5993; (b) U. H. F. Bunz, Poly(p-phenyleneethynylene) by Alkyne Metathesis, Acc. Chem. Res., 2001, 34, 998–1010; (c) M. Ortiz, C. Yu, Y. Jin and W. Zhang, Poly(aryleneethynylene)s: Properties, Applications and Synthesis Through Alkyne Metathesis, Top. Curr. Chem., 2017, 375, 73–96.
10. Examples of recent work: (a) S. von Kugelgen, I. Piskun, J. H. Griffin, C. T. Eckdahl, N. N. Jarenwattananon and F. R. Fischer, Templated Synthesis of End-Functionalized Graphene Nanoribbons through Living Ring-Opening Alkyne Metathesis Polymerization, J. Am. Chem. Soc., 2019, 141, 11050–11058; (b) S. von Kugelgen, R. Sifri, D. Bellone and F. R. Fischer, Regioselective Carbyne Transfer to Ring-Opening Alkyne Metathesis Initiators Gives Access to Telechelic Polymers, J. Am. Chem. Soc., 2017, 139, 7577–7585; (c) H. Jeong, S. von Kugelgen, D. Bellone and F. R. Fischer, Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization, J. Am. Chem. Soc., 2017, 139, 15509–15514.
11. (a) S. Huang, Z. Lei, Y. Jin and W. Zhang, By-design molecular architectures via alkyne metathesis, Chem. Sci., 2021, 12, 9591–9606; (b) Y. Hu, C. Wu, Q. Pan, Y. Jin, R. Lyu, V. Martinez, S. Huang, J. Wu, L. J. Wayment, N. A. Clark, M. B. Raschke, Y. Zhao and W. Zhang, RETRACTED ARTICLE: Synthesis of γ-graphyne using dynamic covalent chemistry, Nat. Synth., 2022, 1, 449–454; (c) A. J. Greenlee, H. Chen, C. I. Wendell and J. S. Moore, Tandem Imine Formation and Alkyne Metathesis Enabled by Catalyst Choice, J. Org. Chem., 2022, 87, 8429–8436; (d) X. Jiang, J. D. Laffoon, D. Chen, S. Perez-Estrada, A. S. Danis, J. Rodriguez-Lopez, M. A. Garcia-Garibay, J. Zhu and J. S. Moore, Kinetic Control in the Synthesis of a Möbius Tris((ethynyl)[5]helicene) Macrocycle Using Alkyne Metathesis, J. Am. Chem. Soc., 2020, 142, 6493–6498; (e) S. Huang, J. Y. Choi, Q. Xu, Y. Jin, J. Park and W. Zhang, Carbazolylene-Ethynylene Macrocycle based Conductive Covalent Organic Frameworks, Angew. Chem., Int. Ed., 2023, 62, e202303538.
12. R. Lhermet and A. Fürstner, Cross-Metathesis of Terminal Alkynes, Chem. – Eur. J., 2014, 20, 13188–13193.
13. For early work: (a) O. Coutelier and A. Mortreux, Terminal Alkyne Metathesis: A Further Step Towards Selectivity, Adv. Synth. Catal., 2006, 348, 2038–2042; (b) O. Coutelier, G. Nowogrocki, J.-F. Paul and A. Mortreux, Selective Terminal Alkyne Metathesis: Synthesis and Use of a Unique Triple Bonded Dinuclear Tungsten Alkoxy Complex Containing a Hemilabile Ligand, Adv. Synth. Catal., 2007, 349, 2259–2263.
14. P. Persich, J. Llaveria, R. Lhermet, T. de Haro, R. Stade, A. Kondoh and A. Fürstner, Increasing the Structural Span of Alkyne Metathesis, Chem. – Eur. J., 2013, 19, 13047–13058.
15. F. Ungeheuer and A. Fürstner, Concise Total Synthesis of Ivorenolide B, Chem. – Eur. J., 2015, 21, 11387–11392.
16. J. Willwacher and A. Fürstner, Catalysis-Based Total Synthesis of Putative Mandelalide A, Angew. Chem., Int. Ed., 2014, 53, 4217–4221.
17. J. Willwacher, B. Heggen, C. Wirtz, W. Thiel and A. Fürstner, Total Synthesis, Stereochemical Revision, and Biological Reassessment of Mandelalide A: Chemical Mimicry of Intrafamily Relationships, Chem. – Eur. J., 2015, 21, 10416–10430.
18. C. Bittner, H. Ehrhorn, D. Bockfeld, K. Brandhorst and M. Tamm, Tuning the Catalytic Alkyne Metathesis Activity of Molybdenum and Tungsten 2,4,6-Trimethylbenzylidyne Complexes with Fluoroalkoxide Ligands OC(CF₃)_nMe_3−n (n = 0–3), Organometallics, 2017, 36, 3398–3406.
19. B. Haberlag, M. Freytag, C. G. Daniliuc, P. G. Jones and M. Tamm, Efficient Metathesis of Terminal Alkynes, Angew. Chem., Int. Ed., 2012, 51, 13019–13022.
20. D. P. Estes, C. Bittner, Ò. Àrias, M. Casey, A. Fedorov, M. Tamm and C. Copéret, Alkyne Metathesis with Silica-Supported and Molecular Catalysts at Parts-per-Million Loadings, Angew. Chem., Int. Ed., 2016, 55, 13960–13964.
21. C. Bittner, D. Bockfeld and M. Tamm, Formation of alkyne-bridged ferrocenophanes using ring-closing alkyne metathesis on 1,1′-diacetylenic ferrocenes, J. Org. Chem., 2019, 15, 2534–2543.
22. S. Hötling, C. Bittner, M. Tamm, S. Dähn, J. Collatz, J. L. M. Steidle and S. Schulz, Identification of a Grain Beetle Macrolide Pheromone and Its Synthesis by Ring-Closing Metathesis Using a Terminal Alkyne, Org. Lett., 2015, 17, 5004–5007.
23. A. Neitzel, T. Ludwig, D. Bockfeld, T. Bannenberg and M. Tamm, Importance of Ligand Fine-Tuning: Alkyne Metathesis with Molybdenum Alkylidyne Complexes Supported by Phenyl-tert-butoxysilanolate Ligand, Organometallics, 2025, 44, 255–267.
24. A. Mortreux, F. Petit, M. Petit and T. Szymanska-Buzar, Reactions of W(CCMe₃) (OCMe₃)₃ with terminal alkynes: metathesis and polymerization, J. Mol. Catal. A: Chem., 1995, 96, 95–105.
25. H. Strutz, J. C. Dewan and R. R. Schrock, Reaction of Mo(CCMe₃)[OCH(CF₃)₂]₃(dimethoxyethane) with tert-butylacetylene, an aborted acetylene polymerization, J. Am. Chem. Soc., 1985, 107, 5999–6005.
26. K. Weiss, R. Goller and G. Loessel, Alkene metathesis and alkyne polymerization with the carbyne complex Cl₃(dme)W≡CCMe₃, J. Mol. Catal., 1998, 46, 267–275.
27. A. Bray, A. Mortreux, F. Petit, M. Petit and T. Szymanska-Buzar, Metathesis vs. polymerization of terminal acetylenes over [W(CBu-tert)(OBu-tert)₃], J. Chem. Soc., Chem. Commun., 1993, 197–199.
28. L. McCullough, R. R. Schrock, J. Dewan and J. Murdzek, Preparation of Trialkoxymolybdenum(VI) Alkylidyne Complexes, Their Reactions with Acetylenes, and the X-ray Structure of Mo[C₃(CMe₃)₂][OCH(CF₃)₂]₂(C₅H₅N)₂, J. Am. Chem. Soc., 1985, 107, 5987–5998.
29. (a) L. M. Atagi, S. C. Critchlow and J. M. Mayer, Reactivity of the Tungsten Carbyne W(≡CCH₃)Cl(PMe₃)₄: Double Carbonylation, Carbyne-Alkyne Complexes, and Stoichiometric Acetylene Metathesis, J. Am. Chem. Soc., 1992, 114, 9223–9224; (b) L. M. Atagi and J. M. Mayer, Reactions of the Tungsten—Carbyne Complex W(=CMe)Cl(PMe₃)₄ with pi-Acceptor Ligands: Carbon Monoxide, Alkynes, and Alkenes, Organometallics, 1994, 13, 4794–4803.
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32. (a) M. G. Jafari, J. B. Russell, H. Lee, B. Pudasaini, D. Pal, Z. Miao, M. R. Gau, P. J. Carroll, B. S. Sumerlin and A. S. Veige, Vanadium Alkylidyne Initiated Cyclic Polymer Synthesis: The Importance of a Deprotiovanadacyclobutadiene Moiety, J. Am. Chem. Soc., 2024, 146, 2997–3009; (b) H. Ehrhorn, D. Bockfeld, M. Freytag, T. Bannenberg and C. E. Matthias, Studies on Molybdena- and Tungstenacyclobutadiene Complexes Supported by Fluoroalkoxy Ligands as Intermediates of Alkyne Metathesis, Organometallics, 2019, 38, 1627–1639; (c) R. R. Schrock, High-Oxidation-State Molybdenum and Tungsten Alkylidyne Complexes, Acc. Chem. Res., 1986, 19, 342–348.
33. (a) V. K. Jakhar, Y.-H. Shen, S.-M. Hyun, A. M. Esper, I. Ghiviriga, K. A. Abboud, D. W. Lester and A. S. Veige, Improved Trianionic Pincer Ligand Synthesis for Cyclic Polymer Catalysts, Organometallics, 2023, 42, 1339–1346; (b) C. D. Roland, T. Zhang, S. VenkatRamani, I. Ghiviriga and A. S. Veige, A catalytically relevant intermediate in the synthesis of cyclic polymers from alkynes, Chem. Commun., 2019, 55, 13697–13700; (c) C. D. Roland, S. VenkatRamani, V. K. Jakhar, I. Ghiviriga, K. A. Abboud and A. S. Veige, Synthesis and Characterization of a Molybdenum Alkylidyne Supported by a Trianionic OCO³⁻ Pincer Ligand, Organometallics, 2018, 37, 4500–4505.
34. (a) J. Chen, Z. Huang, Y. Hua, H. Zhang and H. Xia, Synthesis of Five-Membered Osmacycles with Osmium–Vinyl Bonds from Hydrido Alkenylcarbyne Complexes, Organometallics, 2015, 34, 340–347; (b) R. Castro-Rodrigo, M. A. Esteruelas, A. M. López and E. Oñate, Reactions of a Dihydrogen Complex with Terminal Alkynes: Formation of Osmium−Carbyne and −Carbene Derivatives with the Hydridotris(pyrazolyl)borate Ligand, Organometallics, 2008, 27, 3547–3555.
35. (a) C. Zhu, Y. Yang, M. Luo, C. Yang, J. Wu, L. Chen, G. Liu, T. Wen, J. Zhu and H. Xia, Stabilizing Two Classical Antiaromatic Frameworks: Demonstration of Photoacoustic Imaging and the Photothermal Effect in Metalla-aromatics, Angew. Chem., Int. Ed., 2015, 54, 6181–6185; (b) K. Zhuo, Y. Liu, K. Ruan, Y. Hua, Y.-M. Lin and H. Xia, Ring contraction of metallacyclobutadiene to metallacyclopropene driven by π- and σ-aromaticity relay, Nat. Synth., 2023, 2, 67–75; (c) C. Zhu, J. Zhu, X. Zhou, Q. Zhu, Y. Yang, T. B. Wen and H. Xia, Isolation of an Eleven-Atom Polydentate Carbon-Chain Chelate Obtained by Cycloaddition of a Cyclic Osmium Carbyne with an Alkyne, Angew. Chem., Int. Ed., 2018, 57, 3154–3157; (d) Y. Cai, Y. Hua, Z. Lu, J. Chen, D. Chen and H. Xia, Metallacyclobutadienes: Intramolecular Rearrangement from Kinetic to Thermodynamic Isomers, Adv. Sci., 2024, 11, 2403940; (e) Z. Lu, C. Zhu, Y. Cai, J. Zhu, Y. Hua, Z. Chen, J. Chen and H. Xia, Metallapentalenofurans and Lactone-Fused Metallapentalynes, Chem. – Eur. J., 2017, 23, 6426–6431.
36. (a) A. Mayr, K. S. Lee and B. Kahr, Linkage of Alkylidyne, Carbonyl, and Alkyne Ligands at a Tungsten Center to Form a Metallacyclopentadienone and an η³-Oxocyclobutenyl Complex, Angew. Chem., Int. Ed. Engl., 1988, 27, 1730–1731; (b) M. Sivavec and T. J. Katz, Synthesis of phenols from metal-carbynes and diynes, Tetrahedron Lett., 1985, 26, 2159–2162.
37. M. L. Buil, J. J. F. Cardo, M. A. Esteruelas and E. Oñate, Square-Planar Alkylidyne–Osmium and Five-Coordinate Alkylidene–Osmium Complexes: Controlling the Transformation from Hydride-Alkylidyne to Alkylidene, J. Am. Chem. Soc., 2016, 138, 9720–9728.
38. See for example: (a) J. H. Freudenberger and R. R. Schrock, Wittig-Like Reactions of Tungsten Alkylidyne Complexes, Organometallics, 1986, 5, 398–400; (b) T. Kurogi, B. Pinter and D. J. Mindiola, Methylidyne Transfer Reactions with Niobium, Organometallics, 2018, 37, 3385–3388; (c) S. A. Gonsales, M. E. Pascualini, I. Ghiviriga, K. A. Abboud and A. S. Veige, Fast “Wittig-Like” Reactions As a Consequence of the Inorganic Enamine Effect, J. Am. Chem. Soc., 2015, 137, 4840–4845; (d) V. Jakhar, D. Pal, K. A. Abboud, D. W. Lester, B. S. Sumerlin and A. S. Veige, Tethered Tungsten-Alkylidenes for the Synthesis of Cyclic Polynorbornene via Ring Expansion Metathesis: Unprecedented Stereoselectivity and Trapping of Key Catalytic Intermediates, J. Am. Chem. Soc., 2021, 143, 1235–1246.
39. See for example: (a) J. H. Freudenberger and R. R. Schrock, Preparation of Di-tert-butoxytungsten(VI) Alkylidene Complexes by Protonation of Tri-tert-butoxytungsten(Vl) Alkylidene Complexes, Organometallics, 1985, 4, 1937–1944; (b) F. Zhai, K. V. Bukhryakov, R. R. Schrock, A. H. Hoveyda, C. Tsay and P. Müller, Syntheses of Molybdenum Oxo Benzylidene Complexes, J. Am. Chem. Soc., 2018, 140, 13609–13613; (c) J. Hillenbrand, M. Leutzsch, C. P. Gordon, C. Copéret and A. Fürstner, ¹⁸³W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis, Angew. Chem., Int. Ed., 2020, 59, 21758–21768; (d) P. M. Hauser, J. V. Musso, W. Frey and M. R. Buchmeiser, Cationic Tungsten Oxo Alkylidene N-Heterocyclic Carbene Complexes via Hydrolysis of Cationic Alkylidyne Progenitors, Organometallics, 2021, 40, 927–937.
40. As reported in ref. 4b, when PMePh₂ is modelled by PMe₃, the substitution of the phosphine with an alkyne to form an alkyne–alkylidyne intermediate becomes appreciably more difficult. However, the profile for the cycloaddition reactions of alkyne–alkylidyne intermediates is less influenced.
41. The selectivity of the metathesis reaction is not affected by the ligand substitution steps. Therefore, the calculated profiles for the ligand substitution steps are omitted in Fig. 3 for clarity. The details can be found in the ESI.†.
42. (a) D. W. Rogers, O. A. Dagdagan and N. L. Allinger, Heats of Hydrogenation and Formation of Linear Alkynes and a Molecular Mechanics Interpretation, J. Am. Chem. Soc., 1979, 101, 671–676; (b) D. W. Rogers, A. A. Zavitsas and N. Matsunaga, G3(MP2) Enthalpies of Hydrogenation, Isomerization, and Formation of Extended Linear Polyacetylenes, J. Phys. Chem. A, 2005, 109, 9169–9173.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2413226 (3d), 2413480 (5b), 2413479 (5c) and 2413227 (5d). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi01112d

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