Alkyne metathesis: the making and breaking of carbon-carbon triple bonds

Alkyne metathesis: the making and breaking of carbon-carbon triple bonds

Professor Dr Matthias Tamm from the Technische Universität Braunschweig describes his group’s recent progress in the development of highly active alkyne metathesis catalysts.

Olefin metathesis (alkene metathesis) involves the reversible scission and regeneration of carbon-carbon double bonds in the presence of a suitable catalyst. This fundamentally novel organic reaction was discovered serendipitously more than 60 years ago and is now used extensively in the chemical industry, especially for the development and production of pharmaceuticals and advanced plastic materials.

It was shown by Yves Chauvin that metal complexes with a metal-carbon double bond (‘carbene or alkylidene complexes’) may catalyse the metathesis reaction and that the reaction proceeds through metallacyclobutane intermediates (‘Chauvin mechanism’).

The development of the first well-defined homogeneous catalysts was mainly accomplished by two groups: Richard R Schrock introduced tungsten- and molybdenum-based alkylidene complexes, while Robert H Grubbs used ruthenium alkylidene complexes, which proved to be air-stable and tolerant towards a broad range of functional groups. For these achievements, the 2005 Nobel Prize for chemistry was awarded to these three scientists ‘for the development of the metathesis method in organic synthesis’.1

Development of alkyne metathesis catalysts in the Tamm group

The related metathesis of alkynes, however, represents a significantly less developed synthetic method, although the first catalytic systems – for example mixtures of Mo(CO)6 and phenol additives were introduced by André Mortreux as early as in the mid-1970s.2 This reaction involves the reversible making and breaking of carbon-carbon triple bonds and thus requires catalytically active metal complexes with a metal-carbon triple bond (‘carbyne or alkylidyne complexes’). In analogy to the Chauvin mechanism, four-membered metallacyclobutadiene intermediates are involved (‘Katz mechanism’).

Molybdenum and tungsten alkylidyne complexes that catalyse alkyne metathesis were reported in the 1980s by Richard R Schrock and his coworkers; however, their activity and selectivity could not rival those of the best olefin metathesis catalysts. Hence, our own approach to this field was based on a new design strategy that drew on the structure of the highly active Schrock-type catalysts, and we envisaged that substitution of the dinegative arylimido ligand in these alkylidene complexes by a mononegative imidazolin-2-imido ligand allows the concurrent conversion of the metal-carbon double bond into a triple bond, affording alkylidyne complexes with well-preserved structural and electronic integrity and therefore with potentially undiminished catalytic activity. This strategy proved successful and, in 2007, we were able to report efficient room-temperature alkyne metathesis with well-defined imidazolin-2-iminato tungsten alkylidyne complexes.

An improved synthetic protocol was established in 2010, which gave rise to molybdenum and tungsten benzylidyne complexes from the hexacarbonyl complexes Mo(CO)6 and W(CO)6 (‘low-oxidation state route’).6 This method allows the benzylidyne moiety (ArC≡M) to be varied, and the introduction of bulky aryl groups (Ar) affords catalysts which can even be handled in air.

This protocol also furnished fluoroalkoxide molybdenum and tungsten complexes such as MoF6 and WF3, which are among the first catalyst to efficiently promote the metathesis of terminal alkynes.

The conversion-time diagram (Fig. 4) shows the homocoupling of a representative substrate; the formed acetylene is captured by molecular sieves added to the reaction mixture, which drives the equilibrium reaction to completion. The difference in the optimum degree of fluorination for molybdenum and tungsten in MoF6 and WF3 can be rationalised by the increased intrinsic electrophilicity of tungsten compared to molybdenum.

In collaboration with the group of Christophe Copéret (ETH Zurich), MoF6 and related systems were immobilised on silica surfaces, and the molecular as well as the silica-supported catalysts were used to perform alkyne metathesis at parts-per-million loadings.

Since the silanol HOSi(OtBu)3 can be regarded as a model for SiOH sites present on silica surfaces, it was also used for the preparation of the tungsten alkylidyne complex PhC≡W{OSi(OtBu)3}3, which was not only active as a catalyst in alkyne metathesis, but could also promote the metathesis of conjugated diynes with remarkable selectivity. Diyne cross-metathesis (DYCM) was also possible, providing the opportunity to convert symmetrical into unsymmetrical diynes.

Homogeneous alkyne metathesis catalysts

While the previous section summarised the contributions from our group to the field of catalytic alkyne metathesis, numerous other groups have also significantly contributed to this field in recent decades. First and foremost, it was Richard R Schrock who set the basis for the preparation of high-oxidation state molybdenum and tungsten alkylidyne complexes in the 1980s. Alois Fürstner impressively demonstrated the usefulness of alkyne metathesis as a key step in natural product synthesis, mainly by ring-closing alkyne metathesis (RCAM).

His group developed highly active triphenylsilanolate molybdenum alkylidyne complexes, which can be rendered bench-stable by the addition of ligands such as phenanthroline (phen). The active catalyst is generated from this stabilised form by release of the phen ligand in the presence of metal halides such as MnCl2. This catalyst is probably the most widely used homogeneous alkyne metathesis catalyst to date, and its applications also comprise the metathesis of terminal alkynes and conjugated diynes.

The groups of Jeoffrey S Moore and Wei Zhang have demonstrated the usefulness of alkyne metathesis for applications in supramolecular chemistry, while the groups of Colin Nuckolls and Felix R Fischer provided protocols for the controlled ring-opening alkyne metathesis polymerisation of cycloalkynes.

Over the years, alkyne metathesis has clearly grown into a useful synthetic tool, thereby complementing classical olefin metathesis. It is important to emphasise that alkyne metathesis stands out for its orthogonality to olefins and great potential of post metathesis transformations of the C−C triple bonds to form various structural motifs.

With the growing number of available well-defined alkyne metathesis catalysts and numerous novel applications, alkyne metathesis is clearly ‘on the rise’14 and will become an even more useful tool for the synthesis of organic compounds and materials in the future.

References

1 https://www.nobelprize.org/
2 A. Mortreux, M. Blanchard, J. Chem. Soc. Chem. Commun., 1974, 786
3 T. J. Katz, J. McGinnis, J. Am. Chem. Soc., 1975, 97, 1592
4 R. R. Schrock, Chem. Rev, 2002, 102, 145
5 a) S. Beer, C. G. Hrib, P. G. Jones, K. Brandhorst, J. Grunenberg, M. Tamm, Angew. Chem. Int. Ed., 2007, 46, 8890; b) S. Beer, K. Brandhorst, C. G. Hrib, X. Wu, B. Haberlag, J. Grunenberg, P. G. Jones, M. Tamm, Organometallics, 2009, 28, 1534
6 B. Haberlag, X. Wu, K. Brandhorst, J. Grunenberg, C. G. Daniliuc, P. G. Jones, M. Tamm, Chem. Eur. J., 2010, 16, 8868
7 B. Haberlag, M. Freytag, P. G. Jones, M. Tamm, Adv. Synth. Catal., 2014, 356, 1255
8 B. Haberlag, M. Freytag, C. G. Daniliuc, P. G. Jones, M. Tamm, Angew. Chem. Int. Ed., 2012, 51, 13019
9 C. Bittner, H. Ehrhorn, D. Bockfeld, Kai Brandhorst, M. Tamm, Organometallics, 2017, 36, 3398
10 a) D. P. Estes, C. Bittner, Ò. Àrias, M. Casey, A. Fedorov, M. Tamm, C. Copéret, Angew. Chem. Int. Ed., 2016, 55, 13960; b) D. P. Estes, C. P. Gordon, A. Fedorov, W.-C. Liao, H. Ehrhorn, C. Bittner, M. L. Zier, D. Bockfeld, K. W. Chan, O. Eisenstein, C. Raynaud, M. Tamm, C. Copéret, J. Am. Chem. Soc., 2017, 139, 17597
11 M. Tamm, S. Lysenko, B. Haberlag, C. G. Daniliuc, P. G. Jones, ChemCatChem, 2011, 3, 115-118
12 S. Lysenko, J. Volbeda, P. G. Jones, M. Tamm, Angew. Chem. Int. Ed., 2012, 51, 6757
13 a) S. T. Li, T. Schnabel, S. Lysenko, K. Brandhorst, M. Tamm, Chem. Commun., 2013, 49, 7189; b) T. M. Schnabel, D. Melcher, K. Brandhorst, D. Bockfeld, M. Tamm, Chem. Eur. J., 2018, 9022
14 Review ‘Alkyne metathesis on the rise’: A. Fürstner, Angew. Chem. Int. Ed., 2013, 52, 2794
15 Review ‘Well-Defined Alkyne Metathesis Catalysts: Developments and Recent Applications’ see: H. Ehrhorn, M. Tamm, Chem. Eur. J., 2018, in press (DOI: 10.1002/chem.201804511)

Professor Dr Matthias Tamm
Institute of Inorganic and
Analytical Chemistry
Technische Universität Braunschweig
+49 (0)531 391-5309
m.tamm@tu-bs.de
Tweet @tuBraunschweig

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