Mortreux’alkyne metathesis

Alkyne metathesis is a curious reaction in view of the fact that two alkyne triple bonds are cleaved and reconstructed simultaneously leading to different triple bonds. The first reported effective catalyst is a heterogeneous mixture of tungsten oxide and silica. Then Mortreux found that a catalytic system that consisted of Mo(CO)6 and resorcinol was effective for alkyne metathesis. As reported, the added alkynes come into equilibrium with different product... 

A particularly interesting example is the synthesis of alkyne-bridged oligomers and polymers, which are attractive materials for optical and electronic applications. Bunz and coworkers were able to modify the Mortreux catalyst system [Mo(CO)6 and a suitable phenol]11 and reaction conditions to perform acyclic alkyne metathesis of 1,4-dipropynylated benzenes to produce high-molecular-weight poly(p-... 

Scheme 3. Simple examples of natural products synthesis by alkyne metathesis with the Mortreux catalyst.

Mortreux, A., Petit, F., Blanchard, M. Carbon-13 tracer studies of alkynes metathesis. Tetrahedron Lett. 1978,4967-4968. 

Metathesis of Alkynes. - Alkyne metathesis has been studied far less than has olefin metathesis. Mortreux and co-workers °° demonstrated that the metathesis of alkynes occurs at ambient temperatures with photochemical activation on a Mo(CO)6-3ClPhOH catalyst. The reaction proceeds via a two-step mechanism involving activation of the Mo(CO)6 by the alkyne and subsequent catalysis by the phenol. Heterogenization of the transition metal and... 

Parallel to the development of catalysts for olefin metathesis, the first alkyne metathesis catalysts were W and Mo metal oxides or carbonyls suspended on alumina or silica.65 The first homogeneous catalysts were developed by Mortreux and consisted of a mixture of Mo(CO)6 and substituted phenols.66 It was not until the work of Schrock and his collaborators, however, that a well-defined, isolable alkylidyne catalyst (38) was synthesized, characterized, and shown to catalyze alkyne metathesis.67 Later modifications on 38 included substituting the alkoxy groups with fluorinated analogs, and for the corresponding Mo alkylidynes (39), the fluorinated alkoxy groups are essential for catalytic activity.68... 

Comparison of the Schrock versus the Mortreux-Bunz Alkyne Metathesis Systems... 

The in-sUu formation of alkyne metathesis catalysts from cheap, commercially available Mo(CO)g and 4-chlorophenol, as developed by Mortreux [21-24] and Mori [25], is arguably the most popular method for the formation of hnear polymers by alkyne metathesis polymeriTation. In general, the well-defined W and Mo alkyhdyne... 

Stephens-Castro coupling of 12a generated 14a in 4.6% isolated yield [62]. Almost 30 years later, Bunz reported the preparation of 14b from 13b in 6% yield with the Mortreux catalyst system, clearly demonstrating the potential of alkyne metathesis for the preparation of macrocycles [63]. However, isolation of the desired [6]cycle from linear polymers was reported to be rather tedious. In 2004, the preparation was reported of 14b from 13b in 61% yield with EtC=Mo[N(Ar)(CMe3)]3/4-nitrophenol in 1,2,4-trichlorobenzene at 1 mmHg [59]. 

Coutelier, O. and Mortreux, A. (2006) Terminal alkyne metathesis a fiirther step towards selectivity. Adv. Synth. Catal, 348, 2038-2042. 

The first homogeneous alkyne metathesis has been attributed to Mortreux and Blanchard. They reported the high temperature reaction of 4-methyldiphenylacety-lene in the presence of a catalyst generated from a mixture of Mo(CO) and resorcinol (1,3-dihyroxybenzene) to form a mixture of the starting alkyne, 4,4 -dimethyldiphenylacetylene,... 

J. Chem. Soc., Chem. Commun., 786,1974 A. Mortreux, F. Petit, M. Blanchard - C Tracer Studies of Alkynes Metathesis, Tetrahedron Lett. 49, 4967,1978. 

Although alkyne metathesis is now well developed as a synthetic method, it has been limited to internal alkynes. For this reason, methyl alkynes are often specially prepared to set the stage for C-C triple bond formation by metathesis. Andre Mortreux of ENSC-Lille has now designed (Adv. Synth. Cat. 2007, 349, 2259) a binuclear catalyst 22 that works well with terminal alkynes such as 21. What remains to be determined is the functional group compatibility of 22. 

FIGURE 24.6. Mortreux system and tungsten alkyUdyne complex for the alkyne metathesis. 

Finally, one may note the curious behaviour of alkynes. If Mo(CO)6/non-4-yne is irradiated at room temperature and an excess of 3-chlorophenol then added, there is rapid metathesis to give oct-4-yne and dec-5-yne with nearly 100% selectivity (Mortreux 1977). In contrast, W(CO)6/CCl4//iv (350 ran) induces polymerization of hept-2-yne while causing metathesis of pent-2-ene present in the same reaction mixture (Stockel 1978). The difference in behaviour of the two systems presumably lies in the ability of Mo(CO)6/non-4-yne// v to generate a metal carbyne on addition of the phenol, whereas W(CO)6/CCl4/Av gives only a metal carbene. Photoassisted polymerization of terminal alkynes takes place with WCV/iv in hydrocarbon solutions (Landon 1985) photocatalyzed polymerization of substituted alkynes is induced by W(CO)6/SnCl4// v (Tamura 1994). For related systems, see Ch. 10. 

Mortreux, A. and M. Blanchard. 1974. Metathesis of alkynes by a molybdenum hexacarbonyl-resorcinol catalyst. Chem Commun 786. 

It is believed that the Mortreux system, Mo O a phenol best phenols - 1 hOH, -F3 hOH and -F hOH involves some form of Mo R OAr 3 as well as catalytically active species with the alkylidyne ligand coming from O or the alkyne. 11 24 improvement of such a system with a polyether best efficiency 1, -diphenoxyethane over a bed of molecular sieves leads to metathesis of phenylpropyne at 5 Another improvement of this system is... 

Mortreux A, Blanchard M. Metathesis of alkynes hy a molybdenum hexacarbonyl-resorcinol catalyst. J. Chem. Soc. Chem. Commun. 1974 786-787. 

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