Schrock type carbynes

Fig. 13.14. Schematic representation of the dominant orbital interactions in (a) Fischer-type carbene complexes (b) Schrock-type carbene complexes (c) Fischer-type carbyne complexes using charged closed-shell fragments (d) Fischer-type carbyne complexes using neutral open shell (doublet) fragments (e) Schrock-type carbyne complexes using neutral open-shell (quartet) fragments.

The results of the energy decomposition analyses for the Schrock-type carbyne complexes are given in Table 13.28. The absolute values of the interaction energies are... 

A very active heterogeneous olefin metathesis catalyst was prepared by reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes or with Schrock-type carbyne complexes [L3WsC(Bu )j (L = Cl, O(Bu ), neopentyl).Surface species of the type shown in Scheme 2... 

Encouraged by these results we started to use the Schrock type carbyne... 

Very active heterogeneous metathesis catalysts for metathesis of linear olefins were formed by reactions of the Schrock type carbyne complexes Cl3(dme)W=C Bu (A), ( BuO)3W=C Bu (B) and Np3W=C Bu (C) with silicagel [4], Figure 3. 

Figure 2. Synthesis of heterogeneous highly active metathesis catalysts by reaction of Schrock type carbyne tungsten (VI) complexes with silicagel...

The three Schrock type carbyne complexes A, B and C are also active catalysts for ROMP reactions of heteroatom substituted norbornene derivatives, Table 5. 

The results demonstrate that the Schrock type carbyne tungsten(VI) complexes ... 

The Schrock type carbyne complexes Np3W=C Bu, ( BuO)3W=C Bu and Cl3(dme)WsC Bu are very active catalysts for the polymerization of alkylsubstituted 1-alkynes and also for 1-alkynes with heteroatom containing substituents. The polymerization of disubstituted alkynes was only achieved with Cl3(dme)W=C Bu as catalyst. 

We found that methyl or ethyl substituted diynes give Acyclic Diyne Metathesis Condensation (ADIMET) which is similar to the Acyclic Diene Metathesis (ADMET) of dienes. The catalysts we used for ADIMET reactions are the Schrock type carbyne complex ( BuO)3W=C Bu or W2( BuO)6. For ADIMET reactions we used no or only small amonts of solvent. With higher amounts of solvents Ring Closure Metathesis (RCM) of diynes occured. 

In this paper we report of our work on stoichiometric and catalytic reactions of Fischer and Schrock type carbyne complexes. In the following text the term carbyne will be used also for alykidyne complexes. 

In 1976 Fischer had tested the ROMP reactions of cycloalkanes with carbyne tungsten(O) complexes as catalysts (6). With addition of Lewis acids as cocatalysts the Fischer type carbyne complexes were active in cycloalkane metathesis polymerisation. Fischer type carbyne complexes are also active catalysts for alkyne polymerisations, as found by Katz in 1984 (7). The catalytic reactions of Schrock type carbyne tungsten(VI) or molybdenum(VI) complexes were focussed on alkyne metatheses reactions (8). 

Encouraged by our results on stoichiometric and catalytic metathesis reactions of carbodiimides and imines with Fischer type carbene tungsten(O) complexes (9) we started 1984 with metathesis like reactions of the Schrock type carbyne tungsten (VI) complex Cl3(dme)WCtBu with heteroallenes (isocyanates, carbodiimides, isothiocyanates) and with heteroalkenes (imines and nitroso compounds). 

Following the catalytic metathesis of carbodiimides and isothiocyanates with Cl3(dme)WCtBu, we tested the metatheses of alkenes with this complex. Linear 1-alkenes and monocyclic alkenes (scheme 4) give catalytic metathesis with the Schrock type carbyne complex in CH2CI2 at 20°C (12). At 76 °C and in 1,2 dichloroethane as solvent the turnover frequence enhance (Table 1) (13). 

Many alkene metathesis catalysts are active catalysts for alkyne polymerisation. We tested the polymerisation of 1-alkynes with all 3 Schrock type carbyne complexes. Clo(dme)WCtBu, Np3WCtBu and (tBuO)3WCtBu are active catalysts for 1-alkyne polymerisation. In addition Clo(dme)WCtBu) catalyse the metathesis of internal alkynes and (tBuO)3WCtBu gives alkyne metathesis as shown by Schrock (18). 

An alternative method to make PAEs is the acyclic diyne metathesis (ADIMET) shown in Scheme 2. It is the reaction of a dipropynylarene with Mo(CO)6 and 4-chlorophenol or a similarly acidic phenol. The reaction is performed at elevated temperatures (130-150 °C) and works well for almost any hydrocarbon monomer. The reaction mixture probably forms a Schrock-type molybdenum carbyne intermediate as the active catalyst. Table 5 shows PAEs that have been prepared utilizing ADIMET with these in situ catalysts . Functional groups (with the exception of double bonds) are not well tolerated, but dialkyl PPEs are obtained with a high degree of polymerization. The progress in this field has been documented in several reviews (Table 1, entries 2-4). Recently, a second generation of ADIMET catalyst has been developed that allows... 

The ligand perceptions presented in Fig. 3 concern electronic characteristics. Donors, neutral or anionic ligands, 7t-donor neutral ligands, and so on are perceived. Carbene and carbyne ligands are considered as donor neutral ligands, and a distinction between Schrock -type and Fischer -type car-benes is made (9). 

H. Fischer, The Synthesis of Fischer-Type Carbyne Complexes, in Carbyne Complexes (Eds. H. Fischer, P. Hofmann, F. R. Kreissl, R. R. Schrock, U. Schubert, and K. Weiss, YCH Publishers, Weinheim-New York, 1988, pp. 1-38). 

The reactions of carbyne complexes will be discussed, albeit more briefly, along the lines developed for alkylidene complexes, to emphasize the parallels. Once again, as with alkylidenes, early generalizations suggested two classes of complexes, Fischer-type and Schrock-type, which have subsequently been recognized as extremes in a tunable reactivity continuum. Furthermore, within the chemistry of tungsten, Fischer-type... 

Metal alkylidyne complexes undergo a variety of oxidation and reduction reactions as well as redox-induced transformations of the alkylidyne ligands. A method for the direct transformation of Fischer-type carbyne complexes into Schrock-type alkylidyne complexes was developed in our laboratory. Bromine oxidation of the /ra/7, -carbyne bromo tetracarbonyl complexes 49 of molybdenum and tungsten in the presence of dimethox-yethane affords the dme-stabilized alkylidyne tribromo metal complexes 50 [Eq. (42)] (81). For alkyl-substituted complexes (R = Me, CH2CMe3)... 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

Complexes containing metal-carbon triple bonds in which the metal is in a relatively high formal oxidation state are frequently designated alkylidynes. Carbyne complexes are also sometimes classified as Fischer-type or Schrock-type, as in the case of carbene complexes. The distinction between these is discussed in Chapter 10. 

One route to a carbyne (alkylidyne) complex was reaction 23.40 equation 23.91 illustrates the initial method of Fischer. The abstraction of an ot-H atom from a Schrock-type carbene yields the corresponding carbyne complex (equation 23.92). 

A similar dichotomy of bonding models exists for other classes ofTM compounds where the chemical bond can be discussed either in terms of electron-sharing interactions TM-R or as donor-acceptor bond TM-L. Examples are TM compounds with carbenes CR2 or carbynes CR as ligands, which can be considered as either Fischer-type complexes or as Schrock-type alkylidenes and alkylidynes [31, 32). We want to emphasize that the two bonding models should be considered as sketches of two extreme situations, whereas the electronic situation of real molecules has components of both forms. The value of such dichotomic models lies in the fact that they establish an ordering scheme, which is very useful for describing the physical and chemical properties of molecules. 

A detailed bonding analysis of Fischer-and Schrock-type carbene and carbyne complexes using the EDA approach has heen given in Lein, M., Szaho, A., Kovacs, A., and Frenking, G. (2003) Faraday Discuss., 124, 365. 

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