Stoichiometric metathesis reactions

The chemistry of metal carbene and carbyne complexes has undergone very rapid development since the sixties and olefin metathesis reactions now form part of a much larger family of [2 + 2] metathesis reactions between multiply bonded compounds. In Section 4.2 we shall briefly consider this wider context. Some examples will be given of the use of stoichiometric metathesis reactions of such complexes in organic synthesis (Dotz 1984 Schubert 1989). 

In these early studies, however, the concept of c-bond metathesis most probably did not exist, and the results were presented just as observed facts. Mainly in the 1990s, a wide variety of c-bond metathesis reactions of both three- and five-membered zirconacycles were reported. In Scheme 1.4, the reaction of the five-membered zirconacycle with EtMgBr via c-bond metathesis followed by another c-bond metathesis (p-H abstraction) produces the ethylmagnesation product along with ethylene-zirconocene [51], Some representative examples of c-bond metathesis reactions of three-membered zirconacycles are shown in Scheme 1.69. These are examples of stoichiometric c-bond metathesis reactions from which the products have been identified. 

All the metathesis reactions discussed in this section require only catalytic amounts of a carbene complex. The use of stoichiometric quantities of carbene complexes in organic synthesis is limited to cheap metals such as, e.g., titanium. 

Apart from the tandem metathesis/carbonyl o[efination reaction mediated by the Tebbe reagent (Section 3.2.4.2), few examples of the use of stoichiometric amounts of Schrock-type carbene complexes have been reported. A stoichiometric variant of cross metathesis has been described by Takeda in 1998 [634]. Titanium carbene complexes, generated in situ from dithioacetals, Cp2TiCl2, magnesium, and triethylphosphite (see Experimental Procedures 3.2.2 and 3.2.6), were found to undergo stoichiometric cross-metathesis reactions with allylsilanes [634]. The scope of this reaction remains to be explored. 

The second attanpt to synthesize Mg(BH )j has been made in our laboratory by the mechano-chemical activation synthesis (MCAS) (or sohd state metathesis reaction) [175] according to the reaction of (3.37b). The starting powder mixture of as-received NaBH (98% purity) and anhydrous MgCl (99% purity) were mixed together in the 2 1 stoichiometric ratio with the objective of initiating the following metathesis reaction... 

Metal-catalyzed reactions constitute the second major type of reactions in which organolead compounds act as major partners of the reacting systems. The study of these reactions has considerably increased since COMC (1995) review and they can be divided in two subtypes reactions in which the organolead reactant acts as a stoichiometric partner and reactions in which the organolead is only a catalytic species. In this section, only the reactions with stoichiometric organolead will be reviewed, and these reactions are catalyzed by copper, palladium or rhodium species. The second type is the metathesis reactions where the lead compound acts only as a promoter in a complex catalytic system and is reviewed in Section 9.09.4. 

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

Note that there is a preferred direction of reaction with the original complex. The reaction is prevented from developing into a chain reaction by the propensity of the [Ta]=CH2 complex to undergo the homologation reaction (McLain 1977 Schrock 1980 Rocklage 1981) see Abbenhuis (1994) for another example of a tantalum carbene complex which will undergo only stoichiometric metathesis. However, the complexes 2 and 3 (Table 2.2) are good initiators of ROMP as also is the tantalacyclobutane complex derived from 2 by the addition of norbomene. The complexes 1 and 2 are both effective for the metathesis of ds-pent-2-ene. 

Unsupported WO3 is an insulator and does not catalyze the metathesis of propene at 200°C instead it catalyzes the dimerization (Tsuda 1985). Unsupported Nao2sW03 is a conductor and does catalyze the metathesis reaction (Kosaka 1986 Mori, T. 1986). On a series of unsupported non-stoichiometric samples of Na W03 and K WOj (0 maximum activity and selectivity for propene metathesis at 400°C is found at x 0.2, corresponding to the conductor/ insulator transition (Stevenson 1991). However, both metathesis and dimerization occur over conducting Ko,3Mo03 and insulating K0.33M0O3. Below 127°C metathesis is dominant over both oxides. In contrast to Na jW03 there seems to be no correlation between the selectivity (metathesis dimerization) of the reaction and the conductivity of the catalyst (Suzuki, M. 1987). 

The complex W(=CMe)(Cl)(PMe3)4 undergoes stoichiometric metathesis with PhC=CPh but the product PhC=CMe remains coordinated to the metal centre (Atagi 1992). The stoichiometric metathesis between W(=CEt)(OCMe3)3 and MeC=CRuCp(CO)2 has been reported (Latesky 1987) also the metathesis reaction of EtC=CPr with a complex of rhenium containing an Re=Re bond (Diefenbach 1988). 

A microwave-assisted preparation of a series of l-alkyl-3-methylimidazolium halide ILs has been described [17]. The reaction is run in solvent-free conditions with a near-stoichiometric amount of reactants, and the imidazolium halides are obtained in high yield. It is also possible to perform the subsequent metathesis reaction with sodium hexafluorophosphate by means of microwave radiation and then to form the final product in a one-pot reaction [18]. Due to the fact that ILs absorb microwave energy in a very efficient way, they are believed to be well suited for large-scale microwave-assisted synthesis (that is, for reaction mixtures of more than 100 L). 

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). 

Cl3(dme)WCtBu gives not only stoichiometric but also catalytic metathesis reactions with differently substituted carbodiimides or imines (10) ... 

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