Trans-Metathesis

The preferences of the various pathways are dependent on the catalyst used, specifically the electronic and steric factors involved. The electronic contribution is based on the preference of the metallacycle to have the electron-donating alkyl groups at either the a or the carbon of ftie metallacycle [23]. The steric factors involved in the approach of the olefin to the metal carbene also determine the re-giochemistry of the metallacyclobutane formed. These factors include both steric repulsion of the olefin and carbene substituents from each other and from the ancillary ligands of the metal complex. Paths (b), (c), and (e) in Scheme 6.10 are important to productive ADMET. The relative rates of pathways (c) and (e) will determine the kinetic amount of cis and trans double bonds in the polymer chain. Flowever, in some cases a more thermodynamic ratio of cis to trans olefin isomers is attained after long reaction times, presumably by a trans-metathesis olefin equilibration mechanism [31] (Scheme 6.11). 

Both of these complexes can be used in ADMET polymerizations at temperatures up to approximately 55 °C, although decomposition certainly occurs over the time scale of a typical ADMET polymerization (days). A structure-reactivity study was performed on complexes 1 and 2 that revealed a number of features of these complexes [68]. Notably, 2 will polymerize dienes containing a terminal and a 1,1-disubstituted olefin, but never produces a tetrasubstituted olefin. One of the substituents of the 1,1-disubstituted olefin must be a methyl group. In contrast, complex 1 will not react with a 1,1-disubstituted olefin. The tungsten complex is more reactive towards internal olefins than external olefins [23, 63] indicating that secondary metathesis, or trans-metathesis, probably dominates the catalytic turnovers in ADMET with complex 1. 

The trans microstructure is not only a reflection of the kinetic product rationalized by the metallacyclobutane conformation and approach of the olefin to the metal carbene, but is also a reflection of the eventual thermodynamic preference for trans olefins resulting from trans-metathesis olefin equihbration (see Scheme 6.11). 

Another interesting aspect of ADMET polymerization has to do with the nature of the active chain end which is responsible for step polymerization. Once monomer concentration is sufficiently depleted, then other possible reactions can occur with respect to the chain end. One possibility is trans-metathesis which in essence is similar to exchange reactions that are found in classical step polymerizations. For example, trans-esterification in the formation of polyester naturally occurs upon depletion of monomer concentration. The same is true for polyamide chemistry. The result is the most probable distribution of molecular weights, leading to a polydispersity ratio which approaches 2.0. This polydispersity ratio value is typical for ADMET polymers, as well as for polycartosilanes and polycarbosiloxanes. 

A significant problem is the dehydrocoupling reaction, which proceeds only at low yields per pass and is accompanied by rapid deactivation of the catalyst. The metathesis step, although chemically feasible, requires that polar contaminants resulting from partial oxidation be removed so that they will not deactivate the metathesis catalyst. In addition, apparendy both cis- and /ra/ j -stilbenes are obtained consequendy, a means of converting the unreactive i j -stilbene to the more reactive trans isomer must also be provided, thus complicating the process. 

Another example is the metathesis of cyclooctene, which produces poly-octenylene, an elastomor known as trans-polyoctenamer ... 

In a reiterative approach, enol ether epoxidation with DM DO has been coupled with C-C bond fonnation and ring-closing metathesis to provide trans-fused THP ring systems [82],... 

The most thoroughly studied reactions are the metathesis of propene to ethene and 2-butene, and the metathesis of 2-pentene to 2-butene and 3-hexene. Generally, the thermodynamic equilibrium ratio of the trans and cis components of the products is obtained. The reacting alkene molecules need not be identical, two different alkenes react with each other in the same way. 

Recently, a few examples of the metathesis of alkenes carrying functional groups have been reported. According to a patent, acrylonitrile reacts with propene to crotononitrile (cis and trans) and ethene 10) ... 

Many authors have observed that the cis-trans ratio of the products of the metathesis reaction is equal to the thermodynamic equilibrium value. This suggests that the reaction is not highly stereoselective. However, under certain conditions the product distribution is influenced by kinetic factors. For instance, it proves to be possible to prepare from cyclopentene... 

The above studies are consistent with the hypothesis that the metathesis reaction itself brings about cis-trans isomerization (46). This hypothesis is further supported by the results of a kinetic study of the reactions of the three linear butenes on the metathesis catalyst Mo(CO)6-A1203 by Davie et al. (107), who concluded that cis-trans isomerization for their system is a bimolecular reaction. 

Photolysis of chromium alkoxycarbenes with azoarenes produced 1,2- and 1,3-diazetidinones, along with imidates from formal azo metathesis (Eq. 21) [85, 86]. Elegant mechanistic studies [87-89] indicated the primary photoprocess was trans-to-cis isomerization of the azoarene followed by subsequent thermal reaction with the carbene complex. Because of the low yields and mixtures obtained the process is of little synthetic use. 

The reaction is called metathesis of alkenes. In the example shown above, 2-pentene (either cis, trans, or a cis-trans mixture) is converted to a mixture of 50%... 

The use of stoichiometric ruthenium-NHC complexes generated in situ from [Ruljd-COCKp-cymene)], an imidazohnm salt [4] or an imidizol(idin)ium-2-carboxylate [4] has been applied in the cyclopropanation of styrene 5 with ethyl diazoacetate (EDA) 6 (Scheme 5.2). No base was necessary when imidazolium-2 carboxylate were employed. The diastereoselectivity was low and the cis/trans ratio was around 50/50 (Table 5.1). Although the diastereoselectivity was moderate, the reaction was highly chemoselectivity as possible side reactions (homologation, dimerisation and metathesis) were totally or partially suppressed. 

MagUI AM, Yates BF, CaveU KJ, Skelton BW, White AH (2007) Dalton Trans 3398-3406 Nielsen DJ, CaveU KJ, Skelton BW, White AH (2002) Inorg Chim Acta 327 116-125 Grubbs RH (ed) (2003) Handbook of metathesis, 1st ed. Wiley-VCH, Weinheim Huang J, Stevens ED, Nolan SP, Petersen JL (1999) J Am Chem Soc 121 2674-2678... 

Several hydrido(phenoxo) complexes of nickel, trans-[NiH(OPh)L2] (6) (a L= P Prs b L = PCys c L = PBnj), have been prepared by the metathesis reaction of NaOPh with trans-[NiHClL2] (Eq. 6.6). The complex 6c was obtained as the phenol-solvated complex whose structure was determined by X-ray analysis [9]. An analogous platinum complex trans-[PtH(OPh)(PEt3)2] (7) was prepared by the reaction of trans-[PtH(N03)(PEt3)2] with NaOPh (Eq. 6.7). The complex 7 is air-stable but thermally sensitive and decomposes at room temperature. The structure was elucidated by X-ray analysis [10]. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

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