Metathesis chemistry

The obviation of side reactions is essential to the success of ADMET, and this can be realized if the proper catalyst is chosen. Catalyst choice must avoid the possibility of cation formation,13 vinyl addition, and/or formation of multiple catalytic species, all of which are detrimental to clean metathesis chemistry. Over the past 10 years, our group has utilized a variety of different catalysts, several of which are illustrated in Fig. 8.4. 

Metathesis chemistry, histoiy of, 431-432 Metathesis depolymerization, 456-457 Metathesis polymerization, general conditions for, 440-441 Metathesis reactions, types of, 432 Methanol, 377 Methanolysis, 535... 

The tricyclic compound 20-C, a potential intermediate for alkaloid synthesis, has been prepared by an intramolecular Diels-Alder reaction of the ketone obtained by deprotection and oxidation of 20-B. Compound 20-B was prepared from 20-A using alkyne-ethene metathesis chemistry. Show the mechanistic steps involved in conversion of 20-A to 20-B. 

An interesting combination of ring-closing metathesis chemistry with the aza-Baylis-Hillman reaction has recently been described by Balan and Adolfsson and is shown in Scheme 6.70 a [149]. The authors reported that functionalized 2,5-dihydro-... 

I. Alkenes. 2. Ring-opening polymerization. 3. Metathesis (Chemistry)... 

Although the bulk of this review is concerned with well-defined metal carbene catalysts, it is important to note the contributions made to cross-metathesis chemistry by ill-defined or multicomponent catalysts. A brief discussion of the cross-metathesis reactions of functionalised alkenes using catalysts of this type will therefore be included here [1]. 

During the past 2 years several research groups have published research that either uses or expands upon Crowe s acyclic cross-metathesis chemistry. The first reported application of this chemistry was in the synthesis of frans-disubstitut-ed homoallylic alcohols [30]. Cross-metathesis of styrenes with homoallylic silyl ethers 15, prepared via asymmetric allylboration and subsequent alcohol protection, gave the desired trans cross-metathesis products in moderate to good yields (Eq. 15). 

The ruthenium catalyst system, 14, shown in Fig. 3, also carries out ADMET condensation chemistry, albeit with higher concentrations being required to achieve reasonable reaction rates [32]. The possibility of intramolecular compl-exation with this catalyst influences the polymerization reaction, but nonetheless, ruthenium catalysis has proved to be a valuable contributor to overall condensation metathesis chemistry. Equally significant, these catalysts are tolerant to the presence of alcohol functionality [33] and are relatively easy to synthesize. For these reasons, ruthenium catalysis continues to be important in both ADMET and ring closing metathesis chemistry. 

For a comprehensive review of metathesis chemistry, see Ivin KJ, Mol JC (1997) Olefin metathesis and metathesis polymerization, Academic, San Diego, London and references therein... 

Ofstead EA, Wagener KB (1992) Polymer synthesis via metathesis chemistry. In Mijs WJ (ed) New methods for polymer synthesis. Plenum, New York, Chap 8... 

Olefin metathesis chemistry has had a profound impact in several areas of chemical research, including organome-tallics, polymer chemistry, and small molecule synthesis,many of which have industrial applications. For example, CM is currently utilized in the commercial preparation of several agrochemicals, polymer and fuel additives, and pharmacophores. Unlike RCM reactions, which are typically conducted under dilute... 

A strictly related example of multiple bond metathesis chemistry between C02 and isocyanides proceeds from the binuclear complex Ni2( x-CNMe)(CNMe)2(dppm)2 (dppm = l,2-bis(diphenylphosphino)methane) which contains a reactive g-MeNC ligand [111d—f]. This species, in the presence of liquid C02 (298 K, 10-15MPa, >48h), was converted into Ni2( X-CO)(CO)2(dppm)2 and polymeric... 

Catalytic alkene metathesis chemistry has recently been combined with an additional step in a single pot to further modify natural oils. A metathesis-isomerisation-methoxycarbonylation-transesterification reaction sequence has been performed to yield high-value oxygenates (Zhu et al., 2006a). A palladium catalyst is added to the reaction mixture once maximum conversion in the metathesis step is achieved, and it is heated under 400 psi of carbon monoxide. 

Eleuterio, H. S., Scientific Discovery and Technological Innovation an Eclectic Odyssey into Olefin Metathesis Chemistry , J. Macromol. Sci. - Chem. A, 28, 907-915 (1991). 

Ofstead, E. A. and Wagener, K. B., Polymer Synthesis via Metathesis Chemistry , in New Methods for Polymer Synthesis, Plenum Press, New York, 1992, Ch. 8. 

Chodorge, J.A., Cosyns, J., Commerceuc, D., Debuisschert, Q., and Travers, P., in Novel Metathesis Chemistry, Imamoglu, Y. and Bencze, L., Eds., Kluwer Academic Publishers, Dordrecht, 2003. 

Imamoglu, Y. and Bencze, L., Eds., Novel Metathesis Chemistry, Kluwer Academic Publishers, Dordrecht, 2003. 

Ring opening metathesis polymerization, which has been known since the discovery of the alkene metathesis reaction, has been given the acronym ROMP in recent years. In fact, the ROMP reaction was the first observation made in alkene metathesis chemistry, while the discovery of the exchange reaction in equation (1) actually occurred later. Acychc diene metathesis (ADMET) polymerization (equation 3) has only recently been shown to be a viable method for polymer synthesis, and it has been termed ADMET polymerization. ROMP reactions are driven by the release of ring strain from the monomer, while ADMET polymerization is driven by a shift in the equilibrium caused by the removal of one of the reaction products. 

The synthesis of d° Alkylidene complexes by Schrock demonstrated that carbene complexes could be isolated that were electronically similar to those postulated to be involved in the alkene metathesis reaction see Schrock-type Carbene Complexes). Eventually, this pioneering work led to the synthesis of a class of compounds that are among the most active catalysts known for metathesis chemistry. The first observation that a d° carbene complex was involved in metathesis chemistry was when Tebbe showed that the Ti complex (2) would catalyze the degenerate metathesis of... 

The discovery that Ti complex (4) was an effective catalyst for living ROMP chemistry resulted in the synthesis of several new types of polymers that were inaccessible with conventional catalysts. Narrow polydispersity polymer and di- and triblock copolymers were synthesized soon after living ROMP was discovered. Because polymer chains that are formed in a living ROMP reaction are terminated with metal carbene groups, fimctionalization of the chain ends is possible. Reaction of the carbene with an aldehyde occurs in a Wittiglike fashion, making a metal oxo complex that is inert to finther metathesis chemistry and terminates the chain with the alkylidene group of the aldehyde. 

Tungsten alkylidenes have also been shown to accomplish carbonyl alkylidenation. Aldehydes, ketones, esters and amides have been used as the carbonyl moiety. These reactions involve metallacycle intermediates and appear to parallel much of the metathesis chemistry (equation 32). 

Wallace DJ. Relay ring-closing metathesis—a strategy for achiev- 127. ing reactivity and selectivity in metathesis chemistry. Angew. 

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