Alkene metathesis effects

Ethers, esters, amides and imidazolidines containing an epithio group are said to be effective in enhancing the antiwear and extreme pressure peiformance of lubricants. Other uses of thiiranes are as follows fuel gas odorant (2-methylthiirane), improvement of antistatic and wetting properties of fibers and films [poly(ethyleneglycol) ethers of 2-hydroxymethyl thiirane], inhibition of alkene metathesis (2-methylthiirane), stabilizers for poly(thiirane) (halogen adducts of thiiranes), enhancement of respiration of tobacco leaves (thiirane), tobacco additives to reduce nicotine and to reduce phenol levels in smoke [2-(methoxymethyl)thiirane], stabilizers for trichloroethylene and 1,1,1-trichloroethane (2-methylthiirane, 2-hydroxymethylthiirane) and stabilizers for organic compounds (0,0-dialkyldithiophosphate esters of 2-mercaptomethylthiirane). The product of the reaction of aniline with thiirane is reported to be useful in the flotation of zinc sulfide. 

Scheme 6.3. Zr-catalyzed enantioselective ethylmagnesation and metal-catalyzed alkene metathesis make effective partners. In the two cases shown here, the alkene substrate is synthesized and enantioselectively alkylated in the same vessel.

Initially, it was thought more likely that the electron poor metal atom would be involved in the electrophilic attack at the alkene and also the metal-carbon bond would bring the alkene closer to the chiral metal-ligand environment. This mechanism is analogous to alkene metathesis in which a metallacyclobutane is formed. Later work, though, has shown that for osmium the actual mechanism is the 3+2 addition. Molecular modelling lends support to the 3+2 mechanism, but also kinetic isotope effects support this (KIEs for 13C in substrate at high conversion). Oxetane formation should lead to a different KIE for the two alkene carbon atoms involved. Both experimentally and theoretically an equal KIE was found for both carbon atoms and thus it was concluded that an effectively symmetric addition, such as the 3+2 addition, is the actual mechanism [22] for osmium. 

Coordinated nitrenes RN2- have been thoroughly investigated in recent years285,286 owing to their formal similarity to carbenes and to their potential importance, e.g. as catalysts in the Haber ammonia process. Theoretical studies have suggested that imido ligands may also prove effective in promoting alkene metathesis.287... 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

The expense of ruthenium and the current rise of importance attached to environmentally friendly and cost-effective synthetic protocols gives the search for recyclable alkene metathesis catalysts ever-increasing impetus. Since the... 

Ruthenium is not an effective catalyst in many catalytic reactions however, it is becoming one of the most novel and promising metals with respect to organic synthesis. The recent discovery of C-H bond activation reactions [38] and alkene metathesis reactions [54] catalyzed by ruthenium complexes has had a significant impact on organic chemistry as well as other chemically related fields, such as natural product synthesis, polymer science, and material sciences. Similarly, carbonylation reactions catalyzed by ruthenium complexes have also been extensively developed. Compared with other transition-metal-catalyzed carbonylation reactions, ruthenium complexes are known to catalyze a few carbonylation reactions, such as hydroformylation or the reductive carbonylation of nitro compounds. In the last 10 years, a number of new carbonylation reactions have been discovered, as described in this chapter. We ex-... 

The transition metal chemistry of y-carbenes (14) is of interest because of the possible involvement of such species in Fischer-Tropsch synthesis (15) and alkene metathesis (16,17). However, apart from y-CH2 and one example of y-CHMe, simple hydrocarbon species have, until the work described here, been generally unavailable and the reactivity of y-carbenes is effectively unexplored. An opportunity was therefore presented for such study, in which carbon-carbon bond formation has taken precedence. 

Combination of WCl6 and EtAlCb (or Et3Al) is also effective for acetylene polymerization . This catalyst shows high activity towards linear and cyclic alkene metathesis and a metathesis propagation step is to be invoked also in this case. However, this combination is a typical Ziegler-Natta catalyst system, and it may be probable that, in this particular case, the components are acting as a simple coordinated anionic catalyst system. 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

Alkene-alkene metathesis reactions are a valuable method to construct cyclic compounds (see Section 2.10). Alkene-alkyne reactions can also be effective. Explain the formation of the bicyclic product 16. 

The advent of stable carbene complex catalysts has had a profound effect on the general utility of alkene metathesis. Early metathesis catalysts displayed a low tolerance for functional groups, and successful metathesis reactions were primarily restricted to relatively unfunctionalized substrates. Early demonstrations that discreet carbene complex catalysts could initiate alkene metathesis inspired the development new transition metal carbene complex catalysts. Some of the more commonly employed alkene metathesis initiators are depicted in... 

The general equation and mechanism for alkyne metathesis is depicted in Scheme 31. Alkyne metathesis is considerably less well developed in comparison to alkene metathesis. Garbyne complexes or carbyne complex precursors are among the most effective alkyne metathesis catalysts representative catalysts are depicted in Scheme 32. Tungsten carbyne complex 276 is one of the earliest alkyne metathesis catalysts, and has frequently been employed to initiate... 

According to the nature of alkene metathesis, this general reaction can be divided into different subgroups cross-metathesis (CM), ring-closing metathesis (RCM), ringopening metathesis polymerization (ROMP), and acyclic diene metathesis polymerization (ADMET). " From an industrial perspective, a more cost-effective method for alkene metathesis is to generate the metallo-carbene in situP This reaction has been extensively reviewed. ... 

While vinylsilanes undergo productive cross-metathesis (Mo and Ru carbenes) with allyl-substituted functionalized alkenes, their effective transformation with derivatives containing a fimctionalized group attached directly to a carbon -carbon double bond can be achieved only via silylative coupling catalyzed by metal complexes containing (or generating) M-H and/or M-Si bonds (M = Ru, Rh, Ir). 

The results of ab initio calculations on alkene metathesis catalysts M(OH)2(XH)(=CH2), MCl2(Y)(=CH2), and Mo(OH)2(NH)(=CHZ) are used to address the effects on the bonding caused by modifications in the metal, ligands, and alkylidene substituents. For instance the greater M =C bond polarity in W(OH)2(NH)(=CH2) versus Mo(OH)2(NH)(=CH2) correlates with the greater metathetical activity of the tungsten species.252... 

Alkene Metathesis. Alkene metathesis is a simple and effective method of the synthesis of useful compounds, which otherwise would require complex synthetic procedures. With the commercial availability of various catalysts of high activity and selectivity, the importance of alkene metathesis keeps up growing (190). Interestingly, although there are many highly active water-soluble metathesis catalysts known (71,72,191) the number of aqueous-organic bipha-sic procedures is relatively small. 

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