Alkene metathesis groups

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. 

Exchange of alkylidene groups of alkenes—metathesis of olefins... 

Applications of the cross-metathesis reaction in more diverse areas of organic chemistry are beginning to appear in the literature. For example, the use of alkene metathesis in solution-phase combinatorial synthesis was recently reported by Boger and co-workers [45]. They assembled a chemical library of 600 compounds 27 (including cisttrans isomers) in which the final reaction was the metathesis of a mixture of 24 oo-alkene carboxamides 26 (prepared from six ami-nodiacetamides, with differing amide groups, each functionalised with four to-alkene carboxylic acids) (Eq.27). 

The evidence that mthenium-allenylidenes were easy to make and eflEcient alkene metathesis precursors motivated several groups to design new allenylidene metal complexes and to explore their impact on alkene metathesis. Nolan first reported... 

The trend of structural selectivity can be summarized as degenerate metathesis of terminal alkenes (exchange of methylene groups) > cross-metathesis of terminal and internal alkenes > metathesis of internal alkenes > productive metathesis of terminal alkenes (formation of internal alkene and ethylene).87 Since different catalyst systems exhibit different selectivities, a simple general picture accounting for all stereochemical phenomena of metathesis is not feasible. 

One of the most curious catalytic reactions of alkenes ever discovered is alkene metathesis or alkene dismutation, in which two alkenes exchange alkyli-dene groups, usually over a tungsten catalyst. The essence of the reaction is illustrated by a commercial process for converting excess propene to a mixture of ethene and butenes ... 

Two tandem alkene metathesis-oxidation procedures using Grubb s second-generation ruthenium catalyst resulted in unique functional group transformations. Use of sodium periodate and cerium(III) chloride, in acetonitrile-water, furnished cis-diols. Oxidation with Oxone, in the presence of sodium hydrogencarbonate, yielded a-hydroxy ketones.296 Secondary alcohols are oxidized to ketones by a hydrogen... 

Construction of organic nanotubes starting from porphyrin dendrimers with core/shell architecture is also feasible. Figure 8.29 also shows how covalent nanotubes can be produced by removal of the dendritic component of the molecule. A coordination polymer is first synthesised from a dendritic metallopor-phyrin with alkene end groups. This is subjected to intramolecular and intermo-lecular crosslinking by ring-closing metathesis at the periphery. 

The elimination of a-hydrogen is not general and observed only with limited numbers of metal complexes. The elimination of a-hydrogen from the methyl group in the dimethylmetal complex 68 generates the metal hydride 69 and a carbene that coordinates to the metal. Liberation of methane by the reductive elimination generates the carbene complex 70. Formation of carbene complexes of Mo and Wis a key step in alkene metathesis. The a-elimination is similar to the 1,2-hydride shift observed in organic reactions. 

Reaction of the complex 24 with terminal alkene 25 generates styrene and the real catalytic species 27 via the ruthenacyclobutane 26. The complex 24 is commercially available, active without rigorous exclusion of O2 and water, and has functional group tolerance. Carbonyl alkenation is not observed with the catalysts 22 and 24. Their introduction has enormously accelerated the synthetic applications of alkene metathesis [11]. 

The symmetric alkenes 11 and 15 are formed by homometathesis of the unsymmetric alkene 6. Alkene metathesis is an equilibrium reaction, and the homometathesis of internal alkene 6 may be a useful one only when separation of the products 11 and 15 from the starting alkene 6 is easy, namely when R] and R are clearly different functional groups. 

Sophorolipid lactone (34) was obtained from an open-chain bisalkyne precursor (concomitant formation of 2-butyne) in a macrocyclization yield of 78% [29], The epothilones have been obtained through non-stereoselective alkene metathesis [26d], making them worthwhile targets for the combination alkyne metathesis/Lindlar hydrogenation to assemble the C12-C13 Z double bond. Furthermore, epothilone C (35) shows the most complex array of functional groups among the substrates of alkyne metathesis. The macrocyclization yield, starting from the expected OTBS-protected precursor (not shown) was 81% [23, 30]. Neither... 

In the mid to late 1980s, transition-metal catalysts were developed that were particularly useful for carrying out olefin (alkene) metathesis reactions (Rouhi 2002). Metathesis is a reaction in which two molecules containing carbon-carbon double bonds exchange carbon atoms along with any groups attached to them ... 

R and R refer to organic groups that contain some number of carbon atoms and may or may not be different from each other. With cyclic alkenes, metathesis leads to polymerization (specifically, ring-opening metathesis polymerization [ROMP]), shown here for cyclopentene ... 

Besides enyne metathesis [66] (see also the chapter Recent Advances in Alkenes Metathesis in this volume), which generally produces 1-vinylcyclo-alkenes, ruthenium-catalyzed enyne cycloisomerization can proceed by two major pathways via hydrometallation or a ruthenacycle intermediate. The RuClH(CO)(PPh3)3 complex catalyzed the cyclization of 1,5- and 1,6-enynes with an electron-withdrawing group on the alkene to give cyclized 1,3-dienes, dialkylidenecyclopentanes (for n=2), or alkylidenecyclopentenes (for n= 1) [69,70] (Eq. 51). Hydroruthenation of the alkyne can give two vinylruthenium complexes which can undergo intramolecular alkene insertion into the Ru-C bond. 

A substantial portion of the early interest in MTO pertained to its use in alkene metathesis.12 Of more relevance to oxidation processes, much work has also involved examining Lewis base adducts of this electrophilic, (formally) 14-electron complex. Generally, nitrogen bases coordinate trans to the methyl group. Bidentate ligands such as bipyridyl lead to the fac-Me(N N)Re03 isomer of the octahedral complex. Table I summarizes some of the structural information from X-ray crystallographic and infrared studies. 

Evidence for the carbene mechanism is now so overwhelming (as discussed below) that the pairwise mechanism is only mentioned in this review for historical reasons. All alkene metathesis reactions are catalyzed by a metal carbene complex of some description, and the widely variable compositions used as catalysts are necessary to generate an active metal carbene group. 

The alkene metathesis reaction see Alkene Metathesis) exchanges alkylidene groups between different alkenes, and is catalyzed by a variety of high oxidation state, early transition metal species (equation 40). The reaction is of interest because it is the strongest bond in the alkene, the C=C bond, that is broken during the reaction. It is also commercially important in the Shell higher olefins process and in the polymerization of cycloalkenes. It is relevant to this article because carbenes are the key intermediates, and the best-known catalyst, (1), is a carbene complex. 

Related molybdenum catalysts appear to show even more functional group tolerance. To date, the major test of functional group compatibility has been in the synthesis of polymers however, it is anticipated that this activity will persist into acyclic metathesis. Later transition metals are active in the metathesis polymerization of highly functiondized cyclic alkenes. These catalyst systems, which appear to tolerate almost all functional groups, show very low activity for acyclic alkene metathesis. If these systems can be activated, the problems associated with the use of alkene metathesis in the synthesis of multifunctional organics will be solved. 

The metathesis reaction between carbon-carbon double bonds (alkene metathesis) is well established in commercial scale synthesis. It is a key component of some polymerization processes and is the route to nonfunctionalized alkenes which find applications in fine chemical synthesis. The development of well-defined, functional group tolerant catalysts will lead to a much greater role for alkene metathesis in synthesis. 

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