Witting olefination

Methylanecyclopentane lormation from siloxymelhylallylsilane or acetoxymelhylattylwlane 4 wit i Micfiael acceptor olefins and Pd catalysts (via trimethylene methane equivalent)... 

In an alternative syntliesis of panaatistaliti (S7) by Trost et al. [52], fSdieme 9.15) addilioti of tlie Grlgtiatd teagetil 63 [53] lo a mixture of tlie azide 62 and copper cyanide reprodudbly gave tlie desired adduct 64. Because of tlie difliciilties associated witli purification of adduct, tlie overall yield of tlie two steps ftlie next being diliydroxylation of tlie olefin) was 6 296. 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

The past thirty years have witnessed great advances in the selective synthesis of epoxides, and numerous regio-, chemo-, enantio-, and diastereoselective methods have been developed. Discovered in 1980, the Katsuki-Sharpless catalytic asymmetric epoxidation of allylic alcohols, in which a catalyst for the first time demonstrated both high selectivity and substrate promiscuity, was the first practical entry into the world of chiral 2,3-epoxy alcohols [10, 11]. Asymmetric catalysis of the epoxidation of unfunctionalized olefins through the use of Jacobsen s chiral [(sale-i i) Mi iln] [12] or Shi s chiral ketones [13] as oxidants is also well established. Catalytic asymmetric epoxidations have been comprehensively reviewed [14, 15]. 

The Witting reaction has been investigated in aqueous conditions.305 Wittig olefination reactions with stabilized ylides (known as the Wittig-Homer or Homer-Wadsworth-Emmons reaction) are sometimes performed in an organic/water biphase system.306 Very often, a phase-transfer catalyst is used. Recently, the use of water alone as solvent... 

As for heterogeneous olefin polymerization catalysis, the activity of rare-earth metal catalysts may be also enhanced in organic transformations by the use of silica supports or other carriers [7]. Indeed, several catalytic C-C and C-X (with X = H/D, Si, O) bond formation reactions as weU as functional group transformations witness to the potential of SOLn/AnC-based heterogeneous catalysts for fine chemical synthesis. 

One of the landmark achievements in the area of enantioselective catalysis has been the development of a large-scale commercial application of the Rh(I)/BINAP-catalyzed asymmetric isomerization of allylic amines to enamines. Unfortunately, methods for the isomerization of other families of olefins have not yet reached a comparable level of sophistication. However, since the early 1990s promising catalyst systems have been described for enantioselective isomerizations of allylic alcohols and aUylic ethers. In view of the utility of catalytic asymmetric olefin isomerization reactions, I have no doubt that the coming years will witness additional exciting progress in the development of highly effective catalysts for these and related substrates. 

The first step is a Wittig reaction in which the ketone is converted to the terminal olefin by reaction witli a phosphorus ylide (also called a phosphorane). Phosphoranes are resonance-stabilized by overlap between the carbon p-orbital and one of the d-orbitals of the phosphorus. 

Dtehlorocarbene. Polish chemists have reported a new method for generation of dichlorocarbenc (or a ca rbenoid species) it in volves the reaction of an olefin wit h chloroform in the presence of a 50 % aqueous NaOH solution and a catalytic amount of bcnzyl-triethylammonium chloride. For example, dichloronorcaranc was obtained from cyclohexene in this way in 72% yield. In the absence of the catalyst, a yield of only 0.5% has been reported. ... 

Although the past three years have witnessed many ingenious syntheses of the two insect juvenile hormones, the preparation of the naturally-occurring dextrorotatory Cl8 hormone (cis-epoxide) has only recently been achieved. Indeed, in two independent studies, both enantiomeric pairs of the cis- and trans-epoxides. have been prepared and it has been firmly established that the two chiral centres have the 10R,11S configurations in the naturally-occurring material. Findlay et al. have now published full details of their previously announced synthesis of the two juvenile hormones and other double-bond isomers. In an earlier synthesis of the C18 hormone, Corey et al. used the dienol (30 R = Et) as a key intermediate and now they have described two new stereospecific routes to this compound (Scheme 2). In the first synthesis the lactone (27) was converted into the hydroxy-olefin (28) by hydrolysis, esterification, tosylation, and lithium... 

Substituted olefins were obtained in equal yields compared to those obtained by analogous low molecular weight reactions. In contrast to Witting reagents on insoluble polymeric aipports, these soluble polymeric Wittig rea nts reveal tiie same kinetic behaviour as that of the low molecular weight system. 

Chain extension can only occur by the connection of two terminal olefins and is accompanied by the release of a molecule of efhylene, fhus each chain extension event must start with fhe alkylidene and generate fhe methyUdene (paths C and D in Scheme 6.8). Reaction of the methylidene with a terminal olefin regenerates the alkylidene and liberates fhe ethylene molecule (path A and B). Thus both the methylidene and the alkylidene are present in an ADMET reaction. The relative amounts of the methylidene and alkylidene in an ADMET reaction will depend on the rates of reaction of all steps in the cycle, principally the rates of reaction of the mediylidene and alkylidene witli a terminal olefin, and is thus a function of the particular catalyst used. Obviously the rates of bofh reactions should be as... 

The high trans content witnessed in ADMET polymers produced by these catalysts can be explained by the preference of the all-pseudoequatorial configuration in the metallacyclobutane intermediates, which is suggested by NMR and X-ray data [23] (Fig. 6.2). The all-equatorial conformation of the metallacyclobutane leads to the production of trans olefins. 

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