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Vinyl-metal derivatives

In chapter 15 we saw that dienes could be made by the Wittig reaction and also by the addition of vinyl lithiums or Grignard reagents to ketones followed by dehydration of the allylic alcohol product. Derivatives of acetylenes can do the same job. The first disconnection is the same but a reagent for the synthon 40 replaces the vinyl metal derivative. [Pg.117]

This reaction will be useful only if we are able to prepare these compounds with a fixed (E or Z) configuration. The first part of the chapter concerns the preparation and reactions of simple vinyl metal derivatives and we shall then progress to a study of stereochemistry. [Pg.256]

Alkyne complexes of low oxidation state metals react with electrophiles generally to produce alkene derivatives resulting from addition of the electrophilic species to the coordinated triple bond. These reactions often proceed by way of (x-vinyl-metal derivatives, and in several cases such intermediates have been isolated. The vinyl complexes, in turn, may be cleaved in a subsequent reaction to afford substituted alkenes [Eq. (29)]. [Pg.16]

A retrosynthetic analysis of fragment 152 can be completed through cleavage of the C16-C17 bond in enone 155, the projected precursor of epoxide 152. This retrosynthetic maneuver furnishes intermediates 156 and 157 as potential building blocks. In the forward sense, acylation of a vinyl metal species derived from 156 with Weinreb amide 157 could accomplish the construction of enone 155. Iodide 153, on the other hand, can be traced retrosynthetically to the commercially available, optically active building block methyl (S)-(+)-3-hydroxy-2-methyIpropionate (154). [Pg.603]

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... [Pg.230]

For vinyl monomers two methods can be used to initiate polymerization, both involve alkali metal derivatives, or more rarely alkaline-earth metal derivatives, and differ only by the mechanism of formation of the primary carbanionic... [Pg.47]

Metal-Oxygen Compounds. Clear examples of the addition of transition metal alkoxides to acetylenes are not known however, the addition of trialkyltin alkoxides has been reported. Triethyltin methoxide, for example, reacts with dimethyl acetylenedicarboxylate to give the vinyl tin derivative XVI (63). [Pg.199]

The VEEL spectrum of a surface vinyl group derived from low-temperature decomposition of vinyl iodide on Pt(lll) has been reported with features at 2920 cm 1 (s), eCH 1600 cm 1 (w), vCC 1380 cm-1 (m), 8= CH2 scissors 1255 cm1 (mw), <5C=CH 955 cm 1 (s), -y=CH2 and 690 cm 1 (mw), y=CH2 twist (208). The strength of the out-of-plane y = CH2 mode shows that the plane of the vinyl group is more nearly parallel than perpendicular to the surface, probably o--bonded to one metal atom and 77-bonded to another. This inference is confirmed by the relatively low wavenumbers of the eCH modes. However, in such a situation, the vCC wavenumber... [Pg.227]

But this strategy is doomed too as the allylic bromide will almost certainly react at its less hindered end on the vinyl group. However, we might choose the alternative branchpoint disconnection 38a and consider conjugate addition of some vinyl-metal (copper ) derivative to the enone 41 that could be made by some aldol process from the ketone 42 and an enol(ate) of acetone 43. [Pg.211]

The allylmetallation of vinyl metals, y-heterosubstituted with a methoxy-methyl ether as the chelating group, leads to the corresponding gembismetal-lic derivatives,20 but now, warming the reaction mixture to room temperature promotes an internal nucleophilic substitution, leading to a metallated cyclopropane which can react with different electrophiles21 (Equation 7.5 and Protocol 9). [Pg.121]

Biocides. The attack of fungi on the several components of a vinyl formulation may manifest itself as an unsightly growth looking much like dirt or may result in the formation of discolored areas (yellow, pink, blue, clear) on the finished goods. Outdoor exposure, indoor exposure in humid atmosphere, soil burial or contact, and electrical insulation compounds are typical areas where biocide addition is indicated. Since most commercial biocides are heavy metal derivatives, care must be taken to assess their effect on heat and light stability and compatibility with the vinyl system in which they are used. [Pg.280]

Intramolecular insertion of carbon monoxide into the metal-carbene bond of the (Ej-isomer of D leads to the t/4-vinyl ketene complex intermediate E. Experimental support for this type of intermediate has been provided by the isolation of Cr( CO) 3-coordinated dienyl ketenes related to 5 (Scheme 4) [15a], and by trapping the vinyl ketene intermediates as vinyl lactone derivatives in the course of the reaction of chromium carbene complexes with 1-alkynols [15b]. [Pg.254]

This is a reaction that seems veiy attractive for synthesis but, in the absence of a transition met catalyst, the yields are very low. We showed in the last chapter how vinyl silanes can be made wit control over stereochemistry and converted into lithium derivatives with retention. Neither of thes vinyl metals couple with vinyl halides alone. But in the presence of a transition metal—Cu (I) for I and Pd(0) for Sn—coupling occurs stereo specifically and in good yield. [Pg.1324]

While the allylmetalation of alkynes is a well-described reaction, the addition of vinyl metal to a triple bond has remained comparatively unexplored [1-8,21-25]. It stems from the fact that the metalated dienyl derivative 3, which results from the vinylmetalation of the alkyne 1, exhibits similar reactivity to that of 2 toward the alkyne 1. Therefore, 3 reacts also with the initial substrate 1 to give first the dimer, and after subsequent following additions, oligomers are formed (Scheme 2). Several alternatives were reported in the literature, and the most representative examples are described below. [Pg.134]

The addition of diorganozinc compounds to alkynes can be promoted by different metallic species snch as Cp2Zrl2 and Ni(acac)2 to give the corresponding vinylic zinc componnds (equation 72). A snbsequent hydrolysis leads to the alkene, whereas iodinolysis provides the vinyl iodide derivative. Copper-zinc species, readily obtained by the... [Pg.5239]

One of the most important contributions in metal enolate catalysis is the conjugate addition of arylboronic acids (115) to vinyl ketone derivatives (116) to give the corresponding aryl ketones (117) (equation number of advances have recently... [Pg.567]

A preliminary formation of a mixed vinyl-allylzinc derivative can be also considered, however, undergoing a rearrangement akin to the Claisen rearrangement where zinc plays the role of oxygen [97, 98, 101] (Scheme 7-88). Various metals may be used on the vinylic part as MX, = Li, Mg, B, Al, Cu, but the presence of zinc (or cadmium) is compulsory for the addition to proceed [103J. [Pg.169]

The chemical reactivity of several heterocyclic derivatives is closely associated with a reactivity one might expect for open-chain unsaturated derivatives. The boron heterocycles are sensitive to oxygen or hydrogen peroxide 111, 169) and so apparently is 1,1,2,5-tetraphenylsilacyclo-pentadiene 17, 246). These reactions probably lead to ring cleavage but the products have not been characterized. Vinyl-metal bonds are frequently cleaved by electrophilic reagents and the heterocycles are no exception. [Pg.168]

Phase-transfer catalysis is a useful procedure for a variety of interesting metal-catalyzed reactions. - However, only one example of this approach has been reported for the synthesis of diynes by the sp-sp carbon coupling reaction. Thus vinylic dibromides derived from aromatic aldehydes have been shown to react with carbon monoxide, in the presence of zerovalent palladium compounds as the metal catalyst, and under phase-transfer conditions in a two-phase system (benzene, 5 M NaOH), to give the corresponding diynes in reasonable yields (equations 21 and 22). ... [Pg.559]

This reaction was suggested to proceed by alcohol interchange involving the vinyl alcohol formed by enolization of the acetaldehyde. Naturally, the instability of vinylic alcohols relative to tautomeric aldehydes or ketones precluded synthesis of their metal derivatives for a very long time. [Pg.391]

Treatment of vinyl Sn, B, or Al compounds with BuLi results in effective addition of Bu to the metal to form a hypervalent anion such as 154. These are often referred to as ate complexes. The analogy is with the names of anions such as sulfate or carbonate. You are already familiar with the copper analogues, usually called cuprates. Lithium now replaces tin at the vinyl group 155 to form a vinyl-lithium derivative -156. The reaction is an electrophilic substitution at carbon - the lithium atom attacks the C-Sn bond and does so with retention of configuration. [Pg.269]

Each vinyl metal reagent was prepared by thermodynamic hydrostannylation of the corresponding alkyne.41 42 The simpler lithium derivative 165 R = CH2SMe came from the protected propargyl alcohol 167. [Pg.270]


See other pages where Vinyl-metal derivatives is mentioned: [Pg.7]    [Pg.107]    [Pg.64]    [Pg.7]    [Pg.107]    [Pg.64]    [Pg.432]    [Pg.102]    [Pg.264]    [Pg.186]    [Pg.102]    [Pg.79]    [Pg.243]    [Pg.105]    [Pg.519]    [Pg.2]    [Pg.25]    [Pg.4560]    [Pg.5230]    [Pg.25]    [Pg.102]    [Pg.848]    [Pg.440]    [Pg.432]    [Pg.424]    [Pg.186]    [Pg.157]    [Pg.173]   
See also in sourсe #XX -- [ Pg.107 ]




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