Vinylic substitution productions from

Table III. Vinylic Substitution Productions from 1-Hexene and Various Vinylic Bromides and Morpholine ...

In systems such as (241) which are capable of prototropic change, basic nucleophiles could cause preliminary rearrangement to (242). The migration of the double bond places the leaving group in the reactive allylic position, and its replacement becomes very facile. The primary substitution product (243) is the allylic one, but further prototropy could form the vinylic substitution product (244) formally derived directly from (241) (equation 24). This route, which requires highly... 

In the addition-elimination routes, either via a carbanionic intermediate (I) or via a neutral adduct (II), the anionic nucleophile Nu or the neutral nucleophile NuH attacks the /3-carbon with the expulsion of X. In the a,/8-route (IV), the /9,/3-route (VI) and the /8, y- elimination-addition routes (VII), HX is eliminated in the initial step, and the nucleophile and hydrogen are then added to the intermediates. Substitution occurs also by heterolytic C—X bond cleavage in an SN1 process (X). Initial prototropy followed by substitution can also give vinylic substitution products (XII, XIV), as well as two consecutive Sn2 reactions (XV) where the leaving group leaves from an allylic position. 

The stereochemistry of the reactions of vinyl cations with nucleophiles is predictably different depending on their geometry, (a) High stereospecificity is expected from bridged ions (trans addition to acetylenes and retention of configuration in substitution reactions of vinyl derivatives) and is experimentally observed in the case of thiirenium ions, (b) From free linear cations with two j3 substituents of equal size, complete racemization is expected and is fully verified in the substitution products from l,2-dianisyl-2-phenylvinylbromide (section II,C,2). 

Grant et a/.397 examined the reactions of hydroxy radicals with a range of vinyl and a-methylvinyl monomers in organic media. Hydroxy radicals on reaction with AMS give significant yields of products from head addition, abstraction and aromatic substitution (Table 3.8) even though resonance and steric factors combine to favor "normal tail addition. However, it is notable that the extents of abstraction (with AMS and MMA) arc less than obtained with t-butoxy radicals and the amounts of head addition (with MMA and S) are no greater than those seen with benzoyloxy radicals under similar conditions. It is clear that there is no direct correlation between reaclion rale and low specificity. 

Scheme 2.11 shows some examples of Robinson annulation reactions. Entries 1 and 2 show annulation reactions of relatively acidic dicarbonyl compounds. Entry 3 is an example of use of 4-(trimethylammonio)-2-butanone as a precursor of methyl vinyl ketone. This compound generates methyl vinyl ketone in situ by (3-eliminalion. The original conditions developed for the Robinson annulation reaction are such that the ketone enolate composition is under thermodynamic control. This usually results in the formation of product from the more stable enolate, as in Entry 3. The C(l) enolate is preferred because of the conjugation with the aromatic ring. For monosubstituted cyclohexanones, the cyclization usually occurs at the more-substituted position in hydroxylic solvents. The alternative regiochemistry can be achieved by using an enamine. Entry 4 is an example. As discussed in Section 1.9, the less-substituted enamine is favored, so addition occurs at the less-substituted position. 

Activation of one the double bonds of the allene by coordination to an electrophilic metal center such as Hg(II), Ag(I), Pd(II), Rh(I), Cu(I) or Au(III). Then an intramolecular nucleophile can attack and the product is formed by protodemetallation of the intermediate (Scheme 15.1). Depending on electronic and steric factors, either the proximal or the distal Jt-bond of the allene 1 is activated in that way (2 and/or 3). For each of these two possibilities now an exo or endo attack of the nucleophile is conceivable, leading to intermediates 4—7. An equilibrium between both 5 and 6 and 9 is possible. Finally, from 4 the vinyl-substituted 8 is formed. From 5, 6 or 9 the exocyclic alkene 10 and/or the endocyclic alkene 11 can be observed. Compound 7 would deliver the endocyclic alkene 12. 

Vinylic and divinylic tellnrides react with alkenes in MeOH in the presence of a combination of catalytic amounts of PdCl2 together with AgOAc and EtjN to afford the corresponding vinyl-substituted alkenes The products result respectively from the coupling of the styryl and phenyl moieties of the starting telluride with retention of its double-bond stereochemistry. 

To be really satisfactory, a Friedel-Crafts alkylation requires one relatively stable secondary or tertiary carbocation to be formed from the alkyl halide by interaction with the Lewis acid, i.e. cases where there is not going to be any chance of rearrangement. Note also that we are unable to generate carboca-tions from an aryl halide - aryl cations (also vinyl cations, see Section 8.1.3) are unfavourable - so that we cannot nse the Friedel-Crafts reaction to join aromatic gronps. There is also one further difficulty, as we shall see below. This is the fact that introduction of an alkyl substitnent on to an aromatic ring activates the ring towards fnrther electrophilic substitution. The result is that the initial product from Friedel-Crafts alkylations is more reactive than the... 

Vinyl ether radical-cations also react in a radical substitution fashion with an adjacent electron rich benzene ring [59]. However the reaction products from simple examples such as 33 themselves readily undergo a further anodic oxidation... 

These reactions involve metallate rearrangements , migratory insertion and transition metal-catalysed vinylic substitution reactions. They also perform well in applications in natural product synthesis . Many useful synthetic possibilities arise from application of ring-closing olefin metathesis (RCM) to unsaturated homoaldol products and their derivatives by means of the Grubbs catalyst 3942 4-286 Equation 105 presents some examples. ... 

If two hydrogen atoms on a (3-carbon atom are available for elimination, ( )-alkenes are strongly preferred. If only one hydrogen atom is present the product stereochemistry will be predictable on the basis of a syn addition of the organopalladium group to the double bond followed by a syn elimination of a palladium hydride group, provided the reaction is conducted under the proper conditions as described in Section 4.3.5.1.2.i. Yields of substituted alkenes from these reactions generally decline as the number and size of the substituents on the vinyl carbons increase. 

Intramolecular vinyl substitutions are commonly used to form cyclic products. In a typical reaction, p-in-doleacetic acid has been prepared in up to 43% yield from an o-bromoaniline derivative, as shown in equation (23).74... 

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