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Vinyl carbon substitution, addition-elimination mechanisms

Nucleophilic substitution at a vinylic carbon is difficult (see p. 433), but many examples are known. The most common mechanisms are the tetrahedral mechanism and the closely related addition-elimination mechanism. Both of these mechanisms are impossible at a saturated substrate. The addition-elimination mechanism has... [Pg.428]

Alkenyl(phenyl)iodonium salts are highly reactive in vinylic nucleophilic substitution reactions because of the excellent leaving group ability of the phenyliodonium moiety. Only a few examples of non-catalytic alkenylation of carbon nucleophiles are known [50,51]. In most cases these reactions proceed with predominant retention of configuration via the addition-elimination mechanism or ligand coupling on the iodine [42,50]. [Pg.111]

Nucleophilic vinylic substitutions of 4//-pyran-4-onc and 2,6-dimethyl-4//-pyran-4-one with a hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G (d) basis sets. The aqueous solution was modelled by a supermolecular approach, where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of OH- at the C(2) vinylic carbon of the pyranone ring with an activation barrier of 10-11 kcalmol-1 (B3LYP) has been identified as the rate-determining step, in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step and, conversely, decrease the barrier of the elimination step.138... [Pg.313]

Stereospecificity in the addition-elimination mechanism Towards Carbon Nucleophiles and Vinyl Cation Equivalents Vinyl cation equivalents Conjugate Substitution... [Pg.307]

Study of structure-activity relationships in nucleophilic vinylic substitution reactions (S nV) that proceed by the addition-elimination mechanism has been furthered by the study of acid-catalysed breakdown of alkoxide and thiolate ion adducts of benzylidene Meldrum s acid (36a), methoxybenzylidene Meldrum s acid (36b), and thiomethoxybenzylidene Meldrum s acid (36c). Catalysis of alkoxide or thiolate expulsion is in competition with protonation of the a-carbon or one of the carbonyl oxygens to form an enolate. The positive Ag values for H+-catalysed RO and RS departure imply an imbalanced transition state in which proton transfer leads C-O or C-S bond cleavage. The pA values of the various adducts are much lower than for Meldrum s acid, in view of the stabilizing influence of OR, SR, and OMe groups on the carbanion. [Pg.383]

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... [Pg.322]


See other pages where Vinyl carbon substitution, addition-elimination mechanisms is mentioned: [Pg.349]    [Pg.53]    [Pg.23]    [Pg.102]    [Pg.567]    [Pg.273]    [Pg.171]    [Pg.532]    [Pg.363]    [Pg.370]    [Pg.269]    [Pg.1414]    [Pg.1127]   
See also in sourсe #XX -- [ Pg.428 ]




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Addition-Substitution-Elimination:,

Addition-elimination

Additive mechanism

Additives carbon

Carbon addition

Carbon elimination

Carbon mechanism

Elimination 1,6-addition, eliminative

Elimination addition mechanism

Mechanism elimination

Mechanisms addition

Substitution mechanisms elimination-addition

Substitution, vinyl

Substitution, vinyl elimination-addition

Substitution-elimination

Vinyl addition

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Vinyl carbonates

Vinyl mechanism

Vinylation mechanism

Vinylic carbon

Vinylic substitution

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