Disubstitued Olefins

What is metathesis polymerization Explain the mechanism and show the reaction on a disubstitued olefin. 

More recently, oxidative aminations of vinylarenes with amides and imides have been reported by Stahl. - In this case, the regioselectivity of the process depends on tiie presence or absence of a tertiary amine additive. As shown in Equation 16.117, the 1,1-disubsti-tuted olefin is formed in the presence of amine, but a 1,2-disubstitued olefin is formed in the absence of amine. This change in regioselectivity is proposed to arise from the degree of reversibility of the addition of the nucleophile to the coordinated vinylarene in the presence and absence of base. As shown in Scheme 16.31, the kinetic site of nucleophilic attack is proposed to be the internal carbon, but the thermodynamic site of attack is proposed to be the terminal carbon. The thermodynamic site of attack is proposed to be the terminal carbon because attack at this position forms an ti -benzyl complex. In the presence of base, the kinetic product is rapidly deprotonated, and the 1,1-disubstituted olefin is formed. In the absence of base, the deprotonation step is slow enough that the thermodynamic product of attack is generated. 

During the carbopalladation two possible alkylpalladium(II) intermediates can be formed resulting in two different products 1,1- or 1,2-disubstitued olefins. A number of strategies have been disclosed regarding the control of the regiose-lectivity in the Heck reaction, which can be influenced by factors such as the ligands, solvents, additives and the nature of the electrophile and the olefin. The difference between the two possible outcomes originates from the fundamental mechanisms of the carbopalladation, represented by the neutral and the cationic pathway (Schemes 1.11 and 1.19) [73, 74]. 

These ketone precursors can also serve as chiral auxiliaries. The dioxirane from the fructose-derived ketone 20 converts trisubstituted and rram-disubstitued alkenes (e.g., 22) to the corresponding epoxides in very good yields and enantioselectivities, but is less effective for terminal and cis-disubstituted substrates. Fortunately, the oxazolidino analog 21 exhibits a complementary scope, providing high enantioselectivities for these latter olefins (e.g., 24). The stereochemical outcome of the reaction has been rationalized on the basis of a spiro transition... 

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