Metal induced elimination alkenes

The nitrogen source for the aziridination of alkenes, a nitrene or nitrenoid, can be generated in various ways (1) oxidation of a primary amine (2) base-induced -elimination of HX from an amine or amide with an electronegative atom X (X = halogen, O) attached to the NH group or by -elimination of metal halides from metal A-arenesulfonyl-A-haloamides (3) metal-catalyzed reaction of [A-(alkane/arenesulfonyl)imino]aryliodanes (4) thermolytic or photolytic decomposition of organyl azides and (5) thermally induced cycloreversion reactions . 

In spite of the disadvantages, acid-catalysed dehydration of alcohols and base-induced eliminations from halides and sulfonates are used widely in the preparation of alkenes. Typical bases include alkali-metal hydroxides and alkoxides,... 

General Considerations. A sequence involving metalation, alkylation, and fluoride-induced elimination of benzenesulfinate allows the conversion of (2) to a terminal alkene. An analogous sequence involving a double alkylation of (2) provides a 1,1-disubstituted alkene (eq 2). The Uthio derivative of (2) has also been used to prepare cyclopropyUdene derivatives, homo-allylic alcohols, and aUyl silanes via the Julia alkenation. ... 

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... 

Examples of reductive elimination are present in the literature mostly for group 10 metals, e.g. Fig. 4.28. As outlined above, the reaction can be induced by the addition of electron-withdrawing ligands in this case electron-poor alkenes are very effective as ligands. 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

Based on the diene substrates and the reaction conditions, two distinct mechanisms can be followed to induce alkene activation and consequent cycloisomerization. The first mechanism invokes the hydrometallation of an alkene, followed by its subsequent carbometallation into the second one, terminated by a p-hydride elimination. The second mechanism implies the formation of a it-allylic complex either by its prior or later to the cycloisomerization formation (Scheme 7.35). Dienes possessing heteroatom functionality in the allylic position is able to provide it-allylic complex with a metal prior its cyclization. On the other hand, polyenes bearing 1,3-dienes in their system are commonly producing a 1,3-metal complex, which is readily cycloisomerize to give a ii-allyl metal complex after the formation of the new carbon—carbon bonds. 

The process of the diastereoselective formation of 66/68 involves a rhodium-catalyzed C—H activation after precoordination of the metal to the nitrogen of 67 to form the species 70 followed by a iyK-alkene insertion and a reductive elimination. The secondary chiral silylether group should induce the diastereoselective formation of the C (sp ) C(sp ) bond to afford the product with the correct relative configurations (Scheme 23.16). 

---