Alkene displacement

In competition, the less substituted alkenes displace more highly substituted alkenes (this can be virtually 100%, for example, 2,3-dimethylcy-clohexene versus 1,2-dimethyl cyclohexene.)... 

A few other interesting and potentially important consequences of the reversible formation of five-membered zirconacycles include stereo- and regioselective skeletal rearrangement, as exemplified by Scheme 1.57 [197], and 1,3-C=C bond and Zr migration (Scheme 1.58) [191,192], supporting the associative mechanism for alkene displacement (Generalization 22 ). 

Palladium (II) compounds coordinate to alkenes to form 71-complexes. Roughly speaking, the decrease of alkene electron density caused by coordination to an electrophilic Pd(II) compound enables an attack by nucleophiles on the coordinated alkenes. The attack of a nucleophile with concomitant formation of a carbon-palladium e-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. Unlike the products of mercuration which are stable and isolable, palladation product 1 is usually unstable and undergoes rapid decomposition. Palladation is followed by two reactions. The elimination of H-Pd-X from 1 to form the vinyl compounds 2 is one path, resulting in nucleophilic substitution of the alkene. Displacement of the Pd in 1 by an other nucleophile effects nucleophilic addition of the alkene to give 3. Depending on the reactants and conditions, either nucleophilic substitution of the alkene or the nucleophilic addition to the alkene takes place [4,5]. 

Another imponant application of the alkene displacement process is in the synthesis of a whole series of alkyldiethoxyboranes of very high optical purity by displacement of a-pinene from 3-pinanylalkyl-... 

This mesylate cyclizes an aqueous base. The more nucleophilic end of the other alkene displaces -e mesylate with inversion to make the cis ring junction much preferred by a 5/5 fused system,. ater then attacks the tertiary cation to give the next intermediate. 

In a homolytic cyclization, a side-chain alkene displaces a neighbouring nuclear bromine atom on heating the compound in toluene with tributyltin hydride and AIBN. An optical purity of 39% is observed when an asymmetric A/ -substituent is present. Palladium dia< tate-catalysed cyclization (reviews [B-41,3069,3505] of the bromoester (39.1) gives the oxindole in moderate yield the product may consist of a mixture of stereoisomers for example, when R = Ph, one isomer predominates in the ratio 60 13. 

Thus, using an alkene-displacement approach, a range of P-diketiminato- and NHC-stabilized K -alane complexes of [M(CO)4]... 

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