Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Soft carbon nucleophiles stereochemistry

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. [Pg.15]

The stereochemistry of the Pd-catalyzed allylation of nucleophiles has been studied extensively[5,l8-20]. In the first step, 7r-allylpalladium complex formation by the attack of Pd(0) on an allylic part proceeds by inversion (anti attack). Then subsequent reaction of soft carbon nucleophiles, N- and 0-nucleophiles proceeds by inversion to give 1. Thus overall retention is observed. On the other hand, the reaction of hard carbon nucleophiles of organometallic compounds proceeds via transmetallation, which affords 2 by retention, and reductive elimination affords the final product 3. Thus the overall inversion is observed in this case[21,22]. [Pg.292]

Loss of stereospecificity in the addition of soft carbon nucleophiles can occur if the rate of nucleophilic attack is slow, due, for example, to extreme steric bulk, e.g. NaCH(SChPh>2,167 of the nucleophile (equation 154). In this case, the initially displaced OAc has sufficient time to return and attack the ir-allyl complex. Acetate anions (vide infra) are capable of either ligand or metal addition, thus scrambling the stereochemistry of the starting allyl acetate. [Pg.616]

Substitution reactions of allylic substrates with nucleophiles have been shown to be catalyzed by certain palladium complexes [2, 42], The catalytic cycle of the reactions involves Jt-allylpalladium as a key intermediate (Scheme 2-22). Oxidative addition of the allylic substrate to a palladium(o) species forms a rr-allylpal-ladium(n) complex, which undergoes attack of a nucleophile on the rr-allyl moiety to give an allylic substitution product. The substitution reactions proceed in an Sn or Sn- manner depending on catalysts, nucleophiles, and substituents on the substrates. Studies on the stereochemistry of the allylic substitution have revealed that soft carbon nucleophiles represented by sodium dimethyl malonate attack the TT-allyl carbon directly from the side opposite to the palladium (Scheme 2-23). [Pg.119]

Palladium-catalyzed displacement reactions with carbon nucleophiles are not only regioselective but also highly stereoselective. In the first step, displacement of the leaving group by palladium to form the 7i-allylpalladium complex occurs from the less hindered face with inversion. Subsequent nucleophilic substitution of the intermediate 7t-allylpalladium complex with soft nucleophiles such as amines, phenols, or mal-onate-type anions also proceeds with inversion of the stereochemistry. The overall process is a retention of configuration as a result of the double inversion. [Pg.344]


See other pages where Soft carbon nucleophiles stereochemistry is mentioned: [Pg.300]    [Pg.193]    [Pg.802]    [Pg.331]    [Pg.497]    [Pg.47]    [Pg.64]    [Pg.497]    [Pg.496]    [Pg.397]   


SEARCH



Carbon nucleophile

Carbon nucleophiles

Carbon stereochemistry

Soft carbon

Soft carbon nucleophile

Soft carbon nucleophiles

Soft nucleophile

Soft nucleophiles

© 2024 chempedia.info