Catalytic cycle nucleophilic displacement reactions

Once the nucleophile or base (represented by Nu) is in solution in nonpolar (organic) media, the displacement or deprotonation can take place with product formation. In the case of a nucleophilic displacement reaction, would ultimately be ion-paired with the nucleofuge. If the leaving group were X, the ion pair QX would be generated and would be subject to the equlibria formulated above. Starks has offered a now classic diagram of the phase transfer catalytic cycle [10a]. 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

Frequently, Pd(PPh3)4 (or another Pd" complex) is used as a catalyst for the displacement of aUyUc acetates or halides by nucleophiles. A general catalytic cycle is depicted in Scheme 30. If chloride ions are present, no ( 7r-allyl)PdL2 forms. Instead, a ((( -allyl)PdL2Cl intermediate is formed. Thus, different precursors (such as aUylic chlorides or aUylic acetates) or different reaction conditions can lead to different reactivities, regioselectivities, and enantioselectivities. 

The catalytic cycle accounting for the essential features of the cyanide displacement reaction is illustrated in equation 7.2. Note that this is the cycle shown in Sect. 1.4, except that the anions are identified. In essence, the process involves an equilibrium between sodium cyanide and quaternary ammonium cyanide in the aqueous phase, followed by a phase transfer equilibrium occurring across the phase boundary. Once the quaternary ammonium cyanide is present in the organic phase, nucleophilic dis-... 

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