Nucleophile common

The Lewis base that acts as the nucleophile often is but need not always be an anion Neutral Lewis bases can also serve as nucleophiles Common examples of substitutions involving neutral nucleophiles include solvolysis reactions Solvolysis reactions are substitutions m which the nucleophile is the solvent m which the reaction is carried out 8olvolysis m water (hydrolysis) converts an alkyl halide to an alcohol... 

The mechanism involves initiation and propagation steps. Most commonly, the reactions are initiated by SET between the nucleophile and the a-nitroalkane (Equation 10.16). Strongly basic nucleophiles commonly undergo SET without photostimulation but weakly... 

A key stage in the biosynthesis of piperidine alkaloids is reached with the formation of A -piperideine. For the elaboration of diverse alkaloids, this intermediate undergoes condensation with a variety of nucleophiles, commonly a /3-keto-acid. (A similar situation is found for pyrrolidine alkaloid biosynthesis see, e.g., Scheme l).1,2 Existing evidence on Lythraceae alkaloid biosynthesis, taken up again below, indicated that condensation occurred in this case between A piperideine (17) and acetoacetic acid to give pelletierine (26), further elaboration yielding alkaloids like (22). In the event, however, labelled pelletierine was found not to be a precursor for (22) or (23).8 Negative evidence is always difficult to interpret, but is here made persuasive by the fact that other precursors that were fed concurrently were incorporated. Conclusive support for these results depended on others outlined below. 

In summary, to rank the nucleophilicity of nonbonding electron pairs reacting in protic solvents with soft electrophOes such as R-X, rank first by softness, then by basicity (within the same attacking atom). However, for nonbonding electron pairs reacting with harder electrophiles such as a proton or a carbonyl, rank by basicity. Very reactive electrophiles like carbocations are not selective and react with the most abundant nucleophile (commonly the solvent). 

Synthetic Methods.—The preparation of phosphate esters by 5n2 attack of phosphate ester anions on carbon is receiving more attention following the realization that the poor nucleophilicity commonly associated with such anions is due to solvation and ion-pairing effects. Thus tetra-methylammonium di-t-butyl phosphate reacts with primary and secondary alkyl iodides in aprotic solvents to give the corresponding triesters (1) from which the t-butyl groups are readily removed by trifluoracetic acid. The proposal of a similar S 2 mechanism in the reaction of triphenylphosphine and ethyl azodicarboxylate with a phosphate diester in the presence of an... 

A list of nucleophiles commonly employed in Sn reactions is given in Table 3.2, together with examples of typical reactions involving these nucleophiles. Look at this list, and check that all the nucleophiles shown are encompassed by the above definition. 

Table 3.2 Nucleophiles commonly employed in reactions and some typical examples for each...

Fig. 1.2b) classes wherein an enolizable nucleophile (commonly an enamine, nitronate or 1,3-dicarbonyl) cyclizes onto an sp carbon. Enolexo-exo-tet cyclizations (Fig. 1.2c) are less common however, and tend to occur predominantly in cyclopropanation reactions. Indeed, alkylations using secondary amine catalysis are difficult under standard organocatalytic conditions owing to problems associated with the alkylation of the catalyst itself, although various methods have been adopted to address this. Finally, exo-trig cyclizations of heteroatoms onto sp centres (Fig. 1.2d) are a useful way of constructing enantiopnre heterocycles. 

In catalytic allylic alkylation, a nucleophile, commonly an enolate of a doubly activated a-carbon, replaces an allylic leaving group, commonly a carboxylate such as acetate. The reaction occurs in the presence of a catalytic amount of Pd(0). 

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