Bases weak bulky

Figure 11.26 PEB stability of a conventional DUV 248-nm resist based on poly (4-hydroxystyrene-co-tert-butylcarbonyloxy styrene) versus ESCAP based on poly(4-hyd-roxystryene-co-tert-butyl acrylate). Note that the acid generated in the conventional DUV resist is trifluoromethane sulfonic acid (a very strong, small, volatile acid), while that of the ESCAP resist is camphor sulfonic acid (a very weak, bulky, nonvolatile acid). While the conventional DUV resist polymer is nonannealing, that of the ESCAP is an annealing polymer. [Reproduced from R. Dammel, Practical photoresist processing, SPIE Short Course No. SC616 (2005).]...

R2CHX strong base and nucleophile strong bulky base weak base and nucleophile Sn2 + E2 E2 SnI + El... 

In many reactions, transfer of the anion across the interface and subsequent diffusion into the bulk of the organic phase will not be the rate-determining step when lipophilic catalysts are used, but the effect of less lipophilic catalysts may be influenced more by the anion and the mechanism of the transfer process. Thus, for example, the reactive anion is frequently produced in base-initiated reactions by proton extraction from the substrate at the two-phase interface and diffusion of the ion-pair contributes to the overall kinetics of the reaction. Additionally, the reactivity of the anion depends on its degree of hydration and on its association with the quaternary ammonium cation. In most situations, the activity of the transferred anion is enhanced, compared with its reactivity in aqueous media, as its degree of hydration is reduced, whereas a relatively weak electrostatic interaction between the two ions resulting from the bulkiness of the cation enhances the reactivity of the anion by making it more available for reaction and will be a major factor in the ratedetermining step. 

N[Si(CH3)3]2 is a reasonably strong base it is bulky, which inhibits addition to the carbonyl and it also forms a weakly basic amine, HN[Si(CH3)3]2, which does not interfere in the subsequent reactions. 

Secondary alkyl halides can undergo both SN2 and E2 reactions to give a mixture of products. However, the substitution product predominates if a polar aprotic solvent is used and the nucleophile is a weak base. Elimination will predominate if a strong base is used as the nucleophile in a polar, protic solvent. In this case, bulky bases are not so crucial and the use of ethoxide in ethanol will give more elimination product than substitution product. Increasing the temperature of the reaction favours E2 elimination over Sn2 substitution as explained above. 

R-Li (alkyllithium) Methyllithium Butyllithium sec-Butyllithium fert-Butyllithium Strong base Strong nucleophile when R is not bulky. Deprotonation of weak organic acids, E2 eliminations Sn2/Sn2 displacements, addition reactions, addition-elimination reactions... 

R2N-Li (lithium dialkylamide) Lithium diisopropylamide Strong base, not nucleophilic when R is bulky. Deprotonation of weak organic acids with acidities as high as pKa 35... 

The synthesis of 61 by Lambert and Zhao [135] proofs that it is possible to generate free silylium cations in solution. The success of this work is based on fulfilling the requirements listed in section 8, namely (1) internal stabilization of the cation, (2) steric blocking of the Si+ center, (3) use of a weakly nucleophilic solvent with (4) not too small solvent molecules, and (5,6) use of bulky, weakly coordinating counterions with delocalized anionic charge. 

Classify the base or nucleophile as strong, weak, or bulky. 

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