Linear free energy relationships nucleophilicity scales

Recent works of Mayr and co-workers4-14 have illustrated this trend. In fact, these authors have established, in contrast to the accepted opinion about the relative character of the experimental electrophilicity/nucleophilicity scales for many reactions in organic and organometallic chemistry, that it would be possible to define nucleophilicity and electrophilicity parameters that are independent of the reaction partners. Mayr et al. proposed that the rates of reactions of carbocations with uncharged nucleophiles obey the linear free energy relationship given by 4-14... 

The nucleophilicities are found to be dependent on electronic, steric, and symbiotic effects, and limited series obeyed a constant selectivity , a reactivity-selectivity or a dual-parameter linear free-energy relationship. The conclusion made was that because of different blends of the effects, the construction of a substrate-independent nucleophilicity scale was impossible at present, but an approximate scale was presented. In nucleophilic reactions on relatively long lived vinyl cations, the steric effects predominate, but at constant steric effects, reactivity-selectivity relationships were found for very short series of substrates. Additional data are required for constructing more reliable nucleophilicity scales toward neutral and positively charged vinylic carbons. 

The philicity of singlet carbenes is an important concept to classify carbenes that was systematically studied by Moss. [9-11] The relative reactivity (selectivity) of a series of singlet carbenes in cyclopropanation reactions with electron rich and electron poor carbenes was used to quantify the carbene philicity. An empirical carbene philicity scale with a parameter niQ- (where X and Y are the substituents at the carbene center) was defined (Figure 1). Electrophilic carbenes show Wqxy values below 1, nucleophilic carbenes above 2, and ambiphiles are between. [10] Ambiphilic carbenes act as an electrophile towards electron-rich alkenes and as a nucleophile towards electron-poor alkenes. The niQ- Y values obey an empirical linear free energy relationship with the Taft substituent parameters and Oj. This allows to estimate the niQ- Y values of unknown carbenes. 

Just as with the pK and pKb scales for Bronsted acids and bases, other scales have been developed to relate the strength of one Lewis acid (electrophile) to another, or one Lewis base (nucleophile) to another. For example, scales based upon the relative reactivities of various nucleophiles and electrophiles in common organic reactions, such as Sn2 transformations, have been defined. We will cover these scales in Chapter 8 after we examine linear free energy relationships. Further, after examining the scales, we will discuss what factors make for a good nucleophile and electrophile. 

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