Research Prof. H. Mayr

Research

The basis for the presently most comprehensive quantitative approach to organic reactivity is provided in

Angew. Chem. 2002, 114, 97-102; Angew. Chem. Int. Ed. 2002, 41, 91-95.

We are now trying to extend the emerging scales of nucleophilicity and electrophilicity, employ them for developing new synthetic methods, and search for the theoretical basis of the underlying correlations.

The approach is based on eq. (1), which derives the rate constants for the combinations of electrophiles with nucleophiles from the electrophilicity parameter E, the nucleophilicity parameter N and a nucleophile-specific slope parameter s, which typically adopts values of 0.6 < s < 1.2 and can be neglected (i.e. s = 1) for qualitative considerations.

log k_20°C = s (N + E) (1)
E = electrophilicity parameter
N = nucleophilicity parameter
s = nucleophile-specific slope parameter

From correlations as shown in Figures 1 & 2, E parameters for reference electrophiles and N- and s-parameters for reference nucleophiles have been derived (Figure 3) which are recommended as basis for deriving E, N, and s-parameters of further reagents.

Figure 1. Correlation Analysis of the Reactions of Reference Electrophiles with Neutral Carbon Nucleophiles.
Each correlation line corresponds to a p-nucleophile. Seven of 23 benzhydrylium cations are depicted on the abscissa (ref. #180).

Figure 2. Correlation Lines for the Reactions of Benzhydryl Cations and Quinone Methides with Neutral Nucleophiles and Carbanions.
Each correlation line coresponds to an electrophile (ref. #183).

Figure 3. Reactivity parameters N and E of the recommended reference nucleophiles and electrophiles (s in parentheses).
(PDF download, 117 kB)

Reactivity Scales

Nucleophilicity Scales

Nucleophilicity parameters N and s are obtained by plotting log k(20°C) of the reactions of the corresponding nucleophile with several reference electrophiles towards the electrophilicity parameters E of the electrophiles (see Figure 4).

Figure 4.

Since s adopts similar values for nucleophiles with similar reaction centers, nucleophilicity parameters N can be derived from rate constants by using estimated values of s. The results of such one-point calibrations are given in parentheses in Figure 5. Examples for determining N in this way are shown in publication #191.

Figure 5. Reactivity parameters N and s for p-nucleophiles

(for the complete scheme from -5 < N < 14: PDF download, 115 kB).

Electrophilicity Scales

The rate constants of the reactions of an electrophile with reference nucleophiles (which are depicted in the Figure 3) can be used for determining E according to the relationship

log k_20°C = s (N + E) (eq. (1)).

Though a single rate constant is in principle sufficient for determining E, the use of more than one rate constant is recommended in order to examine the validity of eq. (1). The results of one-point calibrations are marked by parentheses in the following Schemes.

Typical examples for evaluating E parameters are given in publications #181 , #187, and #188.

Figure 6. Electrophilicity parameters E of carbocationic electrophiles (PDF download, 103 kB).