Ritchie nucleophilicity

Linear free energy relationships, see Bronsted equation, Dual substituent parameter (equations), Hammett equation(s), Quantitative structure-activity relationships, Ritchie nucleophilicity equation... 

Revolutions, scientific (Th. Kuhn) 214, 217 Ritchie nucleophilicity equation 158ff. Ruthenium complexes, in Pschorr-type cyclizations 282... 

K). C.D. Ritchie, Nucleophilic Reactivity toward Cations, Acc. Chem. 

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

Turning to cation-anion recombination reactions we find that most of the quantitative studies have been by Ritchie,who defined a nucleophilic constant by Eq. (7-71),... 

Table 7-5. Values of N+ for nucleophilic systems at 23 °C (after Ritchie, 1972).

The N+ relationship, as discussed above, is a systematization of experimental facts. The equation of Scheme 7-4 has been applied to nearly 800 rate constants of over 30 electrophiles with about 80 anionic, neutral, and even cationic nucleophiles covering a range of measured rate constants between 10-8 and 109s 1 (Ritchie, 1978). Only about a dozen rate constants deviated from the predicted values by more than a factor of 10, and about fifty by factors in the range 5-10. It is therefore, very likely that this correlation is not purely accidental. Other workers have shown it to be valid for other systems, e.g., for ferrocenyl-stabilized cations (Bunton et al., 1980), for coordinated cyclic 7r-hydrocarbons (Alovosus and Sweigart, 1985), and for selectivities of diarylcarbenes towards alkenes (Mayr, 1990 Mayr et al., 1990). On the other hand, McClelland et al. (1986) found that the N+ relationship is not applicable to additions of less stable triphenylmethyl cations. 

This is not the only equation that has been devised in an attempt to correlate nucleophilic reactivity. For reviews of attempts to express nucleophilic power quantitatively, see Ritchie, C.D. Pure Appl. Chem., 1978, 50, 1281 Duboc, C. in Chapman Shorter Correlation Analysis in Chemistry Recent Advances, Plenum NY, 1978, p. 313 Ibne-Rasa, K.M. J. Chem. Educ., 1967, 44, 89. See also Hoz, S. Speizman, D. J. Org. Chem., 1983, 48, 2904 Kawazoe, Y. Ninomiya, S. Kohda, K. Kimoto, H. Tetrahedron Lett., 1986, 27, 2897 Kevill, D.N. Fujimoto, E.K. J. Chem. Res. (S), 1988, 408. 

What this implies is that given one equilibrium constant for addition of a nucleophile of known 7 to a carbonyl compound, one could estimate the equilibrium constant for addition of another nucleophile to the same carbonyl compound. This requires knowing the slope of the plot of log K versus y this slope is not very sensitive to the nature of the carbonyl compound, but it is at least known that A H2o/ MeOH depends on the electron-withdrawing power of the groups bonded to the carbonyl, and thus more information is needed to estimate an equilibrium constant for strongly electron-withdrawing substituents. From Ritchie s studies of nucleophile addition to trifluoroacetophenone," we can derive a slope for log K versus 7 of 0.42, distinctly less than the value of 1 for formaldehyde or simple benzaldehydes. 

It is more difficult to interpret micellar effects upon reactions of azide ion. The behavior is normal , in the sense that k /kw 1, for deacylation, an Sn2 reaction, and addition to a carbocation (Table 4) (Cuenca, 1985). But the micellar reaction is much faster for nucleophilic aromatic substitution. Values of k /kw depend upon the substrate and are slightly larger when both N 3 and an inert counterion are present, but the trends are the same. We have no explanation for these results, although there seems to be a relation between the anomalous behavior of the azide ion in micellar reactions of aromatic substrates and its nucleophilicity in water and similar polar, hydroxylic solvents. Azide is a very powerful nucleophile towards carboca-tions, based on Ritchie s N+ scale, but in water it is much less reactive towards 2,4-dinitrohalobenzenes than predicted, whereas the reactivity of other nucleophiles fits the N+ scale (Ritchie and Sawada, 1977). Therefore the large values of k /kw may reflect the fact that azide ion is unusually unreactive in aromatic nucleophilic substitution in water, rather than that it is abnormally reactive in micelles. 

Quantitative data on the nucleophilic reactivity of PhSO towards aryl diazonium ions (127) show that it is also much less reactive in this reaction than such nucleophiles as CN or PhS- (Ritchie and Virtanen, 1972). 

Ritchie (1975) has found that the reactivity of nucleophiles toward a number of different types of electrophilic centers can be correlated remarkably well by a very simple equation (192), where kNu is the rate constant for reaction of a... 

The nucleophilicity of the amine is another factor affecting reactivity, and changes in it have been sometimes responsible for the observed scattering in the Brpnsted plots. The Ritchie equation80 (equation 11) has been applied to a variety of reactions in which nucleophilic addition to, or combination with, an electrophilic center is rate-limiting. 

RITCHIE EQUATION ELECTROPHILE NUGLEOPHILIC GATALYSIS NUCLEOPHILIC COMPETITION NUCLEOPHILICITY ALPHA EFFECT ELECTROPHILICITY... 

Ritchie was the first to directly measure the absolute reactivity of cations toward solvent and added nucleophiles. The cations were highly stabilized examples, triarylmethyl cations bearing stabilizing substituents such as 30 and 31, xanthylium ions (e.g., 32) and tropylium ions (e.g., 33). The feature (and requirement) of these cations was that they had a lifetime in water such that kinetics could be followed by conventional or stopped-flow spectroscopy whereby one solution containing the pre-formed cation was added to a second solution. The time required to mix these solutions was the important factor and limited measurements to cations with lifetimes longer than several milliseconds. The lifetimes in water for 30-33 are provided below. Lifetime is defined as the reciprocal of the first-order rate constant for the decay of the cation in solvent. 

One of the surprising features of Ritchie s studies was a constant selectivity. While the absolute rate constants differed considerably over the series of cations, the selectivities toward pairs of nucleophiles were constant and independent of the reactivities of R" ". Rate constants were correlated by a simple two parameter equation... 

Rate constants for the reactions of carbocations with added nucleophiles are obtained in LFP experiments as the slopes of linear plots of first-order rate constants for cation decay against the concentrations of added nucleophile. One of the first detailed studies using LFP showed that rate constants for the parent triphenylmethyl cation did not adhere to the simple Ritchie N+ relation of Eq. 13, but that the slope of a plot of log Nu versus N+ was significantly < 1 This finding has been verified... 

Mt is, however, possible to measure the rates of reaction of nucleophiles with fairly stable carbocations see Ritchie Acc. Chem. Res. 1972, 5. 348-354 Ritchie Minasz Kamego Sawada J. Am. Chem. Sac. 1977, 99, 3747 McClelland Banait Steenken J. Am. Chem. Soc. 1986, 108, 7023. 

For further examples of the difficulty in assigning a reagent with an intrinsic nucleophilicity, see C. D. Ritchie, Accts. Ckem. Res., 5, 348 (1972). 

The extent of this failure is evident from comparisons of experimental measurements of rate and equilibrium constants. One comparison in the literature is provided by Ritchie and coworkers study of the relatively stable cation, pyronin (the 3,6-bis(dimethylamino)xanthylium cation 71) with a series of nucleophiles.252 Another example is McClelland s measurements of rate and equilibrium constants for the reactions of halide and acetate ions with the trityl cation.19 As already mentioned fluoride and acetate are less reactive than bromide and chloride despite their equilibrium affinities being much greater. This is reflected indeed in the much lower rates of solvolysis of the fluoride and acetate than bromide or chloride as leaving groups... 

Far from confirming a dependence of nucleophilic selectivity on the reactivity of the carbocations, Ritchie observed that selectivities were unchanged over a 106-fold change in reactivity.15 He enshrined this result in an equation (29) analogous to that of Swain and Scott, but with the nucleophilic parameter n modified to N+ to indicate its reference (initially) to reactions of cations, and with the selectivity parameter s taken as 1.0, that is, with no dependence of the selectivity of the cation on its reactivity (as measured by the rate constant for the reference nucleophile, kn2o for water). 

Rappoport and TaShma s work removed a major difficulty for Ritchie s analysis and helped pave the way for Mayr to exploit fully the wide applicability and simplicity of Equation (29) for predicting rates of reactions of electrophiles with nucleophiles. Mayr pointed out that Equation (29) could be rewritten as Equation (30), in which log ka corresponds to the rate constant for reaction of the electrophile under study with a reference nucleophile266 (chosen as water by Ritchie) which, in so far as it is characteristic of the... 

Mayr initially defined a set of electrophilic parameters for the benzhydryl cations using a reference nucleophile, which was chosen as 2-methyl-1-pentene.268,269 Values of E were then defined as log k/k0, where k0 refers to a reference electrophile (E= 0), which was taken as the 4,4 -dimethoxybenzhydryl cation. Plots of log k against E for other alkenes are thus analogous to the plots of logk against p fR in Fig. 7 except that the correlation is referenced to kinetic rather than equilibrium measurements. However, they differ from plots based on the Swain-Scott or Ritchie relationships in which log k is normally plotted against a nucleophilic parameter, that is, n or N+, rather than E. 

It seems clear therefore that more reactive cations than those for which Ritchie s N+ relationship was developed, show a distinct dependence of selectivity between nucleophiles upon the stability and reactivity of the carbocation. Richard has confirmed that for a very stable benzylic carbocation, represented by the bis-trifluoromethyl quinone methide 57, the N+ regime is restored and that a plot of log k against N+ for reactions of nucleophiles, including amines, oxygen and sulfur anions, the azide ion, and a-effect nucleophiles, shows a good correlation with N+.219... 

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