Solvent nucleophilicity, scale

A comparison of the rates of solvolysis of the tert-butyldimethylsulfonium ion and the 1-adamantyldimethylsulfonium ion presents strong evidence that the solvent dependence of the tert-butyldimethylsulfonium ion solvolysis rates is governed primarily by solvent nucleophilicity effects. Leaving-group contributions based upon 1-adamantyldimethylsulfonium ion solvolyses are better incorporated into the establishment of the solvent nucleophilicity scale based upon triethyloxonium ion solvolysis. Alternative solvent nucleophilicity scales based upon the solvolysis of S-methylbenzo-thiophenium ions are discussed. Analyses of the extent of nucleophilic participation by the solvent in the solvolyses of methyldiphenyl-sulfonium and benzhydryldimethylsulfonium ion will be presented. The relative nucleophilicities of various anionic and neutral nucleophiles toward the triethyloxonium ion in ethanol have been determined. 

Solvent Nucleophilicity Scales Based upon R-X+ Substrates... 

The solvolysis rate constants of 2,2,2-trichloroethyl chloroformate CClgCHjCXZICXZ in 30 different solvents were well correlated with the extended GW equation, using the NT solvent nucleophilicity scale and the YCl solvent ionizing scale, with sensitivity values of 1.28 0.06 and 0.46 0.03 for I and m, respectively. The kinetic solvent isotope effect ( MeOH/ MeOD) of 2.39 was in accordance with an 5 2 mechanism probably assisted by general base catalysis. " The specific rates of solvolysis of 2-butyn-l-yl... 

Many other solvent parameters have been defined in an attempt to model as thoroughly as possible solvent effects on the rate constants for solvolysis. These include (a) Several scales of solvent ionizing power Tx developed for different substrates R—X that are thought to undergo limiting stepwise solvolysis. (b) Several different scales of solvent nucleophilicity developed for substrates of different charge type that undergo concerted bimolecular substitution by solvent. (c) An... 

The development of these various solvent parameters and scales has been accompanied by the realization that there are uncertainties in the physical property of the solvent that is correlated by a particular parameter in cases where systematic changes in solvent structure affect several solvent properties. Consider a reaction that shows no rate dependence on the basicity of hydroxylic solvents, and a second reaction that proceeds through a transition state in which there is a small transition state stabilization from a nucleophilic interaction with the hydroxyl group. The rate constants for the latter reaction will increase more sharply with changing solvent nucleophilicity than those for the former, and they should show a correlation with some solvent nucleophilicity parameter. This trend was observed in a comparison of the effects of solvent on the rate constants for solvolysis of 1-adamantyl and ferf-butyl halides, and is consistent with a greater stabilization of the transition state for reaction of the latter by interaction with nucleophilic solvents. ... 

Other theoretical studies discussed above include investigations of the potential energy profiles of 18 gas-phase identity S 2 reactions of methyl substrates using G2 quantum-chemical calculations," the transition structures, and secondary a-deuterium and solvent KIEs for the S 2 reaction between microsolvated fluoride ion and methyl halides,66 the S 2 reaction between ethylene oxide and guanine,37 the complexes formed between BF3 and MeOH, HOAc, dimethyl ether, diethyl ether, and ethylene oxide,38 the testing of a new nucleophilicity scale,98 the potential energy surfaces for the Sn2 reactions at carbon, silicon, and phosphorus,74 and a natural bond orbital-based CI/MP through-space/bond interaction analysis of the S 2 reaction between allyl bromide and ammonia.17... 

Various scales of nucleophilic reactivity for anions have been in use for several years (Swain and Scott, 1953 Edwards, 1956 Pearson et al., 1968 Ritchie, 1972 see also Bunnett, 1963 Ibne-Rasa, 1967 Hartshorn, 1973b), but quantitative scales of solvent nucleophilicity have only recently been developed. Peterson and Waller (1972) derived a scale of solvent nucleophilicity (APW) based on the... 

As already established for combinations of cations with n-nucleo-philes [33,160,219], the situation is less complicated for the reactions of carbocations with 7r-systems. Solvent polarity plays only a minor role (Section III.D.3) and, for many 7r-systems, the relative reactivity has been found to be electrophile-independent (Fig. 10, Section III.D.4.b). Also for these systems, the construction of a universal nucleophilicity scale is not unproblematic, however. Remember Fig. 11, which shows that An2CH + reacts 3.4 times faster with allyltrimethylsilane than with 2-methyl-2-bu-... 

The relative nucleophilicities of anionic reagents towards standard vinylic substrates have been tabulated and from these it is apparent that azide ion is a moderately strong nucleophile in protic solvents, and even stronger in dipolar aprotic solventsThe need for caution in construction of such nucleophilicity scales has been stressed, however, since trends appear to be dependent on the structure of the substrate and some anomalies in the effects of solvent have been noted. 

That a relative nucleophilicity scale based on one solvent and one substrate may not be applicable for different conditions and systems is clearly demonstrated by the work of Beltrame et al. (1968). 

The piperiaiae(DMF)/ N7(98%DMr) ratios for (125a), (125b) and (125c) at 0°are 10,1-3 and 0-08, respectively, i.e. decrease in the same order and to about the same degree as the ratios in ethanol. While the change in the ratio in dimethylformamide may be accounted for by the enhancement of reactivity in the (slightly aqueous) dipolar aprotic solvent (Parker, 1965), the occurrence of the same trend in ethanol, and especially the dependence on the structure of the attacked substrate, points to the need for caution in the construction of nucleophilicity scales. 

In Chapter 18, Bentley reviews development of scales of solvent nucleophilicity and applies the Bentley-Schleyer equation to determine N and Y values for sulfuric acid. Also, Bentley examines here the validity of the Taft... 

The development of quantitative scales of solvent nucleophilicity based on solvolysis reactions is reviewed. Effects of solvent nucleophilicity are illustrated by product studies, by correlations of kinetic data, and by quantitative estimates of competing nucleophilic pathways, including competing solvent-assisted and anchimerically assisted pathways. The problem of separating quantitatively the nucleophilic and electrophilic solvent contributions to reactivity is discussed. Recent results on the nucleophilicities of aqueous sulfuric acid mixtures are presented. 

Quantitative Scales of Solvent Nucleophilicity. Solvolytic studies in solvents of low nucleophilicity led to renewed interest in quantitative measures of solvent nucleophilicity. Peterson and Waller (44) derived a scale of solvent nucleophilicity (Npw) from the rates of displacement by solvent of tetramethylenehalonium ions (VI) in liquid sulfur dioxide. The reaction is approximately half-order in carboxylic acid, possibly because dimer-monomer preequilibrium occurs (44). More recently, hydrolysis of the iodonium salt (VIII) in competition with anionic or solvent nucleophiles was studied. A scale of nucleophilicity relative to water was obtained by quan-... 

For situations where solvent nucleophilicity may be a factor, Kevill (8) favors the use of the extended Grunwald-Winstein equation (equation 1). Scales of NOTs and OTs values based upon the use of methyl tosylate and 2-adamantyl tosylate as model SN2- and SNl-reacting substrates have been developed (15, 16). Also Y scales have been developed for other anionic leaving groups using 1-adamantyl or 2-adamantyl derivatives (17-19), where Sn2 reaction is impossible or severely hindered. 

Kevill and Lin developed (29) an NKL scale of solvent nucleophilicities using equation 6. 

From the deviations, a scale of nucleophilicity was derived. Halogenated acetic acids were included, on the basis of reactivities with halonium ions. Other scales appeared from the Schleyer group (5, 6) at about the same time. The various nucleophilicity scales were used to correlate solvolysis rates by now familiar four-parameter equations AG = N + mY or AG = sN + mY. (G — free energy N = solvent nucleophilicity Y = solvent ionizing power s = sensitivity m = sensitivity.) Previously, parameters for such equations had not been determined. 

We next investigate the effect of the ratio R in equation 5. In the original nucleophilicity scales, the nucleophilicities of acetic and formic acids were set equal, based on interpretations of solvolysis data in the literature and on the nucleophilicities of these in S02 solvent, a property perhaps somewhat distantly related to the nucleophilicity in a pure solvent. We have now calculated yA as a function of R yA changes sign at an R value near 1.2 (Figure 7). Equation 3 shows that N values become scaled A values when yA becomes 0 at an R value near 1.2. We note that changing R is merely a roundabout way to vary the proportion of A and B in the N and Y parameters. 

Taking into account these reservations (as much as possible), we searched the literature to answer the following questions (1) Is there a substrate-independent nucleophilic scale toward vinylic carbon (2) If not, are different scales applicable for addition and substitution (3) What is the role of electronic and steric effects and hard-soft interactions on the nucleophilic order Because of the lack of space, we will mention only briefly the solvent dependence of the nucleophilicity. 

Table III gives kN3 /kpiperidine rati°s- The values for the three (5-chlo-rovinyl ketones (35) demonstrate two important features. First, they increase in the dipolar aprotic solvent, by relative increase in the reactivity of the anionic nucleophile. Second, and less expected, the order of the ratios changes from N3 < piperidine for E-PhCOC(Ph) = CHCl to N3 > piperidine for the cyclic P-haloketone. Consequently, difficulties are expected in attempts to construct even a qualitative nucleophilicity scale toward vinylic carbon.

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