Cesium cyclohexylamide

An extensive series of hydrocarbons has been studied in cyclohexylamine, with the use of cesium cyclohexylamide as base. For many of the compounds studied, spectroscopic measurements were used to determine the relative extent of deprotonation of two hydrocarbons and thus establish relative acidity. For other hydrocarbons, the acidity was derived by kinetic measurements. It was shown that the rate of tritium exchange for a series of related hydrocarbons is linearly related to the equilibrium acidities of these hydrocarbons in the solvent system. This method was used to extend the scale to hydrocarbons such as toluene for which the exchange rate, but not equilibrium data, can be obtained. Representative values of some hydrocarbons withpAT values ranging from 16 to above 40 are given in Table 7.2. 

Allylic conjugation stabilizes carbanions, and pAT values of 43 (in cyclohexylamine) and 47—48 (in THF-HMPA) have been determined for propene. On the basis of exchange rates with cesium cyclohexylamide, cyclohexene and cycloheptene have been found to have pAT values of about 45 in cyclohexylamine. The hydrogens on the sjp-... 

FIGURE 1. Relative rates of tritium exchange with cesium cyclohexylamide in cyclohexylamine31... 

Further experiments designed to elucidate acid-base relationships among weak acids have been carried out more recently by Streitwieser and his coworkers.50 They studied the equilibrium shown in Equation 3.44, with cyclo-hexylamine as solvent and lithium or cesium cyclohexylamide as base. Using spectrophotometric methods to evaluate the position of the equilibrium, they were able to find relative pKa values for a number of hydrocarbons in which the conjugate base is, in most cases, a conjugated aromatic anion. In order to attach... 

Streitwieser et al. [160] and BordweU et al. [161] used the lyate ions of organic solvents such as cyclohexylamine and dimethyl sulfoxide in the determination of the C—H acidity of weak organic carbon acids. Using super base systems such as alkali metal salts of cyclohexylamine [i.e. lithium and cesium cyclohexylamides) [160] and dimethyl sulfoxide (sodium dimsyl) [161] in an excess of these non-HBD solvents, relative acidity scales for weak carbon acids have been established. In this way, values for the ionization of over a thousand Bronsted acids in dimethyl sulfoxide have... 

TABLE 24. Kinetic acidities of cyclopropane and cyclohexane toward cesium cyclohexylamide at... 

Figure 3 Comparison of the structure-reactivity profile for several reaction types, (CsCHA, cesium cyclohexylamide). (From Ref. 81b.)...

The relative equilibrium ion-pair acidity of benzo[b]thiophene toward cesium cyclohexylamide in cyclohexylamine has been determined the pKa value for the 2-position relative to 9-phenyfluorene (= 18.49) is found to be 37.05.260 Surprisingly, the pKa value for the 2-position in benzo[i>]furan is lower than this, showing that, in contrast to the results just described, thermodynamic acidity seems to be enhanced more by an oxygen atom than by a sulfur atom. 

We expect electron density at hydrogen to be related to the acidity of a C-H bond, and Streitwieser and co-workers demonstrated a correlation between 7i3c h and the kinetic acidities (the rates of exchange of C-H protons for tritium catalyzed by cesium cyclohexylamide). The data in Table 1.14 show a good correlation of log fcrei (rate constant relative to that of cyclohexane) with fi3c-H shown in equation 1.53. ... 

Allylic conjugation provides carbanion stabilization, and values of 43 (in cyclohexylamine) and 47-48 (in THF-HMPA) have been determined for propene. On the basis of exchange rates with cesium cyclohexylamide, cyclohexene and cycloheptene have been found to have pK values of about 45 in cyclohexylamine. The hydrogens on the sp carbons in benzene and ethylene would be expected to be more acidic than the hydrogens in saturated hydrocarbons. A pK of 43 has been estimated for benzene on the basis of extrapolation from a series of fluorobenzenes. Electrochemical measurements have been used to establish a lower limit of about 46 for the pK of ethylene. ... 

A number of reviews of carbon acidity have been published during the past decade (1-7). For more than two decades we have measured a number of equilibrium ion pair acidities using cesium cyclohexylamide in cyclohexylamine. These acidities, symbolized as pKcsCHA defined by equation (3) in which 18.49 is the value for the pK of 9-phenylfluorene in aqueous solvents using the method. A more complete discussion of this acidity scale with a summary of our pK g values has recently been published(8). 

To further examine the nucleophilic side of the CH activation continuum, the our group and the Cundari group undertook a computational study [59] of alkali metal amide and alkaline earth metal amide CH activation reactions with alkanes. This study is directly related to classic superbase chemistry using cesium cyclohexylamide-type reagents to induce CH bond deprotonation of alkanes [60]. 

The simplest reaction characteristic of compounds that are regarded as salts of carbanions is that of proton transfer (see Proton Transfers Involving Anions and Dianions) and a number of these have been studied. A typical example is the reaction of methane with lithium amide. This reaction is a model for many known hydrogen isotope exchange reactions of hydrocarbons with lithium and cesium cyclohexylamide whose kinetics have been measured in cyclohexylamine. The model reaction does involve a preliminary weakly bound complex that isomerizes via a transition structure to methyllithium coordinated to ammonia (Figure 3). 

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