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An accurate spectrophotometric method of determining relative equilibrium acidities of carbon acids in DMSO has been developed. The pK scale in DMSO has been anchored by comparisons of values obtained by the spectrophotometric method with those obtained potentiometrically in the 8 to 11 pK range. As a result, the pK of fluorene, formerly arbitrarily taken as 20.5, has been raised to an absolute value of 22.6. The pA"s of other carbon acids previously reported, including nitromethane, acetophenone, acetone, phenylacetylene, dimethyl sulfone, acetonitrile, and the corresponding indicator pX's must also be raised. The pK's have been found to be correlated with heats of deprotonation in DMSO by potassium dimsyl, and evidence is presented to show that pK measurements in DMSO are free from ion association effects. Data are presented which indicate a pK of 35.1 for DMSO. In the methane carbon acids, CHyEWG, the order of acidities is NO2 » CH3CO > CN, CH3SO2. The differences amount to 12.2 and 6.8 kcal/mol, respectively, which are believed to be of a comparable magnitude to gas-phase substituent effects. Carbon acids wherein the charge on the anion resides mainly on oxygen, such as ketones and nitroalkanes, are found to be weaker acids in DMSO than in water by 5.5 to 9.6 pK units. On the other hand, carbon acids wherein the charge on the anion is delocalized over a large hydrocarbon matrix, such as in the anion derived from 9-cyanofluorene, are stronger acids in DMSO than in water. Factors that may contribute to this reversal are discussed. The scale of pX's for 9-substituted fluorenes in DMSO is shown to be expanded when compared to the earlier pK scale determined by the Hmethod. A rationale is presented. The apparent relative acidities of fluorenes and phenylacetylene differ by 6 and 11 pK units, respectively, for cyclohexylamine (CHA) vs. DMSO solvents and benzene vs. DMSO solvents. Similarly, in benzene, acetophenone is a stronger acid than fluorene by ca. 6 pK units, whereas in DMSO acetophenone is a weaker acid by 3.2 pK units. These differences result from ion association effects that occur in solvents of low dielectric constant (benzene, ether, CHA, etc.) causing relative acidities to be dependent on the reference base, as well as the solvent. This is not true in strongly dissociating solvents of high dielectric constant, such as DMSO. A list of 13 indicators covering the pAT range 8.3 to 30.6 in DMSO is presented. Equilibrium acidities of weak (i.e., pX ^15) carbon acids have been measured by a variety of methods3 in a variety of solvents including ether,4a benzene,4b diglyme,5 cyclohexylamine (CHA),6 mixtures of dimethyl sulfoxide (DMSO) with ethanol, methanol, or water,7•8•9 and pure DMSO.10 We have chosen DMSO for our studies because it allows accurate measurements to be made spectrophotometrically for many different types of carbon acids over a wide range of pK (ca. 30 pK units) with apparently little or no interference from ion association effects.1 Furthermore,

The syntheses and properties of novel, extremely strong uncharged polyaminophosphazene bases up to a high level of steric hindrance are described. Most of the systems were readily prepared in up to molar quantities and conveniently recovered from their salts. They are of appreciable to high chemical and thermal stability. Crystal structures of their salts were determined in order to parametrize a force field, which is utilized in molecular modeling studies. The latter offer a rationalization of the high conformational mobility of these systems. These bases cover a range of ca. 15 pK units in basicity and extend the range of uncharged auxiliary bases by ca. 19 pK units up to DMsopKsH+ values of 34-35. They are proposed as a novel class of auxiliary bases for deprotonation of very low acidic compounds where chemists have been so far left to classical metalorganic bases. Depending on the basicity range and the degree of steric protection of the basic center, these systems are particularly applicable to E2 elimination or to in situ generation of highly reactive "naked" anions.Uncharged nitrogen bases have a long tradition as widely used and often irreplaceable standard reagents in organic synthesis; many attempts to improve basicity and to reduce nucleophilicity have been reported"] since the classical work of Hunig et a1.L21. Until recently, amidines and guanidines as described by Eiter et al.f31, Eschenmoser et al.f4I, and Barton et al. ['] were generally considered the strongest synthetically useful auxiliary bases. In the early sixties there was a single report by Flynn et a1.L8] concerning applications of a somewhat stronger isobiguanide base. This base was even commercially available, but surprisingly has not been accepted by synthetic chemists.In connection with our own activities in this field we exploited among other s t r u~t u r e s [~~'~] the structural type of peralkylated triaminoiminophosphoranes. The simplest representative 1 was already known" l] and in our hands turned out to be of unprecedented base strength among kinetically active uncharged bases. The derivatives of this leading structure we synthesized s~bsequently['~,'~] proved to be chemically very stable, highly versatile and easy to recover auxiliary bases with a very broad range of steric shielding of the basic enter[',^*'^]. The Concept of Phosphazene BasesAt the outset there was the question, whether it would be possible to further enhance basicity by the same formal "homologization" which converts weakly basic tertiary amines to strongly basic triaminoiminophosphoranes.

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