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.

No abstract

No abstract

A comparison of acidities of six series of analogous oxygen, nitrogen, and carbon acids in dimethyl sulfoxide (DMSO) solution and the gas phase has shown that the element effect usually causes nitrogen acids to be more acidic than their carbon acid counterparts by an average of 17 f 5 kcal/mol, and oxygen acids to be more acidic than their nitrogen counterparts by a like amount. A much smaller difference was observed between the NH acidities of carboxamides and the CH acidities of ketones (1-2 kcal/mol in DMSO and 7-8 kcal/mol in the gas phase). Equilibrium acidities in DMSO for a number of substituted benzamides, acetamides, N-phenylacetamides, acetohydroxamic acids, benzohydroxamic acids, carbohydrazides, and benzenesulfonamides are reported. Acetoand benzohydroxamic acids were found to be 9.8 and 10.1 pKHA units more acidic in DMSO, respectively, than acetamide and benzamide. In each instance the effect of N-alkylation decreased the acidity more than did 0-alkylation, which indicates that the parents are NH, rather than OH, acids in DMSO. Conclusive supporting evidence for the NH acid assignment was provided by the observation that the N-alkylhydroxamic acids exhibited strong homo-H-bonding, whereas the parent acids and their 0-alkyl derivatives did not. Oxidation potentials of hydroxamate anions in DMSO are close to those of 0-alkylhydroxamate ions, confirming that their conjugate acids are NH acids, but in MeOH they are close to those of N-alkylhydroxamate ions showing that their conjugate acids can act as OH acids in hydroxylic solvents. The N-alkyl-and 0-alkylhydroxamic acids exhibited much stronger chelating power toward K+, Na+, and Li+ ions than did the parent acids.

Understanding the inherent response of water to an external electric ͑E͒-field is useful towards decoupling the role of E-field and surface in several practically encountered situations, such as that near an ion, near a charged surface, or within a biological nanopore. While this problem has been studied in some detail through simulations in the past, it has not been very amenable for theoretical analysis owing to the complexities presented by the hydrogen ͑H͒ bond interactions in water. It is also difficult to perform experiments with water in externally imposed, high E-fields owing to dielectric breakdown problems; it is hence all the more important that theoretical progress in this area complements the progress achieved through simulations. In an attempt to fill this lacuna, we develop a theory based on relatively simple concepts of reaction equilibria and Boltzmann distribution. The results are discussed in three parts: one pertaining to a comparison of the key features of the theory vis a vis published simulation/experimental results; second pertaining to insights into the H-bond stoichiometry and molecular orientations at different field strengths and temperatures; and the third relating to a surprising but explainable finding that H-bonds can stabilize molecules whose dipoles are oriented perpendicular to the direction of field ͑in addition to the E-field and H-bonds both stabilizing molecules with dipoles aligned in the direction of the field͒.

No abstract

The properties of the 2-(l-methoxyethyl)-3,4-dimethylthiazolium cation (3) and seven related thiazolium cations have been investigated as models for the behavior of 2-( 1 -hydroxyethyl)thiamin (1). The equilibrium acidity (pAHA+) of 3 in Me2SO solution was found to be 14.1 ± 0.05, and evidence is presented to indicate that the value in aqueous solution is within ±1 unit of this value. These values are 3 units, or more, lower than previous estimates for the pAHA+ at C-2a of the thiazolium moiety in 1, which were based on kinetic measurements. Related thiazolium cations, where Me, RO, or Ph groups were introduced into the C-2 side chain, were found to have acidities in the 9-15 pAHA+ range.The conjugate base obtained by removing the acidic C2a hydrogen atom of 2-(l-hydroxyethyl)thiamin ( 1) is effective in both enzymic and nonenzymic catalysis.1 Its acidity and the acidities of related models, such as the 2,3-dimethylbenzothiazolium cation (2) and the 2-( 1 -methoxyethyl)-3,4-dimethyl-thiazolium cation(3) , have therefore been a matter of considerable interest for the past two decades.

No abstract