Schwesingers’ phosphazene base

The highly hindered phosphazene base 85, termed P4-f-Bu, is one of a family of similar compounds introduced by Schwesinger, which are completely non-nucleophilic and are useful in performing alkylation and elimination reactions.50-52 Its use in the latter application is illustrated in Protocol 22 by the... 

The A -diphenylmethylene protection has also been used in the solid-phase mode for the synthesis of either unnatural amino acidst 1 or peptides (Scheme 67)J 1 Thereby, both Merrifield or Wang resins were used and the best base proved to be the organic soluble, nonionic phosphazene bases of Schwesinger, e.g. 2-tert-(butylimino)-2-(ethylamino)-l,3-di-methyl-l,3,2-diazaphosphinane (BEMP). As electrophiles alkyl halides,aldehydes, and Michael acceptors have been used. 

Further increase of nitrogen atom basicity was achieved by the combination of Schwesinger s phosphazene base concept and the Alder s idea of the disubstituted 1,8-naphthalene spacer, as reported by Raab [94]. l,8-Bis(hexamethylttiaminophosphazenyl) naphthalene (HMPN) (126), represents the up to date most basic representative of this class of proton sponges , as evidenced by the theoretically estimated proton affinity of 274kcal mol and the measured pABH (MeCN) = 29.9 (Figure 2.12). HMPN is by nearly 12 orders... 

Calculations have revealed that the proazaphosphatrane bases are approximately equal in thermodynamic basicity to the Schwesinger P2 phosphazene bases (Table 2.14). The RHF/ 6-3IG calculations of proton affinities indicate that phosphazene (Z = NH) and phosphorus ylide (phosphorane, Z = CH2) counterparts (146 and 145) are stronger than the parent Verkade superbase 139. Higher basicity is associated with the higher degree of delocalization of the positive charge in the protonated iminophosphoranes and phosphorus ylides as a result of the more electropositive character of phosphorus atom [67]. On the other hand, phosphoms oxides 147 and 150 are weaker than 139, but still above the superbasicity borderline. The strained polycycles 138, 149 and 148, as representatives of superbases... 

Schwesinger, R., Hasenfratz, C., Schlemper, H. et al. (1993) How strong and how hindered can uncharged phosphazene bases be Angewandte Chemie-International Edition,32,1361-1364. Rodima, T, Kaljurand, I., Pihl, A. et al. (2002) Acid-base equilibria in nonpolar media. 2. Self-consistent basicity scale in THF solution ranging from 2-methoxypyridine to EtPl(pyrr) phosphazene. The Journal of Organic Chemistry, 61, 1873-1881. 

Diaryl sulfides in high yields were obtained in the reaction of aryl iodides and activated aryl bromides under CuBr-catalyzed conditions using Schwesinger s phosphazene base, which, most likely, was also coordinated as a ligand (Scheme 3.27) [58]. Similar results, but with longer reaction time, were observed using DBU (l,8-diazabicydo[5.4.0]undec-7-ene). 

An increasingly important aspect of the chemistry of phosphazenes such as Bu -Pl(37), Bu -P2 (38) and Bu -P4 (39), is the basicity of the iminic N atoms that makes them very strong neutral or non-ionic bases (Schwesinger s bases). " The rates of proton transfer from l-nitro-l-(4-nitro-phenyl)alkanes NO2C6H4-CHRNO2 (R = H, Me, Et, Pr ) to the phosphazene (37) are unexpectedly slow and the kinetic parameters of the process indicate the importance of steric effects on the hindered reaction centre. ... 

Hartmann, F., Mootz, D. and Schwesinger, R., Grystal chemistry of uncharged phosphazene bases. 1. Structures of two hydrates and an acetate of... 

Phosphonium salts and phosphazene bases developed by Schwesinger and Schlemper and Schwesinger et (Scheme 14) are of broader interest for epoxide polymerization. These soft and highly delocalized organic compounds reduce aggregation phenomena and favor ionic dissociation by increasing the interdistance between charges in ion pairs. 

In 1985, Schwesinger [10] introduced phosphazenes (triaminoiminophosphorane skeletons), which contain a phosphorus atom [P(V)] bonded to four nitrogen functions of three amine, and one imine substituents, as organobases containing a phosphorus atom. They are classified as P bases, based on the number (n) of phosphorus atoms in the molecule [11]. 

The exceptional basicity of phosphazenes (iminophosphoranes) has been discovered by Schwesinger [83]. The phosphazene derivatives have been proved to be chemically very stable, kinetically active and highly versatile, and the large number of these bases has been synthesized. The gas phase and solution equilibrium basicity measurements for a large number of phosphazenes are conducted by Kaljurand et al. [84], and their PA and pAia values are published in various papers. These measurements show that phosphazenes surpass in their basicity the derivatives of acychc or bicyclic guanidines, amidines and vinamidines (Table 2.11). Extraordinary basicity of phosphazenes was theoretically rationalized by Maksic et al. [85], in terms of effective resonance stabilization of protonated molecules [86]. 

Schwesinger, R., et al. Extremely Strong, Uncharged Auxiliary Bases Monomeric and Polymer-Supported Polyaminophosphazenes (P2-P5), Liebigs Ann. 1996,1055-1081. A review on phosphazenes. 

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