Phosphonium coupling base effect

The resulting phosphonium salts, 79 (n = 0,l), containing up to 2 phenylenevinylene moieties, were then coupled with terephthalic dialdeyde, 80 (m = 0), or 4,4 -stilbene dicarbaldehyde, 80 (m=l), to produce the target structures, terminally f-butylated OPVs (78) up to the heptamer (x=6). With results based on these model oligomers, the effective conjugation length of unsubstituted PPV 60... 

HYDROXYBENZOTRIAZOLE AS ADDITIVE IN COUPLINGS OF N-ALKOXYCARBONYLAMINO ACIDS EFFECTED BY PHOSPHONIUM AND URONIUM SALT-BASED REAGENTS... 

Phosphonium and uronium salt-based reagents effect couplings by first reacting with the anion of the starting acid (Figure 2.22, path A). The benzotriazolyl ester is then one of the two possible precursors of the peptide. Operating in a research climate in which HOBt was commonly used as additive, Hudson rationalized that if the ester is the precursor of the peptide, additional HOBt in the form of the anion would be beneficial because it would by mass action promote formation of the ester. A favorable effect from adding HOBt was reported, so one variant of the use of... 

The phosphonium and carbenium salts are efficient reagents for activating and coupling A-alkoxycarbonylamino acids as well as peptide acids. However, the requirement for tertiary amine to effect the reaction has several implications. The base renders hydroxyl groups subject to acylation. Hence, the side chains of serine and threonine and any hydroxymethyl groups of a resin that have not been derivatized... 

Quaternary ammonium and phosphonium halides were used as the phase transfer catalysts. For effective coupling, high-shear mixing and high concentrations of polymer and base were used. 

The Heck reaction is a C-C coupling reaction where an unsaturated hydrocarbon or arene halide/triflate/sulfonate reacts with an alkene in presence of a base and Pd(0) catalyst so as to form a substituted alkene. Kaufmann et al. showed that the Heck reaction carried out in presence of ILs such as tetra-alkyl ammonium and phosphonium salts without the phosphine ligands, resulted in high yields of product. They attributed the activity to the stabilizing effect of ammonium and phosphonium salts on Pd(0) species. Carmichael et al. used ionic liquids containing either A,A -dialkylimidazolium and A-alkylpyridinium cations with anions such as halide, hexafluorophosphate or tetrafiuoroborate to carry out reactions of aryl halide and benzoic anhydride with ethyl and butyl acrylates in presence of Pd catalyst. An example of iodobenzene reacting with ethyl acrylate to give trans-et vy cinnamate is shown in Scheme 14. 

Contemporary methods for sohd-phase couplings of peptide fragments are based on the use of phosphonium or uronium salts. HOBt was introduced as an additive with BOP, and more recently HOAt was used as an alternative in carbodiimide couplings instead of HOBt and shown to be more effective. Phosphonium and uronium salts of HOAt, namely PyAOP, HATU, and HAPyU, have also been used with success in CSPPS. 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

Phosphonium-based reagents PyBOP and PyBroP were found to be very effective in this transformation, affording oxytripyrrolidino-phosphonium intermediates in high yields, with PyBroP being slightly superior to PyBOP. Weak nucleophiles such as iV-methyl benzenesulfonamide, imidazole, indole, diethyl malonate, and phenol were coupled smoothly with oxytripyrrolidino-phosphonium in high yields. Sodium terr-butoxide was found to be an excellent base to ensure a clean and complete reaction. 

Thus, the role of strong acidic moieties in the stability of the 0 02 redox couple in [PeeewlCl has been studied via addition of hydrochloric acid, (a common side product in phosphonium-based ILs synthesis) [31] by cyclic voltammetry. Figure 18.11 depicts that in the presence of HCl the reduction process for oxygen is irreversible [28] as opposed to the pure [PeeewlCl where chemical reversibility of the electrogenerated superoxide anion is observed which exemplify of the detrimental effect of HCl on the reversibility of the O2 reductimi process. 

---