Polymer supported dipolar aprotic solvent

Closely related to supported PTC are the so-called polymer-supported dipolar aprotic solvents [99]. These are polymers carrying strongly dipolar species which are able to solvate alkali metal cations in hydrocarbon solvents and hence function like solvents. This then activates the attendant anion so the effect overall is very similar to that of PTC. Bound phosphoramides were reported some time ago [103] and more recently bound formamides were shown to be effective [104]. Perhaps somewhat surprisingly phosphine oxide groups have now also proved to form the basis of useful polymer cosolvents [105]. This could prove to be a significant advance because the triphenylphosphine oxide residue is particularly stable and the polymeric cosolvent has been readily re-used with good retention of activity. 

At almost the same time as other polymer-supported phase transfer catalysts were first reported, polymer-supported solvents and cosolvents were found to be effective catalysts for phase transfer reactions 155-156>. Dipolar aprotic solvents such as hexa-methylphosphoramide (HMPA)157, dimethylsulfoxide (DMSO)158), and tertiary amides159,1601 are well known to coordinate strongly with alkali and alkaline earth metal cations, and hence promote nucleophilic displacement reactions of the anions161). Catalysts 44 155-162>163> and 45163),... 

The use of thiazolium salts enables the benzoin condensation to proceed at room temperature. It can also be performed in dipolar aprotic solvents or under phase transfer conditions. Thiazolium salts such as vitamin Bi, thiazolium salts attached to 7-cyclodextrin, macrobicyclic thiazolium salts, thiazolium carboxylate, naphtho[2,l-rf thiazolium and benzothiazolium salts catalyze the benzoin condensation and quaternary salts of 1-methylbenzimidazole and 4-(4-chlorophenyl)-4//-1,2,4-triazole are reported to have similar catalytic activity. Alkylation of 2-hydroxyethyl-4-methyl-l,3-thiazole with benzyl chloride, methyl iodide, ethyl bromide and 2-ethoxyethyl bromide yields useful salts for catalyzing 1,4-addition of aldehydes to activated double bonds. Insoluble polymer-supported thiazolium salts are catalysts for the benzoin condensation and for Michael addition of aldehydes. " Election rich al-kenes such as bis(l,3-dialkylimidazolidin-2-ylidenes) bearing primary alkyl substituents at the nitrogen atoms or bis(thiazolin-2-ylidene) bearing benzyl groups at the nitrogen atoms are examples of a new class of catalyst for the conversion of ArCHO into AiCHOHCOAr. 

Polyamide resin, an alternative polymer support to polystyrene in SPPS. Polyamide resin is chemically more similar to the peptide chain itself The polyamide resin and the peptide chain would then be well solvated in dipolar aprotic solvents. A very successful member of polyamide resins consists of crosslinked polydimethylacry-lamide bearing sarcosine methyl ester side chains. These methyl ester moieties can then be modified by reaction with ethylene diamine. The resulting primary amino groups provide the points of attachment to the resin. In general, the properties of... 

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