Guanidine transesterification

The guanidinate complex (310) polymerizes LA at RT.893 Mn values increase with M0/Io ratios, but control is only moderate, with Mw/Mn typically >1.5. Transesterification occurs to a large extent as shown by NMR studies on the isolated PLA. Potentially tridentate 2,5-bis(N-arylimino-methyl)pyrroles have been explored as ancillary ligands for yttrium.894 Their reactions with... 

A carbodiimide-grafted polystyrene resin was reacted with tetramethyl-guanidine to give an interesting biguanide structure (Scheme 13). This was assayed as a catalyst for a transesterification reaction.33 Incidentally, resin-bound guanidines are useful bases for processes involving resin capture.34... 

Schuchardt, U. and Lopes, O.C. 1988. Tetramethyl-guanidine catalyzed transesterification of fats and oils A new method for rapid determination of their composition. JAOCS 65 1940-1941. 

Derrien, A., Renard, G. and Brunei, D. Guanidine linked to micelle-templated mesoporous silicates as base catalyst for transesterification. Stud. Surf. Sci. Catal., 1998,117, 445-452. 

Guanidines are strong bases whose use as catalysts is convenient because of their solubility in organic media. Since their basicity lies in the range of common inorganic bases, such as alkaline hydroxides and carbonates, they are suitable for use in a wide range of base-directed reactions such as Michael additions, esterifications and transesterifications. 

Biguanides, which are even stronger bases, open new perspectives but they require special preparations. Novel syntheses of poly-N-alkylated biguanides have been devised from carbodiimides, and recent examples of catalyses with soluble and immobilised guanidines and biguanides are presented with emphasis on the transesterification of triglycerides from vegetable oils. 

Table 1 Comparative efficiencies of soluble and immobilised guanidines and biguanides in the transesterification of rapeseed oil with methanola.

Guanidines have been implemented early as recognition elements, guided by the apparent function of arginine in protein structures. The C2-symmetric, chiral anion receptor 52 was introduced by Lehn, Schmidtchen and de Mendoza consecutively and studied in various modifications (Scheme 13) [23c]. For example, an elaborate system based on 52 provided reasonable enantioselective recognition of amino acids [23c, 28]. Furthermore, bis(guanidinium) compounds catalyze RNA hydrolysis in the presence of external base via phosphodiester complexation [29]. The,se functional elements were joined in receptor 53 to yield a functional transesterification catalyst [30]. 

Tetrasubstituted guanidines catalyse the methylation of phenols using dimethylcarbonate. When DBU is used as catalyst, the reaction temperature is lower. Methylation of acids has also been accomplished. TBD has been demonstrated to be an excellent catalyst for acyl transfer and transesterification reactions. Vinyl acetate with TBD forms AT-acetyl TBD that hy adding benzyl alcohol results in formation of the corresponding acetate and regeneration of the TBD. TBD has also heen used for the ring-opening polymerisation of cyclic esters. ... 

The kinetic picture obtained shows that a high level of synergism results from the preorganization of the guanidine-guanidinium catalytic dyad at the upper rim of the cone calix[4]arene scaffold. The transesterification rate of HPNP catalyzed by 1 mM 27H or 28H is three orders of magnitude higher than that obtained when the two active units are disconnected, i.e. in a buffer composed by equimolar amounts of 31 and 31H at millimolar catalyst concentration. 

Silica-supported guanidine as heterogeneous eatalyst was employed for the transesterification of soybean oil with methanol for biodiesel produetion. The reaction conditions of oil to methanol ratio of 1 12, catalyst amount of 3.5% were used at a temperature of 80° C for 3 h, which gave 98% eon version [95]. 

Recent developments in organocatalytic pathways for the ROP of lactide and several lactones, without adverse transesterification creating polymers that are metal-free and therefore perfect candidates for biomedical and microelectronic applications, have been developed using N-heterocyclic carbenes, thiourea-tertiary amines, and amidine and guanidine bases. Here, the exquisite control, the absence of metal ions, the ready synthetic availability of the catalysts, and the mild reaction conditions are of major importance for tailor-made polyesters, and also have high potential for functional polycarbonates [91, 92]. 

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