Other Supported Guanidines

The formation of a bis(guanidinate)-supported titanium imido complex has been achieved in different ways, two of which are illustrated in Scheme 90. The product is an effective catalyst for the hydroamination of alkynes (cf. Section V.B). It also undergoes clean exchange reactions with other aromatic amines to afford new imide complexes such as [Me2NC(NPr )2]2Ti = NC6F5. ... 

Aminosilicas have been widely studied for use in catalysis, either as a base catalyst or as a support for metal complexes (12). For example, amine functionalized silica can be used to catalyze the Knoevenagel condensation, an important C-C bond forming reaction. Also, the amine sites on the silica can be further functionalized to form supported imines, guanidine, and other species... 

A quaternary ammonium hydroxide ion exchange resin 6 was shown to sequester phenols, hydroxypyrazoles, and other weakly acidic heterocycles.25 The sequestered nucleophiles could also be used as polymer-supported reactants. Similarly, the guanidine-functionalized resin 7 was also shown to be a useful capture agent for weakly acidic nucleophiles, including phenols and cyclic iV-acyl sulfonamides.26... 

Carbamate-bound guanidines have been prepared by the condensation of amines with thioureas linked to insoluble supports as carbamates [519]. The direct reaction of guanidines with resin-bound carbonates or other alkoxycarbonylating agents requires the use of chloroformates or other reactive carbonic acid derivatives [520,521], Carbamate-bound amidines (Entries 7 and 8, Table 3.28) have been prepared by the reaction of amidines with resin-bound 4-nitrophenyl carbonates (0.9 mol/L amidine in DMF/DIPEA, > 4 h [522-524]). 

The metathesis of symmetrical aliphatic carbodiimides is also catalyzed by tungsten imido complexes above 140 Other carbodiimide metathesis catalysts include Cr(ll)/Si02/ iminophosphoranes, imido circonocenes, guanidine supported titanium complexes and group 14 amide complexes. ... 

In more recent work a different polystyrene supported dehydrating agent (Fig. 16.4a) was used for synthesis of 1,3,4-oxadiazoles under thermal and microwave conditions [107]. Addition of a homogeneous base, for example guanidine, improved the cyclodehydration reaction and led to completion under thermal and, even more significantly, microwave conditions. This required subsequent purification steps to remove the base for clean and quantitative isolation of the desired products, however. On the other hand, use of a polymer supported phosphazene base (Fig. 16.4b) as a base additive circumvents the need for additional purification of the reaction mixture for product isolation. 

Scheme 4.4). Other reagents useful for terminal guanidinylation of primary and secondary amines have been reported, in which 5-methylisothiourea [15], protected guanidine [16], pyrazole-l-carboxamidine [17] and benzotriazole-1-carboxamidine derivatives [18] are explored. A solid support-linked guanidinylating reagent, consisted of a urethane-protected... 

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