Guanidine-based catalysts

For an example of asymmetric Michael reaction using guanidine-based catalysts, see Ma, D. Cheng, K. Tetrahedron Asym. 1999, 10, 713-719. 

Thereafter, Terada applied related binaphthyl guanidine base catalyst 24 to the asymmetric 1,4 addition of diphenylphosphite to nitroalkenes (Scheme 5.45) [81]. 

Finally, we wish to state that our intention in this exercise is to highlight potentially high E-factors associated with the route for the synthesis of a particular catalyst, and therefore the importance of catalyst recycling and low catalyst loading. The readers should not attempt to judge the capability of each guanidine-based catalyst based on data we provided due to the assumptions given above. 

Recently, an a-hydroxylation reaction of tetralone-derived ketoesters 31 with cumene hydroperoxide 32 in the presence of bis-urea guanidine based catalyst 33 was developed (Scheme 3.28). ... 

The wide applicability of the PK reaction is apparent in the synthesis of pyrroles, for example, 45, en route to novel chiral guanidine bases, levuglandin-derived pyrrole 46, lipoxygenase inhibitor precursors such as 47, pyrrole-containing zirconium complexesand iV-aminopyrroles 48 from 1,4-dicarbonyl compounds and hydrazine derivatives. The latter study also utilized Yb(OTf)3 and acetic acid as pyrrole-forming catalysts, in addition to pyridinium p-toluenesulfonate (PPTS). 

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... 

Furthermore, the same sol-gel matrices have been used in a system where acid and base catalysis occur in the same pot without quenching either catalyst [29]. In this case, the acids were either entrapped Nafion (perfluorinated resin sulfonic super acid, a3) or entrapped molybdic acid (M03-Si02, a2), while the bases were two ORMOSILs (organically modified silica sol-gel materials), one with H2N (CH2)2NH(CH2)3 groups (bi) and the other guanidine base residues (b2) (Scheme 5.12). 

While the significance of the bifunctional Brpnsted base catalysts has been illustrated in the previous sections, few examples rely solely on a Brpnsted base interaction for asymmetric catalysis. However, in the past few decades, a novel catalyst system has emerged as a powerful promoter of chiral transformations. The guanidines have gained the reputation as super bases in organic transformations. 

Ooi has recently reported application of chiral P-spiro tetraaminophosphonium salt 37 as a catalyst for the highly enantio- and diasterioselective direct Henry reaction of a variety of aliphatic and aromatic aldehydes with nitroalkanes (Scheme 5.51) [92]. Addihon of the strong base KO Bu generates in situ the corresponding catalyhcally active triaminoiminophosphorane base A. Ensuing formation of a doubly hydrogen-bonded ion pair B positions the nitronate for stereoselective addition to the aldehyde. This catalyst system bears many similarities to guanidine base catalysis. 

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. 

Stronger solid base catalysts can be prepared by grafting guanidine bases to mesoporous silicas. For example, the functionalization of MCM-41 with 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD), as shown in Fig. 2.40, afforded a material (MCM-TBD) that was an effective catalyst for Michael additions with ethylcya-noacetate or diethylmalonate (Fig. 2.41) [136]. 

With 2-methyT1,1,3,3-tetrabutyl guanidine as catalyst, the decarboxylation proceeds at temperature lower than those described with conventional base catalysts. The result is even better compared with using 4-N,N-dimethy-lamino pyridine (DMAP) as shown in scheme 42. 

The strongly hindered guanidine base l,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was supported to MCM-41 silica by the same post-modification methodology and the catalyst was utilized in the condensation reaction between benzaldehyde and heptanal to produce jasminaldehyde 21 (Scheme 3.7). ... 

Zhu A, Jiang T, Han B, Huang J, Zhang J, Ma X (2006) Study on guanidine-based task-specific ionic liquids as catalysts for direct aldol reactions without solvent. New J Chem 30 736-740... 

Guanidine participating organic reactions could be schematically classified into two types of reactions catalytic and stoichiometric, in which a guanidinium salt composed of guanidine like 2 and either an acid or nucleophile plays an important role as a common active complex. In the former type of reaction, 2 is repeatedly used as a free base catalyst, whereas a guanidinium salt is formed in the latter (Figure 4.2). 

Preparation and use of supported TMG (1) as a novel base catalyst is discussed in a review elsewhere [6]. Heterogeneous guanidines are provided as environmentally friendly base catalysts and, thus, precise discussion on supported superbases is given in Chapter 6. Guanidine chemistry has been excellently surveyed in books [7]. This chapter focuses on the synthetic utility of TMG (1) and its analogues in organic synthesis and application of modified guanidine to asymmetric reactions. 

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