Michael addition, montmorillonite-catalyzed

A diverse group of organic reactions catalyzed by montmorillonite has been described and some reviews on this subject have been published.19 Examples of those transformations include addition reactions, such as Michael addition of thiols to y./bunsatu rated carbonyl compounds 20 electrophilic aromatic substitutions,19c nucleophilic substitution of alcohols,21 acetal synthesis196 22 and deprotection,23 cyclizations,19b c isomerizations, and rearrangements.196 24... 

Reactions of furan (5) under solvent-free conditions, catalyzed by Montmorillonite K10, have been described by Cintas [27]. The reaction with methyl vinyl ketone (32) produced Michael addition in positions 2 and 5, whereas reaction with symmetrically substituted cyclic dienophiles produced a mixture of the endo and exo adducts with the kinetically favored endo adduct predominating, except when maleic anhydride (39) was used as the dienophile (Scheme 9.2). 

Molecular iodine-promoted Michael addition is a simple and efficient method for generating 2-pyrrolyl-2-phenyl-l-nitroalkanes in good yields (Scheme 67) [86]. Cr+3-Catsan (Cr+3 exchanged commercially available montmorillonite clay) and ZnCl2, which were first used as Lewis acids for Michael reactions of pyrrole, showed different selectivity under the same conditions [221], In general, while the reactions catalyzed by Cr+3-Catsan... 

P,P] The Michael addition of silyl ketene acetals to enoates requires more strongly Lewis acidic conditions. Both aluminum triflate (98) and clay montmorillonite (78) have been used to catalyze stereoselective reactions (Table 18, Scheme 49). In general, the selectivities obtained for ketene acetals... 

The first Lewis acid-catalyzed asymmetric Michael addition in water was developed by Kobayashi et al, who reported ee s up to 83%. Very recent developments show great promise for further improvement of Michael addition reactions in water. In an elegant study, Kaneda and coworkers used montmorillonite-enwrapped metal triflates to execute C—C bond forming Michael additions. When scandium triflate was employed, adducts were obtained in quantitative yield within a 0.5-3 h at or slightly above room temperature. The catalysts were reusable with no appreciable loss in activity.In another recent study, Lind-strdm and coworkers observed a remarkable ligand acceleration effect in aqueous ytterbium triflate-catalyzed Michael additions. A number of 1,2-diamines and 1,2-aminoalcohols were shown to have a positive influence on the rate of the reaction, the most efficient being tetramethylethylenediamine, which induced a nearly 20-fold rate acceleration. 

In reversing the two steps, i.e. first imidazole addition on a conjugated double bond, then a N-alkylation, the same derivatives may be obtained. The Michael addition of imidazole or its derivatives on an activated double bond can be catalyzed by various reagents enzyme catalysis (Cai et al., 2004), Montmorillonite catalysis (Martin Aranda et al., 1997, 2002) and activated microwave or ultrasound (Zaderenko et al, 1994), or molecular sieve graft (Blasko-Jimenez et al, 2009), KF(Yang et al, 2005), Cu (Acac)2(Lakshmi Kantam et al, 2007), liquid ion ([bMIM] OH) (Xu et al, 2007). 

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