Cross-Aldol and Reformatsky-Type Addition

Taking into account the competitive hydrolysis of the silyl enol ether, this reaction is remarkable. The method was shown to be general and was extended to a variety of aldehydes and several a,j9-unsaturated carbonyl compounds giving uniformly 1,4-addition with aldehydes and a mixture of 1,4- and 1,2-adducts in the case of ketones [187]. Later, this aqueous version of the Mukaiya-ma reaction was shown to give near quantitative yields in the presence of a water-tolerant Lewis acid such as ytterbium triflate [188]. Keeping with the same concept,copper(II) triflate [189],indium(III) trichloride [190],tris(pentafluoro-phenyl)boron [191] and scandium(III) triflate in the presence of a surfactant [192] have proved to be active catalysts. 

Related to the aldolization reaction is the Mannich reaction, which proceeds readily in water [193]. Allylsilanes [194] and allylstannanes [131] were also shown to add to iminium salts under Mannich-like conditions. As for the Mukaiyama reaction, the reaction between a vinyl ether with iminium salts was catalyzed by ytterbium triflate in a water/THF mixture. The Henry reaction which can be performed in an aqueous medium has been made more efficient in the presence of surfactants. The nitroaldol products were obtained in 65 -95 % yields [195]. 

Of interest also is the aqueous Reformatsky cross-coupling reaction between a-halo ketones or aldehydes and carbonyl compounds in the presence of zinc, tin and indium. Like the Barbier-type reactions, indium was found to be the most efficient metal and it strongly reduced undesirable side reactions such as the reduction of the halide [134]. As usual in aqueous reactions implicating carbonyl compounds, lanthanum(III) triflate promoted the reaction [146]. 

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