Triflate salts, amine neutralization

Similar aza-Diels-Alder reactions of Danishefsky s diene with imines or aldehydes and amines in water took place smoothly under neutral conditions in the presence of a catalytic amount of an alkaline salt such as sodium triflate or sodium tetraphenylborate to afford dihydro-4-pyridones in high yields (Eq. 12.49).117 Antibodies have also been found to catalyze hetero-Diels-Alder reactions.118... 

Pyridine can become involved in nucleophilic substitution when very reactive trifiates are being synthesized. One approach to minimize this disadvantage is to replace it with sterically hindered bases, such as 2,6-di-f-butyl-4-methylpyridine, 2,4,6-trisubs-tituted pyrimidines, or nonnucleophilic aliphatic amines (usually W,W-diisobutyl-2,4-dimethyl-3-pentylamine). No salt formation appears to take place under these conditions. The triflic anhydride seems to be the direct triflating agent and the base only neutralizes the triflic acid formed. Numerous alkyl triflates have been prepared in the literature by the above method. Some recent examples of triflates prepared from alcohols are illustrated in eqs 2 and 3. As an exception, 2,6-dinitrobenzyl alcohol does not react with Tf20 although similar sulfonyl esters could be prepared. ... 

The aza-Diels-Alder reaction of imines with diene of Danishefsky is an important route to 2,3-dihydro-4-pyridones. A number of Lewis acids have been used to catalyze the reaction in organic solvents. In water the reaction was realized by acid catalysis via iminium salts or by Bronsted acids. The montmorillonite K-10 catalyzed this cycloaddition in water or in aqueous acetonitrile in excellent yield. Recently Kobayashi has performed the reaction in water at room temperature under neutral conditions in two (imine - - diene) or three (aldehyde -b amine -b diene) component versions by using sodium triflate as catalyst. Imine intermediates from the indium-mediated reaction, in aqueous medium at 50° C, between aromatic nitro compounds and 2,3-dihydrofuran undergo aza-Diels-Alder cycloadditions to give tetrahydroquinoline derivatives in good overall yields. ... 

Kobayashi and coworkers showed that Lewis add surfactant-combined catalysts such as scandium tris(dodecyl sulfate), Sc(03SCi2H25)3, or copper bis(dodecyl sulfate), Cu(03SCi2H25)2, were efficient catalysts for the three-component Mannich-type reaction, with 73-95% yield being obtained in neat water.Neutral salts such as sodium triflate and sodium iodide catalyzed the condensation reaction in water between preformed imines and silicon enolates, or the three-component Mannich-type reaction using aromatic amines, with 49-93% yields and 0-80% diastereoselectivities. Mechanistic studies indicated that both sodium triflate and the Mannich adduct itself cooperatively promote the reaction. 

A third and very practical alternative is the resolution of racemic acyliron complexes by crystallization with (5)-(+)- or (/ )-(-)-camphersulfonic acid via the diastereomeric hydroxycarbene salts and subsequent neutralization. Conversion of the enantiomerically pure acetyliron complexes to a-alkoxy complexes and subsequent trimethylsilyl triflate-mediated removal of the methoxy group provides enantiomerically pure (ethylidene)iron complexes.Methylation of alkynoyliron complexes with Meerwein s salt has been reported to give cationic methoxy-substituted alkynylcarbenes, which can be transformed into the corresponding (aminocarbene)iron complexes upon reaction with primary amines. " Meerwein s salt can also be used to form cationic... 

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