Ammonium-salt-catalyzed reactions nature

The nature of the ammonium-salt-catalyzed deamination and cross-linking of silazanes and aminosilanes is not well understood. Experiments indicate clearly that NH3 is evolved when [CH3(R)SiNH] -type silazanes participate in the reaction and that CH3NH2 is evolved when methylami-nosilanes or -silazanes are used. Such eliminations would lead to some cross-linking through conversion of Si2N units to Si3N, as shown in equation 6. 

Although chiral ammonium salts have proven their utility in catalyzing a variety of different reactions on numerous occasions, applications in natural product synthesis have so far been limited mainly to a-alkylations and Michael-type reactions. Nevertheless, over the past several years increased efforts to broaden the scope of chiral PTCs towards even more complex reactimis have been undertaken (362-364). 

Some other reported curing agents are quinone [81], chloranil [81], anhydroformaldehyde aniline [82], methylolureas and melamines [83], ethylenediamine-formaldehyde products, and paraformaldehyde. Resorcinol novolac resins can be cured with paraformaldehyde [84] and the mildly acid nature of resorcinol helps to catalyze the curing reaction [84]. Ammonium salts have also been reported as accelerators for the curing of phenolic resins [85]. 

In 1991, Lee et al. reported the synthesis of (—)-esermethole (35) [39], which is an important precursor for the synthesis of the naturally occurring anticholinesterase agent (—)-physostigmine (36). Again a phase-transfer-catalyzed enantioselective a-alkylation was the key step in this procedure. Using the quaternary ammonium salt 39, a reasonably enantioselective cyano-methylation of indole 37 to obtain intermediate 40 could be carried out. CycU-zation and subsequent reactions lead to the synthesis of (—)-esermethole (35) (Scheme 6). 

One of the most important reactions in the synthesis of 77 -allylic palladium complexes is the oxidative addition of allylic substrates, such as allylic halides, carboxylates, carbonates, ammonium or phosphonium salts, to Pd(0) complexes. Besides its preparative value, this reaction is the origin of 77 -allyl palladium intermediates in most Pd-catalyzed transformations of allylic substrates, and the nature of the Pd allyl often determines the outcome of the reaction. 

Jones et al. (67) also prepared a wide range of water soluble ammonium dithiocarbamate salts, [NH4][S2CNR2] (Fig. 5). They result from the iifitial reaction of the amine with concentrated aqueous ammonia in ethanol, followed by later addition of carbon disulfide at low temperatures. For example, diethanolamine, HN(CH2CH20H)2, forms a yellow precipitate in 65% yield. Castro et al. (68) studied the kinetics and mechanism of the reactions of piperidine, pyrrolidine, morpholine, and benzylamine (69) with carbon disulfide in ethanol (Fig. 6). They proceed via a dithiocarbamic acid intermediate (4), which in turn yields the dithiocarbamate anion (5) upon proton loss to the amine. While for pyrrolidine, formation of the dithiocarbamic acid is rate determining and proceeds to the dithiocarbamate irreversibly, for both morpholine and benzylamine, the transformation is reversible. Further, in these cases the ethoxide anion is found to catalyze the transformations. They have also determined that pyrrolidine is 200 times more nucleophilic toward carbon disulfide than piperidine, despite the later being only slightly more basic, a feature that may relate to the irreversible nature of the formation of pyrrolidine dithiocarbamate. 

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