Salts aminium

Treating the iminothiazoles with acyl chlorides yields the aminium salts (415>, which are acylation and alkoxycarbonylation reagents (Scheme 237) (731). 

The aminium salt-induced cyclodimerization of stilbenes in 1,1,1,3,3,3-hexafluoropropanol gave mixtures of indane and tetrahydronapthalenes. The solvent effects are consistent with an electron-transfer mechanism via a radical cation. ... 

Cycloadditions can also be catalyzed by cation radicals generated from aminium salts.537-539 In fact, these are cation-radical chain reactions mimicking Diels-Alder reaction. Interestingly, 2,4-dimethyl-1,3-pentadiene undergoes either a Brpnsted acid-catalyzed cycloaddition [Eq. (6.93)] or cation-radical-catalyzed cycloaddition [Eq. (6.94)] to yield different addition products 539... 

As for halogens as oxidizing reagents, bromine has proved more useful than its homologs chlorine and iodine. It was employed as early as 1879 on di- and tetra-methyl-p-phenylenediamine [27-29] and early in this century, Wieland used it to generate the aminium salts of triarylamines and tetraarylhydrazines [30, 31]. Since bromine adds readily to unsaturated as well as to some strained ring compounds, it is not expected to be very useful in the context of the radical cations discussed here. 

This type of reaction can be induced also by radiolysis [133,134] or by chemical oxidation, particularly with tris-(p-bromophenyl)aminium salts (cation radical catalyzed Diels-Alder reaction) [10]. The scope of this reaction and its synthetic utility have been delineated in detail. The results unambiguously support a free radical cation chain mechanism [10]. 

A possible reconciliation of these seemingly conflicting results lies in the lifetimes of the individual radical cations under the different experimental conditions. In the PET experiment the lifetime is dictated by the rate of intersystem crossing, a hyperfine induced process, which often falls into the range 10-9 to 10-8 sec. The aminium salt catalyzed rearrangement is a free radical cation chain reaction. Under these conditions the radical cation lifetime is determined by the diffusion-limited encounter with a neutral molecule, which may be quite slow at the low temperatures of these experiments. Although any barrier to isomerization is larger at the lower temperatures, it is well-known that the barriers to many radical cation reactions are reduced drastically. 

In the past decade, there has appeared only one report, which can be classified under this heading, on the monodeoxygenation of spiro-adamantane-l,2-dioxetanes induced by catalytic amounts of tris-(2,4-dibromophenyl)-aminium hexachloroantimonate <1998T6939>. An electron-transfer mechanism from the aminium salt has been proposed for the observed formation of various ketones. Reports citing the electrophilic ring opening of dioxetanes by boron trifluoride, trifluoroacetic acid, and various other Lewis acids have been summarized in CHEC-II(1996) <1996CHEC-II(1B)1041>. 

Amino-2-imino-4-methylthiazoline hydrochloride (112) and aminium salts (114) react with carboxylic acid anhydrides to give thiazolo[3,2-6]-s-triazoles (113) (76JHC1225, 74JHC459). 

The halogens were also used as oxidizing reagents bromine in particular has proved quite useful. It was employed in 1879 on di- and tetramethyl-p-phenylene-diamine, [49-51] and Wieland used it to generate numerous aminium salts derived from triarylamines and tetraarylhydrazines [52,53]. 

ESR spectroscopies. In fact, the addition of one equivalent of 22a-c to an acetonitrile solution of aminium salt causes the replacement of the ESR signal of the aminium radical cation with a broad singlet signal shifted at a higher field (Fig. 3). 

Table 10. Product distribution in the reaction of 70 with various aminium salts of different oxidation strength in acetonitrile/methanol 9 1 [170]...

For example, addition of 2 equivalents of a nonnucleophilic base, 2,6-di-tert-butylpyridine, to the reaction system almost totally suppressed the reaction (Table 9). This proposes that acid catalyzed enolization is important for the a-Umpolung. In support of mechanism 1 the yields of the a-methoxylated products increased with decreasing oxidation strength of the oxidant (Table 10), since endergonic oxidation of the ketone 70 is slowed down (Fig. 2). On the other hand, in the presence of stronger aminium salts the ketone tautomer is oxidized which however does not lead to deprotonation in the benzylic position... 

In a comparative study the authors demonstrated that aminium salt oxidation nicely complements the regioselectivity of other a-hydroxylation methods [173-180], which do not occur via electron transfer. Thus, using the Vedejs reagent [178] two positional isomeric products 83 and 84 were obtained from 82, whereas aminium salt oxidation leads to only one product. 

Under oxidative conditions (anode, aminium salts) in the presence of methanol/base most enol ethers are known to be dimethoxylated, with or without dimerization [235,251-255], thus opening up a way to formal a-Umpolung products 158. 

Reaction of various thiiranes 208, catalyzed by aminium salts (tris-(4-bromophenyl)aminium hexachloroantimonate 209 or tris-(2,4-dibromophenyl)aminium hexachloroantimonate 210) in freshly distilled dichloromethane, led to desulfurization of starting thiiranes to alkenes 211 in good yield (typically 80%) (Scheme 53) <1995T8935>. The mechanism of desulfurization of thiiranes catalyzed by aminium salts is presented in Scheme 54. 

These derivatives were described as uronium salts by analogy with phosphonium salts, but X-ray analysis demonstrated that HBTU, HATU, and HAPyU crystahize as aminium salts (guanidinium A-oxides) rather than uronium salts.t NMR studies of HAPyU in solution show the same structure as that observed in the crystahine form.P This discovery changes the nomenclature of these compounds, but their acronyms are unchanged, and they are described in the literature as uronium or aminium salts without discrimination. HBTU should be reformulated as A-[(l//-benzotriazol-l-yl)(dimethylamino)methylene]-A-methyl-methanaminium hexafluorophosphate A-oxide, HATU as A-[(dimethylamino)(l//-l,2,3-triazolo[4,5-b]pyridin-l-yl)methylene]-A-methylmethanaminium hexafluorophosphate A-oxide, and HAPyU as 0-(7-azabenzotriazol-l-yl)-A,A,A, A -bis(tetramethylene)uronium hexafluorophosphate. 

The stereochemistry of cyclobutanation was first studied using the aminium salt (3+ ) method. The cyclobutadimerizations of both trans- and ci5-anethole were found (Scheme 9) to be stereospecific [28]. 

Scheme 10. Mechanism of the aminium-salt induced cation radical cyclobutadimerization of trans-anethole.

When the latter mixture is treated with the aminium salt at 0°C, the mixture isomerizes completely to the more stable trans,anti,trans isomer. Evidently, the cyclodimerization is reversible at the higher temperature. It is also noteworthy that even the reversal is stereospecific, since none of the other possible diaster-eoisomers are formed. This result is nicely interpreted in terms of the ionization of the trans,syn,trans cyclobutadimer to a long bond cation radical, followed by concerted fragmentation to trans-anethole and the trans-anethole cation radical, avoiding the formation of any cw-anethole. The cyclodimerization of phenyl vinyl ether gives both trans (or anti) and cis (or syn) isomers initially, but subsequent reaction induces some isomerization of the less stable cis isomer to the more stable trans isomer. This is believed to occur through the intervention of a distonic 1,4-butanediyl type cation radical. 

When the cation radical of this alkyne is generated by y radiolysis in a solid matrix at 77 K and then warmed to 150 K, the ESR spectrum of the 1,2,3,4-tetramethyl-1,3-butadiene cation radical is observed. An analogous intramolecular reaction was also observed even in a rigid matrix at 77 K. The feasibility of the cycloaddition step itself is therefore indicated, but little work has yet been done in respect of the aminium salt or PET induced cycloadditions of alkynes in solution at ambient or near-ambient temperatures. Whether a chain or catalytic alkyne cyclodimerization can be effected is yet unclear, as is the potential fate of the cyclobutadiene products. 

The aminium salt catalyzed cation radical Diels-Alder reaction... 

It is noteworthy that the latter cyclodimerization cannot be carried out using the aminiiiTTi salt method, because both the reactant and the Diels-Alder product react with the aminium salt in a destructive manner. 

---