Heterocyclic salts

The importance of the solvent, in many cases an excess of the quatemizing reagent, in the formation of heterocyclic salts was recognized early. The function of dielectric constants and other more detailed influences on quatemization are dealt with in Section VI, but a consideration of the subject from a preparative standpoint is presented here. Methanol and ethanol are used frequently as solvents, and acetone,chloroform, acetonitrile, nitrobenzene, and dimethyl-formamide have been used successfully. The last two solvents were among those considered by Coleman and Fuoss in their search for a suitable solvent for kinetic experiments both solvents gave rise to side reactions when used for the reaction of pyridine with i-butyl bromide. Their observation with nitrobenzene is unexpected, and no other workers have reported difficulties. However, tetramethylene sulfone, 2,4-dimethylsulfolane, ethylene and propylene carbonates, and salicylaldehyde were satisfactory, giving relatively rapid reactions and clean products. Ethylene dichloride, used quite frequently for Friedel-Crafts reactions, would be expected to be a useful solvent but has only recently been used for quatemization reactions. ... 

The same approach allows preparation of various pyrylium carboranes from the corresponding 4/f-pyran carboranes 174a,b and 175b by the action of acetyl perchlorate,245 perchloric acid,244 and triarylamine radical cation salts,244,245 as well as electrochemically.243 The oxidation of condensed 4H-pyran 345 with trityl perchlorate, 2,3,5,6-tetra-substituted 4f/-pyrans 431 and 153 with tropylium tetrafluoroborate or 153 with heterocyclic salt 393 led to useful preparations of pyrylium salts 394,330 395a,359 and 395b,360 respectively. 

Asymmetrical dyes can be obtained by condensation of the y-formyl-substituted dinudear polymethine with the corresponding heterocyclic salt. 

In [5, 6] the emphasis of the authors was on the reactions of cyclic emanine 4 with some a,(3-unsaturated carbonyl compounds. These reactions involve both noncyclic chalcone 5 and arylidenecyclohexanones 7 (Scheme 3.3). The solvent used was acetonitrile and the duration of the reaction varied from 15 to 48 h. Zhang et al. [6] also showed that at room temperature the reaction of enamine 4 with unsaturated ketone 9 led to the isolation of 5-hydroxyhexahydroimi-dazo[l,2- ]pyridine 10 without further dehydration. Subsequently, compound 10 reacted with another equivalent of unsaturated ketone to yield a heterocyclic salt 11 (Scheme 3.4). 

The thermolysis of iodonium salts in which their counteranion is a halide may be performed in the molten state or in solution the products are an iodoarene and a haloarene. The reaction which is a nucleophilic aromatic substitution is, however, not preparatively useful an exception was 3-indolyl phenyliodonium trifluoroace-tate which on heating with various chlorides and bromides in DMSO afforded variable mixtures of 2- and 3-haloindoles. By contrast, the jV-methyl and N-benzyl analogues gave only 2-chloro derivatives [58], Sometimes useful products may be obtained from the thermolysis of dibenziodolium or other heterocyclic salts, as exemplified in the preparation of l-iodo-2-(2-iodophenyl)naphthalene [59] ... 

Many new heterocyclic salts are novel polynuclear compounds having multiple heteroatoms, and have been described during the past 20 years by research groups in the former Soviet Union, and more recently by groups in Egypt. These salts are primarily intermediates for cyanine and merocyanine dyes and have been converted to these dyes, but usually not to spiropyrans. New classes of spiropyrans should be obtainable from these salts. 

N-heterocyclic salt such as chiral imidazolium salt 2.87 is the precursor of N-heterocyclic carbene, chiral imidazol-2-ylidene ° 2.88. 

Addition of aluminium and gallium trichloride to phosphorus ylides 96a-96c leads to the formation of the heterocyclic salts 97a-97c (Scheme 34). 

Related heterocyclic salts, imidazolinium chlorides, have recently been prepared through a solvent free reaction of a formamidine with dichloroethane and a base. Solvents are used in this reaction as excess dichloroethane is used and the residue is triturated and washed using either acetone or toluene. However, it is a superior route to these valuable A-heterocyclic carbene... 

Using the above synthesis, a wide variety of triarylsulfonium salt photoinitiators can be prepared. In Table 1 are shown some representative triarylsulfonium salt photoinitiators which were prepared during the course of this research. All the compounds in this table are well characterized crystalline compounds with well defined melting points, elemental analyses, ultraviolet, proton, and nmr spectra (T7). The ability of triarylsulfonium salts bearing non-nucleophilic counterions to serve as photoinitiators is completely general and includes all the symmetrical, unsymmetrical, substituted and unsubstituted as well as polynuclear and heterocyclic salts shown in Table 1. 

It has been reported that dichloromethane, frequently used in the reaction of poly(methimazolyl)borates, is not an innocent solvent, but is able to form slowly heterocyclic salts (Fig. 5.26) [H2C(mt)2BR2]Cl, (BR2 = BH2, BH(mt), 9-borabicyclononyl) which have been characterized by X-ray single-crystal studies.103... 

Upon heating, dithiazine 137 suffers thermal rearrangement to give heterocyclic salt 138 (Equation 31) <2004RJ01222>. 

Salts of o-halogeno-N-heterocyclics, such as 2-fluoro-l-methylpyri-dinium tosylate, and related compounds have been used successfully in a variety of reactions, particularly as agents for esterification, lactoni-zation, and the preparation of amides and thiolic esters They have been used also for the preparation of iodides from alcohols and acid fluorides from the acids as well as for Beckmann rearrangements and reductions, e.g. of a-hydroxyketones to ketones . Various condensed heterocyclic salts have been prepared from l-acylmethyl-2-chloropyridinium salts 2-Dialkylaminopyridinium salts have been used as phase transfer catalysts... 

Ring expansion of heterocyclic salts with diazo compds. 

A -azirines, review 28, 643s31 benzofuran-2(3H)-ones 31,199 bridgehead azides 31,176 heterocyclic salts 31,779... 

Addition of cyanide ion to cyanogen gave the novel heterocyclic salt (53) [125]. 

Energetic heterocyclic salts containing SF5-propyl substituents on imidazole, 4-amino-1,2,4-triazole (134a-c, 135a-d) and pyridine (136) moieties were synthesized (Scheme 33). These salts have moderate densities (>1.4 g/cm ). The calculated heats of formation for dinitramide-containing salts are higher than perchlorate and nitrate salts (Table 12) [124]. 

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