Stibonium salts, quaternary

Electrochemical studies of quaternary arsonium and stibonium salts have mostly been carried out in aqueous or bufiered aqueous solution using polarographic methods, i.e. reduction at a dropping mercury electrode . A few studies in polar aprotic solvents have been carried out. 

Preparative oxidation of EtjAs and PhjAs or PhjSb in the presence of aromatic or heteroaromatic compounds leads to formation of quaternary arsonium or stibonium salts . The oxidations were carried out in nominally dry MeCN in the presence of an excess of the aromatic compound which, in the case of the arsines, were naphthalene, thiophene, pyridine and furan , and in the case of Ph3Sb were thiophene and toluene . ... 

Crum Brown and Fraser (6) in 1868 attributed curariform activity to the onium ion. Even at that early date, they suggested therapeutic possibilities of synthetic quaternary salts which could be prepared of constant purity and effectiveness. Boehm (7, 8) first showed that the active alkaloids of curare preparations are quaternary ammonium compounds. Since these discoveries, a great many quaternary compounds have been investigated, and even the simplest quaternary ammonium salts, as well as sul-fonium, phosphonium, arsonium, and stibonium salts, exhibit curare activity (9). Quaternary ammonium compounds widely varied in character have been found to exhibit curare activity. Hunt and Renshaw and collaborators (10, 11) have published a series of papers on onium compounds and their effects on the autonomic nervous system. In addition to curare action, they were shown to exhibit muscarinic and nicotinic actions in varying degrees. 

Arsonium and stibonium salts have properties consistent with their ionic constitution. The hydroxides are strong alkalis like quaternary ammonium hydroxides. The tetrahedral structures of their cations have been confirmed in some instances by X-ray analysis. Of the few bismuthonium salts known, the tetraphenylborate Ph4Bi BPh4 (from Ph5Bi+Ph3B) is among the more stable. The halides readily lose halobenzene, e.g. 

The stibonium compounds are the most difficult to prepare and are the least common. These quaternary salts, excepting the hydroxides, which are obtained as sirupy masses, are white crystalline compounds. The tetra-phenyl-phosphonium and -arsonium ions are useful for precipitating large anions such as ReO, CIO and complex anions of metals. Tetraphenyl-stibonium hydroxide has been shown55 to contain tbp molecules, with the OH group in an axial position. 

Of the partially unsaturated systems, only the 2- and 3-arsolenes (8) and (9) have been prepared, while no examples of stibolenes or bismolenes have been reported. Examples of the fully saturated systems (10), known as arsolanes, stibolanes, and bismolanes, and their anions (II), where E = As, Sb, and Bi, exist. Arsonium and stibonium quaternary salts are routinely found. Bis(methylene)-stibolane and -bismolane derivatives (12) and their anions have been prepared. Dimeric systems such as (13) and (14) have been prepared (for E = Sb, Bi) and are known as 1,1 -biheterolanes. 

Although this story proved a little too complex to be widely appreciated, and few were inclined to generalize from the evidence, the topic acquired a new facet when Ing (1936) published his much-quoted review. In this he pointed out that the alkaloid tubocurarine (2.6), which blocks neuromuscular transmission (see Section 2.0), must do so by competing with acetylcholine for a receptor in voluntary muscle (both substances are quaternary amines). In addition, Ing postulated that the somewhat weaker curariform action of innumerable quaternary ammonium, phosphonium, arsonium, stibonium, and sulphonium salts arose from their competition with acetylcholine (Ing, 1936). (For further reading on the physiological... 

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