4,4 -bipyridyl quaternary salt

The noncoplanar 4,4 -bipyridyl quaternary salts substituted at the carbon atom adjacent to the bond linking the rings cannot be reduced, which proves that biological activity is connected, at least in the first step, with a reduction process (Homer and Tomlinson, 1959 and Homer et al., 1960). 

The Menschutkin reaction was carried out as a test reaction to show the feasibility of such novel micro flow concepts that allow to process fouling-sensitive reactions (see also Section 4.2.6 here another test reaction is decribed for the same purpose) [78]. The reaction of alkyl bromide with ternary bases such as pyridine or triethylamine gives quaternary salts insoluble in most solvents. Often, fairly rapid precipitation of this salt occurs, hence ideally serving as a test reaction for fouling sensitivity of micro-channel devices. The reaction of 4,4 -bipyridyl and ethyl bromoacetate [78] belongs to the category of fast-predpitating Menschutkin reactions, as the halide function is activated by the carbonyl fimction. 

Organic synthesis 7 [OS 7] Formation of quaternary salts from 4,4 -bipyridyl and ethyl... 

Paraquat, a bipyridyl quaternary ammonium salt, was introduced by ICI in 1965 (Fig. 3). It is a non-selective, contact herbicide on plant foliage, but is immediately inactivated when applied to soil. It is used in minimum tillage programs and as a postemergence directed spray in sugarcane and in fruit tree crops. 

In present agricultural use the 2,2 - and the 4,4 -bipyridyl diquaternary salts are prominent, the two most important are the quaternary salts of diquat and paraquat (Reglone and Gramoxone ). 

Michaelis and Hill (1933) showed that though the reduction of the quaternary salts of 4,4 -bipyridyl proceeds as a two-electron process, these compounds differ from other organic compounds in that the addition of the first and of second electrons proceeds at different, nonoverlapping electrode potentials. The first reaction product is the relatively stable free radical of intensive colour, formed by the addition of one electron. 

It is well known that the reducibility of a substance and the free radical formed as a result of reduction both depend on the redox potential. The more difficult the reduction of a compound, the more negative is the value of the redox potential. Of the bridge-form quaternary salts of 2,2 -bipyridyl the redox potential of diquat dibromide (E ) is -349 3 mV, while the redox potential of the l,r-trimethylene analogue, difficult to reduce, is -548 3 mV. The difference in the two redox potentials is well reflected in their herbicidal activity. 

With the aim of increasing selectivity, the substituted derivatives of 4,4 -bipyridyl and their quaternary salts have also been synthesised. 

Bookbinder and Wrighton have described highly efficient and reversible photoreduction processes on p-type silicon electrodes derivatized with a silicon-containing quaternary salt of 4,4 -bipyridyl. Many thousands of redox cycles are reported to be possible without significant deterioration. 

Paraquat and diquat are both quaternary salts based on bipyridyls. Pyridine is the starting material for both compounds. They are non-selective, rapid action herbicides that act on all green plants through interference with the photosynthetic electron-transport system. 

Although both are quaternary salts of bipyridyls, diquat is a 2,2 -bipyridyl whereas paraquat is a 4,4 -bipyridyl. 

It was pointed out that the ultra-violet spectra of 2-arylpyridine quaternary salts and 1-oxides indicate the presence ofsteric inhibition of resonance, and the same is true of those of 4-aryl-3,5-diethoxycarbonyl-2,6-lutidines4 . These examples recall, of course, the hindered diphenyls, and in pursuing the similarity a number of attempts have been made to achieve optical resolutions of suitably substituted arylpyridines . Success was achieved with the 3,3 -bipyridyl (68), and salts of (69) with different rotations were... 

Paraquat (1,1 dimethyl, 4,4 bipyridyl) is a nonselective contact herbicide. It is used almost exclusively as a dichloride salt and usually is formulated to contain surfactants. Both its herbicidal and toxicological properties are dependent on the ability of the parent cation to undergo a single electron addition, to form a free radical that reacts with molecular oxygen to reform the cation and concomitantly produce a superoxide anion. This oxygen radical may directly or indirectly cause cell death. Diquat, l,T-ethylene-2,2 -dipyridylium, is a charged quaternary ammonium compound often found as the dibromide salt. The structure of diquat dibromide and that of the closely related herbicide paraquat can be seen in Fig. 4.5. 

However, the relationship between the actual herbicidal action and the redox potential of the compounds is not completely consistent. The redox potential of the quaternary diamyl salt of 4,4 -bipyridyl is nearly identical to that of paraquat nevertheless, its limiting concentration is ten times that of paraquat. The redox potential of benzyl-viologen dichloride (l,r-dibenzyl-4,4 -bipyridylium dichloride) is the same as that of diquat, but the compound has no herbicidal action. This irregularity is probably connected with the uptake by the plant. 

In the USSR the quaternary phosphate salts of 4,4 -bipyridyl, methylated on one or both of the nitrogen atoms, depending on reaction conditions, have been developed (Melnikov, 1971). 

Up to now, almost exclusively terminal olefins have been screened in ruthenium-catalyzed hydroformylation. One of the notable exceptions concerns early investigations of Knifton [115]. He used ruthenium carbonyl melt catalysts, wherein the ruthenium carbonyls are dispersed in quaternary phosphonium salts with a low melting point. By the addition of chelating N-donor ligands such as 2,2 -bipyridyl, -aldehydes came out. 

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