Sulfenimides

Fluoride-catalyzed condensations of aldehydes and ketones such as benzaldehyde with N,N-bis(trimethylsilyl)sulfenamide 529 furnish sulfenimides such as 530 in 82-96% yield [103] (Scheme 5.34). 

The direct electrosynthesis of S—N bonds from disulfides and amines has been shown to occur through a reaction of the amine with the oxidized disulfide being a strong electrophile. In contrast to the results, the cross-coupling of phthahmide (16) with disulfide does not proceed in a direct electrolysis. However, the electrosynthesis of sulfenimides (17) can be achieved by a [Br]" "-mediated cross-coupling reaction of imides with disulfides (Scheme 7). The electrolysis of a mixture of (16) and dicyclohexyl disulfide in an MeCN-NaBr-(Pt) system affords... 

Bromide and iodide ions are not only effective as redox catalyst in the indirect electrochemical formation of carbon-hetero atom bonds but also of hetero-hetero atom bonds. Thus, the nitrogen-sulfur bond in sulfenimides (Eq. (53) Table 4, No. 33) and sulfenimines (Eq. (54) Table 4, No. 34) can be generated. The electrogenerated active bromine species forms a positively polarized sulfenyl intermediate from disulfides. 

Similar vulcanisation chemistry is observed with the N- -butyl-2-benzothiazole sulfenimide (TBSI) accelerated sulfur-vulcanisation of HR [26] compared to the TBBS accelerated systems... 

If chance was involved in this discovery, let it be emphasized that it is a component in all inventions (7), and this one proved in time to be highly significant. Subsequent development and derivative invention (8) and discovery have provided biocides used in this country, in the agriculture of all the developed countries, in much of undeveloped Asia, Latin America and Africa, and in the Communist domains throughout the globe. The present installed capacity over the world for sulfenimide group fungicides is estimated as above 30 million lbs but less than 100 million lbs/year. 

Examples of these sulfenimide compositions are given in Figures 2 and 3. Although it is normally proper to utilize generic names, we will employ the usual names by which these compounds are best known throughout the world for the remainder of this paper. In these figures we have indicated some major areas of usefulness. 

In fact this genus of useful sulfenimide fungicides requires a low aqueous solubility to protect against its inherent appreciable hydrolytic instability (14). 

This abnormal non-appearance of practical resistance of fungal organisms to the sulfenimide group merits exploration. This we shall do by examining some current explanations by... 

In a recent book devoted to biochemical mechanism of pesti-cidal action, the author states (quoting primary sources) that resistance to systemics is caused by "the high selection pressure exerted by these fungicides on the fungal population with the result that,in some circumstances only, resistant strains survive. Older fungicides seem to have been less effective as fungicides than the new compounds"(27)This means that most of the chemical protectants such as the sulfenimides left both resistant and sensitive strains and presumably, and as is usual, the sensitive population adapts better to the total environment. 

Were this not the case, either a phytotoxic effect would result (as for 2,4-D) or an ineffective fungicide would result. Exactly these effects are achieved when through synthesis the solubility of the sulfenimides is increased. This author has, alas, quite frequently converted good fungicides into mediocre herbicides by such synthetic effort on the solubility modifications of the sulfenimide generic structure. A systemic must then be preferably a chemically relatively stable substance and not a highly reactive nucleophile or electrophile. 

Sulfenimides + + + + + + (captan, Difolatan) Tentative Arbitrary Values vary between + + + for highest to for absent or lowest. ... 

Obviously, one can not precisely quantify the elements of Figure 8. It is nevertheless highly indicative. Resistance of a practical nature is rarely, if ever, encountered in normal field practice by the sulfenimides (or for example, the dithio-carbamates, Figure 9) - insoluble, multisite protectives. The water soluble single site and variably translocatable antibiotics, benzimidazoles, oxathiins, etc., etc., all exhibit degrees of field resistance. As far as this author knows, this analysis can be extended over the whole class of plant protection fungicides. 

The sulfenimides are a broad genus providing opportunities for much synthetic modification. The compounds presently exploited, particularly captan, Phaltan and Difolatan, are characterized by the relative simplicity of their structures. 

Many biochemical interactions of the sulfenimides with enzyme systems and biological structures have been described in the literature and can be amply demonstrated with isolated systems. At this stage, we must conclude that the sulfenimides are involved in biochemical multisite attack, most likely with the sulfhydryl associated enzymes and co-enzyme systems. 

Finally, we have surveyed particularities that pertain to the fungicide resistance field. From this survey we have presented some generalizations. These all point to the low probability of organism resistance in the future to these sulfenimide fungicides and to fungicides that possess similar physical and biochemical characteristics. 

From the particularities above outlined, we know which chemicals can be utilized with minimal probabilities of resistance development. Among these compositions are the subject fungicides of this paper - the sulfenimides. 

The formation of nitrogen-sulfur bonds, as in the synthesis of sulfenimides and sulfenimines [173], and phosphorous-sulfur bonds [174] is achieved by electrolyses of activated N-H and P-H compounds in the presence of an RSSR/Br mixture ... 

The utility of sulfenimides has been demonstrated. The procedure has the particular advantage of using practically neutral conditions for the final deblocking step (Scheme 44). 

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