With Sulphur Nucleophiles

Reactions with Sulphur Nucleophiles. The addition of Et2AlSPh to the vinyl-oxiran (159) in benzene at 25 °C for 1 hour gives the (Z)-vinyl alcohol (160) (91%) with about 2% of the ( )-isomer. A novel route to the phenoxathiins 

The unusual reactivity of selenoboranes towards epoxides gives new selective routes to /3-hydroxy-selenides and allyl alcohols.Thus, 1,2-epoxyoctane with B(SeMe)3 at 0°C for 0.7 h, followed by aqueous NaHCOa, gave (164) (81%) whereas the same reaction with styrene oxide gave (165) (63%) as the major product. For trisubstituted oxirans, however, the products were allylic alcohols, e.g. (167) (76%) from (166) on treatment with B(SePh)3 at 20 °C for 1.5 h. 

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When the pentamer (66) reacts with alkoxide anions at low temperatures (-30 to -40°C), then the products of kinetic control (102) are isolated, whereas at higher temperatures, thermodynamic control prevails and the products (103) are obtained [131,132] (Scheme 68). Similar observations have been made with sulphur nucleophiles [132], and complex products are obtained with amines, including the formation of heterocycles [132]. Reaction of (66) with ethyl acetoacetate gave a pyran derivative (104) in a reaction that may be rationalised as shown in Scheme 69 [133]. In an analogous way.furan derivatives are formed from perfluoro-2-butene and -cyclohexene in base-induced reaction with 1,3-dicarbonyl derivatives [133]. 

Whereas oxygen nucleophiles gave poor yields of alkenylated products with alkenyl iodonium salts, the reactions with sulphur nucleophiles proceeded more efficiently, leading to unsaturated sulphides and sulphones. Thus, 4-t-butylcyclohexenyl phenyliodonium salts afforded with sodium thiophenoxide 4-t-butylcyclohexenyl phenyl sulphide (81%) [3] and with sodium phenylsulphinate the corresponding sulphone (29%) in the presence of 18-crown-6, the yield of the latter rose to 80% [45]. jS-Phenylsulphonylalkenyl iodonium salts with sodium phenylsulphinate at 0°C, without any catalyst, afforded Z-l,2-bis(phenylsulphonyl)alkenes, in high yield with retention of the stereochemistry [45] ... 

Brown, B.R., and M.R. Shaw Reactions of Flavonoids and Condensed Tannins with Sulphur Nucleophiles. J. Chem. Soc., Perkin Trans. 1, 2036 (1974). 

A new route to thiols employs the l-methyl-2-fluoropyridinium salt (69) which reacts with alcohols to give the 2-alkoxy-derivatives ° (Scheme 49). Treatment with sulphur nucleophiles as shown proceeds with inversion and thiols are available from the products by reduction. 

Brown B R, Shaw M R 1974 Reactions of flavanoids and condensed tannins with sulphur nucleophiles. J Chem Soc Perkin Trans I 2036-2049... 

As with chlorine-containing oxidants, JV-bromo species have been used to oxidize sulphoxides to sulphones (with no bromine incorporation) through the initial formation of a bromosulphonium ion, by nucleophilic attack of the sulphoxide sulphur atom on the electrophilic halogen atom. Such reactions involve JV-bromosuccinimide ° bromamine-T, iV-bromoacetamide ° and iV-bromobenzenesulphonamide. All reported studies were of a kinetic nature and yields were not quoted. In acid solution all oxidations occurred at or around room temperature with the nucleophilic attack on the electrophilic bromine atom being the rate-limiting step. In alkaline solution a catalyst such as osmium tetroxide is required for the reaction to proceed . ... 

L-metilionine to -adenosylmethionine. In this process a positively charged sulphur is produced and facilitates the nucleophilic reaction. By the activity of diamine oxidase, the A -methyl-A -pyrrolinium cation is formed and after that the first alkaloid, hygrine. From hygrine, by way of acetyl CoA, hydrolysis and intramolecular Mannich reactions, other pyrrolidine and tropane alkaloids are synthesized cuscohygrine, hyoscyamine or tropinone, tropine and cocaine. The Mannich reaction involves the combination of an amine, an aldehyde or a ketone with a nucleophilic carbon. This reaction is typical in alkaloid synthesis, and can be written as follows ... 

The observed structure effects are similar, as in the reaction catalysed by sulphuric acid. On this basis and with the notion of the strong acidity of the heterogeneous catalysts used, it is possible to assume a mechanism similar to olefin hydration (Sect. 3.1) or alkylation (Sect. 3.3). Olefin protonation by the catalyst seems to be the first step, which is followed by the interaction with the nucleophile, in this case the alcohol. 

Treatment of a-cyclopropyl acyl silanes with sulphuric or triflic acids results in rearrangements to give cyclobutanones and 2-silyl-4,5-dihydrofurans, respectively, in processes formally involving intramolecular nucleophilic attack at the acyl silane moiety (Scheme 80)190. 

On the other hand, some evidence for the longer life-time of the intermediate formed with amines may be inferred from the formation of small amounts of (116a) from piperidine and /J-chloroacrylonitrile, in addition to the main substitution product. (116a) is also the sole substitution product with aniline (R1=Ph, R2 = H) but no such products were reported for reactions with oxygen or sulphur nucleophiles. For formation of (116a) the initially-formed carbanion should be long-lived enough as to attack another j8-chloroacrylonitrile molecule (Scotti and Frazza, 1964). 

Another sulphur nucleophile, 2-mercaptobenzimidazole, gave similarly 2-phen-acylthio-benzimidazole [29]. Acetophenones on treatment with methanolic HTI underwent rearrangement affording methyl arylacetates, Ar H2C02Me the same compounds were obtained with iodosylbenzene in acidic methanol (Section 5.2.3). 

Compounds with a nucleophilic sulphur are readily arylated by iodonium salts in their anionic form, and less readily in other cases, notably in sulphides triaryl sulphonium salts are useful photoinitiators in cationic polymerizations. A list of chalcogen compounds arylated by iodonium salts appears in Table 8.7. 

In other reactions, cyanide functions rather simply as a nucleophile. The organic examples are numerous. Reaction at sulphur to form thiocyanate is a convenient inorganic example this occurs with thiosulphate tetrathionate and pentathionate, and with sulphur (Sg and Sg) itself The reactions which... 

Reaction of electrophilic cyclopropanes with sulphur and selenium nucleophiles 526... 

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