Mercaptans, nucleophilic reactions

The amine-catalyzed mercaptan-epoxide reaction (Equation 4) proceeds exothermally at room temperature (27. 28). The order of average relative nucleophile-displacement rates (Table II) further suggests that mercaptans react significantly faster than amines and that the addition of the mercaptlde (RS ) ion to the epoxide group is the rate determining step (30). 

There is some question about the mechanism involved in an extensive series of nucleophilic reactions discovered by Umezawa and Hoshino and their co-workers.58,77 78 In these reactions, the 4,7-diacetoxy compound (30) was treated, in base, with alcohols (to yield 31), amines (to yield 32), mercaptans (to yield 33), KCN (to yield 34), nitromethane (to yield 35), malononitrile (to yield 36), malonic ester (to yield 37), cyclohexanone (to yield 38), acetophenone (to yield 39), and dimethyl sulfoxide (to yield 40). The reaction with cyclopentanone was anomalous in that a dimer of the ketone reacted with the isoquinoline. Since the acetoxy group on C-7 was almost always lost and the reaction failed when a 7-methoxy group was present, a quinone methide intermediate (41) was proposed. Some of the reactions were done in aqueous or alcoholic base and some were done under anhydrous conditions with NaH. 

As to the nucleophiles that can be applied in the nucleophilic reactions, ammonia and amines, water and alcohols, and mercaptans have been mentioned already. Sulfide and thiourea have been used only with the achiral ferrocenylmethylium ion [97]. To obtain chiral derivatives of 1-ferrocenyl-ethylmercaptan, substitution of the acetate with potassium thioacetate in acetic acid, followed by reduction with LiAlH4, was found appropriate [98, 99]. Reaction of (R)-l-ferrocenylethanol with NaH and... 

Shemyakin has made a comprehensive study of nucleophilic reactions of 4-halooxazolinones (53) with a variety of alcohols, amines and mercaptans 100, see also 316). Nucleophilic exchange is frequently accompanied by opening of the oxazolinone ring and formation of substituted a-amino acid esters, amides or thiolesters (54). 

Many of these reactions are reversible, and for the stronger nucleophiles they usually proceed the fastest. Typical examples are the addition of ammonia, amines, phosphines, and bisulfite. Alkaline conditions permit the addition of mercaptans, sulfides, ketones, nitroalkanes, and alcohols to acrylamide. Good examples of alcohol reactions are those involving polymeric alcohols such as poly(vinyl alcohol), cellulose, and starch. The alkaline conditions employed with these reactions result in partial hydrolysis of the amide, yielding mixed carbamojdethyl and carboxyethyl products. 

Nucleophilic Addition Reactions. Many nucleophiles, including amines, mercaptans, and alcohols, undergo 1,4-conjugate addition to the double bond of methacrylates (12—14). 

Rifamycin S also undergoes conjugate addition reactions to the quinone ring by a variety of nucleophiles including ammonia, primary and secondary amines, mercaptans, carbanions, and enamines giving the C-3 substituted derivatives (38) of rifamycin SV (117,120,121). Many of the derivatives show excellent antibacterial properties (109,118,122,123). The 3-cycHc amino derivatives of rifamycin SV also inhibit the polymerase of RNA tumor vimses (123,124). 

This reaction is useful in the preparation of anionic derivatives from the chlorides when the nucleophilic displacement route is unsatisfactory. Even weak acids, eg, phenols, mercaptans, and cycHc nitrogen compounds, can be made to undergo reaction with triorganotin hydroxides or bisoxides if the water of reaction is removed a2eotropicaHy as it forms. 

The catalytic effect of protons has been noted on many occasions (cf. Section II,D,2,c) and autocatalysis frequently occurs when the nucleophile is not a strong base. Acid catalysis of reactions with water, alcohols, mercaptans, amines, or halide ions has been observed for halogeno derivatives of pyridine, pyrimidine (92), s-triazine (93), quinoline, and phthalazine as well as for many other ring systems and leaving groups. An interesting displacement is that of a 4-oxo group in the reaction of quinolines with thiophenols, which is made possible by the acid catalysis. 

Reaction of 2-chloromethyl-4//-pyrido[l,2-u]pyrimidine-4-one 162 with various nitronate anions (4 equiv) under phase-transfer conditions with BU4NOH in H2O and CH2CI2 under photo-stimulation gave 2-ethylenic derivatives 164 (01H(55)535). These alkenes 164 were formed by single electron transfer C-alkylation and base-promoted HNO2 elimination from 163. When the ethylenic derivative 164 (R = R ) was unsymmetrical, only the E isomer was isolated. Compound 162 was treated with S-nucleophiles (sodium salt of benzyl mercaptan and benzenesulfinic acid) and the lithium salt of 4-hydroxycoumarin to give compounds 165-167, respectively. 

All reactions of benzotriazole derivatives of the type Bt-CR RbS discussed above are based on electrophilic or nucleophilic substitutions at the ot-carbon, but radical reactions are also possible. Thus, the first report on unsubstituted carbon-centered (benzotriazol-l-yl)methyl radical 841 involves derivatives of (benzotriazol-l-yl)methyl mercaptan. 3 -(Benzotriazol-l-yl)methyl-0-ethyl xanthate 840 is readily prepared in a reaction of l-(chloromethyl)-benzotriazole with commercially available potassium 0-ethyl xanthate. Upon treatment with radical initiators (lauroyl peroxide), the C-S bond is cleaved to generate radical 841 that can be trapped by alkenes to generate new radicals 842. By taking the xanthate moiety from the starting material, radicals 842 are converted to final products 843 with regeneration of radicals 841 allowing repetition of the process (Scheme 134). Maleinimides are also satisfactorily used as radical traps in these reactions <2001H(54)301>. 

The first stage of the synthesis involves the interaction of a nitro compound with sodium sulfide. When used alone, sodium sulfide is only slightly effective The reactions proceed slowly and the yields of mercaptanes are small. If elemental sulfur is added, the conversion accelerates markedly and the yield increases to 75-80%. The promoting effect of elemental sulfur can be easily explained by the radical-chain mechanism. The reaction starts with one-electron transfer from the nucleophile to the nitro compound further conversions resemble other chain ion-radical substitutions. 

Since thiolates are far more reactive than thiols (pK 8-9.5) in nucleophilic attack on coordinated NO, these rates are pH dependent [53]. The reaction of alkaline solutions of mercaptans with nitroprusside to yield reddish-purple solutions has long been used as a test for cysteine, glutathione or other thiol-containing compounds. However, if the solution is too basic, the nitroprusside/ hydroxide reaction becomes competitive [3]. 

The 2,3-double bond in benzo[6]thiophene-1,1-dioxides undergoes addition reactions with nucleophiles in a manner comparable to that of other a,/9-unsaturated sulfones no aromatic properties are detectable in this way for the thiophene ring.718-723 For example, thiophenol and p-thiocresol give the adducts (340 R = H or Me) in the presence of base.723 However, if the aryl mercaptan and the sulfone are heated together, a radical reaction occurs to give the corresponding 2-substituted compound.723 In contrast to the behavior of aryl mercap-... 

Very recently, the reaction of styrene 3,4-epoxide (245) with ethyl mercaptan has been reported.148 A mixture of 2-, 3-, and 4-ethylthiostyrenes is formed in the ratio of 1 9 7 along with small amounts (18%) of 4-vinylphenol. These results can be explained as nucleophilic attack on the intact arene oxide and the reaction of the zwitterion formed by the spontaneous reaction. 

The interaction of alkyl halide with mercaptans or alkali mercaptides produces thioalkyl derivatives. This is a typical nucleophilic substitution reaction, and one cannot tell by the nature of products whether or not it proceeds through the ion radical stage. However, the version of the reaction between 5-bromo-5-nitro-l,3-dioxan and sodium ethylmercaptide may be explained only by an intermediate stage involving electron transfer. As has been found (Zorin et al. 1983), this reaction in dimethylsulfoxide leads to diethyldisulfide (yield 95%), sodium bromide (quantitative yield), and 5,5 -bis(5-nitro-l,3-dioxanyl) (yield 90%). 

A broad spectrum of hydrogen-containing nucleophiles react with both aromatic and aliphatic isocyanates compounds containing OH groups (H20, alcohols, phenols, oximes, acids), SH groups (H2S, mercaptans), NH groups (NH3, amines, hydrazines, amides, ureas, urethanes), enolizable compounds such as malonic and aceto acetic esters, etc. Some reactions are given in Table 2.5. 

Reaction kinetics with the various reagents becomes faster as their nucleophilicity is increased. The following order of reactivity can be given primary aliphatic amine > primary aromatic amine > secondary aliphatic amine primary alcohol > secondary alcohol > water > tertiary alcohol phenol > mercaptan. 

The chemistry of epoxy/mercaptan systems involves the tertiary amine catalyst forming a salt with the mercaptan to generate a mercaptide anion, which is a strong nucleophile. The mercaptide will readily open the epoxy ring. Reaction with another mercaptan group can regenerate the mercaptide anion, as shown in Fig. 5.13. 

Mechanism 10-2 Hydride Reduction of a Carbonyl Group 454 Summary Reactions of LiAIH4 and NaBH4 455 Summary Alcohol Syntheses by Nucleophilic Additions to Carbonyl Groups 457 10-12 Thiols (Mercaptans) 458 EssentialTerms 461 Study Problems 462... 

Methoxyl Groups Lignin is partially demethylated by the action of hydrosulfide ions forming methyl mercaptan which is convertible to dimethyl sulfide by reaction with another methoxyl group. In the presence of oxygen, methyl mercaptan can be oxidized further to dimethyl disulfide (Fig. 7-28). Because the hydroxide ions are less strong nucleophiles than hydrosulfide ions, only small amounts of methanol are formed. Methyl mercaptan and... 

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