Thiophenols acceptors

The process implications of equation 3 go beyond the weU-known properties (27—29) of NMP to faciUtate S Ar processes. The function of the aminocarboxylate is also to help solubilize the sulfur source anhydrous sodium sulfide and anhydrous sodium hydrogen sulfide are virtually insoluble in NMP (26). It also provides a necessary proton acceptor to convert thiophenol intermediates into more nucleophilic thiophenoxides. A block diagram for the Phillips low molecular weight linear PPS process is shown in Eigure 1. 

These reactions produce free radicals, as follows from the fact of consumption of free radical acceptor [42]. The oxidation of ethylbenzene in the presence of thiophenol is accompanied by CL induced by peroxyl radicals of ethylbenzene [43]. Dilauryl dithiopropionate induces the pro-oxidative effect in the oxidation of cumene in the presence of cumyl hydroperoxide [44] provided that the latter is added at a sufficiently high proportion ([sulfide]/[ROOH] > 2). By analogy with similar systems, it can be suggested that sulfide should react with ROOH both heterolytically (the major reaction) and homolytically producing free radicals. When dilauryl dithiopropionate reacts with cumyl hydroperoxide in chlorobenzene, the rate constants of these reactions (molecular m and homolytic i) in chlorobenzene are [42]... 

Irradiation accelerates the reactions of Scheme 4.1, and the substitution products are formed in 70-80% yields. Acceptors of radicals (e.g., di-tert-butylnitroxyl) or electrons (e.g., m-dinitro-benzene [DNB]) completely inhibit the snbstitution even if the acceptors are added to the reaction mixture in small amonnts. The mentioned snbstitution reactions do not take place when no cyano groups are present in the initial a-phenylsnlphonyl cumene. Hence, the cyano groups send the reaction via the ion-radical pathway. Like the nitro gronp, the cyano group promotes the formation of anion-radical, which originates on one-electron transfer from the thiophenolate or malonate ions to the substrate. 

The (oxidizing) peroxyl radicals behave in a similar fashion. They do not react readily with thiols, not even with thiophenols whose H atoms are very weakly bound, but they are readily reduced by the corresponding thiolate ion in contrast to alkyl radicals which are poor electron acceptors and hence do not react with thiolate ions (Simic and Hunter 1986). 

A-Acyloxazolidinone 23 was condensed with p-anisaldehyde to give, after dehydration, the Michael acceptor 24, mainly in the shown (Z) form. Addition of the 5-lithium salt of 2-amino-thiophenol afforded an 82 18 mixture of 2,3-syn and -anti adducts 25. From the major isomer, diltiazem was obtained upon ring closure, MEM group removal, hydroxy group acetylation, and nitrogen alkylation. 

Recendy, we described a useful sequence where Michael-acceptor sulfoxides 30 were obtained in two steps from homopropargylic alcohols 29 by radical addition of thiophenol and oxidation with sodium periodate. The unsaturated sulfoxides were used in a highly stereoselective intramolecular oxa-Michael reaction. The sequence provided stereoselective functionalization of the sulfoxide moiety, and the products 31 proved to be useful in the synthesis of modified furanosides 32. This represents a good exanple where sugars are prepared from acyclic precursors. The Michael addition was followed by a hydrolytic Pummerer reaction, yielding protected a-hydroxy aldehydes tScheme 20.8) that upon acidic treatment afforded 3-substituted ribofuranoses. 

In a preliminary report, Chen and co-workers [30] obtained moderate enantioselectivity in the addition of thiophenol to acyclic benzimides, catalyzed by 11 (55-75% ee). The Deng group [35] successfully exploited a,(3-unsaturated A-acyloxazolidin-2-ones as acceptors. After a careful screening of bifunctional cinchona alkaloid derivatives, they identified the novel thiourea 14 as the most... 

Radical acceptors such as thiophenol, in the absence of oxygen, have an effect similar to that of oxygen with the molecular weight falling on mastication towards, once again, a limiting value of about 70 000. 

Fig. 3.5 Influence of temperature on efficiency of rubber mastication in a size-B Banbury under nitrogen with 0.0925 m/1000 gm radical acceptors of different reactivity ( ) thiophenol (O) benzoquinone (O) azobenzene. Time of mastication, 30 min [8].

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