Butyl Thioethers

Triphenylmethylcysteine is readily oxidized by iodine (MeOH, 25°) to cystine." The 5-triphenylmethylcysteine group can be selectively cleaved in the presence Of a —Cys(Acm)— group (Acm = acetamidomethyl). 5-Ben-zyl and 5-/-butyl thioethers are stable to the action of iodine. 

To further exploit the potential usefiilness of this new family of clusters, monoadduct 54 was saponified into 55 (0.05 M NaOH, quant) and condensed to L-lysine methyl ester using 2-ethoxy-l-ethoxycarbonyl-l,2-dihydroquinoline (EEDQ) to give extended dimer 56 in 50 % yield together with monoadduct in 15 % yield [75]. Additionally, tert-butyl thioethers 52 could be transformed into thiols by a two step process involving 2-nitrobenzenesulfenyl chloride (2-N02-PhSCl, HOAc, r.t, 3h, 84%) followed by disulfide reduction with 2-mercaptoethanol (60%). Curiously, attempts to directly obtain these thiolated telomers by reaction with thioacetic acid f ed. These telomers were slightly better ligands then lactose in inhibition of binding of peanut lectin to a polymeric lactoside [76]. 

Protection of thiols. The r-butyl group has been of limited service for protection of thiols because removal has involved rather drastic treatment with liquid HF. r-Butyl thioethers can be cleaved by treatment with o-nitrobenzene-sulfenyl chloride in acetic acid at 25° to give S-o-nitrophenylsulfeny derivatives. These products are reduced to thiols with NaBH4, HSCH2C02H, or HOCH2CH2SH. 

Most recently, Monteiro et al. have reported that cyclopalladated compounds derived from the ortho-metalation of benzylic tert-butyl thioethers are excellent catalyst precursors for the Suzuki cross-coupling reaction of aryl bromides and chlorides with phenylboronic acid under mild reaction conditions. A broad range of substrates and functional groups are tolerated in this protocol, and high catalytic activity is attained (Eq. (58)) [93]. 

The normal members of the thioether series are again almost linear while the branched chain isomer have lower value of V consistent with their boiling points. This is in contrast to the o gen series on Apiezon L, where the Vg values of hexamethyl-siloxane and the isopropyl ether are almost identical. In the sulphur series, the retention volume of hexamethyIdisilthane was greater than either of the propyl thioether isomers, and only slightly smaller than that of the tert-butyl thioether. 

Phenylvinyl butyl thioether, MA copolymerization, 318, 319 2-Phenylvinyl ethyl ether, MA copolymerization, 318, 319... 

Phenylvinyl jec.-butyl ether, MA copolymerization, 318, 319 2-Phenylvinyl sec. butyl thioether, MA copolymerization, 318, 319 Phenyl o-vinyl formal, 328 2-Phenylvinyl isopropyl ether, MA copolymerization, 318, 319 2-Phenylvinyl isopropyl thioether, MA copolymerization, 318, 319... 

These results provide added support for the earlier observations of Cain et al. [54] who studied the perchloric acid-catalyzed cleavage of di-t-but.yl sulphide and other t-butyl thioethers (Table 7.3). These studies indicate, as observed by Olah et al., a difference in the reactivity of di-t-butyl sulphide as compared to f-butyl isopropyl sulphide. Furthermore the effect of increasing cation stability on the rate of cleavage is apparent. In these cases, as well as in those studied by Konig et al. [55], dimerization and subsequent cyclization of the cation has a marked effect on the yield of thiol produced. These data indicate that the acid-catalyzed rate of removal of thiol protective groups will depend on the strength of the acid... 

Tarbell and Hamish [79] reported that S-trityl thioethers were cleaved by alcoholic iodine at room temperature whereas the S-benzhydryl and S-t-butyl thioethers were stable even on boiling. The utility of this type of direct oxidative conversion of... 

These results indicate that the order of reactivity toward acid is generally acetals > monothioacetals dithioacetals. The ease of removal of the latter two groups is determined by essentially the same factors that govern the removal of the thioethers namely, ease of protonation, the stability of the cation formed and the possibility of irreversible reactions which can shift the equilibria in favor of the cleavage products. On the basis of available data the S-monothioacetal derivatives of cysteine appear to react somewhat more cleanly than the S-trityl thioethers but are of similar reactivity whereas the S-dithioacetal derivatives react more nearly like the S-benzyl or S-f-butyl thioethers. 

Thioethers. Bridgehead positions in hydrocarbons are functionalized as butyl-thioethers by reaction with lead tetra-acetate and butanethiol (c/. ref. 1). Reductive sulphidation of aldehydes is accomplished by conversion to a silyl monothio-acetal and cleavage of the C-O bond as outlined in Scheme 38. Reaction of primary alcohols with aryl isothiocyanates and triphenyl phosphine produces arylalkylthioethers, presumably via the sequence of Scheme 39. In secondary and tertiary cases elimination (to ArSH and alkene) from (76) is a competing process. 

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