Bridging sulfur attack

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

In the oxidative Eschenmoser sulfide contraction (Scheme 11), thioamide 59 is oxidized by benzoyl peroxide to give either a symmetrical disulfide or the O-benzoate of the thiolactam-S-oxide. In any event, the once-nucleophilic thioamide sulfur atom is now forced to adopt the role of electrophile a reactivity umpolung has, in effect, been achieved.13 The nucleophilic enamide 65 attacks the sulfur atom leading to the formation of sulfur-bridged intermediate 66. The action of a phosphine or a phosphite thiophile on the putative episulfide then gives vinylogous amidine 67. 

Examples of the behavior of other substituted vinyl substrates upon exposure to the action of trifluoroacetic acid and triethylsilane are known. For example, -butyl vinyl ether, when reacted at 50° for 10 hours, gives -butyl ethyl ether in 80% yield (Eq. 65).234 In contrast, -butyl vinyl thioether gives only a 5% yield of n-butyl ethyl sulfide product after 2 hours and 15% after 20 horns of reaction.234 It is suggested that this low reactvity is the result of the formation of a very stable sulfur-bridged carbocation intermediate that resists attack by the organosilicon hydride (Eq. 66). 

Type III bridging 82 ligands are more susceptible to nucleophilic attack with extrusion of a neutral sulfur atom than are type la ligands. This is consistent with a generally lower electron density on the S atoms of type III ligands (bonded to two metal atoms) than of type la ligands (bonded to one metal atom). 

A further development of the DMSO/H+ method for oxidation of cysteine peptides led to the cysteine-sulfoxide acid-catalyzed intermolecular disulfide formation with a second S-unprotected or acid-labile protected cysteine component as shown in Scheme 19. 1471 The protonation of the sulfoxide by TfOH in the case of 5(0)-Mob or TFA in the case of 5(0)-Acm derivatives provides electrophilicity to the sulfur atom to allow attack by the second S-unprotected cysteine component (formed by the fast deprotection of the 5-Mob group with TfOH in presence of dimethylsulfide) to generate in a site-directed manner the interchain disulfide bond. Although extensive experience with this method has not been accumulated for interchain disulfide bridging, it has been successfully applied for intrachain site-directed disulfide bond formation in chicken calcitonin-gene-related peptide.1 79 ... 

The problem stipulates that a bridged sulfonium ion is an intermediate. Therefore, use the tt electrons of the double bond to attack one of the sulfur atoms of thiocyanogen and cleave the S—S bond in a manner analogous to cleavage of a Br—Br bond in the reaction of bromine with an alkene. 

Although nuoleophilio attack takes place preferentially in these two positions, very thiophilic groups must also be expected to attack one of the sulfur atoms, leading either to cleavage of the disulfide bridge or (less likely) to occupation of a d orbital of the sulfur.80... 

Figure 9.35. Stereochemistry of Cleaved DNA. Cleavage of DNA by coRV endonuclease results in overall inversion of the stereochemical configuration at the phosphorus atom, as indicated by the stereochemistry of the phosphorus atom bound to one bridging oxygen atom, one one and one sulfur atom. This configuration strongly suggests that the hydrolysis takes place by the direct attack of water on the phosphorus atom.

Deprotonated bridging dithiolate ligands in diiron carbonyl complexes of type 101 (R = H, Ph) rearrange intramolecularly by nucleophilic attack on an iron atom while an electron pair is displaced onto a sulfur atom with formation of an Fe—C bond in 102 (76). Neutral derivatives can be formed upon further alkylation, leading to type Ij complexes (Table 1). The bridg-... 

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