Silanethiols

Tri-tert-butoxidesilanethiolate forms complexes with both Co11 and Co111. Reaction of the thiol with a Co11 ammine in water yields a dimer which undergoes oxidation in an ammonia-saturated atmosphere to form octahedral [Co(SSi(0-t-Bu)3)2(NH3)4]+, the first silanethiolate characterized structurally.1044... 

The addition of trisubstituted silanes to carbonyl sulfide has been applied to the synthesis of the corresponding silanethiol derivatives (Reaction 5.40) [78]. In Scheme 5.12 the mechanism is depicted, starting from the silyl radical attack to the sulfur atom of 0=C=S and ejection of carbon monoxide. The resulting silanethiyl radical abstracts hydrogen from the starting silane, to give the silanethiol and to generate fresh silyl radical (see Section 3.4). 

Trimethylsilyl-substituted silanethiols, such as (2) and (3), were shown to be extremely efficient radical-based reducing agents [265]. These agents... 

FIGURE 20. Bond lengths in silanethiol derivatives (left) and Si—S—X bond angles in acyclic silan-ethiol derivatives (right)... 

Reductions with tris(trimethylsilyl)silanethiol 18 [38J and with heptamethyltrisilane-2-thiol 19 [39] are also effective, but again the cost of these reagents would be prohibitive per hydrogen delivered for larger-scale synthesis. 

In contrast to the afore-mentioned types of compounds, unsubstituted silanethiol (730) can be obtained either by treatment of disilathiane (729) (for preparation of 729 see Section XII, B.2 below) with H2S418 (equation 373) or via the reaction of chlorosilane (731) with alkali- or magnesium hydrogen sulphides (equation 374)419. 

These results were said to be consistent with efficient (p-d)7r bonding in the silanethiols, as in the silanols. The high Bronsted acidity of silanethiols was postulated to be due to increased 7r-bonding in the silanethiolate anions relative to the parent silanethiols. 

TABLE 2. Half-neutralization potentials for some silanethiols and silanols23... 

The facts that thiols are good H-atom donors toward alkyl radicals and that silyl radicals are among the most reactive known species for abstraction and addition reactions suggest that any class of compounds which allows the transformation of a thiyl to a silyl radical via a fast intramolecular rearrangement will potentially be a good radical-based reducing agent. The silanethiols 11 and 12 are found to have this property [84, 85]. The reductions of bromides, iodides and isocyanides by thiol 12 are demonstrated to follow the expected mechanism [85]. 

Silanethiols are much more stable towards condensation reactions than their oxygen counterparts and may be isolated from insertion reactions of Si-H containing compounds with S at high temperature or by high-temperature reactions with H2S (Scheme 39). ... 

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