Silyl radical with thiol

Rate constants for the reaction of thiyl radicals with the t-BuMePhSiH were also extracted from the kinetic analysis of the thiol-catalysed radical-chain racemization of enantiomerically pure (S)-isomer [34]. Scheme 3.2 shows the reaction mechanism that involves the rapid inversion of silyl radicals together with reactions of interest. The values in cyclohexane solvent at 60 °C are collected in the last column of Table 3.5. 

Allyloxysilanes (14) undergo radical chain cyclization in the presence of di-tert-butyl hyponitrite as radical initiator and thiol as a catalyst at ca 60 °C (Reaction 6.3) [5]. The thiol promotes the overall abstraction from the Si—H moiety as shown in Scheme 6.4 and the silyl radical undergoes a rapid 5-endo-trig cyclization. Indeed, EPR studies on the reaction of t-BuO radical with silanes 14 detected only spectra from the cyclized radicals even at — 100°C, which implies that the rate constants for cyclization are > 10 s at this temperature. 

Radical reactions have some stereochemical features that can be compared directly with their ionic counterparts, especially when the radical centre is adjacent to an existing stereogenic centre. The tris(trimethylsilyl)silyl radical adds to chiral ketones like 3-phenyl-2-butanone 7.59 to give a radical 7.60 flanked by a stereogenic centre. The hydrogen atom abstraction from a thiol, determines the relative stereochemistry, and the products 7.61 and 7.62 are analogous to those from the hydride reduction of the ketone. They are formed in the same sense, and the stereochemistry is explained by the Felkin-Anh picture 7.60. 

Again, no reaction was observed in the absence of mercaptoethanol. The mechanistic steps for the transformation were elucidated by the Chatgilialoglu et ah and are represented in Scheme 4.13, in analogy with the pathway reported for the radical reduction of aromatic azides with triethylsilane in toluene, with the addition of silyl radicals to the azide function, liberation of nitrogen and formation of silyl-substituted aminyl radical. The thiol is the hydrogen atom donor to this intermediate and it can be regenerated by its interaction with the silane, thus propagating the chain. The hydrolysis of the silylamine occurred... 

Silyloxy-substituted alkyl radicals, which are generated via addition of tris(trimethylsilyl)silyl radicals to chiral ketones, abstract hydrogen from thiols with moderate diastereoselectivitics37. The svnjunti ratio is dependent on the steric bulk of the neighboring alkyl substituents. 

Reactions at acid centers to create 5 -2-(trimethylsilyl) ethanethiolesters are also common. Carboxylic 5-thiolesters are formed in high yield by the DCC/DMAP mediated coupling of 2-(trimethylsilyl)ethanethiol and carboxylic acids or by thiol substitution on a carboxylic acid chloride. 5 -2-(Tiimethylsilyl) ethyl p-toluenethiolsulfonate is formed by treating 2-(trimethyl-silyl)ethanethiol with tosyl bromide (eq 4). The product is a useful electrophile for carbon nucleophiles, allowing the introduction of the 2-(trimethylsilyl)ethylthio unit by an alternative mechanism. Independent of the thiol, aryl and alkyl 2-trimethylsilylethyl thioethers may be prepared by the radical addition of the appropriate arene- or aikanethiol to vinyl trimethyl-silane in a reaction comparable to that of eq l7 ... 

Radical addition of thiol or thiol-modified support to the vinyl group gives the respective thioether linkage 34 and represents one of the most convenient ways to immobilize Cinchona alkaloids [163, 172]. There are also few examples of platinum-catalyzed hydrosilylation of Cinchona alkaloids toward 11-silyl-substituted derivatives 35 with the use of monomeric silanes or polysiloxane polymers [173-175]. De Vries reported rhodium-catalyzed hydroformylation of the four main members of Cinchona alkaloids carried out on a hundred gram scale. Under optimized condition, linear aldehydes 36 were selectively obtained with the yield over 80% [176]. 

Athene Hydrogenations. Treatment of olefins with Zr catalysts effected their saturation in 50% yield along with 50% dehydrogenative silylation as calculated by GC (eq 20). Ru car-bene complexes also afforded the reduced product in the presence of PhsSiH. Reduction of an olefin via radical-chain reductive car-boxyalkylation proceeded with PhsSiH and a homochiral thiol catalyst upon TBHN initiation (eq 21). The olefin of vinylstan-nanes could be reduced without any protodestannylated product produced (eq 22). ... 

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