Reversible 2, catalysis

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

Geminal bis(hydrostannation) is attributed to quenching of the bis-2-tributylstannylethyl radical by the thiol, with polarity reversal catalysis (Equation (16)). 

A majority of radical addition occurs with electron-poor alkenes using alkyl halides in the presence of BusSnH. These reactions are feasible due to a proper matching between the radical acceptor and the donor. However, when the alkene is electron-rich and since simple alkyl radicals are considered as nucleophilic, the reaction is not a practical method for carbon-carbon bond formation. By applying the concept of polarity-reversal catalysis, an additional reagent is introduced which alleviates the mismatch between the partners and makes the reaction feasible. A few examples illustrating this concept have been described in this review. 

Polarity reversal catalysis by tri-r-butoxysilanethiol has been applied to promote... 

Scheme 4.6 Propagation steps for polarity-reversal catalysis...

The addition of silanes across alkenes has been investigated both experimentally145 and theoretically.146 The effect of optically active thiol catalysts to catalyse radical hydrosilylation (polarity reversal catalysis) has been studied. The use of 2,3,4,6-tetra-O-acetyl-thio-jS-D-glucopyranose as the chiral thiol (used to reduce the intermediate carbon-based radicals) furnished the hydrosilylated alkenes in low to moderate enantiomeric excesses.145 In addition to this work a theoretical study on the reactions of SiH3 with ethene and propene has been undertaken using PMP2(6-31G ) and QCISD(T)(6-31G ) methods. Results indicated that the alkene-addition pathway is favoured over the alternative possible mode of reaction (H-abstraction). This is contrary to that previously suggested for the reaction of SiF with propene.146... 

Cai Y, Roberts BP (1998) Radical-chain racemisation of tetrahydrofurfuryl acetate under conditions of polarity-reversal catalysis possible implications for the radical-induced strand cleavage of DNA.Chem Commun 1145-1146... 

J. D. Wissmann, and H.-D. Jakubke, Reverse catalysis of elastase from porcine pancreas in frozen aqueous systems,... 

Of direct relevance to the debate described above, Skidmore examined extensively the mechanism of polarity reversal catalysis described in Scheme 11 and concluded that the key step (step 2, Scheme 11) in the proposed mechanism, namely the transfer of hydrogen atom from a trialkylsilane to a thiyl radical, is severely endothermic54,55. 

Phenylselenyl radicals are also formed when benzeneselenol is used as a reducing agent. These radicals are usually unable to propagate chain reactions since they dimerize rapidly. Two important exceptions are known the propagation with acyl thiohydroxamates ( Barton esters ) and the use of polarity reversal catalysis. 

On Polarity Reversal Catalysis and the Benefits for Silicon-based Reducing Agents in Aqueous Media... 

Scheme 4.9 Proposed mechanism of polarity reversal catalysis using the (TMS)3SiH-thiol system.

Polarity-reversal catalysis of hydrogen-atom abstraction reactions concepts and applications in organic chemistry. 

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