Polarity reversal catalyst

Roberts and co-workers have employed a number of chiral carbohydrate-derived thiols as polarity reversal catalysts in the radical hydrosilylation of electron-rich prochiral alkenes [68-70]. In these thiols, the SH group is attached to the anomeric carbon atom. Scheme 21 demonstrates the non-catalyzed reaction and in step b, the hydrogen atom transfer from the silane... 

Scheme 21 Hydrosilylation thiols as polarity reversal catalysts...

The stereochemical outcome for addition of r-l,3-dioxolan-4-yl and oxiranyl radicals to phenyl vinyl sulfone has been probed. The results indicated that the symanti selectivity could be altered by changing the group next to the radical in the diox-olanyl case but not in the oxiranyl case (bulky groups had a large xyn-directing effect) (Scheme 39). Several alkenyl-lactones and -lactams have been subjected to hydrosilylation conditions using carbohydrate-derived thiols as homochiral polarity reversal catalysts (yields 25-96% ee 5-95%) " ... 

Addition of 10 mol% of diphenyl diselenide to hydrostannylation reactions involving electron-rich alkenes resulted in a dramatic improvement in yield. For example, reaction of o - [(f-butyl)dimethylsilyl]oxy styrene with PhsSnH (1.1 equiv.) in the presence of PhSeSePh and 2,2/-azobis(2-methylpropanenitrile) (AIBN) afforded 2- [(f-butyl)dimethylsilyl]oxy -2-phenylethyl triphenylstannane in 95% yield after 2 h. This reaction is believed to benefit from the increased rate of H-atom transfer, resulting from the in sz fM-generated polarity-reversal catalyst benzeneselenol.55... 

This substitutes very well for most tin hydrides in many reactions. Roberts, on the other hand, has demonstrated that thiols act as polarity reversal catalysts for hydrogen-atom transfer from organosilanes to alkyl radicals.185 The use of triethyl borane and oxygen as radical initiators166 has allowed the reaction to be carried out at much lower temperatures and hence mechanistic elucidations have been possible.187... 

It has long been known that unsymmetrical ketones can be prepared by the reaction of aldehydes with alkenes under free-radical reaction conditions. Recently the revision of this chemistry has been reported by the Roberts group [42], They introduced thiols as a polarity reversal catalyst for the addition of aldehydes to alkenes. Thiyl radicals are electrophilic, and therefore a polar Sh2 type transition state for the hydrogen transfer step from an aldehyde would be ideal in this situation. Indeed, the addition of aldehydes to a variety of alkenes can be effected by... 

Benzeneselenol can be used as a polarity-reversal catalyst for abstraction of hydrogen from tin hydrides by carbon-centered radicals (Scheme 15.56) [134]. 

Triethylsilane reduction of alkyl halides is even better in the presence of thiols, which act as polarity reversal catalysts for hydrogen atom transfer from silane to alkyl groups (equation 41). This reagent in the presence of AICI3 reduces even em-dichlorides to alkanes (equation 42) . The powerful hydrogen donor reagent tn s-(trimethylsilyl)silane has been introduced as a superior alternative to triethylsilane for the hydrogenolysis of... 

Examples of application of the iert-dodecanethiol/silane couple include typical reductive dehalogenations, but also Barton-McCombie deoxygenations of alcohols, through conversion of the latter into xanthate esters (eq 6), hydrosilylations of alkenes (eq 7), and preparation of silanethiols. The thiol can also be used as a polarity reversal catalyst in conjunction with tris(trimethylsilyl)silane and hexabutyldistannane/malonic acid. ... 

The presence of an electron withdrawing group increases the SH bond dissociation energy (BDE) relative to that of a simple alkanethiol. For this reason, MTG is a good candidate to serve as a polarity reversal catalyst (PRC) that promotes the overall hydrogen atom transfer from a substrate R-H to a carbon centered radical. This reactivity has been applied to the addition of aldehydes to alkenes (eq A)] to the alkylation of electron-rich alkenes in the presence of silane (eq 5), to the preparation of j8-lactams via aminoacyl radical generation (eq 6), and to hydroamination of double bonds (eq 7). ... 

When heated to 125 °C in a hydrocarbon solvent in the presence of a thiol as the catalyst and DBPB as initiator, the c/s-cyclic ketal 7 underwent selective epimerization at the C-H center a to oxygen to give the thermodynamically more stable trans-epimer 8. Silanethiols were found to be more effective protic-polarity reversal catalysts than alkanethiols (eq 4). Thus, in the presence of TBST and a small amount of collidine as the scavenger of adventitious acid formed under the reaction conditions, the epimerization of 7 proceeded smoothly to give 8 in 84% conversion after 1 h. Interestingly, when triphenylsilanethiol was used as the catalyst, the coadministration of collidine proved to be detrimental, resulting in a suppression of the isomerization. This is probably because this latter thiol is susceptible to nucleophilic attack by the base. The improved performance of TBST is therefore attributed to its stability toward nucleophilic substitution at the silicon center. 

Hydroacylation of Alkenes Using NHPI as a Polarity Reversal Catalyst... 

Figure 6.11 Hydroacylation of alkenes using NHPI as a polarity-reversal catalyst.

Scheme6.20 A possible reaction path for hydroacylation using NHPI as polarity-reversal catalyst.

Polarity-reversal catalysis has been used to mediate a cyclization reaction that otherwise could not be accomplished." Reaction of (31) with the peroxide (32) failed to produce any of the desired cyclic material (33). However, when the reaction was repeated in the presence of tri-r-butoxysilathiol (5 mol%) as a polarity-reversal catalyst, the cyclic compound (33) was furnished in 92% yield (Scheme 17)." ... 

Use in Radical Reactions. The radical reduction of alkyl halides by silanes is promoted by thiols (eq 14). According to Roberts, the thiol acts as a polarity-reversal catalyst. The direct atom transfer in eq 15 is replaced by the sequence of the more rapid reactions in eqs 16 and 17. 

The use of silanethiols as polarity-reversal catalysts is highly recommended in the addition of nucleophilic carbon-centered radicals to double bonds. This is especially so for the radical-chain addition of aldehydes to alkenes. In this particular reaction, tri-organosilanethiols are very efficient catalysts for the addition of butanal to both electron-rich isopropenyl acetate, or to electron-deficient ethyl crotonate. The results obtained with HSTIPS are given in eqs 23 and 24. ... 

HSTIPS and tri-f-butoxysilanethiol are equally effective and robust acidic polarity-reversal catalysts since they are both remarkably stable to hydrolysis. The HSTIPS catalyst is preferred owing to the fact that it is commercially available. The reaction is conducted under inert atmosphere, at reflux in dry octane, in the presence of 5 mol % of the catalyst. It is to be noted that, when DTPB is used, 50 mol % of the initiator is necessary. 

An alternative procedure for the reduction of xanthates, as proposed by Roberts and co-workers, was to employ a catalytic amount of a thiol, in combination with triethylsilane. Using 1,1-di-f-butyl peroxide (DTBP) as radical initiator, /-dodecanethiol as an acceptor polarity-reversal catalyst, followed by triethylsilane as stoichiometric reductant. The thiol serves to transfer H-atom from the silane reductant to the intermediate radical species (R-). "... 

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