Reductive cleavage halide salts

The triazole 76, which is more accurately portrayed as the nucleophilic carbene structure 76a, acts as a formyl anion equivalent by reaction with alkyl halides and subsequent reductive cleavage to give aldehydes as shown (75TL1889). The benzoin reaction may be considered as resulting in the net addition of a benzoyl anion to a benzaldehyde, and the chiral triazolium salt 77 has been reported to be an efficient asymmetric catalyst for this, giving the products (/ )-ArCH(OH)COAr, in up to 86% e.e. (96HCA1217). In the closely related intramolecular Stetter reaction e.e.s of up to 74% were obtained (96HCA1899). 

A disadvantage is that solutions of silyl anions prepared from the halosilanes by one of these methods inevitably contain metal halides as by-product. Salt-free perarylated silyl anions can be prepared by the reaction of a disilane with lithium, sodium or potassium in polar solvents such as THF or DME (Table i)28b,29 Alternative procedures for the preparation of halide-free Ph3SiK utilize the reductive cleavage of the Si—C bond of Ph3Si—CMe2Ph by a sodium-potassium alloy in Et20 (equation 17), as well as via the reaction of PhsSiH with KH in DME . 

Reaction of halides with oxide ion 0-68 Cleavage of oxonium salts 0-79 Reduction of acetals or ketals 0-92 Reaction between Grignard reagents and acetals or ketals dimerization of acetals... 

Sulfonium salts of thiepanes are readily formed by electrophilic attack of alkyl halides on the cyclic thioether. Thus, thiepane (35) was found to yield a sulfonium iodide (123), which at elevated temperatures and in the presence of excess methyl iodide underwent ring cleavage to yield 1,6-diiodohexane (isolated as the 1,6-diphenoxy derivative Scheme 24) (53M1206). The alkoxysulfonium salt (124) formed by reaction of (35) with t-butyl hypochlorite (equation 23) was characterized as a stable hexachloroantimonate (67JOC2014). Reduction of thiepane 1-oxide (115) to thiepane has been achieved using an aqueous solution of NaHSC>3 (72JOC919). A hydroxysulfonium salt intermediate (125) has been proposed in the latter reduction reaction which provides a general method for sulfoxide reductions under mild conditions (equation 24). 

It is believed that the electrochemical reductive of aliphatic halides [58], benzyl halides and aryldialkylsulfonium salts [89] are concerted, i.e., electron acceptance is concomitant with bond cleavage, due in part to the a nature of the LUMO as well as the instability of the anion-radical species and stability of the products. If the anion-radical is not a discrete chemical entity back ET cannot take place. Therefore, the efficiency of PET bond cleavage reactions would be expected to be greater for the reasons mentioned above. However, due to the localized nature of the a molecular orbitals the probability for intermolecular and intramolecular ET, for example, to a a MO may be quite low. However, the overall efficiency of PET concerted bond cleavage reactions may approach unity provided that ET to the This topic clearly requires further consideration and research using fast kinetic laser spectrophotometric techniques to go beyond the qualitative discussion provided here. 

A number of low-valent metal ions have been shown to reduce a-halocarbonyl compounds. The most commonly used species for this purpose have been chromium(II) and low-valent titanium " salts, although vanadium(II), samarium(II), iron(II) and tin(II) salts have also been used. 7 222 chloro, bromo and iodo ketones can all be reduced by chromium(II) and titanium(III) salts. Selective reductions are possible axial halides are reduced in preference to equatorial, and a,a-dihalo ketones can be selectively reduced to the corresponding monohalides (equation 10). 7 The use of samarium(II) iodide has recently been advocated for such a-cleavages.72 a-Halo esters and ketones are reduced instantaneously at -78 °C in excellent yields. a-Acetoxy esters are stable to this reagent. 

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