Selenides, phenyl reduction

In contrast to the cathodic reduction of organic tellurium compounds, few studies on their anodic oxidation have been performed. No paper has reported on the electrolytic reactions of fluorinated tellurides up to date, which is probably due to the difficulty of the preparation of the partially fluorinated tellurides as starting material. Quite recently, Fuchigami et al. have investigated the anodic behavior of 2,2,2-trifluoroethyl and difluoroethyl phenyl tellurides (8 and 9) [54]. The telluride 8 does not undergo an anodic a-substitution, which is totally different to the eases of the corresponding sulfide and selenide. Even in the presence of fluoride ions, the anodic methoxylation does not take place at all. Instead, a selective difluorination occurs at the tellurium atom effectively to provide the hypervalent tellurium derivative in good yield as shown in Scheme 6.12. 

As the rate of cyclization becomes slower, the reactivity of the precursor becomes more important. To ensure that the radical generation step does not break the chain, it is important to use the most reactive precursor available. For very slow cyclizations, the advice is simple use iodides whenever possible. The purity of the precursor is also critical for slow cyclizations because tin hydride can sometimes react with impurities to generate hydrogen atom sources that are much more reactive than itself. Any impurities that might generate thiols or selenols may cause undue amounts of reduction (thus, the purity of phenyl sulfides and selenides is especially important). Metal impurities, which may form transition metal hydrides, can be devastating, even for fast cyclizations.41 Empirically, it seems that breaking of the chain is less of... 

Danishefsky and coworkers showed that selenide (52) undergoes reduction upon treatment with Bu3SnH in the presence of allyltributyltin, rather than allylation, as expected (equation 32)197. In this last example, Uriel and Santoyo-Gonz lez utilised Bu3SnH reduction of a glycosidic phenyl selenide (53) in their synthesis of 2-deoxyglycopyranoyl thioureas (Scheme 8)199. This example typifies the synthetic utility of phenyl selenides,... 

The reductions of organic halides, phenyl selenides, isocyanides and thionocarbonates with (TMS)2Si(H)Me119 and (RS)3SiH120 are achieved under normal conditions. Two examples are shown in equations 62 and 63. (TMS)2Si(H)Me is an effective reducing agent which allows the formation of the desired product to be favoured due to a slower hydrogen transfer (Table 4). 

Tetraphenyldisilane has also been introduced as a diversified radical reagent for the reduction of alkyl bromides and phenyl chalcogenides124. An example with 3-choles-tanyl phenyl selenide is given in equation 68. 

Starting with picrotoxinin (1) Yoshikoshi developed an improved synthesis of ( )-picrotin (2). Epoxidation of picrotoxinin (1) with peracid at room temperature led to a 5 2 mixture of the epimeric epoxides. Regioselective cleavage of the epoxide was achieved with sodium phenylselenyl triethoxy boronate. Radical reduction of the phenyl selenides 414 with stannane completed this three-step sequence to picrotin (2) in 87% overall yield. 

Overall reductive decarboxylation of a carboxylic acid may be achieved by the reaction of the derived acyl chloride with triisopropyisilane (equation 12). Relatively high temperatures are required to bring about efficient decarbonylation of die intermediate acyl radical. A related mediod involves the reaction of acyl phenyl selenides with tri-it-butyltin hydride. Here again relatively high temperatures are required for primary and secondary, although not for tertiary, acids (equation 13). 

These acetals are reduced by triphenyltin hydride to the corresponding hydrocarbons in high yield. Raney nickel is less efficient for this purpose. This reaction constitutes a useful alternative to Wolff-Kishner reduction of carb onyl compounds. Phenyl selenides are also reduced by tin hydrides. ... 

The stereospecific conversion of cyclohexene into the corresponding amido selenide 54 is illustrated in Scheme 8. These amidoselenenylation reactions are commonly employed for the preparation of allylic and saturated amides by oxidative or reductive deselenenylation. Propionitrile, butyronitrile, benzonitrile and ethyl cyanoacetate may be used in place of acetonitrile. Styrene gave poor results and other electron-rich olefins such as 1-methylcyclohexene or 2,3-di-methylbut-2-ene did not give the amidoselenenylation products. The reaction can also be effected starting from the hydroxy- or methoxyselenenylation products of alkenes, in the presence of water and trifluoromethanesulfonic acid in this case the nitriles are used in stoichiometric amounts [48c]. This methodology was employed to prepare the amidoselenenylation products of styrene, 55, and of electron-rich olefins. It was necessary, however, to replace the phenyl-... 

This naked selenolate is more reactive than the one complexed with borane 1 [1]. For example, in the presence of HMPA, 4 undergoes an SN2-type ester cleavage to produce the corresponding acids and alkyl phenyl selenides (Sect. 3.3) [6 a]. Uncomplexed selenolate 4 can also be prepared by the reduction of benzeneselenol (PhSeH) with sodium hydride (NaH) (Scheme 4b) [6aj. 

Reduction of diphenyl diselenide with the Sm-Me3SiCl-H20 system also leads to a benzeneselenolate, which reacts with organic halides, epoxides, a, -un-saturated esters or cr, -unsaturated nitriles to afford unsymmetrical phenyl selenides in good yields under mild and neutral conditions as shown in Scheme 13 [19]. This method is easier to handle than the use of air-sensitive Sml2. 

Selenolates prepared from diphenyl or dimethyl diselenide by reduction with NaBH4 smoothly transform various benzylic alcohols 24 into the corresponding selenides 25 in the presence of aluminum chloride (Scheme 30 a) [41]. AICI3 is considered to activate the alcohol substrate by coordinating to the oxygen. Similar transformations are possible by the reaction of alcohols with phenyl selenocyanate in the presence of tributyl phosphine [52]. When the selenolate is reacted with aromatic aldehydes or ketones 26 in the presence of AICI3, the corresponding benzylic selenides 27 are obtained in moderate yields (Scheme 30b) [41]. 

From halides. Halides are easily converted into selenides. Since halides are also suitable radical precursors, this transformation is usually done when side reactions of halides with nucleophiles can occur. An example of this type is reported in Eq. (1). The bromide 5 was converted into a phenyl selenide, which could stand DIBALH reduction and imine formation. Tin mediated cyclization of 6 afforded the cyclopentylamine 7 in 72% yield [5]. 

Phenyl propargyl sulfide " and selenide are rapidly deprotonated by 2 equiv. of base. The resulting dilithio derivatives react at their allylic rather than at their alkynic carbon centers (Scheme 34, entries c and d). Reduction of the sulfides allows the synthesis of 1,5-enynes (Scheme 34, entry b), whereas oxidation of the selenides leads to a-phenylseleno-a,P-unsaturated carbonyl compounds (Scheme 34, entry... 

Alternatively, 1-lithio-l-alkenyl phenyl and methyl selenides have been obtained on reduction of the C—Se bond of l,l-di(seleno)alkenes with alkyllithiums this reaction is the only one which allows the synthesis of the methylseleno derivatives (Scheme 41). ... 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

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