Selenides, acyl phenyl

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). 

Acyl radicals are very useful synthetic intermediates. Their preparation is not simple since the corresponding halides are highly electrophilic and cannot be used as radical precursors. Organocobalt compounds were proposed as suitable source of acyl radicals [44]. However, the use of acyl selenides proved to be more general [45, 46]. These radical precursors can be efficiently prepared from the corresponding carboxylic acids and esters [47]. Acyl phenyl selenides should be preferred, when possible, relative to acyl methyl selenides due to the consumption of two equivalents of tin hydride with this last system (Scheme 1) [4]. Acyl selenides have found many applications in tandem radical additions to alkenes. Examples of intermole-cular [Eq. (18)] [48,49] and intramolecular reactions [Eq. (19)] [50a] are reported. The enoyl selenide 68 give the unsaturated acyl radicals 69. This intermediate... 

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]. 

Free-radical cyclization of phenyl selenide 15 to indolizidinone 16 represented a key step in the total synthesis of (—)-slaframine (equation 52). The two pairs of diastereomers were first separated and then hydrolyzed to the corresponding alcohols in 76% overall yield77. (TMS)3SiH-mediated acyl radical reactions from phenylseleno esters 17 have recently been utilized for the stereoselective synthesis of cyclic ethers78. In fact, the experimental conditions reported in equation 53 are particularly good for both improving cis diastereoselectivity and suppressing decarbonylation. 

Reaction of acyl or aroyl halides with thallium(I) phenyl selenide, prepared by the reaction of either thallium(I) ethoxide or thdlium(I) phenoxide with benzeneselenol, gave the corresponding phenyl sele-nol esters in yields over 97% (equation 3). ... 

Tetraethylammonium borohydride reduces diphenyl diselenide to phenylselenolate anion. When an acyl halide is added to the toluene solution of phenylselenolate anion, the corresponding acylation product is isolated in good yield (equation 4). Resin-supported phenyl selenide anion underwent acylation with acetyl chloride under mild reaction conditions. ... 

In 1980, Detty and co-worker revealed the synthesis of selenol esters by the reaction of thallous phenyl selenide with aroyl and acyl halides (Eq. 35) [70]. 

Thallous phenyl selenide is prepared by the dropwise addition of thallous ethoxide to a solution of benzeneselenol in hexanes-Et20, and is obtained as an orange solid in quantitative yields. Acyl and aroyl halides react readily with thallous phenyl selenide to give the corresponding selenol esters in >97% isolated yields. 

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