Selenols acylation

Friedel-Crafts acylation.1 The complex (CuOTf)2C6H (5, 151-152) induces acylation of aromatics by selenol esters. Other copper salts (Cu20, CuCI2) are... 

The syntheses of 2-ketobenzo[ >]selenophene (134) (76JCS(P1)2452) and its benzo[c] counterpart (135) (64BSB491, 81JHC343) both depend on intramolecular acylation of an intermediary selenol, while in the benzo[Z>]selenophene (137) preparation from (136) the ring closure depends upon the intramolecular addition of the intermediary selenol to the carbonyl group (70BSB235). 

Acyl radical cyclization to cyclohexanones.n Acyl radical cyclization to five-membered rings is well known, but this reaction is also useful for synthesis of substituted cyclohexanones as shown by a recent synthesis of the a-methylenecyclo-hexanone 3. Thus treatment of the selenol ester 1 with Bu3SnH and AIBN in C6H6 at 80° provides a 1 1 mixture of cyclohexanones 2 in 91 % yield. Oxidation of 2 with... 

Alternatively polymer-bound sodium selenide 6 served as the starting point for an acylat-ing protocol (Scheme 3) [12]. Transformation into selenol ester 9 afforded an active polymer-bound intermediate which was cleaved in the presence of an alkinylcopper species to generate a,/ -alkinyl ketones 10 while the copper selenide can be reacylated using acyl chlorides. 

Acyl radical has a nucleophilic character and cyclizes via SOMO-LUMO interaction. Eq. 3.22 shows the cyclization of acyl radicals formed from the reaction of selenol esters (67) with Bu3SnH/AIBN or Bu3SnH/Et3B, to give the cyclic ketones (68) via 5-exo-trig or 6-exo-trig manner through the transition state [II] [84-90]. 

Keywoids Radicals, Group transfer reactions. Photoelectron transfers, Selenosulfonation, Acyl radicals, Azidoselenenylation, Selenides, Selenol... 

Selenol esters are useful as active acyl equivalents, particularly with regard to macrocycle formation. Equation (21) outlines the conversion of esters to selenol esters via aluminum reagent (48). ... 

As well as thiol esters, selenol esters (1) frequently exhibit a high and selective reactivity toward nucleophiles, which is enhanced even further by activation with heavy ions or oxidizing agents. These properties make selenol esters valuable acyl transfer agents. This review deals with general methods for the synthesis of selenol esters and their reactivity as acyl transfer agents. Furthermore, selenoesters (2), isomeric compounds of selenol esters, and their derivatives selenoamides (3) are also described. These compounds show the characteristic reactivity based on the carbon-selenium double bond. 

In 1962 Renson et al. reported that a variety of selenol esters can be synthesized by the reaction of the appropriate acyl chlorides with selenols in the presence of pyridine. - Selenols can easily be prepared from elemental selenium and the corresponding Grignard reagents. In addition to simple selenol esters, a,P-unsaturated selenol esters (4) and (5) and o-substituted aromatic selenol esters (6) have been obtained by this method, as shown in Scheme 1. Butyl selenol esters are generally colorless or light yellow liquids, whereas the phenyl or substituted phenyl selenol esters are white solids, which are easily purified by recrystallization. More recently other groups have used Renson s method to synthesize similar selenol esters. " ... 

It has been reported that some selenols are directly acylated by carboxylic acids to give selenol esters. " For example, heating of 4-hydroselenobutyric acid at about 160 C resulted in ring closure to give the corresponding y-lactone (equation 1). ... 

Selenol esters have also been prepared from trimethylsilyl selenides and acyl halides, as shown in equation (5). ... 

As well as acylation of selenols, alkylation of selenocarboxylates with alkyl halides is one of the most straightforward methods for synthesis of selenol esters. However, known examples of selenol esters synthesized using this strategy have been limited to a few cases, due to the difficulty of the preparation of selenocarboxylates. 

Selenol esters are expected to be a more reactive species than the corresponding thiol esters or 0x0 esters due to the comparatively weak bonding between carbon and selenium. The ability of the selenol esters to serve as active acyl transfer agents has been readily demonstrated by the easy conversion of the selenol ester (34 equation 15) to its corresponding acid (35), ester (36) or amide (37). This acyl-selenium bond cleavage has also been promoted by Cu and Cu" salts. - The isopropylidene derivative (38 equation 16) of ribofuranosylacetate has been converted to a lactone (40) in good yield via the selenol ester (39). 

The copper or copper(I) iodide-catalyzed insertion of diazomethane into the acyl-selenium linkage of selenol esters was reported to afford the corresponding ketones as the chief products. - A nonconcerted mechanism involving a tetrahedral intermediate (42 Scheme 11) has been suggested. 

Moreover, the selenol esters can acylate reactive arenes and heteroaromatic compounds when cop-per(I) triflate is employed as the selenophilic metal cation. - The acylation of aromatics by use of the benzene complex of copper(I) triflate (43 Scheme 12) was complete within an hour at room temperature, with benzene as solvent, and the acylation products were obtained in high yields. Intramolecular acylation was examined successfully, as shown in equation (20). 

Intramolecular capture of the acyl radical formed in situ from a selenol ester and tin hydride has been examined successfully. Phenyl selenol esters (51), with an unsaturated functional group (C—C etc.) at... 

Instead of alkylating reagents, the use of acyl or aroyl halides in fhis reaction conveniently leads to the synthesis of selenoesters as useful acylating reagents in organic synthesis. A mild and convenient method is reaction of trimethylsilyl selenides with acyl halides [27] or reaction of selenols with imidazolide (or triazolide) of carboxylic acids (Scheme 15.8) [28]. 

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