Selenol esters

Hydrozirconation of terminal alkynes R-C=CH (R= aryl, alkyl) with 1 affords terminally ( )-Zr-substituted alkenes with high efficiency and excellent stereochemical and regiochemical control (>98%). These alkenylzirconocene complexes are of particular interest for synthetic use [136, 143, 144]. Moreover, beside the electropositive halogen sources [145] and heteroatom electrophiles [3] used in the pioneering studies to directly cleave the Zr-C bond, ( )-vinyl-Zr complexes were recently transformed into a number of other trans-functionalized alkenes such as ( )-vinyl-sul-fides[146], vinylic selenol esters [147], vinyl-sulfones [148], vinyl-iodonium [149], vinyl-(R0)2P(0) [150], and vinilic tellurides [143]. 

Example 2, in addition to oxyselenide formation, carbo- and heteroseleno cycliza-tion, A -PSP can be used to generate selenides from alcohols and selenol esters from carboxylic acids, respectively, in the presence of a stoichiometric amount of n-Bu,P. ... 

Oxazoles. Activated isonitriles react with selenol esters in the presence of triethylamine and Cu20 at 25° to form oxazoles, presumably via a j8-keto isonitrile (equation I).1... 

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

Alkyl phenyl setenides selenol esters. N-PhcnylscIcnophthalimidc is superior to aryl selenocyanates for conversion of alcohols to alkyl phenyl selenides (6, 252-253) and of carboxylic acids to selenol esters (7, 396-397). When conducted in the presence of an amine the latter reaction provides amides in high yield (equation l). ... 

Relatively little is known about the photochemistry of phosphorus containing organic substrates. Continuing our photochemical work on compounds of structure 1a and b which leads to thio- and selenoxanthones 3a,b via thiol ester-thiopyrone-, selenol ester-seleninone-transformations ( 1, 2, 3), or photo-Friedel-Crafts-reactions ( 3-6J, respectively, "(ScFeme 1) we have extended our interest to aroyl diphenyl phosphines 1c - k (Schemes 2-5) in which X in 1a and b stands for P-phenyl. 

As 1c behaves photochemically similar to certain thiol and selenol esters (e.g. 1a and b), yielding heterocycles of type 3 in the first photo-Friedel-Crafts-reaction of an organic phosphorus compound (Scheme 2), the aroyl diphenyl phosphines id - k show other reactions on u.v. irradiation (7, 8, 9J. 

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]. 

Seleno esters are also produced in good yield by reactions of dimethy-laluminim methanoselenolate103 (CH3)2AlSeCH3, with the protected C-ribofuranosyl acetate 269. Treating the selenol ester 270 with either cuprous or mercuric chloride produced the (Gensler) lactone.110 271, a product useful in the synthesis of various C-nucleoside antibiotics.111,112... 

Se/eno/eslers, R SeCfiHs. Oxidation of N-acylhydrazines with benzeneseleninic acid in CH2CI2 leads to selenol esters, presumably via a diazene. Yields are usually increased by addition of triphenylphosphine and are then in the range... 

Selective activation of carboxylic acids. Carboxylic acids add exclusively to the triple bond of 1 to form an adduct (a) that rearranges to the (Z)-enol ester 2. Hydroxyl, phenolic, amino, and mercapto groups do not react with 1 under the same conditions. The esters (2) react with alkali metal salts of thiols and of selenophenol in THF at 0° to form thiol and selenol esters (3). 

SELENOL ESTERS Benzeneseleninic acid. Dimethylaluminum methyl-selenolate. 

On the other hand, the TiCl4-Sm-THF system was applied for the preparation of selenol esters from diaryl diselenides and acid chlorides (Scheme 14) [20]. 

Dimethylaluminum methylselenolate (38) is prepared in situ by the reaction of trimethylaluminum with elemental selenium in refluxing toluene [66]. The yellow-colored solution can be used in various reactions, such as the conversion of esters to the corresponding selenol esters, the ring opening of epoxides, and Michael addition to cyclohexenone (Scheme 36) [66]. 

A similar reagent, phenylselenotris(trimethylsilyl)silane (52), reacts with chloroformates in the presence of tetrakis(triphenylphosphine)palladium to produce (phenylseleno)formates 53 (Scheme 40) [79]. This reaction is also applicable to the preparation of selenol esters and selenol thionoformates [79]. 

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. 

Hitherto known reviews for the chemistry of selenol esters and their derivatives are shown in refs. 1-5. 

Many methods for the preparation of selenol esters have been developed. These can be classified into the following five general methods according to the types of reaction. 

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). ... 

Areneselenyl magnesium bromide has been reported to react with /7-substituted benzoyl halides, giving aromatic selenol esters " as the first examples exhibiting liquid crystal properties (equation 2). ... 

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