Selenols, aryl

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]. 

The aryl-substituted derivatives of [Bi(SeC6H2R3-2,4,6)3] (R = Me, Pr, Bu) are the only isolated examples of selenolate complexes. They have been characterized by mp, elemental analysis, NMR spectroscopy, and thermal gravimetric analysis, and the solid-state structure of the isopropyl-substituted derivative reveals a tricoordinate environment for bismuth [Bi-Se 2.630(8)-2.711(8) A Se-Bi-Se 92.3(2)-103.3(2)°], imposed by the steric bulk of the ligands (38). 

Unfortunately, the appeal of solid phase extractions on small scale fades as the scale increases due to the cost and inconvenience of using large amounts of fluorous silica gel. Here, modified techniques to reduce the tedium of repeated extractions are attractive. For example, Crich has recently introduced the minimally fluorous selenide C6Fi3CH2CH2C6H4SeH[171. This selenol is added in catalytic quantities to tin hydride reductions of reactive aryl and vinyl radicals. The high reducing capacity of the aryl selenide suppresses undesired reactions of product radicals without suppressing the reactions of the aryl and vinyl radicals themselves. After the reaction is complete, the selenol can be recovered by a modified continuous extraction procedure. 

The reactivity of the tellurolate ion is pecuhar, since the corresponding aryl selenolate gives only the disubstituted o-selenoxylenes, and sodium hydrogen telluride gives only poor yields. 

The strong nucleophilicity of aryl thiolate and aryl selenolate anions of tetraphenylphos-... 

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). ... 

The 4-(2-hydroxyaryl)-l,2,3-selenadiazole 56 undergoes ready decomposition by the action of potassium carbonate to form benzofuran-2-selenolate 177. The intermediate 176 can be alkylated with methyl iodide and benzyl chloride and arylated with 2,4-dinitrochlorobenzene (Scheme 14) <2001RJ01643, 2000RJ0605, 1999TL3903>. Intermediate formation of 2-(o-hydroxyphenyl)ethyneselenolate 174 during decomposition of 1,2,3-selenadiazoles was proven by... 

Carbonsulfur bonds can be formed by the reaction of elemental sulfur with a lithio derivative, as illustrated by the preparation of thiophene-2-thiol 446. If dialkyl or diaryl disulfides are used as reagents to introduce sulfur, then alkyl or aryl sulfides are formed sulfinic acids are available by reaction of lithium derivatives with sulfur dioxide. In the pyrrole series, a 2-thiol and the corresponding selenol can be prepared via reaction of 2-lithio-l-methylpyrrole with sulfur or selenium, then trapping with Me3SiCl leading to the 2-Me3SiX-substituted derivatives <1997T13079>. 

Reaction of alcohol 4.12 with o-nitrophenyl selenocyanate (4.13) and tributylphosphine gives 4.14. Oxidation of 4.14 with hydrogen peroxide gives selenoxide 4.15. Aryl selenoxide 4.15 bearing a (3-hydrogen atom is unstable and undergoes thermal syn-elimination to give styrene (4.16) with the expulsion of a selenol 4.17 in a fashion similar to that of the Cope elimination (Scheme 4.13). 

Alkyl selenides are most conveniently prepared by the dialkylation of Na2Se or by the monoalkylation of selenolates. Common routes to aryl derivatives include the reaction of a selenolate with a diazonium salt, the SrnI reactions described earlier, and the reaction of aryUithiums with aryl selenocyanates (ArSeCN). A different approach employs the reaction of alcohols with aryl selenocyanates or N-(phenylseleno)phthalimide (see Section 6) in the presence of tri-n-butylphosphine. Similar conditions can also be used to convert carboxylic acids to selenoesters (8). These reactions are illustrated in Scheme 3. 

Diselenides are generally prepared by the aerial oxidation of selenols or selenolates (see Section 2) or by the reaction of Li2Sc2, Na2Se2, or other 802 species with alkyl or aryl halides. In the latter case, elevated temperatures and DMF as the solvent are recommended. Aryl diselenides may also be obtained from the reaction of 802 with diazonium salts (Scheme 6). Relatively few unsymmetrical diselenides have been reported. Triselenides are also known but have not been as widely studied. 

Reaction of diphenyl diselenide or dimethyl diselenide with hydrazine hydrate and sodium hydroxide generates the corresponding selenolates smoothly in solvents like DMF or diethyl ether and in the presence of tetrabutylammonium chloride as a phase-transfer catalyst [13]. The selenolates react with organic halides to give various selenides (Scheme 9). Similar conditions have been applied to the synthesis of aryl vinyl selenides from diaryl diselenides and acetylene [14]. 

Substitutions to halide compounds are the most typical reactions of nucleophilic selenium species (Scheme 18). Alkyl or aryl selenolates (R SeM), which are prepared in situ by the methods described in Sect. 2, react with halides to give... 

Aryl diselenides are similarly cleaved to selenols (ArSeH) with Cp2TiH followed by Ph2l+X . ... 

It has been reported that aryl selenocyanate (25 Scheme 7) reacts with carboxylic acids in the presence of tributylphosphine under mild conditions, giving rise to selenol esters in good yields. The process may involve the reaction of carboxylic acids with selenophosphonium salt (26), which is the key intermediate. 

The reaction of A -acylhydrazines with benzeneseleninic acid (29 Scheme 9) in the presence of triphe-nylphosphine afforded high yields of diverse selenol esters. " Alkyl, cycloalkyl and aryl selenol esters were prepared, even in the case of the highly hindered compounds (30) and (31). 

Selenols (R-SeH) have rather small Se chemical shifts which are strongly dependent on the alkyl or aryl residue attached to Se, and incremental influences of individual groups in a, p, or y position can be identified The chemical shifts range from S = —130 for MeSeH to +278 for /BuScll the... 

---