Sodium phenylselenide

It is not surprising that chloro esters 1, 2 readily add thiols, catalyzed by sodium thiolates or triethylamine, to give the corresponding 2-(r-organylthiocy-clopropyl)-2-chloroacetates 85,86 (Scheme 22) [15 b, 22b, 27]. This reaction with thiophenol has been used to quantify the Michael reactivity of 1-Me, 2-Me, 3-X in comparison to simple acrylates (see above). With an excess of PhSH, the nucleophilic substitution of the chlorine in 85 a (but not in 85h) proceeded to give the corresponding bis(phenylthio) derivative in 63% yield [15bj. Alkali thiolates (e.g. NaSMe, NaSBn) add smoothly onto 1-Me, 2c-Me and 2p-Me at - 78 °C, because at this temperature subsequent nucleophilic substitution of the chlorine is much slower [7l, 9]. The Michael additions of sodium phenylselenide and sodium arylsulfenates onto 1-Me and their synthetic utility have been discussed above (see Table 1). 

Ring opening of the oxaspiropentane 343 upon treatment with sodium phenylselenide (vide supra, Sect. 4.5, Eq. (34)) 59) and O-silylation produce the vinylcyclopropanol trimethylsilyl ether 344 which, on flash thermolysis at 670 °C, gave the siloxycyclo-pentene 345 as a 2 1 mixture of epimers at C(8). Then, allylation of the more substituted enolate arising from 345, opens a convenient way to the antitumor agent, aphidicolin 346 181>. 

With a-phenylselenolactones, endocyclic elimination is preferred if the syn elimination is possible (equation 44), perhaps because the conformation leading to the exocyclic product involves greater dipole-dipole interaction between the selenoxide and the carbonyl group. However, a-methylenelactones are obtained if syn elimination cannot be achieved (equation 45). The conjugate addition of sodium phenylselenide has been recommended for the protection of ot-methylenelactones, the double bond being reintroduced by selenoxide elimination. ... 

Diethyl l-(phenylseleno)- and l-(phenyltelluro)methylphosphonates " have been prepared in 77-92% yields by the reaction of diethyl iodomethylphosphonate with sodium phenylselenide and lithium phenylteUuride, respectively. 

Limonene cfr-epoxide (549) is favored by employing molybdenum-catalyzed epoxidation of limonene, and it undergoes franj-diaxial opening with sodium phenylselenide. Both the ring-opened selenides were readily converted to trans-... 

Simmons-Smith cyclopropanation, 456 Simmons-Smith reaction, 455 six-centered transition state, 264 Smiles-type rearrangement, 7 SOCI2, 19, 159 sodium azide, 365, 375 sodium borohydride, 416 sodium hexamethyldisilazide, 290 sodium hydride, 372, 375. 383 sodium in liquid ammonia, 440 sodium iodide, 379 sodium phenylselenide, 458 sodium trifluoroethoxide, 447 solid phase synthesis, 25 Sonogashira conditions, 409 Sonogashira coupling, 411 spartadienedione, 144 spontaneous csdodimerization, 23 S-shaped and C-shaped diastereomers, 83 stainless steel reactor, 283... 

A convenient and high yielding carbonyl homologation involves the reaction of excess sodium phenylselenide with a phenyl-sulphinyl epoxide. The required epoxides are most easily prepared by the reaction of the anion of an a-chloro-sulphoxide with a ketone and ring closure of the resulting chlorohydrin with potassium hydroxide (equation 12). 

Other heteroatomic bond cleavages have found synthetic applications. One of these is the reduction of diphenyldiselenide by sodium under irradiation at 50 kHz, giving sodium phenylselenide (Eq. 30, see p. 371).1 8... 

Sodium phenylselenide 1 Sodium (50% dispersion in paraffin, Aldrich, 433 mg, 9.4 mmol) and benzophenone (25 mg, 0.14 mmol) in a minimal volume of dry THF (2 mL) were placed in a 25-mL roimd-bottom flask and sealed under an argon (dried by passing through a CaC column) atmosphere with a rubber septum. The flask was then suspended in a Semat 80-W, 50-kHz ultrasonic cleaning bath in a position so as to cause maximum agitation of the flask contents. After a few seconds of sonication the formation of sodium benzophenone ketyl was observed as evidenced by the evolution of a deep blue color. 

A solution of diphenyldiselenide (1.400 g, 4.7 mmol, from Aldrich, used without further purification) in the minimum volume of dry THF was then added dropwise to the sonicated mixture. After 15 min the reaction mixture had developed a pale mauve tinge. Sonication was continued for 45 min or until all traces of color had disappeared. The suspension of sodium phenylselenide was used immediately in the next step. 

Sodium phenylselenide generated by this route has been shown to react smoothly with a variety of organic substrates capable of undergoing nucleophilic substitution, including halides, epoxides, and sulfonates. The yields of selenide products are generally excellent and the mild conditions employed ensure that other potentially sensitive functional groups such as THP ethers, ketones, and dioxanes remain intact. 

Interestingly, solvent effects appear to be extremely marked. Luche and co-workers noted that dispersion could not be effected in THF and other reports suggest that the process is extremely sluggish in benzene [106]. Similarly, sodium could only be dispersed in xylene and lithium could not be persuaded to disperse in either of the three solvents tried. Ley et al. have also observed this whilst attempting to form sodium phenylselenide by reaction of sodium with diphenyl diselenide [107]. Using solid sodium, the reaction time was halved when using xylene in place of THF (Table 4). 

Sodium phenylselenide was prepared in 1 h by sonolysis of diphenyl-diselenide with sodium in the presence of benzopheneone, which acts as an electron transfer agent (Scheme 110) [107]. The reaction between diphenyl-diselenide and solid sodium is virtually negligible at room temperature. However, initial studies showed that the reaction could be brought to completion within four days in the presence of ultrasound. A brief investigation of the effect of solvent on the reaction was carried out in line with those described by Luche and co-workers [83]. Thus it was discovered that the reaction time could be halved by using xylene in place of THF. However, from a practical point of view, the difference in boiling points between that of xylene and THF is considerable. This would severely restrict the applicability of the method as isolation of volatile or thermally unstable selenides would be virtually impossible. 

The greatest enhancement in the rate of reaction was observed when a catalytic amount of benzophenone was added to the reaction mixture. This modification had the added advantage that the reaction became self-indicating as a result of the characteristically dark blue sodium benzophenone ketyl radical. Dropwise addition of a solution of diphenyldiselenide in a minimal volume of THF appeared to result in virtually instantaneous formation of the cream-coloured sodium phenylselenide and the reaction was complete when the last mauve tinges had disappeared. 

Furthermore, treatment of ultrasonically generated sodium phenylselenide with acid gives the parent selenol which, although commercially available, is currently more than six times the price of diphenyldiselenide. 

New methods for preparing known selenium reagents are always interesting. For example, phenyl selenocyanate is readily prepared by addition of trimethyl-silylcyanide to phenylselenyl chloride. Two new preparations of sodium phenylselenide have also appeared. In the first, selenide ion is photolysed in the presence of iodobenzene, whereas the second preparation involves... 

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