Phenylselenyl

Alkylthiocyanates and alkylselenocyanates are obtained by treatment of trialkylboranes with potassium thiocycanate (260) and sodium selenoisocyanate (261), in the presence of iron(III) compounds, respectively. Unsymmetrical trialkylboranes react preferentially at the more highly branched alkyl group. Alkenylphenyl selenides are obtained by the reaction of alkenylboronic acids with phenylselenyl bromide (262). 

This ether was prepared from an alcohol and 2-(phenylselenyl)ethyl bromide (AgN03, CH3CN, 20°, 10-15 min, 80-90% yield) it is cleaved by oxidation (H2O2, 1 h ozone or NaI04), followed by acidic hydrolysis of the intermediate vinyl ether (dil. HCl, 65-70% yield). ... 

Ionic Inflate derivatives of nonmetallic elements such as selenium, sulfur, phosphorus, and iodine form an important class of reagents lor organic chemistry. Highly electrophilic phenylselenyl triflate can be used in the cyclization of 5- and 6-hydroxyalkenes, affording the corresponding tetrahydrofurans and pyrans [132] (equation 68). 

Trimethylsi ylethyl, 64 2-(Benzylthio)ethyl, 65 2-(Phenylselenyl)ethyl, 65... 

Simple 1-hetero-substituted allyllithium derivatives, such as 1-alkoxy-94"96, 1-alkyl-thio-50,97, 1-phenylselenyl-54,98 show insufficient regio- and simple diastereoselectivity in their reaction with aldehydes. The rcgiosclectivity is greatly enhanced in favor of the a-products by in... 

Entry 21 involves addition to a glyoxylic hydrazone and the cis ring junction is dictated by strain effects. The primary phenylselenyl group is reductively removed under the reaction conditions. Entry 22 involves generation of a stannyloxy radical by addition of the stannyl radical at the carbonyl oxygen. Cyclization then ensues, with the cis-trans ratio being determined by the conformation of the cyclization TS. 

A phenylselenyl group was eliminated by deactivated Ra-Ni. Deactivatation was achieved by heating the ethanolic suspension at 60°C for 3-4 days after which the starting compound was treated with this ethanolic Ra-Ni suspension for 30 minutes.415 429... 

This strategy is applied to a general method for the preparation of 2,2-disubstituted 1-ni-troalkenes. Conjugate 1,4-addition of complex zinc cuprates to l-nitroalkenes, followed by trapping with phenylselenyl bromide and subsequent oxidative elimination, affords the corresponding 2,2-disubstituted l-nitroalkenes in good yields (Eq. 5.82).132... 

Reaction of the quinoxalinylamidine 135 with phenylselenyl chloride in aqueous triflic acid-acetonitrile gives the benzo-annulated imidazopteridine 136 <1996JHC169> (Equation 30). 

Synthesis of 31 by Method I (107,108) and its conversion to the related anti and syn diol epoxide derivatives (32,33) has been reported (108). The isomeric trans-1,lOb-dihydrodiot 37) and the corresponding anti and syn diol epoxide isomers (38,39) have also been prepared (108) (Figure 19). Synthesis of 37 from 2,3-dihydro-fluoranthene (109) could not be accomplished by Prevost oxidation. An alternative route involving conversion of 2,3-dihydrofluoranthene to the i8-tetrahydrodiol (3-J) with OsO followed by dehydration, silylation, and oxidation with peracid gave the Ot-hydroxyketone 35. The trimethylsilyl ether derivative of the latter underwent stereoselective phenylselenylation to yield 36. Reduction of 3 with LiAlH, followed by oxidative elimination of the selenide function afforded 3J. Epoxidation of 37 with t-BuOOH/VO(acac) and de-silylation gave 38, while epoxidation of the acetate of JJ and deacetylation furnished 39. 

Another useful compound is the 1 2 telomer of malonate and butadiene, 137. The first example is the synthesis of pellitorine (138), a naturally occurring pesticide (126). The terminal double bond was hydrogenated selectively with RuCl2(PPh3)3 as a catalyst. Partial hydrolysis afforded the monoester, which was treated with PhSeSePh to displace one of the carboxyl group with phenylselenyl group. Oxidative removal of the phenylselenyl group afforded 2,4-decadienoate (139), which is converted to pellitorine (138) ... 

Queen substance (140) was synthesized from the same telomer 137 (127). The PdCl2-catalyzed oxidation of the terminal double bond produced the methyl ketone. Reduction of the internal double bond was followed by partial hydrolysis and the displacement of the carboxyl group with phenylselenyl group, which was removed to produce queen substance (140) ... 

One of royal jelly acids (10-hydroxy-2-decenoic acid) (141) was prepared from the telomer of acetoacetate, 142 (128). The terminal double bond was converted to terminal alcohol by hydroboration. The internal double bond was reduced and then reintroduced at the conjugated position by the addition of phenylselenyl group, and its oxidative removal completed the synthesis ... 

Scheme 5.1 Mechanism of thioglycoside activation (a) by thiophiles X1 such as /V-bromosuccinimicle (NBS), 11,12 methyl triflate,13 dimethyl(methylthio)sulfonium triflate (DMTST),14 phenylselenyl triflate (PhSeOTf),17,18 iV-iodosuccinimide/triflic acid (MS/TfOH),19 20 and iodonium di-sym-collidine perchlorate (IDCP)21 (b) by tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA,+)25 and (c) via anomeric sulfoxides.26 The stereochemical outcome of these glycosylations follows the same general trends as with many other glycosyl donor/promoter combinations (m-CPBA = mcta-chloroperbenzoic acid).

The phenylselenyl radical adds irreversibly to the central carbon atom of 2-methylbuta-l, 2-diene (Id) with a rate constant of 3 x 106 M-1 s-1 (23 1 °C) (Scheme 11.7) [45], On a synthetic scale, PhSe addition to cumulated Jt-bonds has been investigated by oxidizing phenylselenol with air in the presence of mono-, 1,1-di- or 1,3-di-substituted allenes to provide products of selective fi-addition. Trapping of 2-phenyl -selenyl-substituted allyl radicals with 02 did not interfere with the hydrogen atom delivery from PhSeH (Scheme 11.7) [31]. 

Selective ring closure of cyclic secondary alkyl radicals onto the central carbon atom of allenes have been investigated in the course of pyrrolizidine alkaloid syntheses [69]. Thus, reduction of the phenylselenyl-substituted N-(l,2-buten-4-yl)pyrroli-done 42 with Bu3SnH via a radical chain mechanism provides 51% of target compound 44 as a 78 22 mixture of diastereomers (Scheme 11.14). The stereoselectivity... 

Scheme 14 a-Phenylselenyl carbonyl compounds from ketones. 

In this context, an elegant alkylative carbonylation of alkenes accompanied by phenylselenyl transfer is described by Ryu and Sonoda. This three-component coupling involves the addition of a methoxycarbonylmethyl radical to an alkene, the trapping of the produced alkyl free radical by CO, and termination of the reaction by phenylselenenyl group transfer from the starting material. Yields are... 

It has been shown that phenylselenyl halides easily reacted with 0-allyl oximes 221 to give cyclic iminium salts 222, which by reaction with water afforded isoxazolidines 223 in moderate to good yields (equation 96) . Compounds 222 can be reduced in situ by sodium borohydride to produce Ai-alkyl-substituted isoxazolidines 224 in 50-95% yields . ... 

On account of this instability it is most lucky that various elementorganic groups, for instance the diphenylarsanyl group or phenylselenyl group, can act as halogen equivalents. Such equivalents have to fullfil mainly three suppositions ... 

Bromination of norperistylane-5,ll-dione (835) gives rapidly and quantitatively the Cj -symmetric dibromide 836a. Similarly, reaction with phenylselenyl chloride delivers 836b . Decomposition of the bis(selenoxide) in glacial acetic acid led to diketo diacetate 837. This product enters into twofold exchange reactions with representative nucleophiles. 

Oxidation of the 7/3-phenylselenyl-A -steroids (135) with H2O2 gave the two configurationally stable R- and 5-selenoxides (136) and (137) respectively which react by independent pathways. The R-isomer (136) underwent the familiar 2,3-sigmatropic shift leading, after solvolysis, to the allylic alcohol (138) whereas the 5-isomer (137) gave the A -diene owing to pronounced steric hindrance to the... 

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