Radical, Phenylseleno

A study on the kinetics of the reactions of phenylseleno radicals with vinyl monomers has also been reported.464... 

Anti-Markovnikov addition of phenylseleno radical (generated frombenzeneselenol and catalytic diphenyl diselenide by irradiation with 500 W tungsten lamp in a Pyrex glass tube) to trimethylsilylacetylene furnished a 44% yield of 1 as a 93 7 mixture of E Z isomers(eq 2). ... 

The initiator for the reaction was the phenylseleno radical, which was generated from PhSeSePh. The cyclopentane derivative was achieved in relatively low yields through photolysis of a benzene solution containing the alkyne, olefin, and PhSeSePh in a 1 10 0.2 molar ratio. In an attempt to increase the yield of the cycloadduct, 50 equiv of the butyl vinyl ether was used but this only resulted in a 10% increase. 

The mechanistic pathway for the reaction is shown in Scheme 10.56, where the generated phenylseleno radical attacks the terminal alkyne, followed by ring-opening to afford the malonate radical substituted with a phenyl-selenoalkene. The malonate radical then adds to the alkene generating a mdical that cyclizes back onto the allene, which is subsequently followed by the loss of the phenylseleno radical to afford the cyclopentane derivative. 

Se-phenyl areneselenosulfonates (24) undergo facile free-radical addition to alkenes to produce / -phenylseleno sulfones (25) in excellent yield86,87 (see Scheme 7). The addition occurs regiospecifically and affords anti-Markovnikov products contrary to the analogous boron trifluoride catalyzed reaction which produces exclusively Markovnikov and highly stereospecific products86 (equation 37). Reaction 36 has been shown to have the radical... 

Free-radical addition of Se-phenyl areneselenosulfonates to acetylenes is also a facile process, occurring regiospecifically and stereoselectively to afford the E-isomer of a -(phenylseleno) vinyl sulfone (26) in high yield88. 

Phenylseleno derivatives 71 are found to be good precursors of phosphonodi-fluoromethyl and phosphonothiodifluoromethyl radicals (Scheme 11). Moreover, when generated by the (TMS)3Si attack on 71 and in the presence of alkenes or... 

The (TMS)3SiH mediated addition of phosphorus-centered radicals to a number of alkenes has been investigated in some detail. Reaction (73) is an example of phosphorous-carbon bond formation using four structurally different phenylseleno derivatives with 3 equiv of (TMSlsSiH and AIBN in refluxing benzene for 2h. Comparative studies on the reaction of the four phosphorus-centered radicals have been obtained. Although the reaction with 1-methylene cyclohexane is efficient with all four derivatives, different selectivity is observed with electron-rich or electron-poor alkenes. 

Radical cyclization is compatible with the presence of other functional groups. Treatment of XCH2CON(R)-C(R )=CH2 derivatives (X = Cl, Br, 1) with Ph3SnH and AIBN led to formation of a lactam via radical cyclization. " Cyclization of N-iodoethyl-5-vinyl-2-pyrrolidinone led to the corresponding bicyclic lactam, " and there are other examples of radical cyclization with molecules containing a lactam unit " or an amide unit. Radical cyclization occurs with enamines as well. Photochemical irradiation of A,A-dialIyl acrylamide leads to formation of a lactam ring, and in this case thiophenol was added to generate the phenylthio derivative. Phenylseleno N-allylamines lead to cyclic amines. co-Iodo acrylate esters cyclize to form lactones. " ... 

Acyl radicals can be generated and they cyclize in the usual manner. A polyene-cyclization reaction generated four rings, initiating the sequence by treatment of a phenylseleno ester with Bu3SnH/AIBN to form the acyl radical, which added to the first alkene unit. The newly formed carbon radical added to the next alkene, and so on. Acyl radicals generated firom Ts(R)NCOSePh derivatives cyclize to form lactams. ... 

The Homer - Emmons reagent (52) is effective in the one carbon homologation of ketones possessing acidic a-hydrogen atoms <96SL875> and electron-deficient alkenes add to 2-phenylseleno-l,3-dithiane in a photo-initiated heteroatom stabilised radical atom transfer process, giving products of considerable synthetic potential <96TL2743>. 

Alternatively, pyranosyl radicals can be generated through the reduction of 3,4,6-tri-O-benzyl glucal epoxide with Cp2TiCl2 and manganese metal.136 With the conformationally restricted 1-phenylseleno-D-xylose derivatives 151 and 152 (4Ci conformation) their reaction with Bu3SnCH2 CH=CH2 in the presence of AIBN (Scheme 51) affords the corresponding a-C-pyranosyl derivatives (153) preferentially.137... 

Secondary alkyl selenides are reduced by (TMS)3SiH, as expected in view of the affinity of silyl radicals for selenium-containing substrates (Table 4.3) [40]. Reaction (4.23) shows the phenylseleno group removal from the 2 position of nucleoside [50]. Similarly to 1,3-dithiolanes and 1,3-dithianes, five- and six-membered cyclic selenoacetals can be monoreduced to the corresponding selenides in the presence of (TMS)3SiH [51]. The silicon hydride preferentially approached from the less hindered equatorial position to give transicis ratios of 30/70 and 25/75 for the five-membered (Reaction 4.24) and six-membered cyclic selenoacetals, respectively. 

Scheme 6.9 shows the radical clock methodology approach used for obtaining the rate constant of the ring expansion (k e)- Radical 37 was obtained by the reaction of the corresponding phenylseleno derivative with (TMS)3SiH. A relative rate constant of = 20.2 was obtained at 80 °C under first-... 

Free-radical cyclization of phenyl selenide 15 to indolizidinone 16 represented a key step in the total synthesis of (—)-slaframine (equation 52). The two pairs of diastereomers were first separated and then hydrolyzed to the corresponding alcohols in 76% overall yield77. (TMS)3SiH-mediated acyl radical reactions from phenylseleno esters 17 have recently been utilized for the stereoselective synthesis of cyclic ethers78. In fact, the experimental conditions reported in equation 53 are particularly good for both improving cis diastereoselectivity and suppressing decarbonylation. 

The formation of bicyclo compounds by the radical cyclization of alkenes with strategically placed phenylseleno groups, promoted by Bu3SnH, occurs in high yields and gives... 

Acyl radicals from RCOSeCfH. Acyl radicals generated from phenylseleno-esters can participate in inter- and intramolecular alkene addition. The latter tandem... 

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