Phenylseleno derivatives

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. 

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-... 

When phenylseleno derivatives are employed to prepare tetrahydrofurans from 4-alkenols, moderate stereoselectivity is generally observed. Cyclization of syn-(Z,Z)-7-benzyloxy-3,9-di-en-6-ol (22) with phenylselenyl chloride in dichloromethane at — 78 °C affords the corresponding 2,5-disubstituted tetrahydrofuran in 89% yield and 75 25 (trans/cis) diastereomeric ratio12. 

Total 2,3-trems selectivity is observed for the cyclization of 22 with phenylselenyl chloride in dichloromethane. The phenylseleno derivative 23 is formed as a unique diastereomer owing to the steric constriction of the isopropylidene group that biases the formation of the 23-trans-r -bose derivative28. 

When the naphthalene derivative 34 is treated with phenylseleneny] bromide in acetic acid the fram -6-acetoxy-7/)-phenylseleno derivative 35 is formed as the sole product 9. 

As indicated in Scheme 39, the initial process is the formation of the a-phenylseleno derivative 249 tvhich gives a second addition reaction to afford products 245, identical to those formed from terminal alkynes. This reaction seems to be of general application since alkyl, vinyl, and aryl methyl ketones give the a-ketoacetals of the type 250,251 and 252 with excellent results in every case. 

The reactions are usually easier with methylseleno than with pheiwlseleno, P Chlorophenylsele no or (p-trifluoromethyl)phenylseleno derivatives, and allenes and alkylidenecyclopropanes are much more difficult to synthesize than other alkenic compounds. Thus, although allyl selenides resulting from selective elimination of the hydroxy and the methylseleno moiety are usually produced from P-(methylseleno)-p -(phenylseleno) alcohols, - - in the case of l-seleno-l-(r-hy[Pg.702]

The reaction involving dichlorocarbene also provides a wide range of epoxides, " including terminal. a.a- and a,p-dialkyl-substituted and tiialkyl-substituted compounds, from a large variety of 3-hy-drcxyalkyl methyl and phenyl selenides (Scheme 162, d). The thallous ethoxide reaction, although it takes place more slowly (especially with phenylseleno derivatives) than under phase transfer catalysis condirions, " has to be in several instances preferred since it avoids the concomitant formation of... 

The first approach to the seleno series taken advantage of the wide availability of 1-(l-seleno)cyclopropyl carbinols and the small tendency (discussed in Section Syntheses of alkylidene cyclopropanes from 3-hydroxy alkyl selenides), of the phenylseleno derivatives to produce alkylidene cyclopropanes on reaction 37), with for example, thionyl chloride/triethylamine mixture (Scheme 51). However, the best results have... 

Unfortunately the stereochemistries of the vinylcyclobutanes prepared by this method have not yet been determined. An interesting observation was made with the parent phenylseleno derivative during this work117, since it was found (Scheme... 

Alkylidenephosphoranes react with phenylselenyl bromide to yield selenophos-phoranes (59), which react normally with aldehydes. However, with ketones, the corresponding 2-phenylseleno-derivatives, e.g. (60), are formed. ... 

A number of interesting reactions have been carried out on ring systems of which the azepine ring is just one example of a number of cyclic amines. For example, treatment of (70) with potassium hexamethyldisilazide in THF at —78°C followed by trisyl azide gives a 65% yield of the /ru/ii-azide (71) <92TL6859>. A phenylseleno derivative is formed in 78% yield when trisyl azide is replaced by phenylselenylchloride. 

---