Selenium compounds reaction with alkenes

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

Alkenes can be aminated in the allylic position by treatment with solutions of imido selenium compounds R—N—Se=N—R. The reaction, which is similar to the allylic oxidation of alkenes with Se02 (see 14-4), has been performed with R = t-Bu and R=Ts. The imido sulfur compound TsN=S=NTs has also been used... 

Thiols add to alkenes under photochemical conditions to form thioethers, and the reaction can be done intramolecularly to give cyclic thioethers. Thiols also add to alkynes and with a palladium catalyst, vinyl sulfides can be formed. " Selenium compounds (RSeH) add in a similar manner. ... 

Hypofluorites, fluorinated, 26 119-137, see also individual compounds bisfluoroxy compounds, 26 134-137 of carbon, 26 134-136 of selenium, 26 137 monofluoroxy compounds, 26 123-133 of carbon, 26 123-127 inorganic, 26 128-133 Hypofluorus acid, 26 121-123 MO calculations, 26 122 reactions, 26 122-123 with alkenes, 26 123... 

Reactions of phosphines and phosphites have received some attention but their preparative value is limited. The zwitterion formed from diphenylmethylphosphine and benzyne rearranges to ylide (124) which can be captured by Wittig alkenation, with cyclohexanone, in about 20% yield.159 Some synthetically useful reactions of tellurium and selenium compounds with arynes have been reported. For example, heating diphenyl iodonium carboxylate and bis(p-ethoxyphenyl) ditelluride in dichlorobenzene affords the compound (125).160 The corresponding reactions with diphenyl selenide and diphenyl sulfide... 

Selenium-containing six-membered ring heterocycles have proved to be useful catalysts in a variety of transformations. The Baylis-Hillman reaction involves the reaction of alkenes containing electron-withdrawing groups such as a,/3-unsaturated carbonyl compounds with aldehydes leading to carbon-carbon bond formation (Equation 79). The reaction is promoted by tertiary amines such as l,4-diazabicyclo[2.2.2]octane (DABCO), or tertiary phosphines and Lewis acids. Unfortunately, the Baylis-Hillman reaction is severely limited because it proceeds only very slowly <1998CC197>. Much research has been carried out in attempts to increase the rate of this reaction. 

Well before the wide use of organoselenium compounds in chemistry, it was discovered that electrophilic selenium compounds of the type RSeX add stereospecifically to alkenes.45 Since that time this reaction has been an important tool in the portfolio of organic chemists and has been used even for the construction of complex molecules. Comprehensive reviews on this chemistry have appeared46-49 and in recent times the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. As shown in Scheme 1, the addition reactions of selenium electrophiles to alkenes are stereospecific anti additions. They involve the initial formation of seleniranium ion intermediates 1 which are immediately opened in the presence of nucleophiles. External nucleophiles lead to the formation of addition products 2. The addition to unsymmetrically substituted alkenes follows the thermodynamically favored Markovnikov orientation. The seleniranium ion intermediates of alkenes with internal nucleophiles such as 3 will be attacked intramolecularly to yield cyclic products 4 and 5 via either an endo or an exo pathway. Depending on the reaction conditions, the formation of the seleniranium ions can be reversible. 

Chloriuatton of alkenes, Allylic chlorination is usually conducted by a free-radical reaction with NCS. Chlorination can be effected with catalysis by C6H5SeCl, in which case the main product is usually the rearranged allylic chloride. However, the formation of rearranged allylic chloride is sensitive to the structure of the alkenes and also to the particular selenium compound used as catalyst. See also N-phenylselenosuccinimide, this volume. 

Reviews on the following aspects of four-membered heterocycles have appeared 1,2-dioxetans and the reaction of singlet oxygen with alkenes/ /8-lactam antibiotics, small ring compounds of sulphur and selenium," recent developments in the synthesis of azetidines, the chemistry of benzazetes and of thiacyclobutenes (thietes) and their valence tautomers, and conformational barriers and interconversion pathways in some small ring systems. ... 

Selenium dioxide is a very useful reagent for allylic oxidation of alkenes. The products are either carbonyl compounds or allyl alcohols or esters, depending upon reaction conditions. The basic mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a sigmatropic rearrangement which restores the original location of the double bond, and (c) breakdown of the resulting... 

Scheme 88 demonstrates the compatibility of selenium linker systems with reductive, eliminative and nucleophilic conditions. The linker has been synthesized from polystyrene 604 via lithiation and quenching with dimethyld-iselenide 605 as mentioned above. Reaction to terminal alkenes 610 is performed via an oxidation-elimination process, induced by the addition of H2O2. Alkyl compound 609 is released using radical chemistry with a combination of AIBN and n-BusSnH [323]. 

As presented for the type 1 reactions in Scheme 6, selenium- and sulfur diimido compounds, 68-71, can undergo a two-step reaction sequence when treated with an allyl alkene to form the allyl amine. 

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