Selenation alkenes

Allylic oxidation is carried out by addition of one equivalent of selenium dioxide. First Se02 will react with the alkene in a [4 + 2] cycloaddition reminiscent of the ene reaction. The initial product is an allylic selenic acid 40, which undergoes - like an allylic sulfoxide -allylic rearrangement to give an unstable intermediate, which decomposes rapidly to the allylic alcohol 42.16... 

The thermal decomposition (pyrolysis) of alkylaryl selenoxides (selenoxide pyrolysis) to an alkene and an aryl selenic acid Ar—Se—OH often takes place even at room temperature (Figure 4.10). This reaction is one of the mildest methods for introducing a C=C double bond by means of a /3-elimination. The mechanism is described by the simultaneous shift of three electron pairs in a five-membered cyclic transition state. One of these electron pairs becomes a nonbonding electron pair on the selenium atom in the selenic acid product. The Se atom is consequently reduced in the course of the pyrolysis. 

An electrochemical synthetic process (see Qiapter 7.1, this volume) has been reported which requires only a catalytic amount of the selenating agent, and converts an alkene to the allylic alcohol in an aqueous cell, or to the allylic methyl ether if the electrolysis is run in methanol, as in equaticxi (9).t ... 

Selenium dioxide oxidation of alkenes with a hydrogen in an a-position involves the formation of the allyl selenic ester (X = OH) by an ene reaction. [2,3] Sigmatropic rearrangement of the allyl selenic ester to the selenium(II) ester and its hydrolysis also resulted in the formation of allylic alcohols. The oxidation of alkenes with selenium dioxide is covered in Section D.4.10. 

The two main systems of this kind used in the deselenation of selenides and diselenoketals are Li-and Na in liquid NH3. Na in HMPA or DMA has also been used with some special aryl alkyl selenides. = This procedure does not deserve special comment since, broadly speaking, the behavior of the reagents towards selenated substrates was very similar to that observed with sulfur-containing substrates (see Section 4.3.1.4). As expected from the difference of bond energies, C—Se bonds are more easily cleaved than C—S bonds. However, few selective deselenations have been performed in the presence of sulfur groups. C—O bonds vicinal to C—Se bonds are, of course, removed with alkene formation, and this property has found interesting applications. ... 

From the in situ reaction between RSeOzH and H3PO2, selenic acid and alkenes interact to yield 3-hydroxyselenides, which undergo selective transformation to oxiranes (Eq. 70). ... 

Uneyama and coworkers reported electrochemical generation of PhSeF for fluoro-selenation of alkenes and alkynes and recycling of the selenenylating reagent [52] ... 

SELENIC ANHYDRIDE (13768-86-0) OgSe Noncombustible solid. Violent reaction with water, forming selenic acid. A strong oxidizer. Violent reaction with many substances including reducing agents, hydrides, nitrides, and sulfides cyanides, esters, combustible materials, active metals, organic substances, aldehydes, alkenes, carboxylic acids, isocyanates, and thiocyanates. Attacks most metals in the presence of moisture. 

Electrochemically induced reactions have been a feature of selenium chemistry in the past two or three years. This year a potentially useful synthesis of ap-unsaturated aldehydes (279) has been reported and involves electrochemical generation of the active selenating species from diphenyl diselenide. Propargyl alcohols are thus selenated to give (279) in good to excellent yields. The ap-unsaturated aldehydes (279) are also formed when 1-lithioselenoalkenes, RCH=C(Li)SePh, react with DMF. Various other a-substituted alkenes are obtained in a similar fashion when DMF is replaced by other electrophiles. 

We have foimd that selenenyl chloride la and tellurenyl chloride lb reacts with equimolar amounts of alkenes 2-6 in CH2CI2 at 20°C to give the derivatives of 2,3-dihydro[l,3]selen(telliU )azolo[3,2-fl]pyridin-4-ium 8-12a,b, the products of cyclization with ring closiu e at the nitrogen atom of the pyridylchalcogeno moiety (Scheme 1, Table 1). 

Selenium. —Selenation and Oxidation. Three studies have appeared which are of general interest in the field. The first considers the fundamental reaction of PhSeCl addition to alkenes, where it is shown that the addition occurs initially in an anti-Markovnikoff fashion, isomerization occurring to the thermodynamically more stable isomer [RCH(Cl)CH2SePh] at higher temperatures.Oxidation and elimination of selenium provides a general alkene synthesis, and it has now been found that Bu OOH-alumina-THF is an efficient oxidation-elimination combination, and that addition of EtsN to a selenoxide prior to thermal elimination is advantageous. Thirdly, reductive removal of Se by Ph3SnH is preferable to the more conventional Raney nickel procedure. ... 

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