Selenates rearrangement

Selenoxide-Selenate Rearrangement 4.63.3 Introduction of Allylic Nitrogen... 

As)fmmetric oxidation of allylic selenides gives allylic alcohols via a [2,3]-sigmatropic selenoxide-selenate rearrangement (eq 5). In both oxidations, eqs 4 and 5, the configuration at the selenoxide is that predicted based on the sulfoxide model. 

Similarities are noted between the mass spectra of aliphatic and aromatic selenides with analogous sulphides and phosphines. " Selenoxides show the expected fragments derived from the selenate rearrangement product " " Extensions of these studies cover selenium dihalides (no moleculm ion is seen " ), seleninic acids and esters, " and selenones. Mass spectra of diaryl tellurides are very much like those of S and Se analogues. ... 

Oxidative rearrangement of 5a-cholestan-3-one (62) with hydrogen peroxide and a catalytic amount of selenic acid affords 2a-carboxy-A-nor-5a-cholestane, isolated in about 35 % yield as the methyl ester (63)." However, the reaction gives a complex mixture of A-nor- and seco-acids, and under... 

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... 

Selenals, selones, and tellurals generated in situ via [3,3] sigmatropic rearrangements of allyl alkenyl selenides and tellurides 167 can be trapped by 2,3-dimethyl-l,3-butadiene affording the expected cycloadducts (Equation 68) <1995CL135>. Higher yields were noted in less-hindered cases where the selenium and tellurium aldehyde... 

Mechanism The reaction of the enol form of the carbonyl compound A with selenium dioxide gives selenous enol ester B. The oxidative rearrangement of selenous enol ester B gives C. Loss of selenium and water from C gives the dicarbonyl compound (Scheme 7.16). 

Both classes of selenium compounds have either been postulated as reaction intermediates or could usually only be observed as short-lived intermediates. Allyl selenoxides (X = R), the oxidation products of allyl selenides, display fast rearrangement to allylic selenates, which are hydrolyzed to allylic alcohols under the standard reaction conditions of the oxidation. 

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. 

Allylic selenoxides were observed in special cases as short-lived intermediates at low temperature5. Kinetic measurements showed that the rearrangement occurs with reasonable rates4 at temperatures between — 80 °C and —20 C. The rearranged allyl selenates are easily hydrolyzed to the allyl alcohols, i.e., special reagents for cleavage, as in the case of the sulfenates, are not necessary. 

Selenium forms weaker a-bonds than sulfur and many reactions which involve the cleavage of such bonds are faster than for analogous sulfur compounds and proceed under milder reaction conditions. The sj -elimination of selen-oxides was discovered in 1970 [9] and had a major impact on organoselenium chemistry. This reaction is about three orders of magnitude more rapid than the elimination of the corresponding less polar and less basic sulfoxides. Sigmatropic rearrangements proceed at markedly lower temperatures. These reactions are discussed in detail in Chap. 8 by Y. Nishibayashi and S. Uemura. 

Step. In fact, several asymmetric reactions via this process have recently been reported for the preparation of chiral allylic alcohols. In this section, typical results of asymmetric [2,3]sigmatropic rearrangement via chiral allylic selen-oxides to afford the corresponding chiral allylic alcohols are described. 

To use a selenate instead of a sulfoxide rearrangement for the preparation of allylic alcohols can be advantageous if silicon is present in the starting material. Vedejs found that the allylsilane (213) could be selenylated to give (214 equation 79) this reaction includes a rapid 1,3-shift of the initially formed selenide. Oxidation and 2,3-rearrangement to the alcohol (215) followed uneventfully. 

This limitation can be overcome using the anion of alkyl phenyl selenoxides which add to ketones and rearrange in situ (equation 34).The amalgam reduction is required to remove the small amount of a-phenyl selenated ketones formed in the reaction. 

The enolate may attack the selenium atom of the selenium dioxide to form a selenate ester of the enol. This intermediate may rearrange to re-form the carbonyl group, and transfer an oxygen atom to the a-carbon. Suggest how this may occur. 

Enantioselective oxidation of Z-aryl cinnamyl allylic selenide (83) with oxaziridine (—)-(69) gave 1-phenyl allyl alcohol (85) via an allyl selenoxide-selenate [2,3] sigmatropic rearrangement (Scheme... 

A second major application of selenium dioxide is the oxidation of aldehydes and ketones to the corresponding l,2-dicarbonyl.533 7 55 conversion proceeds by reaction of the enol form of the carbonyl (386) with selenium dioxide to give the selenous enol ester, 387. Oxidative rearrangement to 388 is followed by loss of... 

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