Selenoxides, vinyl

Cleavage of carbonyl-containing selenoxides and sulfones Fragmentation of epoxy hydrazones Rearrangement of vinylic hydroxycyclopropanes Rearrangement of 3-hydroxy-1,5-dienes (oxy-Cope)... 

Pyrolysis of bisquaternary ammonium hydroxides 7-12 Cleavage of selenoxides 7-13 Dehydrohalogenation of dihalides or vinylic halides... 

Vinyl sulfones.1 The photoadducts of 1 to alkenes on oxidation (H202) undergo selenoxide elimination to provide vinyl sulfones in 60-95% yield. Example ... 

Addition to alkynes. Se-Phenyl areneselenosulfonates undergo anri-addition to alkynes at elevated temperatures to afford /Hphenylseleno)vinyl sulfones (equations I, II). The products can undergo selenoxide elimination to alkynes.3... 

Vinyl-glycine with 100% e.e. can also be prepared from low-temperature elimination of the selenoxide, derived from seleno-amino acid. 

The sulfur analog of the selenoxide pyrolysis is also known. In this sulfoxide pyrolysis the C-S bond is broken. The C-S bond is stronger than the C-Se bond and this explains why sulfoxides must typically be pyrolyzed at 200 °C to achieve elimination. Figure 4.13 shows the transformation of protected L-methionine into the corresponding sulfoxide, which then undergoes sulfoxide pyrolysis. This two-step sequence provides an elegant access to the nonnatural amino acid L-vinyl glycine. 

Similar reactions were also achieved by the formation of diastereomeric optically active selenoxides as intermediates in the elimination reaction. Optically active ferrocenyl diselenide 19 was used in selenenylations of alkynes generating vinyl selenides of type 164. Oxidation of the selenides was performed with mCPBA under various reaction conditions which afforded the corresponding chiral selenoxides, which, after elimination, afforded axial chiral allenecarboxylic ester derivatives 165 in high enantioselectivities (R = Me 89% ee, R=Et 82% ee, R = C3H7 85% ee) (Scheme 47)>85 87... 

Aminal 309 was oxidized to selenoxide, and then heated in refluxing toluene with DBU to give the protected 9-substituted azoninone 310 in 75% yield as a result of Claisen rearrangement of the vinyl-substituted intermediate (Equation 44) <1996J(P1)123>. 

Mono- and 1,2-di-substituted alkenes react with PhSeQ/Hg(SCN)2 in benzene (0.5-96 h, at 20 C), giving -tra/if-phenylselenoalkyl isothiocyanates in 70-94% yields.2 Terminal alkenes generally give the product with the selenium terminal (an exception is the prc uct from Bu CH H2) internal alkenes show the expected stereochemistry (cis to threo, trans to erythro). Oxidation to selenoxides could be achieved cleanly only with ozone, and the products cis eliminate in the usual manner to give predominantly the vinylic isothiocyanates (Scheme 73). 

Several different functional groups can be introduced by selenenylation. The benzeneselenenyl ion tolerates a wide range of anionic groups. However, with differing anionic moieties the reactivity of the benzeneselenenyl ion varies greatly. All adducts can be subsequently converted to either allylic or vinylic moieties by the syn elimination of the corresponding selenoxide (oxidized selenide). 

The isolable and diermally staUe selenoxides are, dKiefore, rather limited. Stable examples are as follows those derived from selenides which have no hydrogen atoms on the p-carbon, such as dimediyl sel-enide, aryl methyl selenides, diaiyl selenides - and benzyl fdienyl selenides, those with an intramolecular hydrogen bonding, such as (36) and (37), and those leading to an unfavorable double bond such as (38). Vinylic selenoxides (39) and (40) are also generally isolable. 

MCPBA and peracetic acid are also effective oxidants, " particularly at low temperature (-78 C) where the selenoxides are stable. These reagents may be used in the presence of a double bond, a triple bond or an amino group. Various selenoxides, especially vinylic selenoxides such as (40), have been isolated using this route. " r-Butyl hydroperoxide is an especially mild oxidant which can be used in excess as a replacement for H7.O2 without undesirable overoxidation side-reactions taking place. ... 

Another, rarely employed route to allylic selenoxides is the deconjugation of vinyl selenoxides 5. 

As a consequence of the selenoxide instability, the oxidation into selenones is only possible if the syn-elimination reaction becomes difficult, as in the case of strained structures. Under these conditions, the selenone is deprotonated and the a-selenonylalkyllithiums formed can react as other selenium-stabilized car-banions. It must be added that the seleninyl [PhSe(O)-] and selenonyl [PhSe(02)-j substituents behave as very good leaving groups. Vinylic selenoxides and selenones can undergo an intramolecular displacement of the selenium moiety after nucleophilic addition to the double bond under basic conditions. Cyclopropanes and oxetanes have been synthesized in this way [1,2]. 

Asymmetric selenoxide elimination of the optically active vinyl selenoxides affords optically active allenes and cyclohexylidenes. On the other hand, asymmetric [2,3]sigmatropic rearrangement of allylic selenoxides, selenimides, and selenium ylides leads to the formation of the corresponding optically active allylic alcohols, amines, and homoallylic selenides, respectively. 

The effects of aryl groups were kinetically analyzed by comparing the rate constants of both steps (ki for oxidation step and /C2 for elimination step) which were determined by NMR analysis of the concentration of vinyl selenides, the intermediate selenoxide, and allenic sulfones [16b]. This kinetic study indicates that the rates of both oxidation and elimination steps were accelerated by the introduction of an electron-withdrawing group. Such acceleration has been known in the overall selenoxide elimination as well as in the selenoxide elimination step of alkyl aryl selenides. As a result, it was disclosed that the ratio of these rate constants (/C1//C2) was closely related to the enantiomeric excess of the products the smaller the ratio, the larger the enantiomeric excess becomes. Thus, the introduction of o-nitrophenyl group as an aryl moiety, which suppresses sterically the racemization of the intermediate chiral selenoxide and accelerates the selenoxide elimination step, is necessary to achieve a higher asymmetric induction. 

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