Elimination reactions selenoxide

By elimination reactions of selenides or selenoxides Diphenyl diselenide, 125 Hydrogen peroxide, 145 Other elimination reactions Chloromethyldiphenylsilane, 74 m-Chloroperbenzoic acid, 76 Hydrogen peroxide, 145 Iodomethyltrimethylsilane, 315 Lithium 4,4 -di-/-butylbiphenylide, 162... 

An even milder cycloelimination uses a ring of five atoms 6.28 instead of six, but still involves six electrons. This is no longer a retro-ene reaction, but it is still a retro group transfer and it is allowed in the all-suprafacial mode 6.29. The pyrolysis of N-oxides 6.30 is called the Cope elimination, and typically takes place at 120°, the corresponding elimination of sulfoxides 6.31 (X=S) typically takes place at 80°, and, even easier, the elimination of selenoxides takes place at room temperature or below. All these reactions are affected by functionality making these numbers only rough guides, but they are all reliably syn stereospecific. 

Monoclonal antibodies raised against proline derivatives (94a) and (94b) have been used to catalyse the selenoxide. vyw-elimination reaction (95) — (97). It was reasoned that the flexible selenoxide (95) would be constrained within the low dielectric... 

As already mentioned, the ry -elimination of selenoxides was discovered around 197010 and had a major impact on the development of 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 Section 9.11.2.5. 

As shown in Scheme 42, selenoxides are crucial intermediates in the selenoxide elimination reactions. These are syn-eliminations which proceed via an intramolecular mechanism to yield alkenes as reaction products. The regioselectivities of these eliminations are dependent on the nature of the substituent Y in the / -position as shown in Scheme 44.286 The mild reaction conditions for these elimination reactions make them highly useful in organic synthesis and theoretical studies on this reaction have been carried out as well.286... 

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... 

E2 elimination reactions occur preferentially when the leaving groups are in an anti copla-nar arrangement in the transition state. However, there are a few thermal, unimolecular sy -eliminations that produce alkenes. For example, pyrolysis of several closely related amine oxides, sulfoxides, selenoxides, acetates, benzoates, carbonates, carbamates and thio-carbamates gives alkenes on heating (Scheme 4.10). The syn character of these eliminations is enforced by a five- or six-membered cyclic transition states by which they take place. 

Selenoxides undergo elimination at temperatures even lower than room temperature. The Grieco elimination is an organic reaction describing the elimination reaction of an aliphatic primary alcohol through a selenide to a terminal alkene. 

Phenylselenoetherification (8,26-28). A related reaction leading to a potential precursor to deoxy sugars has been reported by Current and Sharpless. Thus the reaction of p-chlorobenzeneselenenyl bromide (6, 421) with (E)-4-hexenal (1) in CCI4 containing benzyl alcohol at reflux leads to 2 in about 65% yield. This product is converted into 3 on selenoxide elimination. When benzeneselenenyl bromide itself is used, this elimination reaction is very slow. The unsaturated compound can be functionalized in various wa3 . Of course, alcohols other than benzyl alcohol can be used. 

A side reaction is syn elimination of selenoxides, which is also faster for selenoxides than for sulfoxides. The competition between rearrangement and elimination, if possible, is generally in favor of the rearrangement, but in some special cases the diene is the major or exclusive product1. 

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]. 

Among the available methods for introducing an unsaturated carbon-carbon bond into organic molecules, selenoxide elimination reaction has been shown to be quite useful because of its simple procedure and its characteristic regioselec-tivity. Jones et al., who discovered the first selenoxide elimination, proposed an intramolecular mechanism entailing a five-membered ring structure to explain its syn nature [11]. This proposition was shown to be correct by Sharpless et al. who applied the method that was utilized by Cram to determine the stereochemistry of elimination in amine oxides [12]. Thus, the oxidation of erythro-selenide afforded only Z-olefin and that of f/zreo-selenide gave only -olefin (Scheme 4). 

Furanoid glycals utilizing elimination of selenoxides were prepared from unusual substrates, the 4-phenylselenyltetrahydrofurans. These intermediates were obtained with good yields from D-glyceraldehyde (O Scheme 21). Their oxidations have led to selenoxides, however, the elimination reaction required heating. The best yields were noted for reactions refluxed in 1,2-dichloroethane and yields ranged from 62 to 95%. 

An elimination reaction was also successfully used in the synthesis of furanoid glycals from 2-deoxyribose. The substrates used were 2-deoxy-l-seleno-furanosides 75, which were transformed to their respective glycals 76 via selenoxide elimination in yields ranging from 71 to... 

In the case presented in Scheme SI the di erences in behavior of the p-hydroxyalkyl selenoxides of different origin may be due to different diastereoisomeric ratios the equilibration of which is hampered by steric crowding. Therefore, there is a pronounced stereochemical effect on the regiochemistry of sele-noxide syn elimination reactions. This elimination reaction leads to allyl alcohols where the a,3-disub-stituted double bond possesses exclusively the ( )-stereochemistry when reasonably bulky groups are present (Scheme 52 heme 105, c Scheme 174, Those substituted with stnaller groups... 

Alkenes are formed by the thermal decomposition of esters, xanthates, amine oxides, sulfoxides, and selenoxides that contain at least one (3-hydrogen atom. These elimination reactions require a cw-configuration of the eliminated group and hydrogen and proceed by a concerted process. If more than one (3-hydrogen is present, mixtures of alkenes are generally formed. Since these reactions proceed via cyclic transition states, conformational effects play an important role in determining the composition of the alkene product. 

Selenoxide elimination occurs under relatively mild conditions in comparison to the elimination reactions described above. Selenoxides undergo spontaneous yn-elimi-nation at room temperature or below and thus have been used for the preparation of a variety of unsaturated compounds. The selenide precursors can be obtained by displacement of halides or sulfonate esters with PhSeNa. Oxidation of the selenides with hydrogen peroxide or tert-huiyX hydroperoxide, sodium periodate, or peroxycar-boxylic acids furnishes the corresponding selenoxides. Their eliminations usually favor formation of the less substituted olefin in the absence of heteroatom substituents or delocalizing groups. Since selenium compounds are toxic, they should be handled with care. 

Elimination reactions leading to olefins are usually performed on the corresponding selenoxides 3 9,1 12) (Scheme 2 a). These are often unstable and decompose at room temperature to olefins and selenenic acid (further oxidized to the more stable seleninic acid by excess of oxidant). Hydrogen peroxide in water-THF, ozone and further treatment with an amine or tert-butyl hydroperoxide without or with alumina proved to be the method of choice for such a synthesis of olefins. The reaction is reminiscent of the one already described with aminoxides or sulfoxides 22) and occurs via a syn elimination of the seleninyl moiety and the hydrogen attached to the P-carbon atom. However it takes place under smoother conditions. 

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