Selenoxides thermal elimination

The preparation of the requisite y-keto-p-toluenesulfonate rac-35 as homo-Favorskii precursor commenced with commercially available 2,5-dihy-droanisole (36) that was protected and epoxidized to acetal rac-31 (Scheme 11). Regioselective opening of the epoxide with p-chlorophenylse-lenide followed by sequential oxidation to the selenoxide and thermal elimination generated an allylic alcohol that was protected to give pivaloate rac-38. 

Further, medium-sized lactones have been prepared by a thermal elimination-Claisen rearrangement sequence, of unsaturated selenoxide cyclic acetals (equation 198)710. The reaction affords reasonable yields of these useful lactones upon treatment with DBU and a siloxy species at 185 °C. The reaction has been used as the key step in the synthesis of (-l-)-laurencin, which contains an 8-membered cyclic ether moiety711. 

Oxidation to the selenoxide 39 and thermal elimination (chapter 32) produces a new alkene 40 that forms an epoxide 41 on the outside face again. At this point every carbon atom in the six-membered ring is functionalised, five oxygen-based and one nitrogen, and this all started with symmetrical cyclohexadiene. Each functional group was selectively introduced from an alkene. 

Oxidation of the sulfur or selenium atom to the sulfoxide or selenoxide stage allows thermal elimination to take place by a cyclic mechanism 115. Because the PhS(e) group is trans to the alkyl group it must be cis to the hydrogen atom at C-2 and so cis elimination is possible. The selenoxides are unstable and decompose spontaneously but the sulfoxides usually need heating. The double bond is put back in its original position 116 so the strategy of the sequence is addition... 

Most of the published syntheses of necine bases have been directed towards fully saturated pyrrolizidine derivatives. Robins and Sakdarat have developed a method for the conversion of saturated pyrrolizidine esters into their 1,2-didehydro-derivatives. Thermal elimination of a phenylseleno-group was used to introduce the unsaturation, and ( )-supinidine (17) was synthesized using this technique. The ester (5) is most conveniently prepared by the two-step stereospecific route of Pizzorno and Albonico. Phenylselenenylation of the lithium enolate derived from the ester (5) gave the phenylseleno-ester (15), which was reduced to the corresponding alcohol (16) (Scheme 5). syn-Elimination of the selenoxide derived from (16) gave ( )-supinidine (17). Each step proceeded in about 60% yield. None of the isomeric 1,8-didehydro-base was detected. 

Holmes and co-workers have also applied the Claisen rearrangement to the synthesis of mediumsized lactams. The thermal elimination of phenylselenic acid from the selenoxide (366) gave the unisolated ketene aminal (367), which rearranged to the unsaturated nine-membered lactam (368) (Scheme 56) <92TL6857, 96JCS(P1)123>. 

In some cases, eliminations occur in non-ionizing solvents and without the addition of any base. In these cases the reactant itself has an internal base and a cyclic transition state leads to elimination. The symbolism for the reactions is Ei, standing for elimination, intramolecular. Only heat is required to induce the reaction, and hence these reactions are called thermal eliminations (the term pyrolysis is also sometimes used). Thioesters, xanthates, selenoxides, and N-oxides are common in these reactions. The Cope elimination involves the formation of an N-oxide and subsequent elimination via the pathway shown in Eq. 10.91, and the Chugaev elimination involves xanthate esters [ROC(S)SR]. The Chugaev elimination was shown to follow a syn elimination pathway based on the stereospecific nature of the reaction (Eqs. 10.92 and 10.93). 

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. ... 

Sulphoxides and selenoxides undergo syn elimination under thermal conditions. A 1,4-elimination of sulphenic acid from an allyl sulphoxide leads to dienes (equation 20)50. Precursor sulphoxides are generated by oxidation of corresponding sulphides. This reaction, however, did not give good results when applied to more complicated systems51. 

Oxidation of alkyl iodides, bearing electron-withdrawing groups such as car-bomethoxy and sulfonyl at the a-carbon, with m-chloroperbenzoic acid results in clean elimination to give olefins [Eq. (27)]. This reaction involves reductive / -elimination of the intermediate iodosylalkanes, as observed in thermal peri-cyclic -elimination of sulfoxides and selenoxides. Exclusive syn stereochemistry in the reductive /1-elimination was established by deuterium labeling... 

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. 

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. 

Reaction of alcohol 4.12 with o-nitrophenyl selenocyanate (4.13) and tributylphosphine gives 4.14. Oxidation of 4.14 with hydrogen peroxide gives selenoxide 4.15. Aryl selenoxide 4.15 bearing a (3-hydrogen atom is unstable and undergoes thermal syn-elimination to give styrene (4.16) with the expulsion of a selenol 4.17 in a fashion similar to that of the Cope elimination (Scheme 4.13). 

The selenosulfonates (26) comprise another class of selenenyl pseudohalides. They are stable, crystalline compounds available from the reaction of selenenyl halides with sulftnate salts (Scheme 10) or more conveniently from the oxidation of either sulfonohydrazides (ArS02NHNH2) or sulftnic acids (ArS02H) with benzeneseleninic acid (27) (equations 21 and 22). Selenosulfonates add to alkenes via an electrophilic mechanism catalyzed by boron trifluoride etherate, or via a radical mechanism initiated thermally or photolytically. The two reaction modes produce complementary regioselectivity, but only the electrophilic processes are stereospecific (anti). Similar radical additions to acetylenes and allenes have been reported, with the regio- and stereochemistry as shown in Scheme 11. When these selenosulfonation reactions are used in conjunction with subsequent selenoxide eliminations or [2,3] sigmatropic rearrangements, they provide access to a variety of unsaturated sulfone products. 

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. 

This reaction is just a thermal internal syn E2 elimination, and a relative of path 6e. There are many possible reactants for this internal syn elimination path. Five-membered transition state examples include Y equals oxygen and Z is NR2 (amine oxides), SPh (sulfoxides), or SePh (selenoxides). Six-membered transition state examples include both Y and Z being oxygen (esters), or Y is sulfur and Z is oxygen (xanthates). 

Trifunctionalization of alkenes. Addition to a double bond using the combination of anhydrides leads to a thermally unstable selenoxide as the primary product. yyn-Elimination occurs and the allylic trifluoroacetate is susceptible to a second... 

C.vi. Selenoxide Pyrolysis.218 just as sulfides are oxidized to sulfoxides, selenides (R—Se—R) can be oxidized to selenoxides. It is reasonable to assume, therefore, that replacing the sulfoxide with a selenoxide will also lead to thermal syn-elimination to the less substituted alkene. The increased polarity of the Se-O bond of the selenoxide, relative to the S-0 bond of the sulfoxide, and the loss of the unstable R—Se—OH allows even lower temperatures for thermal syn-elimination (typically 0-25 °C). Elimination of PhSeOH from 245 is clearly analogous to the sulfoxide pyrolysis of 243, and the exocyclic-methylene derivative (246) rather than... 

Sol 1. (b) Selenites on oxidation with hydrogen peroxide (or ozone or m-CPBA) give selenoxides. The latter in the presence of a p-hydrogen undergoes an intramolecular syn elimination to leave behind an alkene and selenenic acid. The diene component is generated in situ by thermal sulfur dioxide extmsion from sulfolene in a reverse reaction and is then trapped by the dienophile in a Diels—Alder reaction. 

Thermal )6-elimination reactions of acetates, benzoates, xanthates, sulfoxides, selenoxides, and N-oxides are also group transfer reactions. All these elimination reactions are yn-stereospecific and proceed through a cyclic six membered—or five membered—ring transition state of 6e process by intramolecular transfer of hydrogen atom, where all the participating orbitals have suprafacial interactions. These reactions are fundamentally retro-group transfer reactions. 

Regioselectivity is also an important issue with the most important thermal sulfoxide (or selenoxide) reaction, which is their elimination as illustrated in 494. 

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