Pyrolysis selenoxide

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 

The retrosynthetic transform for syn elimination is that shown for the anti elimination reactions except that the less substituted alkene is the target  

Chapter 2. Acids, Bases, Functional Group Exchanges 

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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. 

Fig. 4.11. Regiocontrol in a selenoxide pyrolysis based on its mechanism-imposed syn-selectivity.

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. 

Fig. 17.35. A one-pot combination of a selenide selenoxide oxidation and a selenoxide "pyrolysis" (see Figures 4.10-4.12 for the mechanism).

The reaction from Figure 4.11 proves that a selenoxide pyrolysis is a sy -elimination. The cyclohexylphenyl selenoxide shown reacts regioselectively to produce the less stable Hofmann product (D). The Saytzeff product (E) is not produced at all, although it is more stable than D. This observation shows that the transition state C of an anti-... 

Sol 2. (i) In selenoxide, pyrolysis cycloelimination uses a ring of five atoms therefore, the reaction is necessarily yu-selective. Hence, on heating, cyclo-hexylphenyl selenoxide undergoes iyn-elimination regioselectively to produce the less stable Hofmann product. 

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

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. 

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. 

Another example of alkene synthesis by the pyrolysis of selenoxide is given in Scheme 4.14. The enolate derived from 4.18 reacts with either PhSeBr or PhSeSePh to form selenide 4.19. Oxidation of 4.19 gives selenoxide 4.20, which undergoes sy -elimination to give a,P Unsaturated carbonyl compound 4.21. 

With respect to the latter strategy, several types of elimination reaction have been described for intermediates having one or two leaving groups in the aliphatic chain. An example is the elimination of the selenoxide derived from an a-phenylselenyl ester to give compound (38), which is then transformed into pellitorine (1) [62] (Scheme 18). The starting diester (37) is obtained by telomerization of butadiene and malonate. The pyrolysis of 2-(phenylsulfinyl)enoates to 2,4-dienoates has also been applied to the synthesis of pellitorine (1) [63]. 

The pyrolysis of selenoxides takes place at room temperature or below. In the presence of a p-hydrogen, a selenite will give an elimination reaction after oxidation to leave behind an alkene and a selenenic acid (Scheme 6.22). Oxidizing agents such as hydrogen peroxide, ozone, or m-CPBA are quite often used. This reaction type is commonly used with ketones leading to the formation of enones. 

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). 

Pyrolysis of N-oxides known as Cope elimination takes place at lower temperatures (100-150 °C). The pyrolysis of sulphoxides and selenoxides takes place easily below 100 °C because of weaker C-S and C-Se bonds. As for example, ery/firo-N-oxide 6 and eryT/iro-sulfoxide 7 on pyrolysis give cis and trans olefins as major product, respectively. 

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