Sulfoxide pyrolysis

We have mentioned previously (see Section 3.1) the easy formation of a double bond by sulfoxide pyrolysis. Associated with an alkylative step of a metallated sulfoxide, the sequence is of great value in alkene synthesis [44U, 441], as in the one-pot reaction given here. 

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. 4. 13. Sulfoxide pyrolysis for the formation of the C=C double bond of protected l-vinyl glycine, a non-proteino-genic amino acid.

So far in this chapter sulfoxide pyrolysis has been discussed in connection with alkene synthesis. However, the other products of these reactions, the sulfenic acids, are also of interest, and have been generated for study by several methods including sulfoxide pyrolysis. Thermolysis of di-f-butyl sulfoxide (79 ... 

Alternatively, oxidation of the sulfide to the sulfoxide followed by desilylation without allylic migration using CsF and sulfoxide pyrolysis gave 50, having the zygosporin G ring system in 63% yield. 

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... 

Chloromethyl ketones. The ketones can be prepared by reaction of an aldehyde wilh lilhio chloromethylphenyl sulfoxide in THF at —78° to form a -hy-droxy- t-chloro sulfoxide. Pyrolysis of the product in xylene (160°) leads to a chloromethyl ketone.-... 

Nucleophilic addition to the P-position of satisfactorily unsaturated sulfoxides has been known for many years [62-64]. Both carbon- and hetero-nucleophiles have been successfully added in such a manner, often followed by sulfoxide pyrolysis, the synthetic equivalent of addition of a vinyl group to a nucleophile. 

The use of free-radical reactions for this mode of ring formation has received rather more attention. The preparation of benzo[Z)]thiophenes by pyrolysis of styryl sulfoxides or styryl sulfides undoubtedly proceeds via formation of styrylthiyl radicals and their subsequent intramolecular substitution (Scheme 18a) (75CC704). An analogous example involving an amino radical is provided by the conversion of iV-chloro-iV-methylphenylethylamine to iV-methylindoline on treatment with iron(II) sulfate in concentrated sulfuric acid (Scheme 18b)(66TL2531). 

Sulfenyl chlondes react with allyl alcohols to yield allyl sulfenates, whtch are in equihbnum with the allyl sulfoxides [12] (equation 9a) These products can be oxidized to the corresponding sulfones (equation 9b) Pyrolysis of the sulfoxides gives sulfines or evidence for the presence of sulfmes Pyrolysis of sulfones leads to unsamrated compounds by extrusion of sulfur dioxide [12] (equation 9c)... 

A method for the stereospecific synthesis of thiolane oxides involves the pyrolysis of derivatives of 5-t-butylsulfinylpentene (310), and is based on the thermal decomposition of dialkyl sulfoxides to alkenes and alkanesulfenic acids299 (equation 113). This reversible reaction proceeds by a concerted syn-intramolecular mechanism246,300 and thus facilitates the desired stereospecific synthesis301. The stereoelectronic requirements preclude the formation of the other possible isomer or the six-membered ring thiane oxide (equation 114). Bicyclic thiolane oxides can be prepared similarly from a cyclic alkene301. 

Though the PECH decomposes to indefinite fragments with n-butyl lithium or sodium hydride in THF at room temperature, it reacts with sodium methoxide with liberation of Cl in which the -elimination of hydrogen chloride predominates instead of nucleophilic substitution. For instance, PECH in DMSO was reacted with double the molar quantity of sodium methoxide at room temperature for 24 h to give the unsaturated polyether (DS 92.3%,v(C=C) 1630,5 (=CH2) 795 cm" ) after purification by dissolution(DMF)-precipitation (H20) technique. A similar unsaturated polyther was obtained by the pyrolysis of the sulfilimine 13 (110-130°C) but not of sulfoxide 12 (100-150°C). When the polymer 26, was heated to 90°C, the absorption of C=C and =CH2 decreased and a new absorption at 1720 cm appeared and increased. This is explained as a result of [3.3] sigmatropic rearrangement of to afford including C=CH2 and C=0 structure as shown in equation 7. 

The first example of asymmetric induction in transfer of chirality from the chiral sulfur atom to the prochiral carbon atom was described by Goldberg and Sahli in 1965 (197). It concerns the pyrolysis of the optically active p-tolyl tra s-4-methylcyclohexyl sulfoxides 258. It was found that on pyrolysis at 200 to 250°C, optically active sulfoxides (R)-258 and (5)-258 yield optically active 4-methylcyclohexenes-l 259, with the absolute R and S configurations, respectively, at the newly formed chiral carbon atoms (Scheme 25). The optical purities of the 4-methylcyclohexenes-l that were formed depended largely on the temperature of pyrolysis. Thus, the values of 42 and 70% optical purity were noted for 259 at 250° and 200°C, respectively. The formation of the cycloolefins 259, whose absolute configurations are the same as those of the starting optically active sulfoxides 258, indicates that the pyrolysis reaction proceeds... 

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