Pyrolysis of sulfoxides

Sulfenic acids also react with alkynes, and (r-butylsulfinyl)alkynes (85) eliminate 2-methylpropene on thermolysis to give the ( )-alkenyl sulfoxides (87) stereoselectively via cyclization of the intermediate sulfenic acids (86 Scheme 20)7 The stereochemistry of the adduct (87) is established by the mechanism of sulfenic acid addition. For the intermolecular trapping of sulfenic acids by alkynes, generation of the sulfenic acids by thermolysis of the corresponding 3-sulfinylpropionitrile (88 equation 35) is the preferred route. Sterically hindered sulfenic acids have also been generated by flash vacuum pyrolysis of sulfoxides and trapped intermolecularly by alkynes. ... 

Pyrolysis of sulfoxides provides a convenient method for introducing unsaturation at the position a- to carbonyl compounds. Formation of the enolate and reaction with dimethyl (or diphenyl) disulfide gives the a-methylthio (or phenylthio) derivative. Oxidation with a suitable oxidant, such as mCPBA or NaI04, gives the sulfoxide, which eliminates sulfenic acid on heating to give the a,(3-unsaturated carbonyl compound. For example, the methyl ester of a pheromone of queen honey bees was synthesized from methyl 9-oxodecanoate after initial protection of the ketone as the acetal (2.24). The -isomer usually predominates in reactions... 

Another complication with gas phase pyrolyses is that many possible nonconcerted reaction pathways are possible. Alkyl halides xmdergo elimination in the gas phase, and some compounds, such as ethyl chloride, appear to undergo unimolecular elimination. Their unimolecular decompositions may involve transition structures with significant carbocation character. For example, p5u-olysis of (-l- )-2-chlorooctane in the gas phase at 325-385°C was found to produce racemization of the starting material as well as elimination of HCl. Some compounds appear to react by radical chain mechanisms, and heterogeneous radical reactions often complicate studies that are not carried out in "well-seasoned" (i.e., coated with a layer of organic material) vessels. Furthermore, there appears to be a significant radical (but not radical chain) component to the pyrolysis of sulfoxides. These complications mean that many control studies are necessary to clarify the mechanism of gas phase elimination reactions. 

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... 

It was mentioned previously that, unlike most arylnitrenes, polyfluorinated arylnitrenes have bountiful bimolecular chemistry. Banks and Sparkes found that pyrolysis of 4-azido-2,3,5,6-tetrafluoropyridine in cyclohexane produced the product of formal CH insertion in 45% yield. Tetrafluoropyridyinitrene was also captured by alkenes, dimethyl sulfoxide and even benzene. Pyrolysis of perfluorophenyl azide (15e) yields a product of formal insertion with a CH-bond of benzene and the formation of adducts with dimethyl sulfoxide.Abramovitch et demonstrated that... 

The cleavage of sulfoxides and sulfones bearing a (3-hydrogen, on treatment with an alkoxide [212] or on mild pyrolysis for a sulfoxide, allows the formation of a double bond. 

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. 

Sulfenic acid (3) has been synthesized in the gas phase by low-pressure, high-temperature pyrolysis of di-t-butyl sulfoxide (4) and characterized by means of matrix isolation and gas-phase IR spectroscopy (Scheme 4).32 The mechanism of formation of (3) by flash pyrolysis of (4) has been studied by quantum chemical calculations, and different pyrolysis experiments monitored by mass spectrometry. In agreement... 

The syn-periplanar eliminations by pyrolysis of esters, xanthates, sulfoxides and amine oxides are symmetry-allowed. With respect to the alkene portion of the transition state, the centers presumably are or... 

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