Sulfur dioxide reaction with dienes

Hogeveen and Heldeweg found a rare example of a Diels-Alder reaction with sulfur dioxide where the [4 + 2] adduct is reasonably stable. Addition of sulfur dioxide to diene 76 at low temperature yielded [4 -I- 2] adduct 77 as the kinetic product. At temperatures above 2(J C products resulting from the dihydrothiophene dioxide 78 were observed (Scheme 1-XVI). 

Sulfur dioxide does not homopolymerize, but on reaction with olefins it yields copolymers.Terminal olefins react more readily than those with an internal double bond. The presence of various substituents affects the rate of polymerization. Conjugated dienes copolymerize with sulfur dioxide to give linear polymers containing residual double bonds. 

Surprisingly allene gave no polymer on reaction with sulfur dioxide [46] but a polymer was prepared from diallyl (1,5-hexadiene) [47]. Some typical diene-sulfiir dioxide copolymers are shown in Table X. 

An organic reaction of interest is the Diels-Alder reaction that sulfur dioxide undergoes with butadiene and other acyclic dienes. With butadiene, the product is suhblene, C4H6S, a five-membered S-heterocyclic ring compound which is hydrogenated to form sulfolane, C4H8S. 

Since the first report in 1914 of the formation of a butadiene-sulfur dioxide adduct [538], much work has been carried out on the reaction of conjugated dienes with sulfur dioxide and its applications in synthetic strategies. Two pathways can and have been observed a cheletropic reaction (to 2s + ir 4s) yielding 3-sulfolenes and (4 -ns + tt 2s) hetero-Diels-Alder addition yielding sultines (see [539] and [540] and references... 

The intermediate case where a complex is formed but opening occurs spontaneously to only a limited extent is illustrated by the reaction of butadiene or other conjugated dienes with sulfur dioxide (16). When the two components react, a cyclic adduct and a linear alternating copolymer are produced simultaneously. 

Cycloaddition reactions of 1,3-dienes with sulfur dioxide, unlike its mono and bis imino derivatives, usually produce five-membered ring 2,5-dihydrothiophene dioxides rather than Diels-Alder adducts [Eq. (36)]. 

Durst has shown that a Diels-Alder adduct is in fact the kinetic product of reactions of sulfur dioxide with conjugated dienes, but is thermally unstable and readily undergoes a retro-Diels-Alder reaction to butadiene and sulfur dioxide. A thermodynamically more favorable addition then ensues, producing a stable five-membered ring adduct. Thus, the apparent inability of sulfur dioxide to form Diels-Alder adducts is not one of reactivity, but of product instability. 

Diels-Alder reactions. Heldeweg and Hogeveen have shown that, at least in the case of highly reactive dienes, the initial products of cycloaddition of sulfur dioxide are sulfinic esters rather than sulfones. Thus reaction of the tricyclic diene (1) with SO2 at room temperature leads to the sulfinic ester (2). This ester is unstable thermally and rearranges mainly to the aromatic ester (3). They also point out the similarity between cycloaddition reactions of sulfur dioxide and selenium dioxide. Thus (1) reacts with the latter reagent to give the seleninic ester (4), probably formed via the intermediate (a). 

Oxidation of a thiophene leads to a dioxide, which in the case of thiophene itself dimerises in low yield, but with substituted thiophenes can be isolated peracids or 2,2-dimethyloxirane, which improves yields from 2,5-disubstituted thiophenes, can be used. The 5,5-dioxides are no longer aromatic thiophenes and react as dienes in Diels-Alder reactions generally sulfur dioxide is extruded from the initial adduct, leading to further reaction" - eventual aromatisation in the example shown. ... 

An alternative method for incorporating a sulfone moiety into a carbon framework for the RBR involved an elegant four-component coupling between sulfur dioxide, silyl enol ethers 19 and 20, and iodide 21 (Scheme 8.71. The mechanism presumably comprises addition of 1-siloxy-1,3-diene 19 to sulfur dioxide, reaction of the resulting aldehyde with silyl enol ether 20, and then attack of the resulting sulfinate on the a ky iodide 21. The product 22, obtained as four diastereomers, was converted by the RBR to an intermediate that comprised a formal total synthesis of apoptolidin (see Section R.fiT... 

SO2 Activation. At low temperatures, l-alkoxy-l,3-dienes add to sulfur dioxide activated by TBDMS triflate to generate a zwitterionic intermediate that can be quenched with silyl enol ethers to give, y-unsaturated silyl sulfinates. DesUylation and reaction with methyl iodide provide a stereoselective route to polyfunctional sulfones in an overall four-component coupling sequence (eq 50). ... 

In contrast to the [4-I-2] cycloaddition reactions of sulfur dioxide with 1,3-dienes, the sulfine derived cycloadducts are stable compounds. 

For a long time, the organic chemistry of sulfur dioxide was limited to the formation of sulfinate salts and the sulfones derived therefrom. As shown, the sulfones are highly valuable synthetic intermediates for the construction of carbon-carbon single and double bonds, and very often with high stereoselectivity. More recently, the H-ene and hetero-Diels-Alder reactions of sulfur dioxide have been realized under conditions that avoid polymerization of alkenes, dienes, and other unsaturated compounds. Thus, alk-2-ene-1-sulfonyl chlorides are obtained readily from alkenes... 

The reversible reaction of 1,3-dienes with sulfur dioxide [59] provides another means to mask dienes for use in cycloaddition reactions [60]. An early example of this strategy in an intramolecular context was reported by Nicolaou, leading to a total synthesis of the estratrienone derivative 100 (Equation 7) [61]. Cheleotropic extrusion of SO2 from 98 formed 99, which participated in a diastereoselective Diels-Alder cycloaddition to give 100 in 85 % isolated yield. 

Sulfur dioxide acts as a dienophile ia the Diels-Alder reaction with many dienes (253,254) and this reaction is conducted on a commercial scale with butadiene. The initial adduct, sulfolene [77-79-2] is hydrogenated to a solvent, sulfolane [126-33-0] which is useful for selective extraction of aromatic hydrocarbons from... 

Reaction of myrcene and sulfur dioxide under pressure produces myrcene sulfone. This adduct is stable under ordinary temperatures and provides a way to stabilize the conjugated diene system in order to hydrate it with sulfuric acid. The myrcene sulfone hydrate produced is pyrolyzed in the vapor phase in order to regenerate the diene system to produce myrcenol [543-39-5] (50). 

Interestingly, benzonitrile oxide does not react with thiirene dioxide 19b even in boiling benzene, whereas the electron-rich diene l-piperidino-2-methyl-l, 3-pentadiene (177) does react under the same reaction conditions to give the expected six-membered [4 + 2] cycloadduct 178, accompanied by sulfur dioxide extrusion and 1,3-hydrogen shift to form the conjugated system 179175 (equation 70). 

The product 303 from disulfur dichloride and 2,3-dimethylbuta-l,3-diene rearranges spontaneously to the tetrahydrothiophene 304 (equation 159)150. The reaction of liquid sulfur dioxide with conjugated dienes 305 (e.g. butadiene, isoprene) results in cyclic sulfones which dissociate into their components on heating (equation 160)151 152. 

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

Clearly, there is a fine balance between [2 + 2]- and [4 + 2]-cycloaddition in the reaction of TCNE with vinylindoles. The cycloaddition of 1-methyl-3-vinyl compound and TCNE in benzene gives mainly the [4 + 2]-product, although a small amount of the [2 + 2]-adduct also appeared to be formed. In liquid sulfur dioxide this [4 + 2]-adduct completely isomerized to the [2+ 2]-product. It is probable that in the reaction between diene 205 and TCNE the transition state for [4 + 2]-cycloaddition is destabilized by the steric requirements of the bulky N-benzyl group. 

This has been essentially accomplished in a case of the diene-dienophile monomer polymerization (6, 7, 8, 16). This excellent work makes use of the reaction of such dienes as cyclopentadienones, a-pyrones, and thiophene dioxides with dienophiles. The resulting adduct, a protected or inactive diene, loses carbon monoxide, carbon dioxide, or sulfur dioxide at elevated temperatures to form a diene. Thus when a Ws-malei-mide reacts with one mole of a cyclopentadienone, the resulting adduct will lose carbon monoxide at 150—260° in an inert solvent to form an active intermediate diene-dienophilic monomer which readily poly-... 

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