Cheletropic reactions, cyclic

The reaction of ADC compounds with carbenes and their precursors has already been discussed in Section IV,A- In general, the heterocyclic products are not the result of 1,2-addition but of 1,4-addition of the carbene to the —N=N—C=0 system.1 Thus the ADC compound reacts as a 4n unit in a cheletropic reaction leading to the formation of 1,3,4-oxadiazolines. Recent applications include the preparation of spiro-1,3,4-oxadiazolines from cyclic diazoketones and DEAZD as shown in Eq. (14),133 and the synthesis of the acyl derivatives 85 from the pyridinium salts 86.134 The acyl derivatives 85 are readily converted into a-hydroxyketones by a sequence of hydrolysis and reduction reactions. 

If the fragmentation of the two atoms from the group, to which they were bonded, takes place in a concerted process resulting in the formation of a conjugated system, this is a cheletropic extrusion. Thus, the two possibilities can be found in the case of the loss of a SO2 group SO2 extrusion from an episulfone, as in the Ramberg-Backlund reaction, is an extrusion reaction, but SO2 extrusion from a cyclic sulfolene is a cheletropic reaction. ... 

The term "cheletropic reaction" defines reactions in which two a bonds to a single atom are made or broken conceitedly. A widely studied example, theoretically as well as experimentally, is the cheletropic elimination of sulfur dioxide from a cyclic sulfolene to generate stereochemically pure dienes or polyenes. ... 

A further method for the synthesis of cyclic compounds in the phosphinic acid series, and investigated within the last decade, is the clearly not unrelated cheletropic reaction which takes place between alka-1,3-dienes and phosphenium salts. The latter are based on dicoordinate phosphorus, [R R P ], the commonly encountered counter ion being the tetrachloroaluminate anion. Such salts are obtained in situ through the interaction of AICI3 and a hosphorus(III) chloride R R PCl, for which R = Cl, R = R2N, or R = R =... 

A cycloaddition reaction is actually a type of pericydic reaction, but the term peri-cyclic includes other types of reactions. The textbook definition of pericydic is a reaction whose transition state has a cyclic structure (i.e., the electrons are flowing in a closed loop). In addition to cycloaddition reactions (which exchange two Jt-bonds for two o-bonds, or vice versa), pericydic reactions include sigmatropic reactions, electro-cyclic reactions, and cheletropic reactions (and a few others which we ll ignore). 

The cheletropic reaction between conjugated dienes and sulfur dioxide yields cyclic sulfones. However, the isolated adducts of selenium dioxide and dienes possess the selenalactone structures (174). The only apparent explanation for the different stabilities of the cheletropic products (175) is that selenium is softer and tends to avoid a higher oxidation state. 

The irreversible extmsion of nitrogen from the cyclic diazene and the easy loss of carbon monoxide from norbomadienone are important examples of six-electron cheletropic reactions (Scheme 5.2). The driving force for these reactions is often the entropic benefit of gaseous evolution (e.g., N2 or CO). 

The reverse reaction is a linear cheletropic extrusion of SO2 with conrotatory twisting of the terminal methylenes. With cyclic triene SO2 forms only the 1,4 adduct. In this case the antarafacial addition to the triene is geometrically impossible and the alternative non-linear n s + (O s process does not compete with the concerted 71 5 + (O s linear cheletropic addition to a diene component. So a nonlinear cheletropic addition containing eight electrons becomes highly unfavorable. This is illustrated by the fact that SO2 is eliminated 60,000 times more slowly (even at 180°) from (c) than from (b). In (b) + (ip s, retro process is allowed. 

Cheletropic Cycloadditions. If an atom can both donate an electron pair and accept an electron pair to move to a higher covalency, it can act as a dienophile to a diene. Such is the case with the sulfur atom in sulfur dioxide and with phosphoms in tricovalent halides. This type of cycloaddition is called cheletropic, and results in the formation of 5-membered rings. The reaction of butadiene with SO2 is particularly important it is conducted on a commercial scale to produce the cyclic sulfone 5.18 (called sulfolene). Its hydrogenation product 5.19 (sulfolane) is widely used as a nonaqueous highly polar solvent. Substituted dienes also participate in the cycloaddition. Here, the sulfur can donate its electron pair while accepting an electron pair to form a new carbon-sulfur bond. 

D-A-substituted a-quinquethiophene 2.110 capped by an anthracene and Ceo unit has been synthesized via a [4-1-2] cycloaddition reaction of Ceo with anthrylquinquethieno-o-quinodimethane intermediate, which was generated in situ by cheletropic extrusion of SO2 from the precursor cyclic sulfone 2.109 (Scheme 1.19) [212]. Energy transfer from the anthracene donor to the fullerene acceptor has been observed for the triad. Photoinduced electron transfer processes in the triad were observed in low-temperature ESR measurements [213]. 

Cheletropic and other cydoaddition reactions conserve orbital symmetry and can occur both intramolecularly as well as tmermolecularly. Both versions are peri-cyclic reactions. The imermolecular set is discussed here (since reactions in that set may be viewed as addition reactions between two molecules). The m/ramolecular reactions, which can be considered rearrangements, will be treated in that section of this chapter. 

Cycloadditions Interestingly, sulfur dioxide participated as a dienophile in the [4+2] cycloaddition reaction with 1,3-dienes. In this manner, sulfur dioxide reacts similarly to the related selenium dioxide and the other sulfur dienophiles RN=S=0, RN=S=NR and R2C=S=0 (sulfines). However, the [4+2] cycloadducts derived from 1,3-dienes and sulfur dioxide are only obtained at low temperatures (—80 °C) in a kinetically controlled reaction and the cycloaddition reactions often require the presence of a Lewis acid (CF3COOH or BF3). Above —50 °C the Diels-Alder adducts undergo a cycloreversion and a cheletropic addition of the generated sulfur dioxide to the diene occurs with formation of the corresponding 2,5-dihydrothiophene-1,1-dioxides (sulfolenes). According to ab-initio computations, electrostatic solvent effects are predicted to be of importance in the control of the selec-tivities in this reaction . From linear dienes, the [4+1] cycloadducts are usually obtained. For example, from 1,3-butadiene and SO2 at -20 °C, the cyclic sulfone 25 is obtained in 95% yield. ... 

We are interested here in the elimination process. In these reactions the atom X is bound to o her atoms in such a way that it is part of a small stable molecule. The most common examples involve five-membered cyclic transition states, although the term cheletropic is not restricted with respect to ring size so long as the process can be concerted. 

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