And cheletropic reactions

Two-component methods represent the most widely applied principles in sulfone syntheses, including C—S bond formation between carbon and RSOz species of nucleophilic, radical or electrophilic character as well as oxidations of thioethers or sulfoxides, and cheletropic reactions of sulfur dioxide. Three-component methods use sulfur dioxide as a binding link in order to connect two carbons by a radical or polar route, or use sulfur trioxide as an electrophilic condensation agent to combine two hydrocarbon moieties by a sulfonyl bridge with elimination of water. 

Sigmatropic, electrocyclic, cycloaddition and cycloelimination, and cheletropic reactions have all been carried out with organosulfur compounds and often used for synthetic puiposes. A chapter of Block s monograph (203] is devoted to this topic, and most of the pericyclic processes include examples with sulfur compounds. The treatises by Barton and Ollis [482], Trost and Fleming [483] and Klamann [484] are guides to the more specialized literature. Some reviews deal with specific cases thiocarbonyl compounds [120] or cycloaddition reactions [485],... 

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 nomenclature we need to examine electrocyclic reactions (and cheletropic reactions, see below) is one that describes how p orbitals at the termini of a tt system rotate. As can be seen in Figure 15.15, if the two p orbitals rotate in the same direction (both clockwise or both counterclockwise) the process is termed conrotatory. If they rotate in opposite directions (one clockwise and one counterclockwise), the process is termed disrotatory. The same terms are used to describe the direction of rotation of atoms involved in a bonds. Figure 15.15 also shows their use in this context. 

A particularly interesting system where nitrogen is lost cheletropically after formation of the initial [4 + 2] cycloadduct involves the thermal reaction of azirines with tetrazines (82) (74CC45, 74TL2303, 74CC782, 75JHC183). A variety of heterocyclic products are produced depending on the structure of the azirine and tetrazine used and the reaction conditions. 

Since sulfoxides and sulfones are versatile synthetic intermediates, and since in both the thiolene oxide and dioxides the reverse dethionylation114 ( — SO), and cheletropic extrusion of sulfur dioxide296, respectively, readily take place thermally, these cycloadditions are expected to find a useful place in organic synthesis. It should be kept in mind, however, that the retrograde SO-diene reaction and interconversion of the thiolene oxides compete effectively against SO extrusion on heating, and that diene isomerization accompanies the forward reaction (SO + diene). 

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry analysis in the same way as cycloadditions and sigmatropic processes. In the elimination processes of interest here, the atom X is normally bound to other atoms in such a way that elimination gives rise to a stable molecule. In particular, elimination of S02, N2, or CO from five-membered 3,4-unsaturated rings can be a facile process. 

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. 

Cheletropic reactions include both addition and elimination reaction. The number of elimination reactions that have been studied in detail is not large but there is sufficient information to establish that orbital symmetry controls are operating. 

The cycloaddition of an atom or group X to an olefine to form a three-membered ring and the reverse process constitutes an example of four electron cycloaddition or elimination and if the reaction is concerted it becomes an example of cheletropic reaction. 

Some of its special chapters are the Pericyclic Reactions, which includes Cheletropic, Electrocyclic, Sigmatropic and Cycloaddition reactions. The concept of Stereochemistry and Conformation deserve special attention not because they cater to the needs of higher students, but they are immensely useful for candidates trying for UGC and CSIR sponsored competitive examinations, but also those preparing for Union Public Service Commission and State Public Service Commission Exams. The candidates will find the chapters immensely useful and is sure to rouse interest in them in knowing more about mechanistic chemistry. 

Another method to prepare allenyl ketones uses flash vacuum pyrolysis of the heterocycles 121 (Scheme 7.19) [163], This elimination of carbon monoxide is at least formally a cheletropic reaction. Highly reactive allenes such as esters and nitriles of type 124 or unsubstituted butadienal can be generated if retro-Diels-Alder reaction of 123 or similar precursors, respectively, is performed by flash vacuum pyrolysis [164]. 

The most important cheletropic reactions are those that involve the reaction of an electro-defficient species -as a carbene, nitrene or "atomic oxygen"- with an olefinic double bond and lead to three-membered rings. 

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry restrictions in the same way that cycloadditions and sigmatropic processes are. 

A general cheletropic reaction is shown in Figure 12.2. This reaction involves the addition to, or extrusion from, a conjugated system of a group bound through a single atom. The reaction usually involves the elimination of simple stable molecules such as SO2, CO, or N2. The atom to which there were two a bonds carries away a pair of electrons, usually in a spn hybrid orbital. The addition of a carbene to a simple olefin to form a cyclopropane is also a cheletropic reaction which, as discussed in Chapter 14, is not predicted to be concerted. Cheletropic reactions incorporate features of both cycloaddition and electrocyclic reactions. 

Cheletropic reactions, in which a single atom is added or extruded, comprise a special case of cycloaddition reactions. Figure 14.4 displays correlation diagrams for two typical cheletropic reactions, the loss of SO2 from a thiirane dioxide (Figure 14.4a) and the loss of CO from a norbornadienone (Figure 14.4b). The addition of a carbene to an olefin is another example which is discussed below (Figure 14.9a). 

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... 

A number of cheletropic reactions also appear to be anomalous, including the best known of all cheletropic reactions, the stereospecific insertion of a carbene into a double bond, as in the reaction of dichlorocarbene 2.173 with alkenes. Here we have a reaction involving only four electrons, which is known to be suprafacial on the alkene, preserving the geometry of the substituents in the starting alkenes in the cyclopropanes 2.174 and 2.175. Furthermore, the [2+2] reaction takes place even with a diene, which could. undergo an allowed [4+2] reaction, but chooses not to. 

We shall find that this reaction is again a rather special anomaly, needing special treatment, but there are straightforward 6-electron cheletropic reactions, such as the irreversible extrusion of nitrogen from the diazene 2.176, and the easy loss of carbon monoxide from norbornadienone 2.177. 

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. 

Thus hydroboration is not a pericyclic reaction, because the boron atom makes use of two AOs. Similarly, the reaction between a carbene and a double bond is not pericyclic because the carbon atom uses two AOs. Cheletropic reactions are not pericyclic either. Consider, for example, the fragmentation of 3-cyclopentenone to give CO and butadiene. Not only does the expelled carbon atom use two AOs to bond with its neighbours, but also the oxygen atom, which is an intervening atom (it was initially linked to the carbon atom by a double bond, which becomes a triple bond in CO) is exocyclic in the transition state. 

Cheletropic reactions are cyclizations - or the reverse fragmentations - of conjugated systems in which the two newly made o bonds terminate on the same atom. However, a cheletropic reaction is neither a cycloaddition nor a cycloreversion. The reason is that the chelating atom uses two AOs whereas in cycloadditions, each atom uses one and only one AO. Therefore, Dewar-Zimmerman rules cannot apply to cheletropic reactions. Selection rules must be derived using either FO theory or correlation diagrams 38 The conjugated fragment39 of 4n + 2 electron systems reacts in a disrotarory (conrotarory) mode in linear (nonlinear) reactions. In 4n electron systems, it reacts in a disrotarory (conrotarory) mode in nonlinear (linear) reactions. 

This is no longer true for cheletropic reactions. When applying the selection rules, one must always consider that the chelating fragment X contributes two electrons. Erroneous conclusions can be made otherwise. Thus, in the cyclopropanation reaction, which should be considered a four-electron cheletropic reaction, only one n bond is broken. In the formation of diazene 32, a six-electron cheletropic reaction, butadiene uses two double bonds and N2 one lone pair and one n bond the two components thus employ a total of eight electrons ... 

If X always contributes two electrons, its chemical nature should be unimportant. This is contradicted by experimental results. Whereas fragmentations of diazenes give good yields and are stereospecific,41 heating of nitrosopyrroline 33 gives, in addition to polymers, only traces of butadiene and N20.42 It can be then be expected that the validity of the selection rules is better for pericyclic than for cheletropic reactions. 

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