Cheletropic reactions 8-electron

The linear cheletropic reactions in which the polyene is a suprafacial component (i.e., involving disrotatory motion of the termini) will be allowed if it has a total of (4n + 2) electrons. But linear cheletropic reactions in which the polyene is an antarafacial component (i.e., involving conrotatory movement of the termini) are allowed if it has a system of 4n electrons. 

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. 

Another example of 6 electron cheletropic reaction is the addition of trivalent phosphorus compounds with the dienes... 

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. 

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. 

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. 

Cheletropic reactions are a special group of cycloadditions in which the two o bonds are made or broken to the same atom. Thus sulfur dioxide adds to butadiene to give an adduct, for which the sulfur has provided a lone pair to one of the o bonds and has received electrons in the formation of the other. It is an oxidative addition to the sulfur dioxide, changing it from SIV to SVI. The reaction is readily reversible on heating. 

An anomalous cycloaddition is the insertion of a carbene into an alkene. Some cheletropic reactions are straightforwardly allowed pericyclic reactions, which we can illustrate with the drawing 6.127 for the suprafacial addition of sulfur dioxide to a diene, and with the drawing 6.128 for the 8-electron antarafacial addition of sulfur dioxide to a triene. The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial on the alkene with singlet electrophilic carbenes [see (Section 4.6.2) page 149]. This is clearly a forbidden pericyclic reaction if it takes place in the sense 6.129. 

Symmetry Allowed Cycloaddition of SO to a diene Cheletropic Reactions involving 4 Electrons Addition of Carbenes or Nitrenes Reactions involving 6n Electrons Cycloaddition and Elimination Reactions involving 8ji Electroncs... 

The application of the Woodward-Hoffmann rules to cheletropic reactions is not straightforward. In the [2+1] cycloaddition of singlet carbenes to alkenes, the stereochemistry of the alkene is preserved in the product, so the alkene must react suprafacially. The Woodward-Hoffmann rules suggest that the carbene component of this thermal, four-electron reaction must react antarafacially. However, what this means for a species lacking a 77 system is difficult to interpret. 

Following Woodward, such conversions are called cheletropic reactions. The LUMO of the 1,3-diene surrounds the nonbonding electron pair of sulfur like the claws of a crab (Greek chele). 

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 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 selection rules for the thermal cheletropic reactions are given in Table 5.1, where m is the number of electrons in the TT-system and n is an integer including zero. 

The most important cheletropic reaction is the addition of singlet carbenes to olefins to make cyclopropanes. Only singlet carbenes will be considered here the pericyclic selection rules cannot be applied to triplet states. The electronic structure of a singlet carbene involves an empty p orbital and a roughly sp hybrid that has two electrons (see, for example, the two lone pair orbitals of the water molecule in Appendix 3). We know from Chapter 10 that singlet carbenes add stereospecifically to olefins, and that the olefin stereochemistry is retained in the cyclopropane product. As such, in the present context, the reaction would be described as suprafacial on the olefin. 

Various other pericyclic reactions are analysed in Fig. 5.3 using the Zimmerman method. Cases (a) and (b) represent respectively the linear and non-linear (see Fig. 3.16, p. 95) cheletropic reactions of an acyclic polyene with singlet carbene. The polyene contains m ir-electrons the carbene, of course possess two frontier electrons which are placed in the jp -hybrid orbital, the p-orbital being empty. 

In the linear approach (a) the carbene orbitals are interacting in a supm-facial manner. The topology of the interaction with the polyene orbitals can be suprafacial, which therefore requires a disrotatory twisting about the terminal bond axes, or antarafacial which can result only if the canrotatoiy mode applies. The linear disrotatory process involves a Hiickel interaction, whereas the linear conrotatory reaction has a Mobius transition state. Hence if (m + 2), that is the total number of participant electrons, is equal to (4n+2) the Hiickel-type disrotatory closure will be preferred. If m + 2) = 4n, then the conrotatory (Mobius) closure is predicted. Hence butadiene should undergo a linear cheletropic reaction with singlet carbene (or similar electron deficient species - e.g. SO2) with disrotatory closure, whereas the analogous reaction of hexatriene requires the operation of the conrotatory mode. 

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