Cheletropic reactions carbene cycloadditions

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). 

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. 

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. 

Cheletropic Reactions. Cheletropic reactions are really a sub-class of cycloadditions. The only difference is that, on one of the components, both new bonds are being made to the same atom. The best-known reaction of this type is the general reaction of a carbene (133) with an olefin. At first sight, this... 

Cheletropic reactions are special type of concerted cycloadditions or cycloreversions in which two bonds are formed on or fissioned from same atom. For example, reaction of a singlet carbene with olefin. 

In addition to the representative [3 + 2] cycloaddition reactions shown in Table I, the delocalized singlet vinyl carbenes have been shown to participate as it2a components of non-Hnear cheletropic [tt 2s + tt 2a] cycloadditions to provided cyclopropanes with an observable endo effect, and as7r2s components of [tt4s + tt 2S] cycloadditions with selected dienes to provide cyclo-... 

Cheletropk Reactions and Reverse Cycloadditions.—The cheletropic addition of unsaturated carbenes, which were generated by Bu OK treatment of the appropriate vinyl triflate (e.g. Me2C=CHOTf) at low temperature, to dialkylacetylenes affords... 

The cycloaddition of a conjugated it system to an electrophilic molecule by the formation of two new a bonds to an atom of the electrophile in a concerted manner is known as cheletropic addition reaction and its reverse process in which two a bonds are broken from the same atom of the adduct is known as cheletropic elimination reaction. In cheletropic elimination, the driving force is often from the entropic benefit of gaseous elimination of N2, CO, and SO2. For example, the cycloaddition of 1,3-butadiene and its derivatives with SO2 and of alkene with a carbene are cheletropic addition reactions. 

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