Cheletropic reactions carbene additions

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. 

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 organomercurial (67) is decomposed by diphenylmercury and, in the presence of added olefin, methylenecyclopropanes are obtained. The cheletropic reactions of carbenes with bicyclic dienes have been further investigated. Thermal generation of difluorocarbene and subsequent addition to norbornadiene results in a 34% yield of the 1,4-homo-addition product cf. (58)] as well as the anticipated exo- and endo-tricyclo[3,2,0,0 ]octenes. With 7-methylnorbornadiene, where steric interference to difluorocyclopropanation is increased, the homo-adduct represents 61 % of the product yield chlorofluorocarbene behaves similarly, but, because of an increased... 

The cheletropic reactions of dihalogenocarbenes with norbomadienes have been the subject of further investigations. The main reactions involved are 1,2-addition, which can occur from the endo and exo directions, and homo-1,4-addition. For reaction with norbomadiene itself it has been found that the exo endo selectivity ratio increases som hat in the series difluoro-, fluorochloro-, dichloro-, and dibromo-carbene (respectively 1.2,1.8,4.0, and 2.0), whereas the selectivity ratio for the 1,2 1,4... 

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. 

The reactions in this category are 4-centre 4-electron processes. The related 3-centre 4-electron process involved in the addition of sin et carbenes to olefinic 7r-bonds, is discussed under cheletropic reactions. 

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. 

Cheletropic additions at the C,C double bond of alkenyl azides, for example, reactions with carbenes to generate azidocyclopropanes or epoxidation are weU-known for several decades. Recently, the 1,4-addition of sulfur dioxide to 2-azidobuta-l,3-dienes 92 has been investigated (Scheme 5.27). Moderate yields of the corresponding 3-sulfolenes 226 were obtained. 

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