Cheletropic reactions linear

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. 

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. 

The stereochemistry of cheletropic reactions has been discussed in Section 3.3.4. The linear cheletropic reactions usually offer the best scope for orbital overlap and would normally be expected to be the preferred reaction pathways. Generally the extniaon reactions (i.e. the loss of the species Xyz) are more numerous than cheletropic cyclo-additions because the Xyz species is often a molecule of high thermodynamic stability, and in such cases the reactions are essentially uni-directional. ... 

Consider Fig. 3.i6 it is seen that k + 2) electrons are involved in the reaction, and further that two electrons are delivered from the hybrid (sp") orbital of Xyz. This orbital, considered alone, can be acted upon in the supra-facial or antarafacial senses, and so can the k 7r-electron olefin. Therefore, there are the four usual combinations, namely supra-supra, antara-antara, antam-supra, and supra-antara (respectively Fig. 3.16(a) (i), (b) (ii), (a) (ii), and (b) (i)). When (/c + 2) - (4 + 2) electrons it will be expected, because of the general Wood ward-Hoffmann rule, that the supra-supra or antara-antara interactions will occur in the thermal cheletropic reactions. These respectively correspond to a linear cheletropic reaction with disrotatory cleavage, and to a non-linear cheletropic reaction with conrotatory cleavage. The alternative pathways are reserved for the cases (/ -I- 2) = An electrons. In the photochemical reactions the usual cross-over relationship should apply. 

The formation or fragmentation of five-membered rings characterizes this category. The cyclization process is mainly the province of SO2, but even here the extrusion reaction occurs readily. The supra-supra mode is likely to be the preferred pathway, that is a linear cheletropic reaction with disrotation. The transformations summarized in Equation (6.128) amply verify these predictions,... 

The molecule of S02 must lie in a plane which bisects the suprafacial cis conformation of the diene. This attack of S02 will be from below, so that a concerted suprafacial, suprafacial addition is allowed. Such reactions are called linear cheletropic processes. 

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. 

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. 

Nonlinear (left) and linear (right) cheletropic reaction. 

However, from the structures of the products of cheletropic reaction of SO2 with a 4n tt-system, it appears that the reagent approaches the ir-system in a linear fashion. 

Of the theoretical studies that have appeared, the MINDO/3 semiempirical MO treatment, developed by Dewar, has been expanded to unrestricted open-shell treatment by Bischof and applied to a series of radicals including the cyclopropyl species. A MINDO/3 study of the thermal decarbonylation of cyclopropanone by Shevlin and co-workers predicts the non-linear cheletropic mode of the reaction (A/f 41 kcalmol" ) to be favoured by ca. 45kcalmol over the linear mode, in... 

PMO-treatment of Cheletropic Reactions Cheletropic reactions can be easily explained on the basis of aromatic and antiaromatic transition states by PMO-method. Transition states in each mode of linear and non-linear approach of carbene both suprafacially and antarafacially to n-system are drawn below ... 

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 non-linear approach (b) the carbene orbitals are interacting in an antarafacial manner, and the polyene orbitals can interact either suprafacially or antarafacially as before. The disrotatory process this time relates to a Mobius system, and the conrotatory process to a Hiickel system. The nonlinear cheletropic reaction with conrotation will be preferred for the case (m +2) = 4n + 2), and with disrotation for the case (rn + 2)- 4n. Theffi conclusions are in full agreement with those obtained using the general Woodward-Hoffmann rule. 

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. ... 

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