Cheletropic additions

Finally, it should be mentioned that from a purely synthetic point of view the reactions involving "transfer" of methylene groups -like the Simmons-Smith reaction [27] or Seyferth s reagents [28]- may be considered formally as cheletropic additions of a [2 + 1] type. 

Figure 14.4. a) Orbital correlation diagram for n2s + 0J2S cheletropic addition of SO2 to an olefin. The symmetry element preserved is a vertical mirror plane, (b) Orbital correlation diagram for elimination of CO from a norbornadienone. Two vertical planes of symmetry are preserved. 

It has been found that dihalocarbenes add to l263, ll264 and 50 . The products, ring expanded cyclobutarenes, are plausibly accounted for by way of cheletropic addition to the strained bridge bond and subsequent rearrangement as illustrated for 1 in equation 27. However, the reaction mechanism has not been established. 

Dihalogermylenes and dihalostannylenes are supposed to be rather inert species, yet MNDO calculations predict a low activation barrier ( a = 19.5 kcalmol-1) for the cheletropic addition of Br2Sn to butadiene with formation of cyclo-Br2SnC4H6 (equation 21)381. 

Azadienes, synthesis of heterocycles from (review), 57, I I-Aza-1,3-dienes cheletropic addition to... 

Stereoselective cheletropic addition of SO2 to vinylallene 534 gives the cyclic 1,1-dioxide 535 (Equation 21) <20050L1565>. 

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. 

When the dihalocarbene is generated thermally or photochemically the product of cheletropic addition to the alkyne is usually isolable in adequate yield. For example, pyrolysis of (trichloromethyl)trifluorosilane at 140°C and 6.5 atm in the presence of perfluorobut-2-yne provides " 3,3-dichloro-l,2-bis(trifluoromethyl)cyclopropene in 93 % yield. The corresponding perfluorocyclopropene is available from addition of di-fluorocarbene which can be generated by thermolysis of perfluoropropene oxide ... 

The photolysis of 3-allyl substituted cyclopropenes contrasts with their thermal behaviour (Section IV.A.6b). Direct irradiation of 246 affords the bicyclohexenes 248. Concerted intramolecular cheletropic addition of 247 cannot be involved since both C(6)... 

Roberson, M., Jepsen, A. S., Jorgensen, K. A. On the mechanism of catalytic enantioselective hetero-Diels-Alder reactions of carbonyl compounds catalyzed by chiral aluminum complexes-a concerted, step-wise or Mukaiyama-aldol pathway. Tetrahedron 2001, 57, 907-913. Monnat, F., Vogel, P., Rayon, V. M., Sordo, J. A. Ab Initio and Experimental Studies on the Hetero-Diels-Alder and Cheletropic Additions of Sulfur Dioxide to (E)-I-Methoxybutadiene A Mechanism Involving Three Molecules of S02. J. Org. Chem. 2002, 67, 1882-1889. 

Monnat, F., Vogel, P., Sordo, J. A. Hetero-Diels-Alder and cheletropic additions of sulfur dioxide to 1,2-dimethylidenecycloalkanes. Determination of thermochemical and kinetics parameters for reactions in solution and comparison with estimates from quantum calculations. Helv. Chim. Acta 2002, 85, 712-732. 

SCHEME 5.4 Stereochemistry of cheletropic addition of SO2 to diastereomeric hexa-2,4-dienes. 

The mechanism of formation of benzvalene from cyclopentadienyl-lithium and dichloromethane has been studied in detail. On employing dideuteriodichloro-methane, the deuterium label is found to be stereospecifically located at the C-1 position of the product, in contrast to earlier reports. These data are compatible with two routes (i) chlorocyclopropanation of the cyclopentadienyl-lithium and subsequent nucleophilic displacement of the chloro-substituent and (ii) attack of the cyclopenta-dienyl anion on the dichloromethane to produce a cyclopentadienyl carbenoid A distinction between these two routes comes from a study of the reaction with indenyl-lithium. The exclusive formation of 1- and 2-deuterionapthalene (0.6 1) as byproducts is compatible only with the carbenoid path (Scheme 6) in which 1,2-cheletropic addition affords benzobenzvalene uniquely labelled at C-1, as is observed. 

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. 

Fig. 3.14 Orbital interactions in the TS for cheletropic addition reactions in (4n+2) and 4n electron systems...

Cheletxopic addition of SO2 to a polyene depends on the geometry of the intrusion adduct. For example, cheletropic addition of SO2 to 1,3,5-cyclooctatriene 156 gives cycloadduct 157 using 4jt e system in a disrotatory motion process [163]. 

Cheletropic addition of SO2 to divinylallenes takes place preferably at the most substituted vinyl allene and at the -site of the vinyl groups. For example, divinylaUene 160 having both -and Z-vinyl units selectively undergoes addition at the -site to give 161 [164]. 

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