6-Membered rings cyclodimerizations

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + rt2a -I- pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

The reaction of germylene 134 with /< rt-butylphosphaalkyne gave germadiphosphacyclobutene 33, possessing both two- and three-coordinated phosphorus atoms. It was assumed that the reaction started with the addition of 134 to the phosphaalkyne to afford a three-membered ring system. Cyclodimerization of this intermediate yielded the final product 33 (Scheme 49) <2001CC215>. 

In the cyclodimerization of methylenecyclopropanes and carbon dioxide, the three-membered ring is opened to form five-membered lactones. In the presence of palladium (O)-triphenylphosphine. the 0-mcthy enc-Ucionc was obtained in 3... 

In the presence of Ni(0) catalysts, methylenecyclopropanes cyclodimerize at temperature as low as —15 °C. The chemoselectivity of these reactions strongly depends on the substitution pattern of the substrates. [2+2]-Cyclodimerizations are restricted to methylenecyclopropanes bearing no further substituents at the exo-cyclic double bond. The substitution pattern at the threemembered ring (R = alkyl, aryl) determines whether four-membered rings, five-membered ring or open-chain products (or a mixture of all those) are obtained. 

Ni(cod)2 or mixtures of Ni(cod)2 with an electron deficient alkenes (e.g. dialkyl fumarate or maleic anhydride) have been found to be the most efficient catalysts for the cyclodimerization of methylenecyclopropane and 2-methylmethylenecyclopropane no. i7i) Ni(cod)2 the combined yields of cyclodimerization products afe lower, but the ratio or four-membered to five-membered rings is higher. The reverse holds for the modified catalysts (Eq. 66). 

While 2,2-dimethyImethylenecyclopropane can be cyclodimerized in 80 to 90% yield, tetramethylmethylenecyclopropane gives mainly the ring-opened product 2,3,3-trimethyl-l,4-pentadiene 172>. Control of stereochemistry in these reactions is low. However, the observed regiospecifity of four-membered ring formation is surprising (Eq. 69). 

The irradiation of 2-methylisoindole in moist hexane with a high-pressure mercury lamp through Pyrex gives two cyclodimerization (4n + 4n) products.115 The structure assigned to the major product (105) involves 1,3 V,3 cyclization. The minor product (106), which is formed by 1,3 4, 7 cycliza-tion, is unusual in involving cycloaddition across the 4,7-positions of the six-membered ring. A valence isomer of the isoindole has not been detected. 

In 2010, Ohashi and Ogoshi found an interesting nickel-catalyzed [3+3] cyclodimerization of ester-substituted MCPs giving six- and five-membered ring derivatives (see (43)) [98]. 

The seven-membered ring tricyclic carbodiimide 9, obtained in the thermolysis of tetrazolophenanthridene, is stable up to —40 °C. Above this temperature it undergoes cyclodimerization to give 10 . 

A formal [3 + 3]-cyclodimerization of cyclopropenes via ring-opening (Eq. 36) to give six-membered carbocycles has only been observed in the case of 1,2-diphenyl-cyclopropene72) (Eq. 43). 

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