Oxidative cyclizations/cycloadditions dimer

Furoxans are not formed by direct oxidation of furazans, but they can readily be prepared by ring closure or cycloaddition pathways. The most synthetically useful routes are the oxidative cyclization of 1,2-dioximes, the dehydration of a-nitroketoximes and, for symmetrically substituted furoxans, dimerization of nitrile oxides. For asymmetrically substituted analogues care must be taken in selecting the route in order to avoid formation of mixtures of 2- and 5-oxide isomers. 

Cyclization of butadiene catalysed by Ni(0) catalysts proceeds via 7r-allylnickel complexes. At first, the metallacyclic bis-7i-allylnickel complex 6, in which Ni is bivalent, is formed by oxidative cyclization. The bis-7r-allyl complex 6 may also be represented by cr-allyl structures 7, 8 and 9. Reductive elimination of 7, 8 and 9 produces the cyclic dimers 1, 2 and 3 by [2+2], [2+4] and [4+4] cycloadditions. Selectivity for 1, 2 and 3 is controlled by phosphine ligands. The catalyst made of a 1 1 ratio of Ni and a phosphine ligand affords the cyclic dimers 1, 2 and 3. In particular, 1 and 3 are obtained selectively by using the bulky phosphite 11. 1,2-Divinylcyclobutane (1) can be isolated only at a low temperature, because it undergoes facile Cope rearrangement to form 1,5-COD on warming. Use of tricyclohexylpho-sphine produces 4-vinylcyclohexene (2) with high selectivity. 

Scheme 29 describes a plausible mechanism for the formation of the products which fit the observed coulometric (n 0.45 F/mol) and preparative results. The intramolecular cyclization process involves a dimerization between a radical cation 52a and the ketene imine 52 to form the intermediate radical cation 52b which then cyclizes to the radical 52c which can abstract a hydrogen atom leading to 54 or can be further oxidized and transformed through a cyclization and deprotonation reaction to 53 which involves 1 F/mol. However, it seems that the [2 -1- 3]-cycloaddition between the parent compound 52 and the cation 52d giving rise to 55 is the fastest reaction as compared with the intramolecular cyclization of 52d to 53. This can also explain the low consumption of electricity. 

Metal-mediated cyclizations that rely on the initial complexation of an alkene or alkyne around a low oxidation state metal center are often sensitive to the presence of additional substituents (particularly electron-donating substituents), and relatively more stringent reaction conditions are often required for successful cychzation. This effect was noted in the Ni-catalyzed formal [4 -I- 4] cycloaddition reactions developed by Wender and Tebbe and is apparent when one compares the reported facility of Pd-catalyzed linear dimerization of 1,3-butadiene versus that of substituted 1,3-dienes. Similarly, the initial attempts at Pd-catalyzed cyclization of bisdiene 70a (Scheme 22) were rather disappointing. Using 0.05 equiv of [Pd(OAc)2/3 PhjP] (THF, 65 °C, 12 h), only a small... 

---