Cyclobutane ring oxidative

SO2CIF (eq 28). In these systems, separation of the two cation centers by at least two methylene groups is necessary for the ions to be observable. Aromatic dications are usually prepared by oxidizing the corresponding aromatic compounds with SbFs (eq 29). In the case of pagodane containing a planar cyclobutane ring, oxidation leads to the formation of cyclobutane dication which was characterized as a frozen two-electron Woodward-Hoffmann transition state model (eq 30). ... 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Cyclic amino acids 139, when heated in acetic anhydride, probably form initially mesoionic oxazolium 5-oxides (munchnones) subsequent 1,3-dip olar cycloaddition of 1,2-dicyanocyclobutene, loss of carbon dioxide, and opening of the cyclobutane ring lead to dinitriles 140 (80JHC1593). Pyridone 141 is the by-product (together with an indolizine) of the mono-cyclic pyridone dicarboxylate and acrylic ester (73JHC77). 

Due to the ring strain in the cyclobutane ring, /(-carbonyl cyclobutanones made by oxidation of methylenecyclobutane in methanol will readily undergo retro-Claisen-type reactions. For ozonolysis, this can be avoided by changing the solvent from methanol to dichloromethane treatment of 6-methyl-8-methylene-m-3-oxabicyclo[4.2.0]octan-2-one with ozone in methanol gave methyl 2-(4-methyl-2-oxo-4-tetrahydropyranyl)acetate (1), while the same treatment in dichloromethane gave 6-methyl-m-3-oxabicyclo[4.2.0]octane-2,8-dione (2).16... 

The cyclobutane ring in propellanes may be cleaved by electrochemical221 or chemical222 oxidation. Thus, dehydrohomoadamantane 1 was converted to disubstituted homoadaman-tanes on oxidation with nitronium ions in the presence of nucleophiles. 

Addition of diethylaminopropyne to C60 resulted in a [2+2] cycloadduct that can be ring-opened under acidic conditions, while, when followed by oxidative cy-clization in the presence of activated carbon, novel fullerene lactones were formed [62]. Further work on the reaction of C60 with propiolates in the presence of triph-enylphosphine resulted in the construction of bismethanofullerenes and a [2+2] cycloadduct consisting of a cyclobutane ring fused to a 6,6 ring junction [63]. 

The stepwise, oxidative cycloaddition mechanism [particularly with d metal systems 1 )] could intervene in the valence isomerizations of strained, cyclobutane ring systems where energy factors and difficulties in attaining bidentate coordination work in its favor. For the other processes, however, where bidentate coordination is either very favorable or guaranteed, its contribution to catalytic chemistry would seem to be significantly less. 

As discussed by Ruckel et al., (14) the catalytic cationic polymerizations of a-plnene oxide and B-plnene oxide both Involve the concomitant opening of the epoxide and cyclobutane rings In the propagation step. This rearrangement mechanism also clearly operates In the radlatlon-lnduced polymerization of these monomers, and Is depicted below for a-plnene oxide (Scheme ). 

Enamines (e.g. 377) have been shown to react with conjugated nitroolefins 378 to give mainly dihydro-l,2-oxazine A -oxide derivatives 379 as products of kinetic control (sometimes a cyclobutane ring is formed in these reactions see Section II.B). The stability of these heterocycles is largely dependent on the parent enamine and the type of substituent used on the nitro olefin as has been extensively studied by Valentin and coworkers " . Usually they open into the corresponding nitroalkylated enamines 380 (equation 82), in particular in a solution of methanol or deuteriated chloro-and often an equilibrium between the two forms is established. Stable 1,2-oxazine A -oxides have been obtained in the reaction of 2-nitro-l,3-dienes with cyclic enamines . 

This sequence has been shown because there are examples known of substituted olefins being cyclized to cyclobutane derivatives by certain transition metal complexes (Schrauzer Heimbach, Jolly and Wilke °W). In addition, metal complexes will catalyze the opening of cyclobutane rings to olefins and other rearranged products, as shown particularly by Cassar, Eaton and Halpern f). It is likely that the sequence shown in Fig. 15, both forward and reverse, is the mechanism for many of these reactions. Metal ions such as silver(I) which do not undergo oxidation readily probably involve a different mechanism (Paquette 2a) Gassmann 2b)). 

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