Hydrocarbon structures cycloaddition reactions

In the gas phase, 2 is a thermally very stable compound up to 850 °C. Pyrolysis at 880°C/10 Torr generates styrene (55-62%) and o-xylene (6%) along with small amounts of phenylacetylene, benzene, toluene and unidentified hydrocarbons. Cycloaddition reactions with dienophiles were among the first reactivity studies on 2 they were of course driven by the expectation to generate a cyclobutadiene structure by a twofold (4 + 2) cycloaddition. However, while 2 reacts readily with electron-deficient alkenes such as TCNE, A -phenyhnalermide, 4-phenyl-l,2,4-triazolinedione and diethyl azodicarboxylate to form 1 1 adducts 115, a second Diels-Alder reaction... 

Metal-assisted cycloaddition reactions involving meso-perfluoroalkylated porphyrinic templates define a versatile methodology for the syntheses of both cofacial porphyrin structures and facially-functionalized (porphinato)metal species [62], and corresponding compositions that offer new opportunities for development of small molecule redox catalysts [63, 64]. Scheme 21 shows the versatile metal-mediated [2+2+2] cycloaddition step for the isolation of meso-perfluoroalkyl cofacial porphyrins that results in the purified indane complexes in 10-12 % yields. These cofacial complexes are directly applicable to multi-electron redox reactions such as oxygen or nitrogen reduction and even hydrocarbon oxidation. 

Bicyclo[3.2.0]hept-l-ene (151) can be prepared from the tosylhydrazone of norbornan-7-one via norbornan-7-ylidene 150 in 74% as a stable olefin (144). Similarly, 152 has been obtained from norborn-2-en-7-one in a yield of 67% (144a). Introduction of a further double bond leads to the strained triene 153, which has been prepared from 105b (Scheme 3). Compound 153 dimerizes readily at room temperature to a 1 1 mixture of hydrocarbons 154 (145). The most likely structures of these dimers arise from a formal [2 + 2]cycloaddition of the bridgehead double bonds of 153 with each other. Support for the intermediacy of 153 comes from trapping experiments with 1,3-diphenyl-isobenzofuran, which yield a mixture of stereoisomers via reaction with the bridgehead double bond of 153. 

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