Cyclobutadienes cycloreversion

The impact of (2 + 2)-cycloaddition and (2 + 2)-cycloreversion reactions of heterocyclic compounds on organic chemistry over the last 10 years is clearly illustrated by several examples. Various members of the important /Hactam antibiotics, penicillin and cephalosporin C, as well as structurally related heterobicyclic compounds have been obtained by (2+ 2)-cycloaddition of heterocycles with ketenes (Section II,D,l).n Intramolecular photochemical (2 + 2)-cycloadditions of 2-pyrones yield 2-oxabicyclo 2.2.01hex-5-en-3-ones, which upon further irradiation afford cyclobutadienes (Section III,D,2).12 Intermolecular (2 + 2)-cyclo-additions of vinylene carbonates with olefins and with acetylenes offer a simple route to cyclobutanes and cyclobutenes, respectively (Sections III,B,3 and 5).13 (2 + 2)-Cycloaddition and (2 + 2)-cycloreversion reactions have contributed substantially to the development of the chemistry... 

On irradiation in butane at -15°C, pyridine is converted into Dewar pyridine 96 (half-life 2.5 min at 25°C). Its structure has been spectroscopically confirmed and it was chemically characterized by reduction with NaBH4 to give the bicyclic azetidine 97, as well as by hydrolysis to the 5-amino-2,4-pentadienal 99 (via the hemiaminal 98). Matrix photolysis of pyridine at 8K leads to acetylene and HCN as the result of a [2+2] cycloreversion of the Dewar pyridine 96, resulting in cyclobutadiene formation. 

MINDO/3 calculations of the cycloreversion of vinylcyclobutane to ethylene and butadiene show a biradical transition state to be involved, and the cycloaddition of cyclobutadiene and acetylene, affording Dewar benzene (4), was also examined. The equilibrium geometry of the latter, predicted by semi-empirical INDO calculations, was in good agreement with experimental results and the calculated dipole moment of < 0.04 D suggests that the likelihood of obtaining a microwave spectrum is marginal at best. ... 

Cycloaddition of species with triple bonds, which should logically be addressed at this point, will be postponed to later chapters. The reluctance of acetylene to dimerize to cyclobutadiene (CBD) on the ground-state surface follows directly from Fig. 6.2. It is sufficient to note that when two acetylene molecules approach one another in the plane-rectangular (D2/1) orientation, the two additional tt orbitals in acetylene are retained as such in CBD, so they cannot alleviate the forbiddenness of the [ 2g + pathway [5, Fig. 4]. Discussion of the reaction between dioxygen and acetylene to form 1,2-dioxetene and the cycloreversion of tetraalkyl-l,2-dioxetanes to two ketonic fragments has to be postponed until the relation between space and spin symmetry has been introduced in Chapter 9. 

Azete 6 crystallizes as reddish needles of mp 37°C. In its chemical properties, 6 shows analogies to cyclobutadienes. Its flash pyrolysis at 700°C gives di-tert-butyl acetylene and tBu-CN AG for this decomposition is very high, since a concerted (2 + 2)-cycloreversion is not allowed according to the Woodward-Hoffman rules ... 

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