Bicyclo pentanes diradicals

A practical synthesis of bicyclo[2.1.0]pentane (6) by thermolysis of 2,3-diazabicyclo[2.2.1]-hept-2-ene has been reported in detail.13 It has already been shown that the conversion of 2,3-diazabicyclo[2.2.1]hept-2-ene to bicyclo[2.1.0]pentane involves a diradical intermediate.14... 

Bicyclo[2.1.0]pentane (6) was also generated by benzophenone sensitized photolysis of2,3-diaz-abicyclo[2.2.1]hcpt-2-ene.15 The existence of the intermediate diradical in this reaction was substantiated by a trapping experiment.15... 

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

After a brief historical recapitulation, the substantial body of experimental and theoretical work on these thermal epimerization reactions reported over the past 40 years is summarized. Of primary concern here are examples of stereomutations involving monocyclic, stereochemically unconstricted and minimally substituted molecules. Experimental studies of more heavily substituted cyclopropanes attempts to generate trimethylene diradical intermediates from pyrazolines " and the fascinating and still incompletely understood thermal chemistry of bicyclo[2.1.0]pentanes, 2-methylenebi-cyclop.l.OJpentanes", bicyclo[3.1.0]hex-2-enes and related reactions such as the pyrolysis of cyclopropane at 1200 °C to give products such as cyclopentadiene and toluene are neglected, in spite of obvious mechanistic interrelationships. 

Bicyclo[2.1.0]pentane (62) reacts thermally (120°C, 48 h), with electron-poor olefins via a stepwise, diradical mechanism to afford bicyclo[2.2.1]heptane products derived from the addition of the olefins on the endo side of the bicyclo envelope . On the other hand in the presence of nickel(O) catalysts, the addition occurs under milder reaction conditions and with an alternative stereochemistry, i.e. predominantly on the exo face (equation 42) No class of transition metal catalysed rearrangements has been the subject of more controversy than those of bicyclo butane. A general mechanistic picture, consistent with the experimental facts, has, however, been presented ... 

It has been demonstrated that the addition of olefins and acetylenes to bicyclo[2.1.0]pentane occurs via the formation of a diradical intermediate Literature data support this conclusion for the reactions of bicyclobutane as well. Thus, in the reaction of 35(R = Me) with acrylonitrilethere are two possible cycloadducts, 50 and 51 (equation 69). An ionic reaction in which the olefin serves as an electrophile should lead to an intermediate in which the positive charge is located a to the methyl rather than to the cyano group of the cyclobutyl moiety. Ring-closure in the following step will result in the formation of 51. Yet this product was not observed and the reaction yielded only the second isomer, 50, thus supporting a radical course for the reaction. 

On the other hand, cycloaddition of cyclobutene by bis(methoxycarbonyl)carbene yielded dimethyl bicyclo[2.1.0]pentane-5,5-dicarboxylate (10) as the sole product (55% yield) in the direct photolysis, whilst the triplet carbene reaction in addition to 10 produced two vinylcy-clopropanes 11 and 12 which probably arise from the same intermediate 1,3-diradical. 

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