Cyclopropane from trimethylene diradicals

Thus a substantial body of experimental evidence shows that 1,2-disubstituted cyclopropanes, including vinylcyclopropanes, react thermally to give isomeric cyclopropanes through both one-center and two-center epimerizations, with (kt + k2) kl2 ratios from 1.4 to 4. Rate constants for both (, + k2) and kl2 events respond to the capacity of substituents to stabilize adjacent radicals in a regular fashion consistent with trimethylene diradical transition structures. Rate constants for vinylcyclopropane structural isomerizations do as well, thus reinforcing the notion that these reactions are nonconcerted diradical mediated reactions. 

In addition some p.opane and n-hexane are formed. These are also believed to originate from the trimethylene diradical. A direct comparison of these results with the results reported by other workers is not possible since the only other study in the presence of a foreign gas involved water vapor at a pressure of 70 mm. (5). In the latter instance the ketone pressure was 136 mm., which is 5 to 10 times greater than the pressure used in ref. (17). The presence of water vapor does not seem to have affected the yields of propylene and of cyclopropane. Since the diradical mecha-... 

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. 

Enough substituted cyclopropanes have now been subjected to careful kinetic studies so that a characteristic pattern of reactivity and stereochemical preferences has emerged. Substituents facilitate stereomutations in proportion to their ability to stabilize 1,3-trimethylene diradical structures. The values for both k 2 and (k + k2) stereomutation rate constants relate linearly with consistent measures of substituent radical stabilization energies with equal sensitivities. Experimentally determined (A , + / 2)- i2 ratios do not vary widely they range from 1.4 to 2.5 over a fair diversity of substituents. Neither do kf.kj ratios vary widely. The majority fall between 1 1 and 2.5 1 the largest yet reported gives 2(CHD) a symmetry corrected kinetic advantage over A i(CDPh) in 1-phenyl-1,2,3-d3-cyclo-propanes of 5 1. 

The involvement of trimethylenemethane diradicals in deazetization of diazoalkane-allene adducts or trimethylene diradicals in the deazetization of the adducts of acyclic alkenes often leads to mixture of regioisomers and stereoisomers and from the standpoint of cyclopropane syntheses, this is undesirable. Far fewer problems of this type attend deazetization of the adducts of cyclic or polycyclic alkenes and, furthermore, even a modest amount of strain in the system activates the alkene to diazoalkane addition so that there is no need for activating substituents on the double bond. Cyclopropene is highly reactive towards diazoalkanes (see also Section 1.1.5.1.5.3.1.) and cycloaddition reactions of this type provide a ready entry into the bi-cyclo[1.1.0]butane series. The addition of diphenyldiazomethane to cyclopropene gave 4,4-diphenyl-2,3-diazabicyclo[3.1.0]hex-2-ene (1), which on photolysis gave a mixture of 2,2-diphenylbicyclo[1.1.0]butane (2) and 1,1-diphenylbuta-l,3-diene (3). ... 

Calculations and Experiments on the Stereomutation of Cyclopropane. In 1965, Hoffmann published a seminal paper on trimethylene, another name for propane-1,3-diyl (8). He used extended hiickel (EH) calculations and an orbital interaction diagram to show that hyperconjugative electron donation from the central methylene group destabilizes the symmetric combination of 2p-n AOs on the terminal carbons in the (0,0) conformation of this diradical. Hoffmann s calculations predicted that the resulting occupancy of the antisymmetric combination of 2p-n AOs in 8 should favor conrotatory opening of cyclopropane (7), as depicted in Figure 22.8. 

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