Cyclopropanes diradical opening

Can Hyperconjugation in a 1,3-Diradical Control the Stereochemistry of Cyclopropane Ring Opening and Make a Singlet the Electronic Ground State of the Diradical ... 

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

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. 

Figure 22.10. Rearrangement products (13), predicted to be formed from 1,1-disilyl-cyclopropanes (11) by conrotatory ring opening, followed by a 1,2-sbift of a trimethylsilyl group in the 1,3-diradicals (12) generated. Pyrolysis of 11b should lead to a mixture of E/2) isomers (13b), but pyrolysis of 11c is predicted to form only the E stereoisomer (13c). ...

A related example is given in equation 1751. Here, both roles of the n-7t excited state are involved. The antibonding electron density opens that a,/J bond which will afford a dicyano-stabilized odd-electron center. Its oxygen p v orbital attacks the ipso carbon of the phenyl group on the cyclopropane ring to give the spiro-diradical species shown. This then opens to photoproduct. 

While there remain open questions on a few experimental aspects of early work on the stereomutations of 1 -phenyl-2-d-cyclopropanes and 1,2-d2-cyclopropanes, the preponderance of data and theory now provides a consistent understanding of the thermal stereomutations of cyclopropanes multiple paths and three types of diradical transition structures are involved. Evolving theory relevant to cyclopropane stereomutations and to vinylcyclopropane to cyclopentene isomerizations, and to other 1,3-carbon shifts283 288, may well provide more detailed insights, rationales and predictions. 

Fig. 5. Potential energy curve for the ring opening from cyclopropane to the face-to-face diradical. The energies are in kcal/mol...

In this case, one of the two ir-bonds is a cyclopropane unit and a vinyl cyclobutane is formed (equation 17), the photorearrangement still resembling closely the di-i -methane process of equation (1). Such an unsymmetric substrate should in principle afford, besides the cyclobutane (path b), also dicyclopropane (path a) by opening of the other o-bond in the cyclopropyl diradical intermediate this pathway has not been observed so far. 

On the basis of the general mechanistic concept used in Charts 11 and 12, the transformations of the steroid ketones 21 and 22 (Chart 3) can be rationalized as follows photoproducts 23, 24, and 26 are derived from ketone 21 via the common intermediate species 93, which may be produced upon 1,5-bridging in the excited state 91 and subsequent homolytic fission of the 1,10-cyclopropane bond in the resulting diradical 92 (Chart 13). Bonding between C-1 and C-6 in the key intermediate 93 (->- 94) and opening of the three-membered ring gives access to struc-... 

The cyclopropane ring is cleaved via the induced ring-opening of a cyclopropylcarbinyl radical to form a diradical which in turn cyclizes intramolecularly to afford the reaction product. 

Dispiro[anthrone-10,r-cyclopropane-2, 10"-anthrone] (15) in benzene or dichloromethane at room temperature ring opens to give a diradical 16 that undergoes cycloaddition with molecular oxygen. ... 

Cycloaddition reactions of dienophiles to extremely strained cyclopropanes, such as bicyclobutanes or bicyclopentanes (Table 1), have been reviewed by Gassman. The rate-determining step of these reactions, most probably, is the formation of a diradical intermediate for which mainly two pathways are open for recombination to a closed-shell product a) ring closure to give bicyclic structure 34 (formal [27r- -2ff] cycloaddition), and b) hydrogen shift to give the homo-ene-type product 35 (formal [27 -l-2(r- -27t] addition). 

Unlike the photochemical [27r + 2ff] reaction, there is no significant rate dependence on exo or endo orientation of the cyclopropane ring in the phenyl-substituted systems (Table 3). Most probably, cycloaddition is preceded by homolytic ring opening of the cyclopropane ring to give a diradical, which is stabilized by the phenyl substituents. Nonactivated systems react at temperatures from 180 to 200 "C in solution or at 500-600 °C under flash pyrolysis conditions. 

Cyclopropane (and its derivatives) can undergo thermally induced ring opening to a 1,3-diradical with subsequent H-shift to propene. Stereochemical, kinetic and mechanistic aspects of this reaction for cyclopropane, substituted cyclopropanes and cyclopropane-containing polycyclic structures have been exhaustively reviewed. ... 

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