Diradicals vinylcyclopropane

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 ... 

In addition to cyclopentenes, other types of compounds may be formed upon heating of vinylcyclopropanes. For example pentadienes 6a/b may be formed by a competitive route from a diradical intermediate. 

A further point of interest is the reaction regiochemistry. For the reaction in Scheme 2 we might consider two possible vinylcyclopropane products. Thus the intermediate cyclo-propyldicarbinyl diradical might open in two ways. However, only one product is formed. In Scheme 3 this is shown more definitively where the two modes of unzipping the cyclopropyl dicarbinyl diradical are depicted. A general rule is that the less stabilized diradical center utilizes its odd-electron density to open the three-membered ring. This leads onwards to the more stabilized Diradical II 3. 

The relatively constant kinetic disadvantage experienced by an average vinylcyclopropane rearrangement, compared with a (kt + k2) stereomutation, amounts to a A A (7 of about 3 kcal moT1. This may well be associated with configurationally distinct sets of diradical structures with those of E stereochemistry favored thermodynamically over those of Z stereochemistry by about 3 kcal mol 1. Only the latter may lead to cyclopentene products212. 

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. 

The AG1 values for vinylcyclopropane to cyclopentene isomerizations show the same sensitivity to radical stabilizing substituents, implying that they too involve diradical transition structures. 

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. 

The di-7r-methane rearrangement, takes place on structures that possess two -groups connected with a single carbon (the central carbon), upon direct and/or sensitized irradiation of a C=C double bond chromophore, affording vinylcyclopropane products. Zimmerman proposed a stepwise cyclization of the excited --system via a 1,4-diradical of... 

Vinylcyclopropane-Cyclopentene, Bergman, Di-jr-methane, and Related Diradical Rearrangements... 

Fig. 7. The [1,3] sigmatropic shift of vinylcyclopropane and two transition structures involved in the reaction. Left transition structure for the concerted process right transition structure for the bond closure in the diradical pathway...

Most of the radical stabilization energy parameters utilized in Figure 2 (R = H or D, -2.65 Me or Et, - 0.5 CN, 6.6 vinyl or propen-2-yl, 13.2 (E )-propenyl, 15.4) are SE values calculated by Leroy and coworkers " those for phenyl (9.4) and cinnamyl (19.1) were set empirically. These particular parameters are serviceable and reasonable, but cannot be considered preferred . Other plausible radical stabilization energy values lead to very similar correlations which demonstrate, as Figure 2 demonstrates, that the rate constants for two-center epimerizations of cyclopropanes and vinylcyclopropanes are sensitive to the radical stabilizing capacities of substituent groups in a simple additive fashion. Whatever 1,3-disubstituted trimethylene diradical intermediates or transition structutes may be involved respond kinetically to substituents as though each locus of radical character were thermochemically independent. 

Halogen and thiol migration, and radical bridging 29 Diradicals in the vinylcyclopropane rearrangement 30 Trimethylenemethane 30... 

An extension of the simplistic but practical diradical mechanism of equation (1) leads to the generalized di-TT-methane process given in equation (43). Thus, the initial 1,3-diradical, instead of cyclizing across C-3 and C-5 to the final vinylcyclopropane, bonds across C-2 and C-5 to generate a cyclopro-pyldicarbinyl diradical, which on opening of the C-2/C-4 bond affords the isomerized 1,4-diene. A num-... 

The retention of stereochemistry at C-1, as, for example, in the direct photolysis in equation (41), implies that rotation between C-1 and C-2 in the cyclopropyldicarbinyl diradical does not occur. Study of the stereochemical event at the central carbon atom requires optically active substrates containing an asymmetric C-3 carbon atom. The example of equation (49) and all other cases studied demonstrate that in the two diastereomeric vinylcyclopropanes which form, the central carbon atom has inverted its configuration. Knowledge of the stereochemical fate of C-5 can be gained by inspection of the examples shown in equation (50). ° Rearrangement of the cis and trans isomers of the diene gave the cis and trans vinylcyclopropanes, respectively. ... 

A similar acceleration has most recently been observed in the rearrangement of vinylcyclopropanes of type (39 Scheme 8). This fluoride-mediated vinylcyclopropane-cyclopentene isomerization proceeds at -78 C to give (40) in 85% yield this is to date the mildest condition available. Two possible intermediates, the enolate anion (39a) or the diradical anion (39b), may be responsible for such acceleration in analogy to the enolate anion accelerated divinylcyclobutane rearrangement recently reported." The mechanism of this transformation is unclear but may involve anion acceleration similar to that observed in the rearrangement of sulfonyl anions derived from (42 Scheme 8). By comparison the thermolysis of (39) produced exclusively the endo isomer of (41) at 580... 

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