Vinylcyclopropanes structure

Enantioselective cyclopropanation of monoolefins 214 has also been performed. With the already mentioned chiral catalysts 195a and 209-213 rather high enantiomeric excess was achieved in some cases (Table 16), and the vinylcyclopropane structure was obtained in a subsequent dehydrohalogenation step. 

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. 

Roth et aJ [10] have used CIDNP to study the structures of vinylcyclopropane radical cations fomied from precursors such as sabinene (1). 

The vinylcyclopropane rearrangement is of synthetic importance, as well as of mechanistic interest—i.e. the concerted vs. the radical mechanism. A reaction temperature of 200 to 400 °C is usually required for the rearrangement however, depending on substrate structure, the required reaction temperature may range from 50 to 600 °C. Photochemical and transition metal catalyzed variants are known that do not require high temperatures. 

Vinylcyclopropane reacts with HBr to yield a rearranged alkyl bromide. Follow the flow of electrons as represented by the curved arrows, show the structure of the carbocation intermediate in brackets, and show the structure of the final product. 

Viagra, preeclampsia and, 164 structure of, 1 Vicinal, 261, 662 Vinyl group, 178 Vinyl monomer, 241 Vinylcyclopropane, rearrangement of, 1202... 

A more detailed evaluation of the diverse structures proposed for the secondary species goes beyond the scope of this review. We mwely emphasize that the ESR results provide detailed evidence for the nature of the radical center, but fail to elucidate the cationic site. The identity of this center is left to secondary considerations or speculation. We also note that any alternative structure has the virtue of not contradicting the ab irutio calculations the potential c ture of chloride ion has precedent in the nucleophilic substitution at a cyclopropane carbon (see Section 7). Another type of ring-opened structure has been postulated as an intermediate in the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes (see Section 5). 

On the other hand, the CIDNP approach and recent ab initio calculations have provided what we consider unambiguous evidence for the nature of the vinylcyclo-propane radical cation as well as for some simple derivatives. We have carried out ab initio calculations on the prototype 19 and several simple derivatives, and probed by CIDNP the structures of three simple vinylcyclopropane radical cations in which the two functionalities are locked in a syn configuration. 

The nature of vinylcyclopropane radical cations was elucidated via the electron transfer induced photochemistry of a simple vinylcyclopropane system, in which the two functionalities are locked in the anri-configuration, viz., 4-methylene-l-isopropylbicyclo[3.1.0]hexane (sabinene, 39). Substrates, 39 and 47 are related, except for the orientation of the olefinic group relative to the cyclopropane function trans for 39 versus cis for 47. The product distribution and stereochemistry obtained from 39 elucidate various facets of the mechanism and reveal details of the reactivity and structure of the vinylcyclopropane radical cation 19 . 

Basic structures used so far comprise bicyclo[l. 1 OJbutanes1 and bicyclo[2.2.0]hexanes2 for the cleavage of electrophilic attack on jr-bonds. 

TABLE 3. Structural parameters of vinylcyclopropane (37) and trans-1,2-di vinylcyclopropane (38) ... 

The history of vinylcyclopropane goes back to Gustavson s reported synthesis of vinyl-cyclopropane ( vinyltrimethylene ) in 189678, a publication which occasioned considerable controversy, for the major C5H8 compound actually formed in his preparation proved to be spiropentane. Authentic vinylcyclopropane was secured by Demjanow and Dojarenko in 1922", but nevertheless both structures remained in dispute. Authorities as eminent as... 

C. K. Ingold denied the existence of spiropentane, even as an unstable intermediate product80 F. C. Whitmore considered both spiropentane and vinylcyclopropane to be incapable of existence 81. Whatever the doubts then, they have not survived the structures of cyclopropane, vinylcyclopropane and spiropentane are now known in considerable detail82". ... 

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. 

All of the experimental and theoretical work on the stereomutations of cyclopropanes and vinylcyclopropanes covered above seems consistent with and understandable in terms of kinetically significant involvements of Cj(ts), Cs(ts) and EF(ts) structures and partitionings of EE trimethylene intermediates resulting in the formation of klt k2 and kl2 products at comparable rates. For trans-1,2-disubstituted cyclopropanes, neither the Smith mechanism (one-center stereomutations only) nor any two-center-only formulation can be correct, as demonstrated by Crawford and Lynch in 1968143 and reinforced by numerous subsequent studies (Figures 2 and 3). 

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. 

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