Cyclopropylcarbinyl systems

Since Roberts . work, A,great deal of evidence, both experimental and theoretical, has accumulated that indicates that the hicyclobutoninm ion is not the first-formed ion upon solvolysis of unstrained cyclopropylcarbinyl systems. Instead, the structure of the ion apparently is the bisected cyclopropylcarbinyl cation, which is shownin ajlatted-line formulation in 67 aniirr.airorbital dia-, gram fn Figure 9. (See also Section 10.6.)... 

Figure 6.12 (a) Rearrangement in the cyclopropylcarbinyl system, (b) Rearrangement in the dimethyl cyclopropylcarbinyl system. From W. J. Hehre and P. C. Hib-berty, J. Amer. Chem. Soc., 96, 302 (1974). Reprinted by permission of the American Chemical Society. 

The 8,9-dehydro-2-adamantyl cation (101) incorporates a cyclopropylcarbinyl system in the preferred bisected (97) conformation. As would be expected, then, 8,9-dehydro-2-adamantyl derivatives (e.g.102) are highly... 

Substituted cyclopropylcarbinyl systems that are free to adopt the optimum conformation around the bond from cyclopropyl to the cationic center are considred in this section. 

FIGURE 4. Overlap angles and relative reactivities for cyclopropylcarbinyl systems... 

Long-lived ions derived from the substrates 89 and 90 have also been studiedand a topic of particular interest has been the 7-norbornenyl cation (98a) which, as shown in 98b, is a cyclopropylcarbinyl system in which 7r-donation from one of the ring C-C bonds is effective. These ions may be derived from cyclopropylcarbinyl precursors (93) or, more usually, from 7-norbornenyl substrates and have been exhaustively studied, as summarized in some recent reports ". ... 

The area of free-rotating cyclopropylcarbinyl systems can be separated into two major categories. The first is where the cyclopropane ring is not contained as part of another ring. 

The investigations carried out in this area were done primarily to determine the magnitudes of steric and electronic effects on the solvolytic rates and products of reaction in the cyclopropylcarbinyl cation system. The goal of most of these studies was to learn more about the nature of the charge delocalization in the cyclopropylcarbinyl system and of the stereochemistry of the cyclopropylcarbinyl-cyclobutyl and cyclopropyl-carbinyl-allylcarbinyl cation rearrangements. Key papers in these studies were those in 1966 by Schleyer and Van Dine, in 1971 by Majerski and Schleyer and in 1974 by Poulter and Spillner which demonstrated that in the simple cyclopropylcarbinyl system... 

Although the previous reviews adequately cover the effects of simple alkyl substituents in cyclopropylcarbinyl systems, they do not mention the case where the cyclopropane ring is part of a spiro system. Thus, Wilcox and Hsu compared the hydrolysis rate in 70% aqueous acetone at 125°C of 30 having a cyclopropylcarbinyl 3,5-dinitrobenzoate attached spiro to the 7-position of norbornane with that of 3,3-dimethylcyclopropylcarbinyl 3,5-dinitrobenzoate (29). It was found that with a larger spiro ring as in 30, the effect on rate was small ( 29/ 30 = 1 ) No products were reported in this study. 

Finally, in a recent paper DeLucca and Paquette have examined the solvolyses of several trimethylsilyl-substituted cyclopropylcarbinyl trifluoroacetates in 2,2,2-tri-fluoroethanol. This study was initiated to learn of the nature of trimethylsilyl substituent effects in cyclopropylcarbinyl systems. The kinetic results obtained are shown below... 

A second important class of free-rotating cyclopropylcarbinyl systems is that in which the cyclopropane ring is also part of another ring or rings as in 32 and 33. These studies... 

An interesting recent case of a solvolytic study of a fused-ring cyclopropylcarbinyl system is that of Hudrlik and coworkers on 38 as part of a synthesis of a-methylene-a-butyrolactones. 

The solvolytic rates and products for a variety of mono- and dispiro-cyclopropylcarbinyl systems have been investigated. These were done primarily to assess the importance of the geometrical orientation of a cyclopropyl group for maximum electron release to an adjacent developing cationic center. 

In similar studies by Martin and Ree and Schleyer and Buss " which were published simultaneously, the problem of locking the cyclopropylcarbinyl system into the energetically unfavorable perpendicular conformation was solved by use of a spiroadam-antyl system. The relative rates shown in Scheme 6 were measured in acetic acid at 45 °C. 

For the various methyl-substituted 2-bicyclo[4.1.0]heptyl 3,5-dinitrobenzoates , in all cases the exo isomers were slightly more reactive than the endo. The k Jky values for the various methyl substituents, which were as expected based on studies involving open-chain cyclopropylcarbinyl systems were 1-methyl = 2 2-methyl = 10 6-methyl = 10 antz-7-methyl = 6. In a study using a 1-phenyl substituent a rate decrease by a factor of about 10 was observed. This may be due in part to electron-withdrawing inductive effects of the phenyl group and in part to steric hindrance to solvation for the cyclopropylcarbinyl system. 

As part of their study of geometrically constrained cyclopropylcarbinyl systems, Rhodes and DiFate studied a tertiary 2,5-ethano bridged 2-bicyclo[4.1.0]heptyl tosylate (82). 

Starting with a cyclopropylcarbinyl system, ring-closure via formation of a side C-C bond in bicyclobutane can be affected by several processes. 

An interesting application of this chemistry involves cyclopropylcarbinyl systems that generate cyclo-pentenes via a [l,3]-alkyl sigmatropic rearrangement. The prototype examples involves heating a vinyl-... 

As can be seen from the above data, the exo-epimer 214 solvolyzes at a rate typical for secondary cyclopropylcarbinyl systems e.g., 216 ai the same time the endo epimer 215 is by 6 orders of magnitude more reactive. This difference is explained since the participation of the C —C is only possible in compound 215. 

The interaction of the cyclopropane ring and the neighbouring vacant p-orbital in a-cyclopropylcarbinyl systems has been well investigated in both nucleophilic and superacid media . 94 -697) stable long-living a-cyclopropylcarbinyl ions have been generated and studied by Olah in recent years the research has been... 

Cleavage of cyclopropylcarbinyl systems (5, 317). This useful step in a prostaglandin synthesis has been improved by conversion of the diol (1) into the... 

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