Cyclopropylcarbinyl cations secondary

Several secondary and tertiary substituted cyclopropylcarbinyl cations such as dicyclo-propylmethyl cation, (a-methylcyclopropyl)carbinyl cation and l,a,a-trimethylcyclo-propylcarbinyl cation were discussed in the early review11. 

There are several examples of secondary and tertiary cyclopropylcarbinyl cations where equilibria are degenerate i.e., the structures that are equilibrating are identical in the absence of deuterium or any other labels. The degenerate equilibria can be broken at lower... 

The 3-homonortricyclyl cation 70 was prepared by the isomerization of bicyclo[3.2.1]oct-3-en-2-yl cation 71 at 20 °C in SbFs/SC ClF solution83. The ion shows a threefold degenerate rearrangement between -85 °C to 20 °C. At 20 °C the C4, C6, C8 and Cl, C3, C7 carbons become equivalent with an average of 36.19 ppm and 135.8 ppm, respectively (equation 43). Below -80 °C the cation is a static secondary cyclopropylcarbinyl cation with the cationic center chemical shift at <5I3C 234.1. 

Long-lived cyclopropylcarbinyl cation chemistry, including spiro cations and dications, has been reviewed,7 and some of the more interesting newer carbocations, such as (1), are the subject of a short survey.8 The use of secondary deuterium isotope effects in the study of carbocation-forming reactions has been revisited,9 and the... 

The rotational barrier for the secondary I -methyl cation 30 could not be measured experimentally, but the barrier for the tertiary r,T-dimethyl cation 31 could be estimated from the observed line broadening at + 80 °C as 18 kcalmoT Theoretical calculations at the STO-3G level gave a value of 17.7 kcalmoT for the latter cation. The secondary cationic system is a mixture of two isomers (cw-30a and trans-3 h) due to the restricted rotation across the Cl—CT bond. In the tertiary dimethyl cation 31, the methyls expectedly show distinct NMR absorptions <5 H 3.14 (cw-Me), 2.60 trans-Mo) <5 C 30.2 (cw-Me), 33.7 trans-MQ). The cationic center s absorptions are also interesting in that they are highly shielded compared with other related cations, including cyclopropylcarbinyl cations. The tertiary cationic center has <5 C of 249.6, the secondary cationic center <5 C of 220.8 and the primary cationic center <5 C of 191.4. Calculations at MNDO and STO-3G suggested that these cations have enhanced vinyl-bridging character compared to the simple cyclopropylcarbinyl model". ... 

Olah et al. (1976a) continued the studies of potentially degenerate secondary cyclopropylcarbinyl cations incorporated into polycyclic carbon skeletons with the preparation of 2,4-dehydro-5-homoadamantyl cation [144 R = H]. This ion and two 5-substituted tertiary derivatives were prepared from the corresponding 5-R-2,4-dehydrohomoadamantyl alcohols [145 R = H, CH3... 

The cyclopropane ring in both epimers occupies the same position with respect to the departing group both epimers form a stable bisected cyclopropylcarbinyl cation. The exo endo rate ratio can reflect the contribution ratio of steric factors on the formation of the unsubstituted 2-benzonorbomenyl cation out of the two epimers. The hydrolysis of compounds 269 and 270 has resulted in an exo endo rate ratio of 12 and agrees fairly well with the previous value. A comparison of these data with the value of 15000 for the unsubstituted compounds clearly shows the steric and electrostatic effects to have a low value for secondary systems and in the absence of the ring 7i-participation the exo-isomer solvolyzes faster than its endo epimer only 2- to 10-fold. 

Cyclobutyl-Cyclopropylmethyl-Allylmethyl Systems. The long-accepted picture that interconversions amongst the cations in this system take place via a rapid equilibrium of classical structures has recently been criticized by Olah. He has argued, on the basis of n.m.r. evidence, in favour of non-classical a-delocalized structures for primary cyclopropylcarbinyl cations, although recognizing that localized, and even static, structures are involved in secondary and tertiary cases, and especially in the 8,9-dehydro-2-adamantyl series. Hehre and Hiberty have now questioned the (T-delocalized picture of the parent ion presented by Olah, and provide ab... 

The second step in the squalene biosynthesis is the reductive rearrangement of presqualene diphosphate (36a) to squalene. Initial diphosphate abstraction formally gives a primary cyclopropylcarbinyl catimi 37a that rearranges via a secondary cyclobutylium intermediate 38a to the tertiary cyclopropylcarbinyl cation 39a (Scheme 87.12) [176-178]. Hydride attack from NADPH at C3 terminates the squalene biosynthesis. Evidence for the existence of the tertiary... 

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