Cyclopropylcarbinyl cations tertiary

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

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. 

The tertiary 2-methyl-8,9-dehydro-2-adamantyl cation 25, R = Me is stable up to 10 °C, and shows a much deshielded absorption for the cationic center (<5I3C 274.4), which is relatively shielded compared with the 2-methyl-2-adamantyl cation (<513C 323) showing some charge delocalization into the cyclopropane ring. The C8 and C9 carbons are also deshielded (<5I3C 100.7), comparable to other static cyclopropylcarbinyl cations. 

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... 

Evidence has been obtained467 for the involvement of a tertiary cyclopropylcarbinyl cationic intermediate in the rearrangement of presqualene diphosphate to squalene. 16-Oximino-17a-benzyl-17//-hydroxy derivatives in the androstane and estrane series have been converted into 16-oxo-17//-benzyl-17a-hydroxy derivatives with inversed configuration at C(17), on treatment with titanium trichloride. It has been suggested468 that the rearrangement occurs through the key intermediate (401) (see Scheme 97). 

Evidence has been obtained467 for the involvement of a tertiary cyclopropylcarbinyl cationic intermediate in the rearrangement of presqualene diphosphate to squalene. 

Similar results are obtained for the corresponding tosylates 281).In conforma-tionally nonrigid systems, on the other hand, where the preferred bisected (97) conformation may be adopted, cyclopropyl substituents enhance solvolysis rates by a factor of approximately 104 30S). Therefore, a free energy difference between 97 and 98 of about 10 kcal/mole (1.36 log 107) may be estimated for tertiary cyclopropylcarbinyl cations at 25 °C28°). 

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 stereochemistry just described also applies to the transformations of optically active (521) which ionizes to give two distinct bisected cyclopropylcarbinyl cations in a 3 1 ratio407. The tertiary cyclopropylcarbinyl derivative (522), on the other hand, solvolyzes without equilibration of the C(CH3)2 groups4087. A cyclopropylcarbinyl rearrangement with inversion at the migrating carbon is also necessary for... 

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... 

Although the homoallylic- cyclopropylcarbinyl cyclization is well-precedented in carbonium ion chemistry (101, 102) there seem to be no reports of the direct cyclization of the tertiary 4-terpinenyl carbonium ion. However, deamination of cyclohex-3-enyl amine and solvolysis cyclo-hex-3-enyl tosylate gives exo- and n /o-bicyclo[3.1.0]hex-2-yl derivatives as 6—43% of the products resulting from nucleophilic capture (101, 103, 104). The modest yield of bicyclic products in these reactions apparently is the result of competing nucleophilic capture prior to cyclization and hydride shift to the 2-cyclohexenyl cation. More efficient cyclization occurs in the acetolysis of 2-bicyclo[2.2.2]oct-5-enyl tosylate (49) owing to the rigid boat-like conformation of the precursor (105). The high efficiency of the base-catalyzed cyclization of the epoxide of 4-iso-... 

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