Cyclopropylcarbinyl cations primary

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). ... 

III. PRIMARY CYCLOPROPYLCARBINYL CATIONS A. Equilibrating (Degenerate) Cations... 

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". ... 

The parent spirocyclopropylbenzenium ion 102 can be prepared by the ionization of -phenylethyl chloride in HF-SbF5-S02ClF at -90 °C followed by warming to -60 °C. At higher temperatures (-27 to -5 C), the ion isomerizes to the a-methylbenzyl cation 103 with an activation energy of 13 kcalmol as shown by the NMR kinetic study (equation 61). The rearrangement probably proceeds through a partially delocalized primary 2-phenylethyl cation 104 ". The NMR spectrum of the ethylenebenzenium ion 102 showed (5 C 68.8 (Cl), 171.8 (C2, C6), 133.4 (C3, C5), 155.4 (C4), 60.7 (CH2). The equivalence of the C3, C5 and C2, C6 carbons as well as the methylene carbons show that the ion has Qv symmetry. The deshielded absorptions for the CH2 carbons are similar to those in other cyclopropylcarbinyl cations. 

All cyclopropylcarbinyl cations show some charge delocalization into the three-membered ring, but only the primary system (29) is unique in its chemical shifts which suggest a delocalized structure61. The C—1 —H coupling constants, however, do not support this conclusion. The non-equivalence of the methyl groups establishes the bisected geometry of (31), with slow rotation about the exocyclic C—C bond. The... 

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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