Substituent group, crystallographic studie

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroanthnonate salt at —124 °C. It is nearly planar (8 ° twist), with a short bond between the C " and the ring (1.41 A), consistent with benzylic delocalization. The Me—C bonds are also shortened, indicative of hyperconjugative interaction. However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5). The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation. The heats of formation of some a-methylbenzyl cations indicate that the primary stabilization in these species comes from the a-substituents, and that the stabilization provided by the aromatic ring is secondary. ... 

The introduction of a methyl group at C(2) of aminocyclohexenes causes a large downfield shift (+ 22 to +26 ppm) of the resonance of this carbon which can be explained by a large allylic 1,3-strain ( 4 ) in the tetrasubstituted coplanar enamine. Theoretical calculations - as well as the results of photoelectron spectroscopy and X-ray crystallographic studies indicate that in unhindered enamines with the nitrogen lone pair nearly orthogonal to the C=C plane, one of the substituents of the flattened... 

Scheme 13) in which one ester substituent is isopropyl and and the other is sec-butyl (43) this material was prepared in both optically active form using (S)-(+)-sec-butanol and in racemic form using ( )-sec-butanol. X-ray crystallographic studies revealed that both foims are isostructural with the diisopropyl derivative 22b and crystallize in the chiral space group P2 2 2 with disorder in the sec-butyl group. Compound 26 thus resembles diacrylate 6c studied by Addadi and Lahav (19a,b) in that it... 

Since chasmanine and neoline form identical derivatives on methylation with methyl iodide-sodium hydride,chasmanine (18) must also contain a la-substituent, and it is therefore clear that the reported correlation of chasmanine with browniine must be in error. The previous conversion of chasmanine diacetate into homochasmanine permits assignment of structure (19) to the latter. A n.m.r. study of chasmanine (18), delphonine (20), neoline (12), and delphisine (11) supports the assignment of the 1 a -groups in structures (11),(12),(18), and(19). In view of the X-ray crystallographic studies on delphisine, these correlations also define the absolute stereochemistries of these alkaloids. 

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