An NMR study of the rotational barriers in cobalt-stabilized carbocations: X-ray crystal structures of (η4-C4Ph4)Co-(η5-C5H4R), where R is CH3C=O, CH=O, CH(t
- 作者：Ortin ; Yannick ; Ahrenstorf ; Kirsten ; O'Donohue ; Paul ; Foede ; Daniel ; Mü ; ller-Bunz ; Helge ; et. al.
- 关键词：Cyclobutadiene&ndash ; cobalt complexes ; Metal-stabilized carbocations ; NMR ; X-ray crystallography
- 刊名：Journal of Organometallic Chemistry
- 出版年：2004
- 期刊代码：64_0022328x
- 类别：ch
- 出版时间：May 1, 2004
- 卷：689
- 期：9
- 页码：1657-1664
- 文件大小：404 K
摘要
Treatment of the aldehyde (η4-C4Ph4)Co(η5-C5H4-CH
O) (4b) with tert-butyllithium or phenyllithium yields the secondary alcohols (η4-C4Ph4)Co(η5-C5H4-CH(R)OH), where R=tert-butyl (5) or phenyl (6). Protonation of 5 and 6 at −80 °C furnishes the deep purple, cobalt-stabilized cations, 7 and 8, respectively, both of which exhibit restricted rotation about the external C5H4–CHR+ linkage on the NMR time-scale. These data indicate a minimum value for the barrier to rotation of 15 kcalmol−1, but it is certainly much higher, indicating a considerable degree of C–C double bond character. X-ray crystal structures of 4b, 5 and also of the ketone (η4-C4Ph4)Co(η5-C5H4-C(O)CH3 (4a) are reported. The secondary alcohol 5 exhibits disorder in the solid state because of the presence of diastereomers as a consequence of the stereogenic center at the α-carbon and the clockwise or anticlockwise propeller orientations of the tetraphenylcyclobutadiene ligand.
O) (4b) with tert-butyllithium or phenyllithium yields the secondary alcohols (η4-C4Ph4)Co(η5-C5H4-CH(R)OH), where R=tert-butyl (5) or phenyl (6). Protonation of 5 and 6 at −80 °C furnishes the deep purple, cobalt-stabilized cations, 7 and 8, respectively, both of which exhibit restricted rotation about the external C5H4–CHR+ linkage on the NMR time-scale. These data indicate a minimum value for the barrier to rotation of 15 kcalmol−1, but it is certainly much higher, indicating a considerable degree of C–C double bond character. X-ray crystal structures of 4b, 5 and also of the ketone (η4-C4Ph4)Co(η5-C5H4-C(O)CH3 (4a) are reported. The secondary alcohol 5 exhibits disorder in the solid state because of the presence of diastereomers as a consequence of the stereogenic center at the α-carbon and the clockwise or anticlockwise propeller orientations of the tetraphenylcyclobutadiene ligand.