Equilibria and barriers to interconversion between the syn and anti conformers in 9-(o-substituted phenyl)fluorenes obtained by photorearrangement of triptycenes
✍ Scribed by Hideyuki Tukada; Michiko Iwamura; Tadashi Sugawara; Hiizu Iwamura
- Publisher
- John Wiley and Sons
- Year
- 1982
- Tongue
- English
- Weight
- 684 KB
- Volume
- 19
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
The population ratios of the syn to the anti conformers in 9-(2-~ubstituted phenylMuorenes have been determined by 'H NMR spectroscopy to range from 0.25 (for 2'-NHz) to >20 (2'-CH=NNH-CJ&(NOZ)-2,4). They appear to increase, firstly, with the steric bulk of the 2'-substituents and, secondly, with the electronic repulsion between the substituent and the IT electrons on the fluorene ring. The unusually low population of the syn isomer in the 2'-NHZ compound was found by IR absorption to be due to stabilization of the anti form by N-H --IT hydrogen bonding. Whereas the substituents on the fluorene ring do not affect the population of the two isomers in 9-(2-tolyl)flUorenes, the same series of substituents exhibit a positive slope in the Hammett plots of the equiliirium coush~ts in 9-(2-methoxymethylphenyl)~orenes.
Through-space interaction between the methoxymethyl group and the wdonating fiuorene ring is postulated.
The free energy of activation ( A d ) for the interconversion between the two conformers in 9-(2-substituted pheny1)fluorenes have been obtained from the he-shape dependence on temperatures. The A@ values ranged from 56.8 (for 2'-NHZ) to 78.6 W mol-' when there is no fluorene ring substbent In tbree typical examples, a full line-shape analysis gave, additionally, the A H and A 9 data of the interconversion. Introduction of a methyl group at position 1 of the fluorene ring raised A d by c. 13 kJ mol-'.