Measuring aromaticity with the dimethyldihydropyrene ring current probe. experimental and computational studies of the fulvenes and the strongly antiaromatic cyclopentadienone reveal large mills-nixon-type Bond localization effects. synthesis of fulvene-fused dihydropyrenes
Journal of the American Chemical Society, ISSN: 0002-7863, Vol: 131, Issue: 1, Page: 189-199
2009
- 24Citations
- 22Captures
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Article Description
The synthesis of the methylfulvene-and phenylfulvene-annelated dihydropyrenes 10 and 22 from the cyclopentadiene-fused dihydropyrene 7 in 68% and 80% yields, respectively, are reported. However, the attempted formation of the parent fulvene-fused dihydropyrene 18 failed, both from the cyclopentadiene 7 with formaldehyde and from the cyclopentadienone 5 in Wittig-type reactions. Chemical shift data for the methylfulvene (35) and phenylfulvene (36)-fused dihydropyrenes 10 and 22 were used to estimate the reduction in the dihydropyrene nucleus aromaticity (DHPN) (relative to benzene fusion) in 10 and 22 12-16% and 22-25% respectively). Calculations revealed that this reduction in diatropicity, contrary to the situation with benzene fusion, is not due to any aromaticity of the annelating fulvenes but instead is caused by Mills-Nixon-type effects. We conclude that methyl-and phenylfulvene are nonaromatic. An improved synthetic route to the cyclopentadienone 5 was found in an unprecedented cyclization of the frans-cinnamic acid analogue 29 in 80% yield. This enabled an X-ray structure of 5 to be obtained, for comparison to that of the saturated ketone 4. Even though crystals of 5 and 4 show diastereomeric disorder, when the average bond length data of cyclopentadienone 5 is compared with those of cyclopentenone 4 and the parent and benzo dihydropyrenes 6 and 33, it is clearly evident that 5 has the opposite bond-alternation pattern, consistent with a [4n] fused annulene. From the bond length data, cyclopentadienone has ∼87% of the effect of a benzene ring on bond alternation, which is in reasonable agreement with the previously found NMR value (78%). Structure and nucleus-independent chemical shift calculations support these results. © 2009 American Chemical Society.
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