41983-AC1
The Quest for Hydrocarbon Analogues of the Porphyrins
Porphyrin analogues are widely studied, including carbaporphyrins and N-confused porphyrins (NCPs). In carbaporphyrins, NCPs and related systems like azuliporphyrins, one of the interior nitrogen's of the porphyrin macrocycle has been replaced by a carbon atom.1-3 Several examples of dicarbaporphyrinoid systems with two internal carbons are known, but tri- and tetracarbaporphyrinoid structures have yet to be synthesized.1-3 In our recent studies, two new dicarbaporphyrinoid systems have been synthesized and related heterocarbaporphyrins have been investigated in detail. A series of azuliporphyrins were prepared from 6-tert-butyl and 6-phenylazulene, and these showed superior solubility characteristics that allowed high quality proton and carbon-13 NMR spectra to be obtained.4,5 The internal CH and NH resonances were observed near 3 ppm, although the precise values were dependent upon substituent effects. The presence of a tert-butyl group on the azulene moiety slightly enhanced the diatropicity of the macrocycle compared to the phenyl substituted azuliporphyrins. A tert-butyl substituted azuliporphyrin also gave X-ray quality crystals and this allowed the first structural analysis of a free base azuliporphyrin to be conducted.5 The macrocycle is near planar and the azulene unit was only tilted out of the plane by 7.4o. An analysis of the bond lengths suggests that a 17 atom delocalization pathway significantly contributes to the aromatic properties of this system. Protonation, oxidative ring contraction, and metalation studies were also conducted on these azuliporphyrins.4,5 These studies have been extended to the synthesis of heteroazuliporphyrins.6 The first examples of 23-oxaazuliporphyrins were obtained by reacting azulitripyrranes with 2,5-furandicarbaldehyde. 23-Thia- and 23-selenaazuliporphyrins were also prepared by this approach and these analogues were further characterized by X-ray crystallography.6 As was the case for azuliporphyrins, the presence of a tert-butyl substituent did not inhibit oxidative ring contractions to heterocarbaporphyrins.6
A general route to fulvene aldehydes was developed by reacting an indene enamine with aromatic aldehydes in the presence of Bu2BOTf.7 This methodology was used to prepare an azulene-fulvene dialdehyde that reacted with a dipyrrylmethane in the presence of TFA to give a 22-carbaazuliporphyrin.7 This strategy was also used to prepare a 22-oxacarbaporphyrin using a furan derived fulvene dialdehyde.8 The adj-dicarbaporphyrin system with an indene unit adjacent to an azulene moiety proved to be far more stable than previously synthesized dicarbaporphyrins. This encouraged us to investigate the synthesis of adj-diazuliporphyrins from a diformyl diazulenylmethane using a MacDonald '2 + 2' condensation. Excellent yields of mesoionic porphyrinoid products were obtained.9 These fully conjugated macrocycles showed significant diatropicity, and the internal CHs gave an upfield resonance near 0 ppm. Preliminary investigations show that the corresponding palladium(II) complex can also be formed under mild conditions. This stable organometallic derivative has two carbon-palladium bonds and must exist as a mesoionic structure. This unprecedented derivative has been further characterized by X-ray crystallography, and takes on a near planar conformation.9 Investigation of higher order carbaporphyrinoid systems continues to produce novel structures with unique characteristics and future investigations are likely to lead to further modified structures. Significant investigations on the synthesis of other carbaporphyrinoid systems, including benziporphyrins,10 23-carbabenziporphyrins,11 pyriporphyrins12 and pyrazole-containing porphyrin analogues,13 have also been accomplished.
References
1. Lash, T. D. in The Porphyrin Handbook, Ed. Kadish, K. M.; Smith, K. M.; Guilard, R.; Academic Press: San Diego, CA, 2000, Vol. 2, pp 125-199.
2. Lash, T. D. Eur. J. Org. Chem. 2007, 5461-5481.
3. Lash, T. D. Macroheterocycles 2008, 1, 9-20.
4. El-Beck, J. A.; Lash, T. D. Eur. J. Org. Chem. 2007, 3981-3990.
5. Lash, T. D.; El-Beck, J. A.; Ferrence, G. M. J. Org. Chem. 2007, 72, 8402-8415.
6. Lash, T. D.; Colby, D. A.; Idate, A.; Davis, R. N. J. Am. Chem. Soc. 2007, 129, 13801-13802.
7. Lash, T. D.; El-Beck, J. A.; Ferrence, G. M. Manuscript in preparation.
8. Jain, P.; Lash, T. D. Unpublished work.
9. Zhang, Z.; Ferrence, G. M.; Lash, T. D. J. Am. Chem. Soc. 2008, submitted.
10. Lash, T. D.; Szymanski, J. T.; Ferrence, G. M. J. Org. Chem. 2007, 72, 6481-6492. El-Beck, J. A.; Lash, T. D. Org. Lett. 2006, 8, 5263-5266.
11. Xu, L.; Lash, T. D. Tetrahedron Lett. 2006, 47, 5263-5266.
12. Lash, T. D.; Pokharel, K.; Serling, J. M.; Yant, V. R.; Ferrence, G. M. Org. Lett. 2007, 9, 2863-2866.
13. Lash, T. D.; Young, A. M.; Von Ruden, A. L.; Ferrence, G. M. Chem. Commun. 2008, in press.
