alkynes - aromaticity - carbobenzenes
Significance
<P>Chauvin and co-workers report the first synthesis of hexaalkynyl [6]pericyclyne
and carbobenzene. The selected method involves an [8+10] ring formation that proved
to be efficient for the preparation of other [6]pericyclynes. The final step in the
synthesis is a reductive aromatization that was achieved by the use of only two equivalents
of Co
2(CO)
8. Subsequent treatment with SnCl
2/HCl and direct oxidative decomplexation with ceric ammonium nitrate (CAN) afforded
the hexaalkynyl carbobenzene in 12% yield over three steps. The
1H NMR spectrum showed that the triethylsilyl groups are particularly deshielded as
a result of a remote ring-current effect. The attempted desilylation of hexa(trialkylsilyl)
carbobenzene with tetrabutylammonium fluoride afforded only insoluble black material.</P>
Comment
<P>The synthesis of carbobenzene reported by the authors via a hexaalkynyl [6]pericyclyne
demonstrates the power of organic chemistry to synthesize complex structures. The
stereochemical resolution of hexaalkynyl [6]pericyclyne and its crystal packing open
new horizons for the syntheses of novel expanded carbon nanotubes. The controlled
desilylation of the reported carbobenzene could lead to a genuine carbomer of benzene
or to novel carbon allotropes derived from deprotected carbobenzene. Carbobenzene-type
systems might interact with gas molecules in a selective fashion and thereby have
potential applications in sensors as previously reported for macrocycles (T. Naddo,
Y. Che, W. Zhang, K. Balakrishnan, X. Yang, M. Yen, J. Zhao, J. S. Moore, L. Zhang
J. Am. Chem. Soc. 2007,
129, 6978).</P>
