in-[3(4,10)][7]Metacyclophane
Sheltered deep within the boring hydrocarbon exterior of this
cyclophane is an inwardly-directed methine hydrogen
forced into the face of a benzene ring. The calculated hydrogen-to-ring
distance is only 1.66 Angstroms (ab initio HF/3-21G geometry). Among
the unusual spectroscopic consequences are a proton chemical shift of -4.0
ppm and an infrared C-H stretching frequency of 3325 wavenumbers. For a
brief communication describing its synthesis and properties, see
"Synthesis of in-[3(4,10)][7]Metacyclophane:
Projection of an Aliphatic Hydrogen Toward the Center
of an Aromatic Ring." R. A. Pascal, Jr., R. B. Grossman,
and D. Van Engen, J. Am. Chem. Soc. 1987, 109,
6878-6880.
For those who may doubt the calculated geometry of this compound (there
is no X-ray structure), the X-ray crystal structure of a VERY closely
related molecule, 2,6,15-trithia-in-[3(4,10)][7]metacyclophane, which is
only slightly less strained, is reported in "Small, Strained Cyclophanes
with Methine Hydrogens Projected toward the Centers of Aromatic Rings."
R. A. Pascal, Jr., C. G. Winans, and D. Van Engen, J. Am. Chem. Soc.
1989, 111, 3007-301.
Theory has at last caught up with experiment, and the
in-[3(4,10)][7]metacyclophane appeared on the cover of
Chemical & Engineering News (September 28, 1998) in the context of
the calculation of NMR chemical shifts!
