Synthesis and Molecular Structure of (Z)-[6]Paracycloph-3-enes

(Z)-[6]Paracycloph-3-ene (la) and its ester derivatives lb and lc, the smallest bridged [«]paracyclophanes having a double bond in the bridge, have been synthesized successfully by utilizing the thermal valence isomerization of the corresponding Dewar benzene isomers 5a-c. Examination of the spectroscopic data, especially that of la, has provided qualitative but important information concerning the conformation of the bridge and the deformation of the benzene ring and the bridge double bond. Namely, the upfield shift in the^]H NMR spectra of the vinyl protons and the aromatic protons located on the same side with the bridge double bond and the downfield shift in the¹³C NMR spectra of the aromatic carbons on the same side with the double bond indicate that the double bond is located closely over the aromatic ring. Moreover, the vinyl—allylic and benzyl-allylic coupling constants in the 1H NMR spectra suggest that the plane of the double bond tilts toward the opposite direction against the benzene plane. The large olefinic vicinal coupling constant (³/(H(3)-H(4)) = 12.2 Hz) and low wavenumber C=C stretching band in the IR spectrum of la suggest that the bond angle of the double bond (C(3)-C(4)-C(5)) is remarkably expanded (128°) and the bond length of the double bond is coniderably stretched, respectively. The remarkable bathochromic shift in the long wavelength region of the electronic spectra of la-c relative to the corresponding [6]paracyclophanes 2a-c indicates that the out-of-plane bending of the benzene ring of la-c is greater than that of 2a-c. The rate of mutual isomerization between the conformational isomers, anti-1 b and syn-lb, which corresponds to the inversion of the bridge of this system, has been determined from which the barrier for the inversion has been estimated to be 24.6 kcal/mol. More precise information about the structure of this system has been elucidated by the single crystal X-ray analysis of the diester lc. The crystal structure of lc has the conformation which is in accord with that deduced from 1H NMR coupling constants; the dihedral angle between planes C(2A)-C(3A)-C(3B^/)-C(2B^/) and C(8)-C(9)-C(8')-C(9') is 17.7°, and the nonbonded distance between two x-systems is 3.04 (2) (C(3A')-C(8)) or 3.03 (1) (C(3B)-C(9)) A. The bending angle of the para carbon out of the base plane of the benzene ring (C(8)-C(9)-C(8')-C(90) is 20.5° and that of the benzyl carbon from the plane of C(7)-C(8)-C(9) is 24,1°. The total bending angle (44.6°) represents the most highly deformed benzene ring so far isolated. The bond angles of the bridge double bond are considerably widened from the normal; C(2A)-C(3A)-C(3B0 = 130 (2)°, C(2B)-C(3B)-C(3A0 = 132 (1)°. Moreover, the bond length of the double bond is remarkably longer than the normal one: C(3A)-C(3B') = 1.37 (2) A. This represents one of the longest unconjugated C—C double bond hitherto known. Thus it has been clarified that the bridge double bond of this system is expanded and stretched by the inherently planar benzene ring which is in turn bent by the short rigid bridge having a double bond in the center.