Arenes proton

The chemical shifts of arene protons (6.5 ppm to 8.0 ppm) characteristically are toward lower magnetic fields than those of protons attached to ordinary double bonds (4.6 ppm to 6.9 ppm). The difference of about 2 ppm cannot be easily explained because the hydrogens in both types of systems are bonded to carbon through sp 2-a bonds (Sections 6-4C and 6-5A). 

The 1H—NMR-spectrum of the less symmetric 2-isomer 11 described by Truesdale and Cram34) is, again, characterized by a distinct upfield-shift of the arene proton signals. Molecular models indicate a nonplanar arrangement of the benzene rings. 

The asymmetric hydrocarbon phane 13 reported recently35 is extremely strained according to the molecular model and is inclined to resinify even at ambient temperature 1. The unusually high field shifted H-NMR-absorption of one arene proton (6 = 5.04 ppm) indicates, in accord with the model, a noncongruent arrangement of the benzene rings. 

Since all the arene protons of 26 resonate at lower field than those of 25, the wider bond angle at sp2 carbon is assumed to keep the arene units in the olefin at a distance greater than in the CH2CH2-bridged phane ). 

In the H—NMR-spectrum of compound 4144 44b an AA BB multiplet is found at the unexpected 5-value of 6.39 ppm it is assigned to the triphenylethane arene protons, which, according to molecular models, come to lie in the shielding region of an opposing phenylene ring of the triphenylbenzene unit. 

The 7c-clouds of anti-6 penetrate one another only partly, so that protons 4-H, 5-H and 6-H (12-H, 13-H and 14-H respectively) absorb in the same range as the arene protons in 158 (Figs. 39, 40), whereas the inner proton 8-H (16-H respectively) are affected by the shielding cone of the opposite benzene ring, resulting in an upheld shift by about AS x 3 (Fig. 40) [35, 89, 140]. Absorption of the arene protons in the syn-conformation (sy -6) is comparable to that in 8, due to similar conformation (Fig. 39) [41a]. 

Complexation with transition metals leads to a significant upfield shift of the arene protons [16, 35, 89, 142, 143] as well as the ring carbons [89, 142-144]. Examination of proton NMR-spectra of chromium-, iron- and ruthenium complexes particularly with anti-6 as ligand provide an insight into the reason for this upfield shift [89, 90]. 

Polymer 5 is isolated as a deep red solid showing limited solubility in chlorinated solvents (e.g. dichloromethane chloroform). Elemental analysis of a polymer prepared from imbalanced stoichiometry (1 0.95, stannane chromium monomer, respectively) gave data consistent with an average DP of 8 and the expected 0% chlorine content. Proton NMR data showed a downfield shift in the 17 -arene protons consistent with replacement of the chlorides with acetylenic moieties based on model compounds. 

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