Carbon 13 chemical shifts phenyl complexes

Todd et al. (23) have studied the NMR spectra of complexes of the type (CO)sMCRR, M = Cr, W R = CHs, Ph, m- andp-Ph R = NH2, OR (Tables XXXVI and XXXVII). They have also found (cf Ref. 123) that substitution of a methyl for a phenyl group (R) causes an 8 to 11 ppm downfield shift. A unique solvent effect was observed for this system the carbene carbon chemical shift was 6 to 7 ppm upfield in THF from the value in CHCI3. This effect was attributed to the formation of a solvent-solute complex in THF solution. 

We note upon inspection of the data in Table XIV that the chemical shifts in these carbonium ions (Via, b, and c) follow the normal order observed when substituents on a fully substituted carbon atom are varied from R = H (most shielded) to CH3 to C6H5 (least shielded), rather than for substituents on an electron-deficient trivalent carbon atom, where the CH3-substituted carbon atom is less shielded than the analogous phenyl-substituted carbon atom (58). This is a rather clear-cut indication that the carbinyl carbon atoms in complexes Via, b, and c are nearly fully bonded and only slightly electron-deficient. The slight increase in shielding of the CH3 carbon when VIb is formed from the alcohol and of the C-l carbon of the phenyl group of Vic provides further confirmation of this. 

The carbon-boron heterocycle, 3-phenyl-3-benzoborepin, exhibits oxidative stability upon exposure to air, an unusual feature for a trivalent boron compound. In Table XVI are recorded the chemical shift data for the vinyl protons for the benzometallepins of B, Sn, and Si. The PMR spectrum of 3-phenyl-3-benzoborepin exhibits vinyl proton resonances at lower fields than would be expected for an olefinic boron compound (compared to trivinylboron or 4,5-dihydroborepin see Table XV), and also at lower field than the benzostannepin derivative (217). The shift to lower field of 0.4 to 0.8 ppm may be consistent with the presence of a ring current, which would require the participation of the Bp orbital in the 7r-electron system. Support for increased electron density at boron might be provided from B NMR measurements, but such data have not yet been reported. Complexation of boron, which converts the... 

The NMR studies of Group IIA compounds center on investigations of Grignard complexes. Carbon-13 chemical shifts for phenyl and alkyl Grig-nard complexes are given in Tables IV and V, respectively. 

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