Carbon 13 chemical shifts vinylic complexes

As we saw with metal complexed vinyl carbons, or-ganometallic complexes of aromatic compounds exhibit l3C chemical shifts upfield of the uncomplexed compound. For example, metal-coordinated benzene carbons are found in the range 8 74-111, depending on the exact structure of the complex. Similarly, metal-coordinated cyclopentadienide (C5H5-, abbreviated Cp ) appears in the range 8 75-123. 

The spectroscopic data for 2 support the proposed structure. In particular, the 29Si NMR chemical shift of 37.17 ppm as a doublet of doublets (Jp,.Si = 1281.6 Hz, JSi-p(i xm)= 148.82 Hz, /sl-pc ) = 12.80 Hz) is close to the literature values for cis-PtSizPz complexes.6 Compound 2 was found to be a good precursor for the double-silylation reaction.7 Thus, thermolysis of a toluene solution of 2 and 1-hexyne at 120 °C afforded complex 3 as a colorless oil. A key feature in the H NMR spectrum of 3 is a singlet at 6.24 ppm assigned to the vinyl proton. A characteristic low-frequency l3C NMR resonance at 5 138.50 provides evidence for a tethered carbon atom of the two silicon moieties8 (Scheme 1). 

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... 

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