Carbon 13 chemical shifts olefin complexes

The fact that we have three olefinic hydrogens means that our compound is a primary olefin, the fact that the other two carbons are both methylene carbons means that our substituent, bromine, is terminal. Thus the only possibility we have is that we are dealing with 4-bromo-1-butene (try to find another isomer that fits ). But this simple molecules has a highly complex proton spectrum, which can only be interpreted completely (exact chemical shift, coupling constants) by spectrum simulation. 

The effect on the coordination chemical shift of varying the phosphine has been studied for two series of trigonal metal-olefin complexes, viz., (CH2 CH2) PtL (46) and (CH2 C(CH3)C02C2H )NiL2 (49). In both cases the chemical shift is found to correlate with the basicity of the phosphine [Table IV (41, 45, 46, 49-52)]. The chemical shift of the unsubstituted olefinic carbon atom (CH2 ) of the ethyl methacrylate complexes is more strongly phosphine-dependent than the substituted olefinic carbon atom, and this effect has been attributed to electron withdrawal from this site by the ethoxycarbonyl substituent. 

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... 

In some cases it may be useful to reduce the catalyst activity level to facilitate the observation of early reaction steps. An example of this approach is shown by the two in situ studies illustrated in Fig. 28 [101]. When acetaldehyde-1,2- C was heated on a zeolite sample activated to 673 K, a complex product distribution was formed, which decomposed to CO, COj, and other products at higher temperatures. If a small amount of water was first adsorbed uniformly on the zeolite, acetaldehyde was converted almost quantitatively to crotonaldehyde by a similar in situ protocol. It seems that water levels the acidity of the zeolite in a manner analogous to that seen in nonaqueous acid-base chemistry. As an aside, note that the C chemical shifts of the carbonyl and 3 olefinic carbons are shifted downfield owing to the protonation equilibrium. This effect was discussed previously as a caveat to chemical shift interpretation. 

This weaker and stronger interaction of the olefinic bonds with the metal centre is reflected in the different chemical shifts of the olefinic carbon and the olefinic proton signals in the NMR spectra (Table 3). One of the olefinic carbon signals of the NBD ligand shifts more upfield than the other ca. 43 ppm). Similarly, one of the olefinic proton signals shifts more upfield than the other [ca. 1.3 ppm). The NMR and X-ray data suggest different lability of the two double bonds in this kind of complex. 

The C chemical shifts of olefin carbon atoms change as a result of olefin coor-dination to the metal (coordination shift A3) in the ca +30 ppm to ca -115 ppm range. The signs + and — represent changes in chemical shift in the direction of a weaker and a stronger field, respectively. The value of the coordination shift approximately corresponds to the strength of the metal-olefm interaction. The smallest coordination shifts were found for weak Ag(I) compounds, and the largest shifts for stable complexes of Pt(0), Pt(II), and... 

Relatively few data are available concerning NMR spectra of protons connected to the alkyne carbon atoms in acetylene metal complexes. In contrast to olefins, coordinated acetylenes have their proton signals shifted to lower t values. The shift to lower fields generally equals 2.5-4 ppm for coordinated acetylenes. Therefore, acetylene protons in alkynes bonded to the central atom have chemical shifts which are typical for olefin hydrogen atoms. This is in agreement with theoretical predictions. X-ray data, IR spectra, and the metal-alkyne bond model. The chemical shift of protons for some alkyne complexes are given in Table 6.21. 

The NMR spectra of allyl complexes of transition metals are distinctive. The terminal carbon atoms have chemical shifts in the 35-80 ppm range, while the middle cabon atom has chemical shifts in the 80-140 ppm range (see Table 7.3). These values may be explained on the basis of the valence bond theory. The chemical shift of terminal carbon atoms have values which are intermediate between shifts of carbon atoms creating the a bond with the metal and those of free olefins. This is in agreement with the following canonical forms ... 

Pitcher, Buckingham, and Stone 285) have discussed the anomalous chemical shift of fluorine atoms bonded to the a-carbon atom of perfluoro-alkyl-transition metal derivatives in terms of mixing of nonbonding electrons of the halogen with orbitals of the metal. Bennett, Pratt, and Wilkinson (27) have discussed the shielding of protons of olefins in the complexes of these ligands with transition metals. 

As a matter of fact, olefin-consuming reactions (by H2) may be a serious problem in some technical reactions. Palladium complexes and Co2(CO)g (commercial products) are typical catalysts. Problems may also arise in the Fischer-Tropsch reaction [19, 20] where iron oxides of a certain basicity (alkaline-metal doping) are being used to catalyze the formation of hydrocarbons according to (the simplified) eq. (15). More details are provided in Section 3.1.8. Since water is inevitably formed, carbon dioxide can also occur. On the other hand, it is doubtful whether the CO/H2O system will be used for directed reductions of organic compounds, since hydrogen is an extremely abundant industrial chemical. The water-gas shift reaction is thus to be avoided in the vast majority of cases. 

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