Problems with quantitation scheme

As with the NOE experiment, one proton or a group of protons i.s selected for selective perturbation prior to the acquisition time. Among several variations the simplest is the one using an initial selective 90 pulse and a of series of experiments may be performed with the selected target spin(s) to be irradiated varied from experiment to experiment. In contrast to the ID NOE experiment ID ROE spectra are directly obtained after Fourier transformation of the corresponding FIDs and no difference spectra need to be calculated with this simple variant. However problems with quantitation occur which may be partially circumvented by using alternative experimental schemes. 

As should be evident, part of the problem in dealing with structures that contain atoms other than carbon is what values to use for a and /3. The values that have been suggested are based on correlating calculated properties with other known data. Because the Hiickel method is not a quantitative scheme for calculating properties of molecules, we will not address the issue of correcting the values of a and /3 further. 

At 80 °C, the value of the pseudo second order rate constant, k, was 2.05 X 10 mole" hr" (gram cat)" These observed kinetics provide firm quantitative support for a Rideal mechanism, which requires fast, reversible, and noncompetitive adsorption of the olefin on the catalyst acid sites see Scheme 5 above). The strictly first order dependency of toluene proves freedom from intracrystalline diffusion problems with respect to ethylene. Likewise, the product distribution, which contains only traces of disubstituted toluenes, shows absence of diffusion limitation for the product, ethyltoluene. Intracrystalline diffusion problems may, however, exist in the isopropylation experiments with extremely active catalysts. 

The hthium ylide generated from methyl (2,2-dimethylpropylideneamino)ace-tate and LiBr/DBU reacts with an a,p-unsamrated ester bearing a perhydropyr-rolo[l,2-c]imidazole chiral auxiliary at the p-position at —78°C in THF to give a quantitative yield of the Michael adduct as a single diastereomer (Scheme 11.25) (112). When the reaction is performed at room temperature, the diastereoselectivity decreases to 95 5 and some epimerization is observed. Accordingly, epimerization is not a serious problem as long as the reaction temperature is kept at or below -78 °C. 

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