Enantiotopic hydrogen atoms

Some hydrogen atoms may appear to be equivalent because they are bonded to the same carbon atom, yet they are not equivalent. Such hydrogen atoms are often diastereotopic (Section 8.12), and they have different chemical shifts. We can determine if two hydrogen atoms are diastereotopic by replacing either one by a deuterium atom. For example, the two geminal hydrogen atoms of 2-bromopropene are diastereotopic. 

The chemical shift of methylene chloride as measured with a 300 MHz instrument is 5.30 d. What is the separation in Hz from TMS What is the d value when measured with a 400 MHz instrument  

How many sets of nonequivalent hydrogen atoms are contained in each of the following ketones  

Classify the two hydrogen atoms bonded to the chlorine-bonded carbon atom of the following compound. 

The benzylic carbon atom is a chiral center. Thus, the two hydrogen atoms bonded to the adjacent carbon atom are diastereotopic. These hydrogen atoms do not have the same chemical shift. 

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The symmetry planes (a) in molecules 30, 32, 34, 36, 38, Fig. 13 should be readily evident. It is possible to have both homotopic and enantiotopic ligands in the same set, as exemplified by the case of cyclobutanone (34) HA and HD are homotopic as are HB and Hc, HA is enantiotopic with HB and Hc HD is similarly enantiotopic with Hc and HB. The sets HAjB and HC>D may be called equivalent (or homotopic) sets of enantiotopic hydrogen atoms. The unlabeled hydrogens at position 3, constitutionally distinct — see Section 3.4 — from those at C(2, 4), are homotopic with respect to each other. Enantiotopic ligands need not be attached to the same atom — viz. the case of mew-tartaric acid (32) and also the just-mentioned pair Ha, Hc [or Hb, Hd] in cyclobutanone. 

The mechanism of this isomerization involves an intramolecular 1,3-supra-facial hydrogen migration via an imminium complex 11.21 (Figure 11.6). The enantioselection takes place during the removal of one of the two enantiotopic hydrogen atoms of 11.22, as shown by deuterium labeling experiments (Figure... 

Recent development of asymmetric C-H hydroxylation was discussed in this chapter. Although the discussions were limited to C-H hydroxylation at active methylene via a radical intermediate, it was demonstrated that efficient differentiation of enantiotopic hydrogen atoms could be achieved by using a well-crafted molecular catalyst as discussed in Section 2.2.2. This may open a gateway to enantioselective C-H hydroxylation at non-activated methylene. On the other hand, studies of asymmetric Kharasch-Sosnovsky reaction proved that transform of a topos-selective issue to the face-selective one by generating an allyl radical intermediate is another useful approach to asymmetric C-H hydroxylation. 

Enantiotopic hydrogen atoms have the same chemical shift and give only one NMR signal ... 

Enantiotopic hydrogen atoms may not have the same chemical shift if the compound is dissolved in a chiral solvent. However, most H NMR spectra are determined using achiral solvents, and in this situation enantiotopic protons have the same chemical shift. 

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