Proton-bearing solvent

This situation is illustrated in Eq. 7 where B represents the conjugate base of the solvent, BH, used in the kinetic experiment. After the base, B, removes the deu-teron from the acid donor, A, (Eq. 7a) it is still complexed to it. At this point, the deuterated base, DB, may diffuse away and be replaced by a proton-bearing analogue, HB (Eq. 7b), or it can return the deuteron to the conjugate base of the acid. Under the typical conditions, the step in Eq. 7b is irreversible (the concentration of HB is always much greater than DB) and the rate of isotope exchange can be expressed in terms of the following elementary rate constants. 

Factor (c) appears to depend on complex interacting effects and deserves detailed discussion. When the proton nmr spectrum of a polar substance, typically but not always a ketone (83), dissolved in an aromatic hydrocarbon is compared with that obtained in a saturated hydrocarbon, large shifts of up to 1.5 ppm are frequently observed. These shifts are either upfield or downfield, depending on the stereochemistry of each proton-bearing group (84). This general behavior, known as ASIS (aromatic solvent-induced shifts), has been reviewed by Foster and Lazio (85). 

Figure 25. Effective correlation times (Tgff) of proton-bearing carbons in proline dissolved in a mixture of D2O and glycerol as a function of solvent composition (% v/v) (Deslauriers and Smith, 1977).

Recently, a rhodium-catalyzed tandem cyclization has been reported with an arylboronic ester bearing a pendant Michael-type acceptor olefin and acetylenic65 or olefinic66 derivatives. This transformation proceeds in a water-containing medium as solvent and proton source. This catalyst system is optimized with electron-rich and bulky ligands to stabilize the rhodium intermediate and decrease the protonolysis of boron derivatives in a protic solvent. 

These conclusions are reinforced by measurement of natural abundance 15N chemical shifts in piperidines and decahydroquinolines (77JA8406,78JA3882,78JA3889). Lack of correlation between 13C shifts of cyclohexanes and 1SN shifts of piperidines bearing the same methyl substituents are attributed to, among other factors, solvent effects and the difference between H-lone pair and H-H interactions. Protonation served to cancel these stereoelec-tronic effects. Correspondence between 1SN shifts in N- and C- methyl substituted piperidines and decahydroquinoline hydrochlorides and the analogous 13C values were, however, generally much closer than for saturated aliphatic amines. 

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