Sulfur exchanges

If the metal-halogen interconversions were originally used to prepare magnesium enolates, other heteroatoms than halogen can undergo the exchange. Sulfur atom is often employed. 

Correlation between the amount of exchangeable sulfur (Sexc) in and HDS activity of the catalyst ... 

Different from the observation on the absence of a correlation between sulfur uptake and HDS activity, a definite correlation was found (Fig. 6) between the amounts of exchangeable sulfur atoms (Sexc) and the thiophene HDS activity of the different catalysts, as mentioned before in the case of some other catalysts. This observation suggests some parallelism of exchangeable sulfur with the catalyst sulfur, and with the ratio of catalytic sites among sites of sulfur uptake. 

Experimental data indicated that the isotope exchange process is described by first order equations, and in most cases, e.g., on supported C0M0/AI2O3 catalysts by a superposition of two curves representing two types of sulfur mobility, the more and the less mobile i.e., rapidly and slowly exchangeable sulfur. In general, the H2S molar radioactivity a (in percent of the initial molar radioactivity of catalyst sulfide sulfur), as a function of the produced H2S-[X(cm )], is given (Fig. 7) as a superposition of curves... 

Note that a near to similar ratio of rapidly and slowly exchangeable sulfur was determined when the catalyst was sulfided originally with thiophene instead of elemental irrespective of the 2.5 times lower S content of the sample, sulfided by thiophene. This indicates the similarity of active site distribution for catalysts of equal chemical composition. 

It is of interest to compare the Smob/Scat data with Sexc/Scat obtained in studies on the S-exchange of catalyst sulfide with H2S.I 1 The degree of MoOx sulfidation was 63% if sulfided with thiophene, [ 1 that by the H2S-sulfided sample — 48%.t l The fraction of exchangeable sulfur in the... 

Comparison of thiophene HDS conversion with the ratio of mobile (exchangeable) sulfur indicated that the higher ratio of mobile sulfur was paired with higher HDS activity of the C0M0X/AI2O3 catalysts. 

The effect of methyl substituents is a widely discussed issue in HDS studies.Kabe et al. foundl different S-exchange values for 4-methyl-DBT, 4,6-dimethyl-DBT and DBT. The amount of exchangeable sulfur was higher in the case of DBT than that in the case of the methyl-substituted DBT-s. In agreement with this, the HDS rate of DBT was higher than those of the methyl substituted ones. The lower amount of mobile sulfur is presumably the consequence of steric hindrance caused by methyl groups, as it follows from the sequence of the HDS rates... 

Compartmentalization at the molecular level one can imagine a transconjugating enzyme(s) that could exchange sulfuric and glucuronic acid without ever making free dehydroepiandrosterone available fOr metabolism to androsterone. This enzyme has never been described and is improbable. 

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