Base-catalysed H-D exchange

Base-catalysed H/D exchange experiments for a series of haloforms [80] (Table 4.10) demonstrate that carbanion formation is stabilised by halogen in the order 1 > Br > Cl > F. When these results are combined with acidity measurements which show that CF3H (pKa 31) is little more acidic than methane (pKa 40) [81], we can conclude that, in these systems, fluorine attached to a carbanion centre is stabilising with respect to hydrogen but destabilising compared with the effects of other halogens. Similar conclusions can be drawn from pKa measurements of a number of halobis(trifluoromethyl) methanes [82]. 

Sulphinyl carbanions are generally observed to show greater stability when the carbanion lone pair bisects the S-lone pair/S—O bond angle, behaviour consistent with MO predictions. However, rates of base-catalysed H-D exchange in the system (9 X = SO, R = Me, R = H) now indicate a significant solvent dependence such that a change from t-butan[ H]ol to [ H4]methanoI results in a substantial reversal of the relative rates at which the two a-hydrogen atoms are abstracted. ... 

Reactions.— The stereoselectivity of the abstraction of diastereotopic protons a to trico-ordinated sulphur and the stereochemistry of electrophilic reactions at the carbanion centres of cFlithio-thian 1-oxides still excite considerable interest. The rate factors for base-catalysed H-D exchange of a-protons in several conforma-tionally biassed or rigid thianium cations have been measured with increasing rigidity, the reactivity of decreases, while that of remains about constant. It... 

Partial rate constants for H-D exchange at the methyl and methine sites of cycloalkyl methyl ketones have been measured chiefly in order to examine the ring-size effect on exocyclic enol and enolate formation. Under base-catalysed... 

Base-catalysed hydrogen/deuterium exchange is still probably the only definitive probe for the ElcB process in which H/D exchange occurs in the starting material at a rate faster... 

Bronsted acidity, the authors suggested that dissociative adsorption over a Lewis acid-base pair occurs. This conclusion is similar to that reached for the H/D exchange occurring between methane-d4 and OH sites, catalysed by basic oxides. In 1965, Larson and Halll reported that such exchange, which could be observed even at room temperature, was catalysed only... 

Now, we return to Scheme 3 to consider Vf and Vj, whose stability may determine the stereochemical outcome of a nucleophilic attack. Based on elimination and hydrogen exchange data, an estimate of the barrier to isomerization (Vi) > 30 kcal/mol was given for HCBr=CBr . It has been reported the base-catalysed D for H... 

It has been observed that the lithium salts of phosphono acetates (497) and (498) undergo base catalysed D/H proton exchange and C-OR esterolysis as well as acidic hydrolysis mediated by Th and (Figure 95). ... 

L-3-(3-Deoxy-l,2 5,6-di-0-isopropylidene-a-D-allofuranos-3-yl)alanine (290) (R = R = H) has been synthesized by a route that involved the addition of the carbanion derived from methyl (methylthio)methyl sulphoxide to 3-C-cyano-methyl-3-deoxy-l,2 5,6-di-0-isopropylidene-a-D-allofuranose. Subsequent treatment of the resulting (glycosyl)enamino sulphoxide with acetic anhydride, base-catalysed ester-exchange, and reductive desulphurization afforded the A-acetyl-3-(glycos-3-yl)alanate (290) (R = Ac R = Me), which was hydrolysed to (290) (R = R = H) with base. 

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