Base-catalysed hydrogen exchange

Kinetic studies of base-catalysed hydrogen exchange have been carried out by Roberts, by Shatenshtein, and by Streitweiser and their coworkers. In earlier work, potassium amide was used as base in liquid ammonia as solvent, whereas later workers used lithium and caesium cyclohexylamides in cyclohexylamine. The reaction can be represented by equilibria (239) and (240) 

The first studies were concerned with deuteration of aromatics by deuterated potassamide (0.02 M) in liquid ammonia (Table 176)582,583. From this data it was not wholly apparent that electron-supplying substituents decrease the reaction rate and vice versa as has been subsequently confirmed. A further study of 

FIRST-ORDER RATE COEFFICIENTS (lO7 ) FOR REACTION OF ArH WITH 0.02 M KN(2H)j 

(° C) PhH PhMe PhEt Ph-i-Pr Ph-t-Bu Tetralin Bibenzyl Naphthalene Biphenyl 

FIRST-ORDER RATE COEFFICIENTS (107A j) FOR DEDEUTERATION OR DEUTERATION (ITALICISED) OF AROMATICS BV POTASSAMIDE IN 

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Kinetic studies of base-catalysed hydrogen exchange of heterocyclic compounds have been carried out. Paudler and Helmick515 measured second-order rate coefficients for deuteration of derivatives of imidazo[l,2-a]pyridine(XXXIII), imidazo[l,2-a]pyrimidine(XXXIV), and 1,2,4-triazolo[1,5-a]pyrimidine(XXXV)... 

The rate constant for the base-catalysed hydrogen exchange of [Ir(NH3)6] at 298 K in D2O (reaction 64) has been determined (fc = 1.5 x 10 M s" ). The chloride anation of [Ir(H2)(NH3)5] + is three times faster than the water exchange rate, and has been attributed to an associative interchange mechanism. ... 

Measurements of the rate of detritiation in (46)—(48) have provided data concerning the base-catalysed hydrogen exchange in thiazolium salts. The results may have some bearing on the interpretation of enzymatic reactions involving thiamine. ... 

Many base catalysed isotope exchange or racemization processes involve either rate-determining proton abstraction or rapid preequilibrium formation of the anion (19) followed by ratedetermining reaction with solvent (20) (L is an isotope of hydrogen). 

Acid-catalysed hydrogen-deuterium exchange in norcamphor has also been investigated by Werstiuk and Banerjee (1977) (DOAc—D20—DC1 medium). It was observed that exo-deuteron addition to the enol is also preferred, but with a slightly smaller selectivity (x 190). This would mean that, if torsional factors cause preferential base-catalysed exo-exchange, they also occur for acid-catalysed keto-enol tautomerism. However, the absence of important torsional strain effects on the rate constants of acid-catalysed enolisation of cyclic and bicyclic ketones contradicts this assumption. 

The following imidazoles have been tritiated by base-catalysed hydrogen isotope exchange routes at the specified positions. 

A large part (65%) of the label located at the C(2)(H) position in 152 is probably due to base-catalysed hydrogen isotope exchange still operating even in the presence of prereduced Pt02. The specific activities of the tritium-labelled epoxides by the same procedure are lower than for the imidazoles. The specific activities of the tritiated imidazoles can be improved by using tritiated water of higher specific activity. 

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... 

Base-catalysed deuterium exchange proceeds at C-3 and C-5 ° preparative lithiation occurs at C-3, or if C-3 is blocked, at C-5 or C-8 depending on other substituents, but the 2,6-dichloro compound reacts selectively at C-5, even in the presence of hydrogen at C-3. ... 

The main features of NMR spectroscopy are well known (Table 2). To illustrate several of the points made above, Fig. 1(A) shows the NMR ( H decoupled) of a sample of acetophenone, tritiated by a base-catalysed hydrogen isotope exchange procedure. The sample contains 11 p,Ci of... 

A useful compilation of 220 MHz n.m.r. data of substituted bicyclo[3,2,l]octan-6-ones has been published. The bicyclo[3,3,0]octyl cation appears to be implicated in the isomerization of bicyclo[3,2,l]oct-2-ene over SiOj-PaOj at 250 °C as deduced from the scrambling of label in [3- C]-tagged material and the distribution of label in the product, namely bicyclo[3,3,0]oct-2-ene. It has been found that 2,4-diphenyl-bicyclo[3,2,l]octa-2,6-diene (630) and its 6,7-dihydro-derivative undergo base-catalysed hydrogen-deuterium exchange at C-4 at essentially the same rate. Hence, the phenyl groups result in the total elimination of charge delocalization from C-4 to the C-6-C-7 double bond that is found in the unsubstituted bicyclo-octadiene. The... 

General.—Aliphatic seleno-esters RCSeOEt condense with o-phenyl-enediamine, o-aminophenol, or o-aminothiophenol to yield the benzazoles (510 X = NH, O, or S, respectively). Cyclic voltammetry of the salts (511 X, Y = O, S, or Se R = SEt or SeEt) results in electrochemical reduction to dimers. The rates of base-catalysed hydrogen-deuterium exchange in the 2-position of compounds (510 X = 0, S, or Se R = H) have been determined. Successive treatment of 1,8-dibromonaphthalene with butyl-lithium, sulphur, butyl-lithium, and selenium gives naphtho[l,8-Cif]-l,2- selenathiole (512 X = S, Y = Se) the tellurathiole (512 X = S, Y = Te) and the telluraselenole (512 X = Se, Y = Te) were prepared similarly. ... 

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