Toluene, hydrogen-deuterium exchange

Data from competitive hydrogen deuterium exchange reactions of toluene and benzene over DHY (66)... 

Fio. 20. Plot of log pt and selectivity factor, Sf. data from competitive hydrogen deuterium exchange reactions of toluene and benzene over DHY (7f) O, data from competitive ethylation reactions of toluene and benzene over REX (12S). 

Some experimental observations on hydrogen-deuterium exchange are summarized below. The reactions were carried out in toluene solution (10 M) (one at. ) was used in large excess, and the products were isolated and characterized as pure deuteride complexes. 

Hydrogen-deuterium exchange is catalysed by an unusually stable mercury-toluene complex that contains a GaMe3 moiety. The first homoleptic platinum complex with terminal and bridging Cp Ga ligands has also been made. This is the compound [Pt2(GaCp )2( i2-GaCp )3]. 

The cationic rhenium complexes [Re(NO)2(PR3)2] (R = Cy, Pr) [50] indeed show great potential for hydrolytic activation of dihydrogen. When [Re(NO)2(PR3)2l is treated with a mixture of H2 and D2 in toluene or chlorobenzene, hydrogen-deuterium scrambling is observed, and HD can be traced in the NMR spectrum of the reaction mixture [51]. The proposed mechanism for this catalytic exchange is illustrated in Scheme 4. 

Toluene-d4, -d5, and -d6 remained constant -d3 decreased, and - 2 increased. This indicates some exchange, probably by way of a benzyl radical that abstracts protium in place of the original deuterium. Toluene-a-d3, refluxed for 2 hours in acetic acid with the same concentration of cobalt acetate and cobalt bromide, was recovered unchanged. Hydrogen, of course, could be furnished by the acetic acid however, in the Ci4 hydrocarbons, the product of molecular weight 187 amounted to about 15% of that of 186. Presumably these are chiefly methyldiphenylmethane and bibenzyl, respectively. 

Cinneide and Clarke (770) have studied the activity of Pd-Au films for the deuteration and exchange of benzene and the hydrogenation of p-xylene. The authors report that the activity for the exchange reaction between benzene and deuterium persists to the palladium-lean compositions, which is in agreement with results obtained by Honex et al. (Ill) in a study of the exchange of toluene over alloys of the same kind. The rates are much reduced (by 102 to 103) compared to those found with palladium-rich films. 

Yakushin and Shatenshtein (1960) and Yakushin et al. (1959) have determined the ratio of the rate constants for tritium and deuterium replacement by protium in ammonia solutions of fluorene and methyl 2-naphthyl ketone at 25°. Yakushin has obtained data for these reactions in anhydrous methylamine. Streitwieser et al. (1960, 1962b) have measured the ratio fcD/ T for the hydrogen exchange of ethylbenzene and toluene catalysed by a solution of lithium cyclohexylamide in cyclohexylamine at 49-9°. 

It is interesting to note that C-H activation on ruthenium NHC complexes is not limited to intramolecular protons located in the N-sidechain of the carbene, but occurs inter-molecularly as well. Leimer et al. reacted [MesIRuH PCyj] with toluene-dg at ambient temperature and observed a rapid H/D exchange reaction involving the four hydride hydrogen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene ligand and the deuterium atoms on the meta positions of toluene-dg. The ortho-, para- and methyl-deuterium atoms of the solvent did not participate [145]. 

We have presented results for the dependence upon activation of the chemisorption of ammonia, carbon monoxide, carbon dioxide, and oxygen in Section VI, of hydrogenation and isomerization of hexenes and of exchange between deuterium and the ring and side chain of toluene in Section VII. Despite our attempts to control the activation conditions carefully there is a regrettable degree of scatter in our results. However, the bulk and surface condensations are clearly complicated in detail and the surface area of chromia is known to be a rather sensitive function of the exact details of activation (28). 

In hydrogenations with H2 in D20 the product showed only CHD— stretches in the infrared. This observation excludes a fast H/D exchange on Pd, and implies a monohydridic mechanism of hydrogenation. With the same catalyst in an aqueous (D20) solution, itaconic acid is reduced under H2 to yield multiply deuterated methyl succinic acid having 1.97 deuterons at C3, 0.66 at C2 and none at Cl (Eq. 32) [83]. On the other hand, in an H20/ethyl acetate biphasic solvent mixture, the catalyst prepared in situ from [Rh(cod)Cl]2 and TPPTS catalyzed the reduction (with D2) of dimethyl itaconate with deuterium incorporation at C3 (2.06), C2 (0.78) and at Cl (0.18) [84], Similar results were obtained in toluene/methanol (1 1) with the Rh(I)-BPPM cationic catalyst [85], Again, these findings could be explained by a fast /3-elimination from the intermediate Rh(I)-alkyl. 

Further acceleration of isotope exchange, by three to four powers of ten, can be achieved116-118 by working in liquid deuterium fluoride, particularly if it contains BF3.108-110 The properties attaching to deuterium bromide apply also to this system. For toluene, equilibrium is reached in 15 minutes,108 and, as in exchange in DBr, only the hydrogen atoms attached directly to the aromatic ring are replaced, even in presence of BF3. 

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