Acidity of hydrocarbons. XLVII. Primary kinetic isotope effects and internal return in hydrogen isotope exchange of toluene and triphenylmethane with lithium cyclohexylamide

Abstract

Experimental primary isotope effects for LiCHA-catalyzed proton exchange are compared for toluene and triphenylmethane. For toluene at 25°, /cDexPti/A:TexPti = 2.82 in cyclohexylamine (CHA) and fcHexPti/kTexP« = 27.2 in cyclohexylamine-W/V-if (CHA-A). For triphenylmethane at 25°, kH,.,rjtl/7cTcxpii = 2.79 in CHA and &HexPti/&Te*Pti = 19.2 in CHA-,. Derived internal return values show that internal return is greater for triphenylmethane than for toluene. Mechanistic primary isotope effects for the proton transfer step are also derived. The resulting kiH/kiD value is about 11 for both hydrocarbons.Base-catalyzed hydrogen isotope exchange reactions are frequently complicated by internal return phenomena3•4 (eq 1). The primary hydrogen isotope(1) This work was supported in part by Grant No.

Intermolecular Insertion of an N,N‐Heterocyclic Carbene into a Nonacidic CH Bond: Kinetics, Mechanism and Catalysis by (K‐HMDS)2 (HMDS=Hexamethyldisilazide)

“…It is thus concluded that whilst the formal deprotonation of toluene by 2 is thermodynamically very unfavourable, the reaction between 2 and toluene may be closer to a concerted process (pathway II , Scheme ) than a fully ionogenic process (pathway III ) to facilitate a sufficiently low activation barrier (116 kJ mol −1 ) for reaction to proceed at a reasonable rate. The magnitude of the primary and secondary deuterium isotope effects18,31 are consistent with this in that the transition state for proton transfer would be “late”. Indeed the primary KIE ( k H / k D =4.2(±0.6)) is similar in magnitude to that reported by Moss et al for the insertion of dimethoxy carbene into the OH bond of methanol ( k H / k D =3.3(±0.5),21e for which a pathway of type III (with some characteristics of II ) was suggested 21f.…”

“…Deuterium labelling at the toluene methyl group ([D 8 ]‐, [D 1 ]‐ and [D 0 ]toluene) results in moderate primary and large secondary KIEs for [D 1 ]toluene in C 6 D 6 (primary ( k H / k D )=4.2(±0.6)) and secondary ( k H / k D )=1.18(±0.08)), and for [D 0 ]toluene/[D 8 ]toluene as reactant and solvent ( k H / k D =10.9±1.9 on rate and k / k =9.5±0.8 for partitioning) 30. These are consistent with rate‐limiting CH cleavage (${k{{{\rm \bf II}\hfill \atop 1\hfill}}}$ or ${k{{{\rm \bf III}\hfill \atop 1\hfill}}}$ in Scheme ) proceeding via a late transition state,18 with an accompanying decrease in p character at the benzylic carbon and little competing internal return, which would reduce the primary KIE to an equilibrium isotope effect (${K{{{\rm \bf III}\hfill \atop 1\hfill}}}$ in Scheme ) 31,32. Analysis of the chemical shift of the 13 C{ 1 H} NMR signal arising from C(2) in [ 13 C 1 ]‐ 2 indicates that there is rapid and reversible complexation of carbene 2 to the (K‐HMDS) 2 with an association constant of K a =62(±6) M −1 .…”

Kinetic acidity of methane

“…The average value, k 2 = 1.9 ± 0. 7 but the lithium amide is aggregated and only the small amount of monomer present is the active catalyst. The solvent isotope effect in that case is probably an effect on the aggregation equilibrium.…”

“…The preparation has been described previously. 7 Cesium cyclohexylamide-N-d was prepared by reaction with cesium metal. Note, however, that this reaction is much slower than the reaction of cesium metal with undeuteriated cyclohexylamine.…”

Stereochemical properties of 1,3-bissulfonium salts and 1,3-bissulfonium ylides

“…For personal use only. significant contribution to the exchange process of 'internal return' (31), with this being more pronounced for HZe than HZa Because of its high acidity, trifluoroacetic acid can hardly be regarded as a typical medium for the formation of a carbanion. Yet there is no doubt that hydrogen exchange of 26 takes place in the TFA-D20 mixture.…”