| The rhodium(III) alkyl deuterides {HCnRh(alkyl)(D)[(POMe) 3]}BAr4F (HCn = 1,4,7-triazacyclononane, alkyl = ethyl, propyl, BAr4F = tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate) were synthesized in approximately 20% yield from RhCl3·H 2O. Investigations were conducted into the synthesis of analogous cyclopentyl and cyclohexyl derivatives. These alkyl deuterides underwent thermolysis reactions in benzene at 39.6°C, eliminating alkane-d1 and forming the benzene activation product {HCnRh(phenyl)(H)[P(OMe) 3]}BAr4F quantitatively. During this thermolysis reaction, the deuterium label isomerized very rapidly into the alpha-position of the alkyl chain, and more slowly into the o-position---both isomerizations were significantly faster than alkane elimination. The reaction was monitored by 2H and 31P NMR spectroscopy over time to determine the rate of deuterium isomerization into the o-position of the alkyl (ko). In a similar reaction, {HCnRh(propyl)(H)[(POMe) 3]}BAr4F was thermolyzed in benzene at 39.6°C to determine the rate of alkane loss (kelim).;These rates for the ethyl (ko = 9.61 x 10 -6 s-1) and propyl (ko = 2.32 x 10-5 s-1, k elim = 2.01 x 10-6 s-1) alkyl chains were compared to previously determined values for the butyl, hexyl, and decyl chains. The isomerization and elimination processes are postulated to proceed through an alkane sigma-complex intermediate (wherein the complex undergoes reductive coupling to form the intact alkane, which remains coordinated to the metal through electronic interactions with the methyl and methylene C-H bonds of the alkane). The ko and kelim values of the alkyl series were used to determine key energy barriers for the interconversion of the various intermediates and draw conclusions about the behavior of the alkane sigma-complex. |
