β-Deuterium kinetic isotope effects for identity processes: bromide ion substitution at 1-bromo-1-arylethanes and 2-bromooctane

Abstract

. While deuterium kinetic isotope effects for solvolyses have been extensively studied, other nucleophilic substitutions have received less attention, and identity processes, that is, substitutions where the nucleophile and leaving group are the same, have rarely been examined. Identity reactions must pass through a truly symmetrical stage, a transition state or an intermediate, so that data will be of interest to both theoretical and experimental chemists. Values of kHlkD have been determined by polarimetry for bromide exchange-racemization at ArCHBrCH3/CD3 (Ar = C6H5, 4-Br-and 4-Me-C6H4, and 3,4,-dimethyl-C6H3) in acetone, acetonitrile, and nitromethane. Observed values are analogous to values seen in solvolyses. They range from 1.01 to 1.35 and, in some cases, increase markedly as the concentration of Bu4NBr decreases. Solvolyses are either first order or pseudo first order whereas plotting observed racemization rate versus [Bu4NBr] allows separation of first-and second-order components; those species giving more stable carbocations in the more dipolar solvents, the systems showing kHlkD variation with Br-concentration, alone show an appreciable first-order component. The second-order kHlkD ratio averages 1.062 2 4 . 0 1 8 at temperatures ranging from 25 to 50°C for all substrates in the three solvents, very analogous to the values seen for racemization of 1,1,1 -d3-2-bromooctane or solvolysis of ethyl substrates but considerably lower than the typical solvolysis values of 1.15-1.25 for secondary, and 1.35-1.5 for tertiary substrates. The first-order kHlkD values obtained are higher, 1.1-1.5. These and other results are discussed. de Bu4NBr. Les solvolyses sont soit du premier ordre ou du pseudo premier ordre alors que, dans notre cas, la courbe de la rackmisation en fonction de la concentration de Bu4NBr permet de sCparer les composantes du premier et du deuxieme ordre; seules les especes conduisant B carbocations les plus stables dans les solvants les plus dipolaires, les systemes presentant une variation du kH/kD en fonction de la concentration du Br-, prksentent une portion importante des rCaction du premier ordre. Pour tous les substrats, dans les trois solvants, ides temptratures allant de 25 a 50°C, le valeur du rapport kH/kD des rkactions du deuxikme ordre rend une moyenne de 1,062 5 0,018 et elles sont tres semblables ? I celles observkes lors de la ractmisation du I ,1,1-d3-2-bromoCthane ou de la solvolyse des substrats avec des groupes Cthyles; elles sont toutefois beaucoup plus basses que les valeurs typiques de 1,15 1,25 ou 1,35 B 1,5 obsewtes lors de solvolyses de substrats secondaires ou tertiaires. Les valeurs des kH/kD du premier ordre sont plus ClevCes (1,l-1,5). On discute de ces valeurs et d'autres rtsultats.[Traduit par la redaction]

Reaction Pathways and Energetics of Etheric C–O Bond Cleavage Catalyzed by Lanthanide Triflates

“…Yb(OTf) 3 and Pd nanoparticles on alumina (Pd@ALD) containing 2% Pd by weight were synthesized according to previously published procedures. The labeled substrates 1- d 3 -2-octanol and 1-methyl- d 3 -heptyl phenyl ether were synthesized according to modified literature procedures as outlined below. The 1 H NMR integration internal standard, 1,1,2,2-tetrachloroethane, was purchased from Aldrich Chemical Co., distilled under reduced pressure, and stored under N 2 .…”