Solvolysis secondary kinetic isotope effects

Sialosides have a distinct mechanism of hydrolysis for its unusual sugar structure of sialic acid. For example, the large 8-dideuterium and small primary kinetic isotope effects observed at the anomeric carhon and the large secondary kinetic isotope effect observed at the carboxylate carbon in the acid-catalyzed solvolysis of CMP-Af-acetyl neuraminate 24 support an oxocarbenium ion-like transition state 25 having the 5S conformation without nucleophilic participation of carboxylate and with the carboxylate anion in a looser environment than in the ground state [15] (O Fig. 3). Such a zwitterion structure is consistent with the results from calculations using the COSMO-AMI method for aqueous solutions [16]. 

The bond between a-carbon and deuterium is not broken in the solvolysis transition state, but the presence of deuterium, bonded to the reaction center, causes the reaction rate decrease. Isotope effects such as in (1.14.20) and (1.14.21) are called secondary kinetic isotope effects, to be clearly distinguished from primary isotope effects. Secondary isotope effects were defined as rate effects caused by isotope substitution on the bond not broken in the rate-determining step. 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

Resume. Alkyl participation in the solvolysis of exo-2-norbornyl derivatives is not unambiguously detected by rate measurements. Neither monocyclic analogs nor the corresponding enr/o-2-norbornyl compounds appear to be good models for comparison. Secondary deuterium kinetic isotope effects, however, point to distinct ionization mechanisms of exo- and -norbornyl derivatives. 

Secondary deuterium kinetic isotope effects obtained for the solvolysis... 

The low values obtained for the secondary deuterium kinetic isotope effects in the comparative solvolyses of (141) and (142) in formic acid (kn/ko = 1.154 at 29.9 °C) and in aqueous ethanol kji/ko = 1.116 at 64.4 °C) are regarded as evidence for extensive bridging (143) in the solvolytic transition state the deuterium atom at the C-1 bridgehead in (142) obviates the scrambling problem.As the contribution from the P-deuterium is unknown, then assuming the effects of a- and -deuterium are cumulative the tt-kinetic isotope effect is further decreased from the value expected for reaction occurring without nucleophilic assistance. The products of solvolysis of ent-methyl-12p-p-tolylsulphonyloxybeyeran-19-oate, which is a related bicyclo [3,2,1 ] -octan-2-yl system, have been discussed in terms of intermediate carbocation stabilities and lifetimes in the solvent employed. ... 

The secondary P-deuterium kinetic isotope effect is cumulative, as it has been shown by Shiner.Thus, the rates of the tert-amyl chloride solvolysis depend on the degree of deuteration ... 

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