Yutaka Tsuji

Dynamics for the reactions of ion pair intermediates of solvolysis

Publisher Summary This chapter discusses the dynamics for the reactions of ion pair intermediates of solvolysis. It focuses on determining partition rate constant ratios for a variety of reactions of ion pairs, and of absolute rate constants from these ratios. This has been accomplished by use of one of the rate constants from this product ratio as a “clock” for the second reaction. The addition of water to a free carbocation intermediate of solvolysis has been distinguished from addition to an ion-pair intermediate by an examination of common ion inhibition of solvolysis. The observation of “hidden” reactions during solvolysis, through the use of chiral or isotopically labeled substrates has created considerable excitement in communities interested in the mechanisms of non-enzymatic and enzyme catalyzed reactions. The racemization of chiral substrate or the exchange of bridging and nonbridging oxygen during solvolysis may occur through an ion-pair reaction intermediate that is sufficiently long-lived to undergo reorganization in a solvent cage, or it may proceed by an effectively “concerted” mechanism over an energy maximum that closely resembles the carbocation–anion pair intermediate. Another historically important reaction is the reorganization of “chiral” ion pair intermediates of solvolysis of a chiral substrate that leads to racemization of substrate during solvolysis.

Resonance effect in the solvolysis of α-α-di-t-butylbenzyl p-nitrobenzoates

Abstract The substituent effect on the solvolysis rates of α,α-di-t-butylbenzyl p-nitrobenzoates in 50% aq. EtOH was correlated to give ρ=−2.19 and r=0.26 in terms of the LArSR equation. The reduced r value reflects the significant deviation from the coplanarity of the benzylic π-system.

The unexalted resonance scale of substituent effect in the benzylic solvolysis

Abstract The substituent effect on the solvolysis of 4-methylbenzobicyclo[2,2,2]octen-1-yl triflates where no through-conjugative aryl-cation center interaction is possible, can be described in terms of resonance-unexalted σo parameters of the aryl substituents.

Substituent effect on the solvolysis of 2,2-dimethylindan-1-yl chlorides

Abstract The substituent effect on the solvolysis of 2,2-dimethylindan-1-yl chlorides was analyzed in terms of the LArSR Eq., to give ϱ=−5.81 and r=1.14. The coplanarity of α-t-butylbenzyl cation system was discussed.

Substituent effects on the solvolysis of o-methyl- and o,o'-dimethyl-α-t-butylbenzyl tosylates

Abstract Substituent effect in the solvolysis of o-methyl- and 2,6-dimethyl-α-t-butylbenzyl tosylates was analyzed compared with that of the parent α-t-butylbenzyl tosylates. No significant steric loss of resonance was observed by introducing o-Me groups

Oxygen-18 scrambling studies in the solvolysis of benzyl tosylates

The ion-pair return process during the solvolysis of benzyl tosylates was studied in typical solvents by means of the sulfonate18O-scrambling experiments using carbon-13 NMR spectroscopy. The substituent and solvent dependency of amount of ion-pair return in this borderline solvolysis was discussed.

Solvent effects on the solvolysis of α-t-butylbenzyl tosylate

Abstract The solvolysis rates of α-t-butylbenzyl tosylate resulted in a single straight-line correlation with unit slope against 2-adamantyl Y OTs value. The result is compatible with k c solvolysis mechanism without nucleophilic solvent and/or β-methyl participation.

Formation and Stability of the 4-Methoxyphenonium Ion in Aqueous Solution

The reaction of 2-methoxyphenylethyl tosylate (MeO-1-Ts) is first-order in [N(3)(-)]. A carbon-13 NMR analysis of the products of the reactions of MeO-1-[α-(13)C]Ts shows the formation of MeO-1-[β-(13)C]OH and MeO-1-[β-(13)C]N(3) from the trapping of a symmetrical 4-methoxyphenonium ion reaction intermediate 2(+). An analysis of the rate and product data provides a value of k(az)/k(s) = 83 M(-1) for partitioning of 2(+) between addition of azide ion and solvent. These data set a limit for the lifetime of 2(+) in aqueous solution.

Oxygen-18 scrambling studies in the solvolysis of α-(t-butyl)benzyl tosylates

Abstract The sulfonate 18 O-scrambling during the solvolysis of α-(t-butyl)benzyl tosylates was investigated in typical solvents by 13 C-NMR spectroscopy using 18 O/ 16 O isotope shift on the alkoxy 13 C-NMR signals. The existence of extensive ion-pair return comparable to product formation was explored in this k c solvolysis.

Kinetic effects of thiourea addition on benzylic solvolyses

Effects of thiourea addition on the solvolysis of benzyl, α-methylbenzyl, and α-t-butylbenzyl chlorides were investigated in 50 % aqueous acetone at 45 °C. Logarithmic plots between the second-order rate with thiourea (kN) and the corresponding solvolysis rate (ksolv) provide a scattered pattern of deviations from a straight line defined for the kc mechanism. Mechanistic change in three solvolyses was discussed based on characteristic deviation behaviors.