Interconversion of Contact and Separated Ion Pairs in Silyl- and Arylthio-Substituted Alkyllithium Reagents

Abstract

Ether-solvated contact and separated ion pairs (CIP and SIP) for two lithium reagents, tris(trimethylsilyl)methyllithium (1) and bis(3,5-bistrifluoromethylphenylthio)methyllithium (2), have been characterized and observed for the first time under conditions of slow exchange by NMR spectroscopy, and barriers to interconversion have been measured. A Saunders isotope perturbation experiment was used to support identification of the CIP and SIP species for 2.

What’s Going on with These Lithium Reagents?

“…The only exception was 42, where the barrier for triple ion dissociation was >16 kcal/mol. 34e, 51,52 This chemistry is discussed in a separate section on RINMR below.…”

“…We were not able to study their reactivity, since in almost all cases they were in sufficiently rapid equilibrium with other species that even under optimum conditions direct study with our rapid-inject NMR (RINMR) apparatus (see below) was not possible. The only exception was 42 , where the barrier for triple ion dissociation was >16 kcal/mol. ,, This chemistry is discussed in a separate section on RINMR below.…”

“…The latter effect should be especially important in epoxide openings, in which the leaving group is very basic (lithophilic), and less so or absent in halide or sulfonate displacements. Our study involved three lithium reagents: bis(3,5-bistrifluoromethylphenylthio)methyllithium 79 , a SIP in THF, bis(phenylthio)methyllithium 80 , a weak CIP (half ionized at 1 equiv of HMPA, Figure , a), and 2-lithiodithiane 81 , a strong CIP (half ionized at 4.5 equiv of HMPA) . All three species are monomeric.…”

“…Some typical measured barriers of lithium species interconversion determined by DNMR methods: (a) CIP/SIP; (b) CIP/SIP; (c) D/M; (d) D/M; , (e) D/M; (f) T/D; ,, (g) TI/monomers; (h) TI/monomers; , (i) racemization; (j) racemization; (k) TI/quadruple ion . In all of these cases, both species are present in sufficient concentrations for individual study if under non-Curtin–Hammett conditions.…”

Synthetic Potential of Configurationally Unstable Chiral Carbanions

“…Strong solvating agents such as THF, 1,2-dimethoxyethane (DME), and diglyme facilitate charge delocalization into the double bond and the phenyl ring by conversion of a contact ion pair (CIP) 22 into a SIP 23. 17,18 In bulkier solvents such as MTBE and CPME, the solvation becomes less effective due to the steric repulsion with 23. 19 Although a similar type of repulsion can also exist in 22, this is reduced when the solvent molecule is replaced with the allyl ether oxygen, as found in 25.…”