Chelated aryllithium reagent: ortho- and para-pyrrolidinomethylphenyllithium

Hans J Reich,Aaron W Sanders,Wayne S Goldenberg,Joachim Schantl

doi:10.3998/ark.5550190.0005.d11

Abstract

Multinuclear NMR studies have been performed on two isomeric aryllithium reagents, one with an ortho amine chelating group (5-ring chelate, 4) and the second, a non-chelating model (5) with the same group in the para position. Both compounds were prepared isotopically enriched in 6 Li as solutions in THF or mixtures of THF with ethyl and/or methyl ether. Solution dynamics (aggregate interchange, chelation averaging) were measured by DNMR experiments. The model compound 5 is a mixture of dimer and monomer in THF (KMD = 180 M -1 ). Several bidentate chelating cosolvents (TMEDA, dimethyl-2-methoxyethylamine and 2-dimethylamino- methyltetrahydrofuran) interacted with 5 to give mono and bis complexes of the dimers as well as mono-complexed monomer, with only minimal changes in the monomer to dimer ratio. Compound 4 in THF solution forms a strong dimer as a single chelation isomer of the B-type. The dynamics of decoordination of the pyrrolidine groups was measured by DNMR coalescence of the diastereotopic pyrrolidine ring carbons ( ‡ 190 = 8.5 kcal/mol), and the dynamics of aggregate exchange were measured both by loss of C-Li coupling, and by exchange of 4 dimer with the mixed dimer 16 ( ‡ 246 = 12.5-13.0 kcal/mol). Compound 4 interacts only very weakly with TMEDA, but HMPA causes significant conversion of dimer to monomer. The Li/I exchange of 4 and the analogous iodide 17 was slower by 5 orders of magnitude than the PhLi/PhI exchange. A 119 Sn NMR study of a series of trimethylstannylaryl compounds with ortho substituents that could form a 5-membered chelate with an ether or amino group were weakly chelated.

Highlights

The detailed consideration of chelation effects in organolithium reagents began with the work of Hauser[1] who recognized that the dimethylaminomethyl group could facilitate the metalation of ortho aryl hydrogens which otherwise could not be metalated.Numerous subsequent studies have involved many presumably chelated lithium and other organometallic reagents.[2,3] N BuLi 1 N Li Scheme 1The structural aspects of compound 2 and some of its analogs have been studied extensively.[4,5] The X-Ray structure of crystals grown from ether-hexane showed 2 to be tetrameric, with each lithium vertex coordinated by a nitrogen.[4a]
It resembles the structure of tetrameric PhLi[6] except that the amino group replaces the coordinated ether molecule on each Li atom
The aryl bromides were not used directly to prepare lithium reagents for spectroscopic studies because of interference from LiBr or bromobutane. Both lithium reagents were prepared in ether by Li/Sn exchange, and the lithium reagent was purified by crystallization from diethyl or dimethyl ether before solutions were prepared for spectroscopic study, using solvent mixtures containing THF, dimethyl ether, and/or ether

Summary

Introduction

The solution structure of 5 is similar to that of PhLi,[7c,8a,9,10] with both dimer and monomer present in THF (Figure 3a). 13C variable temperature NMR spectra of 17, 4, and a sample containing 0.13 M 4 and 0.26 M 17 in THF/ether (3:2).

Results

Conclusion