Variations in the solid-state, solution and theoretical structures of a laterally deprotonated aromatic tertiary amide

Abstract

Reaction of 2-ethyl-N,N-diisopropyl-1-naphthamide 3 with Bu t Li in tetrahydrofuran (thf) affords a laterally metallated derivative which exists as a tris(thf) solvated monomer with no Li-C interaction and an sp 2 hybridised carbanionic centre in the solid-state; NMR spectroscopy suggests that this structure is viable in solution but that Li-C bonded atropisomers are also possible and calculations corroborate these data.

Controlling Chemoselectivity in the Lithiation of Substituted Aromatic Tertiary Amides

“…As with 6, [14] NMR spectroscopy reveals diastereotopic isopropyl groups at low temperature for samples of both 8 and 9 in [D 8 ]THF solution. In fact, for both of these complexes, data indicate that dissolution is accompanied by significant structural reorganization to afford three solution entities in a 2:10:1 ratio and 1 H NMR spectra that, notwithstanding the presence of THF (see 8) or pmdeta (see 9), are essentially identical (see the Supporting Information).…”

“…[13] Contrastingly, the laterally deprotonated salt of 5 reveals a tris(thf) solvate, 6, in which the metal center is only coordinated by O atoms with no C···Li interaction, thus allowing the amide and aromatic planes to be near to perpendicular in the solid state. [14] These data suggest a link between the number of donor atoms per solvent molecule (solvent denticity) and reaction chemoselectivity and lead us to report here on the competitive deprotonation of 2-ethyl-N,N-diisopropyl-1-benzamide, 7.…”

The Directed Lithiation of Benzenoid Aromatic Systems

“…Crystallography reveals that (iPr) 2 NC(O)C 10 H 6 CH-(Li·3THF)Me-2 (86) is monomeric in the solid state, there being no LiϪC interaction and the metal instead being entirely oxygen-supported. [102] Calculations (at the 6-311G**/ B3LYP level) on (iPr) 2 NC(O)C 10 H 6 CH(Li·3OMe 2 )Me-2 corroborate the stability of the crystallographically characterised tris(solvate) and suggest that cleavage of the LiϪC α interaction occurs in order to allow coordination of the third solvent molecule. In consequence, the calculated enthalpy of complexation for the introduction of this donor molecule is largely negated.…”

“…[98] The establishment that lateral metallation occurs under the influence of a tertiary amide directing group has recently been probed both theoretically and in the solid state (see below). [102] This follows on from formative studies, which established that enantiomerically pure α-stanyllated anilides undergo tin-lithium exchange and electrophilic workup with retention and inversion of configuration, respectively. [103] Thus, 2-ethyl-N,N-diisopropyl-1-naphthamide has been metallated using s-BuLi to give a pair of configurationally stable diastereomeric syn atropisomers of (iPr) 2 NC(O)C 10 H 6 CHLiMe-2 (85a).…”

“…However, at Ϫ48°C in [D 8 ]-toluene all 1 H NMR resonances split into two in a 60:40 ratio, suggesting that diastereoisomers interconvert very much faster, with NMR spectroscopy showing rapid exchange in solution above this temperature. [102] Metallation of the tertiary phosphane MeP(C 6 H 4 CH 2 NMe 2 Ϫ2) 2 using tBuLi yields a dimer of the benzyllithium product MeP(C 6 H 4 CH 2 NMe 2 Ϫ2)-(C 6 H 4 CHLiNMe 2 Ϫ2) 2 (87) that exhibits novel dynamics in [D 8 ]toluene solution. [105] In spite of this it is apparent that the main features of the solid-state structure are retained at 203 K. However, 31 P and 7 Li NMR spectroscopy reveal that at elevated temperatures each metal can be associated with either phosphorus atom in the dimeric structure, but not with both simultaneously.…”