X-Ray structure of tetrameric 1-lithio-3-methoxypropane

Directed ortho-lithiation: observation of an unexpected 1-lithio to 3-lithio conversion of 1-lithio-naphthyllithium compounds with an ortho-directing 2-(dimethylamino)methyl group

Regioselectivity is an important aspect in the design of organic protocols involving Directed ortho-Lithiation (DoL) of arenes, in particular with those arenes containing heteroatom substituents as directing groups. The DoL of 2-[(dimethylamino)methyl]naphthalene (dman) that proceeds with low regioselectivity was revisited by varying both the nature of the lithiating reagent (either n-BuLi or t-BuLi) and/or the solvent (pentane or diethyl ether); the 3-deuterated substrate, 3-Ddman, was also investigated as a substrate to compare to that of dman. The 3-lithio regioisomer exists as tetranuclear [2-(Me2NCH2)C10H6Li-3]4, 1, both in the solid state (X-ray) and in solution (NMR). The 1-lithio regioisomer, 2a, is insoluble; in the presence of additional coordinating solvents (Et2O) or ligands (dman), it exists as dinuclear [2-(Me2NCH2)C10H6Li-1]2·L (coordinated L = Et2O: 2b, dman: 2c) in apolar solvents. Heating solutions of 2c in toluene-d8 (to 90 °C) induced a surprisingly clean and quantitative 1-lithio to 3-lithio conversion of the 1-lithio-naphthalene isomer. This type of reaction is rare in organolithium chemistry and has obvious significant implications for the design of regioselective DoL protocols; this thus represents the synthetically useful protocol for the DoL of dman in a one-pot/two-step process in toluene solution. The results of the use of 3-Ddman in these reactions gives strong credence to a mechanism involving formation of the heteroleptic species [(2-(Me2NCH2)C10H6-1)(2-(Me2NCH2)C10H6-3)Li2]·[dman], A, as the key intermediate. Intramolecular trans-lithiation takes place with A; dman becomes selectively lithiated at its 3-position, while the formerly 1-lithio-naphthalene fragment, acting as a highly unusual ortho-lithiating reagent, is converted into the N-coordinated amine, dman. In this intramolecular DoL process, free dman can be considered to act as a catalyst.

(MeLi)4(dem)1.5]infinity] and [(thf)3Li3M3[(NtBu)3S--how to reduce aggregation of parent methyllithium

Organolithium compounds play the leading role among the organometallic reagents in synthesis and in industrial processes. Up to date industrial application of methyllithium is limited because it is only soluble in diethyl ether, which amplifies various hazards in large-scale processes. However, most reactions require polar solvents like diethyl ether or THF to disassemble parent organolithium oligomers. If classical bidentate donor solvents like TMEDA (TMEDA= N,N,N',N'tetramethyl-1,2-ethanediamine) or DME (DME=1,2-dimethoxyethane) are added to methyllithium, tetrameric units are linked to form polymeric arrays that suffer from reduced reactivity and/or solubility. In this paper we present two different approaches to tune methyllithium aggregation. In [[(MeLi)4(dem)1,5)infinity] (1; DEM = EtOCH2OEt, diethoxymethane) a polymeric architecture is maintained that forms microporous soluble aggregates as a result of the rigid bite of the methylene-bridged bidentate donor base DEM. Wide channels of 720 pm in diameter in the structure maintain full solubility as they are coated with lipophilic ethyl groups and filled with solvent. In compound 1 the long-range Li3CH3...Li interactions found in solid [[(MeLi)4]infinity] are maintained. A different approach was successful in the disassembly of the tetrameric architecture of [((MeLi)4]infinity]. In the reaction of dilithium triazasulfite both the parent [(MeLi)4] tetramer and the [[Li2[(NtBu)3S]]2] dimer disintegrate and recombine to give an MeLi monomer stabilized in the adduct complex [(thf)3Li3Me-[(NtBu)3S]] (2). One side of the Li3 triangle, often found in organolithium chemistry, is shielded by the tripodal triazasulfite, while the other face is mu3-capped by the methanide anion. This Li3 structural motif is also present in organolithium tetramers and hexamers. All single-crystal structures have been confirmed through solid-state NMR experiments to be the same as in the bulk powder material.