Cyclopropyllithium aggaregation and¹⁹⁹Hg NMR as a tool for exchange studies with organo‐mercury compounds

Abstract

Low-temperature NMR studies revealed aggregate formation in cyclopropyllithium compounds. For C 3 H 5 Li (1), a static tetramer [ 1 J( 6 Li, 13 C) = 6.5 Hz] was found in diethylether (DEE), while in tetrahydrofuran (THF) a dimer and a fluxional tetramer [ 1 J( 6 Li, 13 C) = 9.3 and 4.7 Hz, respectively] were detected. Trans-2,3-dimethyl-1 (2) yielded in DEE/THF(1:1) diastereomeric dimers [ 1 J( 6 Li, 13 C) = 9.3 Hz] and 2,2,3,3-tetramethyl-1 (3) formed dimers in DEE and DEE/THF [ 1 J( 6 Li, 13 C) = 10.0 and 9.6 Hz, respectively]. For bis-(trans-2,3-dimethylcyclopropyl)-mercury (7), ligand exchange could be detected by the isotopic fingerprint method with 199 Hg NMR at room temperature. 199 Hg NMR in connection with specific deuteration may thus be used for ligand exchange studies in the field of organomercury compounds.

NMR spectroscopy of organolithium compounds, part XXXIII: trans‐2,3‐dimethylcyclopropyllithium aggregation in diethylether: An equilibrium between three different complexes of comparable energy and the influence of LiBr on aggregate structure

“…The triplet and the doublet result from a dimer and a monomer , respectively, while the quintuplet characterizes a fluxional tetramer . A dimer and a fluxional tetramer was already observed for 1 in THF, but a monomer was in this case not found . In the fluxional tetramer with intra ‐aggregate exchange, the number of “next” neighbors is 4 compared with 3 in the static tetramer and thus provides the 6 Li environments dddd , ( dddh , ddhd , dhdd , hddd ), ( ddhh , dhdh , dhhd , hdhd , hddh , hhdd ), ( hhhd , hhdh , hdhh , dhhh ), and hhhh that lead to a 1:4:6:4:1 quintuplet for the 6 Li resonance.…”

“…58.88‐MHz 6 Li NMR spectrum of cyclopropyllithium in the presence of LiBr (1:1) without 1 H decoupling at various temperatures in DEE [23a] ; external reference .0 M LiBr in THF…”

“…Recently published examples of monomer stabilization by THF were found for trisolvated 1‐penylcyclopropyllithium and trisolvated iso ‐propyllithium . In DEE/THF (1:1), the methyl‐substituted cyclopropyl compounds 2 and 3 form only dimers …”

“…We have recently reported that cyclopropyllithium ( 1 ) forms static tetramers in DEE and dimers as well as fluxional tetramers in THF . In a fluxional aggregate, intra ‐aggregate exchange of lithium cations is still fast on the NMR timescale while inter ‐aggregate exchange is slow.…”

Triangles and Squares for a Unique Molecular Crystal Structure: Unsupported Two‐Coordinate Lithium Cations and CC Agostic Interactions in Cyclopropyllithium Derivatives

“…In hydrocarbon solution, the tetrameric structure of the tertiary alkyllithium t BuLi is preserved whereas equilibria between hexameric and tetrameric aggregates are established for secondary alkyllithiums i PrLi and ( c ‐C 5 H 9 )Li . ( c ‐C 3 H 5 )Li was recently shown to be a static tetrahedral tetramer in [D 10 ]diethylether …”