“LiZn(TMP)₃”, a Zincate or a Turbo‐Lithium Amide Reagent? DOSY NMR Spectroscopic Evidence

Abstract

LiCl counts! DOSY NMR studies (see picture; TMP=2,2,6,6‐tetramethylpiperidide; TMEDA=N,N,N′,N′‐tetramethylethylenediamine) reveal the true nature of the synthetically useful basic mixture formed by reacting three equivalents of LiTMP with one of (TMEDA)⋅ZnCl2 in THF. Surprisingly, Zn(TMP)2 is just a spectator of the mutual interactions shown between LiTMP and the LiCl coproduct released from the transmetalation/“salt elimination” reaction.

Adding a Structural Context to the Deprotometalation and Trans‐Metal Trapping Chemistry of Phenyl‐Substituted Benzotriazole

“…Dimerisation occurs through four bonds, Li1ÀN2' (the central Na tom of the triazole unit), Li2ÀN4' (the Na tom of the amide) and their symmetrical equivalents Li1'ÀN2 and Li2'ÀN4, respectively.B ondingt ot wo Scheme1.Stepwise deprotozincation/iodination reactionso f1 -phenyl-1H-benzotriazole as was reported by Mongin and co-workers. [5] Figure (18);N4'-Li2-C81 10.4(2); N4'-Li2-O2 121.9(2); N4'-Li2-O31 02.8(2); O2-Li2-O39 7.9(2); O2-Li2-C8 108.9(2);O 3-Li2-C81 14.3(2); Li2-C8-C71 30.1(2); N1-N2-N3 109.38(18); C1-C2-C19 124.6 (2);N 4-C24-C19 125.8(2); C24-C19-C2 122.7(2). (b) Cut out of the core of 2 highlightingi ts 6-5-6-5-6f usedr ing system.…”

“…From other literature on this base mixture, it is certain that al ot of chemistry is concealedi nt he reaction coordinateb etween reactant 1 and the final 4,2'-diiodo product. Intuitively,o ne might expect that the base mixture would undergo ac omplete metathesis to generate the ate "LiZn(TMP) 3 " [17] especially as ate reagents have been fundamental to many of the recenta dvances made in metalation chemistry.H owever,D OSY NMR data in the reaction solvent (THF) point to an alternative partial metathesis terminating at neutral,w eakly basic Zn(TMP) 2 ,w ith" LiZn(TMP) 3 "f orbidden on steric grounds, leaving LiTMP (or ac o-complexed/solvated version) [18] as the only starting base in the mixture strong enough to CÀHd eprotonate ab enzotriazole. Following deprotonation of presumably the benzo-4 position( using less base Mongin and co-workers found that this regioselectivity was greatly favoured [5] ), the lithiated benzotriazolec ould undergo transmetalation with Zn(TMP) 2 to make am ono-zincated benzotriazole and LiTMP (Scheme 4).…”

[Li2{CH2S(O)Ph}2(TMEDA)2] – An α-sulfinyl-functionalized methyllithium carbenoid

“…S1). This method allows to estimate -as a type of ''NMR chromatography'' -molecular masses of compounds in solution [26][27][28] and has been used several times in organolithium chemistry, too [29][30][31][32][33][34][35]. Here, to obtain reliable diffusion coefficients, as internal standards of known molecular The 1 H DOSY NMR spectrum of 1 exhibits a diffusion coefficient D of the coligand TMEDA (D = 18.77 Á 10 À10 m 2 /s) which significantly differs from that of 1 (D = 7.28 Á 10 À10 m 2 /s).…”

Long-range Effect of Bromine in the Deprotonative Metalation of Aromatic Compounds

“…and LiTMP (1.5 equiv. ), [25] and postulated to be 1:1 LiTMP-Zn(TMP) 2 (±TMEDA), [26] 2,6-dibromopyridine (19a) is attacked at its 4 position. The 4-iodo derivative 19d is isolated in 87% yield after iodolysis.…”