Advanced NMR Methods and DFT Calculations on the Regioselective Deprotonation and Functionalization of 1,1′‐Methylenebis(3‐methylimidazole‐2‐thione)

Abstract

Depending on the temperature, the twofold deprotonation of 1,1′‐methylenebis(3‐methylimidazole‐2‐thione) (1) and the subsequent reaction with 2 equiv. of trimethylsilyl chloride (TMSCl) gives two different bis‐TMS‐functionalized isomers, namely, 1,1′‐methylenebis(3‐methyl‐4‐trimethylsilylimidazole‐2‐thione) (2) and 1,1′‐methylenebis(3‐methyl‐5‐trimethylsilylimidazole‐2‐thione) (3). The cyclic dimethylsilyl‐bridged derivative 1,1′‐methylene‐5,5′‐dimethylsilylenebis(3‐methylimidazole‐2‐thione) (4) can also be obtained, corroborating the 5/5′ addition under certain conditions. All compounds have been examined by multinuclear 1D and 2D NMR experiments (1–4) together with single‐crystal X‐ray diffraction (3 and 4). Additionally, the dilithiated species 5 was synthesized by reacting 1 with 2 equiv. of nBuLi at ambient temperature in solution (THF). 1H and 7Li pulsed field‐gradient spin‐echo (PGSE) NMR, 7Li–1H heteronuclear Overhauser spectroscopy (HOESY), gradient heteronuclear multiple quantum correlation (gHMQC) and gradient heteronuclear multiple bond correlation (gHMBC) experiments showed that 5 exists as a monomeric contact ion pair (CIP) in THF solution. On the contrary, the X‐ray diffraction analysis of 5 revealed a polymeric chain, which can be described as [{5(thf)2}2]∞. Quantum chemical DFT and MP2 calculations were also conducted to determine the energies required for the deprotonation of 1. These results explain the regioselective deprotonation of 1 by CIP formation depending on the temperature and fully support the results of the synthetic and spectroscopic experiments.

Synthesis and Properties of Backbone Silylated Imidazol‐2‐thiones

“…The observed bond lengths and angles are unexceptional and compare well to those found in the structures of the parent imidazol‐2‐thione 1 or the C‐functionalized imidazol‐2‐thiones III and VII [15,17,26] . Silyl substitution produces a slight elongation of the endocyclic C=C bonds by <0.6 Å.…”

“…An alternative approach to silylimidazole-2-thiones has recently been published by Kuzu et al (Scheme 2). [17] In close analogy to Streubel's procedure for the synthesis of P-functionalized 1,3-diorganoimidazole-2-thiones, methylenebis(3-methylimidazole-2-thione) was deprotonated with nBuLi followed by the addition of Me 3 SiCl to give the backbone silylated bridged species VI. The fused tricyclic species VII was obtained analogously with Me 2 SiCl 2 .…”

NHCs in Main Group Chemistry

“…The lithium–C4 bond length (2.116(5) Å) is in a range similar to that of the BR 3 adducts 51 . Studies about regioselective backbone deprotonation of methylene-bridged bis­(imidazole-2-thiones) leading to a NHC lithium complexes were also reported . Lithium a NHC complexes have been utilized in the postcoordination backbone functionalization of NHCs …”

“…Studies about regioselective backbone deprotonation of methylene-bridged bis(imidazole-2-thiones) leading to aNHC lithium complexes were also reported. 135 Lithium aNHC complexes have been utilized in the postcoordination backbone functionalization of NHCs. 136 Lithium NHC complexes with an (anionic) backbonesubstituent have also been reported.…”

Hydrosilylation of Carbonyl Compounds Catalyzed through a Lithiated Hydrazone Derivative

“…We have occasionally observed that the solution and solid-state structures of group 1 salts may be different and are strongly dependent on the choice of solvent . Consequently, we have studied the solution structure of 2 via multinuclear NMR spectroscopy together with 1 H and 7 Li PGSE NMR diffusion measurements.…”