Solution Structures of Hauser Base ⁱPr₂NMgCl and Turbo-Hauser Base ⁱPr₂NMgCl·LiCl in THF and the Influence of LiCl on the Schlenk-Equilibrium

Abstract

Grignard reagents that are at the simplest level described as "RMgX" (where R is an organic substituent and X a halide) are one of the most widely utilized classes of synthetic reagents. Lately, especially Grignard reagents with amido ligands of the type R1R2NMgX, so-called Hauser bases, and their Turbo analogue R1R2NMgX·LiCl play an outranging role in modern synthetic chemistry. However, because of their complex solution behavior, where Schlenk-type equilibria are involved, very little is known about their structure in solution. Especially the impact of LiCl on the Schlenk-equilibrium was still obscured by complexity and limited analytical access. Herein, we present unprecedented insights into the solution structure of the Hauser base (i)Pr2NMgCl 1 and the Turbo-Hauser base (i)Pr2NMgCl·LiCl 2 at various temperatures in THF-d8 solution by employing a newly elaborated diffusion ordered spectroscopy (DOSY) NMR method hand-in-hand with theoretical calculations.

Solution Structure of Turbo‐Hauser Base TMPMgCl⋅LiCl in [D8]THF

“…Our investigations confirm that lithium chloride indeed co‐coordinates to the magnesium amides in solution as observed in the Turbo‐Hauser base 2 . A detectable population of the solvent separated lithium cation Li2 can also be excluded for Turbo‐Hauser base 1 (MW calcd =295 g mol −1 , MW det =387 g mol −1 , MW err =−31 %).…”

“…At least, in the solid state the Turbo‐Hauser bases TMPMgCl ⋅ LiCl 1 C and DAMgCl ⋅ LiCl 2 A (Figure b) supported the existence of a contact ion pair (CIP). Recently, we showed that redissolved crystals of 2 A do not form SSIPs in THF solution either . Instead, the dimeric crystal structure 2 A equilibrates with its monomeric counterpart 2 C at room temperature.…”

“…Furthermore, it is independent of diversities in temperature and viscosity and offers an easy and robust approach to determine accurate MWs in THF solution with an error of less than ±9 % . Employing this method, we were already able to characterize the complex oligomeric mixture of lithium diisopropylamide (LDA) in toluene solution and the structure of DAMgCl and DAMgCl ⋅ LiCl 2 in [D 8 ]THF solution . In the current paper, we describe DOSY‐ECC‐MW‐determinations of the TMP‐Turbo‐Hauser base TMPMgCl ⋅ LiCl 1 in [D 8 ]THF solution.…”

“…Changing the steric bulk and the electronic properties of the amide strongly controls the structural features, both in the solid state as well as in solution. While 1 is a monomer in the solid state and in solution, 2 is a dimer in the crystal structure and forms predominately dimeric aggregates in THF solution . The deviation in the aggregation state and steric demand is also reflected in the different reactivity and regioselectivity of both Turbo‐Hauser bases (Scheme ).…”

A Novel Bulky Heteroaromatic‐Substituted Methanide Mimicking NacNac: Bis(4,6‐tert‐butylbenzoxazol‐2‐yl)methanide in s‐Block Metal Coordination

“…Starting materials were purchased commercially and were used as received, unless stated otherwise. ( i Pr) 2 NMgCl and KC 8 were synthesized according to literature procedures . 1 H‐ and 13 C NMR spectra were recorded with a Bruker Avance 300 or 400 MHz spectrometer and are referenced to the residual proton signal of the deuterated solvent .…”