Mixed Lithium Complexes: Structure and Applicationin Synthesis

Abstract

IntroductionFor half a century, organolithium (C-Li bonded) reagents and lithium amides (N-Li bonded) have played a pivotal role in the development of modern day synthetic chemistry. Reagents in these categories, in particular the butyllithiums (the normal-, secondary-, and tertiary-isomers), lithium diisopropylamide (LDA), lithium 2,2,6,6-tetramethylpiperid-1-ide (LiTMP), and lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS) are utilized in most synthetic laboratories around the world and are especially important in the metallation reaction (that is, the conversion of a relatively inert C-H bond into a more reactive (more chemically pliable) C-Li bond) and in the kinetically operative metal-halogen exchange reactions. The past two decades have witnessed several new classes of constitutionally more complex reagents that contain multimetal and/or multianion formulations that have challenged the dominance of these unimetallic/unianionic reagents in deprotonative/metal-halogen exchange chemistry. This chapter covers some of the rich structural and synthetic chemistry of these new and still emerging systems, detailing only a small selection of what these classes of compounds can offer the chemist at the present time. Structural Chemistry of Heterometallic Lithium ComplexesStructure is inextricably and mutually linked to reactivity; hence, much recent research activity has focused on the elucidation of solid-state and solution structures of lithium organometallics and amides. This section concentrates on the structural chemistry of lithium-containing mixed-metal complexes, focusing particularly on mixed s-block (Group 1/Group 1 or Group 1/Group 2) complexes, and those containing the Group 12 d 10 metal zinc, which is commonly regarded as a displaced alkaline earth metal. In recent years, heterometallic sodium reagents (particularly sodium zincates, magnesiates, aluminates, etc.) have received a great Lithium Compounds in Organic Synthesis: From Fundamentals to Applications, First Edition. Edited by Renzo Luisi and Vito Capriati.

C−N Bond Activation and Ring Opening of a Saturated N‐Heterocyclic Carbene by Lateral Alkali‐Metal‐Mediated Metalation

“…Contrastingly, K forms a series of π‐K…C interactions, engaging in a η 6 ‐fashion with the mesityl fragments of the two amido ligands. These well‐defined distinct bonding modes of Li and K contrast with those found in other mixed lithium/potassium amides such as [LiK(TMP) 2 (PMDETA)] where both metals are bridged by amide N atoms . Potassium is further solvated by two molecules of THF while another THF completes the distorted trigonal planar geometry around Li (sum of the angles around Li, 359.8°), with a noticeably obtuse NLiN angle [133.3(4)°].…”

“…Contrastingly,Kforms as eries of p-K…C interactions,e ngaging in a h 6 -fashion with the mesityl fragments of the two amido ligands.T hese well-defined distinct bonding modes of Li and Kcontrast with those found in other mixed lithium/potassium amides such as [LiK(TMP) 2 -(PMDETA)] where both metals are bridged by amide N atoms. [28] Potassium is further solvated by two molecules of THF while another THF completes the distorted trigonal . This distortion is probably imposed by the formation of ac oordination pocket for the {K(THF) 2 } + cation with the mesityl rings.S tructurally defined potassium lithiates are extremely rare,w ith ar ecent example of ap otassium diboryllithiate reported by Nozaki [29] in 2016, where similar Li-B/K-aryl interactions are observed.…”

Lithium, Sodium, and Potassium Magnesiate Chemistry

“…have come to the fore as a new class of compounds capable of smoothly performing deprotonation reactions. [5][6][7][8][9][10][11][12] These reagents often offer enhanced functional group tolerance, greater stability in common laboratory solvents, and also reactions can be performed at ambient temperature (rather than at −78°C). The bimetallic compounds are often referred to as 'ate' complexes, a term coined by Wittig in 1951 when he studied bimetallic compounds such as the lithium magnesiate LiMgPh3, lithium zincate LiZnPh3 and 'higher-order' lithium zincate Li3Zn2Ph7.…”

“…Several reviews have been published in this area. [6][7][8][9][10][11][12] In this chapter, an overview of the recent structural chemistry (from 2007-2015) is presented focusing specifically at the metal pairs utilized.…”