Heavy Grignard Reagents: Synthesis, Physical and Structural Properties, Chemical Behavior, and Reactivity

Abstract

The Grignard reaction offers a straight forward atom-economic synthesis of organomagnesium halides, which undergo redistribution reactions (Schlenk equilibrium) yielding diorganylmagnesium and magnesium dihalides. The homologous organocalcium complexes (heavy Grignard reagents) gained interest only quite recently owing to several reasons. The discrepancy between the inertness of this heavy alkaline earth metal and the enormous reactivity of its organometallics hampered a vast and timely development after the first investigation more than 100 years ago. In this overview the synthesis of organocalcium reagents is described as is the durability in ethereal solvents. Aryl-, alkenyl-, and alkylcalcium halides are prepared by direct synthesis. Characteristic structural features and NMR parameters are discussed. Ligand redistribution reactions can be performed by addition of potassium tert-butanolate to ethereal solutions of arylcalcium iodides yielding soluble diarylcalcium, whereas sparingly soluble potassium iodide and calcium bis(tert-butanolate) precipitate. Furthermore, reactivity studies with respect to metalation and addition to unsaturated organic compounds and metal-based Lewis acids, leading to the formation of heterobimetallic complexes, are presented.

Low‐Coordinate Barium Boryloxides: Synthesis and Dehydrocoupling Catalysis for the Production of Borasiloxanes

“…Complexes of the large alkaline earths (Ae) calcium, strontium, and barium are increasingly used in molecular catalysis as alternatives to mainstream late transition metal catalysts. [1] These developments can be paralleled with the many breakthroughs in the organometallic chemistry of calcium, [2][3][4] while in comparison progress with strontium [5] and barium [6] has been slow owing to greater synthetic challenges.C a 2+ (r ionic = 1.00 ), Sr 2+ (1.18 )a nd Ba 2+ (1.35 )a re large,e lectropositive ions that generate highly ionic d 0 complexes.D espite their trademark reactivity that generally increases upon descending group 2, heteroleptic Ae complexes may be plagued by ligand scrambling.H ence, although they often demonstrate the best aptitude in catalysis, [1,7] barium complexes can also suffer from excessive basicity,k inetic lability,a nd propensity to form polynuclear, insoluble species. [8] Discrete molecular barium alkoxides are scarce.Barium is characterized by its large size,a ffinity for high (8 or more) coordination numbers,a nd pronounced oxophilicity and electropositivity.W ith regular alcohols,f or example, t BuOH, it yields polymeric alkoxides of low solubility that sometimes decompose to oxoclusters.…”

“…[11] Yet, nothing is known about barium boryloxides,i ns pite of occurrences for other main group metals, [17] including magnesium. [18] Due to delocalization of the Oa tom lone pairs in the empty p z orbital at boron, boryloxides R 2 BO À may be seen as electron poor analogues of alkoxides,o re ven of ketones or ketyl radicals.Onthis account, and in conjunction with its great steric demands,w er easoned that {(Me 3 Si) 2 CH} 2 BO À should afford soluble,l ow coordinate, highly electrophilic and hence reactive barium species.T his strategy met with success,a nd we report herein on the first barium boryloxides,t hat exhibit low coordination numbers (2)(3)(4)…”

Kudos and Renaissance of s-Block Metal Chemistry

“…This fact led to the increasing impact of this chemistry in all fields of chemistry ( Figure 1), especially in organometallic and catalytic applications [15,16]. Thus, the delayed development of a sophisticated organic [17] and coordination chemistry of calcium [18] led to the nickname of "sleeping beauty" for this dormant element [18,19]. Lithium (and to a lesser extend sodium and magnesium) will become the foundation for car batteries, ensuring electric-based transport and, hence, the demand (as well as the price) will increase significantly.…”

“…Future tasks related to the chemistry of these s-block organometallics include stability and reactivity [17], catalysis and stoichiometric conversions [18,21], Lewis acidity and nucleophilicity [24]. Quantum chemical calculations are valuable tools to elucidate the agostic and π-interactions between the s-block metal ions and multiple bonds [25,26].…”

Low‐Coordinate Barium Boryloxides: Synthesis and Dehydrocoupling Catalysis for the Production of Borasiloxanes

“…[1] These developments can be paralleled with the many breakthroughs in the organometallic chemistry of calcium, [2][3][4] while in comparison progress with strontium [5] and barium [6] has been slow owing to greater synthetic challenges. Ca 2+ (rionic = 1.00 Å), Sr 2+ (1.18 Å) and Ba 2+ (1.35 Å) are large, electropositive ions that generate highly ionic d 0 complexes.…”