An Efficient General Synthesis of Halide‐Free Diarylcalcium

Langer, Jens; Krieck, Sven; Görls, Helmar; Westerhausen, Matthias

doi:10.1002/anie.200902203

Cited by 56 publications

(38 citation statements)

References 21 publications

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“…A similar trend was also observed for other arylcalcium species containing bridging aryl groups. 14,15 Additionally, an analogous shift was also found for related phenyllithium species containing terminal or bridging phenyl groups, respectively. 16 This calcium species most likely represents another example of an oxo-centered cluster species, occasionally observed for the aryl derivatives of the heavier alkaline-earth metals.…”

Section: ■ Introductionmentioning

confidence: 60%

Arylcalcium Iodides in Tetrahydropyran: Solution Stability in Comparison to Aryllithium Reagents

et al. 2012

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Reduction of para-substituted iodobenzene in tetrahydropyran (THP) with finely dispersed calcium powder yields arylcalcium iodides of the type [(THP) 4 Ca(C 6 H 4 -4-R)I] with R = CH 3 (1), Cl (2), Br (3), I (4), OCH 3 (5). A 2-fold insertion of calcium into dihalobenzenes was not observed. The β-naphthylcalcium iodide [(THP) 4 Ca(β-Naph)I] (6) is also accessible by direct synthesis in THP. The durability of arylcalcium compounds in THP was studied in comparison to that in THF, and a slightly enhanced lifetime in THP at ambient temperature was observed. Furthermore, the relative reactivity and selectivity of 1 and its lithium counterpart [{(THP) 2 Li} 2 (μ-Tol)(μ-Br)] (7) in the reaction with THP and THF were studied. α-Metalation and subsequent cycloreversion was the major pathway observed for THF in both cases. In the degradation reaction induced by 7, several byproducts arising from carbolithiation and, surprisingly, from β-metalation reactions were identified, while 1 was found to be more selective. The related [(THP) 2 Li(μ-Ph)] 2 (9) was prepared and used to unambiguously identify some of the products. In order to verify the formation of benzyllithium as one of the byproducts, an authentic sample of [(dme)Li(μ-CH 2 Ph] 2 (8) was prepared. In THP, an inversion of the relative reactivity of 1 and 7 was observed and the calcium compound was found to be more reactive than its lithium analogue. The crystal structures of 1−9 were determined by X-ray diffraction studies, and a trans arrangement of the anionic ligands due to electrostatic reasons was observed in case of the hexacoordinated calcium complexes. ■ INTRODUCTIONThe year 2005 marked the occurrence of the first structurally characterized arylcalcium derivatives, independently prepared by the groups of Niemeyer, 1 Harder, 2 and Westerhausen, 3 using different synthetic strategies. During the following years arylcalcium derivates emerged from elusive academic curiosities to an easily accessible group of substances. 4 These compounds show tremendous potential and might be able to challenge the well-established aryllithium derivatives in organic and organometallic syntheses. The worldwide accessibility of calcium, its low price, and its nontoxic behavior regardless of concentration further encourage subsequent investigations. Given its position in the periodic table, the alkaline-earth metal calcium is expected to combine typical properties of s block metals (saltlike behavior, highly heteropolar metal−carbon bonds) and early transition metals (Lewis acidity of cations, d orbital participation, and catalytic activity) within its compounds. Considering a few preconditions, convenient high-yield syntheses of solutions of these post-Grignard reagents have been developed recently. 5 These preconditions are as follows:(i) Activation of calcium succeeds with ammonia, yielding a finely divided highly reactive metal powder. (ii) The direct synthesis has to be performed in ethereal solutions, preferably in THF.(iii) Iodoarenes represent the most suitable substrates; br...

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Section: ■ Introductionmentioning

confidence: 60%

Arylcalcium Iodides in Tetrahydropyran: Solution Stability in Comparison to Aryllithium Reagents

et al. 2012

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“…9−12 Amines were also used to stabilize arylcalcium complexes. 13 As also observed for classical Grignard reagents, a Schlenk-type equilibrium converts arylcalcium halides into homoleptic diarylcalcium and calcium dihalides depending mainly on solvent and temperature. Substitution of iodide by an alkoxide shifts this equilibrium quantitatively to the side of the homoleptic complexes.…”

Section: ■ Introductionmentioning

confidence: 90%

Synthesis and Molecular Structures of Meta-Substituted Arylcalcium Iodides

et al. 2012

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The reduction of meta-methyl-substituted iodobenzene with activated calcium in tetrahydrofuran (THF) yields [(3-MeC 6 H 4 )CaI(thf) 4 ] (1) and [(3,5-Me 2 C 6 H 3 )CaI(thf) 4 ] (2). The reaction of 3-halo-1-iodobenzene with calcium powder leads to the formation of the corresponding post-Grignard reagents [(3-XC 6 H 4 )CaI(thf) 4 ] [X = F (3), Cl (4), Br (5), and I (6)]. The synthesis of the thf adducts of 3-methoxyphenylcalcium iodide (7) and β-naphthylcalcium iodide (8) follows the same strategy. All post-Grignard reagents show a characteristic low-field shift for the calciumbound carbon atoms in 13 C NMR spectra. The molecular structures of 1, 2, 4, and 8 show distorted octahedral environments for the calcium centers with the aryl and halide anions in a trans arrangement. ■ INTRODUCTIONGrignard reagents represent widely used organomagnesium compounds that were introduced into preparative organic and organometallic chemistry more than a century ago. 1−3 Because of the importance of these reagents, Grignard was honored with the Nobel Prize in 1912. Despite the fact that early attempts to prepare arylcalcium halides also date back more than a 100 years, 4 the first structurally characterized arylcalcium complexes consisted of oxygen-centered oligonuclear arylcalcium clusters. 5,6 Nevertheless, this success of isolation of arylcalcium derivatives intensified the efforts to also isolate and characterize arylcalcium halides that can be considered as post-Grignard reagents with mesitylcalcium iodide being the first structurally characterized representative 7 and entering the textbooks shortly thereafter. 8 Knowing the requirements to isolate crystalline arylcalcium complexes, many of such derivatives were prepared and investigated spectroscopically as well as via derivatization reactions. Several recent review articles summarize this vastly growing arylcalcium (post-Grignard) chemistry. 9−12The synthesis of arylcalcium complexes succeeds with high yields if iodoarenes are reduced with activated calcium in Lewis basic solvents, such as ethers. 9−12 Amines were also used to stabilize arylcalcium complexes. 13 As also observed for classical Grignard reagents, a Schlenk-type equilibrium converts arylcalcium halides into homoleptic diarylcalcium and calcium dihalides depending mainly on solvent and temperature. Substitution of iodide by an alkoxide shifts this equilibrium quantitatively to the side of the homoleptic complexes. 14 Because of the high reactivity of the arylcalcium complexes, degradation of ethers often was observed, 7,15,16 as is also known for organyllithium 17 and, in minor extent, for Grignard reagents, 18 justifying to consider these organometallics as superbases. 19 On the basis of similar electronegativities of lithium and calcium, a comparable reactivity can be envisioned that should be higher than that of Grignard reagents due to a larger ionic character of the metal−carbon bonds.Despite the vastly growing knowledge of post-Grignard reagents, the influence of the substitution pattern on reactivity an...

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“…In this aim, two fundamental steps are well identified. Following the landmark description of Ca{CH(SiMe 3 ) 2 } 2 •(dioxane) 2 by Cloke and Lappert,2 Westerhausen and coworkers revealed that more than mere heavier analogues of Grignard's magnesium reagents, non‐cyclopentadienyl3 organometallic calcium‐hydrocarbyl species displayed a unique reactivity of their own 4–10. Elsewhere, reliable access to a range of stable bis(amido) Ae precursors,11 amongst which, is the highly influential Ae{N(SiMe 3 ) 2 } 2 •(THF) 2 ,12 triggered the development of synthetic tools for the rational design of heteroleptic Ca‐Ba complexes with well‐controlled coordination spheres.…”

Section: Introductionmentioning

confidence: 99%

Calcium, Strontium and Barium Homogeneous Catalysts for Fine Chemicals Synthesis

Sarazin

Carpentier

2016

The Chemical Record

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The large alkaline earths (Ae), calcium, strontium and barium, have in the past 15 years yielded a brand new generation of heteroleptic molecular catalysts for the production of fine chemicals. However, the integrity of these complexes is often plagued by ligand redistribution equilibria in solution. This personal account retraces the paths followed in our research group towards the design of stable heteroleptic alkalino-earth complexes, including the use of intramolecular noncovalent Ae···H-Si and Ae···F-C interactions. Their implementation as homogenous precatalysts for reactions such as the intramolecular and intermolecular hydroamination and hydrophosphination of activated alkenes, the hydrophosphonylation of ketones, and the dehydrogenative coupling of amines and hydrosilanes that enable the efficient and controlled formations of CP, CN, or SiN σ-bonds, is presented in a synthetic perspective that highlights their overall outstanding catalytic performance.

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An Efficient General Synthesis of Halide‐Free Diarylcalcium

Cited by 56 publications

References 21 publications

Arylcalcium Iodides in Tetrahydropyran: Solution Stability in Comparison to Aryllithium Reagents

Arylcalcium Iodides in Tetrahydropyran: Solution Stability in Comparison to Aryllithium Reagents

Synthesis and Molecular Structures of Meta-Substituted Arylcalcium Iodides

Calcium, Strontium and Barium Homogeneous Catalysts for Fine Chemicals Synthesis

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