Direct Synthesis of Heavy Grignard Reagents: Challenges, Limitations, and Derivatization

Abstract

The direct synthesis of organocalcium compounds (heavy Grignard reagents) by the reduction of organyl halides with activated calcium powder succeeded in a straightforward manner for organic bromides and iodides that are bound at sp -hybridized carbon atoms. Extension of this strategy to alkyl halides was very limited, and only the reduction of trialkylsilylmethyl bromides and iodides with activated calcium allowed the isolation of the corresponding heavy Grignard reagents. Substitution of only one hydrogen atom of the methylene moiety by a phenyl or methyl group directed this reduction toward the Wurtz-type coupling and the formation of calcium halide and the corresponding C-C coupling product. The stability of the methylcalcium and benzylcalcium derivatives in ethereal solvents suggests an unexpected reaction behavior of the intermediate organyl halide radical anions. Quantum chemical calculations verify a dependency between the ease of preparative access to organocalcium complexes and the C-I bond lengths of the organyl iodides. The bulkiness of the trialkylsilyl group is of minor importance. Chloromethyltrimethylsilane did not react with activated calcium; however, halogen-exchange reactions allowed the isolation of [Ca(CH SiMe )(thf) (μ-Cl)] . Furthermore, the metathetical approach of reacting [Ca(CH SiMe )I(thf) ] with KN(SiMe ) and the addition of N,N,N',N'',N''-pentamethyldiethylenetriamine (pmdeta) allowed the isolation of heteroleptic [CaCH SiMe {N(SiMe ) }(pmdeta)]. In the reaction of this derivative with phenylsilane, the trimethylsilylmethyl group proved to be more reactive than the bis(trimethylsilyl)amido substituent.

Super‐Bulky Penta‐arylcyclopentadienyl Ligands: Isolation of the Full Range of Half‐Sandwich Heavy Alkaline‐Earth Metal Hydrides

“…Neutral donor solvents have played a crucial role in the stabilization of the heavy alkaline‐earth alkyl and hydride complexes . A series of ethereal co‐solvents (S=THP, Et 2 O, DME, Morph), amine (S=DABCO) and phosphine (S=DMPE) were examined.…”

“…2-Sr and 4-Sr exhibited comparable activities towards alkenes hydrogenation, indicating that THF and DABCO played asimilar effect on catalytic reactions.Compared with 2-Ca, 4-Sr and 4-Ba showed higher reactivity as well as wider substrate scope (entry [13][14][15][16][17][18][19][20]. Di-substituted conjugated alkenes (trans-stilbene and a-Me-styrene) and tri-substituted activated alkenes (trans-1,2-diphenylpropene and phenylcyclohexene) could be fully converted within 1-10 hw ith the highly reactive 4-Ba (entry 14,16,18,20).…”

Chemistry of alkaline earth metals: It is not all ionic and definitely not boring!

“…In analogy to magnesium, calcium is able to form a large number of different Grignard type compounds. It is however interesting that the direct synthesis of methyl calcium complexes in analogy to magnesium is not as simple and rather leads to the formation of Wurtztype coupling products CaX 2 and R-R [59]. Instead, a metathetical approach between LiMe and Ca{N(SiMe 3 ) 2 } 2 allowed to yield dimethylcalcium [60].…”

Straightforward One-Pot Syntheses of Silylamides of Magnesium and Calcium via an In Situ Grignard Metalation Method

“…However, aliphatic hydrocarbylcalcium halides (RCaX) are not accessible via this approach (with the exception of cyclopropylcalcium halide). 27 Instead, Wurtz-type C-C coupling reactions dominate and alkanes R-R form nearly quantitatively. The reason for this strikingly different be-havior of aromatic and aliphatic hydrocarbyl iodides and bromides R-X (X = Br, I) lies in the nature of the hydrocarbyl halide radical anions after the first electron transfer from the metal onto R-X.…”