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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 sp2-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(CH2SiMe3)(thf)3(m-Cl)]2.
Furthermore, the metathetical approach of reacting
[Ca(CH2SiMe3)I(thf)4] with KN(SiMe3)2 and the addition of
N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (pmdeta) allowed
the isolation of heteroleptic [CaCH2SiMe3{N(SiMe3)2}-
(pmdeta)]. In the reaction of this derivative with phenylsilane,
the trimethylsilylmethyl group proved to be more reactive than the bis(trimethylsilyl)amido substituent.