Calcium Hydride Insertion Reactions with Unsaturated C-C Bonds

The dimeric β-diketiminato calcium hydride, [(BDI)CaH]₂ (BDI = HC{(Me)CN-2,6-i-Pr₂C₆H₃}₂), reacts with terminal alkenes to provide the corresponding alkyl derivatives. With 1-octene, 3,3-dimethyl-1-butene, 3-phenyl-1-propene, and 4-phenyl-1-butene, the ultimate products are dimeric alkyl species, which form sequentially via dinuclear calcium hydrido-alkyl intermediates. Reaction of diphenylacetylene with [(BDI)CaH]₂ provides a tetra-calcium complex in which a stilbene dianion bridges symmetrically between two hydrido-bridged bis-β-diketiminato calcium units, while treatment with trans-stilbene yields a dicalcium benzyl-hydride derivative, [{(BDI)Ca}₂(H)(PhCHCH₂Ph)]. Reaction of [(BDI)CaH]₂ and 1,5-hexadiene has previously been observed to provide facile 5-exo-trig cyclization to calcium cyclopentylmethyl derivatives. In contrast, analogous reaction with 1,7-octadiene provides exclusive intermolecular insertion to yield a dimeric open chain calcium oct-7-en-1-yl derivative. Reaction with the internal alkene, norbornene, enables the identification of a highly reactive dinuclear calcium norbornyl-hydride. The greater basicity of the calcium secondary alkyl species is emphasized by the subsequent isolation of a product of intramolecular deprotonation of a BDI iso-propyl methyl substituent.