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

Reduction of para-substituted iodobenzene in tetrahydropyran
(THP) with finely dispersed calcium powder yields arylcalcium iodides
of the type [(THP)4Ca­(C6H4-4-R)­I]
with R = CH3 (1), Cl (2), Br
(3), I (4), OCH3 (5). A 2-fold insertion of calcium into dihalobenzenes was not observed.
The β-naphthylcalcium iodide [(THP)4Ca­(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)2Li}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)2Li­(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-CH2Ph]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.