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.在四氢吡喃（tetrahydropyran, THP）中，采用精细分散的钙粉还原对位取代碘苯，可得到通式为[(THP)₄Ca(C₆H₄-4-R)I]的芳基碘化钙，其中R分别为CH₃（化合物1）、Cl（化合物2）、Br（化合物3）、I（化合物4）、OCH₃（化合物5）。未观察到钙对二卤代苯发生两次插入反应的情况。β-萘基碘化钙[(THP)₄Ca(β-Naph)I]（化合物6）同样可通过在THP中直接合成获得。研究了芳基钙化合物在THP中的化学稳定性，并与四氢呋喃（tetrahydrofuran, THF）中的稳定性进行对比，结果显示室温下芳基钙化合物在THP中的稳定寿命略有延长。此外，还考察了化合物1及其锂类似物[{(THP)₂Li}₂(μ-甲苯基)(μ-Br)]（化合物7）与THP和THF反应时的相对反应活性与选择性。两种体系中，THF的主要反应路径均为α-金属化后发生环逆转反应。在化合物7诱导的降解反应中，鉴定出了碳锂化反应生成的多种副产物，以及意外发现的β-金属化反应副产物；而化合物1则表现出更高的反应选择性。制备了相关化合物[(THP)₂Li(μ-苯基)]₂（化合物9），用于明确鉴定部分产物结构。为验证苄基锂作为其中一种副产物的生成，合成了[(二甲氧基乙烷, dimethoxyethane, DME)Li(μ-CH₂Ph)]₂（化合物8）的标准对照样品。在THP中，观察到化合物1与7的相对反应活性发生反转，钙基化合物的反应活性高于其锂类似物。通过X射线衍射（X-ray diffraction）测定了化合物1~9的晶体结构，对于六配位的钙配合物，观察到因静电作用导致阴离子配体呈反式排布。