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

Abstract

Reduction of para-substituted iodobenzene in tetrahydropyran (THP) with finely dispersed calcium powder yields arylcalcium iodides of the type [(THP) 4 Ca(C 6 H 4 -4-R)I] with R = CH 3 (1), Cl (2), Br (3), I (4), OCH 3 (5). A 2-fold insertion of calcium into dihalobenzenes was not observed. The β-naphthylcalcium iodide [(THP) 4 Ca(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) 2 Li} 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) 2 Li(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-CH 2 Ph] 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. ■ INTRODUCTIONThe year 2005 marked the occurrence of the first structurally characterized arylcalcium derivatives, independently prepared by the groups of Niemeyer, 1 Harder, 2 and Westerhausen, 3 using different synthetic strategies. During the following years arylcalcium derivates emerged from elusive academic curiosities to an easily accessible group of substances. 4 These compounds show tremendous potential and might be able to challenge the well-established aryllithium derivatives in organic and organometallic syntheses. The worldwide accessibility of calcium, its low price, and its nontoxic behavior regardless of concentration further encourage subsequent investigations. Given its position in the periodic table, the alkaline-earth metal calcium is expected to combine typical properties of s block metals (saltlike behavior, highly heteropolar metal−carbon bonds) and early transition metals (Lewis acidity of cations, d orbital participation, and catalytic activity) within its compounds. Considering a few preconditions, convenient high-yield syntheses of solutions of these post-Grignard reagents have been developed recently. 5 These preconditions are as follows:(i) Activation of calcium succeeds with ammonia, yielding a finely divided highly reactive metal powder. (ii) The direct synthesis has to be performed in ethereal solutions, preferably in THF.(iii) Iodoarenes represent the most suitable substrates; br...

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

“…When the reduction of 2,4,6‐tri( tert ‐butyl)phenyl halide is performed with calcium, tunneling processes convert the intermediately formed tri( tert ‐butyl)phenyl radical into the 1‐[3,5‐di( tert ‐butyl)phenyl]‐1,1‐dimethylethyl radical . Furthermore, lithium and magnesium form dimetalated benzene by reaction with 1,4‐diiodobenzene, whereas the reaction of 1,4‐I 2 C 6 H 4 with activated calcium selectively yields 4‐iodophenylcalcium iodide . The π‐systems of oligocyclic aromatic halides tend to be reduced by activated calcium (leading to Birch‐type reductions) much more easily than by magnesium …”

Solution Stability of Organocalcium Compounds in Ethereal Media

“…This value, which will serve as a benchmark for further experiments, is in good agreement with the one reported for the closely related derivative [Ca(Tol)(I)(thf) 4 ] under comparable conditions (8 days, see Table 1, entry 12). 20 In contrast, the lithium derivative [(thp) 2 Li 2 (μ-Tol)(μ-Br)] decomposes twice as fast. 20 The simultaneous formation of ethene (δ H = 5.4) during the degradation of [Ca(Ph)(I)(thf) 4 ] suggests that α-deprotonation and subsequent [3+2] cycloreversion of THF is the predominantly occurring degradation mechanism under the applied conditions, in agreement with earlier reports; see Scheme 1.…”

“…20 In contrast, the lithium derivative [(thp) 2 Li 2 (μ-Tol)(μ-Br)] decomposes twice as fast. 20 The simultaneous formation of ethene (δ H = 5.4) during the degradation of [Ca(Ph)(I)(thf) 4 ] suggests that α-deprotonation and subsequent [3+2] cycloreversion of THF is the predominantly occurring degradation mechanism under the applied conditions, in agreement with earlier reports; see Scheme 1. 8,20 Repetition of this experiment, using THF-d 8 as ligand and solvent, resulted in a drastic decrease of the t 50 value and 69% of the initial phenylcalcium iodide was still present after 35 days (Table 1, entry 2), while this degree of decomposition was already reached after 3.9 days in case of nondeuterated THF.…”

“…32 The use of THP indeed decelerated degradation of phenylcalcium iodide by a factor of 3 (23 days; Table 1 (Table 1, entry 11) this positive effect was much less pronounced and only a slight stabilization was achieved. 20 When only the decay of the primary species is taken into account, actually no difference is noticeable. However, the observation of additional broad signals in the aromatic region indicates that the loss of primary [Ca(Tol)-(I)(thp) 4 ] is partially due to the formation of secondary species that contain a significant amount of calcium-bound tolyl groups.…”

1‐Alkenylcalcium Iodide: Synthesis and Stability

“…Bond lengths Ca1C4 and Ca1I1 of 253.0(4) and 318.63(8) pm, respectively, were established. The closely related α‐ and β‐naphthylcalcium iodides [Ca(α‐Naph)(I)(thf) 4 ], [Ca(β‐Naph)(I)(thf) 4 ], and [Ca(β‐Naph)(I)(thf) 4 ] showed similar CaC bond lengths of 255.2(6),19 252.8(5),20 and 253.4(6) pm,21 respectively. The Ca1O bond lengths of 2 varied between 236.9(3) and 241.5(3) pm.…”