New Modes of Reactivity in the Threshold of the Reduction Potential in Solution. Alkylation of Lithium PAH (Polycyclic Aromatic Hydrocarbon) Dianions by Primary Fluoroalkanes: A Reaction Pathway Complementing the Classical Birch Reductive Alkylation

Abstract

Some of the most highly reduced organic species in solution, such as the dianions of PAHs (polycyclic aromatic hydrocarbons) display unexpected reactivity patterns when they react with an appropriate counterpart. As seen before in their reaction with propene and other alkenes, PAHs(-2) apparently react with fluoroalkanes in a nucleophilic fashion in spite of being generally regarded as powerful electron-transfer reagents in their reactions with haloalkanes. This methodology complements the current methodologies on reductive alkylation of polycyclic arenes by allowing access to a new set of regioisomers, the regiochemistry of which can be easily predicted by simple MO calculations.

4‐Biphenylylcalcium Iodide and 9‐Phenanthrylcalcium Bromide: Grignard‐Type Reagents of Polycyclic Aromatic Hydrocarbons

“…The observed (and rather unexpected) inaccessibility of simple and substituted 4-biphenylylcalcium derivatives of polycyclic aromatic hydrocarbons (PAH) clearly limits the synthetic value of organocalcium compounds, especially since the related lithium [8] and magnesium [9] derivatives can be prepared with good yields on a large scale. The unexpected behavior of the naphthyl derivatives seems to be related to the extremely negative second reduction potential of the parent naphthalene, [10] preventing it from being doubly reduced under the applied conditions.In general, bromo-and iodo-arenes with extended p-systems offer two reaction pathways with calcium, namely the electron transfer into the p*-system of the PAH and the formal insertion of this alkaline earth metal into the carbonÀhalogen bond yielding a Grignard-type reagent. This investigation was aimed at identifying reaction conditions to separate the two reactivities from each other and to evaluate conditions for an efficient synthesis of polycyclic arylcalcium halides.Generally, simple polycyclic aromatic hydrocarbons can be reduced to radical anions with biphenyl showing the highest first reduction potentials (E 1 = À2.68 V, E 2 = À3.18 V) except for benzene, lower reduction potentials were determined for instance for phenanthrene (E 1 = À2.49 V, E 2 = À3.13 V).…”

“…This investigation was aimed at identifying reaction conditions to separate the two reactivities from each other and to evaluate conditions for an efficient synthesis of polycyclic arylcalcium halides.Generally, simple polycyclic aromatic hydrocarbons can be reduced to radical anions with biphenyl showing the highest first reduction potentials (E 1 = À2.68 V, E 2 = À3.18 V) except for benzene, lower reduction potentials were determined for instance for phenanthrene (E 1 = À2.49 V, E 2 = À3.13 V). [10] These radical anions and dianions [11] represent intermediates in Birch-type reduction reactions of arenes (Scheme 1). [12] In liquid ammonia also containing THF (to raise the solubility of biphenyl) approximately equimolar amounts of calcium and 4-bromobiphenyl gave a red suspension.…”

On the reactivity of naphthalene and biphenyl dianions: tying up loose ends concerning an SN 2-ET dichotomy in alkylation reactions

“…In previous studies we examined the reaction of LiBp and Li 2 Bp with a radical probe (hex‐5‐enyl fluoride, 1b ) in THF and THP at 0 °C, paying special attention to the distribution of products observed . It was significant that, among the alkylated products (>80% overall yield for Li 2 Bp), no rearranged products were observed (see Schemes and and the structures of R· and rearranged R′· below).…”

“…Under certain conditions, lithium naphthalene (LiC 10 H 8 , LiNp) as well as lithium biphenyl (LiC 12 H 10 , LiBp) can be over‐reduced with Li (s) to afford even more reduced species, i.e. the corresponding dianions Li 2 C 10 H 8 (Li 2 Np) and Li 2 C 12 H 10 (Li 2 Bp), respectively . There are many fundamental aspects that remain to be explored in these compounds, so we became involved some time ago in the study of the structure and reactivity of these species.…”

Magnetism in hydro‐ and dehydrogenated benzene

“…High hydrogenation of PAHs has been proposed as a method for hydrogen storage [9]. Besides, the feasibility of double hydrogenation of those compounds has been investigated theoretically [10] , and a selective reaction pathway on PAHs is being investigated experimentally [11]. The proposal of [8] was inspired on Lieb's theorem for bipartite lattices which proves that, under certain conditions, finite clusters of those lattices will be magnetic whenever they are unbalanced [12].…”