4-biphenylylcalcium iodide and 9-phenanthrylcalcium bromide: Grignard-type reagents of polycyclic aromatic hydrocarbons

Article abstract of DOI:10.1002/chem.201301152

Reduced to the max: Halogenated polycyclic hydrocarbons can be reduced with calcium via two different pathways yielding radical anions due to electron transfer into the π*-orbital or Grignard-type reagents due to insertion of Ca into the carbon-halogen bo

Full text of DOI:10.1002/chem.201301152

COMMUNICATION
DOI: 10.1002/chem.201301152
4-Biphenylylcalcium Iodide and 9-Phenanthrylcalcium Bromide: Grignard-
Type Reagents of Polycyclic Aromatic Hydrocarbons
Mathias Kçhler, Jens Langer, Reinald Fischer, Helmar Gçrls, and
Matthias Westerhausen*[a]
Refined general procedures for the preparation of arylcal-
cium halides were developed recently[1,2] and a variety of
monometalated and even dimetalated derivatives[3] were
synthesized and isolated by reaction of the corresponding
aryl iodides (or bromides) and activated calcium. However,
the availability of derivatives containing coplanar extended
p-systems seems to be limited. Although a few naphthylcal-
cium complexes have been reported,[2a,4] it was noticed earli-
er that introduction of an additional phenyl group in the
para-position of an aryl iodide prevented isolation of the
corresponding 4-biphenylylcalcium iodide.[2b] In contrast,
doubly reduced 1,3,5-triphenylbenzene radical dianions[5]
were isolated from the reduction of 2,4,6-triphenylphenyl
halides with activated calcium and subsequent ether degra-
dation reactions. The formation of arene radical anions
represents the basic reaction step in Birch[6] or Benkeser[7]
reduction reactions. 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 com-
pounds, especially since the related lithium[8] and magnesi-
um[9] derivatives can be prepared with good yields on a
large scale. The unexpected behavior of the naphthyl deriva-
tives 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-sys-
tems 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
Generally, simple polycyclic aromatic hydrocarbons can
be reduced to radical anions with biphenyl showing the
highest first reduction potentials (E1 =À2.68 V, E2 =
À3.18 V) except for benzene, lower reduction potentials
were determined for instance for phenanthrene (E1 =
À2.49 V, E2 =À3.13 V).[10] These radical anions and dia-
nions[11] represent intermediates in Birch-type reduction re-
actions of arenes (Scheme 1).[12]
Scheme 1. Proposed reaction mechanism of the Birch-type reduction of
arenes.
In liquid ammonia also containing THF (to raise the solu-
bility of biphenyl) approximately equimolar amounts of cal-
cium and 4-bromobiphenyl gave a red suspension. After
protolysis with methanol, a mixture of starting 4-bromobi-
phenyl and dihydrobiphenyl was observed with an approxi-
mate 3:1 ratio of 1,4-dihydrobiphenyl and 3,4-dihydrobi-
phenyl.[13] In order to gain more information on the red
solid formed in this reaction, equimolar amounts of biphenyl
and calcium were used in a second run, resulting again in
the formation of a red precipitate, which was isolated and
yielded 1,4-dihydrobiphenyl and 3,4-dihydrobiphenyl (ratio
ca. 3:1) after subsequent protonation with methanol. Under
appropriate conditions, this red compound was isolated in
À
carbon halogen bond yielding a Grignard-type reagent.
This investigation was aimed at identifying reaction condi-
tions to separate the two reactivities from each other and to
evaluate conditions for an efficient synthesis of polycyclic
arylcalcium halides.
[a] M. Kçhler, Dr. J. Langer, Dr. R. Fischer, Dr. H. Gçrls,
Prof. Dr. M. Westerhausen
Institute of Inorganic and Analytical Chemistry
Friedrich Schiller University Jena
Humboldtstrasse 8, 07743 Jena (Germany)
Fax : (+49)3641-948132
form of dark-red crystals of the composition [Ca
[Ph-C6H6 ]2·1.5THF (1) from the reaction mixture
G
À
(Scheme 2). Figure S3 in the Supporting Information shows
the structural motif of 1 consisting of solvent-separated ions.
The seven-coordinate calcium atom shows no short contacts
to the phenylcyclohexadienide anions.
E-mail: m.we@uni-jena.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301152.
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Scheme 2. Solvent-dependent reactivity of activated calcium with biphenyl derivatives.
À
À
The absence of bromo-substituted dihydrobiphenyl deriv-
atives as well as of simple biphenyl after hydrolysis of the
reaction mixture of calcium and 4-bromobiphenyl indicates
that dehalogenation and reduction of 4-bromobiphenyl
occur simultaneously under the applied conditions. A selec-
tive insertion of calcium into the carbon halogen bond was
not observed. Pure THF as solvent was also reported to be
not suitable for the synthesis of arylcalcium derivatives of
biphenyl.[5a] Therefore, we turned our attention to tetrahy-
dropyran (THP), a solvent which was recently used to suc-
cessfully isolate a variety of para-substituted phenylcalcium
derivatives in crystalline form.[2a,14] In the absence of an
excess of the metal, the reduction of 4-iodobiphenyl with ac-
tivated calcium in THP yielded a bluish black solution.
From this reaction mixture colorless crystalline [(4-Ph-
ment. The Ca C and Ca I bond lengths of 256.0(4) and
314.64(8) pm lie in expected ranges.[1] Due to the lack of
À
À
steric strain rather small Ca1 O distances (av. Ca O
238.7 pm) are observed. As is usual in aryl groups attached
to positive metal centers[15] the endocyclic C2-C1-C2A bond
angle of 114.3(4)8 is rather narrow due to electrostatic repul-
sion between the sp2 orbital of C1 (carrying the negative
À
charge) and the neighboring C1 C2/C2A bonds.
The successful synthesis, isolation and characterization of
biphenylylcalcium iodide 2 encouraged us to attempt to
reduce 9-bromophenanthrene (PhenBr) with activated calci-
um in an equimolar ratio. In tetrahydropyran a dark green
reaction mixture was formed and off-white solid phenan-
thrylcalcium bromide [(Phen)Ca(Br)ACTHNUTRGNEUNG(thp)4] (3) was isolated
after work-up. The molecular structure and numbering
C6H4)Ca(I)
G
scheme of 3 is depicted in Figure 2. Again, a trans arrange-
crystallization. The isolated complex is extremely soluble in
THF and THP.
The molecular structure and numbering scheme of 2 is de-
picted in Figure 1. The molecule exhibits crystallographic C2
symmetry with the atoms Ca1, I1, C1, C4, C5, and C8 lying
on the axis enforcing a strictly linear I1-Ca1-C1 fragment.
The hexacoordinate calcium atom is in a distorted octahe-
dral environment with the anionic ligands in a trans arrange-
Figure 2. Molecular structure and numbering scheme of 3. Ellipsoids
represent a probability of 40%. Non-coordinated THP and H atoms are
omitted for clarity reasons.
ment of the anionic ligands is observed in this mononuclear
À
octahedral complex. The lengths of the Ca1 C1 bond
À
(255.9(3) pm) and of the Ca1 Br1 bond (293.07(7) pm) re-
semble those of thf-coordinated phenylcalcium bromide
[16]
À
À
(Ca C1 258.3(3) pm; Ca Br1 288.99(8) pm).
As in 2, a
Figure 1. Molecular structure and numbering scheme of 2. Ellipsoids rep-
resent a probability of 40%. Non-coordinated THP and H atoms are
omitted for clarity reasons.
narrow endocyclic bond angle at C1 of 113.7(3)8 is ob-
served.
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Chem. Eur. J. 2013, 19, 10497 – 10500

Polycyclic Aromatic Hydrocarbon Anions
COMMUNICATION
Recrystallization from THF at ambient temperature yield-
ed dinuclear [(Phen)CaACHTNUGTRNNEUG(m-Br)ACHTUNTGREN(NUNG thf)3]2 (4), which is only spar-
solvent and reaction conditions can be adjusted to allow the
direct synthesis of arylcalcium derivatives of these polycyclic
aromatic hydrocarbons, eliminating one of the limitations of
organocalcium chemistry in comparison to the correspond-
ing chemistry of lithium and magnesium.
ingly soluble in this ether. The mono- or dinuclear nature of
these complexes seems to be temperature-dependent with
the dinuclear species being favored at higher temperatures
for entropic reasons. This equilibrium is depicted in
Scheme 3.
Acknowledgements
This work was supported by the German Research Foundation (DFG,
Bonn, Germany). M.K. is grateful to the Fonds der Chemischen Industrie
(Frankfurt/Main, Germany) for a generous PhD stipend. We also appre-
ciate the financial support of the Verband der Chemischen Industrie
(VCI/FCI, Frankfurt/Main, Germany). The infrastructure of our institute
was provided by the EU (European Regional Development Fund,
EFRE) and the Friedrich Schiller University Jena.
Keywords: Birch reduction · calcium · Grignard reagent ·
polycyclic aromatic hydrocarbons · reduction
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Scheme 3. Equilibrium between solvated mononuclear and dinuclear phe-
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À
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À
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À
kaline earth metal into the carbon iodine bond. In contrast
to THF, which was found to be unsuitable for the prepara-
tion of 4-biphenylylcalcium derivatives earlier,[5] mononu-
clear [(4-Ph-C6H4)Ca(I)
lized. A similar procedure also allowed the preparation and
isolation of 9-phenanthrylcalcium bromide as
[(Phen)Ca(Br)(thp)4] (3). Despite the fact that biphenyl,
ACHTUNGRTEN(NUNG thp)4] (2) was isolated and crystal-
ACHTUNGTRENNUNG
phenanthrene and their halogenated derivatives can be re-
duced by highly reactive calcium in a Birch-type manner,
Chem. Eur. J. 2013, 19, 10497 – 10500
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M. Westerhausen et al.
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Received: March 26, 2013
Published online: June 20, 2013
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Chem. Eur. J. 2013, 19, 10497 – 10500