Rubidium-mediated birch-type reduction of 1,2-diphenylbenzene in tetrahydrofuran

Article abstract of DOI:10.1021/ja2018527

The reaction of 1,2-diphenylbenzene with rubidium metal in THF yields extremely sensitive and pyrophoric [η⁵-{1,2-diphenyl-2,5- cyclohexadienyl}rubidium]∞ (1). Compound 1 characterizes a possible intermediate in a Birchtype reaction and represents a very rare example of a fully characterized organorubidium complex as well as an open main-group metal pentadienide as part of a six-membered ring. In the solid state the rubidium atoms interact with the cyclohexadienyl moiety, whereas the coordination sphere of the soft cation is additionally stabilized exclusively by several metal π-arene interactions despite the presence of strongly coordinating donors. The bonding situation was elucidated by MP2/def2-TZVPP calculations including population analysis.

Full text of DOI:10.1021/ja2018527

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Rubidium-Mediated Birch-Type Reduction of 1,2-Diphenylbenzene in
Tetrahydrofuran
Sven Krieck,† Robert Kretschmer,‡ Helmar G€orls,§ and Matthias Westerhausen*,§
†Institut f€ur Chemie/Bereich Anorganische Chemie, Karl-Franzens-Universit€at Graz, Schubertstrasse 1, A-8010 Graz, Austria
‡Institut f€ur Chemie, Fachgruppe Organische Chemie, Technische Universit€at Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany
§Institut f€ur Anorganische und Analytische Chemie, Friedrich-Schiller-Universit€at Jena, Humboldtstrasse 8, D-07743 Jena, Germany,
S Supporting Information
b
Scheme 1. Possible Intermediates of the Birch-Type Reduction
ABSTRACT: The reaction of 1,2-diphenylbenzene with rubi-
dium metal in THF yields extremely sensitive and pyrophoric
[η5-{1,2-diphenyl-2,5-cyclohexadienyl}rubidium]¥ (1). Com-
pound 1 characterizes a possible intermediate in a Birch-
type reaction and represents a very rare example of a fully
characterized organorubidium complex as well as an open
main-group metal pentadienide as part of a six-membered
ring. In the solid state the rubidium atoms interact with the
cyclohexadienyl moiety, whereas the coordination sphere of
the soft cation is additionally stabilized exclusively by several
metal π-arene interactions despite the presence of strongly
coordinating donors. The bonding situation was elucidated
by MP2/def2-TZVPP calculations including population
analysis.
of Benzene via a Neutral (B) or a Di-anionic Intermediate (C)
reduction of 1,2-diphenylbenzene was performed with lithium and
ammonium chloride (or water) in liquid ammonia, yielding mainly
2,3-diphenyl-1,4-cyclohexadiene.6 Bock and co-workers7 reduced
1,2-diphenylbenzene with potassium in bis(methoxyethyl)ether
(diglyme) and isolated the coordination polymer [{(diglyme)Kþ}2
{Ph2C6H42ꢀ}]¥. 1,3,5-Triphenylbenzene (tpb) can also be re-
duced twice with alkali metals (AM), leading to a disproportionation
equilibrium between {(AMþ)2(tpb2ꢀ)} on the one hand and {(A-
Mþ)(tpbꢀ) þ AM} on the other.8,9 The nature of the metal
(ionization potential, size, hardness) and the arene (availability of
the LUMO π* orbital) as well as of the proton source (pKa value,
Lewis basicity of the remaining anion) play key roles; therefore, we
investigated a Birch-type reduction with 1,2-diphenylbenzene em-
ploying rubidium in tetrahydrofuran (THF) without additional
proton sources. The tendency toward R-deprotonation is expressed
much more for THF than for acyclic ethers.9
he chemistry of radical anions and dianions of aromatic
T
hydrocarbons represents a broad, intensely investigated, and
long-pursued research field.1 These paramagnetic species have
interesting properties, and Birch reduction of them offers a mild
reduction procedure for the synthesis of 1,4-cyclohexadienes. On
the basis of the work of Wooster and Godfrey,2 who reduced
toluene with sodium and water in liquid ammonia, Birch devel-
oped the first preparative protocols.3 Now, the so-called Birch
reduction is a well-established procedure with a wide variety of
applications, making this method one of the most powerful reduc-
tions in synthetic organic chemistry, e.g., for the reduction of arenes
in order to prepare 1,4-hexadienes. The generally accepted mechan-
ism involves an electron transfer from an electropositive metal
(sodium or potassium) to the arene (formation of a radical anion A)
which is protonated by a Brønsted acid (such as ammonia, alcohol,
or water), yielding radical B. Subsequent reduction (formation of a
cyclohexadienide anion D) and protonation steps yield the desired
1,4-hexadienes E according to Scheme 1. The pentadienyl radical4
(B) and pentadienide anions5 (D) represent possible intermediates
on this preparative route. The electron-withdrawing and -donating
properties of substituents determine their position in relation to the
double bonds of the resulting cyclohexadiene. Another type of this
reaction cascade also seems to be feasible: two-fold electron transfer
leads to the formation of an arene dianion C, which then can be
protonated twice. In either case, the cyclohexadienide D represents
an intermediate in the sequence of electron-transfer and protonation
steps leading to partially reduced arenes or heteroarenes. The Birch
Rubidium metal (E0 = ꢀ2.99 V)10 was suspended in a mixture
of toluene and THF, and 1,2-diphenylbenzene (E0(1) = ꢀ2.62 V,
E0 = ꢀ2.72 V)11 was added in small portions at ꢀ40 ꢀC
(2)
(Scheme 2). The solution turned dark violet, and 2 equiv of the
metal dissolved during this exothermic reaction. Crystallization
afforded black opalescent and pyrophoric cube-shaped single
crystals of 1,2-diphenylcyclohexadienyl rubidium 1. This com-
pound is extremely air and moisture sensitive and decomposes
above 59 ꢀC.
Received: March 8, 2011
Published: April 19, 2011
r
2011 American Chemical Society
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|

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Scheme 2. Reduction of 1,2-Diphenylbenzene with Rubi-
dium followed by a Protolytic Workup Yielding Cyclohexa-
dienes; Middle Row Shows the Resonance Forms of the Anion
Figure 1. Asymmetric unit and numbering scheme of [η5-{1,2-diphen-
yl-2,5-cyclohexadienyl}rubidium] (1). The ellipsoids represent a prob-
ability of 40%. H atoms are shown with arbitrary radii. Selected bond
lengths (pm): Rb1ꢀC1 332.1(5), Rb1ꢀC3 329.0(5), Rb1ꢀC4
316.0(5), Rb1ꢀC5 324.0(6), Rb1ꢀC6 319.8(5), C1ꢀC2 152.3(7),
C1ꢀC6 138.7(8), C1ꢀC7 145.3(7), C2ꢀC3 151.5(7), C2ꢀC13
154.7(7), C3ꢀC4 134.8(9), C4ꢀC5 141.3(10), C5ꢀC6 138.7(8).
Because this solution showed the same color as reported for
the 1,2-diphenylbenzene dianion, we assume the formation of
this dianion as the first reaction step, in accordance with NMR
studies of the reduction with lithium in THF-d8 yielding the
dilithium adduct.12 Thereafter, THF molecules serve as proton
sources, leading to ether degradation via R-deprotonation, which
is a common reactivity of THF in the presence of strong reducing
reagents13,14 and was also observed earlier for arene anions.1
Recent investigations show the deprotonation side reactions of
arenes with strong electropositive s-block organometallics as
dominant reaction pathways.14a,15 It is remarkable that, even in
the presence of strong chelating donors such as dimethoxyethane
(dme), 1,3,5-trimethyl-1,3,5-triazinane (tmta), or 1,2-bis(meth-
oxyethoxy)ethane (triglyme), the cosolvent-free complex [η5-
{1,2-diphenyl-2,5-cyclohexadienyl}rubidium]¥ (1) crystallized.
Consistent with alkylation reactions,5c,6,16 protolysis of 1 yielded
a mixture of 2,3-diphenyl-1,4-cyclohexadiene (2) and 2,3-di-
phenyl-1,5-cyclohexadiene (3), as shown in Scheme 2.
Figure 2. Coordination environment of the hydrocarbon anion of 1 and
presentation of the aggregation pattern. The atoms are drawn with
arbitrary radii, and H atoms are omitted for clarity. Rb atoms are
represented as dotted circles and C atoms as shaded ones. Broken lines
represent short contacts between Rb and the carbon atoms.
The asymmetric unit and numbering scheme of 1 are displayed
in Figure 1.17 The hydrocarbon anion binds to three rubidium
cations as shown in Figure 2, which leads to the formation of a
three-dimensional coordination polymer. The cyclohexadienide
anion is bound totwo rubidiumatoms on opposite sides, which are
also surrounded by three additional side-on-coordinated phenyl
groups. The RbꢀC distances to the anionic moiety vary between
316.0(5) (Rb1ꢀC4) and 332.1(5) pm (Rb1ꢀC1), whereas the
Rb1ꢀC distances to the phenyl groups adopt slightly larger values,
>326 pm. Due to charge delocalization, the contribution of
additional electrostatic attraction is rather small. Therefore, the
distances of the Rb1 atom to the aromatic planes also vary within a
rather narrow range ([Rb1 and plane of C1,C3ꢀC6, 302.1(5) pm;
Rb1 and plane of C7ꢀC12, 331.2(5) pm; Rb1 and plane of
C13ꢀC18, 312.0(5) pm].
pentadienide unit (C1,C3ꢀC6). This leads to an envelope-shaped
inner six-membered ring. The C2ꢀC bond lengths are character-
istic for single bonds to sp3-hybridized carbon atoms. Charge
delocalization leads to similar CꢀC bond lengths within the
pentadienide fragment (average 138.4 pm). Comparable values
were observed for a calcium complex of U-shaped 2,4-di(tert-
butyl)pentadienide (average CꢀC bond length 140 pm).18 The
C1ꢀC7 bond length of 145.3(7) pm excludes strong charge
delocalization into the phenyl substituent, even though the twist-
ing angle between the planes of C7ꢀC12 and C1,C3ꢀC6 shows
only a value of 26.1ꢀ.
To elucidate the bonding situation in 1, we performed MP2/
def2-TZVPP calculations including population analysis. The
MP2-optimized structure reproduces the experimental structure
in a satisfying manner, as shown in Figure 3. According to the
The protonated carbon atom C2 expectedly shows a tetra-
hedral environment and lies 49.3 pm above the plane of the
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COMMUNICATION
side reactions (solvent deprotonation and cleavage) occur.23 For
these reasons, structurally characterized heavier alkali organo-
metallics are still a rarity.24 Sufficient description of the bonding
situation in these complexes in terms of traditional coordination
chemistry is challenging due to the number of multi-hapto
interactions.25 Large (soft) metal ions interact with large anions
with at least partially delocalized charge, leading to powerful
nucleophiles.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental and calculation
b
details; crystallographic data in CIF format for 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 3. Alignment of calculated geometry (dark gray) and the X-ray
structure-based molecular presentation (light gray).
’ AUTHOR INFORMATION
Corresponding Author
m.we@uni-jena.de
Table 1. Calculated, Multicenter-Corrected Atomic Charges
and Two-Center Shared Electron Numbers (σ) for Selected
Atoms of 1 at RIMP2/def2-TZVPP Level of Theory
’ ACKNOWLEDGMENT
We thank the Deutsche Forschungsgemeinschaft (DFG,
Bonn/Germany) for generous financial support of this research
initiative. We also acknowledge the financial support of the
Fonds der Chemischen Industrie (VCI/FCI). S.K. and R.K. are
very grateful to the Stiftung Stipendienfonds im Verband der
Chemischen Industrie for a Ph.D. grant.
atom
charge
σ
atom
charge
σ
C1
C2
C3
C4
C5
ꢀ0.2356
0.0492
0.0576
0.0958
0.2082
0.0551
0.1212
C6
C8
C9
ꢀ0.0263
ꢀ0.1924
ꢀ0.0121
0.0955
0.0348
ꢀ
ꢀ0.2289
ꢀ0.0152
ꢀ0.2686
Rb
0.7646
ꢀ
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