Unprecedented ligand anti-bis-benzylation upon thermolytic treatment of 2,3-diphenylbenzo[g]quinoxaline with (η1-benzyl) pentacarbonylmanganese

Article abstract of DOI:10.1016/j.jorganchem.2005.07.077

The treatment of 2,3-diphenylbenzo[g]quinoxaline with PhCH ₂Mn(CO)₅ in boiling n-heptane affords substantial amounts of a biscyclomanganated 5,10-anti-bis-benzylation product, which structure was assessed by X-ray diffraction analysis. In toluene and under similar experimental conditions, the formation of the latter is inhibited and the major product is the corresponding bis-cyclomanganated 2,3-diphenylbenzo[g] quinoxaline. The bis-benzylation reaction seemingly results from a trapping of benzyl radicals formed upon homolytic cleavage of the C-Mn bond in PhCH ₂Mn(CO)₅ by the metallated 2,3-diphenylbenzo[g] quinoxaline.

Full text of DOI:10.1016/j.jorganchem.2005.07.077

Journal of Organometallic Chemistry 690 (2005) 4822–4827
www.elsevier.com/locate/jorganchem
Unprecedented ligand anti-bis-benzylation upon
thermolytic treatment of 2,3-diphenylbenzo[g]quinoxaline with
(g1-benzyl) pentacarbonylmanganese
*
´
Jean-Pierre Djukic , Andre de Cian, Nathalie Kyritsakas Gruber
UMR 7513 CNRS, Universite´ Louis Pasteur, 4 rue Blaise Pascal, F-67000 Strasbourg, France
Received 7 July 2005; received in revised form 20 July 2005; accepted 20 July 2005
Available online 2 September 2005
Abstract
The treatment of 2,3-diphenylbenzo[g]quinoxaline with PhCH2Mn(CO)5 in boiling n-heptane affords substantial amounts of a
biscyclomanganated 5,10-anti-bis-benzylation product, which structure was assessed by X-ray diffraction analysis. In toluene and
under similar experimental conditions, the formation of the latter is inhibited and the major product is the corresponding bis-cyclo-
manganated 2,3-diphenylbenzo[g]quinoxaline. The bis-benzylation reaction seemingly results from a trapping of benzyl radicals
formed upon homolytic cleavage of the C–Mn bond in PhCH2Mn(CO)5 by the metallated 2,3-diphenylbenzo[g]quinoxaline.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Manganese; N ligands; Crystal structures; Helical structures; Radicals
1. Introduction
and often led to divergent conclusions as to the real
nature of the intermediates [3]. According to Rourke
and co-workers [4], the failure to establish a consistent
kinetic law stems from the absence of a clearly defined
rate determining step. In fact, the behaviour of
RMn(CO)5 has not much been investigated in the con-
ditions used for cyclometallation. Although prelimin-
ary ligand coordination to the Mn center upon
substitution of one CO ligand may be accepted as the
reactionꢀs first step, the fate of the resulting putatively
labile (L)(R)Mn(CO)4 species has never been ad-
dressed. Even though it is likely that an homolytic
cleavage of the C–Mn bond yielding a pair of reactive
alkyl and (L)(CO)4Mn radicals may take place [5], the
importance of radicals in the ortho-metallation process
has not yet been established. In this article, we report
the unprecedented trapping of benzyl radicals in the
course of the double cyclomanganation of 2,3-diph-
enylbenzoquinoxaline with PhCH2Mn(CO)5 and show
that the bis-anti-benzylation reaction is sensitive to
the nature of the solvent.
It is well established that the treatment of ligand ap-
pended arenes with alkylmanganesepentacarbonyl com-
plexes Eq. (1) may lead to the formation of [C,Y]
heterochelates of Mn(CO)4 (Y being a two electron do-
nor ligand) [1]. These manganacyclic complexes have
been subjected to a wide range of metal-centered trans-
formations mostly involving insertion of electron-unsat-
urated moieties into the C–Mn bond [2].
R-Mn(CO)5
Y
C
Y
C
Mn(CO)4
H
ð1Þ
CO, RH
The mechanism of this CAr–H bond activation reaction
has been investigated by many authors, in various ways
*
Corresponding author: Tel.: +33 03 90 24 15 23; fax: +33 03 90 24
50 01.
E-mail address: djukic@chimie.u-strasbg.fr (J.-P. Djukic).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.077

J.-P. Djukic et al. / Journal of Organometallic Chemistry 690 (2005) 4822–4827
4823
2. Results and discussion
[11]. However, the case reported herein is unprece-
dented as it involves a double addition of the PhCH2
radical to an heterocycle, which could follow the reac-
tion scheme drawn in Scheme 1.
2.1. The manganation of 2,3-diphenylbenzo[g]-
quinoxaline 1
At this stage, the inhibition of the formation of 1c in
toluene is not clear; we speculate that toluene might pro-
mote the scrambling of the benzyl radical via a predom-
inant hydrogen abstraction process [12].
The treatment of 1 with 3 equiv. of PhCH2Mn(CO)5
Eq. (1) in a boiling mixture of n-hexane and benzene
for 8 h afforded essentially the mono-manganated com-
pound 1b in 51% yield.
2.2. Structural characterization of 1a and 1c
An ORTEP diagram of 1a is displayed in Fig. 1 be-
side two projections showing the stacking of the aro-
matic systems in the crystal lattice. Acquisition
parameters and refinement data for all the structures
presented herein are gathered in Table 1. Worthy to
note, compound 1a, which can be considered as a me-
talla and azabenzo analogue of its parent pentaheli-
cene, exists under two forms in the crystal lattice,
e.g., the inversion center-related helical M and P
enantiomers. The helical orientation of the molecule
stems obviously from the steric hindrance residing be-
tween vicinal phenylene moieties essentially at the
C(28) and C(13) atoms and can be quantified by the
interplanar
phenylene–phenylene
angle,
which
amounts 63.7ꢁ. It is worthy to note that this angle
is larger than in the ‘‘all-carbon’’ parent [5] helicene
– or 3,4-5,6-dibenzophenanthrene – (47.3ꢁ) [13] or
5,10-dihydro-benzo[a]indeno[2,3-c]fluorene (10.2ꢁ) [14].
This helical distortion entails a relatively important
folding of the diaza-heterocycle C(26)–C(15)–N(1)–
C(16)–C(25)–N(2) with the largest atomic shifts from
ð2Þ
A similar experiment carried out in boiling toluene for
14 h with 2 equiv. of PhCH2Mn(CO)5 afforded essen-
tially complex 1a in 41% yield, which pro-helical molec-
ular structure (Fig. 1) was confirmed by X-ray
diffraction analysis (vide infra).
˚
the mean plane residing at atoms C(26) (+0.104(4) A)
˚
and C(15) (ꢀ0.115(4) A). In the crystal lattice, mole-
cules of 1a are p-stacked almost in a parallel head-
to-tail manner with an alternation of M and P enan-
tiomers: the angle between the quinoxalyl planes of
two related enantiomers amounts ca. 2ꢁ. The mean
When the reaction of 1 [6] with PhCH2Mn(CO)5
was carried out in boiling n-heptane for 18 h, the reac-
tion afforded minute amounts of 1a, Mn2(CO)10 and
significant amounts of a new compound 1c, which
C2 symmetric structure was ascertained by NMR spec-
troscopy and X-ray diffraction analysis. An ORTEP
diagram of the structure of 1c is displayed in Fig. 2
(vide infra). Compound 1c stems with certainty from
the anti-double addition of a benzyl radical [7] to
the central aromatic ring of the benzoquinoxaline at
the 5 and 10 positions yielding the corresponding
substituted pyrazine complex. The most plausible
origin of the benzyl radical is the heat-promoted
homolytic cleavage of the C–Mn bond [8] in
PhCH2Mn(CO)5 or its coordinatively unsaturated tet-
racarbonyl derivatives. It is well known that the latter
readily converts to Mn2(CO)10 and dibenzyl upon long
periods of heating in aliphatic solvents [9] or upon
irradiation [10]. The benzyl radical has a well docu-
mented propensity to attack activated aza-heterocycles
˚
interplanar distance is 3.5 A.
A similar trend can be drawn for 1c, which is also
helical with an interplanar phenylene–phenylene angle
of 55.4ꢁ (Fig. 2). The pyrazylene moiety deviates sig-
nificantly from planarity with the largest atomic shifts
˚
from the mean plane showing at C(9) (ꢀ0.142(5) A),
˚
˚
C(16) (+0.132(5) A), C(23) (+0.099(5) A) and C(32)
˚
(ꢀ0.107(6) A). The phenyl moieties of the benzyl
groups are proximal to the polycyclic fragment seem-
ingly to minimize steric interaction with the Mn(CO)4
fragment: they define an interplanar angle with the
central ring C(23)–C(24)–C(25)–C(30)–C(31)–C(32)
amounting ca. 61ꢁ. This arrangement does not prevent
the free motion of the benzyl group though. In the
NMR time scale, the five phenyl protons resonate in
CDCl3 as three separate signals at d 6.86 (t*, 2H),
6.64 (t*, 4H), 5.68 (d, 4H) ppm.

4824
J.-P. Djukic et al. / Journal of Organometallic Chemistry 690 (2005) 4822–4827
Fig. 1. Left: ORTEP diagram of complex 1a displayed at 30% probability level. Hydrogen atoms have been omitted for clarity purpose. Interatomic
˚
distances (A): C(13)–C(28), 3.181(9); Mn(2)–N(2), 2.100(4); C(13)–C(27), 3.297(8); Mn(2)–C(32), 2.037(5); Mn(1)–N(1), 2.120(4); Mn(1)–C(9),
2.058(5); C(14)–C(27), 3.217(9); C(14)–C(28), 3.206(9); C(14)–C(32), 4.562(9). Torsion angles (ꢁ): N(1)–C(15)–C(26)–N(2), 5.6; C(28)–C(27)–C(26)–
C(15), 16.4; C(27)–C(26)–C(15)–C(14), 34.5; C(26)–C(15)–C(14)–C(13), 25.9. Right: (a) side view of the p-stacking in the crystal lattice; (b) top view
showing the alignment of the heterocyclic units; Mn(CO)4 fragments and hydrogen atoms have been omitted for clarity.
Scheme 1.
esting applications in radical-mediated synthesis. At
this stage, it is not possible to anticipate which type
of heterocyclic ligand might undergo similar homolytic
radical additions since in many other reported cases of
double metallation of phenyl substituted quinoxaline
Fig. 2. ORTEP diagram of complex 1c displayed at 30% probability
and pyrimidine derivatives such a side reaction was
not noticed [2n,2o]. The therein reported reaction
bears interesting similarities with the one described
by Iwamura et al. [15], who observed the formation
of a 9,10 dibenzylation adduct to anthracene upon
thermal treatment of the latter with dibenzylmercury.
There are other earlier reports [16] in the literature
which document the trapping of benzyl radicals by
anthracene. This allows us to propose that benzyl rad-
icals do form in the course of the cyclomanganation
reaction of 1 and that they can be trapped by 1a.
level. Hydrogen atoms and molecules of CH2Cl2 have been omitted for
clarity purpose. Interatomic distances (A): Mn(1)–C(11), 2.042(6);
Mn(1)–N(1), 2.177(5); C(16)–C(9), 1.409(7); C(23)–C(32), 1.397(7);
C(24)–C(33), 1.563(9); C(31)–C(32), 1.508(7); C(23)–C(24), 1.510(8).
Interatomic angles and torsion (ꢁ): N(1)–Mn(1)–C(11), 79.56(19);
N(2)–Mn(2)–C(22), 80.2(2); C(17)–C(16)–C(9)–C(10), 32.0(8).
˚
3. Conclusions
In summary, our results show a new aspect of the
cyclomanganation reaction which might lead to inter-

J.-P. Djukic et al. / Journal of Organometallic Chemistry 690 (2005) 4822–4827
4825
Table 1
Acquisitiona and refinement data for the X-ray diffraction analyses of compounds 1a and 1c
Compound
1a
1c Æ 2CH2Cl2
Formula
C64H28Mn4N4O16
1328.71
Monoclinic
P21/c
C46H28Mn2N2O8 Æ 2 CH2Cl2
Molecular weight
Crystal system
Space group
1016.49
Monoclinic
P21/n
˚
a (A)
16.7514(2)
22.0204(4)
14.9158(3)
94.353(5)
5486.2(2)
4
15.3761(2)
12.2420(2)
24.0031(4)
97.238(5)
4482.2(1)
4
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
Color
Orange
0.14 · 0.08 · 0.04
1.61
2672
0.979
Orange
0.20 · 0.18 · 0.08
1.51
2064
0.859
Crystal dimension (mm)
Dcalc (g cmꢀ3
F000
)
l (mmꢀ1
)
hkl limits
h limits (ꢁ)
ꢀ23, 23/ꢀ30, 30/ꢀ20, 20
2.5/29.99
16254
ꢀ21, 21/ꢀ15, 17/ꢀ33, 33
2.5/30.01
21449
Number of data measured
Number of data with I > 3r(I)
8160
7243
Number of variables
793
598
R
Rw
0.070
0.085
0.084
0.101
Goodness-of-fit
Largest peak in final difference (e A
1.463
0.903
1.146
1.368
ꢀ3
˚
)
a
Reflections were collected at 173 K with a Nonius KappaCCD diffractometer using the Mo Ka graphite monochromated radiation
˚
(k = 0.71073 A).
Further investigations are underway to probe the reac-
tivity of both 1a and 1c.
3.2. Manganation of 2,3-diphenylbenzoquinoxaline 1 in
n-heptane
3.1. Experimental
A solution containing 2,3-diphenylbenzoquinoxaline
1 (1 g, 3 mmol) and PhCH2Mn(CO)5 (4.32 g, 15 mmol)
in n-heptane (20 mL) was refluxed for 18 h. During that
time the colour of the reaction mixture changed from
pale yellow to dark red. The solution was stripped of
solvents, dissolved in CH2Cl2 and silica gel was added.
Upon removal of the solvent under reduced pressure
the coated silica gel was loaded on the top of a silica
gel column packed in dry n-hexane. A first yellow band
containing a mixture of unreacted PhCH2Mn(CO)5 and
Mn2(CO)10 was eluted with pure n-hexane. A second
red-colored band containing 1a (79 mg, 4% yield) was
eluted with a 1:9 mixture of CH2Cl2 and n-hexane. A
third brown yellow-colored band containing 1c
(422 mg, 16%) was eluted with the same solvent mixture.
All experiments were carried out under a dry atmo-
sphere of argon with dry and degassed solvents. NMR
spectra were acquired on Bruker DRX 500, AV 400
1
(13C and H nuclei) and AV 300 (1H nucleus) spectro-
meters at room temperature unless otherwise stated.
Chemical shifts are reported in parts per million down-
field of Me4Si and coupling constants are expressed in
Hz. IR spectra were measured with a Perkin–Elmer
FT spectrometer. Mass spectra were recorded at the Ser-
vice of Mass Spectrometry of University Louis Pasteur
(FAB+) and at the Analytical Center of the Chemical
Institute of the University of Bonn (EI). Elemental anal-
yses (reported in % mass) were performed at the Service
dꢀAnalyses of the ‘‘Institut de Chimie de Strasbourg’’
and at the analytical center of the ‘‘Institut Charles Sa-
dron’’ in Strasbourg. Chromatographic separations
were performed with a Merck Geduran silica (Si 60,
40–60 lm) in columns packed in n-hexane or n-pentane
with a maximum positive argon pressure of 0.5 bar.
Purifications required the use of deactivated silica pre-
pared by suspending 200 g of SiO2 in a mixture of dis-
tilled acetone and water (4% in weight); the resulting
silica gel was washed with 200 mL of n-hexane and dried
under reduced pressure prior to use.
3.3. Manganation of 1 in a mixture of n-hexane and
benzene
A
solution of ligand
1
(1 g, 3 mmol) and
PhCH2Mn(CO)5 (2.6 g, 9.1 mmol) in a 2:1 mixture of
n-hexane (10 mL) and benzene (5 mL) was refluxed for
8 h under argon. The red colored solution was then
evaporated under reduced pressure, the residue was dis-
solved in CH2Cl2 and silical gel was added. Upon re-
moval of the solvent under reduced pressure the

4826
J.-P. Djukic et al. / Journal of Organometallic Chemistry 690 (2005) 4822–4827
coated silica was loaded on the top of SiO2 column
packed in dry n-hexane at 2 ꢁC. Compound 1b was
eluted as a red colored band with a 1:4 mixture of
CH2Cl2 and n-hexane and isolated upon removal of sol-
vents and recrystallization from n-hexane as a red pow-
der (770.5 mg, 51%).
Hbenzyl,
3J = 3.9), 3.33 (dd, 2H, CH2–Ph, 2J = 13.3,
3J = 3.6), 3.10 (dd, 2H, CH2–Ph, J = 13.3, J = 3.6).
13C NMR (CDCl3) d 220.8 (MnCO), 214.6 (MnCO),
213.2 (MnCO), 211.3 (MnCO), 176.3, 157.3, 150.1,
147.9, 141.2, 134.6, 134.1, 130.6, 130.0, 128.8, 127.83,
127.81, 127.7, 123.9, 47.9, 46.3.
2
3
3.4. Manganation of 2,3-diphenylbenzoquinoxaline 1 in
toluene
3.5. Experimental procedure for the X-ray diffraction
analysis of 1a and 1c
A solution of ligand 1 (1 g, 3 mmol) and PhCH2-
Mn(CO)5 (2.0 g, 7 mmol) in dry toluene (15 mL) was
refluxed for 14 h under argon. The red colored solution
was then evaporated under reduced pressure, the residue
was dissolved in CH2Cl2 and silica gel was added. Upon
removal of the solvent under reduced pressure the
coated silica was loaded on the top of SiO2column
packed in dry n-hexane at 2 ꢁC. A first yellow band con-
taining Mn2(CO)10 (330 mg, 28%) was first eluted with a
1:4 mixture of CH2Cl2 and n-hexane. Compound 1a was
eluted a deep red band with a 1:1 mixture of CH2Cl2 and
n-hexane and recovered as a red powder upon removal
of the solvents under reduced pressure (830 mg, 41%).
Complex 1a [17]: HR MS (FAB+) Calc. for
C32H14N2O8Mn2: 663.951104. Found: 663.951108. IR
Acquisition and processing parameters are displayed
in Table 1. Reflections were collected with a Nonius
KappaCCD diffractometer using Mo Ka graphite
˚
monochromated radiation (k = 0.71073 A). The struc-
tures were solved using direct methods, they were refined
against |F| and for all pertaining computations, the Non-
ius OPENMOLEN package was used [20]. Hydrogen atoms
were introduced as fixed contributors.
Acknowledgments
The authors gratefully acknowledge the contribution
of Luc Eberhardt to this work and the support provided
by the CNRS.
(CH2Cl2) m(CO): 2076, 1999, 1980, 1940 cmꢀ1
.
1H
NMR(CDCl3) d 9.11 (s, 2H, Hbqx), 8.23 (dd, 2H, HPh
,
,
3J = 8.0, 4J = 1.0), 8.15 (dd, 2H, Hbenzoquinoxaline
3J = 6.0, 4J = 3.0), 7.95 (d, 2H, HPh, 3J = 8.0), 7.67
Appendix A. Supplementary data
3
4
(dd, 2H, Hbenzoquinoxaline, J = 7.0, J = 3.0), 7.29 (td,
2H, HPh, 3J = 8.0, 4J = 2.0), 7.03 (td, 2H, HPh,
NMR spectra of compounds 1 and 1a–c. Crystallo-
graphic data have been deposited with the Cambridge
Crystallographic Data Centre, CCDC Nos. 277448
(1a) and 277449 (1c). Copies of this information may
be obtained free of charge from The Director, CCDC,
12 Union Road, Cambridge CB2 1E2, UK, fax: +44
1223 336 033, deposit@ccdc.cam.ac.uk or www:
www.ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2005.07.077.
4
3J = 8.0, J = 1.0). 13C NMR (CDCl3, 268 K) d 220.7,
214.5, 212.7, 212.6, 182.3, 162.7, 149.5, 141.5, 138.4,
133.5, 132.2, 130.8, 128.5, 128.4, 125.6, 123.6. MS
(FAB+) m/e 664.8 [M + H]+, 580.8 [M ꢀ 3CO]+, 551.8
[M ꢀ 4CO]+,
383.9
[M ꢀ 8CO ꢀ Mn]+,
331.0
[M + 2H ꢀ 8CO ꢀ 2Mn]+. Complex 1b [18]: Anal. Calc.
for C28H15N2MnO4: C, 67.48; H, 3.03; N, 5.62. Found:
C, 67.41; H, 3.27; N, 5.48. IR (CH2Cl2) m(CO): 2071 (w),
1994 (s), 1976 (vs), 1936 (vs) cmꢀ1. 1H NMR (CDCl3) d
9.19 (s, 1H, Hbenzoquinoxaline), 8.68 (s, 1H, Hbenzoquinoxaline),
8.18 (m, 2H), 8.08 (d, 1H, Hbenzoquinoxaline
7.76 (m, 2H), 7.67–7.50 (m, 5H), 7.23–7.16 (m, 2H),
,
3J = 9.0),
References
3
4
6.82, (td, 1H, J = 7.0, J = 1.0). 13C NMR (CDCl3) d
220.8 (MnCO), 214.4 (MnCO), 213.4 (MnCO), 181.5,
163.5, 153.5, 148.4, 141.4, 140.2, 139.3, 137.0, 134.1,
133.6, 132.0, 130.2, 129.9, 129.2 (2C), 129.0(2C), 128.7,
128.6, 128.1, 127.9, 127.4, 124.7, 123.1. Complex 1c
[19]: Anal. Calc. for C46H28N2Mn2O8: C, 65.26; H,
3.33; N, 3.31. Found: C, 65.39; H, 3.66; N, 3.11%. IR
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