Comparison of the reactivity of 2-amino-3-chloro- and 2,3- dichloroquinoxalines towards Ph2PH and Ph2PLi and of the properties of diphenylphosphanyl-quinoxaline P,N and P,P ligands

Article abstract of DOI:10.1016/j.poly.2012.08.089

The synthesis of quinoxaline P,N ligands by monoamination of 2,3-dichloroquinoxaline (1) to 2-amino-3-chloroquinoxalines 2a,b and the subsequent substitution of chlorine by a diphenylphosphanyl group was studied. Whereas the reaction of 2a,b with Ph₂PH in the presence (or absence) of catalytic amounts of palladium acetate furnished only minor amounts of the expected ligands in favor of tetraphenyldiphosphane and dechlorinated quinoxalines, the coupling with Ph₂PLi in ether provided the novel NH-functional P,N hybrid ligands 3a,b with a quinoxaline scaffold in moderate to good yields. 3a is slightly and 3b somewhat more sensitive to air oxidation, leading to the P-oxides 4a,b. The more reactive 1 forms with Ph₂PH only a small amount of 2-chloro-3-diphenylphosphanylquinoxaline 5 and traces of the quinoxaline-bis(phosphane) 6. The main products are 2,2′- bis(quinoxaline) and Ph₂PCl, which converts residual Ph₂PH into tetraphenyldiphosphane. The coupling with Ph₂PLi in diethyl ether, however, gave in a fast reaction high yields of 6, exceeding those of 3a,b, with interfering NH functions. Semi-empirical quantum chemical calculations (PM6) illuminate the background of the air sensitivity of 3a,b, whereas the recently reported 6 is air stable. Preliminary studies for use of the ligands in catalysis with the air-stable 6, showed moderate to good yields in the Pd-catalyzed C-N cross coupling of 2-bromopyridine with mesityl amine. Complex formation was confirmed by isolation of the Pd complex 7. The structure elucidation of the new compounds is based on conclusive NMR data and crystal structure analyses for 2b, 3a, 4a and 4b.

Full text of DOI:10.1016/j.poly.2012.08.089

Polyhedron 50 (2013) 101–111
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Comparison of the reactivity of 2-amino-3-chloro- and 2,3-dichloroquinoxalines
towards Ph2PH and Ph2PLi and of the properties of
diphenylphosphanyl-quinoxaline P,N and P,P ligands
a
b
c
Mohamed Shaker S. Adam a, , Ahmad Desoky Mohamad , Peter G. Jones , Markus K. Kindermann ,
⇑
Joachim W. Heinicke c,
⇑
a Department of Chemistry, Faculty of Science, Sohag University, Sohag-82534, Egypt
b Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, D-38106 Braunschweig, Germany
c Institut für Biochemie, Ernst-Moritz-Arndt-Universität Greifswald, Felix Hausdorffstrabe 4, 17487 Greifswald, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of quinoxaline P,N ligands by monoamination of 2,3-dichloroquinoxaline (1) to 2-amino-3-
chloroquinoxalines 2a,b and the subsequent substitution of chlorine by a diphenylphosphanyl group was
studied. Whereas the reaction of 2a,b with Ph2PH in the presence (or absence) of catalytic amounts of
palladium acetate furnished only minor amounts of the expected ligands in favor of tetraphenyldiphos-
phane and dechlorinated quinoxalines, the coupling with Ph2PLi in ether provided the novel NH-func-
tional P,N hybrid ligands 3a,b with a quinoxaline scaffold in moderate to good yields. 3a is slightly
and 3b somewhat more sensitive to air oxidation, leading to the P-oxides 4a,b. The more reactive 1 forms
with Ph2PH only a small amount of 2-chloro-3-diphenylphosphanylquinoxaline 5 and traces of the quin-
oxaline-bis(phosphane) 6. The main products are 2,20-bis(quinoxaline) and Ph2PCl, which converts resid-
ual Ph2PH into tetraphenyldiphosphane. The coupling with Ph2PLi in diethyl ether, however, gave in a fast
reaction high yields of 6, exceeding those of 3a,b, with interfering NH functions. Semi-empirical quantum
chemical calculations (PM6) illuminate the background of the air sensitivity of 3a,b, whereas the recently
reported 6 is air stable. Preliminary studies for use of the ligands in catalysis with the air-stable 6, showed
moderate to good yields in the Pd-catalyzed C–N cross coupling of 2-bromopyridine with mesityl amine.
Complex formation was confirmed by isolation of the Pd complex 7. The structure elucidation of the new
compounds is based on conclusive NMR data and crystal structure analyses for 2b, 3a, 4a and 4b.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 15 June 2012
Accepted 17 August 2012
Available online 17 October 2012
Keywords:
Phosphane ligands
P,N hybrid ligands
Quinoxaline phosphanes
Palladium
Aryl amination
1. Introduction
genations, were introduced by Imamoto et al. [3]. Subsequently,
various other catalytic applications of QuinoxR⁄ were reported
Phosphanes represent an extensive and varied class of ligands
that has found extensive applications in coordination chemistry
and homogenous transition metal catalyzed organic reactions [1].
The usefulness of quinoxaline substituents or backbones in such
P-ligands, however, has only recently been recognized (Chart 1).
An interesting P,O-quinoxaline hybrid ligand, 3-diphenylphospha-
nyl-quinoxaline-2-carboxylic acid, was synthesized by Sinou et al.
[2] and used as a precursor for the preparation of P,P,N,N-hybrid li-
gands. The chiral 2,3-bis(tert-butylmethylphosphanyl)quinoxaline
(QuinoxR⁄) and a related methyladamantylphosphanyl ligand, that
proved to be excellent ligands in Rh-catalyzed asymmetric hydro-
[3,4]. The redox properties of various transition metal complexes
with this electron-deficient diphosphane chelate ligand were
investigated by Kaim and co-workers [5]. The chiral quinoxaline-
2,3-bis(2,5-diphenylphospholane) proved highly useful in asym-
metric hydroformylation and hydrogenation [6]. In 2011, 2,3-bis
(diphenylphosphanyl)quinoxaline and triosmium clusters and
copper complexes with this ligand were reported [7,8].
Our investigations of quinoxaline phosphorus compounds
started with the phosphonylation of 2-amino-3-chloroquinoxalines
to 3-amino-2-phosphonoquinoxalines and the bis-phosphonylation
of 2,3-dichloroquinoxaline [9]. The aminophosphonoquinoxalines
were intended to be reduced to primary 2-amino-3-phosphanyl-
quinoxalines as starting materials for quinoxaline-annulated
1,3-azaphospholes, in order to extend our knowledge of annulated
1,3-azaphospholes [10–12]. However, reduction with LiAlH4 led to
P–C bond cleavage and elimination of PH3. A literature search failed
to show alternative routes to 2-amino-3-phosphanylquinoxalines.
⇑
Corresponding authors. Tel.: +20 122 8122858; fax: +20 934 601159 (M.S.S.
Adam). Tel: +49 3834 864337; fax: +49 3834 864377 (J.W. Heinicke).
E-mail addresses: shakeradam61@yahoo.com (Mohamed Shaker S. Adam),
heinicke@uni-greifswald.de (J.W. Heinicke).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.089

102
Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
Ph
But
Me
P
P
Ph
Ph
N
N
N
N
P
P
Ph
Ph
N
N
P
COOH
But
Me
Ph
I
III
II
Chart 1. Examples of recently reported phosphanylquinoxalines.
This prompted us to study coupling reactions of 2-amino-3-chloro-
quinoxalines with diphenylphosphane and lithium diphenylphos-
phide. In order to compare the reactivity of 2-amino-3-chloroq
uinoxalines and 2,3-dichloroquinoxaline in these reactions and the
properties of the products, the latter was included in this study.
The conversion of the resulting air stable P,P-ligand to the PdCl2
complex and preliminary tests of its usefulness in a Pd-catalyzed
C–N coupling reaction are also included in this report.
removed in vacuo to give
a
brownish-yellow viscous mass
(220 mg) with a low content of 3a (relative 31P intensity ca. 20%
besides signals of Ph4P2, Ph2PHO and other P compounds). Purifica-
tion under aerobic conditions by column chromatography on silica
gel (ethyl acetate/hexane 95/5%) and removal of solvent gave
180 mg (45%) pale yellow solid 2-aminoquinoxaline. Mp: 156 °C.
1H NMR (CDCl3) d: 5.03 (vbr s, 2H, NH2), 7.45 (td, 3J = 8.4, 7,
4J = 1.2 Hz, 1H, H-6), 7.61 (td, 3J = 8.4, 7, 4J = 1.2 Hz, 1H, H-7), 7.67
(dd, 3J = 8.4, 4J = 1.2 Hz, 1H, H-8), 7.92 (dd, 3J = 8.4, 4J = 1.2 Hz, 1H,
H-5), 8.35 (s, 1H, H-3); these values are in good agreement with
the reported data [17]. 13C NMR (CDCl3) d: 125.05 (CH-6), 125.88
(CH-8), 128.83 (CH-5), 137.43 (Cq-4a), 137.78 (CH-3), 130.29
(CH-7), 140.89 (Cq-8a), 151.97 (Cq-2). HRMS (ESI in MeOH): Calc.
for C8H7N3 [M+H+] 146.0713; found: 146.0713.
2. Experimental
2.1. General considerations
The reactions with diphenylphosphane or -phosphide were con-
ducted in dry and deoxygenated solvents under a nitrogen atmo-
sphere using Schlenk techniques. Diphenylphosphane, palladium
chloride and n-butyl lithium were used as received from commer-
cial suppliers. The starting compounds 1 [13], 2a [9,14] and 2b [15]
were prepared by known procedures. Single crystals of 2b were ob-
tained after column chromatography from a concentrated solution
in hexane/ethyl acetate (M. Ghalib). NMR spectra were measured
at 25 °C on a multinuclear FT-NMR spectrometer Bruker ARX300
or AVANCE300 at 300.1 (1H), 75.5 (13C) and 121.5 (31P) MHz. The
1H, 13C and 31P NMR chemical shifts d are given in ppm relative
to Me4Si and H3PO4 (85%), respectively, as external standards. Cou-
pling constants refer to JHH in 1H and JPC in 13C NMR, unless de-
noted otherwise. Atom numbering follows the nomenclature
rules (with different priorities for phosphanes and phosphine oxi-
des versus amines, see Scheme 3). Mass spectra were measured on
a single-focusing sector-field mass spectrometer, AMD40. HRMS
measurements were performed in Göttingen on a double-focusing
sector field mass spectrometer, MAT 95 (Fa. Finnigan) EI (70 eV), or
a 7T Fourier transform ion cyclotron resonance mass spectrometer,
APEX IV (Bruker Daltonics) (ESI). Elemental analyses were carried
out with a CHNS-932 analyzer from LECO or with a vario MICRO
cube (CHN) from Elementar using standard conditions. Geometry
optimization and calculation of orbital energies of the model com-
pounds Ia,b and II were performed with the semi-empirical PM6
program of MOPAC2009, version 11.052W [16] with improved
parameters (GNORM = 0.01, SCFCRT = 1.D-10, DMAX = 0.01). The
orbitals were presented with the program Gabedit [16c] from the
aux-files of MOPAC2009.
2.1.2. Reaction of 2b with Ph2PH to give 2-tert-butylaminoquinoxaline,
Ph2PCl and Ph4P2
Compound 2b (596 mg, 2.53 mmol) was mixed with Pd(OAc)2
(0.7 mg, 0.1 mol%) and heated with Ph2PH (0.44 mL, 2.50 mmol)
for 2.5 h at 130 °C. An extract of the brownish-yellow product mix-
ture in C6D6 displayed 31P NMR signals of Ph2PCl, Ph2P-PPh2 and
3b, intensity ratio 22:50:28. Further extraction with diethyl ether
left 274 mg of an insoluble pale green powder. Treatment with
aqueous sodium hydroxide in the presence of diethyl ether, drying
of the ether layer over Na2SO4 and removal of the solvent gave
150 mg of a sticky pale yellow solid, consisting of 2-tert-butylami-
noquinoxaline slightly contaminated by Ph2PHO and trace
amounts of 3b, tetraphenyldiphosphane and a compound with
d
31P 34.7 ppm. Chromatography on silica furnished tert-butylami-
noquinoxaline (100 mg, 20%), crystallizing from Et2O in large col-
ourless plates. Mp: 117–118 °C. (Cutting of the extremely soft
crystals for X-ray diffraction led to severe deformation and low
quality diffraction data, so that the structure analysis was unsatis-
factory; R ca. 25%.) 1H NMR (CDCl3) d: 1.51 (s, 9H, CMe3), 5.03 (br s,
1H, NH), 7.31 (td, 3J = 8.1, 7.5, 4J = 1.5 Hz, 1H, H-6), 7.51 (td, 3J = 8.1,
7.5, 4J = 1.5 Hz, 1H, H-7), 7.68 (dd, 3J = 8.1, 4J = 1.5 Hz, 1H, H-8), 7.82
(dd, 3J = 8.1, 4J = 1.5 Hz, 1H, H-5), 8.07 (s, 1H, H-3). 13C NMR (CDCl3)
d: 151.52 (Cq-2), 141.65 (Cq-8a), 139.42 (CH-3), 136.25 (Cq-4a),
129.48 (CH-7), 128.43 (CH-5), 126.56 (CH-8), 123.80 (CH-6). HRMS
(ESI in MeOH): Calc. for C12H15N3 [M+H+] 202.1339; found:
202.1340.
2.1.3. Reaction of 1 with Ph2PH to give 2,20-bis(quinoxaline), Ph2PCl
2.1.1. Reaction of 2a with Ph2PH to give 2-aminoquinoxaline, Ph2PCl
and Ph4P2
and minor amounts of 2-chloro-3-diphenylphosphanylquinoxaline (5)
Compound
1 (515 mg, 2.59 mmol) and Ph2PH (0.45 mL,
Heating a mixture of 2a (495 mg, 2.75 mmol) and Ph2PH
(0.48 mL, 2.76 mmol) in the presence of Pd(OAc)2 (0.8 mg,
0.13 mol%) for 2 h at 130 °C led to a viscous blue-green mass.
Extraction of the soluble part with diethyl ether and NMR monitor-
ing in C6D6 identified Ph2P-PPh2, Ph2PCl, 3a and an unknown phos-
2.59 mmol) were heated in the presence of Pd(OAc)2 (2.0 mg,
0.3 mol%) for 2 h at 130 °C. Extraction of the dark-blue mass with
C6D6 and 31P/13C NMR monitoring displayed Ph2PCl as a major
component (d31P 82.3, int. 80%), small amounts of 5 (d31P ꢀ1.9,
int. 10%) and trace signals of unidentified impurities. Washing of
the insoluble residue with Et2O gave 250 mg of a dark blue powder.
This was extracted with CD3OD/MeOH and filtered to give a yellow
solution. Slow concentration of the solution under conditions that
were not strictly anaerobic gave pale yellow crystal plates (90 mg,
phorus compound
(
31P signals at
d
ꢀ14.9, 82.2, ꢀ12.8 and
ꢀ5.4 ppm, intensity ratio 84:12:2:2). The insoluble hydrochloride
part, 615 mg blue-green powder, was treated with aqueous
NaOH/Et2O. The ether phase was dried with Na2SO4 and the ether

Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
103
10% calc. for 5), identified by 1H, 31P and 13C NMR spectra as a
mixture of 5 and its P-oxide 5ox. The main component of the blue
powder (ca. 150 mg, 40%) was the known hydrochloride of 2,20-
bis(quinoxaline) [18a]. It was identified as 2,20-bis(quinoxaline)
in a second experiment (see Supporting information) after treat-
ment of the blue powder with aqueous sodium hydroxide/diethyl
ether. Concentration of the red-brown solution gave a high-melt-
ing pale brown residue. Mp: 274 °C. 1H NMR (CDCl3) d: 7.82–7.86
(m, 4H), 8.17–8.21 (m, 2H), 8.22–8.27 (m, 2H), 10.11 (s, 2H); these
values are in good agreement with reported proton NMR data
[18b,c]. 13C NMR (CDCl3) d: 129.41, 129.96, 130.52, 130.84 (CH5,
CH-6, CH-7, CH-8), 141.65 (Cq-4a, Cq-8a), 142.77, 144.27 (CH-3),
148.54 (Cq-2). 5: 13C{1H} NMR (CDCl3) d: 128.08, 129.59, 129.85,
130.92 (quinoxaline CH-signals); 128.51 (d, 3J = 7.9 Hz, 4 CH-m),
signals of unconverted 3a, 4a and an unknown compound
(d31P = 32.7 ppm), intensity ratio 80:4:16%. Ethyl acetate was
added (10 mL) and the measurement repeated after 2 d, showing
the same signals in an intensity ratio of 76:7:17. Then CHCl3
(5 mL) and aqueous H2O2 (0.2 mL, 30%) were added to the CDCl3
solution, the mixture shaken for 5 min and dried with Na2SO4.
Filtration, removal of the solvent and subsequent NMR measure-
ment in CDCl3 indicated quantitative oxidation to 4a and a trace
amount of a compound with d31P = 50.2. Crystals precipitated from
the concentrated CDCl3 solution. Mp: 198 °C. 1H NMR (CDCl3) d: 2.2
(vbr, 0.5H, NH), 4.1 (vbr, 0.5H, NH), 6.95 (br s, 1H, NH), 7.36 (m, 1H,
aryl), 7.43–7.61 (m, 7H, aryl), 7.61 (partly superimposed d, 3J = 8–
9 Hz, 1H, H-5), 7.82 (br d, 3J = 8.1 Hz, 1H, H-8), 7.95 (m,
3JPH = 12.3, 3J = 8.4, 6.9, 4J = 1.5 Hz, 4H, o-H), 6.95 (vbr s, 1H, NH).
13C{1H} NMR (CDCl3) d: 124.63 (CH-7), 125.46 (d, 5J = 1.5 Hz, CH-
5), 128.21 (d, 3J = 12.1 Hz, 4 CH-m), 129.49 (CH-8), 131.89 (d,
1J = 105.6 Hz, 2 Cq-i), 131.92 (d, 2J = 9.1 Hz, 4 CH-o), 132.01 (d,
4J = 3.8 Hz, 2 CH-p), 132.06 (CH-6), 136.98 (d, 3J = 16.6 Hz, Cq-8a),
140.12 (d, 1J = 125.3 Hz, PCq-2), 141.77 (br, Cq-4a), 154.56
(2J = 18.9 Hz, NCq-3). 31P{1H} NMR (CDCl3) d: 25.6. Anal. Calc. for
129.46 (2 CH-p), 133.89 (d, 1J = 6.6 Hz,
2 Cq-i), 134.68 (d,
2J = 19.9 Hz, 4 CH-o), 140.94, 141.60 (Cq-4a, Cq-8a), 150.54 (d,
2J = 33.1 Hz, ClCq-2), 159.37 (d, 1J = 15.9 Hz, PCq-2). 31P{1H} NMR
(CDCl3) d: ꢀ2.7. HRMS (ESI in MeOH): Calc. for C20H14ClN2P
[M+H+] 349.0656; found: 349.0655. 5ox 13C{1H} NMR (CDCl3) d:
:
128.29, 130.13, 130.61, 133.08 (CH-5, CH-6, CH-7, CH-8), 128.44
(superimposed d, 3J = 13 Hz, 4 CH-m), 130.64 (d, 1J = 108.8 Hz, 2
Cq-i), 132.09 (d, 2J = 9.1 Hz, 4 CH-o), 132.26 (d, 4J = 2.6 Hz, 2 CH-
p), 139.92 (d, 3J = 15.9 Hz, Cq-8a), 150.86 (d, 1J = 127.4 Hz, PCq-2),
141.82 (br, Cq-4a), 148.79 (2J = 21.3 Hz, NCq-3). 31P{1H} NMR
(CDCl3) d: 27.4. HRMS (ESI in MeOH): Calc. for C20H14ClN2OP
[M+H+] 365.0605; found: 365.0607.
C20H16N3OP (345.33): C, 69.56; H, 4.67; N, 12.17. Found: C,
69.41; H, 4.53; N, 12.02%.
2.1.6. Synthesis of 2-tert-butylamino-3-diphenylphosphanyl-
quinoxaline (3b)
A solution of 2b (430 mg, 1.82 mmol) in Et2O (10 mL) was added
slowly at ꢀ80 °C (rapid addition leads to a brown side product) to a
solution of yellow Ph2PLi, prepared from tBuLi (1.11 mL, 1.82 mmol,
1.64 M in n-pentane) and diphenylphosphane (0.32 mL,
1.82 mmol) in Et2O (10 mL) at ꢀ70 °C. The mixture was allowed
to warm to room temperature and stirred overnight. 31P NMR mon-
itoring of the brown, in thin layers yellow solution displayed signals
of 3b, Ph2PH and 4b in an intensity ratio of 68:30:2%. The precipi-
tate was separated by filtration and washed with Et2O. Extraction
as the sulfate with air-free semi-concentrated aqueous sulfuric acid
(dilute acid remains colourless) and recovery as a base by extrac-
tion with diethyl ether after addition of aqueous potassium hydrox-
ide solution did not completely separate 3b from
diphenylphosphane. Column chromatography under anaerobic
conditions using silica gel and washing with hexanes followed by
elution with hexanes/ethyl acetate (97–95/3–5%) gave 3b as a yel-
low oil. It was still contaminated by residual Ph2PH (10–30%), iden-
tified by its 31P and 13C NMR signals [19]. 3b: 1H NMR (CDCl3) d:
1.35 (s, 9H, CMe3), 5.30 (br d, JPH = 7.8 Hz, 1H, NH), 7.16 (td, 3J = 8,
7, 4J ꢁ 1.2 Hz, 1H, H-6), 7.23–7.30 (m, 6H, aryl), 7.39 (td, 3J = 8, 7,
4J ꢁ 1.2 Hz, 1H, H-7), 7.44–7.52 (m, 4H, aryl), 7.65 (dd, 3J = 8.3,
4J ꢁ 1.2 Hz, 1H, H-8), 7.69 (dd, 3J = 8.3, 4J ꢁ 1.2 Hz, 1H, H-5).
13C{1H} NMR (CDCl3) d: 28.42 (CMe3), 51.93 (CqMe3), 123.55 (CH-
6), 126.25 (CH-8), 128.43 (d, 3J = 8.3 Hz, 4 CH-m), 129.32 (2 CH-p),
129.13, 129.63 (CH-5, CH-7), 133.49 (d, 3J = 6.8 Hz, 2 Cq-i), 134.30
(d, 2J = 19.6 Hz, 4 CH-o), 137.22 (d, 3J = 5.3 Hz, Cq-4a), 141.12 (Cq-
8a), 150.12 (d, 1J = 10.6 Hz, PCq-3), 152.40 (d, 2J = 18.9 Hz, NCq-2).
31P{1H} NMR (CDCl3) d: ꢀ11.7. HRMS (ESI in MeOH): Calc. for
2.1.4. Synthesis of 2-amino-3-diphenylphosphanylquinoxaline (3a)
tBuLi (2.0 mL, 3.28 mmol, 1.64 M in pentane) was added drop-
wise to a solution of diphenylphosphane (0.56 mL, 3.22 mmol) in
Et2O (10 mL) at ꢀ70 °C. After stirring for 15 min at ꢀ30 °C the yel-
low suspension was cooled again, and a solution of 2a (578 mg,
3.22 mmol) in Et2O (10 mL) was added slowly at ꢀ70 °C (rapid
addition leads to a brown side product) and stirred for 1 h. Then
the mixture was allowed to warm to room temperature and stirred
overnight. 31P NMR monitoring displayed signals for 3a and Ph2PH
in an intensity ratio of 17:83%. The mixture was cooled to ꢀ80 °C,
and a second equivalent of cold tBuLi in pentane (2.0 mL) was
added. The mixture was allowed to warm to room temperature
(brown colour) und stirred for 2 d (31P NMR signal ratio of 3a
and Ph2PH, 60:40%). The precipitate was filtered off and washed
with ether. (The brown colour vanishes on contact with traces of
moisture.) After the major part of the solvent was removed in vac-
uum the product started to crystallize. The mixture was overlay-
ered with hexane and after some hours filtered, washed with
hexane/Et2O and dried in vacuum to give 440 mg (42%) orange-yel-
low crystals. Mp: 158–160 °C. Single crystals were obtained from
warm saturated hexane/Et2O solution. 1H NMR (CDCl3) d: 5.31
(br s, 2H, NH2), 7.29 (superimposed td, 3J = 8, 7, 4J ꢁ 1.2 Hz, 1H,
H-6), 7.25–7.35 (m, 7H, aryl), 7.39–7.46 (m, 4H, aryl), 7.48 (td,
3J = 8, 7, 4J ꢁ 1.2 Hz, 1H, H-7), 7.55 (dd, 3J = 8.3, 4J ꢁ 1.2 Hz, 1H, H-
8), 7.72 (dd, 3J = 8.3, 4J ꢁ 1.2 Hz, 1H, H-5). 13C{1H} NMR (CDCl3) d:
124.70 (CH-6), 125.60 (CH-8), 128.57 (d, 3J = 7.4 Hz, 4 CH-m),
129.51 (2 CH-p), 129.46, 130.34 (CH-5, CH-7), 133.25 (d,
1J = 5.4 Hz, 2 Cq-i), 134.45 (d, 2J = 19.6 Hz, 4 CH-o), 138.78 (d,
3J = 3.3 Hz, Cq-4a), 141.05 (Cq-8a), 149.24 (d, 1J = 13.0 Hz, PCq-3),
153.60 (d, 2J = 24.3 Hz, NCq-2). 31P{1H} NMR (CDCl3) d: ꢀ12.1.
C
24H24N3P [M+H+] 386.1781; found 386.1784.
2.1.7. Synthesis of 3-(tert-butylamino)quinoxalin-2-yl-
diphenylphosphine oxide (4b)
UV–VIS (c = 3.4 ꢂ 10ꢀ5 mol/L) kmax(MeOH)/nm
(e
/dm3 molꢀ1
-
A solution of 2b (1.22 g, 5.18 mmol) in Et2O (30 mL) was added
very slowly (over 1 h) at ꢀ78 °C to a solution of yellow Ph2PLi, pre-
pared before from n-BuLi (2.7 mL, 6.75 mmol, 2.5 M in n-hexane)
and diphenylphosphane (1.16 mL, 6.66 mmol) in Et2O (10 mL) at
ꢀ78 °C. The mixture was allowed to warm to room temperature
and stirred overnight. The precipitate was separated by filtration
and the solvent removed under vacuum. The 31P and 1H NMR spec-
tra of the residue, a yellow viscous oil, indicated formation of 3b as
the main product and the sole phosphorus compound apart from
cmꢀ1): 371 (6730), 306 (3560), 244 (19500), 208 (38400). Anal.
Calc. for C20H16N3P (329.33): C, 72.94; H, 4.90; N, 12.76. Found:
C, 73.07; H, 4.89; N, 12.87%.
2.1.5. Synthesis of 3-aminoquinoxalin-2-yl diphenylphosphine oxide
(4a)
A sample of 3a (50 mg, 0.14 mmol) in CDCl3 was exposed to air.
31P NMR measurement of the solid residue after 15 h displayed

104
Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
Table 1
Crystal data and structure refinement for 2b, 3a, 4a, and 4b.
Compound
2b
3a
4a
4b
Empirical formula
Formula weight
T (K)
C
12H14ClN3
C20H16N3P
329.33
100(2)
1.54184
Monoclinic
P21/c
C20H16N3OP
345.33
100(2)
C24H24N3OP
401.43
100(2)
0.71073
Orthorhombic
P212121
235.71
100(2)
0.71073
Orthorhombic
Pbcm
k (Å)
1.54184
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
Triclinic
ꢀ
P1
8.9217(3)
19.2743(6)
6.7743(3)
90
90
90
1164.90(7)
4
1.344
0.303
496
0.40 ꢂ 0.40 ꢂ 0.07
2.28–30.91
ꢀ12 6 h 6 12,
ꢀ27 6 k 6 27,
ꢀ9 6 l 6 9
25807
18.7473(9)
5.5731(3)
17.3641(10)
90
114.405(6)
90
1652.11(15)
4
1.324
8.3570(7)
10.1675(8)
10.8425(9)
101.964(7)
103.546(7)
95.382(7)
866.33(12)
2
1.324
1.503
360
0.10 ꢂ 0.10 ꢂ 0.08
4.32–75.92°
ꢀ10 6 h 6 10,
ꢀ12 6 k 6 12,
ꢀ13 6 l 6 13
29722
8.5508(3)
11.8649(3)
21.0888(5)
90
90
90
2139.55(11)
4
1.246
0.148
848
0.40 ꢂ 0.40 ꢂ 0.20
2.57–30.03°
ꢀ12 6 h 6 12,
ꢀ16 6 k 6 16,
ꢀ29 6 l 6 29
77645
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F (000)
)
1.503
688
Crystal size (mm3)
0.10 ꢂ 0.04 ꢂ 0.04
5.10–75.89°
ꢀ23 6 h 6 23,
ꢀ6 6 k 6 6,
ꢀ21 6 l 6 21
37818
h Range for data collection (°)
Index ranges
Reflections collected
Independent reflections (Rint
)
1897 (0.0296)
3424 (0.0308)
3595 (0.0214)
6218 (0.0495)
Completeness
98.3% to
m
= 30.50°
99.9 % to
m
= 75.00°
99.9% to
m
= 75.00°
99.8% to m = 30.03°
Absorption correction
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Maximum and minimum
transmission
1.00000 and 0. 96803
1.00000 and 0.70815
1.00000 and 0.27626
1.00000 and 0.97710
Refinement method
full-matrix least-squares on
F2
full-matrix least-squares on
F2
full-matrix least-squares on
F2
full-matrix least-squares on
F2
Data/restraints/parameters
1897/0/98
1.053
3424/0/225
1.042
3595/0/234
1.036
6218/0/269
1.054
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
r
(I)]
R1 = 0.0319,
wR2 = 0.0801
R1 = 0.0370,
wR2 = 0.0829
R1 = 0.0313,
wR2 = 0.0817
R1 = 0.0330,
wR2 = 0.0829
R1 = 0.0299,
wR2 = 0.0780
R1 = 0.0305,
wR2 = 0.0785
R1 = 0.0329,
wR2 = 0.0770
R1 = 0.0383,
wR2 = 0.0796
0.00(6)
R indices (all data)
Absolute structure parameter
Largest difference in peak and hole
0.434 and ꢀ0.250
0.448 and ꢀ0.247
0.318 and ꢀ0.372
0.326 and ꢀ0.193
(e Åꢀ3
)
unconverted diphenylphosphane. Further workup was performed
under aerobic conditions. Attempts at crystallization failed, but
after column chromatography 1.55 g (74%) yellow crystals of 4b
were obtained. Single crystals were grown by slow evaporation
of a solution of 4b in ethanol. The crystal data are given in Table 1,
and selected bond lengths and angles in Table 2. 1H NMR (CDCl3) d:
1.54 (s, 9H, CMe3), 7.26 (td, 3J = 8.3, 6.7, 4J = 1.5 Hz, 1H, H-7), 7.42–
7.57 (m, 7H, H-6, 4 m-H and 2 p-H), 7.61 (dd, 3J = 8.3, 4J = 1.5 Hz,
1H, H-5), 7.73 (dm, 3J = 8.5, 4J ꢁ 1, J = 0.7 Hz, 1H, H-8), 7.91 (m,
3JPH = 12, 3J = 8.4, 4J = 1.6 Hz, 4H, o-H), 8.49 (br s, 1H, NH). 13C{1H}
NMR (CDCl3) d: 28.53 (CMe3), 51.79 (CqMe3), 123.67 (CH-7),
126.57 (d, 5J = 1.9 Hz, CH-5), 128.35 (d, 3J = 12.0 Hz, 4 CH-m),
129.48 (CH-8), 131.49 (CH-6), 131.89 (d, 1J = 105.6 Hz, 2 Cq-i),
132.08 (d, 4J = 3.5 Hz, 2 CH-p), 132.17 (d, 2J = 9.4 Hz, 4 CH-o),
135.69 (d, 3J = 17.4 Hz, Cq-8a), 141.33 (d, 1J = 124.6 Hz, PCq-2),
142.25 (d, 4J = 2.3 Hz, Cq-4a), 154.19 (2J = 19.1 Hz, NCq-3). 31P{1H}
NMR (CDCl3) d: 25.5. MS (EI 70 eV, 110 °C) m/z (%): 401 (4), 387
(4), 386 (14), 385 (7), 328 (3), 184 (9), 108 (17), 107 (100). HRMS
(ESI in MeOH/aqueous formic acid): Calc. for C24H24N3OP [M+H]+
402.17298; found 402.17303.
7.53 mmol) in Et2O (20 mL) was added slowly (over 1 h) to the lith-
ium diphenylphosphide solution at ꢀ78 °C. After stirring for 1 h at
the same temperature and then for 1 d at room temperature a
white precipitate was separated, the solvent removed in vacuum
and the product purified by flash column chromatography using
ethyl acetate/n-hexane (1:9) to give 3.0 g (80%) of yellow crystals.
Mp: 109 °C. (Repetition with 4.2 mmol and faster addition of 1 in
Et2O gave 1.4 g (66%) 6). The NMR data are in good agreement with
reported 1H, 31P [7] and 13C NMR [8] values. Our data with assign-
ments: 1H NMR (CDCl3) d: 7.20–7.29 (m, 12H superimposed with
0
0
CDCl3), 7.30–7.37 (m, 8H), 7.65 (mAA , 2H, H-6 and -7), 7.90 (mBB
,
2H, H-5 and -8). 13C{1H} NMR (CDCl3, 298 K) d: 128.11 (pseudotri-
plet, |3J + 6J| = 7.3 Hz, 8 CH-m), 128.77 (s, 4 CH-p), 129.69, 129.87
(2s, CH-5/8 and CH-6/7), 134.60 (pseudotriplet, |2J + 5J| = 20.9 Hz,
8 CH-o), 135.68 (s, 4 Cq-i), 142.2 (s, Cq-4a and Cq-8a), 163.79
(pseudotriplet, |1J + 4J| = 19.0 Hz, Cq-2 and Cq-3). 31P{1H} NMR
(CDCl3) d: ꢀ10.1. MS (EI 70 eV, 340 °C) m/z (%): 499 (32) [M+],
498 (90) [M+], 497 (36), 421 (37), 313 (29), 219 (27), 201 (29),
185 (42) [Ph2P+], 183 (100) [(C6H4)2P+]. Anal. Calc. for C32H24N2P2
(498.49): C, 77.11; H, 4.85; N, 5.62. Found: C, 77.24; H, 4.93; N,
5.53%.
2.1.8. Synthesis of 2,3-bis(diphenylphosphanyl)quinoxaline (6)
A solution of n-BuLi (7.2 mL, 18.0 mmol, 2.5 M in n-hexane) was
added dropwise to a solution of diphenylphosphane (3.05 mL,
17.5 mmol) in Et2O (20 mL) at ꢀ78 °C and stirring was continued
for 15 min at the same temperature. Then a solution of 1 (1.50 g,
2.1.9. Synthesis of k2-{2,3-Bis-(diphenylphosphanyl)quinoxaline}
palladium dichloride (7)
Dichloro-bis(acetonitrile) palladium (0.26 g, 1.01 mmol) in THF
(10 mL) was added slowly under nitrogen to a solution of 6 (0.50 g,

Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
105
Table 2
Selected bond lengths (Å) and angles (°).
Compd.
2b
3a
4a
4b
C3–X, X=
P–C21
P–C11/C31
C9–N2
N2–C2
N1–C2
N4–C3
N1–C8A
N4–C4A
C2–C3
C4A–C8A
C6–C7
C4A–C5
C8A–C8
C5–C6
Cl: 1.7564(14)
–
–
P: 1.8437(12)
1.8219(12)
1.8355(13)
–
P: 1.8253(11)
1.8009(12)
1.8029(12)
–
P: 1.8336(11)
1.8031(13)
1.8021(12)
1.4784(16)
1.3515(16)
1.3291(15)
1.3058(15)
1.3655(16)
1.3734(14)
1.4562(16)
1.4108(17)
1.406(2)
1.4116(17)
1.4203(15)
1.3734(17)
1.3701(19)
126.53(11)
105.19(6)
1.4806(17)
1.3547(17)
1.3184(17)
1.2848(17)
1.3674(17)
1.3816(17)
1.4526(18)
1.4141(18)
1.410(2)
1.4102(19)
1.4163(19)
1.375(2)
1.375(2)
126.52(12)
–
1.3538(17)
1.3188(16)
1.3067(16)
1.3706(16)
1.3829(16)
1.4585(16)
1.4168(17)
1.4101(19)
1.4090(18)
1.4101(18)
1.3750(19)
1.3703(19)
–
104.86(5)
104.86(5)
104.09(6)
P: 122.44(9)
P: 115.68(9)
118.90(11)
119.65(11)
117.32(10)
117.30(10)
121.38(11)
121.85(11)
1.3493(15)
1.3250(15)
1.3049(15)
1.3671(14)
1.3715(15)
1.4566(15)
1.4134(16)
1.4083(18)
1.4082(17)
1.4132(16)
1.3702(18)
1.3715(17)
–
107.54(5)
107.05(5)
106.10(5)
P: 115.99(8)
P: 122.01(8)
118.92(10)
120.58(10)
117.66(10)
117.99(10)
120.42(10)
122.00(10)
C7–C8
C2–N2–C9
C3–P–C21
C3–P–C11/C31
C21–P–C11/C31
N4–C3–X, X=
C2–C3–X, X=
N2–C2–N1
N2–C2–C3
C2–N1–C8A
C3–N4–C4A
N1–C2–C3
N4–C3–C2
–
–
106.21(5)
109.98(6)
Cl: 117.00(10)
Cl: 118.00(10)
121.45(12)
119.52(12)
117.55(12)
116.34(12)
119.03(12)
125.01(12)
P: 114.25(8)
P: 123.69(9)
120.41(11)
119.17(10)
117.38(11)
118.07(10)
120.41(11)
122.05(10)
1.00 mmol) in THF (10 mL) at ꢀ10 °C. The reaction mixture was
stirred for 1 h at this temperature and for 1 d at room temperature.
The color of the reaction mixture changed gradually from pale yel-
low to deep orange. The orange precipitate was separated by filtra-
tion, washed several times with THF and n-hexane and then dried
in vacuum to give 0.62 g (91%) of a yellow powder. Mp: 268 °C. The
complex is air-stable, insoluble in non- or weakly polar organic sol-
vents and sparingly soluble in CDCl3, THF, ethanol, acetone or
DMSO. 1H NMR (D6-DMSO) d: 7.54 (tdd, 3J = 8.4, 7.4, J = 2.8,
J = 1.2 Hz, 8H, m-H), 7.63 (tdd, 3J = 7.5, J ꢁ 2 Hz, 4H, p-H), 7.92
Table 3
C–N Coupling of 2-bromopyridine with mesitylamine catalyzed by 6/Pd.a
No.
Pd compd.
Mol (%) M/L
Yield (%)
1
2
3
4
5
5
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd(Oac)2
Pd(Oac)2
Pd(Oac)2
5
10
2
5
10
2
43
64
40
40
51
36
3
(ddd, JPH = 12.6, 3J = 7.1, J ꢁ 1.3 Hz, 8H, o-H), 8.10 (mAA, 2H, H-6
a
Reaction conditions: 2-Bromopyridine (1.34 mmol), 2,4,6-trimethylaniline
and H-7), 8.29 (m
AA, 2H, H-5 and H-8). 31P{1H} NMR (D6-DMSO)
´
(1.99 mmol), tBuOK (2.76 mmol), 6 and the Pd compound in toluene were heated
for 24 h at 100 °C. Yields refer to the product after isolation by column chroma-
tography (silica gel, ethyl acetate/hexane 2:8).
d: 41.5. HSQC 13C NMR (D6-DMSO) d: 129.3 (8 CH-m), 130.4 (2
CH-quin), 132.8 (4 CH-p), 134.75 (2 CH-quin), 134.9 (8 CH-o). Anal.
Calc. for C32H24Cl2N2P2Pd (675.82): C, 56.87; H, 3.58; N, 4.15.
Found: C, 56.75; H, 3.54; N, 4.05%.
Ka-radiation (k = 1.54184 Å) were used. Absorption corrections
were based on multi-scans. Crystal data are summarized in Table 1.
The structures were solved by direct methods and refined by full-
matrix least-squares on F2 using the program SHELXL-97 [20]. Hydro-
gen atoms were included using a riding model or rigid methyl
groups except for NH hydrogens, which were refined freely.
2.2. Tests for use of 6 in the Pd-catalyzed coupling of 2-bromopyridine
with mesitylamine
The catalytic test reactions were carried out under a nitrogen
atmosphere; workup was performed under aerobic conditions. A
Schlenk flask was charged with 2-bromopyridine (0.13 mL,
1.34 mmol), mesityl amine (0.28 mL, 1.99 mmol) and potassium
tert-butanolate (0.31 g, 2.76 mmol) in toluene (10 mL). Then, 6
and Pd2(dba)3 or Pd(OAc)2 were added (mol% see Table 3). The
reaction mixture was heated at 100 °C for 24 h. Then the reaction
was stopped and the C–N coupling product isolated by flash col-
umn chromatography on silica gel 60 using ethyl acetate/n-hexane
(2:8) for elution. The results are presented in Table 3.
3. Results and discussion
3.1. 2-Amino-3-chloroquinoxaline starting materials
The electron-withdrawing character of the quinoxaline system
makes the nucleophilic replacement of halogen substituents easier
for quinoxalines than for halobenzenes. Therefore, direct phosph-
anylation of chloroquinoxalines appears to be a suitable strategy
to access quinoxaline-based phosphane ligands. The selective syn-
thesis of hybrid ligands with two different nucleophilic groups
from 2,3-dichloroquinoxaline necessitates consideration of the ef-
fect of the first substitution on the reactivity. For the synthesis of 2-
amino-3-phosphanylquinoxalines, we introduced in the first step
the amino group (Scheme 1) which diminished the reactivity and
2.3. Crystal structure analysis of 2b, 3a, 4a and 4b
Crystals of 2b, 3a, 4a and 4b were mounted on a glass fiber in
inert oil. Data were recorded at low temperature on various Oxford
Diffraction diffractometers; for 2b and 4b monochromated Mo
Ka-radiation (k = 0.71073 Å), for 3a and 4a mirror-focused Cu

106
Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
N
N
H
NH3 (NMP)
N
PPh2
N
N
Cl
N
N
Cl
Cl
...
+
+
ClPPh2
(HPPh2)
Ph2P-PPh2
or tBuNH2
+
(H+)
NHR
(H+)
NHR
N
(80 °C)
NHR
Cl
3a (<10, *30%)
3b (<20%)
(R = H: 45, *30%,
tBu: 20%)
2a (R = H), 2b (R = tBu)
Scheme 1. Synthesis of the starting materials.
1
(main product
in Et2O extract)
(X=NHR, n = 1)
Cl
(2a,b)
air
4a,b
N
n HPPh2
(cat.)
+
Ph2P-PPh2
N
X
(n = 1, ~0%,
2, 25%)
130 °C
(X=Cl, n = 1 or 2)
(1)
(HPPh2)
ClPPh2
N
PPh2
Cl
+
+
blue bis(quinoxalin)ium chloride
(main product
N
in Et2O extract)
.
( HCl)
aq.NaOH/Et2O
N
5
(n = 1, 10%,
2, 4%)
N
air
(CD3OD)
...
+
Ph2PHO
+
N
(main product,
up to 60% for n=2)
N
5ox
Scheme 2. Identified products in the mixtures formed from chloroquinoxalines and
Ph2PH (percentages are rough estimates from 31P and characteristic 13C(H) signal
intensities, ⁄for the reaction without Pd(OAc)2 (0.1 mol%) catalyst).
Fig. 1. Molecular structure of 2b in the crystal (ellipsoids represent 50% probability
levels). The molecule, except for C11 and its symmetry-equivalent, lies in
crystallographic mirror plane. The borderline intramolecular hydrogen bond has the
dimensions N2–H02ꢃ ꢃ ꢃCl: N2–H02 0.83(2), H02ꢃ ꢃ ꢃCl 2.48(2), N2ꢃ ꢃ ꢃCl 2.9681(12) Å,
N2–H02ꢃ ꢃ ꢃCl 118.4(19)°; for data of the quinoxaline scaffold, see text and Table 2.
a
(ABX coupling pattern) in the 13C NMR spectrum and the charac-
teristic phosphorus resonance at d31P = ꢀ14.5 ppm [22]. Heating
of the N-basic 2a with Ph2PH and Pd(OAc)2 (0.1 mol%) without
auxiliary base and solvent (2 h 130 °C) led to similar results, with
only trace amounts of the 2-amino-3-phosphanylquinoxaline 3a
in the ether-soluble fraction and minor amounts (31P int. 20%) in
the aq. NaOH/Et2O extract of the insoluble material. The main
product was 2-aminoquinoxaline (45%), isolated as pale yellow
crystals by column chromatography on silica gel. In the absence
of base and palladium acetate under otherwise identical condi-
tions, somewhat more 3a was formed (yield ca. 30%). The ether ex-
tract of the reaction mixture displayed only traces of 3a
(3a:Ph2PH:Ph4P2 31P int. 6:32:62), but treatment of the hydrochlo-
ride residue (79 wt.%) with aq. NaOH/Et2O gave the coupling prod-
uct 3a and 2-aminoquinoxaline in a roughly 1:1 M ratio as the
main components. Both were unambiguously identified by 13C
NMR analysis and HRMS. Contact with air led to partial oxidation
of 3a to its P-oxide 4a. The reactions of 2b with Ph2PH in the
presence of Pd(OAc)2 (0.1 mol%, 2 h 130 °C) proceeded similarly,
forming mixtures with low amounts of the 2-tert-butylamino-3-
phosphanylquinoxaline 3b (31P int. 18–23%). Tetraphenyldiphos-
phane and chlorodiphenylphosphane displayed the strongest 31P
NMR signals in the ether-soluble fraction (50 and 22 int.%), and
Ph2PHO and Ph4P2 the strongest signals of the aqueous NaOH/
Et2O extract (75 and 10 int.%). From this extract almost pure 2-
tert-butylaminoquinoxaline was isolated (20% yield), crystallizing
from the slowly evaporating ethereal solution in large soft colour-
less plates. Formation of the 2-aminoquinoxalines (R = H, tBu) in-
stead of 3a and 3b indicates that the desired P–C coupling
reaction between 2a or 2b and Ph2PH is disfavoured by a compet-
ing replacement of chlorine by hydrogen (Scheme 2). The resulting
Ph2PCl reacts with unconsumed Ph2PH to give the tetraphenyldi-
phosphane observed in the above reactions. This reactivity pattern
is confirmed by the formation of chlorodiphenylphosphane as the
main product (d31P 82.3 ppm, 80 int.%) in the fast 1:1 conversion
of the more reactive 1 with Ph2PH, whereas the slower consump-
tion of the second equivalent of Ph2PH in the 1:2 reaction of 1 with
Ph2PH results in the additional formation of Ph4P2 (Ph2PCl:Ph4P2
allowed selective monoamination. Reaction of 2,3-dichloroquinox-
aline (1) with NH3 in NMP (N-methylpyrrolidone) at 80 °C pro-
vided compound 2a in high yield, and heating of 1 with excess
tert-butylamine in an autoclave furnished only 2b [9]. The main
reason for the diminished reactivity is the
p-donor effect of the
amino group. This is illustrated clearly in its molecular structure,
determined by crystal structure analysis (Fig. 1). Thus, the C2–
N2–C9 angle (126.52(12)°) corresponds to sp2 hybridization, and
the dihedral angle N(1)–C(2)–N(2)–C(9) (0.0°) indicates a coplanar
arrangement of the CtBu–N–C motif and the aromatic ring, allowing
p-delocalization. The shorter bond length of C2–N2 (1.3547(17) Å)
compared to N1–C8A (1.3674(17) Å) or N4-C4A (1.3816(17) Å),
with the same hybridization of the C and N atoms, and the longer
bond C2–N1 (1.3184(17) Å) compared to C3–N4 (1.2848(17) Å)
reflects the
p-conjugation within the N@C–N structure unit. The
exo-arrangement of the tert-butyl group, enforced by steric
repulsion from chlorine, ensures enough space for substitution of
chlorine by other nucleophiles if they are not too bulky. The methyl
carbon C10 is positioned in the anti-position to C2 (C10–C9–N2–C2
180.0°), corresponding to syn-Me(10)–C–N–H, whereas C11 and
C110 are arranged above and below the ring plane.
3.2. Reactivity towards Ph2PH
Inspired by high yields in the Pd-catalyzed phosphonylations of
2a and 2b [9], we attempted to introduce the phosphanyl group,
the second donor group of the 2-amino-3-phosphanylquinoxalines,
by palladium acetate catalyzed coupling reactions of 2a,b with
diphenylphosphane. The same conditions were applied as previ-
ously optimized for the couplings of various aryl halides with
diphenylphosphane [21]. However, heating of 2a with Ph2PH in
NMP (15 min 200 °C, microwave) in the presence of Pd(OAc)2
(0.8 mol%) and Na2CO3 provided tetraphenyldiphosphane as the
main product (75% by 31P integration). It was unambiguously iden-
tified by its typical pseudotriplets of i-, o- and m-carbon nuclei

Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
107
Ph
Ph
N 4
N
PPh2
NHR
1
5
8
8
5
N1
P
4a
8a
8a
4a
/
2 O2 (air)
6
7
7
6
O
H
3
2
2
3
2a,b
(n=1)
or H2O2
N 4
N
R
1
3a,b
n LiPPh2
(Et2O)
4a,b
1
Ph
Ph
(n=2)
N
PPh2
N
P
P
Pd(CH3CN)2Cl2
(THF)
PdCl2
N
PPh2
N
Ph Ph
6
7
Scheme 3. Synthesis of quinoxaline-P compounds 3–7.
60:27 int.%). In the ether-insoluble fraction of the dark blue prod-
uct mixture, 2-chloro-3-diphenylphosphanylquinoxaline 5 (d31P
ꢀ1.9 ppm) was detected as a minor side product in both reactions,
with 1:1 and 1:2 M ratios (10 and 4 int.%, respectively). It co-
crystallized with its P-oxide 5ox (d31P 27.4 ppm, intensity ratio
7:3) by slow concentration of the CD3OD extract (slow air diffusion
through a plastic cap) and was identified along with its oxide by
conclusive 13C NMR and HRMS data. The bis-substitution product
2,3-bis(diphenylphosphanyl)quinoxaline (6) was detected by a
trace phosphorus signal (d31P ꢀ10.2 ppm). For the blue solid,
formed in the 1:2 M reaction, treatment with aqueous sodium
hydroxide/diethyl ether gave a red extract that in the 1H and 13C
NMR spectra displayed signals of 2,20-bis(quinoxaline) [18] as the
main product, accompanied by signals for minor amounts of an
unidentified 2-substituted quinoxaline (13CH-3 intensity ratio
4:1), Ph2PHO and a phosphorus compound with d31P 32.6 ppm.
Clarification of the mechanisms of the Cl–H exchange and the
C–C coupling reaction will need a separate investigation.
and an unknown phosphane (d ꢀ19.1 ppm). Isolation provided 80%
of 6. If 1 was added faster, the yield was 66%. The excellent yield of
85%, recently reported for an inverse procedure in THF [7], could
not be achieved in the above method because of the side products,
in particular the competing formation of tetraphenyldiphosphane
via metal halogen exchange and subsequent coupling of the result-
ing Ph2PCl with Ph2PLi.
The aminophosphanylquinoxaline 3a crystallized as orange-
yellow needles from concentrated diethyl ether solution; the crys-
talline yield could be improved by overlayering with hexanes
(crystal structure analysis, c.f. Figs. 2 and 3). Compound 3b was ob-
tained as a yellow oil, contaminated by unconverted Ph2PH. The
extraction as the sulfate from the ethereal product solution with
air-free semi-concentrated aqueous sulfuric acid and recovery by
extraction with diethyl ether after addition of aqueous potassium
hydroxide solution did not completely separate 3b from diphenyl-
+
phosphane, which can be attributed to the formation of Ph2PH2
sulfate at the high acid concentration required to extract the quin-
oxaline into the aqueous phase. Workup under aerobic conditions
led to air oxidation. The phosphine oxide 4b was isolated in the
pure form by column chromatography and subsequent crystalliza-
tion from ethanol solution (crystal structure analysis, c.f. Fig. 4). A
comparison of the air sensitivity of 3a, 3b and 6 in CDCl3 and ethyl
acetate solution showed that 3b is considerably more sensitive to
air than 3a, whereas 6 is air-stable. 3b was completely oxidized
3.3. P–C couplings with Ph2PLi
To obtain better access to the 2-amino-3-phosphanylquinoxa-
lines, we studied the nucleophilic substitution of 2a and 2b with
lithium diphenylphosphide (Scheme 3). It is known that NH-
functional chloroalkylamines couple in good yields with organo-
phosphides, as do alkyl halides with NH-functional phosphides or
arsenides [23]. The much higher NH-acidity in the electron-
deficient chloroheteroaryl amines 2a or 2b, however, lowers the
amount of phosphide in the protonation equilibrium (R2PLi + R02
NH = R2PH + R02NLi) and disfavours the coupling. To assure at least
high starting concentrations of phosphide, ethereal solutions of 2a
or 2b were added slowly to the Ph2PLi solution in diethyl ether. At
the low temperature (ꢀ70 to ꢀ80 °C), required to suppress initial
side reactions (at room temperature there is an immediate colour
change to brown), the coupling rate was low. Therefore, the mix-
ture was allowed to stir at room temperature for two days. NMR
monitoring after 1 d displayed the 31P signals of Ph2PH and 3a in
an intensity ratio of about 85:15% or those of Ph2PH and 3b in a ra-
tio of 30:70%. After 2 d the ratio increased by about 10% and 5%,
respectively. Addition of a solution of tBuLi in pentane at low tem-
perature (ꢀ80 to ꢀ90 °C) to the reaction mixtures increased the
amount of 3a to 60% (by 1 equiv. tBuLi), whereas the amount of
3b decreased and also became contaminated with an unknown
side product (d31P ꢀ15.9 ppm). For comparison with 2a and 2b,
the more reactive dichlorochinoxaline 1 was also reacted as an
ethereal solution with Ph2PLi in diethyl ether under the same con-
ditions. 31P NMR monitoring after stirring overnight at room tem-
perature indicated the disubstitution product 6 along with (Ph2P)2
Fig. 2. Molecular structure of 3a in the crystal. Ellipsoids represent 50% probability
levels. Hydrogen bond: N2–H02ꢃ ꢃ ꢃN1#1: N2-H02 0.920(19), H02ꢃ ꢃ ꢃN1#1 2.030(19),
N2ꢃ ꢃ ꢃN1#1 2.9501(15) Å; <(N2–H02ꢃ ꢃ ꢃN1#1) 178.0(17)° (symmetry operator #1:
ꢀx, 1 ꢀ y, 1 ꢀ z).

108
Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
was reported to be air stable [24], and the electron-withdrawing
effect of the quinoxaline substituent should further diminish the
susceptibility to air oxidation. A more sophisticated picture of
the factors governing the air oxidation of primary and other phos-
phanes was presented in a recent DFT study [25]. It was demon-
strated that steric effects, involvement of the P lone pair in the
HOMO, and the energy of the SOMO (semi-occupied MO) of the
radical cation, formed by release of one electron, control the air
sensitivity. For triaryl phosphanes with contributions of the P lone
pair to the HOMO, the stability of the radical cation seems to be
decisive. Low SOMO energies favour the formation of phosphane
radical cations (R3P+ꢀ) and thus the air oxidation of the respective
phosphanes. For a comparison of 3a, 3b and 6 in this respect, we
performed semi-empirical quantum chemical calculations using
PM6 in the MOPAC2009 program package [16] with dimethylphos-
phanyl-substituted quinoxalines Ia, Ib and II as more simple model
compounds with one energetically clearly preferred conformer.
The calculations show that the HOMOs of Ia and II involve the P
lone electron pair and are similar in energy, whereas the HOMO
of Ib is less stable and does not include the electron density from
the P atom. The latter is found in the HOMO ꢀ 1 orbital (Table 4).
The SOMO of II+ꢀ is considerably less stable than the SOMOs of
the radical cations of Ia and Ib, explaining the higher stability of
6 to air by the decreased stabilization of the intermediate radical
cation. The higher air sensitivity of 3b compared to 3a, however,
cannot be explained by the energy difference of the SOMOs of
Ia+ꢀ and Ib+ꢀ. In this case the higher energy of the HOMO of Ib com-
pared to Ia may favour orbital interactions with oxygen as the
reactivity-controlling step.
Fig. 3. Base-pairing and packing of 3a in the crystal.
3.5. Structure elucidation
The new compounds 3a,b and 4a,b were characterized by con-
clusive NMR data and HRMS or satisfactory elemental analysis. The
phosphorus resonances of 3a, 3b and 6 are very similar (d ꢀ12.1,
ꢀ11.7, ꢀ10.1 ppm), as are those of 4a and 4b (d 25.5, 25.6 ppm),
with values typical of triarylphosphanes and triarylphosphine
oxides, respectively [26]. The 13C NMR chemical shifts of the quin-
oxaline scaffold of 3a,b and 4a,b are close to those observed in 2-
amino-3-diethylphosphonoquinoxalines [9]. The assignments for
the signals in these compounds, based on CH-COSY experiments
and typical P–C coupling constants, can therefore also be applied
to 3a,b and 4a,b (for atom numbering see Scheme 3). The P–C
coupling constants differ, however, as is often observed for
triarylphosphanes compared to triarylphosphine oxides and
arylphosphonates.
More detailed structural information was obtained for 2b, 3a,
4a and 4b (Figs. 1–6) by single crystal X-ray diffraction (Crystal
data are presented in Table 1). Structural features of 2b (Fig. 1)
have already been discussed above in connection with their influ-
ence on the reactivity. The electronic effect of the amino group on
the bond lengths within the quinoxaline system is similar in the
other 2-aminoquinoxalines, as shown in a compilation of selected
bond lengths and angles of the quinoxaline scaffold (Table 2). In
the pyrazine ring it causes a shortening of the N1–C2 bonds com-
pared to C3–N4, and of N1–C8A compared to N4–C4A. Within the
benzene ring the bonds C5–C6 and C7–C8 are shortened. The gen-
Fig. 4. Molecular structure of 4a in the crystal. Ellipsoids represent 50% probability
levels. Hydrogen bond: (°): P1–O4 1.4918(8); O–P–C3 111.49(5), O–P–C21
112.50(5), O–P–C11 111.82(5); hydrogen bond (N2–H02Aꢃ ꢃ ꢃO@P): N2–H02A
0.863(18), H02Aꢃ ꢃ ꢃO 2.139(18), N2ꢃ ꢃ ꢃO 2.8603(14), <(N2–H02Aꢃ ꢃ ꢃO) 140.9(15);
hydrogen bond (N2–H02Bꢃ ꢃ ꢃN1#1): N2–H02B 0.887(19), H02Bꢃ ꢃ ꢃN1#1 2.224(19),
N2ꢃ ꢃ ꢃN1#1 3.1086(14), <(N2-H02Bꢃ ꢃ ꢃN1#1) 175.6(16), with symmetry operator
#1 ꢀ x, 1 ꢀ y, 1 ꢀ z.
to 4b and an unknown product (d31P = 30.0 ppm) when exposed to
air for 15 h, whereas only 20% of 3a was oxidized to 4a and a spe-
cies with d31P = 32.7 ppm. Even after 2 d, 3a was not completely
oxidized by air. Oxidation was achieved instantaneously, however,
by treatment of a CHCl3 solution of 3a with aqueous H2O2.
erally long C2–C3 bonds give the impression of only weak
p-cou-
pling of the two N@C groups in the benzo-fused pyrazine ring.
The P–C3 bonds are longer in 3a and 4a,b than the P–C bonds to
the phenyl groups and hint at a weaker P–C bond to the quinoxa-
line group. The larger N4–C3–P angle compared to C2–C3–P in 3a
hints at a slight bending of the C–P bond towards the amino group,
although the N–H02Aꢃ ꢃ ꢃP distance is rather long and an intramo-
lecular hydrogen bond is disfavoured by the propeller-like arrange-
ment of the three planar (hetero)aryl groups; the ring systems
3.4. Electronic influence on air sensitivity
The facile air oxidation of 3b was surprising in view of the low
air sensitivity of most triaryl phosphanes. +M-Substituents in the
o-position to phosphorus are expected to increase the electron
density at phosphorus, which might also increase the tendency
for oxidation. However, the related o-diphenylphosphanylaniline

Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
109
Table 4
Selected molecular orbitals of Ia, Ib and II and their radical cations (energies in eV).a
N
N
N
PMe2
PMe2
PMe2
PMe2
N
NH2
N
NHt Bu
N
Ib
II
Ia
HOMO ꢀ 8.723
HOMO ꢀ 8.317
HOMO ꢀ 1 ꢀ 8.962
HOMO ꢀ 8.629
SOMO ꢀ 12.465
SOMO ꢀ 12.396
SOMO ꢀ 10.013
a
Calculations using PM6 (MOPAC2009) [19]. The HOMO energies refer to geometry-optimized neutral Ia, Ib and II, the SOMO energies to geometry-optimized radical
cations Ia+ꢀ, Ib+ꢀ and II+ꢀ. In the most stable conformer of Ib, N–H and C–Me (of NH–CMe3) are arranged in syn-periplanar conformation, as found for 2b in the crystal (Fig. 1).
Fig. 5. Base-pairing and packing of 4a in the crystal (also showing two ‘‘weak’’ hydrogen bonds H7ꢃ ꢃ ꢃO and H26ꢃ ꢃ ꢃO).
subtend interplanar angles of 45°, 56°, 67° with the plane of the
phosphorus substituents. The opposite trend, with a smaller N4–
C3–P angle compared to C2–C3–P, is observed in the two phos-
phine oxides, both of which display intramolecular P@Oꢃ ꢃ ꢃH–N
hydrogen bonds. The oxygen atom is found somewhat outside of
the plane of the heterocycle in both compounds, with O–P–C3–
C2 dihedral angles of 10.76(12)° in 4a and 23.41(11)° in 4b (Figs. 4
and 6). The NH2 group of 3a and 4a allows inversion-symmetric
base pairing via intermolecular hydrogen bonds to the pyrazine
nitrogen of a neighbouring molecule (Figs. 3 and 5), which is not
possible for the N-tert-butyl substituted compounds because the
bulky group at nitrogen is directed away from the substituents at
C3 (e.g. torsion angle C3–C2–N2–C9-167.68(12)° for 4b), thus
favouring an intramolecular over an intermolecular hydrogen
bond. The ring systems subtend much larger angles of 69°, 79°
and 85° to the plane of the direct phosphorus substituents, so that
the propeller form is to a large extent lost.
3.6. Preliminary studies on the use of Pd catalysts with the air-stable
ligand 6; isolation of complex 7
The high stability of 6 in air makes this compound an ideal li-
gand for studies of complex formation and catalytic activity with
transition metals. For our syntheses of N-heterocyclic phosphorus
compounds, Pd catalysts are of particular interest [9–11]. The use-
fulness of ligand 6 in palladium-catalyzed allylic substitutions has
already been shown [4b], but arylaminations [27] have not yet
been reported with this ligand. Although C–N cross-coupling reac-
tions are favoured, particularly by bulky electron-rich phosphanes
[28], chelating bis(diphenylphosphane) ligands such as dppp in
combination with Pd2(dba)3 or palladium acetate have also given
excellent results, e.g. in the coupling of amines with 2-bromopyri-
dines [29]. We screened ligand
6 in combination with the
aforementioned Pd compounds for the coupling of 2-bromopyri-
dine with mesitylamine (2,4,6-trimethylaniline) and obtained

110
Mohamed Shaker S. Adam et al. / Polyhedron 50 (2013) 101–111
addition of one equivalent of tBuLi at low temperature. The 2-ami-
no-3-phosphanylquinoxalines also differ from 2,3-diphosphanyl-
quinoxalines in their slow air oxidation (for N-secondary
aminophosphanylquinoxalines it is relatively rapid). As shown by
the preliminary tests, the air-stable 2,3-diphosphanylquinoxalines
will be useful as ligands in Pd-catalyzed C–N cross coupling reac-
tions, in addition to many other catalytic applications shown re-
cently by other groups. Our new synthetic access to the novel
quinoxaline P,N ligands may also pave the way for investigations
of hemilabile P,N-transition metal complexes and catalysts with
the quinoxaline scaffold.
Acknowledgments
The authors thank M. Ghalib, N. Lense and B. Schröder for
experimental support, J. Dickerhoff for HSQC and G. Thede and
M. Steinich for numerous NMR spectra. For the HRMS measure-
ments we thank Dr. H. Frauendorf and G. Sommer-Udvarnoki
(Georg-August-Universität Göttingen, Institut für Organische und
Biomolekulare Chemie).
Fig. 6. Molecular structure of 4b in the crystal. Ellipsoids represent 50% probability
levels. Selected bond lengths (Å) and angles (°): P1–O4 1.4977(9); O–P–C3
112.24(5), O–P–C21 111.55(6), O–P–C31 111.37(6); intramolecular hydrogen bond
(N–Hꢃ ꢃ ꢃO@P): N–H 0.840(17), Hꢃ ꢃ ꢃO, 2.043(17), Nꢃ ꢃ ꢃO 2.8190(13), <(N2–H02ꢃ ꢃ ꢃO)
153.3(15).
Appendix A. Supplementary data
CCDC 868421–86824 contain the supplementary crystallo-
graphic data for 2b, 3a, 4a and 4b. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2012.08.089.
NH2
Pd2(dba)3
6
+
t-BuOK
Toluene
N
N
H
N
Br
Scheme 4. Pd-catalyzed C–N cross-coupling of 2-bromopyridine with 2,4,6-
trimethylaniline.
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