Quinoxaline-anellated N,N′-dialkylimidazolium salts and iPr2quinox-NHC-Pd halide complexes

Article abstract of DOI:10.1016/j.jorganchem.2020.121487

Excess isopropyl- or neopentylamine react with dichloroquinoxaline to the respective diaminoquinoxalines and further with triethyl orthoformate to quinoxaline-anellated imidazolium salts as precursors for quinoxaline-anellated imidazolin-2-ylidene transition metal complexes. The bulky tert-butylamine replaces only one chlorine atom and opens, via a second amination step, a route to unsymmetrically N,N′-substituted diaminoquinoxalines as potential precursors to quinox-NHCs with different N,N′-substituents. A convenient high-yield access to the new trans-(iPr₂quinoxImy)₂PdX₂ (X = Cl, I) complexes, supplementing an earlier communication on the first quinox-NHC-Rh(COD)Cl and Ag complexes, was provided by the reaction of PdCl₂/NaOAc or Pd(OAc)₂/2KI with iPr₂quinoxImyH⁺ PF₆^? via deprotonation of the C-H acidic precursor salt with acetate. The compounds were characterized by NMR data, assigned via 2D and proton-coupled ¹³C NMR spectra, and/or crystal structure analyses. Short CMe₂-H···Pd and Pd-Cl···C^II distances suggest weak stabilizing interactions, in solution indicated by unusually strong downfield shifts of the CMe₂H protons.

Full text of DOI:10.1016/j.jorganchem.2020.121487

Journal of Organometallic Chemistry 926 (2020) 121487
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Quinoxaline-anellated N,N-dialkylimidazolium salts and
´
iPr quinox-NHC-Pd halide complexes
2
a,†
a
b
Saravanakumar Shanmuganathan , Carola Schulzke , Peter G. Jones ,
a,∗
Joachim W. Heinicke
a
Universität Greifswald, Institut für Biochemie, Felix-Hausdorff-Str. 4, 17487 Greifswald, Germany
b
Technische Universität Braunschweig, Institut für Anorganische und Analytische Chemie, Hagenring 30, D-38106 Braunschweig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Excess isopropyl- or neopentylamine react with dichloroquinoxaline to the respective diaminoquinoxa-
lines and further with triethyl orthoformate to quinoxaline-anellated imidazolium salts as precursors
for quinoxaline-anellated imidazolin-2-ylidene transition metal complexes. The bulky tert-butylamine
Received 21 July 2020
Revised 23 August 2020
Accepted 24 August 2020
Available online 26 August 2020
replaces only one chlorine atom and opens, via
a second amination step, a route to unsymmet-
rically N,N-substituted diaminoquinoxalines as potential precursors to quinox-NHCs with different
´
Keywords:
N,N -´ substituents. A convenient high-yield access to the new trans-(iPr2quinoxImy)2PdX2 (X = Cl, I) com-
Amino-quinoxalines
N-heterocyclic carbenes
Quinoxaline annulation
Palladium
plexes, supplementing an earlier communication on the first quinox-NHC-Rh(COD)Cl and Ag complexes,
was provided by the reaction of PdCl /NaOAc or Pd(OAc) /2KI with iPr quinoxImyH+ PF6− via deprotona-
2
2
2
tion of the C-H acidic precursor salt with acetate. The compounds were characterized by NMR data, as-
signed via 2D and proton-coupled 13 C NMR spectra, and/or crystal structure analyses. Short CMe2-H···Pd
Rhodium
II
and Pd-Cl···C distances suggest weak stabilizing interactions, in solution indicated by unusually strong
Silver complexes
downfield shifts of the CMe2H protons.
©
2020 Elsevier B.V. All rights reserved.
1. Introduction
erocycles suggested that fusion with the π-electron-withdrawing
quinoxaline-ring led to electron-poor NHCs with lowered stabil-
+
−
The great steric and electronic variability of non-, carbo- or
heterocyclically anellated imidazolin-2-ylidenes, discovered follow-
ing the first reports of suitably substituted isolable monomer rep-
resentatives [1], provided the impetus for investigations of their
synthesis (or generation as reactive intermediates) and their re-
activity towards a wide variety of reagents including transition
metal compounds [2,3]. The currently known NHCs cover a broad
range of stereochemical properties [4] and allow access not only to
transition metal or organo catalysts for a wide variety of organic
transformations [2,5] but also to novel polymers, MOFs or redox-
switchable materials [6], and luminescent [7] or otherwise useful
complexes for medicinal chemistry [8]. In the course of our inves-
tigations on synthesis, structure and reactivity of five-membered
heterocycles with dicoordinated main group elements [9,10], sta-
bilized by delocalized (p-p)π-bonds, we have synthesized various
anellated NHCs and transition metal complexes thereof. Evalua-
tion of the influence of anellation with aromatic carbo- or het-
ity, although a LRh(COD)Cl and a L Ag PF6 complex proved sta-
2
ble [10b]. Recent research by other groups on even more π-acidic
N-heterocyclic carbenes, realized by introducing conjugated C=O-
+
+
or cationic N-alkyl-, R E - or RnM - groups into anellated carbo-
3
or heterocycles or the NHC itself [3c,6a,11–13], or by replacing
one N-donor atom by carbon (CAACs) [14], confirmed the desta-
bilization of such NHCs but also the stabilization of their tran-
sition metal complexes by π-backbonding effects [15]. The inter-
esting properties of the recently reported electron-deficient NHC
complexes prompted us to supplement our earlier communicated
results on quinox-NHC precursors and their first Ag and Rh com-
plexes [10b] by a detailed description of two new quinoxaline-
anellated NHC Pd complexes.
2. Results and discussion
2.1. Syntheses
The starting materials 1a,b for N,N-diisopropyl- and
∗
´
Corresponding author.
dineopentyl-substituted quinoxaline-anellated imidazolium salts
E-mail address: heinicke@uni-greifswald.de (J.W. Heinicke).
+
−
†
(quinox-NHC-H X ) and quinox-NHC complexes were conve-
niently accessible from 2,3-dichloroquinoxaline by heating with
Present address: Alagappa University, Department of Industrial Chemistry,
Karaikudi, India
https://doi.org/10.1016/j.jorganchem.2020.121487
0
022-328X/© 2020 Elsevier B.V. All rights reserved.

2
S. Shanmuganathan, C. Schulzke and P.G. Jones et al. / Journal of Organometallic Chemistry 926 (2020) 121487
neous proton transfer to Pd-coordinated acetate and coordination
of the resulting NHC via a pentacoordinated Pd-intermediate or
in another way. Crude 6 and 7 both display NMR signals of the
NHC ligands consistent with trans-configuration. This may be fa-
vored over cis-configuration for steric reasons or, as shown for
trans-(N,N‘-Me bimy)PdI2 [21a], by lower solubility of the trans-
2
compared to the cis-isomer. Yellow crystals of trans-6, suitable for
characterization by XRD, were obtained by simple slow concen-
tration of the solution in CDCl , whereas attempts to crystallize
3
crude trans-7, initially characterized by NMR data and the cor-
+
+
rect isotopic pattern of [L PdI ] peaks in the HRMS DEI spec-
2
2
trum, failed. Finally, after long storage in 1,4-dioxane, very small
and sparingly soluble orange crystals were obtained by slow cool-
ing of hot saturated solutions in this solvent. Crystals analyzable
with CuKα-radiation were found and identified as the 1,4-dioxane
disolvate of 8. This complex may be formed as a side product or,
inferred from the slow color change to orange, via slow dissoci-
ation and decomposition of one quinox-NHC ligand, possibly fa-
vored by the O-donor solvent and low solubility of the μ-iodido
dimeric quinox-NHCPdI2 dioxane solvate. It is known that non- or
benzanellated bis(N,N -´ dialkylimidazolin-2-ylidene)PdI2 complexes
possess limited kinetic stability and undergo slow dissociation of
one NHC ligand, which in turn makes them applicable as pre-
catalysts for slow catalytic reactions [21,23]. Preliminary tests of
quinoxaline-anellated NHC-Pd catalysts for use in Suzuki-Miyaura-
couplings of PhB(OH)2 with some aryl bromides, generated in-
situ from 4b and Pd(OAc)2 in dioxane in the presence of excess
K3PO4, were performed to compare the effects of anellation of the
Np2Imy-ligand and gave good coupling yields with PhBr (70-90%)
even with quinoxaline-anellation. For p-bromoanisol (see Support-
ing Information) and p-bromotoluene [10f], however, the coupling
yields diminished in the catalyst ligand order Np2bImy > Np2Imy
> Np2naphImy >> Np2quinoxImy, so that the complexes 6 and 7
were not tested in these reactions. Finally, it should be mentioned
that silver complexes, exemplified by 9, can be obtained directly
from quinoxaline-imidazolium salts and Ag2O using the protocol
of Wang and Lin [24]. The applicability of various NHC-silver com-
plexes in transmetallation reactions [24,25] may also open an al-
ternative route to other quinoxaline-anellated imidazolin-2-ylidene
transition metal complexes.
Scheme 1. Synthesis of alkylaminoquinoxalines 1-3 (showing in 2 the conjugative
effects of the +M-amino group, which influence the chemical shifts of the benzene
ring nuclei).
excess diisopropyl- or dineopentylamine in an autoclave [10b].
This reaction was also applicable to a few other sterically un-
hindered primary amines [16], whereas the more bulky tBuNH2
replaces only one chlorine, even in large excess and on prolonged
heating in the autoclave, affording 2 [17]. Subsequent reaction
with isopropylamine to 3 (Scheme 1) provides a route to 2,3-
unsymmetrically substituted diaminoquinoxalines. Application of
known arylamination protocols for chloroquinoxalines [18] to 2
could not be included in this supplementing study but might
be relevant for further investigations on routes to new mate-
rials related to the recently reported luminescent N,N´-Ph,Me-
and N,N-Ph -substituted electron-withdrawing pyrido [b]- and
´
2
pyrazine-anellated NHC complexes of platinum [7e,f]. Claims for
luminescent N,N-diaryl-quinox-NHC Ir complexes have already
´
been patented [18d].
+
The N,N-diisopropyl-quinox-NHC-H salts 4a,bPF6 and 4a,bCl
´
were obtained by cyclization of 1a with triethyl orthoformate in
the presence of NH PF or NH Cl in high or moderate yield, re-
4
6
4
spectively. Strong downfield shifts of 2-H in the proton NMR spec-
tra (δ = 9 to 11), large one-bond C-H couplings (for 4a
in
PF6
1
CD CN
J
= 219 Hz) and rapid H/D exchange of this proton in
PF6
3
CH
CD OD solution indicate high C-H acidity. Deprotonation of 4b
3
with excess KH in THF failed to give an isolable N,N-dineopentyl-
´
quinox-NHC. Successful generation of isolable dineopentyl-benzo-
,
-naphtho- and -pyrido-imidazolin-2-ylidenes by the same pro-
cedure [10c–e] hint at diminished stability of the electron-
deficient quinoxaline-anellated imidazolin-2-ylidene. This is in ac-
cordance with the earlier reported isolation of the dimer of
2.2. Structural aspects
The structure of the compounds is proved by unequivocal NMR
data, elemental analysis or HRMS data and by crystal structure
analyses. The assignment of the proton and 13C NMR signals of
2 is based on the H-H and C-H coupling pattern and a 2D NMR
cross peak of CH3 with H-8. It correlates well with similar chemical
shift values of 2-amino-3-chloro-quinoxaline (see Supporting Infor-
mation) and 3-phosphono-substituted 2-aminoquinoxalines with
characteristic nJPC coupling constants, which give additional evi-
dence of the assignments [16a,b]. The upfield shifts of the H6 and
H7 triplets versus the H5 and H8 doublets in 2 suggest analo-
gous assignment of the two multiplets in the diaminoquinoxalines
1 and 3 (upfield m to H6/7 and downfield m to H5/H8). Triplets
for H6/7 (δ = 7.62) and downfield doublets for H5/8 (δ = 8.03
and 8.22) in an unsymmetrical aromatic quinoxaline-anellated imi-
dazole (N-tert-butylphenyl-2-p-dimethylaminophenyl-imidazo [4,5-
b] quinoxazoline) [26] account also for an upfield position of the
N,N-bis(trifluorophenyl)-substituted quinoxaline-2-ylidene by heat-
´
ing the corresponding 2,3-bis(arylamino)quinoxaline with excess
triethyl orthoformate [19]. Trapping the dineopentyl-quinox-NHC
by [Rh(COD)Cl]2 during in-situ deprotonation with potassium hy-
dride in THF furnished the rhodium complex 5, but in rather low
yield. Since 5 proved stable to air, moisture and chromatography
on silica gel with various solvents [10b], it appears probable that
complex formation is accompanied by competing reactions of the
quinox-NHC or of 5 itself with the hydride to unidentified prod-
ucts. Attempts to trap the quinox-NHC by K PdCl via in-situ de-
2
4
protonation of 4bPF6 with KH in THF failed completely. A successful
modification is in-situ deprotonation with acetates, a method that
was applied by the groups of Hahn [20] and Huynh [21] to the
synthesis of N,N-dialkylbenzimidazolin-2-ylidene palladium com-
´
plexes with N,N-methyl, allyl, isopropyl or other groups, which are
´
+
not bulky enough to prevent the dimerization of the correspond-
ing free carbenes [22]. Reactions of 4aPF6 with palladium chloride
and sodium acetate in THF (24 h reflux) or with palladium acetate
and KI in DMSO (2 h 80°C) gave evidence that this strategy is also
applicable to quinox-NHC complexes and provides the Pd halide
complexes 6 and 7 as yellow powders in high yields (Scheme 2).
It is not yet known whether the reaction proceeds by simulta-
H6/7 versus the H5/8 multiplets in the aromatic quinox-NHCH
+
cations 4a,b with a similar effect of the positive charge on the
downfield shifts of both multiplets (ꢀδ = 0.35-0.40). Compared
to the quinoxaline diamine 1a, the downfield shifts of H6/7 and
H5/8 in 4a amount to ꢀδ4a-1a = 0.74 and 0.70. The 13C reso-
nances of C5/8, however, appear in 1a,b and 3 close to those of
+
C6/7 and in 4a,b , as indicated by HSQC cross-peaks and larger

S. Shanmuganathan, C. Schulzke and P.G. Jones et al. / Journal of Organometallic Chemistry 926 (2020) 121487
3
+
Scheme 2. Synthesis of the quinox-NHCH salt 4a,bPF6 and the quinox-NHC Pd halide complexes 5-9.
signal splitting of C6/7 versus C5/8 in the proton-coupled 13C NMR
spectrum even upfield from C6/7. The downfield shift for the C6/7
groups of 6, contrasting two with and two without C-H···Pd in-
II
teractions in the crystal of 6·2 CDCl , reflect a dynamic behav-
3
13C nuclei is in the aromatic quinox-NHCH
+
cations 4a,b consid-
+
ior in solution. The nature of the preagostic (or anagostic) inter-
actions, which become stronger with decreasing radius of the d8-
metal cation, was discussed in detail in [21d] and studied for other
d8-complex types by quantum chemical DFT methods [28].
erably stronger than for C5/8 (ꢀδ4a-1a = 8.01 versus 4.42). Cq4a
and Cq8a of the benzene ring likewise display a downfield shift
(
ꢀδ4a-1a = 4.94) whereas Cq3a/9a of 4a exhibit upfield shift ver-
sus Cq2/3 of 1a (ꢀδ = −5.18) and thus increased π-electron den-
sity. In the quinox-NHC-PdX2 complex 6 (and analogously in 7)
the downfield shifts of the protons and benzene ring 13C nuclei
are considerably smaller (ꢀδH = 0.54, 0.49; ꢀδC = 4.47, 3.34
and 1.57), as are the upfield shifts of Cq3a/9a (ꢀδ = −3.87). In
comparing the chemical shifts of 6 and 7 with those of 4a the
most prominent change is the very strong downfield shift of C2
Direct structural information regarding the molecules in the
crystals was obtained by XRD analysis. Selected bond lengths and
angles are compiled in Table 1. The respective data of 5 and 9,
published in the early communication [10b], are included for com-
parison. Those of 4bPF6 [10e] are, except for the different alkyl
groups, similar to the values found for 4aPF6. The imidazo [4,5-b]
quinoxaline ring system is planar in all of these compounds. Sig-
II
(
ꢀδC2 = 48.9 and 47.8). The larger values compared to those in
nificant differences are displayed between the N-C -N angles of
the closely related naphtho- and non-anellated complexes trans-
4a,bPF6 (112.3-112.0°) and the complexes (107.2-109.5°), and to a
II
(
(
Np naphImy) PdCl (ꢀδC2 = 41.1 [10c]) and trans-(iPr Imy) PdCl
smaller extent also for the C -N-C angle (107.1-107.5 versus 108.9-
2
2
2
2
2
2
ꢀδC2 = 36.3 [27]), versus the respective imidazolium precursors,
110.3°) and the N-CII bond lengths. The latter increase in the or-
der 4a,bPF6 < 6·2 CDCl3 ≈ 8·2 dioxane < 9 < 5 and may re-
flect the extent of π-back bonding by repulsion of π-electrons
involved in the N-CII bonds. Other structural parameters remain
largely unchanged and show that the coordination only affects C2
and the two neighboring N atoms. The orientation of the two iso-
propyl groups in 4aPF6 (Fig. 1) is different. CHCMe2 at N3 exhibits a
staggered conformation (C14/C15-C13-N3-C3a, 62.1/62.3°) with C-
H13 in the same orientation as C-H2, while CHMe2 at N1 adopts
a gauche position with a transverse backwardly directed C10-H10
indicate the electron-withdrawing effect at C2 by anellation with
2
aromatic carbon- and σ N-containing heterocycles, confirmed in
quantum chemical calculations [15e], and resulting in upfield shifts
of the benzene ring atoms, for 6 versus 4a ꢀδ = −3.54, −1.08
and −3.37 for C6/7, C5/8 and Cq4a/8a). The polarization of π-
density from the imidazolin-ylidene ring towards the benzene ring
is caused by electron-electron repulsion from Cl-Pd-Cl bonding
electrons parallel to the C2-pz-orbital, core effects and the close
distance of Cl lone-electron pairs to C2(pz). The unusually strong
downfield shifts of the isopropyl NCH-protons of 6 and 7 (in CDCl3
δ = 6.34 and 5.91), exceeding even those in the aromatic pre-
˚
bond (H10-C10-N1-C9a 38.32°; H10···C9a 2.63A). In the palladium
halide complex 6·2 CDCl (Fig. 2) all isopropyl groups adopt a stag-
3
cursor 4a
(in CD CN δ = 5.21, ꢀδ = 1.13, 0.70) and in the
gered arrangement (C11/12-C10-N1-C9a 72.1(5), 55.4(5), C14/15-
PF6
3
d10-Ag complex 9 (in CD Cl δ = 5.41) may be attributed to a
+
C13-N3-C3a 101.4/111.5°), two with a short contact from the α-H
2
2
˚
combination of the remaining ring-current effect and preagostic C-
atom to palladium (H10-Pd 2.71 A, sum of van der Waals radii
II
8
˚
H···Pd (d ) interactions retained in preferred populations in solu-
2.83 A), the other two with opposite orientation and H13/H13A not
tion, as observed also in related N,N-diisopropyl-benzimidazolin-
far from N4/N4A. Even shorter with respect to the sum of the van
´
II
II
III
II
˚
2
-ylidene Pd , Pt , Au and particularly strong in Ni complexes
21]. These effects were confirmed also by crystal structure analy-
sis (see below), but equal chemical shifts for all four N-isopropyl
der Waals radii (Cl···C 3.45 A) are the distances Cl1···C2 (3.0370(7)
˚
[
and Cl1···C2A 3.1009(7) A) within the coordination plane, corre-
˚
sponding to ꢀ = −0.41 and −0.35 A, respectively, while the C2-

4
S. Shanmuganathan, C. Schulzke and P.G. Jones et al. / Journal of Organometallic Chemistry 926 (2020) 121487
Table 1
Comparison of selected bond lengths ( A˚ ) and angles (°) of 4aPF6, 4bPF6 [10e], 5 [10b], 6·2 CDCl3, 7·2 dioxane and 9 [10b].
[
a]
9[b]
Compound
4aPF6
4bPF6
5
6·2 CDCl3
8·2 dioxane
CII-H/M
0.944(19)
-
90
-
1.998(3)
2.021(3)
1.9591(19)
2.088(4); 2.090(5)
-
M-Xterm; M-Xbridge
2.3756(9)
2.3101(10);-
2.59368(19);
2
2
.6166(2),
.6694(2)
N-CII
1.326(2);
1.333(4),
1.334(4)
1.393(4),
1.394(4)
1.298(4),
1.295(4)
1.414(4),
1.430(4)
1.363(4),
1.362(4)
1.472(4),
1.472(4)
112.0(3)
-
1.395(3),
1.381(3)
1.375(3),
1.393(3)
1.320(3),
1.297(3)
1.450(4),
1.447(4)
1.393(5),
1.367(3)
1.478(3),
1.497(3)
107.2(2)
125.53(18),
1.351(4);
1.353(4)
1.378(4);
1.385(4)
1.305(4);
1.304(4)
1.416(4);
1.422(5)
1.375(4);
1.373(4)
1.488(4);
1.489(4)
108.2(3)
122.8(2);
128.9(2)
109.8(3);
109.4(3)
106.3(3);
106.3(3)
4.1(5); 10.6(6)
85.92/90.35
1.351(2),
1.352(2)
1.351(5), 1.370(5);
1.352(5),1.364(5)
1.385(5), 1.397(5);
1.386, 1.388(5)
1
.3279(19)
1.3875(18);
.3897(19)
1.2982(19);
.3019(17)
N5ring-C5/6ring
N6ring-C5/6ring
1.391(2),
1.394(3)
1
1.297(3),
1.296(3)
1.305(5), 1.306(5);
1.302(5), 1.303(5)
1.412(6), 1.425(6);
1.416(6), 1.425(6)
1.380(5), 1.375(5);
1.374(6), 1.374(6)
1.493(5), 1.487(5);
1.483(6), 1.490(5)
107.1(4); 107.2(4)
127.1(3), 125.7(3);
126.9(3), 125.9(3)
110.1(3), 110.3(3);
110.2(4), 110.4(4)
106.3(4), 106.1(3);
106.0(4), 106.2(4)
1
C=C5/6ring;
C=C6/6ring
1.408(2);
1.432(2)
1.431(3),
1.433(3)
N6ring-C6/6ring
1.3666(18);
1.373(3),
1.374(3)
1
.3667(19)
1.4861(19);
.4848(19)
N-CαR
1.484(2),
1.487(2)
1
II
N-C -N
112.31(13)
-
109.54(16)
125.04(14),
125.41(14)
109.11(16),
108.94(15)
106.17(16),
106.20(16)
5.9(3), 1.1(3)
II
N-C -M
1
27.05(19)
II
C -N-Cring
107.09(12);
107.2(2),
107.5(2)
106.9(2),
106.4(2)
110.9(2),
109.0(2)
105.4(2),
107.5(2)
1
07.26(12)
N5ring-C-C
106.91(12);
1
-
06.42(12)
II
CαR-N-C -M
II
N1/3-C -Pd-Cl1
-
[
a]
b]
CII-Pd-CII 180.0°.
CII-Ag-CII 177.67(19)°.
[
Fig. 1. Molecular structure of 4aPF6 (ellipsoids with 50 % probability).
Pd-Cl angles display almost ideal values for square-planar coordi-
nation (C2-Pd-Cl 88.79(10), C2-Pd-ClA 91.21(10)°). The reason may
be small back-bonding effects by stabilizing interactions between
the formally vacant pz-orbitals of C2/C2A and electron lone-pairs
of the chlorine atoms, arranged perpendicularly to the ligand ring-
Pd-C2 bond lengths are also similar to those in the benzanellated
NHC palladium halide complexes. The μ-I-bridged (quinoxNHC-
PdI2)2 complex 8·2 dioxane (Fig. 3) displays almost the same struc-
tural features with the exception that here the α-H atoms of both
isopropyl groups of the ligands are close to the respective Pd-atom
II
˚
plane (89.92° considering PdCl C and all ring atoms), as known
(C10···H10 2.71, C13···H13 2.75 A, angles to Pd 120.0 and 119.0°).
2
2
ꢀ
for (dialkylbenzimidazolin-2-ylidene)Pd-halide complexes [21]. The
The interplanar angle Pd1, I1, I1 , I2, C2 / C2, N1, N3, C3A, C9A

S. Shanmuganathan, C. Schulzke and P.G. Jones et al. / Journal of Organometallic Chemistry 926 (2020) 121487
5
Fig. 2. Molecular structure of 6·2 CDCl3 (ellipsoids with 50 % probability, solvent omitted for clarity).
Fig. 3. Molecular structure of 8·2 1,4-dioxane (ellipsoids with 50 % probability). The symmetry-related second dioxane molecule is omitted.
amounts to 89.3° and may allow interactions between iodine and
C2, similar to those between Cl and C2 in 6, which might be
the reason for the slightly diminished C2-Pd-I2 angles (86.39(6)°)
for the terminal iodide ligands. The bridging iodides display al-
most ideal values for the square-planar coordination (C2-Pd1-I1
the quinoxaline-anellated imidazolin-2-ylidene transition metal
complexes are less suitable for catalytic reactions such as the
Suzuki-Miyaura coupling they might be useful for transforma-
tions requiring catalysts with electron-withdrawing ligands, for
materials such as the luminescent pyrido[b]- or pyrazine-anellated
imidazolin-2-ylidene platinum complexes [7e,f] and claimed irid-
ium complexes [18d] or for complexes with extended delocalized
aromatic NHC ligands and useful photophysical or electrocatalytic
properties [7c,29]. The above results, including preliminary in-
9
0.01(6), C2-Pd1-I1# 178.85(6)°).
3. Conclusions and outlook
A
convenient synthetic route to N,N-dialkyl-substituted
formation on a practicable route towards N,N-unsymmetrically
´
´
quinoxaline-anellated
complexes is presented. It comprises the bis(amination) of
,3-dichloroquinoxaline with primary amines, cyclocondensation
with orthoformate in the presence of NH PF to the respec-
imidazolin-2-ylidene
transition-metal
substituted 2,3-diaminoquinoxalines, might be helpful for further
developments towards such goals.
2
4. Experimental
4
6
tive quinoxaline-imidazolium salts and coordination of the
quinoxaline-imidazolin-2-ylidene, generated in-situ by acetate
anions, at a suitable transition-metal centre. The deprotonation
4.1. General
+
of the C-H-acidic quinox-Imy-H salts with the acetate anion
All operations with air- or moisture-sensitive compounds were
carried out under nitrogen or argon using Schlenk techniques. Sol-
vents were dried by standard procedures and distilled before use.
NMR spectra were recorded on a multinuclear FT-NMR spectrom-
eter ARX300 (Bruker) at 300.1 ( H) or 75.5 (13C) MHz. Chemical
shifts (δ) are given in ppm related to SiMe4 or calibrated sol-
vent signals. Coupling constants refer to JHH (for 1H NMR) or to
J13CH (for proton-coupled 13C NMR). Assignment numbers follow
nomenclature numbers of quinoxalines (cf. Schemes 1 and 2). Mass
spectra were measured on a single focusing sector-field spectrom-
eter AMD40 (EI, 70 eV), HRMS spectra on APEX IV (Fa. Bruker Dal-
tonik) (ESI in MeOH) or a double focusing sector field mass spec-
avoids competing reductions or other side reactions that lower
the yields or even prevent formation of quinox-Imy-transition
metal complexes when using KH for the in-situ deprotonation.
The very strongly downfield shifted 3CII-resonances of 6 and
1
1
7
versus 4a (ꢀδ = 48.9 and 47.8) compared to those of analo-
gous benzo- or naphtho-anellated Pd complexes versus their C-2
protonated precursors (ꢀδ = 39.0 [10f] and 41.1 [10c]), together
with slight upfield shifts of the benzo-1 C nuclei with respect
3
+
to the corresponding signals of the quinox-NHC-H precursors,
imply stabilization by metal-ligand back-bonding effects, per-
haps strengthened in the case of the Pd halide complexes by
interactions between CII and halide electron lone pairs. Whereas
+
trometer JMS-700 from JEOL (DEI ). Melting points (uncorrected)

6
S. Shanmuganathan, C. Schulzke and P.G. Jones et al. / Journal of Organometallic Chemistry 926 (2020) 121487
orange-yellow solid, mp. 94-95°C, soluble in hexane or pentane. 1H
were determined with a Sanyo Gallenkamp melting point appara-
tus, elemental analysis with a CHNS-932 analyser from LECO or
from Elementar using standard conditions. Compound 4bPF6 in-
cluding its crystal structure was reported in a full paper on anel-
lated dineopentyl-imidazolium salts [10e].
3
NMR (CDCl ): δ = 1.32 (d, J = 6.5 Hz, 6 H, CMe , 1.57 (s, 9 H,
3
2
3
CMe ), 4.01 (d, J = 6.6 Hz, 1 H, NH ), 4.07 (s, 1 H, NH ), 4.39 (d
tBu
3
iPr
sept, 3J(CHNH)
≈
3
J
(CHCH3)
= 6.4 Hz, 1 H, NCH), 7.28 (m, 2 H, H-6/7),
3
7
.60 (m, 2 H, H-5/8); in C D : δ = 1.05 (d, J = 6.6 Hz, 6 H, CMe ,
6
6
2
1
.43 (s, 9 H, CMe ), 3.58 (unresolved br d, 1 H, NH ), 3.83 (s, 1 H,
3
i
P
r
4
.2. 2,3-Diisopropylamino-quinoxaline (1a)
NHtBu), 4.39 (d sept, 3J(CHNH)
3J
≈
= 6.4 Hz, 2 H, NCH), 7.29
(CHCH3)
m, 2 H, H-6/7), 7.93, 8.01 (2 m, 1 H, H-5/8). 13C{ H} NMR (CDCl ):
1
(
3
Isopropylamine (20 mL, reactant and solvent) and 2,3-
δ = 22.92 (CHMe ), 28.99 (CMe ), 42.80 (CHMe ), 52.23 (CMe ),
2
3
2
3
dichloroquinoxaline (2.4 g, 12.1 mmol) were heated in a stainless-
steel autoclave for 2 h at 120 °C. Thereafter the resulting pink
product mixture was transferred to a separation funnel, into which
aqueous NaOH and diethyl ether were added to separate chloride
and the product. The combined ether phase (from 3-4 extractions)
1
24.37, 124.40, 125.70, 126.01 (CH-5 to CH-8), 136.80, 136.72 (Cq-
4
2
a/8a), 143.76, 144.05 (Cq-2/3). MS (EI, 70 eV): m/z (%) = 259 (18),
+
58 (100) [M ], 243 (22), 203 (98), 202 (70), 188 (95), 160 (62)
+
[
M –2 C H ] and fragmentations thereof. Anal. calcd. for C15H22N4
3
6
(
258.37): C 69.73, H 8.58, N 21.69; found: C 69.40, H 8.46, N 21.91.
was dried with Na SO4 and filtered. Evaporation of the solvent
2
furnished 2.8 g (95 %) pale yellow 1a, mp. 155-157°C. 1H NMR
4
.6. 1,3-Diisopropyl-imidazolio[4,5-b]quinoxaline
3
3
(
CDCl ): δ = 1.32 (d, J = 6.4 Hz, 12 H, CH ), 4.06 (d, J = 5.8 Hz, 2
3
3
hexafluorophosphate (4aPF6
)
H, NH), 4.42 (d sept, 3J = 6, 6.4 Hz, 2 H, NCH), 7.29 (m, 2 H, H-6/7),
1
7
.62 (m, 2 H, H-5/8). 13C{ H} NMR (CDCl ): δ = 22.9 (CH ), 42.8
3
3
A mixture of 1a (2.0 g, 8.19 mmol), NH PF6 (1.3 g, 8.0 mmol)
4
(
(
[
NCH), 124.46, 125.67 (CH-6/7, CH-5/8), 137.06 (Cq-4a/8a), 143.79
and triethyl orthoformate (15 mL) was heated for 24 h at 120 ºC
in a rectification apparatus while a slow stream of N2 supported
the separation of ethanol from the reaction mixture. Then the sol-
vent was removed under vacuum, the residue washed with hex-
+
+
Cq-2/3). MS (EI, 70 eV, 150°C): m/z (%) = 244 (89) [M ], 229 (23)
+
+
M –Me], 201 (100) [M –C H ], 187 (51), 160 (17) [M –2 C H ],
3
7
3
6
144 (20). Anal. calcd. for C14 H20N4 (244.34): C 68.82, H 8.25, N
2
2.93; found: C 68.71, H 8.40, N 22.46.
ane (3×10 mL) and the product extracted with CH CN to give 2.5
3
g (76%) colorless crystals of 4a . Crystal data are compiled in
PF6
4
.3. 2,3-Dineopentylamino-quinoxaline (1b) (alternative synthesis
Table 2. 1H (CH-COSY) NMR (CD CN): δ = 1.81 (d, 3J = 6.8 Hz, 12
3
see [16c])
3
H, CH ), 5.21 (sept, J = 6.8 Hz, 2 H, NCH), 8.04 (m, 2 H, H-6/7),
3
8
.32 (m, 2 H, H-5/8), 9.59 (s, 1 H, H-2). 13C{ H}, proton-coupled
1
13
C
Reaction of neopentylamine (21.3 mL, reactant and solvent)
1
2
NMR, HSQC, HMBC (CD CN): δ = 21.59 (quart, quint, J = 129, J ≈
3
with 2,3-dichloroquinoxaline (7.0 g, 35.2 mmol) in a stainless-steel
autoclave (3 h at 120 °C) gave a pale yellow solid and, after re-
peated extraction with hexane and evaporation of the solvent, 9.2
g (87%) of spectroscopically pure crude 1b, after sublimation in
3
1
2
3
J = 4.4 Hz, CH ), 53.27 (d sept d, J = 146, J = 4.4, J = 1.3 Hz,
3
NCH), 130.09 (dm, 1J ≈ 160, |ꢁ(
n>1
J)| = 14.5 Hz, CH-5/8), 132.47
dm, 1J ≈ 167, |ꢁ(
n>1
(
J)| = 17 Hz, CH-6/7), 138.61 (dd, J = 6.8, 3.3
Me2
Hz, Cq-3a/9a, HMBC# to HC
), 142.00 (m similar to ddd, J = 8.3,
3
high vacuum (10–5 Torr, 130°C) mp. 94-96°C. 1H NMR (CDCl ):
5.6, 4.7 Hz, Cq-4a/8a), 148.12 (dt, 1J = 219, J = 5 Hz, CH-2). Anal.
3
3
δ = 1.06 (s, 18 H, CMe ), 3.43 (d, J = 5.7 Hz, 4 H, NCH ), 4.33 (br,
3
2
calcd. for C15H19F N P (400.31): C 45.01, H 4.78, N 14.00; found: C
6
4
13
1
2
H, NH), 7.30 (m, 2 H, H-6/7), 7.63 (m, 2 H, H-5/8). C{ H} NMR
4
5.12 H 4.78, N 13.98.
(
CDCl ): δ = 27.61 (CMe ), 31.60 (CMe ), 52.42 (NCH ), 124.62,
3
3
3
2
1
25.57 (CH-6/7, CH-5/8), 136.96 (Cq-4a/8a), 145.06 (Cq-2/3). MS (EI,
4.7. 1,3-Diisopropyl-imidazolio[4,5-b]quinoxaline chloride (4aCl)
+
7
0 eV): m/z (%) = 301 (11), 300 (61) [M ], 244 8(17), 243 (100),
2
14 (11), 192 (32), 173 (20), 129 (20) and smaller fragments. Anal.
Reaction of a mixture of 1a (1.0 g, 4.09 mmol), NH Cl (219 mg,
4
calcd. for C18 H28N4 (300.45): C 71.96, H 9.39, N 18.65; found: C
1.81, H 9.87, N 17.98.
4.09 mmol) and triethyl orthoformate (10 mL) and workup as de-
7
scribed for 3PF6 gave 0.51 g (43%) colorless crystals of 4aCl, mp.
>
300°C. 1H NMR ([D ]-DMSO): 1.77 (d, 3J = 6.6 Hz, 12 H, CH ),
6
3
5.23 (sept, 3J = 6.6 Hz, 2 H, NCH), 8.03 (m, 2 H, H-6/7), 8.38
4
.4. 2-tert-Butylamino-3-chloro-quinoxaline (2)
m, 2 H, H-5/8), 10.89 (s, 1 H, H-2). 13C{ H} (DEPT) NMR ([D ]-
1
(
6
The preparation of 2 was published earlier [17]. For correct as-
DMSO): δ = 21.18 (CH ), 51.12 (NCH), 128.86 (CH-5/8), 131.14 (CH-
3
signment of the NMR data H,H-NOESY, proton-coupled 13C NMR,
HSQC and HMBC spectra (see Supporting Information) were mea-
sured. 1H NMR (CDCl ): δ = 1.57 (s, 9 H, CMe , NOESY# to H-8),
6/7), 137.71 (C -3a/9a), 140.07 (C -4a/8a), 149.68 (CH-2). In CD OD
q
q
3
exchange of H-2 by deuterium: 13C{ H} NMR (CD OD): δ = 21.75
1
3
3
3
(CH ), 53.52 (NCH), 130.34 (CH-5/8), 132.45 (CH-6/7), 138.91 (C -
3
q
4
3a/9a), 142.52 (C -4a/8a), 149.01 (t, 1JCD = 33.2 Hz, CD-2).
5
.49 (br s, 1 H, NH), 7.36 (td, 3J = 8.3, 7.0, J = 1.5 Hz, 1 H, H-6),
q
4
3
7
4
.55 (td, 3J = 8.4, 7.0, J = 1.5 Hz, 1 H, H-7), 7.70 (ddd, J = 8.3,
5
3
4
J = 1.3, J = 0.3 Hz, 1 H, H-8), 7.76 (ddd, J = 8.4, J = 1.3,
4.8. (1,3-Dineopentyl-imidazo[4,5-b]quinoxaline-2-ylidene)
rhodium(cyclooctadiene-1,5) chloride (5)
5
13
1
J = 0.3 Hz, 1 H, H-5). C NMR (CDCl ): δ = 28.51 (br q, J = 133
3
1
Hz, 3 CH ), 52.52 (br, Cq), 124.71 (dd, J = 161.7, J = 8.3 Hz, CH-
3
6
1
), 126.26 (ddd, 1J = 163, J = 7.7, J = 1.7 Hz, CH-8), 127.80 (ddt,
A suspension of [Rh(1,5-COD)Cl]2 (151.0 mg, 0.306 mmol) and
4b (279.4 mg, 0.612 mmol) in THF was added at –78 °C to a sus-
pension of potassium hydride in THF, freshly prepared from KH
(30%)/mineral oil (98.1 mg, 0.73 mmol) by repeated extraction of
the oil with dry THF. After ca. 15 h at ambient temperature, the
solvent was removed in vacuum and the product purified by col-
1
J = 161.5, J = 7.7, J = 1.7, 1 Hz, CH-5), 129.85 (dd, J = 161.2,
J = 8.8 Hz, CH-7), 135.9 (br m near noise level, Cq-4a), 138.15 (d,
J = 2.2 Hz, Cq-3), 141.1 (br m near noise level, Cq-8a), 147.38 (d,
J = 1.8 Hz, Cq-2); δ values of weak Cq-multiplets were confirmed
13
1
by C{ H} signals.
umn chromatography on silica gel. Elution with CH Cl /1%MeOH
2
2
4
.5. 2-tert-Butylamino-3-isopropylamino-quinoxaline (3)
after initial removal of unconverted ([Rh(1,5-COD)Cl] ) with CH Cl
2 2 2
furnished 139 mg (41%) yellow crystals of 5. 1H NMR (CDCl ): δ
3
Reaction of isopropylamine (20 mL, reactant and solvent) and 2-
1.31 (s, 18 H, Me), 2.05 (m, 4 H, CH ), 2.50 (m, 4 H, CH ), 3.15
2 2
(br q, 2 H, =CH), 4.86 (d, 4J(103Rh H) = 13.6 Hz, 2 H, CH ), 5.25
1
tert-butylamino-3-chloroquinoxaline (2.3 g, 9.8 mmol) and workup
of the resulting orange liquid as described for 1a gave 2.3 g (93%)
2
4
103
1
(d, J( Rh H) = 13.5 Hz, 2 H, CH ), 5.37 (br m, 2 H, =CH), 7.74
2

S. Shanmuganathan, C. Schulzke and P.G. Jones et al. / Journal of Organometallic Chemistry 926 (2020) 121487
7
Table 2
Crystal data and structure refinement for 4aPF6, 6·2 CDCl3 and 8·2 dioxane.
4aPF6
6·2 CDCl3
8·2 dioxane
Formula
Mw
C15H19F6N4P
400.31
C32H36D2Cl8N8Pd
926.71
C38H52I4N8O4Pd2
1405.27
Temperature (K)
Wavelength ( A˚ )
170(2)
170(2)
100(2)
0.71073
Monoclinic
P21/n
0.71073
1.54184
Crystal system
Orthorhombic
Pbcn
Triclinic
Space group
a ( A˚ )
b ( A˚ )
c( A˚ )
P(-1)
8.6949(17)
15.478(3)
13.217(3)
90
12.599(3)
9.3741(3)
13.187(3)
9.6251(3)
23.021(5)
12.8839(4)
α (°)
β (°)
γ (°)
90
83.578(3)
96.66(3)
90
90
79.042(3)
90
87.700(3)
V [ A˚ 3
]
1766.7(6)
4
3824.6(13)
1133.91(6)
Z
4
1
calcd (Mg m 3)
−
1.505
1.609
2.058
ρ
Absorption coefficient (mm 1)
−
0.222
1.081
28.16
F(000)
824
1872
672
Crystal size (mm)
0.227×0.210×0.162
3.056 to 29.528
-9<=h<=12,
-21<=k<=21,
0.231×0.111×0.111
3.090 to 28.280
-16<=h<=16
-17<=k<=16
-30<=l<=30
37863
0.08×0.03×0.03
3.51 to 77.26
-11<=h<=11,
-12<=k<=12,
-16<=l<=16
47302
ꢂ range for data collection (°)
Index
ranges
-
18<=l<=18
Reflections collected
19442
Independent reflections
Completeness (%) to ꢂ (°)
Absorption correction
Max. and min. transmission
Refinement method
4881 [R(int) = 0.0501]
99.7 (to ꢂ = 25.242)
Numerical
4746 [R(int) = 0.0936]
99.9 (to ꢂ = 25.242)
Numerical
4760 [R(int) = 0.040]
99.0 (to ꢂ = 77)
Semi-empirical from equivalents
1.000 and 0.528
Full-matrix least-squares on F2
4760 / 0 / 258
1.08
0.9911 and 0.9535
Full-matrix least-squares on F2
4881 / 0 / 402
0.9585 and 0.8742
Full-matrix least-squares on F2
4746 / 0 / 239
1.042
Data / restraints / parameters
Goodness-of-fit on F2
1.054
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0410, wR2 = 0.0976
R1 = 0.0756, wR2 = 0.1145
n/a
R1 = 0.0456, wR2 = 0.1091
R1 = 0.0930, wR2 = 0.1234
0.0015(2)
R1 = 0.018, wR2 = 0.047
R1 = 0.018, wR2 = 0.047
0.00040(3)
Extinction coefficient
Largest diff. peak and hole (e· A˚ −3)
0.227 and -0.464
0.537 and -1.192
0.49 and -0.71
1
(
m, 2 H, H-6/7), 8.11 (m, 2 H, H-5/8). 13C{ H} NMR (150.90 MHz;
6·2 CHCl3, suitable for XRD, were obtained by slow concentration
DEPT 75.47 MHz) (CDCl ): δ = 28.93 (CH ), 30.17 (CMe ), 32.98
of a solution in CHCl3 (for crystal data see Table 2).
3
2
3
103
13
(
CH ), 33.58 (CMe ), 60.08 (NCH ), 70.72 (d, J( Rh C) = 14.6
2
3
2
Hz, CH=), 102.52 (d, J(103Rh13C) = 6.5 Hz, CH=), 128.86, 128.91
4.10. trans-Bis(1,3-diisopropyl-imidazo[4,5-b]quinoxaline-2-
ylidene)palladium iodide (7) and complex 8
(
CH-6/7, CH-5/8), 138.50, 141.18 (Cq-4a/8a, Cq-3a/9a), 219.46 (d,
1J(103Rh13C) = 52 Hz, Cq-2); assignment of aryl signals in anal-
ogy to those of 6. Elemental analysis: Anal. calcd. for C27
H
ClN Rh
Pd(OAc)2 (119 mg, 0.53 mmol) was added to 4aPF6 (424 mg,
1.06 mmol) and KI (176 mg, 1.06 mmol) in dry DMSO (20 mL).
After heating the resulting mixture for 2 h at 80°C the solvent
38
4
(
557.03): C 58.22, H 6.88, N 10.06; found: C incomplete combus-
tion, H 6.73, N 10.19.
was removed under reduced pressure. CH Cl2 was added to the re-
2
maining yellow solid, the insoluble salt filtered off, and the solvent
removed to give 386 mg (84%) of 7. 1H NMR (CDCl ): δ = 2.00
4
.9. trans-Bis(1,3-diisopropyl-imidazo[4,5-b]quinoxaline-2-
3
3
(
d, 3J = 6.9 Hz, 12 H), 5.91 (sept, J = 6.9 Hz, 2 NCH), 7.77 (m,
ylidene)palladium chloride (6)
H, H-6/7), 8.14 (m, 2 H, H-5/8). 13C{ H} (DEPT) NMR (CDCl ):
1
2
3
δ = 20.13 (CH ), 55.72 (NCH), 128.85, 128.96 (CH-6/7, CH-5/8),
Anhydrous sodium acetate (69 mg, 0.84 mmol) was added to
3
+
1
38.39, 140.21 (Cq-3a/9a, Cq-4a/8a), 195.95 (Cq-2). HRMS (DEI ):
4
aPF6 (300 mg, 0.749 mmol) and PdCl2 (66 mg, 0.372 mmol) in
+
H I N Pd (868.89), calcd. for [M] 868.0182; found: 868.0147
36 2 8
C30
dry THF (20 mL). The mixture was heated for 24 h at 80°C. Then
the solvent was evaporated under reduced pressure, dry CH Cl2
(
av. of 23 scans). Crystallization of 7 was attempted with several
2
solvents. After long storage of a suspension in 1,4-dioxane, small
orange-brown crystals deposited during slow cooling after satura-
tion by heating to reflux; these were identified by XRD as the 1,4-
dioxane disolvate of 8 (for crystal data see Table 2).
added to the remaining yellow solid and the insoluble salt sep-
arated. Removal of the solvent from the filtrate provided 240
mg (83%) of pale yellow microcrystals of 6. 1H NMR (CDCl ):
3
3
3
δ = 2.08 (s, J = 6.9 Hz, 24 H, Me), 6.34 (sept, J = 7.0 Hz,
4
NCH), 7.78 (m, 4 H, H-6/7), 8.16 (m, 4 H, H-5/8, NOESY# to
Me). 13C{ H} (DEPT, HSQC, HMBC) NMR (CDCl ): δ = 21.20 (CH ),
1
4.11. Bis(1,3-Diisopropyl-imidazo[4,5-b]quinoxaline-2-ylidene)silver
hexafluorophosphate (9)
3
3
5
5.29 (NCH), 128.93 (C-6/7), 129.01 (C-5/8), 138.63 (Cq-4a/8a),
1
39.92 (Cq-3a/9a, HMBC# to NCH), 196.95 (Cq-2, HMBC# to NCH).
+
HRMS (ESI in MeOH): C30H36Cl N Pd (685.99), calcd. for [M+H]
CH Cl (15 mL) was added to a mixture of 4a (500 mg, 1.25
2
8
2
2
6
87.1543, 685.1554, 684.1568; found: 687.1542, 685.1537, 684.1562
for the three strongest isotopic peaks). Analytical data of mi-
crocrystals formed in 1,4-dioxane correspond to a monosolvate
mmol), Ag O (289.4 mg, 1.25 mmol) and freshly dried molec-
2
(
ular sieve (3A, 1 g) and stirred for 24 h at ambient temper-
ature. Insoluble components were filtered off and washed with
CH Cl . The filtrate was concentrated under vacuum (to ca. 5
6
·dioxane. Anal. calcd. for C34H44Cl N O Pd (774.09): C 52.75, H
2
8
2
2
2
5
.73, N 14.48; found: C 52.72, H 5.50, N 14.77. Single crystals of
mL) and layered with hexane (10 mL). The white precipitate

8
S. Shanmuganathan, C. Schulzke and P.G. Jones et al. / Journal of Organometallic Chemistry 926 (2020) 121487
was separated and recrystallized from CH Cl to give 850 mg
including 4b for use in Pd-catalyzed Suzuki-Miyaura coupling of
PhB(OH)2 with bromoanisole and bromobenzene. Supplementary
data to this article can be found online at https://doi.org/10.1016/j.
jorganchem.2020
2
2
(
3
89%) colorless crystals, mp. >300°C. 1H NMR (CD Cl ): 1.99 (d,
J = 6.8 Hz, 12 H), 5.41 (sept, J = 6.8 Hz, 2 NCH), 7.93 (m, 2
2 2
3
13
1
H, H-6/7), 8.29 (m, 2 H, H-5/8). C{ H} (DEPT) NMR (CD Cl ):
2
2
δ = 22.63 (CH ), 54.92 (NCH), 129.44, 130.36 (CH-6/7, CH-5/8),
3
107/109
39.57 (d, 3J(
Ag C) = 6.5 Hz, Cq-3a/9a), 140.18 (Cq-4a/8a),
13
1
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The authors declare that they have no known competing finan-
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