A new mixed amino-amido N-heterocyclic carbene based on anthranilic acid

Article abstract of DOI:10.1021/om301152n

The synthesis of a new mixed amino-amido N-heterocyclic carbene (type A) starting from anthranilic acid is presented. A new straightforward synthetic approach to related diaminocarbenes of type B is also described. Both NHCs react with group 6 elements to form heteroureas and coordinate to L₂ClM fragments (M = Rh, Ir; L₂ = COD, (CO)₂). IR spectroscopic analysis of the carbonyl complexes reveals that the diamino-NHC is a better donor ligand (TEP: 2054 cm^-1) compared to the amino-amido NHC (TEP: 2060 cm^-1). In line with this behavior, a carbene dimerization to give the corresponding olefin is observed only for derivatives of type A. X-ray structure determinations are reported for two Rh complexes of ligand A and its cationic precursor.

Full text of DOI:10.1021/om301152n

Article
pubs.acs.org/Organometallics
A New Mixed Amino−Amido N‑Heterocyclic Carbene Based on
Anthranilic Acid
Abdelaziz Makhloufi, Michaela Wahl, Walter Frank, and Christian Ganter*
Institut fur Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universitat Dusseldorf, Universitatsstrasse 1, D-40225
̈
̈
̈
̈
Dusseldorf, Germany
̈
S
* Supporting Information
ABSTRACT: The synthesis of a new mixed amino−amido N-heterocyclic carbene
(type A) starting from anthranilic acid is presented. A new straightforward synthetic
approach to related diaminocarbenes of type B is also described. Both NHCs react with
group 6 elements to form heteroureas and coordinate to L2ClM fragments (M = Rh, Ir;
L2 = COD, (CO)2). IR spectroscopic analysis of the carbonyl complexes reveals that the
diamino-NHC is a better donor ligand (TEP: 2054 cm−1) compared to the amino−
amido NHC (TEP: 2060 cm−1). In line with this behavior, a carbene dimerization to
give the corresponding olefin is observed only for derivatives of type A. X-ray structure determinations are reported for two Rh
complexes of ligand A and its cationic precursor.
Chart 1. Types of N-Heterocyclic Carbenes Reported in This
Contribution
INTRODUCTION
■
N-Heterocyclic carbenes (NHCs) have developed into a well-
recognized class of compounds with applications as ligands for
organometallic complexes and organocatalysts within the last
twenty years.1 The modification of the electronic nature of
NHCs continues to stimulate active current research, and a
number of different backbone structures and substitution
patterns have been reported within this context.2 In
diaminocarbenes the nitrogen lone pairs provide electron
density for the stabilization of the carbene carbon atom,
whereas this electron-donating effect is drastically diminished in
the case of diamidocarbenes.3 Accordingly, when regarded as
ligands in transition metal complexes, the former are classified
as superb σ-donors, while for the latter a π-acceptor interaction
also contributes significantly to the metal−carbene bonding.
Additionally, the diamido substitution pattern leads to a smaller
singlet−triplet gap and endows the diamidocarbenes with some
electrophilic reactivity in addition to the typical nucleophilic
behavior of NHCs. Mixed amino−amido-type NHCs show
properties intermediate between those described above.4 In
continuation of our efforts to develop new backbone structures
for electron-poor NHCs5 we present here a new amino−amido
carbene A, which is easily derived from anthranilic acid. A
related diamino NHC B based upon the same scaffold was also
accessed to enable a comparison of electronic properties. Our
report is stimulated by a very recent publication by Zhang and
Shi,6 who described an alternative synthetic approach to the
diamino carbenes B.
formamide 2 at elevated temperature according to a protocol
described by Sherill.7 Introduction of the substituent at the
amide nitrogen atom was achieved by treatment of 3 with KOH
and an excess of alkylation reagent (MeI, EtI, BzBr) in
acetonitrile at elevated temperature, affording the neutral
derivatives 4 in good yield.
More powerful alkylation reagents were required for the
conversion of derivatives 4 to the respective amidinium salts 5.
Thus, treatment of 4 with either methyltriflate in ether or
Meerwein’s salt (Me3O BF4) in acetonitrile at room temper-
ature provided the desired cations with tetrafluoroborate (5a−
c) or triflate (5d−f) anions as colorless crystalline solids in high
yield.
The 1H and 13C resonances for the amidinium moiety
NCHN were detected at 9.84−10.12 and 153.9−154.5 ppm,
respectively. Crystals of the tetrafluoroborate salt 5a-(BF4)
were obtained by diffusion of ether into a THF solution and
examined by X-ray diffraction analysis. The molecular structure
of the cation is depicted in Figure 1 together with selected
geometrical data. The molecule is entirely flat and shows bond
lengths and angles that are comparable to those in related
amidinium carbene precursors.3a,8 The C1−N1 bond involving
the amide-type nitrogen N1 is significantly longer (1.321(4) Å)
RESULTS AND DISCUSSION
■
Synthesis of Anthranilic Acid-Based NHCs 6. The cyclic
amidinium salts 5 were identified as precursors for the new
carbene structure A, and they were synthesized as depicted in
Scheme 1. The basic heterocyclic scaffold quinazolin-4-one 3
was assembled by the condensation of anthranilic acid 1 and
Received: November 28, 2012
Published: January 22, 2013
© 2013 American Chemical Society
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Scheme 1. Preparation of Quinazoline-4-ones and Cationic Carbene Precursors
The amino−amido carbenes 6 were then successfully
generated in situ by deprotonation of the amidinium salts 5
with NaHMDS in THF at −80 °C and trapped by elemental
sulfur or selenium, resulting in the formation of the heteroureas
7 and 8 (Scheme 2). When 6a was generated in the absence of
a scavenging reagent, the corresponding olefin 9a arising from
carbene dimerization was isolated in 71% yield after workup as
a mixture of cis and trans isomers (ca. 1:1 ratio), as evident
1
from two signal sets in the H and 13C NMR spectra.
After the successful trapping of the in situ generated carbene,
the preparation of metal complexes was studied. NHC
precursors 5a−c were deprotonated with KOtBu in THF at
−80 °C in the presence of [(COD)MCl]2 (M = Rh, Ir),
resulting in the metal complexes 10a−c and 11a−c (Scheme
2).
All (COD)metal complexes were isolated in high yield after
column chromatography and were fully characterized. For the
(COD)Rh complexes 10 doublet resonances were observed in
the 13C NMR spectra for the carbene C atoms as a result of a
1JC−Rh coupling of ca. 50 Hz. Suitable crystals of complexes 10a
Figure 1. Structure of the cation in the crystal of compound 5a-(BF4).
Bond lengths [Å] and angles [deg]: O1−C2 1.210(4), N1−C1
1.321(4), N1−C2 1.408(5), N2−C1 1.297(4), N2−C4 1.409(4), C2−
C3 1.441(5), C3−C4 1.381(5); C1−N1−C2 121.2(3), C1−N2−C4
120.0(3), N2−C1−N1 124.4(3), N1−C2−C3, 115.3(3), C4−C3−C2
120.8(3), C3−C4−N2 118.3(3).
than the C1−N2 bond to the amine-type nitrogen N2
(1.297(4) Å).
Scheme 2. Reactions of Carbenes 6
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and 10c were examined by X-ray diffraction (Figures 2 and 3).
10c crystallizes in the monoclinic space group P21/c with two
containing a ligand of type 18 (vide infra).6 Within the six-
membered heterocycle of the NHC ligand, the C(carbene)−N
bond lengths show comparable differences, as described above
for the precursor molecule 5a: the longer C1−N1(amide) bond
of 1.382(5) Å reflects the reduced interaction of this amide-
type nitrogen with the carbene center compared to the amine-
type N atom with a C1−N2 distance of 1.341(4) Å. In both
carbene complexes the C1−N bonds are significantly elongated
by ca. 5 pm compared to the amidinium precursor, while the
N−C−N angle decreases by 8°.
The Tolman electronic parameter (TEP) is a commonly
applied measure of the donor properties of NHC ligands,1c and
it can be easily calculated from the CO stretching vibrations of
suitable NHC−metal carbonyl complexes.2e,11 Therefore, in
order to classify the new carbenes 6, the dicarbonyl complexes
12 and 13 were prepared by passing a slow stream of CO
through dichloromethane solutions of the (COD)metal
complexes 10 and 11 (Scheme 2). Complexes 12 and 13
were characterized in solution by NMR, mass spectrometry,
and IR. The stretching vibrations of the carbonyl ligands are
listed in Table 1 together with the TEP values derived thereof.
Figure 2. Molecular structure of compound 10a in the solid state. H
atoms are omitted for clarity. Displacement factors are shown at the
30% probability level. Selected bond lengths [Å] and angles [deg]:
Rh1−C1 2.030(4), Rh1−C11 2.111(4), Rh1−C12 2.128(3), Rh1−
C15 2.190(3), Rh1−C16 2.228(3), Rh1−Cl1 2.3710(10), O1−C2
1.224(4), N1−C2 1.397(5), N1−C1 1.382(5), N2−C1 1.341(4), N2−
C4 1.408(4), C2−C3 1.447(5), C3−C4 1.394(5); C1−Rh1−Cl1
87.37(10), N2−C1−N1 116.3(3).
Table 1. CO Streching Vibrations (CH2Cl2 solution) and δ
13C of the Carbene C Atom for Carbonyl Complexes 12, 13,
23, and 24
complex
ligand
ν(CO) (cm−1
)
δ
13C (ppm)
TEP (cm−1
)
12a
12b
12c
13a
13b
13c
23a
23b
23c
24a
24b
24c
6a
2089, 2010
2088, 2010
2089, 2012
2075, 1994
2075, 1993
2075, 1995
2075, 2003
2075, 2002
2082, 2005
2062, 1986
2071, 1989
2070, 1989
219
219
213
210
210
213
198
214
208
201
218
208
2060
2059
2061
2060
2060
2061
2051
2051
2055
2051
2056
2056
6b
6c
6a
6b
6c
18a
18b
18c
18a
18b
18c
Figure 3. Molecular structure of compound 10c in the solid state.
Only one of two independent molecules in the asymmetric unit is
shown. H atoms are omitted for clarity. Displacement factors are
shown at the 30% probability level. Selected bond lengths [Å] and
angles [deg]: Rh1−C1 2.026(4), Rh1−C11 2.131(2), Rh1−C12
2.125(2), Rh1−C15 2.221(2), Rh1−C16 2.218(2), Rh1−Cl1
2.3780(6), O1−C2 1.216(3), N1−C2 1.404(3), N1−C1 1.375(3),
N2−C1 1.345(3), N2−C4 1.402(3), C2−C3 1.452(3), C3−C4
1.392(3); C1−Rh1−Cl1 87.27(6), N2−C1−N1 116.1(2).
Synthesis of Diamino NHC Derivatives 18. In order to
assess the influence of the keto group attached to the scaffold of
NHC 6, the synthesis of related diamino NHCs 18 was
targeted. The appropriate amidinium salt precursors 16 are
straightforwardly available from 2-aminobenzylamine 14, which
was cyclized by treatment with triethylorthoformate at elevated
temperature, affording the dihydrochinazoline 15 in 61%
yield.12 Reaction of 15 in acetonitrile with alkylating reagents
(MeI, EtI, BzBr) in the presence of KOtBu provided the
desired amidinium salts 16 as colorless crystalline solids in
excellent yield (Scheme 3), which were fully characterized by
NMR spectroscopy, mass spectrometry (ESI-MS), and
elemental analysis. Zhang and Shi reported recently the
syntheses of several dihydrochinazolinium salts via imine
formation from aromatic aldehydes.6 Their approach allows
the preparation of derivatives with different substituents
attached to the nitrogen atoms including aryl groups. However,
sophisticated starting materials and/or Pd-catalyzed coupling
reactions are required. Notably, the protocol described here
provides the dihydrochinazolinium salts in two steps
commencing from cheap convenient starting materials.
independent molecules in the asymmetric unit, which show
identical geometrical parameters within experimental uncer-
tainty. In both structures, the plane of the essentially flat
carbene ligand is in a perpendicular arrangement to the
coordination plane of the rhodium atom. Bond lengths and
angles are not significantly affected by the exchange of a methyl
(10a) for a benzyl group (10c); they coincide for both
structures within the 3σ range. In complex 10a, the Rh1−C1
bond of 2.030(4) Å and the Rh−C distances to the olefinic
carbon atoms of the COD ligand cis (ca. 2.12 Å) and trans (ca.
2.20 Å) to the carbene are within the range usually observed for
complexes of this type.1b,c,4g,h,9 More specifically, the Rh−C
distance is identical to that for the complex with the 1,3-
dimethylimidazol-2-ylidene ligand,10 while it is 3 pm shorter
than that reported by Zhang and Shi for a related complex
Characteristic downfield resonances for the C2−H fragment
were observed in the H (8.73−9.87 ppm) and 13C NMR
1
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Scheme 3. Preparation of Amidinium Salts 16 and 17
Scheme 4. Reactions of Carbenes 18
1
spectra (152−153 ppm) of the cations 16a−c. Anion exchange
doublets in the range from 208 to 215 ppm due to a JC−Rh
−
from halide to PF6 did not significantly increase the solubility
coupling of 42−46 Hz, while singlet resonances were obtained
in the case of the iridium complexes 22 between 203 and 206
ppm. All complexes were also identified in the ESI mass
spectra, where a base peak corresponding to the [M − X]
fragment was present.
of the cationic carbene precursors in organic solvents. Both
hexafluorophosphates 17 and halide derivatives 16 were used
for subsequent reactions.
Treatment of the amidinium salts 16a−c with NaHMDS in
THF at −80 °C led to the formation of the corresponding
carbenes 18a−c (Scheme 4), which were trapped by addition of
S8 or selenium to give the corresponding heteroureas 19 and 20
in good yield after standard workup. In the absence of trapping
reagents, formation of the carbenes 18 was evident from the
absence of the characteristic downfield signal for the C2H
The CO stretching vibrations recorded for the carbonyl
complexes 23 and 24 are listed in Table 1 together with the
corresponding TEP values calculated from these data. For the
amino−amido NHCs 6, the TEP values for the Rh (12) and Ir
(13) bound ligands are identical and the influence of the one N
substituent (a, b, c) is within a small range of 2 cm−1. A mean
TEP value of 2060 cm−1 is typical for mixed amino−amido
NHCs.3m,4 For the diamino NHCs 18 an average TEP of 2054
cm−1 is calculated, reflecting the increased donor capacity of
this NHC type. However, much more scattered individual
values are seen: for the same N-substituents, the TEP values
derived for Rh (23) and Ir (24) complexes differ by 3 cm−1,
and a difference of 5 cm−1 is noted when comparing N-Me and
N-Et derivatives 24a and 24b, respectively. While it is
comprehensible that the variation of substituents at both N
atoms of the heterocycleas is the case for carbenes 18has a
more pronounced effect on the donor properties of the carbene
than the permutation of only one N-substituent as in the case
of NHCs 6, a difference of 5 wavenumbers for the Me and Et
NHCs 18a and 18b, respectively, is certainly more than one
would expect for such a structural variation. Care should thus
be taken in order not to overestimate TEP values, especially if
they are obtained under nonidentical conditions.1c Interest-
ingly, Shi and co-workers reported IR data for complexes of
NHCs similar to 18 bearing different substituents on the N
atoms (e.g., iPr and Bz), which were recorded as KBr disks.6
Conversion of their CO stretching vibrations leads to TEP
1
proton of the precursor in the H NMR spectra recorded at
−80 °C. However, the free carbenes 18 turned out to be
extremely moisture sensitive and were hydrolyzed much faster
than a 13C NMR spectrum could be recorded. The resulting
ring-opened N(o-aminobenzyl)formamide was identified by
NMR spectra and MALDI mass spectra. No indication of a
dimerization product was observed. NHC metal complexes
were then obtained as outlined above by reaction of the free
carbeneobtained in situ by precursor deprotonation with
KOtBu at −80 °Cwith [(COD)MCl]2 (M = Rh, Ir) in THF,
affording the COD derivatives [(COD)M(18)Cl] 21 and 22,
which were subsequently converted to the carbonyl complexes
23 and 24 in good yield (Scheme 4).
All COD complexes were purified by chromatography on
silica and completely characterized by NMR and mass
spectrometry because halide exchange at the metal center
from chloride to iodide or bromide (which were present from
the starting materials 16) prevented correct elemental analyses.
The protons of the CH2 group of the carbene ligand in
complexes 21 and 22 appeared to be diastereotopic, giving rise
1
to an AB pattern in the H NMR spectra. In the 13C NMR
spectra of the Rh complexes 21 the carbene carbon appeared as
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values around 2044 cm−1, well apart from our values, which
were recorded in dichloromethane solution.
white solid. Single crystals of 5a were grown by slow evaporation from
acetonitrile/n-hexane. 1H NMR (200 MHz, DMSO-d6): δ 9.84 (s, 1H,
NCHN), 8.38−8.34 (m, 1H, Ar−CH), 8.22−8.15 (m, 1H, Ar−CH),
8.06−8.01 (m, 1H, Ar−CH), 7.92−7.85 (m, 1H, Ar−CH), 4.06 (s,
3H, CH3), 3.65 (s, 3H, CH3). 13C NMR (126 MHz, DMSO-d6): δ
158.1 (s, CO), 154.1 (s, NCHN), 138.0, 136.7, 129.9, 127.7, 119.3,
118.4 (all aromatic C), 40.1 (s, CH3), 35.8 (s, CH3). MS (MALDI):
m/z 176 [M − BF4]+. Anal. Calcd (%) for C10H11BF4N2O: C, 45.84;
H, 4.23; N, 10.69. Found: C, 45.81; H, 4.34; N, 10.58.
Synthesis of Compound 5d. A 100 mL Schlenk flask was
charged with 4a (2.19 g, 13.7 mmol, 1.0 equiv) in 30 mL of diethyl
ether. After stirring for 15 min at room temperature, methyltriflate (2.0
mL, 17.79 mmol, 1.3 equiv) was added dropwise and the mixture was
stirred overnight. A white-yellow solid precipitated and was filtered,
washed with small portions of acetonitrile and diethyl ether, and
subsequently dried in vacuo to yield 3.63 g (82%) of 5d as a white
solid. 1H NMR (200 MHz, DMSO-d6): δ 9.88 (s, 1H, NCHN), 8.40−
7.38 (m 4H, Ar−CH), 4.09 (s, 3H CH3), 3.66 (s, 3H, CH3). 13C NMR
(126 MHz, DMSO-d6): δ 169.7 (CO), 160.8 (NCHN), 138.4,
137.2, 129.3, 128.1 (all aromatic C), 122.3 (q, CF3), 119.8, 118.9 (all
aromatic C), 40.1 (s, CH3), 35.8 (s, CH3). MS (MALDI): m/z 175 [M
− CF3SO3]+. Anal. Calcd (%) for C11H11F3N2O4S·H2O: C, 39.64; H,
3.63; N, 8.41. Found: C, 39.56; H, 4.00; N, 8.47.
Synthesis of Compound 7a. A suspension of 5a (150 mg, 0.56
mmol, 1.0 equiv) and S8 (220 mg, 0.84 mmol, 1.5 equiv) in 20 mL of
THF was cooled to −80 °C, and NaHMDS (2 M in THF, 0.23 mL,
1.13 mmol, 2.0 equiv) was added dropwise. The resulting orange
solution was stirred for 10 min at −80 °C, the cooling bath was
removed, and the solution was allowed to warm to room temperature
within 2.5 h. After evaporation of all volatiles, the crude product was
dissolved in 20 mL of dichloromethane and filtered through a short
pad of Celite. The filtrate was evaporated, and the yellow solid was
purified by flash chromatography on aluminum oxide with dichloro-
methane as mobile phase. All volatiles were evaporated in vacuo, to
yield 7a as a white solid (57.7 mg, 50%). 1H NMR (200 MHz,
CDCl3): δ 8.30−8.25 (m 1H, Ar−CH), 7.78−7.70 (m, 1H, Ar−CH),
7.40−7.32 (m, 2H, Ar−CH), 4.17 (s, 3H CH3), 3.91 (s, 3H, CH3).
13C NMR (126 MHz, CDCl3): δ 178.3 (s, CS), 159.9 (s, CO),
141.2, 135.8, 129.4, 125.1, 117.5, 115.4 (all aromatic C), 39.4 (s, CH3),
36.4 (s, CH3). MS (EI): m/z 206 [M]+. Spectral data were consistent
with literature values.
Synthesis of Compound 8a. A suspension of 5a (300 mg, 1.15
mmol, 1.0 equiv) and Se (132.2 mg, 1.72 mmol, 1.5 equiv) in 20 mL
of THF was cooled to −80 °C, and NaHMDS (2 M in THF, 0.35 mL,
1.72 mmol, 1.5 equiv) was added dropwise. The resulting gray solution
was stirred for 10 min at −80 °C, the cooling bath was removed, and
the solution was allowed to warm to room temperature within 2.5 h.
After evaporation of all volatiles, the crude product was dissolved in 10
mL of CH2Cl2 and filtered through a short pad of Celite. The filtrate
was treated with 20 mL of n-hexane and subsequently dried in vacuo
to yield 174.5 mg (65%) of 8a as a white powder. 1H NMR (200 MHz,
CDCl3): δ 8.34−8.29 (m 1H, Ar−CH), 7.84−7.75 (m, 1H, Ar−CH),
7.50−7.42 (m, 2H, Ar−CH), 4.38 (s, 3H CH3), 4.09 (s, 3H, CH3).
13C NMR (126 MHz, CDCl3): δ 181.2 (s, CSe), 159.0 (s, CO),
141.2, 136.0, 129.5, 126.7, 117.8, 115.9 (all aromatic C), 43.5 (s, CH3),
40.1 (s, CH3). GC/MS (EI): m/z = 254 [M]+. Anal. Calcd (%) for
C10H10N2OSe: C, 47.44; H, 3.98; N, 11.07. Found: C, 48.06; H, 4.36;
N, 10.84.
CONCLUSION
■
Two six-ring NHC systems with benzo anellation are easily
accessible from convenient starting materials in high yield.
Assessment of the electronic properties of the new ligands by
means of IR spectroscopy reveals a higher donor capacity for
the diamino carbenes 18 compared to the mixed amino−amido
NHCs of type 6. Further studies concerning the reactivity and
potential applications especially of the latter type are currently
under way.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed with
standard Schlenk techniques in an oxygen-free dry nitrogen
atmosphere. Glassware was dried at 120 °C in an oven for at least
12 h. Solvents were dried and distilled under nitrogen by using
standard procedures. Diethyl ether and THF were distilled over
sodium/benzophenone, dichloromethane over CaH2, and n-hexane
over sodium. NMR spectra were recorded on a Bruker Avance DRX
200 and a Bruker Avance DRX 500 spectrometer. H and 13C{1H}
1
spectra are referenced to the residual solvent signal. Mass spectra were
recorded on a Thermo Finnigan Trace DSQ 7000 (EI), Ion Trap-API
mass spectrometer, and Bruker Ultraflex I TOF (MALDI). Elemental
analyses were recorded on a Perkin CHN 2400 series II. IR spectra
were obtained with a Shimadzu IR Affinity-1 spectrometer. Reagents
such as potassium tert-butoxide and NaHMDS (2 M in THF) were
purchased from Acros Organics and Sigma Aldrich and used as
received. [RhCl(COD)]2 and [IrCl(COD)]2 were synthesized
according to a literature procedure.13
Synthesis of Compound 3. A 50 mL Schlenk flask was charged
with anthranilic acid (8.2 g, 60 mmol, 1.0 equiv) and formamide (10.0
mL, 250 mmol, 4.2 equiv). The mixture was stirred for 3 h at 150 °C.
After 2 h a white precipitate formed. The solid was filtered off, washed
with 20 mL of diethyl ether and 20 mL of n-hexane, and subsequently
dried in vacuo to yield 5.70 g (65%) of 3 as a white powder. 1H NMR
(200 MHz, DMSO-d6): δ 10.36 (s, 1H, NH), 8.14 (m, 1H, Ar−CH),
8.08 (s, 1H, NCHN), 7.78 (m, 1H, Ar−CH), 7.67 (m, 1H, Ar−CH),
7.49 (m, 1H, Ar−CH). 13C NMR (126 MHz, DMSO-d6): δ 161.1 (s,
CO), 149.1 (aromatic C), 145.8 (s, NCHN), 135.0, 128.0, 127.1,
126.2, 123.0 (all aromatic C). MS (EI): m/z 146 [M]+. Anal. Calcd
(%) for C8H6N2O: C, 65.75; H, 4.14; N, 19.17. Found: C, 65.48; H,
4.38; N, 19.34.
Synthesis of Compound 4a. A suspension of 3 (1.00 g, 6.94
mmol, 1.0 equiv) and KOH (1.17 g, 20.8 mmol, 3 equiv) in 60 mL of
acetonitrile was stirred for 15 min at room temperature, and
iodomethane (0.48 mL, 7.6 mmol, 1.1 equiv) was added dropwise.
The resulting suspension was stirred for 4 h at 65 °C. After removing
the solvent in vacuo, the crude product was suspended in 20 mL of
H2O and 20 mL of CH2Cl2. The organic phase was washed three times
with 20 mL of H2O and dried over sodium sulfate, the combined
organic phases were evaporated in vacuo, and 4a was obtained as a
1
bright yellow solid (720 mg, 65%). H NMR (200 MHz, CDCl3): δ
8.36−8.31 (m, 1H, Ar−CH), 8.08 (s, 1H, NCHN), 7.83−7.69 (m, 2H,
Ar−CH), 7.57−7.49 (m, 1H, Ar−CH), 3.62 (s, 3H, CH3). 13C NMR
(126 MHz, CDCl3): δ 160.6 (s, CO), 147.3 (s, NCHN), 145.8,
133.2, 126. 4, 126.3, 125.5, 121.0 (all aromatic C), 33.1 (s, CH3). MS
(EI): m/z 160 [M]+. Spectral data were consistent with literature
values.
Synthesis of Compound 5a. A mixture of 4a (580 mg, 3.63
mmol, 1.0 equiv) and trimethyloxonium tetrafluoroborate (697 mg,
4.71 mmol, 1.3 equiv) in 30 mL of acetonitrile was stirred at room
temperature overnight. The solvent was removed in vacuo, and the
yellow crude product was dissolved in 5 mL of acetonitrile and treated
with 25 mL of diethyl ether. After a few minutes a white solid
precipitated and was filtered off, washed with 20 mL of diethyl ether,
and subsequently dried in vacuo to yield 780 mg (82%) of 5a as a
Synthesis of cis/trans-Olefin (9a). To a suspension of 5d (240
mg, 0.74 mmol, 1.0 equiv) at 25 °C in 20 mL of THF was added
NaHMDS (2 M in THF, 0.18 mL, 0.89 mmol, 1.2 equiv) dropwise.
After 25 min at 25 °C the clear yellow solution was dried in vacuo and
the yellow crude product was washed with 20 mL of n-hexane and 20
mL of diethyl ether to give 9 as a colorless solid (183 mg, 71%). cis-
1
Isomer: H NMR (200 MHz, CDCl3): δ 8.02 (m, 2H, Ar−H), 7.52−
7.41 (m, 2H, Ar−H), 7.09−6.90 (4H, Ar−H), 3.26 (s, 6H, CH3), 3.16
(s, 6H, CH3). 13C NMR (126 MHz, CDCl3): δ 163.8 (CO), 147.2,
134.0, 129.4, 127.0, 121.3, 119.7, 115.5, 40.0 (s, CH3), 34.6. trans-
1
Isomer: H NMR (200 MHz, CDCl3): δ 8.02 (m, 2H, Ar−CH), 7.47
(m, 2H, Ar−CH), 7.09−6.90 (m, 4H, Ar−H), 3.22 (s, 6H, CH3), 3.14
858
dx.doi.org/10.1021/om301152n | Organometallics 2013, 32, 854−861

Organometallics
Article
(s, 6H, CH3). 13C NMR (126 MHz, CDCl3): δ 164.1 (CO), 147.1,
134.0, 129.2, 127.0, 120.7, 119.21, 114.53, 39.4 (s, CH3), 34.5 (s,
CH3). MS (MALDI): m/z 348 [M]+. HRMS (ESI): [M + H+]+ calcd
for C20H21N4O2 349.41250, found 349.1659.
CO), 166.3 (s, CO), 157.9 (s, CO), 139.5 134.7, 127.9, 126.9, 117.7,
115.0 (all aromatic C), 44.0 (s, CH3), 39.8 (s, CH3). IR (CH2Cl2): ν
(CH2Cl2) 2075 (CO), 1994 (CO), 1701 (NCON) cm−1.
Synthesis of Compound 16a. A 100 mL Schlenk flask was
charged with 15 (1.69 g, 12.9 mmol, 1.0 equiv) and KOtBu (1.59 g,
14.2 mmol, 1.1 equiv) in 60 mL of acetonitrile. Iodomethane (1.61
mL, 25.8 mmol, 2.0 equiv) was added dropwise, and the yellow
solution was stirred for 4 h at 80 °C. After removing the solvent in
vacuo, the crude product was suspended in 5 mL of acetonitrile and
treated with 25 mL of diethyl ether. Directly a white solid precipitated
and was filtered, washed with 20 mL of diethyl ether, and subsequently
Synthesis of Complex 10a. A 50 mL Schlenk flask was charged
with 5a (250 mg, 0.95 mmol, 1.0 equiv), KOtBu (117.3 mg, 1.05
mmol, 1.1 equiv), and [(Rh(COD)Cl)]2 (233.7 mg, 0.47 mmol, 0.5
equiv). The solid mixture was cooled to −80 °C. After 10 min at −80
°C 20 mL of THF was added dropwise with vigorous stirring. The
mixture was stirred for 15 min at −80 °C, the cooling bath was
removed, and the suspension was warmed to room temperature and
stirred overnight. The resulting brown solution was evaporated to
dryness in vacuo, and the crude product was purified by flash
chromatography on silica gel 60 with THF as mobile phase. All
volatiles were evaporated in vacuo to yield 10a as a yellow solid (320
mg, 80%). Single crystals of 10a were grown by slow evaporation from
CH2Cl2/diethyl ether. 1H NMR (200 MHz, CDCl3): δ 8.33−8.29 (m,
1H, Ar−CH), 7.82−7.78 (m, 1H, Ar−CH), 7.55−7.44 (m, 2H Ar−
CH), 5.19 (br s, 2H, CH COD), 4.96 (s, 3H, CH3), 4.65 (s, 3H, CH3),
3.39 (br s, 2H, CHCOD), 2.51 (m, 4H, CH2 COD), 2.05 (m, 4H,
CH2 COD). 13C NMR (126 MHz, CDCl3): δ 219.4 (d, 1JRhC = 50.3 Hz,
NCN), 158.7 (s, CO), 140.7, 135.4, 129.0, 126.9, 118.5, 115.3 (all
aromatic C), 100.4 (d, JRhC = 6.3 Hz, CHCOD), 100.1 (d, JRhC = 6.3 Hz,
CHCOD), 71.0 (d, JRhC = 13.8 Hz, CHCOD), 70.6 (d, JRhC = 13.8 Hz,
CHCOD), 44.7 (s, CH3), 40.8 (s, CH3), 33.2 (s, CH2 COD), 32.6 (s,
CH2 COD), 29.3 (s, CH2 COD), 29.0 (s, CH2 COD). MS (MALDI): m/z
420 [M]+. Anal. Calcd (%) for C18H22ClN2ORh·0.5CH2Cl2: C, 47.97;
H, 5.00; N, 6.05. Found: C, 47.48; H, 4.74; N, 6.10.
Synthesis Complex 11a. A 50 mL Schlenk flask was charged with
5a (154 mg, 0.59 mmol, 1.0 equiv), KOtBu (73 mg, 0.65 mmol, 1.1
equiv), and [(Ir(COD)Cl)]2 (197 mg, 0.29 mmol, 0.5 equiv). The
solid mixture was cooled to −80 °C. After 10 min at −80 °C 20 mL of
THF was added dropwise with vigorous stirring. The mixture was
stirred for 15 min at −80 °C, the cooling bath was removed, and the
suspension was warmed to room temperature and stirred overnight.
The solvent was removed in vacuo, and the red crude product was
purified by flash chromatography on silica gel 60 with diethyl ether/
CH2Cl2 as mobile phase. All volatiles were evaporated in vacuo to yield
11a as bright yellow solid (250 mg, 83%). 1H NMR (200 MHz,
CDCl3): δ 8.36−8.31 (m, 1H, Ar−CH), 7.83−7.79 (m, 1H, Ar−CH),
7.56−7.46 (m, 2H Ar−CH), 4.84 (br s, 2H, CHCOD), 4.76 (s, 3H,
CH3), 4.44 (s, 3H, CH3), 3.00 (br s, 2H, CHCOD), 2.31 (m, 4H,
CH2 COD), 1.87 (m, 4H, CH2 COD). 13C NMR (126 MHz, CDCl3): δ
210.7 (s, NCN), 159.8 (s, CO), 141.5, 135.5, 129.0, 126.7, 118.4,
115.4 (all aromatic C), 87.2 (s, CHCOD), 86.8 (s, CHCOD), 54.9 (s,
CHCOD), 54.5 (s, CHCOD), 44.1 (s, CH3), 40.3 (s, CH3), 33.8 (s,
CH2 COD), 33.2 (s, CH2 COD), 29.8 (s, CH2 COD), 29.5 (s, CH2 COD).
MS (MALDI): m/z 510 [M − Cl]+. Anal. Calcd (%) for
C18H22ClIrN2O: C, 42.39; H, 4.35; N, 5.49. Found: C, 42.29; H,
4.31; N, 5.28.
1
dried in vacuo to yield 3.40 g (91%) of 16a as a white powder. H
NMR (200 MHz, DMSO-d6): δ 8.73 (s, 1H, NCHN), 7.37−7.24 (m,
4H, Ar−CH), 4.83 (s, 2H, Ph−CH2), 3.46 (s, 3H, CH3), 3.24 (s, 3H,
CH3). 13C NMR (126 MHz, DMSO-d6): δ 153.1 (s, NCHN), 132.2,
129.5, 127.8, 127.1, 118.9, 115.6 (all aromatic C), 48.5 (s, Ph−CH2),
41.9 (s, CH3), 37.8 (s, CH3). MS (MALDI): m/z 162 [M − I]+.
HRMS (ESI): [M − I]+ calcd for C10H13N2 161.10732, found
161.10732.
Synthesis of Compound 17a. A solution of 16a (666.1 mg, 2.31
mmol, 1.0 equiv) in H2O (20 mL) was cooled to 0 °C. Ammonium
hexafluorophosphate (570.1 mg, 3.5 mmol, 1.5 equiv) was added in
small portions with vigorous stirring. Immediately a white solid
precipitated, and the suspension was stirred for 15 min at 0 °C. The
white solid was filtered off, washed with 10 mL of diethyl ether and 10
mL of n-hexane, and subsequently dried in vacuo to yield 694.8 mg
(98%) of 16a. 1H NMR (200 MHz, DMSO-d6): δ 8.51 (s, 1H,
NCHN), 7.46−7.20 (m, 4H, Ar−CH), 4.79 (s, 2H, PH−CH2), 3.45
(s, 3H, CH3), 3.22 (s, 3H, CH3). 31P{1H} NMR (81 MHz, DMSO-
−
d6): δ −142.95 (sept, JPF = 711.3 Hz, PF6 ). 13C NMR (126 MHz,
DMSO-d6): δ 153.2 (s, NCHN), 132.1, 129.5, 127.9, 127.1, 118.8,
115.5 (all aromatic C), 48.3 (s, Ph−CH2), 41.9 (s, CH3), 37.7 (s,
CH3). MS (MALDI): m/z 161 [M − PF6]+. Anal. Calcd (%) for
C10H13F6N2P·H2O: C, 37.05; H, 4.66; N, 8.64. Found: C, 36.68; H,
4.18; N, 8.67.
Synthesis of Compound 19a. A suspension of 16a (250 mg, 0.86
mmol, 1.0 equiv) and S8 (41.2 mg, 1.29 mmol, 1.5 equiv) in 20 mL of
THF was cooled to −80 °C, and NaHMDS (2 M in THF, 0.35 mL,
1.72 mmol, 2.0 equiv) was added dropwise. The resulting orange
solution was stirred for 10 min at −80 °C, the cooling bath was
removed, and the solution was allowed to warm to room temperature
within 2.5 h. After evaporation of all volatiles, the orange crude
product was purified by flash chromatography (Al2O3, diethyl ether
100%). All volatiles were evaporated in vacuo and yielded 19a as a
1
white solid (79.6 mg, 47%). H NMR (200 MHz, CDCl3): δ 7.38−
7.32 (m, 1H, Ar−CH), 7.14−7.11 (m, 2H, Ar−CH), 7.02−6.98 (m,
1H, Ar−CH), 4.47 (s, 2H, Ph−CH2), 3.81 (s, 3H, CH3), 3.55 (s, 3H,
CH3). 13C NMR (126 MHz, CDCl3): δ 179.72 (s, CS), 137.40,
127.59, 124.06, 122.63, 119.77, 112.89 (all aromatic C), 50.36 (s, Ph−
CH2), 42.27 (s, CH3), 36.95 (s, CH3). GC/MS (EI): m/z 192 [M]+.
Anal. Calcd (%) for C10H12N2S: C, 62.46; H, 6.29; N, 14.57; S, 16.68.
Found: C, 62.28; H, 6.22; N, 14.36; S, 16.41.
General Synthesis of Dicarbonylhalogenido-Rhodium/Iridi-
um Complexes 12a and 13a. CO was bubbled into a stirred
solution of complex 10a or 11a (100 mg) in CH2Cl2 (5 mL) for 5 min
at room temerature. All volatiles were removed in vacuo, and the
residue was washed with 5 mL of n-hexane. 12a and 13a were obtained
as solids, which were characterized by IR, NMR, and MS spectrometry.
The carbonyl complexes were too sensitive to provide correct
elemental analyses.
Synthesis of Compound 20a. A suspension of 16a (300 mg, 1.04
mmol, 1.0 equiv) and selenium (106.7 mg, 1.35 mmol, 1.3 equiv) in 20
mL of THF was cooled to −80 °C, and NaHMDS (2 M in THF, 0.32
mL, 1.56 mmol, 1.5 equiv) was added dropwise. The resulting gray
solution was stirred for 10 min at −80 °C, the cooling bath was
removed, and the solution was allowed to warm to room temperature
within 2.5 h. After evaporation of all volatiles, the crude product was
taken up in 20 mL of dichloromethane and filtered through a short pad
of Celite. The filtrate was evaporated, taken up in 5 mL of
dichloromethane, precipitated in 10 mL of n-hexane, and subsequently
Complex 12a. Yield: bright yellow solid (76.0 mg, 84%). 1H NMR
(500 MHz, CDCl3): δ 8.21−8.19 (m, 1H, Ar−CH), 7.73−7.70 (m,
1H, Ar−CH), 7.48−7.36 (m, 2H Ar−CH), 4.85 (s, 3H, CH3), 4.54 (s,
3H, CH3). 13C NMR (126 MHz, CDCl3): δ 218.0 (d, JRhC = 49.1 Hz,
NCN), 184.5 (d, JRhC = 54.4 Hz, CO), 180.8 (d, JRhC = 75.2 Hz, CO),
157.4 (s, NCO), 134.6, 127.7, 127.5, 125.5, 114.8, 113.9 (all aromatic
C), 43.3 (s, CH3), 39.4 (s, CH3). MS (ESI): m/z 368 [M − 2CO]+. IR
(CH2Cl2): ν 2089 (CO), 2010 (CO), 1693 (NCON) cm−1.
1
dried in vacuo to yield 200 mg (80%) of 6a as a white powder. H
NMR (200 MHz, CDCl3): δ 7.41−7.33 (m, 1H, Ar−CH), 7.21−7.03
(m, 3H, Ar−CH), 4.49 (s, 2H, Ph−CH2), 3.94 (s, 3H, CH3), 3.68 (s,
3H,CH3). 13C NMR (126 MHz, CDCl3): δ 180.1 (s, CSe), 136.4,
127.8, 124.2, 123.3, 113.0, 50.3 (all aromatic C), 45.6 (s, Ph−CH2),
40.4 (s, CH3), 29.9 (s, CH3). GC/MS (EI): m/z 240 [M]+. Anal.
Calcd (%) for C10H12N2Se·H2O: C, 46.60; H, 5.49; N, 10.89. Found:
C, 46.49; H, 5.18; N, 9.38.
Complex 13a. Yield: bright yellow solid (80.0 mg, 87%). 1H NMR
(500 MHz, CDCl3): δ 8.40−8.38 (m, 1H, Ar−CH), 7.91−7.89 (m,
1H, Ar−CH), 7.65−7.56 (m, 2H Ar−CH), 4.54 (s, 3H, CH3), 4.21 (s,
3H, CH3). 13C NMR (126 MHz, CDCl3): δ 198.8 (s, NCN), 178.2 (s,
859
dx.doi.org/10.1021/om301152n | Organometallics 2013, 32, 854−861

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Article
Synthesis of Complex 21a. A 50 mL Schlenk flask was charged
with 16a (290 mg, 1.01 mmol, 1.0 equiv), KOtBu (124.5 mg, 1.11
mmol, 1.1 equiv), and [(Rh(COD)Cl)]2 (245 mg, 0.5 mmol, 0.5
equiv). The solid mixture was cooled to −80 °C. After 10 min at −80
°C 20 mL of THF was added dropwise with vigorous stirring. The
mixture was stirred for 15 min at −80 °C, the cooling bath was
removed, and the suspension was warmed to room temperature and
stirred overnight. The resulting brown solution was evaporated to
dryness in vacuo, the crude product was taken up in 5 mL of
dichloromethane, the reagent was precipitated in 30 mL of n-hexane,
and the solution was dried in vacuo to yield 264 mg (52%) of 21a as a
imaging plate diffraction system (5a-[BF4]) and an Oxford Diffraction
Excalibur diffractometer, respectively, using graphite-monochromat-
ized Mo Kα radiation (λ = 0.71073 Å). Unit cell parameters were
determined by least-squares refinements on the positions of 3066, 17
838, and 9818 reflections, respectively. For 5a-[BF4], the monoclinic
crystal system and systematic absences are consistent with space group
types P21 and P21/m. In accordance with E-statistics significantly
better results were observed in the centrosymmetric type P21/m in the
course of structure refinement, taking into account a structural model
with a 1:1 disorder of the tetrafluoroborate anion. For 10a, the
monoclinic crystal system and systematic absences are consistent with
space group types Cc and C2/c. The latter proved to be the right
choice in the course of structure refinement. Corrections for Lorentz
and polarization effects and multiscan absorption corrections in the
1
green-yellow solid. H NMR (500 MHz, CDCl3): δ 7.25−6.92 (m,
4H, Ar−CH), 5.15 (br s, 2H, CH COD), 4.36 (d, 1H, J = 15.0 Hz, Ph−
CH2), 4.29 (d, 1H, J = 10.0 Hz, Ph−CH2), 4.26 (s, 3H, CH3), 3.92 (s,
3H, CH3), 3.49 (br s, 2H, CHCOD), 2.31 (m, 4H, CH2 COD), 1.85 (m,
4H, CH2 COD). 13C NMR (126 MHz, CDCl3): δ 210.3 (d, 1JRhC = 45.3
Hz, NCN), 134.1, 127.6, 124.7, 123.3, 118.7, 111.6 (all aromatic C),
94.5 (d, JRhC = 7.5 Hz, CHCOD), 94.39 (d, JRhC = 7.5 Hz, CHCOD),
71.51 (d, JRhC = 13.8 Hz, CHCOD), 71.3 (d, JRhC = 15.09 Hz, CHCOD),
49.1 (s, Ph−CH2), 45.1 (s, CH3), 40.5 (s, CH3), 31.1 (s, CH2 COD),
30.9 (s, CH2 COD), 28.5 (s, CH2 COD), 28.4 (s, CH2 COD). MS
(MALDI): m/z 498 [M − I]+. Halide exchange at the metal center
from chloride to iodide prevented correct elemental analyses.
Synthesis of Complex 22a. A 50 mL Schlenk flask was charged
with 16a (250 mg, 0.86 mmol, 1.0 equiv), KOtBu (106 mg, 0.95
mmol, 1.1 equiv), and [(Ir(COD)Cl)]2 (289 mg, 0.43 mmol, 0.5
equiv). The solid mixture was cooled to −80 °C. After 10 min at −80
°C 20 mL of THF was added dropwise with vigorous stirring. The
mixture was stirred for 15 min at −80 °C, the cooling bath was
removed, and the suspension was warmed to room temperature and
stirred overnight. The solvent was removed in vacuo, and the red
crude product was purified by flash chromatography on silica gel 60
with n-hexane/diethyl ether as mobile phase. All volatiles were
evaporated in vacuo and yielded 22a as a red solid (329 mg, 65%). 1H
NMR (200 MHz, CDCl3): δ 7.88−7.01 (m, 4H, Ar−CH), 4.78 (br s,
2H, CHCOD), 4.48 (d, 2H, J = 6.8 Hz, Ph−CH2), 4.12 (s, 3H, CH3),
3.80 (s, 3H, CH3), 3.08 (br s, 2H, CHCOD), 2.21 (m, 4H, CH2 COD),
1.83 (m, 4H CH2 COD). 13C NMR (126 MHz, CDCl3): δ 203.2 (s,
NCN), 129.3, 128.3, 126.5, 125.8, 123.3, 116.8 (all aromatic C), 79.5
(s, CHCOD), 65.4 (s, CHCOD), 59.5 (s, Ph−CH2), 36.1 (s, CH3), 35.6
(s, CH3), 31.3 (s, CH2 COD), 30.9 (s, CH2 COD), 29.8 (s, CH2 COD),
28.6 (s, CH2 COD). MS (MALDI): m/z 588 [M − I]+. Halide exchange
at the metal center from chloride to iodide prevented correct
elemental analyses.
case of 10a (Tmin = 0.402, Tmax = 0.613) and 10c (Tmin = 0.768, Tmax
=
1.000) were applied. The structures were solved by direct methods14
and subsequent ΔF syntheses. Approximate positions of all the
hydrogen atoms were found in different stages of converging
refinements by full-matrix least-squares calculations15 on F2 (max.
shift/s.u. = 0.001 in each case). Selected crystallographic data are
shown in Table S1. Anisotropic displacement parameters were refined
for all atoms heavier than hydrogen. With idealized bonds lengths and
angles assumed for all the CH, CH2, and CH3 groups, the riding model
was applied for the corresponding H atoms and their isotropic
displacement parameters were constrained to 120%, 120%, and 150%
of the equivalent isotropic displacement parameters of the parent
carbon atoms, respectively. In addition, the H atoms of the CH3
groups were allowed to rotate around the neighboring C−C bonds.
The disordered anion of 5a-[BF4] was treated as rigid groups with
idealized geometry. Appropriate anisotropic displacement restraints
had to be applied for its atoms. CCDC-912080 (5a-[BF4]), CCDC-
912081 (10a), and CCDC-912588 (10c) contain the supplementary
crystallographic data (excluding structure factors) for this paper. These
data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and analytical data for
compounds not described in the experimental section.
Crystallographic data (Table S1, CIF files) for compounds
5a-BF4, 10a, and 10c. This material is available free of charge
via the Internet at http://pubs.acs.org.
General Synthesis of Dicarbonylhalogenido-Rhodium/Iridi-
um Complexes 23a and 24a. The carbonyl complexes 23a and 24a
were prepared according to the protocol described above for the
synthesis of complexes 12a and 13a. Due to slow decomposition, no
elemental analyses could be obtained. The complexes were
characterized by IR and NMR spectroscopy.
AUTHOR INFORMATION
Corresponding Author
*E-mail: christian.ganter@uni-duesseldorf.de.
Notes
■
Complex 23a. Yield: red-orange solid (50 mg, 84%). 1H NMR (500
MHz, CDCl3): δ 7.33−7.02 (m, 4H, Ar−CH), 4.52 (q, 2H, J = 14.8
Hz, PH−CH2), 3.86 (s, 3H, CH3), 3.56 (s, 3H, CH3). 13C NMR (126
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
MHz, CDCl3): δ 198.2 (d, JRhC = 37.7 Hz, NCN), 186.6 (d, JRhC
=
■
52.8 Hz, CO), 180.3 (d, JRhC = 79.2 Hz, CO), 161.8 (s, CO), 133.7,
128.0, 124.9, 124.4, 118.3, 112.3 (all aromatic C), 49.4 (s, Ph−CH2),
45.4 (s, CH3), 40.9 (s, CH3). IR (CH2Cl2): ν 2075 (CO), 2003 cm−1
(CO).
We thank Dr. Guido Reiß for help with the structure
determination of complex 10c and Maximilian Klopotowski
for valuable assistance with the experimental work.
1
Complex 24a. Yield: dark red solid (60 mg, 79%). H NMR (500
REFERENCES
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CDCl3): δ 200.8 (s, NCN), 179.7 (s, CO), 179.6 (s, CO), 166.3 (s,
NCO), 129.3, 128.3, 126.5, 125.8, 124.9, 116.8 (all aromatic C), 49.9
(s, Ph−CH2), 45.0 (s, CH3), 40.5 (s, CH3). IR (CH2Cl2): ν 2062
(CO), 1986 cm−1 (CO).
■
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dx.doi.org/10.1021/om301152n | Organometallics 2013, 32, 854−861