Reactivity of an oxalamide-based N-heterocyclic carbene

Article abstract of DOI:10.1021/om201235c

The reactivity of the oxalamide-based carbene 2 was investigated. Treatment with styrene or methylacrylate proceeded under carbene addition to yield the respective cyclopropanation products 6a,b. With elemental selenium the selenide 7 was obtained, which was characterized by X-ray diffraction. The Rh complex [(COD)RhCl(2)] (10) was also structurally characterized. From the IR spectrum of the dicarbonyl derivative [(CO)₂RhCl(2)] (11) a Tolman electronic parameter of 2068 cm^-1 was calculated for the diamidocarbene 2, which characterizes this ligand as the least electron donating NHC known so far. The dimerization product of carbene 2, the intensly fluorescing tetraamido ethylene 3, showed an unexpected chemical stability. Treatment of the chloro compound 1a with methanol afforded the methoxide 5a, whose structure was determined by X-ray diffraction.

Full text of DOI:10.1021/om201235c

Article
pubs.acs.org/Organometallics
Reactivity of an Oxalamide-Based N-Heterocyclic Carbene
Markus Braun, 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 reactivity of the oxalamide-based carbene
2 was investigated. Treatment with styrene or methylacrylate
proceeded under carbene addition to yield the respective
cyclopropanation products 6a,b. With elemental selenium
the selenide 7 was obtained, which was characterized by X-ray
diffraction. The Rh complex [(COD)RhCl(2)] (10) was also
structurally characterized. From the IR spectrum of the di-
carbonyl derivative [(CO)2RhCl(2)] (11) a Tolman electronic parameter of 2068 cm−1 was calculated for the diamidocarbene 2,
which characterizes this ligand as the least electron donating NHC known so far. The dimerization product of carbene 2, the
intensly fluorescing tetraamido ethylene 3, showed an unexpected chemical stability. Treatment of the chloro compound 1a with
methanol afforded the methoxide 5a, whose structure was determined by X-ray diffraction.
centered at the carbene carbon atom, which in turn should enforce
stabilization through the metal center (Chart 1).
INTRODUCTION
■
Since their isolation by Arduengo in 1991,1 N-heterocyclic
carbenes (NHCs) have found widespread and spectacular use
as ligands in transition-metal catalysis2 and as organocatalysts.3
Apart from having widespread applications, NHCs are fascinat-
ing molecules in their own right and continue to stimulate various
aspects of research. Today, their properties and reactivity are well
investigated, and it is known that they are strong σ-donors and
act as Lewis bases like phosphines. These concepts have been
delineated in a number of excellent reviews.4 However, in
contrast to the strong σ-donor properties, potential π-acceptor
contributions exerted by NHCs have been neglected for a long
time. Meanwhile, theoretical5 as well as experimental work indi-
cates clearly that the π-acceptor character of NHCs can signif-
icantly contribute to the metal−NHC bonding interaction.
Attempts to enforce π-back-bonding interactions included the
attachment of electron-withdrawing groups to either the N-aryl
groups or the heterocyclic carbene skeleton itself.6 In a previous
communication we reported the synthesis of the diamidocar-
bene 2, which was obtained straightforwardly from the neutral
oxalamide precursor 1a (Scheme 1).6e
Chart 1. Effect of the Amide Resonance on Carbene
Stabilization
́
The groups of Cesar and Glorius reported the formation of
a five-membered monoamidocarbene (MAC) independently,7
followed by the reports of anionic and neutral six-membered
diamidocarbenes (DACs) by Ces
membered DACs by the group of Bielawski.9 Most recently,
Cesar described a six-membered diamidocarbene based on 4,5-
́
ar8 and neutral six- and seven-
́
dioxo-1,3,5-triazine and its behavior as an ambidentate Janus-
type ligand system.10
In contrast to the analogous six- and seven-membered DACs,
the five-membered derivative 2 is not stable as a monomeric
species but dimerizes to the corresponding tetraamidoethylene
3 as a result of its small HOMO−LUMO gap11 (Scheme 1).
In this report we present the results of a comprehensive inves-
tigation of the reactivity of the oxalamide-based NHC 2 and the
tetraamidoethylene 3.
Scheme 1. Preparation and Dimerization of
Diamidocarbene 2
RESULTS AND DISCUSSION
■
On the basis of the observation that the carbene 2 is not stable
as a monomeric species, we tried to use more bulky groups attached
to the nitrogen atoms to protect the carbene center and avoid
the dimerization reaction. Like the mesityl compound 1a,12 the
NHC 2 was supposed to be a poorer electron donor com-
pared to diaminocarbenes due to the amide resonance struc-
tures, which lead to a reduced stabilization of the empty pπ-orbital
Received: December 13, 2011
Published: February 9, 2012
© 2012 American Chemical Society
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precursors 1b and 1c with sterically demanding Dipp and
1-adamantyl groups were obtained from the respective form-
amidines 4 and oxalylic chloride in good yield (Scheme 2). In
Scheme 2. Syntheses and Reactivity of the Carbene
Precursor
Figure 1. ORTEP representation of 5a (ellipsoids drawn at the 40%
probability level). Only one orientation of the disordered CHOMe
group is shown. Selected distances (pm) and angles (deg): C1−O2
127.2(3), C12−O2 143.7(6), C1−N1 145.0(2), C2−O1 120.9(2),
C2−C2A 153.3(4), C1−O2−C12 118.0(5), N1−C1−N1A 102.7(2),
C1−N1−C2 112.78(16), O1−C2−C2A 126.37(12).
the case of compound 4c the addition of 1 equiv of triethylamine
was necessary to capture the emerging hydrogen chloride. In the
absence of triethylamine the N,N′-diadamantylformamidine 4c
will be protonated and the conversion to 1c is limited to
50%. Unfortunately, the chloro compounds 1b,c could not be
deprotonated by NaHDMS, KOtBu, LDA, or nBuLi to give the
anticipated diamidocarbenes. For the 1-adamantyl derivative,
addition of the base to a solution of 1c in THF at −78 °C
immediately initiated the formation of a white precipitate,
which was insoluble in all common organic solvents including
DMSO. No meaningful peaks could be detected in the EI-MS
spectrum. When the Dipp derivative 1b was subjected to
deprotonation, a yellowish solid could be isolated after workup,
which was identified to be neither the free NHC nor the dimer
or the product of insertion of the intermediate carbene into the
C−H unit of a Dipp-iPr group, as was observed for a malonic
diamido NHC by Bielawski and co-workers.9a Interestingly, the
solubility of the precursors 1b and 1c was quite different. Like
the mesityl compound 1a, the Dipp derivative 1b was soluble in
all common solvents, while the 1-adamantyl compound 1c was
soluble only in THF. When methanol was added to a suspen-
sion of 1c in chloroform, a colorless solution formed imme-
diately due to the formation of the methoxy product 5c, which
in turn showed good solubility in common organic solvents.
Unfortunately, we were not able to obtain a correct elemental
analysis for this compound. The preparation of the related
methoxide derivative with Dipp substituents attached to the N
atoms has not been attempted. However, the analogous mesityl
derivative 5a, whose synthesis has been described before,6e
could be crystallized from dichloromethane and hexane to give
crystals suitable for an X-ray diffraction analysis. The molecular
structure of the methoxide 5a is shown in Figure 1 together
with selected geometrical data. The structure is affected by a
disorder of the molecule, which appears to be located on a
crystallographic 2-fold axis, leading to disorder of the CHOMe
unit (only one orientation is shown in Figure 1).
with [(COD)RhCl]2 or with CS2 to yield the corresponding
rhodium-carbene complex or the NHC-dithiocarboxylate, re-
spectively. Such reactions have been reported for electron-rich
enetetramines by Lappert, Delaude, and Regitz.15 Surprisingly,
olefin 3 withstands the presence of 3-chloroperbenzoic acid
or potassium permanganate without reaction. In each case the
olefin 3 is recovered in almost quantitative yield after workup.
The group of Beckert reported a photooxygenation reaction of
1,4,5,8-tetraazafulvalenes with singlet oxygen, leading to rupture
of the C−C double bond and concomitant formation of the
corresponding imidazolidine-2-one. In the case of 3, however,
no reaction with singlet oxygen was observed leading to N,N′-
dimesitylimidazolidine-2,4,5-trione.16
An interesting feature of compound 3 is its strong yellow
fluorescence. Figure 2 shows the UV−vis and fluorescence
Figure 2. UV−vis and fluorescence spectra of 3 (6.1 × 10−2 mg/mL)
in CH2Cl2 at room temperature.
spectra recorded in dichloromethane at room temperature. The
absorption is characterized by a strong band at 398 nm with an
extinction coefficient of ε = 27 000 L/mol cm, whereas a Stokes
shift of 75 nm (3984 cm−1) is observed for the emission peak
at 473 nm. Compared to a 1,4,5,8-tetraazafulvalene,17 whose
absorption was detected at 330 nm with an extinction of ε =
8000 L/mol cm, the absorption of the tetraamidoethylene 3
is significantly shifted into the visible region and is also much
more intense. Bielawski and co-workers reported a series of
dibenzotetraazafulvalenes which showed absorption bands be-
tween 400 and 464 nm with extinctions ranging from 8000 to
10 000 L/mol cm.18
The observed bond lengths and angles are close to the values
found for the derivative bearing 2,6-dimethylphenyl groups
instead of mesityl5d and 2-methoxy-2-methylimidazolidine-4,5-
dione13 with trigonal-planar N atoms, shorter OC−N bonds,
and longer N−CHOMe bonds as a consequence of the amide
resonance. Accordingly, in the saturated derivative 2-methoxy-
N,N′-dimesitylimidazolidine14 the N−CHOMe bonds are shorter
(average: 143.5 pm) than the N−CH2 bonds (average: 146.3 pm)
and the geometry around the N atoms is pyramidal.
We also investigated the reactivity of the air- and moisture-
stable olefin 3, which results from the dimerization of carbene 2
in the absence of trapping reagents. Olefin 3 does not react
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Figure 3. CV of 3 obtained in dry CH2Cl2 with 0.1 M nBu4NPF6 electrolyte using a 100 mV s−1 scan rate and referenced to Fc+/Fc.
In addition, the redox properties of olefin 3 were evaluated
by cyclic voltammetry (Figure 3). The CV shows a reversible
redox step at a fairly low potential of E1/2 = −1325 mV, attrib-
uted to the reduction of the carbonyl moiety of 3 in analogy
with the findings reported by Bielawski and co-workers for a
quinone annelated carbene system.6b No oxidation wave was
detected up to 1000 mV.
The reactivity of carbene 2 toward electron-rich as well as
electron-poor olefinic double bonds was also evaluated. Thus,
the carbene was prepared in situ and treated with styrene and
methyl acrylate, respectively, leading to the formation of the
cyclopropanes 6a,b in moderate to good yield (Scheme 3).
crystals were obtained, which were identified by X-ray diffrac-
tion to consist of N,N′-dimesityloxaldiamide, whose structure
was already reported by Połon
ski and co-workers in 2007.22
The formation of this compound is unclear. Remarkably, a solu-
tion of 6a in CDCl3 in the presence of water showed no de-
gradation over a period of several days in the 1H NMR
spectrum.
Similar to the formation of a thione, carbene 2 could also be
trapped by reaction with elemental selenium in good yield.
Dark red crystals suitable for an X-ray diffraction analysis
were grown by slow diffusion of hexane into a solution of 7 in
dichloromethane. The molecular structure of selenide 7 is
shown in Figure 4 together with some representative geometric
parameters.
́
Scheme 3. Reactivity toward Several Substituted Olefins and
Trapping with Selenium or Isonitriles
Figure 4. ORTEP representation of 7 (ellipsoids drawn at 30% prob-
ability; H atoms are omitted for clarity). Selected distances (pm) and
angles (deg): C1−Se1 177.7(4), C1−N1 137.6(3), N1−C2 137.4(4),
C2−O1 119.2(3), C2−C2A 153.6(6), Se1−C1−N1 125.98(15), C1−
N1−C2 111.4(2), N1−C2−O1 127.9(3), O1−C2−C2A 127.55(18).
Compared to the large number of structurally characterized
carbene sulfur adducts, only a couple of related selenium com-
pounds have been described.23 Interestingly, among these derivatives,
the closely related N,N′-diethylimidazolidine-2-selenone-4,5-dione24
features almost identical geometric parameters.
The reaction of 2 with tert-butyl isonitrile leading to the
formation of keteneimine 8 has been described in our previous
report.6e While various attempts to obtain X-ray quality crystals
of ketenimine 8 failed, a degradation product (9) was identified
by X-ray diffraction in one case (Figure 5). Formally, the ether
9 is derived by addition of the two O−H bonds of a water
molecule to the carbene C atoms of two molecules of 2. How-
ever, when in situ generated DAC 2 was reacted with water, the
formation of 9 was not observed. Instead, olefin 3 was isolated
under these conditions in moderate yield as the only identifiable
Cyclopropanation reactions with electron-poor acrylates have
already been reported for (phosphanyl)(silyl)carbenes,19 (alkyl)-
(amino)carbenes,20 and a ferrocene-derived diaminocarbene.21
Interestingly, the electron-poor carbene 2 reacted preferentially
with the electron-poor methyl acrylate, affording the product in
higher yield and without formation of dimer 3 as a side product,
whereas carbene dimerization competed significantly with the
cyclopropanation in the case of styrene. Olefin 3 could not be
removed completely by column chromatography and thus pre-
vented the isolation of 6b in analytically pure form. When 6a
was recrystallized from dichloromethane and hexane, colorless
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Figure 6. ORTEP representation of 10 (ellipsoids drawn at the 30%
probability level; H atoms are omitted for clarity). Selected distances
(pm) and angles (deg): Rh1−Cl1 236.93(9), Rh1−C1 194.2(3),
Rh1−C22 213.3(3), Rh1−C23 214.7(3), Rh1−C26 225.5(3), Rh1−
C27 228.6(3), N1−C1−N2 105.9(2), C1−Rh1−Cl1 94.69(9).
Figure 5. ORTEP representation of 9 (ellipsoids drawn at 30% prob-
ability; H atoms are omitted for clarity). Selected distances (pm) and
angles (deg): C1−O5 142.2(4), C22−O5 141.6(4), C1−N2 145.5(4),
C3−O2 120.2(4), C2−C3 152.2(6), C22−N3 145.9(5), N3−C23
135.5(5), C23−O3 120.5(5), C23−C24 152.9(6), C1−O5−C22
116.2(3), O5−C1−N2 108.2(3), N1−C1−O5 113.7(3), O5−C22−
N3 114.2(3), O5−C22−N4 109.3(3).
The IR spectrum of the dicarbonyl complex 11 shows two
strong CO stretching vibrations at 2017 and 2103 cm−1 (νav =
2060 cm−1), which were converted to a Tolman electronic pa-
rameter (TEP) value of 2068 cm−1 using a relation established
by Nolan and Plenio.6f,26
TEP values are a common measure for the donor properties
of Lewis bases including a large number of NHCs. Selected
values are compiled in Table 1 together with the corresponding
Rh−C(NHC) distances derived from X-ray diffraction analyses.
The parent saturated and unsaturated system (entries 5 and 6)
feature TEP values of ca. 2053 cm−1. The presence of an
anionic alcoholate (entry 2) or alkoxy group (entry 4) leads to
an enhanced donor capability and hence to a significant decline
of TEP values. In contrast, the introduction of keto groups
to the backboneleading to amido functional groupsis
accompanied by a reduced donor strength due to the in-
volvement of the nitrogen lone pair in the amide resonance
(see Chart 1), which in turn leads to a reduced stabilization of
the carbenic center. Accordingly, TEP values shifted to higher
wavenumbers are expected and indeed observed, as can be seen
from the values in Table 1: introduction of one carbonyl group
results in a moderately shifted TEP (entry 9), while the diamido
derivative 2 is characterized by one of the highest TEP values
reported so far for a NHC (entry 12), being on the same order
as the dicyano compound in entry 11. Interestingly, the TEP of
the malonic diamide (entry 7) is on the same order as that of the
five-membered monoamide (entry 9), while the anionic malonic
diamide derivative features a much lower TEP, as expected
(entry 1). It is noteworthy that the neutral seven-membered
DAC derived from phthalic diamide (entry 3) features a lower
TEP than the diamino derivatives in entries 5 and 6. Collectively,
the two neighboring carbonyl groups in the oxalamide NHC 2
exert a strong electron-withdrawing effect of comparable strength
to the two cyano groups in the backbone (entry 11). Just as
anionic charge leads to enhanced donor strength, the opposite
effect is expected for cationic derivatives. Indeed, we recently
reported a benzimidazole-based NHC with a cationic Cp*Ru+
fragment attached to the six-membered ring (entry 10), which
shows a fairly high TEP value. It is tempting to correlate the
observed TEP values with the corresponding Rh−C distances.
At first glance, for the five-membered diamino, monoamido,
and diamido derivatives (entries 6, 9, and 12) an increase of the
TEP is paralleled by a shortening of the Rh−C bond. This is in
accord with theoretical results that especially the DAC 2 is
product. The structural parameters of the two heterocyclic com-
ponents of compound 9 are close to the values observed for
the methoxide 5a (vide supra). In order to avoid steric repulsion
between the bulky mesityl groups, the ring planes of the
heterocycles form an angle of 74.5°.
The propensity of 2 to coordinate to metal fragments has
already been demonstrated in our previous communication by
the preparation of the rhodium complexes 10 and 11 according
to Scheme 4.6e In extension of this report, we were now able
Scheme 4. Generation of the Carbene Metal Complex and
Conversion into the Dicarbonyl Complex
to determine the crystal structure of the (COD)Rh complex
10, which is depicted in Figure 6 together with some relevant
geometrical data.
The Rh atom is coordinated in a slightly distorted square-
planar environment with an exceptionally short Rh1−C1 distance
of 194.2(3) pm. This distance is fairly short compared to other
Rh−C(NHC) distances for MACs, DACs, or NHCs derived
from the imidazole core,6a,g,8a,b,9b,25 which are typically found in
the range from 200 to 205 pm. As is commonly observed, the
Rh−C(COD) bonds trans to chlorine are shorter (213.3(3)
and 214.7(3) pm) than those trans to the NHC ligand (225.5(3)
and 228.6(3) pm) compared with other (NHC)Rh(COD)Cl
complexes. Overall, the geometrical features observed for the Rh
complex 10 are very close to the data found for the analogous Ir
compound.6e
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Table 1. Comparison of Several NHC Ligands Regarding Their Metal Carbene Distances and TEP Values
a
b
c
Rh−C(NHC) distance in pm in (NHC)Rh(COD)Cl. Tolman electronic parameter (TEP) in cm−1. Calculated from ν(CO) in CH2Cl2.
d
e
f
Calculated from ν(CO) in CHCl3. Calculated from ν(CO) in CDCl3. Calculated from ν(CO) in KBr.
at least 12 h. THF and diethyl ether were distilled from sodium/
benzophenone, hexane was distilled from sodium, and dichloro-
methane and chloroform were dried over CaH2 and subsequently
distilled. All solvents were stored under an atmosphere of nitrogen.
NMR spectra were recorded on Bruker DRX200 or DRX500 spec-
trometers. Chemical shifts are reported in ppm (δ) compared to TMS
(1H and 13C) using the residual peak of deuterated solvents as internal
standard (1H: CDCl3 7.26 ppm; THF-d8 3.58 ppm; 13C: CDCl3
77.0 ppm; THF-d8 67.21 ppm). Coupling constants (J) are quoted in
Hz. Infrared spectra were obtained on a Shimadzu IRAffinity-1 spec-
trometer. Elemental analyses were recorded on a Perkin CHN 2400
series II. For MS data a Thermo Finnigan Trace DSQ (EI) and a
Bruker Ultrafelx I TOF (MALDI) were used. UV−vis measurements
were performed on a Hewlett-Packard 84252 diode array spectropho-
tometer, and the fluorescence measurements were performed on a
Perkin-Elmer LS55.
capable of acting significantly in a metal-to-carbene π-back-
bonding interaction, which leads to a short and strong Rh−C
bond despite the fact that the donor strength of this ligand is
reduced compared to diaminocarbenes, which are known to act
as superb σ-donors. The same trend holds for the dicyano and
the cationic derivatives (entries 11 and 10). However, steric
effects can certainly not be ignored. Thus, increasing NHC ring
size (six- and seven-membered, entries 7, 1, and 3) is expected
to push the nitrogen substituents toward the coordination site,
which should result in elongated Rh−C bonds compared to the
smaller five-membered systems. Moreover, care has to be taken
when comparing TEP values from different studies, as the
positions of IR bands are known to depend on the solvent used
for the measurement. Nevertheless, the data provided in Table 1
can certainly be taken as a suitable guideline for the classification
of NHCs according to their ligand properties.
Cyclovoltametric measurements were performed on a Metrohm
μ-Autolab Type III using an Ag/AgCl electrode in dry CH2Cl2 with
0.1 M n-Bu4NPF6 electrolyte using a 100 mV s−1 scan rate and
referenced to Fc+/Fc.
CONCLUSION
■
N,N′-Dimesitylformamidine29 and N,N′-bis(2,6-diisopropylphenyl)-
formamidine29 were prepared according to literature procedures. Oxalyl
chloride was purchased from Merck Chemicals; sodium bis(trimethylsilyl)-
amide (NaHMDS) (2 M in THF), styrene, and methyl acrylate were
purchased from Acros Organics. The commercially available chemicals
were used without further purification. Column chromatography was
performed on Merck silica gel 60 (0.040−0.063 mm), which was oven-
dried for at least 24 h.
In conclusion, the diamidocarbene 2 displays a rich chemistry,
including adduct formation with sulfur and selenium, cyclo-
propanation reactions, and the formation of transition-metal
complexes. In these complexes, carbene 2 is the least electron
donating NHC described so far, and a significant metal-to-
ligand π-back-bonding is assumed to augment the metal−NHC
bonding, in accord with the short Rh−C distance observed in
complex 10.
4c. Acetic acid (710 μL) was added to a round-bottom flask charged
with 1.88 g (12.43 mmol) of 1-adamantylamine and 10 mL of triethyl
orthoformate. The resulting suspension was refluxed for 4 h, and while
cooling to ambient temperature a white solid started to settle. The
precipitation was completed by adding 40 mL of hexane. The crude
product was filtered and dissolved in dichloromethane, and 10 mL of
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations were performed under
an inert gas atmosphere of dry nitrogen by using standard vacuum line
and Schlenk techniques. Glassware was dried at 120 °C in an oven for
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2 N NaOH was added. The phases were separated, and the aqueous
phase was washed three times with 5 mL of dichloromethane. The
combined organic phases were dried over sodium sulfate and dried
in vacuo to obtain a white powder in 60% yield. 1H NMR (500 MHz in
CDCl3): δ 1.64−1.70 (m, 24H, CH2), 2.07 (s, 6H, CH), 7.60 (s, 1H,
C2−H) ppm. 13C{1H} NMR (126 MHz in CDCl3): δ 29.66
(s, CH2 adamantyl), 36.43 (s, CH adamantyl), 44.50 (s, C adamantyl), 145.97
(s, NH-CHN) ppm. MS (EI): m/z 312 (M+, 90%). IR (ATR): ν
2898, 2844, 1653, 1088 cm−1. Anal. Calcd (%) for C21H32N2 (312.15):
C 80.71, H 10.32, N 8.96. Found: C 80.07, H 9.33, N 8.93.
solid (446 mg, 80%). Crystals suitable for an X-ray diffraction analysis
were obtained by slow diffusion of hexane into a saturated solution of
7 in dichloromethane. 1H NMR (200 MHz in CDCl3): δ 2.18 (s, 12H,
CH3 Mes), 2.36 (s, 6H, CH3 Mes), 7.04 (s, 4H, CH Mes) ppm. 13C{1H}
NMR (126 MHz in CDCl3): δ 17.75 (s, CH3 Mes), 19.43 (s, CH3 Mes),
128.32 (s, C Mes), 129.61 (s, CH Mes), 135.71 (s, CCH3 Mes), 140.61
(s, CCH3 Mes), 154.60 (s, CO), 182.09 (s, CSe) ppm. MS (EI,
70 eV): m/z 414 (M+, 85%). IR (ATR): ν 1763 (CO) cm−1. Anal.
Calcd (%) for C21H22N2O2Se (413.37): C 61.02, H 5.36, N 6.78.
Found: C 60.95, H 5.49, N 6.81.
1b. To a chloroform solution (20 mL) of 1.49 g (11.74 mmol) of
oxalyl chloride was added dropwise and with stirring a dilute chlo-
roform solution (20 mL) of 4.0 g (10.97 mmol) of N,N′-bis(2,6-
diisopropylphenyl)formamidine while the reaction temperature was
maintained around 0 °C. The resulting colorless solution was heated
to reflux for 2 h. After removing the solvent in vacuo, the crude prod-
uct was washed with hexane (3 × 5 mL) and dried in vacuo to obtain a
white powder in 98% yield. 1H NMR (200 MHz in CDCl3): δ 1.24 (d,
6H, 3JHH = 6.8 Hz, CH3‑iPr), 1.29 (d, 6H, 3JHH = 6.8 Hz, CH3‑iPr), 1.32
(6a). 2-Chloro-1,3-dimesitylimidazolidine-4,5-dione (505 mg,
1.360 mmol) and 300 μL (3.31 mmol) of methyl acrylate were
dissolved in THF (15 mL) and cooled to −80 °C. NaHMDS (750 μL,
2 M in THF, 1.05 equiv) was added dropwise, and the resulting yellow
solution was stirred at −80 °C. After 45 min of stirring, the cooling
bath was removed and the solution was allowed to warm to room
temperature. After removing the solvent in vacuo, the crude product
was purified by flash chromatography (SiO2; diethyl ether) to yield 6a
1
as a white solid (397 mg, 70%). H NMR (500 MHz in CDCl3): δ
1.44 (dd, 1H, 3JHH = 10.4 Hz, 3JHH = 7.6 Hz, CHCOOMe), 1.93 (dd,
2H, 2JHH = 9.1 Hz, 3JHH = 7.6 Hz, CH2), 1.93 (dd, 2H, 2JHH = 9.1 Hz,
3JHH = 10.4 Hz, CH2), 2.12 (s, 3H, CH3 Mes), 2.23 (s, 3H, CH3 Mes),
2.26 (s, 3H, CH3 Mes), 2.30 (s, 3H, CH3 Mes), 2.31 (s, 3H, CH3 Mes),
3
3
(d, 6H, JHH = 6.8 Hz, CH3‑iPr), 1.34 (d, 6H, JHH = 6.8 Hz, CH3‑iPr),
3
3
2.80 (sept, 2H, JHH = 6.8 Hz, HiPr), 3.10 (sept, 2H, JHH = 6.8 Hz,
HiPr), 6.93 (s, 1H, C2-H), 7.38−7.28 (m, 4H, CH Ring), 7.50 (t, 2H,
3JHH = 7.8 Hz, CH Ring) ppm. 13C{1H} NMR (126 MHz in CDCl3): δ
23.40 (s, CH3 iPr), 24.13 (s, CH3 iPr), 24.99 (s, CH3 iPr), 25.10 (s,
CH3 iPr), 29.53 (s, C(CH3)2 iPr), 29.57 (s, C(CH3)2 iPr), 86.44 (s,
NCN), 124.32 (s, CHRing), 125.37 (s, CHRing), 126.87 (s, CRing),
130.89 (s, CHRing), 145.71 (s, CC(CH3)2 Ring), 148.91 (s, CC-
(CH3)2 Ring), 156.45 (s, CO) ppm. MS (EI, 70 eV): m/z 455 (M+,
3%), 420 ([M − Cl]+, 28%). IR (ATR): ν 1758 (CO) cm−1. Anal.
Calcd (%) for C27H35ClN2O2 (455.03): C 71.27, H 7.75, N 6.16.
Found: C 71.50, H 8.40, N 5.78.
2.34 (s, 3H, CH3 Mes), 3.44 (s, 3H, OCH3), 6.92−7.03 (s, 4H, CHMes
)
ppm. 13C{1H} NMR (50 MHz in CDCl3): δ 10.07 (s, CH2), 17.87
(s, CH3 Mes), 18.07 (s, CH3 Mes), 18.31 (s, CH3 Mes), 20.94 (s, CH3 Mes),
21.07 (s, CH3 Mes), 21.13 (s, CH3 Mes), 21.58 (s, CHCOOCH3), 22.00
(s, CH3 Mes), 52.56 (s, OCH3), 63.59 (s, NCN), 125.75 (s, Cipso Mes),
127.76 (s, Cipso Mes), 129.32 (s, CHMes), 129.84 (s, CHMes), 130.03
(s, CHMes), 130.24 (s, CHMes), 135.27 (s, CCH3 Mes), 136.83
(s, CCH3 Mes), 137.29 (s, CCH3 Mes), 137.61 (s, CCH3 Mes), 139.80
(s, CCH3 Mes), 140.52 (s, CCH3 Mes), 156.63 (s, CO), 157.81
(s, CO), 166.73 (s, OCH3) ppm. MS (MALDI): m/z 421
(M+, 100%), 334 ([M+ − CH2CHCOOCH3], 20%). IR (CDCl3):
ν 1749 (CO) cm−1. Anal. Calcd (%) for C15H28N2O4 (420.50):
C 71.74, H 6.71, N 6.66. Found: C 71.21, H 6.56, N 6.58.
1c. To a chloroform solution (10 mL) of 503 mg (3.96 mmol) of
oxalyl chloride was added dropwise and with stirring a dilute chlo-
roform solution (10 mL) of 1.48 g (3.67 mmol) of N,N′-
diadamantylformamidine and 510 μL (3.67 mmol) of triethylamine
while the reaction temperature was maintained around 0 °C. The
resulting yellow solution was heated to reflux for 2 h. After removing
the solvent in vacuo, the crude product was suspended in THF and the
insoluble triethylammonium chloride was removed over a thin pad of
Celite. After removing the solvent in vacuo, the crude product was
washed with hexane (2 × 5 mL) and dried in vacuo to obtain a yellow
(6b). 2-Chloro-1,3-dimesitylimidazolidine-4,5-dione (500 mg,
1.348 mmol) and 1.5 mL (13.11 mmol) of styrene were dissolved in
THF (15 mL) and cooled to −80 °C. NaHMDS (750 μL, 2 M in THF,
1.05 equiv) was added dropwise, and the resulting yellow solution was
stirred at −80 °C. After 35 min of stirring, the cooling bath was removed
and the solution was allowed to warm to room temperature. After re-
moving the solvent in vacuo, the crude product was purified by flash
chromatography (SiO2; diethyl ether) to yield 6b as a white solid
(266 mg, 45%). 1H NMR (500 MHz in CDCl3): δ 1.29 (s, 3H, CH3 Mes),
1
powder in 80% yield. H NMR (500 MHz in THF-d8): δ 1.70−1.76
(m, 12H, CH2), 2.10 (s, 6H, CH), 2.24−2.32 (m, 12H, CH2), 6.10
(s, 1H, C2-H) ppm. 13C{1H} NMR (126 MHz in THF-d8): δ 30.57
(s, CH2 adamantyl), 37.01 (s, CH2 adamantyl), 39.93 (s, CH adamantyl). 56.65
(s, C adamantyl), 85.66 (s, NCN), 158.05 (s, CO) ppm. MS
(MALDI): m/z 367.1 ([M + Cl−], 100%). IR (ATR): ν 1735 (CO)
cm−1. Anal. Calcd (%) for C23H31N2ClO2 (402.96): C 68.56, H 7.75,
N 6.95. Found: C 69.40, H 8.35, N 7.14.
3
3
1.55 (dd, 1H, JHH = 11.7 Hz, JHH = 7.8 Hz, CH-Ph), 1.93 (dd, 2H,
2JHH = 9.7 Hz, 3JHH = 7.8 Hz, CH2), 2.24 (s, 3H, CH3 Mes), 2.29 (s, 3H,
CH3 Mes), 2.30 (s, 3H, CH3 Mes), 2.33 (s, 3H, CH3 Mes), 2.48 (s, 3H,
2
3
CH3 Mes), 2.37 (dd, 2H, JHH = 9.7 Hz, JHH = 11.7 Hz, CH2), 6.44−
6.64 (m, 4H, CHMes), 6.94−7.13 (m, 5H, Ph) ppm. 13C{1H} NMR
(126 MHz in CDCl3): δ 8.90 (s, CH2), 17.24 (s, CH-Ph), 18.29
(s, CH3 Mes), 18.47(s, CH3 Mes), 18.55 (s, CH3 Mes), 20.96 (s, CH3 Mes),
21.10 (s, CH3 Mes), 63.63 (s, NCN), 126.38 (s, Cipso Ph), 126.96
(s, CH Ph), 127.99 (s, CH Ph), 128.17 (s, Cipso Mes), 128.41 (s, Cipso Mes),
129.14 (s, CHMes), 129.39 (s, CHMes), 130.00 (s, CHMes), 130.08
(s, CHMes), 131.59 (s, CH Ph), 134.90 (s, CCH3 Mes), 136.89
(s, CCH3 Mes), 136.98 (s, CCH3 Mes), 139.14 (s, CCH3 Mes), 134.41 (s,
CCH3 Mes),140.24 (s, CCH3 Mes), 156.92 (s, CO), 157.84 (s, CO)
ppm. MS (MALDI): m/z 439 (M+, 100%).
5c. 4c (100 mg, 0.29 mmol) was suspended in chloroform (30 mL)
before methanol (5 mL) was added. The resulting solution was heated
to reflux for 1 h. After removing the solvent in vacuo, the crude
product was washed with hexane (2 × 5 mL) and dried in vacuo to
obtain a white powder in 99% yield. 1H NMR (500 MHz in CDCl3): δ
1.67−1.74 (m, 12H, CH2), 2.13 (s, 6H, CH), 2.21 (s, 12H, CH2), 3.01
(s, 3H, OCH3), 6.10 (s, 1H, C2-H) ppm. 13C{1H} NMR (126 MHz in
CDCl3): δ 29.42 (s, CH2 adamantyl), 36.03 (s, CH2 adamantyl), 39.07
(s, CH adamantyl), 46.23 (s, OCH3), 57.42 (s, C adamantyl), 87.51
(s, NCN), 158.31 (s, CO) ppm. MS (MALDI): m/z 398
(M+, 100%). IR (ATR): ν 1725 (CO) cm−1. Analysis Calcd (%)
for C24H34N2O3 (398.54): C 72.33, H 8.60, N 7.03. Found: C 70.82, H
8.25, N 6.84.
(7). 2-Chloro-1,3-dimesitylimidazolidine-4,5-dione (500 mg, 1.348
mmol) was dissolved in THF (15 mL) and cooled to −80 °C.
NaHMDS (750 μL, 2 M in THF, 1.05 equiv) was added dropwise.
The red solution was stirred for 5 min, and elemental selenium (red)
(300 mg, 3.800 mmol) was added. After 30 min of stirring, the cooling
bath was removed and the solution was allowed to warm to room tem-
perature. After evaporation of all volatiles, the crude product was purified
by flash chromatography (SiO2; CH2Cl2) to yield 7 as a gray-brown
Crystal Structure Determinations. Crystals of compounds 5a, 7,
9, and 10 suitable for X-ray study were selected by means of a
polarization microscope and investigated with a STOE imaging plate
diffraction system and an Oxford Diffraction Xcalibur diffractometer,
respectively, using graphite-monochromatized Mo Kα radiation (λ =
0.71073 Å). Unit cell parameters were determined by least-squares
refinements on the positions of 8000, 8000, 31 779, and 8000
reflections, respectively. Space groups no. 60, no. 43, no. 19, and no.
14 were uniquely determined for 5a, 7, 9, and 10, respectively. Cor-
rections for Lorentz and polarization effects were applied. The struc-
tures were solved by direct methods30 and subsequent ΔF syntheses.31
1932
dx.doi.org/10.1021/om201235c | Organometallics 2012, 31, 1927−1934

Organometallics
Article
Table 2. Crystal Data and Structure Refinement for Compound 5a, 7, 9, and 10
5a
7
9
10
CCDC no.
855559
855560
855561
855562
empirical formula
fw
C22H26N2O3
366.45
C21H22N2O2Se
413.37
C43H48Cl2N4O5
771.75
C29H34ClN2O2Rh
580.94
temperature (K)
cryst syst
291(2)
291(2)
291(2)
291(2)
orthorhombic
Pbcn
orthorhombic
Fdd2
orthorhombic
P212121
monoclinic
P21/n
space group
unit cell dimensions (Å, deg)
a = 15.3988(9)
b = 8.1980(4)
c = 16.2972(10)
a = 11.2248(5)
b = 32.4746(18)
c = 11.1989(5)
a = 13.0623(4)
b = 16.4429(5)
c = 19.0786(7)
a = 11.3659(7)
b = 15.1471(6)
c = 15.9812(10)
β = 98.205(7)
2723.2(3)
4
volume (Å3)
2057.3(2)
4082.2(3)
4097.7(2)
Z
4
8
4
densitycalc (Mg/m3)
1.183
1.345
1.251
1.417
absorp coeff (mm−1
)
0.079
1.856
0.207
0.753
F(000)
784
1696
1632
1200
cryst size (mm3)
theta range (deg)
reflns collected
indep reflns
0.5 × 0.3 × 0.25
2.50 to 25.00
17 047
0.3 × 0.3 × 0.3
2.51 to 25.00
12 952
0.3 × 0.3 × 0.3
1.99 to 25.00
31 131
0.1 × 0.04 × 0.03
2.07 to 25.85
33 034
1807 [R(int) = 0.0567]
99.7%
1806 [R(int) = 0.0501]
99.9%
7203 [R(int) = 0.0941]
99.9%
5262 [R(int) = 0.0403]
completeness to theta = 25.85°
absorption corr
99.9%
none
none
none
none
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/params
goodness-of-fit on F2
1807/0/136
1806/1/122
7203/0/499
5262/0/322
1.067
1.048
1.215
1.002
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole (e·Å−3
R1 = 0.0482, wR2 = 0.1543
R1 = 0.0650, wR2 = 0.1610
0.148 and −0.155
R1 = 0.0296, wR2 = 0.0632
R1 = 0.0367, wR2 = 0.0645
0.345 and −0.146
R1 = 0.0691, wR2 = 0.1297
R1 = 0.0779, wR2 = 0.1328
0.170 and −0.449
R1 = 0.0338, wR2 = 0.0785
R1 = 0.0484, wR2 = 0.0878
0.480 and −0.406
)
Approximate positions of all but the hydrogen atoms of some methyl
groups were found in different stages of converging refinements
(max. shift/s.u. = 0.000 in all cases) by full-matrix least-squares cal-
culations on F2.31 As explained in the Results and Discussion section, a
1:1 disorder of the methoxy group of 5a was observed. Anisotropic
displacement parameters were refined for all atoms heavier than
hydrogen. With idealized bond lengths and angles assumed for all the
CH, CH2, and CH3 groups, the riding model was applied for the cor-
responding H atoms, and their isotropic displacement parameters were
constrained to 120%, 120%, and 150% of the equivalent isotropic dis-
placement 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 right choice of the polar axis for 7 and
the enantiomorphic form for 9 is indicated by the Flack param-
eter32 −0.016(11) and 0.11(12), respectively. Selected crystal data and
refinement parameters are collected in Table 2. CCDC-855559 (5a),
CCDC-855560 (7), CCDC-855561 (9), and CCDC-855562 (10)
contain the supplementary crystallographic data (excluding structure
factors) for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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