Amino-acrylamido carbenes: Modulating carbene reactivity via decoration with an α,β-unsaturated carbonyl moiety

Article abstract of DOI:10.1021/om300401t

Two 6-methoxy-4-oxo-1,3-diaryl-3,4-dihydropyrimidinium salts (2a and 2b) have been prepared as precursors to novel amino-acrylamido carbenes. Treatment of 2a (where the aryl groups are mesityl groups) with one equivalent of sodium hexamethyldisilazide in aromatic hydrocarbons affords the persistent amino-acrylamido carbene 3a, which has been characterized spectroscopically. This novel carbene has been trapped with Ir(I) transition metal fragments as well as electrophilic carbon disulfide and nucleophilic isocyanides. Infrared spectroscopic studies carried out on 3a-Ir(CO)₂Cl (5a) indicated that this carbene exhibits a Tolman electronic parameter (TEP) of 2049 cm ^-1, a value that suggests that 3a is a stronger donor than both diamidocarbenes (DACs) and a recently reported amino-ureido carbene (DAC TEPs ≈ 2057 cm^-1; amino-ureido TEP = 2058 cm^-1), but similar σ-donating properties to a monoamido-amino carbene (TEP = 2050 cm ^-1). This result has been corroborated by DFT analyses carried out on all four species, which indicated that the HOMO and LUMO energies of 3a are comparable to the amino-ureido and monoamido-amino carbenes, whereas the DAC was shown to be more electrophilic with a much lower energy LUMO than the other three carbenes. Surprisingly, deprotonation of 2b (where the N-substituents are 2,6-diisopropylphenyl groups) does not afford the anticipated carbene. Indeed, ¹H NMR spectroscopic analysis indicates the formation of a novel bent allene or carbodicarbene (3b), which decomposes rapidly in solution at room temperature.

Full text of DOI:10.1021/om300401t

Article
pubs.acs.org/Organometallics
Amino-Acrylamido Carbenes: Modulating Carbene Reactivity via
Decoration with an α,β-Unsaturated Carbonyl Moiety
Ryan M. Mushinski, Brian M. Squires, Katherine A. Sincerbox, and Todd W. Hudnall*
Department of Chemistry and Biochemistry, Texas State University, San Marcos, Texas 78666, United States
S
* Supporting Information
ABSTRACT: Two 6-methoxy-4-oxo-1,3-diaryl-3,4-dihydro-
pyrimidinium salts (2a and 2b) have been prepared as
precursors to novel amino-acrylamido carbenes. Treatment of
2a (where the aryl groups are mesityl groups) with one
equivalent of sodium hexamethyldisilazide in aromatic hydro-
carbons affords the persistent amino-acrylamido carbene 3a,
which has been characterized spectroscopically. This novel
carbene has been trapped with Ir(I) transition metal fragments
as well as electrophilic carbon disulfide and nucleophilic
isocyanides. Infrared spectroscopic studies carried out on 3a-
Ir(CO)2Cl (5a) indicated that this carbene exhibits a Tolman electronic parameter (TEP) of 2049 cm−1, a value that suggests
that 3a is a stronger donor than both diamidocarbenes (DACs) and a recently reported amino-ureido carbene (DAC TEPs ≈
2057 cm−1; amino-ureido TEP = 2058 cm−1), but similar σ-donating properties to a monoamido-amino carbene (TEP = 2050
cm−1). This result has been corroborated by DFT analyses carried out on all four species, which indicated that the HOMO and
LUMO energies of 3a are comparable to the amino-ureido and monoamido-amino carbenes, whereas the DAC was shown to be
more electrophilic with a much lower energy LUMO than the other three carbenes. Surprisingly, deprotonation of 2b (where the
1
N-substituents are 2,6-diisopropylphenyl groups) does not afford the anticipated carbene. Indeed, H NMR spectroscopic
analysis indicates the formation of a novel bent allene or carbodicarbene (3b), which decomposes rapidly in solution at room
temperature.
contributions to this area have focused on the incorporation of
carbonyl moieties into the NHC backbone.11−19,23−26 The
initiative of utilizing oxygen-containing electron-withdrawing
substituents in NHCs was first investigated by Lavigne and
shortly thereafter by Glorius, who demonstrated that the donor
properties of the anionic carbenes I and II (Figure 1) when
complexed to a transition metal could be modulated via
alkylation.24,25,27,28
Following the reports from Lavigne and Glorius, the groups
of Bielawski11−17 and Ganter18,19,22 disseminated a series of
elegant papers that demonstrated that the incorporation of
carbonyl moieties into the backbone of an NHC can facilitate
remarkable small-molecule reactivity.29 Bielawski and Ganter
have shown that the diamidocarbenes (DACs)30 III and IV,
respectively, are capable of reversibly fixing CO,13,14 facilitating
N−H14 and intramolecular C−H activation,13 irreversibly
coupling to isocyanides,15,16,18 and, more recently, [2+1]
cycloadditions with olefins, alkynes, nitriles, and alde-
hydes.11,12,19 Early in 2012, Ganter then reported on the
chemistry of the so-called amino-amido carbene V,22 followed
shortly by another paper from Bielawski on the synthesis and
reactivity of the monoamido-amino carbene (MAAC) VI.17,24
Interestingly, while both authors refer to their respective
INTRODUCTION
■
Since their isolation over twenty years ago by Bertrand,1 the
preparation and utility of stable carbenes have become
increasingly pertinent areas of research globally. This craze
has been spawned primarily by the seminal discovery of the
robust N-heterocyclic carbenes (NHCs) by Arduengo in 1991.2
These highly modular, neutral, carbon-based ligands have
become ubiquitous in homogeneous catalysis,3 organocatalysis,4
and materials science.5,6 Despite the popularity of NHCs,7 it
was the discovery of cyclic alkyl amino carbenes (CAACs) by
Bertrand that unveiled the ability of stable carbenes to activate
small-molecule substrates.8−10 Indeed, in a series of elegant
articles, Bertrand demonstrated that by appropriate electronic
modification stable carbenes can emulate transition metal
behavior and engage in reactivity with small-molecule substrates
atypical of the canonical NHCs developed by Arduengo. In
2007, Bertrand proved that CAACs readily facilitate the
heterolytic cleavage of H2 and NH3.9 Additionally, CAACs
are capable of fixing carbon monoxide (CO), affording stable
ketenes.10
Shortly following Bertrand’s initial results, several groups
provided evidence that the incorporation of π-acidic sub-
stituents into NHCs could drastically alter the electronic
properties and attendant reactivity.11−26 With appropriate
modification, several of these derivatives were shown to engage
in novel small-molecule reactivity.11−20 The most prominent
Received: May 11, 2012
Published: June 18, 2012
© 2012 American Chemical Society
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Figure 1. Representative examples of some known free carbenes with oxygen-containing backbones relevant to this study.
a
Scheme 1. Synthesis of Pyrimidinium Salts 2a and 2b
a
Conditions: (i) malonic acid (1 equiv), dicyclohexylcarbodiimide (2 equiv), CH2Cl2, 25 °C, 3 h; (ii) methyl triflate, CH2Cl2, 25 °C, 12 h. Isolated
yields are indicated.
carbenes as amino amido carbenes, the overall donating ability
(as determined by the Tolman electronic parameter (TEP)) as
well as their reactivity is not similar. Whereas carbene V was
found to exhibit a TEP of 2058 cm−1,22 with no small-molecule
reactivity reported to date, the TEP of VI was found to be 2050
cm−1 and, like other carbenes decorated with carbonyl
functional groups, engages in small-molecule reactivity.17
We became interested in this disparity as we were
investigating the synthesis and reactivity of a series of carbenes
that we also dubbed amino-amido carbenes (VII). The
differences in reactivity among carbenes V−VII led us to
reconsider the classification of each species. We now consider
carbene V as an amino-ureido carbene and VI as an amino-
amido carbene, and herein we report our contribution to this
burgeoning area of carbene chemistry with the synthesis and
reactivity studies of the first free amino-acrylamido carbene of
type VII.25
the aryl substituents are mesityl (2a) and 2,6-diisopropylphenyl
(dipp) (2b)) were rapidly prepared in excellent yield (Scheme
1) via alkylation of their zwitterionic precursors 1a,b with
methyl triflate (MeOTf).31
Both salts were found to be thermally robust with melting
points of 206 and 231 °C for 2a and 2b, respectively.
Interestingly, salts 2a,b exhibited minor sensitivity to water and
were found to demethylate in wet organic solvents over a
period of several days. When stored in a drybox however, these
compounds are indefinitely stable at ambient temperature. Both
salts have been fully characterized by multinuclear NMR
spectroscopy and IR spectroscopy, and 2b has been further
investigated using single-crystal X-ray diffraction (Figure 2,
Table S1). The 1H NMR spectra of each salt exhibit two salient
signals for the C2 and C4 protons. The C2-bound protons
resonate characteristically downfield and appear at 10.01 and
In 2010 Lavigne et al. demonstrated that the anionic carbene
I, once coordinated to a transition metal center, could be
alkylated or silylated to afford amino-acrylamido carbene metal
complexes.25 Remarkably, this simple chemical modification
resulted in a drastic change in the TEP of the (NHC)-
Rh(CO)2Cl complexes from 2042 to 2050 cm−1 (for the
anionic carbene I (R = Me) and the O-methylated analogue,
respectively), which established the ability to effectively
modulate the carbene’s donor properties (vide supra).25 Despite
this account however, no efforts have been reported on the
preparation of the free amino-acrylamido carbenes, thus
precluding any study of their respective reactivity with small-
molecule substrates and prompting our investigation into their
synthesis.
Figure 2. Crystal structure of 2b. Thermal ellipsoids are drawn at the
50% probability level. Isopropyl substituents are drawn as small
spheres, and hydrogen atoms (except H2A and H4A) and OTf− anion
are omitted for clarity. Pertinent metrical parameters are provided in
the text.
RESULTS AND DISCUSSION
■
By following the protocol delineated by Lavigne,29 the 6-
methoxy-4-oxo-1,3-diaryl-3,4-dihydropyrimidinium salts (where
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9.99 ppm (CDCl3) for 2a and 2b, respectively. In contrast, the
C4-bound protons are shifted upfield at 6.08 and 6.27 ppm and
are more characteristic of olefinic protons. Methylation of the
zwitterionic precursors 1a,b is also confirmed by the
appearance of a downfield signal corresponding to the methoxy
protons located at 4.05 and 4.14 ppm for 2a and 2b,
respectively.
IR data collected on 2a and 2b exhibited intense, sharp
signals at 1738 and 1739 cm−1, respectively, which is quite
surprising given that both amides and α,β-unsaturated carbonyl
functional groups typically give rise to bands at much lower
frequency (ca. 1680 cm−1). This observed shift may simply
result from the delocalization of cationic charge across the N−
C2−N centers, which effectively marshals electron density away
from the carbonyl, thus strengthening the CO bond. This
observation was further substantiated by close inspection of the
carbonyl C(3)−O(2) distance in the crystal structure of 2b
(1.190(7) Å), which indicated substantial multiple-bond
character.
shifted upfield at 3.15 ppm in 3b from 4.14 ppm in 2b.
Unfortunately, the identity of 3b could not be conclusively
confirmed, as it rapidly decomposed in solution over a period
of approximately 30 min to an intractable mixture of
unidentifiable compounds, thus precluding further character-
ization including 13C NMR.32
Because both 3a and 3b could not be isolated in the solid
state, we sought to utilize standard carbene-trapping experi-
ments to fully elucidate the structure of these species.
Therefore, the free carbene 3a was treated with [(cod)IrCl]2
(cod = 1,5-cyclooctadiene) in toluene to afford the expected
Ir(I) complex 3a-[(cod)IrCl] (4a) in moderate yield (52%), as
depicted in Scheme 3. The chemical shift of the carbene
nucleus in the 13C NMR of 4a was observed at δ = 216.1 ppm
(C6D6), a value that was downfield relative to known carbene-
[(cod)IrCl] complexes (179.6−208.2 ppm),33 but similar to V-
[(cod)IrCl]22 and VI-[(cod)IrCl]17 (213.6 and 218.6 ppm,
respectively). Like other carbenes decorated with carbonyl
moieties, this downfield chemical shift is a direct result of the π-
withdrawing nature of the acrylamido functional group
incorporated into the carbene backbone.13,18 In agreement
with this view, we note the following observations obtained
from the X-ray crystal structure of 4a (Figure 4, Table S1): (1)
the long C3−O2 distance of 1.231(18) Å, (2) the short C4−C5
distance of 1.32(2) Å, and (3) the short N1−C2 and N2−C3
distances of 1.381(18) and 1.426(18) Å, respectively, all
indicating that a large degree of electron density was being
pulled away from the carbene C2 nucleus into the acrylamide
backbone.
Upon isolation of salts 2a,b, our efforts shifted toward the
preparation of the respective free carbenes. Gratifyingly, upon
treatment with sodium hexamethyldisilazide (NaHMDS) in
aromatic hydrocarbons, deprotonation of the mesityl derivative
2a proceeded cleanly, affording carbene 3a in quantitative yield
(Scheme 2) as observed spectroscopically (1H and 13C NMR in
a
Scheme 2. Deprotonation of Pyrimidinium Salts 2a and 2b
Whereas the metsityl carbene 3a was readily trapped with
[(cod)IrCl]2, an exhaustive series of trapping experiments
carried out on the dipp derivative 3b proved to be unsuccessful.
Indeed, despite the use of several trapping agents including
transition metal fragments (PdCl2, Pd(OAc)2, and [(cod)-
IrCl]2) as well as main group electrophiles (BH3, BF3, and
CS2), the identity of 3b could not be fully elucidated.
Additionally, if our assignment of 3b as a zwitterionic bent
allene was correct, we hypothesized that the compound may be
trapped with strong oxophilic electrophiles. We therefore
attempted to react 3b with the silylating and alkylating agents
trimethylsilylchloride (TMSCl) and MeOTf; however these
reactions led to intractable mixtures of multiple unidentifiable
a
Conditions: (i) NaHMDS (0.9 equiv), C6H6, 25 °C, 30 min.
C6D6). Formation of 3a was confirmed by (1) the loss of the
C2 proton at 10.01 ppm, (2) a large upfield shift of the C4
proton (from 6.08 ppm in 2a to 5.34 ppm in 3a) and the
methoxy resonance (from 4.05 ppm in 2a to 2.76 ppm in 3a),
which indicated the loss of cationic character, and (3) the
downfield NCN resonance in the 13C NMR at 257 ppm
(C6D6), indicative of the free carbene. Efforts to isolate the free
carbene 3a were unsuccessful, as demethylation concomitant
with protonation of the C2 position were observed to afford the
zwitterionic compound 1a in near quantitative yield. This
1
species (observed spectroscopically via H NMR).
With the identity of 3a confirmed as the amino-acrylamido
carbene via the [(cod)IrCl]2 trapping experiment, our attention
became focused on interrogating its electronic properties and
exploring its chemical reactivity toward small-molecule
substrates. With compound 4a (3a-[(cod)IrCl]) in hand, we
decided to measure the Tolman electronic parameter of 3a by
converting 4a into the carbene-[(CO)2IrCl] complex 5a
followed by measuring the iridium-bound carbonyl groups via
infrared (IR) spectroscopy.34 Therefore, exchange of the 1,5-
cyclooctadiene ligand for two carbonyl ligands was facilitated
via bubbling CO into a dichloromethane (CH2Cl2) solution of
4a at room temperature until the solvent had completely
evaporated. Washing the resulting residue with cold hexanes
afforded the desired complex 5a in near quantitative yield
1
process was monitored by H NMR (C6D6), which revealed
that 3a was persistent for nearly two days in solution at room
temperature, but ultimately decomposed to the zwitterion.
Surprisingly, deprotonation of the dipp analogue 2b did not
result in the formation of the expected free carbene, as
1
evidenced by H NMR (C6D6). Indeed, upon treatment of 2b
with 1 equiv of NaHMDS in C6D6, deprotonation of the C4-
bound proton at ∼5.5 ppm was observed (Scheme 2, Figure 3).
Further inspection of the 1H NMR suggested that formation of
the zwitterionic bent allene 3b, depicted in Scheme 2, occurred
instead of the desired carbene. In agreement with this
assessment we note the upfield shift of the C2-bound proton
from ∼10 ppm in 2b to 9.0 ppm in 3b, consistent with the loss
of cationic character. Additionally, the methoxy resonance was
1
(94%, Scheme 3). The structure of 5a was confirmed via H
NMR by the loss of signals corresponding to the cyclooctadiene
ligand, the appearance of two new signals in the 13C NMR
spectrum centered at δ = 178.7 and 170.0 ppm (C6D6), and the
appearance of strong CO stretching bands in the IR spectrum
(2069.3 and 1977.8 cm−1, CH2Cl2).13,23,22,17 Using a method
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Figure 3. Stacked 1H NMR spectra of (a) 2b in CDCl3 and (b) 3b in C6D6. Lines are drawn to indicate shifting of peaks upon deprotonation of 2b.
Scheme 3. Synthesis of the Amino-Acrylamido Ir(I)
a
Complexes
a
Conditions: (i) NaHMDS (0.9 equiv), C7H8, 25 °C, 30 min,
[(cod)IrCl]2 (0.5 equiv), C7H8, 25 °C, 12 h; (ii) CO, CH2Cl2, 25 °C,
30 min. Isolated yields are indicated.
developed by Crabtree35 and modified by Nolan,33c the TEP of
3a was calculated to be 2049 cm−1, a value that corresponded
well with the TEP measured for Lavigne’s methylated anionic
carbene I (TEP = 2050 cm−1, R = Me).25 For comparison, the
Figure 4. Crystal structure of 4a. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms are omitted for clarity.
Pertinent metrical parameters are provided in the text.
TEP values for carbenes V and VI were calculated to be 2058
and 2050 cm−1 using the same methodology.22,17
Having observed that the overall donating ability of carbenes
VI and 3a were similar while the amino-ureido derivative V is a
much weaker donor ligand, we decided to study the overall
electronic structure of these carbenes using density functional
theory (DFT). We performed full geometry optimization of
carbenes V, VI, 3a, and the DAC N,N′-dimesityl-4,6-diketo-5,5-
dimethylpyrimidin-2-ylidene (III) with Gaussian0336 using the
Becke exchange functional and the Lee−Yang−Parr correlation
functional (B3LYP).37 The following basis sets38 were used: 6-
31G for all H atoms and 6-31G+(d′) for all C, O, and N atoms.
The HOMO and LUMO energies for each carbene were
obtained from their respective output files, and the electron
densities of the DFT-optimized structures for each orbital are
shown in Figure 5.
Interestingly, carbenes V, VI, and 3a were all found to have
similar frontier orbital energies. Importantly, the localization of
the LUMO for V was noticeably different than that of VI and
3a. Specifically, the LUMO of V is localized primarily on the
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Figure 5. Energy levels (kcal/mol) and depictions of frontier molecular orbitals of carbenes III, V, VI, and 3a.
a
Scheme 4. Synthesis of the Dithioate 6 and the Amino-Acrylamido-Ketenimine Adduct 7
a
Conditions: (i) NaHMDS (0.9 equiv), C7H8, 25 °C, 30 min, CS2, C7H8, 25 °C, 2 h; (ii) NaHMDS (0.9 equiv), C7H8, 25 °C, 30 min, tert-
butylisocyanide (1 equiv), C7H8, 25 °C, 2 h. Isolated yields are indicated.
benzyl substituent, whereas in VI and 3a there is a large p-type
orbital lobe localized on the carbene carbon. Indeed, the lowest
energy orbital, which corresponds to the vacant p orbital of the
carbene, is the LUMO+2 (−10.02 kcal/mol), which is nearly 10
kcal/mol higher in energy than the LUMOs for VI and 3a. It is
significant to note that this discrepancy may explain the
differing reactivities between amino-ureido carbene V and the
other two carbenes VI and 3a (vide infra). Additionally, the
DFT analyses suggest that while all three carbenes featuring a
single carbonyl moiety exhibit similar HOMO energies to the
DAC III (HOMO energies range from −131 to −140 kcal/
mol), the LUMO energies were found to be much higher
compared to the DAC (LUMO energies for V, VI, and 3a were
−18.24, −22.67, and −19.48 kcal/mol, respectively, DAC III,
LUMO = −46.67 kcal/mol). This suggested that while all four
carbenes exhibit a similar nucleophilic character, DAC III is a
much more electrophilic carbene than the other three, which
may explain why DACs exhibit a much greater breadth of small-
molecule reactivity (vide infra).
Following an interrogation into the electronic structure of
the amino-acrylamido carbene 3a, which revealed a similar
donating strength to MAAC VI along with similar frontier
molecular orbitals, we hypothesized that 3a may exhibit similar
chemical reactivity to that of MAAC VI. We first began our
investigation into the reactivity of 3a with both electrophilic
and nucleophilic substrates. Scheme 4 provides a summary of
this reactivity. When a freshly prepared solution of 3a in
toluene was treated with an excess (2 equiv) of CS2, the
resultant solution turned a deep red color, and after
approximately 30 min, the formation of a reddish-brown solid
appeared. Removal of the solvent in vacuo followed by washing
with hexanes afforded the desired carbodithioate 6 in excellent
yield (96%) as a reddish solid. The identity of compound 6 was
confirmed by 13C NMR, which displayed diagnostic dithioate
(158.96 ppm, CDCl3) and an upfield carbene nucleus (220.04
ppm in 6, CDCl3, vs 257.25 ppm in 3a, C6D6) signals.
With the nucleophilic reactivity of 3a established,25 we then
investigated its ability to react as an electrophilic reagent.
Consequently, a solution of 3a in toluene was treated with tert-
butyl isocyanide (1 equiv) at room temperature, which resulted
in a gradual color change from pale yellow to deep orange. The
resulting solution was stirred for two hours in a nitrogen-filled
glovebox, and then the solvent was removed in vacuo. The dark
yellow residue was then washed with cold hexanes (−30 °C)
and dried to afford the amino-acrylamido ketenimine 7 as a
yellow solid in good yield (71%). The identity of 7 was
1
confirmed via H and 13C NMR as well as IR spectroscopy.
1
Specifically, the H NMR (C6D6) revealed seven singlets, each
integrating to three protons ranging from 1.98 to 2.89 ppm for
the seven inequivalent methyl groups on the carbene backbone
in 7 as well as a sharp singlet centered at 0.59 ppm, which
integrated to nine protons for the tert-butyl methyl groups. The
observed inequivalent methyl peaks suggested a highly
asymmetric molecule with significant steric congestion even
in solution. The 13C NMR data were also consistent with a
congested structure, as 26 different carbon signals were
observed. Most notably was an upfield peak corresponding to
the carbenoid carbon (δ = 118.48 ppm; C6D6) and a downfield
shifted signal at 206.7 ppm (C6D6), which was diagnostic for
the central CCN carbon nucleus. Additionally, the IR spectrum
of 7 exhibited an intense sharp peak centered at 2025 cm−1
(KBr), indicating the presence of the CCN stretch for the
cumulene.15 Interestingly, 7 was found to decompose slowly at
room temperature over time, analogous to what Bielawski
noted for the amino-amido-ketenimines derived from MAAC
VI.17 This instability is presumably due to decreased electro-
philicity of both the MAAC VI and 3a when compared to the
DAC III (vide supra), which forms stable diamidoketenimines
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(Indianapolis, IN, USA). Melting point determinations were
preformed on a Mel-Temp apparatus and are uncorrected.
when coupled to isocyanides. This decreased electrophilicity
most likely results in the formation of a weaker π-bond upon
coupling to the isocyanide. Unfortunately, 3a displayed no
appreciable reactivity toward CO13,14 or ammonia.14 Reactions
with various olefins11,12,19 (e.g., dimethyl fumarate, cyclo-
hexene, or methyl acrylate) however result in decomposition
and not the anticipated [2+1] cycloaddition products.39
Currently, efforts are focused on elucidating the structure of
the products from these reactions.
Crystallography. All crystallographic measurements were carried
out on a Rigaku Mini CCD area detector diffractometer using
graphite-monochromated Mo KR radiation (R = 0.71073 Å) at 150 K
using an Oxford Cryostream low-temperature device. A sample of
suitable size and quality was selected and mounted onto a nylon loop.
Data reductions were performed using Crystal Clear Expert 2.0. The
structures were solved by direct methods, which successfully located
most of the non-hydrogen atoms. Subsequent refinements on F2 using
the SHELXTL/PC package (version 5.1)31 allowed location of the
remaining non-hydrogen atoms. Colorless, single crystals of 2b were
obtained by slow vapor diffusion of n-pentane into a dichloromethane
solution saturated with 2b. This compound cocrystallized in the
monoclinic space group P21/m with a highly disordered dichloro-
methane solvent molecule. Attempts to model this disorder were
unsatisfactory, and the contributions to the scattering due to the
solvent were removed using SQUEEZE in PLATON 98 and the
WinGX software. Yellow, single crystals of 4a were obtained by
layering a dichloromethane (wet) solution saturated with 4a with
hexanes and cooling to −30 °C for several days. This compound
crystallized in the monoclinic space group P21/c with one dichloro-
methane and three interstitial water molecules in the unit cell. Key
details of the crystal and structure refinement data are summarized in
Table S1. Further crystallographic details may be found in the
respective CIF files, which were deposited at the Cambridge
Crystallographic Data Centre, Cambridge, UK. The CCDC reference
numbers for 1a, 2b, and 4a were assigned as 881502, 881503, and
881504, respectively.
Synthesis of 1,3-Dimesityl-4-oxo-3,4-dihydropyrimidin-1-
ium-6-olate (1a). A 250 mL Schlenk flask was charged with malonic
acid (2.00 g, 19.20 mmol), N,N′-dimesitylformamidine (5.36 g, 19.20
mmol), and N,N′-dicyclohexylcarbodiimide (DCC) (7.92 g, 38.39
mmol). To this mixture was added enough CH2Cl2 (60 mL) to
dissolve all the solids. The solution stirred at room temperature for 3
h. After approximately 20 min, the solution became slightly warm and
changed to colorless, and a solid began to precipitate. The yellow
suspension was filtered after 3 h through a plug of Celite. The solid
filter cake was washed with an additional 10 mL of CH2Cl2. The
resulting yellow solution was concentrated in vacuo to approximately
10 mL and then slowly poured into diethyl ether (100 mL),
precipitating the product as a white solid. To ensure complete
precipitation, the ether suspension was then sonicated in an ultrasonic
bath for 30 min. The solid was separated from the ether solution via
filtration and was washed (3 × 10 mL) with diethyl ether to afford 1a
in analytically pure form (6.13 g, 73% yield): mp 305−308 °C (dec).
1H NMR (CDCl3, 400.13 MHz): δ 2.19 (s, 12H, Ar−CH3), 2.29 (s,
CONCLUSION
■
The first free amino-acrylamido carbene (3a) along with its
corresponding iridium complexes (3a-(cod)IrCl) 4a and (3a-
(CO)2IrCl) 5a have been prepared and characterized.
Determination of the TEP for 3a (derived from the IR spectra
of 5a) suggested that 3a exhibits a similar donating ability to
MAAC VI. This assessment was corroborated by DFT
computational analyses, which showed that both amino-
acrylamido carbene 3a and MAAC VI have similar frontier
orbitals. In this same vein, 3a was shown to engage in reactivity
intermediate of NHCs and DACs, but similar to that of MAAC
VI: the amino-acrylamido carbene was found to react with both
organic electrophiles (CS2) and nucleophilic isocyanides. The
latter reaction provided access to relatively unstable keteni-
mines. The instablility of ketenimine 7 can be rationalized via
comparison of the LUMO energies for 3a and DAC III, which
show that 3a is a much less electrophilic carbene than III,
resulting in the formation of a weaker CC π-bond. In
contrast, the amino-ureido carbene V has not been shown to
engage in any small-molecule reactivity to date. On the basis of
our DFT analyses of carbenes V, VI, and 3a we hypothesize
that localization of LUMO in V on the benzyl substituent as
opposed to the carbene nucleus as seen in VI and 3a may be
the source of this disparity in reactivity. In summary, our
findings suggest that small modifications in the backbone of
carbenes decorated with carbonyl functional groups can
drastically alter reactivity. We believe our findings provide
useful insight into the electronic properties of carbonyl-
functionalized carbenes and will ultimately serve to guide
researchers in the design of the next generation of electrophilic
carbenes.
EXPERIMENTAL SECTION
6H, Ar−CH3), 5.39 (s, 1H, CCH), 6.98 (s, 4H, Ar−CH), 8.00 (s,
1H, NCH−N). 13C NMR (CDCl3, 100.61 MHz): δ 18.11, 21.05,
86.15, 129.98, 132.41, 135.02, 140.78, 150.37, 159.62. Anal. Calcd (%)
for C22H24N2O2: C, 75.83; H, 6.94; N, 8.04. Found: C, 75.59; H, 7.04;
N, 8.23.
■
General Considerations. Unless noted otherwise, all procedures
were carried out using standard Schlenk techniques under an
atmosphere of dry nitrogen or in a nitrogen-filled glovebox. Solvents
were dried and degassed by an Innovative Technology solvent
purification system and stored over 3 Å molecular sieves in a nitrogen-
filled glovebox. Benzene-D6 (C6D6) was distilled from sodium−
potassium alloy (Na/K)/benzophenone, and deuterated chloroform
(CDCl3) was distilled over 3 Å molecular sieves. Unless otherwise
noted, all purchased chemicals were used as received. Malonic acid was
purchased from Oakwood Chemical Products; N,N′-dicyclohexylcar-
bodiimide, methyl trifluoromethanesulfonate (MeOTf), and sodium
bis-trimethylsilyl amide (NaHMDS) were purchased from ACROS.
Carbon monoxide was purchased from Airgas. N,N′-Dimesitylforma-
midine and N,N′-bis(2,6-diisopropylphenyl)formamidine were pre-
pared as previously described.40 1,5-Cyclooctadiene iridium(I)
chloride dimer ([(cod)IrCl]2) was prepared as previously described.41
IR data were obtained on a Bruker Tensor 27 infrared spectropho-
tometer. NMR spectra were recorded on a Bruker Avance III 400
MHz spectrometer. Chemical shifts (δ) are given in ppm and are
referenced to the residual solvent (1H: CDCl3, 7.24 ppm; C6D6, 7.15
ppm; 13C: CDCl3, 77.0 ppm; C6D6, 128.0 ppm) or CFCl3 for 19F (0
ppm). Elemental analyses were performed at Midwest Microlabs, LLC
1,3-Bis(2,6-diisopropylphenyl)-4-oxo-3,4-dihydropyrimidin-
1-ium-6-olate (1b). Following an identical procedure to that
described for 1a, malonic acid (1.43 g, 13.74 mmol), N,N′-bis(2,6-
diisopropylphenyl) formamidine (5.00 g, 13.74 mmol), and DCC
(5.67 g, 27.48 mmol) were used to prepare 1b as a white solid (2.38 g,
1
40.1% yield): mp 288 °C (dec). H NMR (CDCl3, 400.13 MHz): δ
3
3
1.19 (d, J = 8.0 Hz, 12H, CH(CH3)2), 1.28 (d, J = 4.0 Hz, 12H,
3
CH(CH3)2), 2.82 (sept. J = 8.0 Hz, 4H, CH(CH3)2), 5.35 (s, 1H,
CCH), 7.20−7.27 (m, 4H, Ar−CH), 7.42−7.46 (t, 3J = 8.0 Hz, 2H,
Ar−CH), 8.10 (s, 1H, NCH−N). 13C NMR (CDCl3, 100.61 MHz):
δ 24.23, 29.30, 85.65, 124.59, 131.59, 131.70, 145.93, 149.82, 159.92.
Anal. Calcd (%) for C28H36N2O2: C, 77.74; H, 8.39; N, 6.48. Found:
due to reasons unknown, we could not obtain clean elemental analyses
for this molecule. We do however have clean analysis for the
methylated form of this compound: see 2b below.
Synthesis of 1,3-Dimesityl-6-methoxy-4-oxo-3,4-dihydro-
pyrimidin-1-ium Triflate (2a). In a nitrogen-filled glovebox, methyl
trifluoromethanesulfonate (494 mg, 3.01 mmol) was added dropwise
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Article
at room temperature to a stirred solution of 1a (1.00 g, 2.87 mmol) in
15 mL of CH2Cl2. The solution was stirred at room temperature for 12
h and then concentrated in vacuo to approximately 5 mL. The
concentrated CH2Cl2 solution was then added dropwise into rapidly
stirred diethyl ether (60 mL), resulting in the precipitation of a
colorless solid, which was filtered and dried in vacuo to afford the
desired triflate salt, which was stored in the glovebox. (1.21 g, 82%
1.86 (s, 3H, Ar−CH3), 2.09 (s, 3H, Ar−CH3), 2.11 (s, 3H, Ar−CH3),
2.19 (s, 3H, Ar−CH3), 2.53 (s, 3H, Ar−CH3), 2.66 (s, 3H, Ar−CH3),
2.86 (s, 3H, O−CH3), 2.89−2.96 (m, 1H, cod-CH), 3.11−3.18 (m,
1H, cod-CH), 4.64 (bs, 2H, cod-CH), 5.08 (s, 1H, CCH), 6.68 (s,
1H, Ar−CH), 6.81 (s, 1H, Ar−CH), 6.84 (s, 1H, Ar−CH), 6.91 (s,
1H, Ar−CH). 13C NMR (C6D6, 100.61 MHz): δ 19.08, 19.63, 20.77,
20.84, 20.99, 28.01, 28.16, 30.17, 33.70, 53.99, 54.34, 55.97, 83.77,
85.13, 85.24, 130.30, 130.43, 134.36, 134.69, 135.70, 136.92, 138.09,
138.59, 138.60, 138.85, 160.58, 161.46, 216.09 (NCN). IR (KBr):
1699 cm−1. Anal. Calcd (%) for C33H42.667N2O2IrCl (4a·1/3(C6H14)):
C, 54.52; H, 5.92; N, 3.85. Found: C, 54.34; H, 5.76; N, 4.04.
Synthesis of (1,3-Dimesityl-6-methoxy-4-oxo-3,4-dihydro-
pyrimidin-2-ylidene)iridium(I) (dicarbonyl) Chloride (5a). In a
10 mL round-bottom flask, 4a (100 mg, 0.143 mmol) was dissolved in
5 mL of CH2Cl2 and the flask was capped with a rubber septum. A
balloon filled with carbon monoxide and fitted to a plastic syringe was
then introduced to the reaction mixture through the use of a long
needle. Carbon monoxide was bubbled through the solution at
ambient temperature until the volume of the dichloromethane had
reduced to approximately 1 mL. Hexanes (8 mL) were then
introduced to the flask and CO was again bubbled through the
solution until a yellowish solid began to form. The solid was isolated
via filtration, washed with cold (−30 °C) hexanes (3 × 2 mL), and
dried in vacuo to afford 5a as a yellow solid (87 mg, 94% yield): mp
190−195 °C. 1H NMR (C6D6, 400.13 MHz): δ 1.96 (s, 3H, Ar−CH3),
2.04 (s, 3H, Ar−CH3), 2.12 (s, 3H, Ar−CH3), 2.34 (s, 3H, Ar−CH3),
2.36 (s, 3H, Ar−CH3), 2.50 (s, 3H, Ar−CH3), 2.61 (s, 3H, O−CH3),
5.15 (s, 1H, CCH), 6.73 (s, 2H, Ar−CH), 6.84 (s, 2H, Ar−CH).
13C NMR (C6D6, 100.61 MHz): δ 18.27, 18.50, 19.73, 19.80, 20.93,
1
yield): mp 205−207 °C. H NMR (CDCl3, 400.13 MHz): δ 2.14 (s,
6H, Ar−CH3), 2.17 (s, 6H, Ar−CH3), 2.32 (s, 3H, Ar−CH3), 2.33 (s,
3H, Ar−CH3), 4.05 (s, 3H, O−CH3), 6.08 (s, 1H, CCH), 7.03 (bs,
4H, Ar−CH), 10.01 (s, 1H, NCH−N). 13C NMR (CDCl3, 100.61
MHz): δ 17.64, 20.62, 59.16, 90.45, 129.25, 129.62, 130.10, 130.24,
134.00, 134.34, 141.70, 142.34, 156.87, 157.61, 160.28. 19F NMR
(CDCl3, 376.461 MHz): δ −78.9. IR (KBr): 1738 cm−1. Anal. Calcd
(%) for C24H27N2O5SF3: C, 56.24; H, 5.31; N, 5.47. Found: C, 56.24;
H, 5.28; N, 5.62.
Synthesis of 1,3-Bis(2,6-diisopropylphenyl)-6-methoxy-4-
oxo-3,4-dihydropyrimidin-1-ium Triflate (2b). Following an
identical procedure to that described for 2a, 1b (250 mg, 0.578
mmol) and methyl triflate (190 mg, 1.16 mmol) were used to prepare
1
2b as a white solid. (307 mg, 89% yield): mp 230−232 °C. H NMR
(CDCl3, 400.13 MHz): δ 1.24−1.29 (m, 24H, CH(CH3)2), 2.58 (sept.
3J = 8.0 Hz, 4H CH(CH3)2), 4.13 (s, 3H, O−CH3), 6.33 (s, 1H, C
CH), 7.31−7.34 (m, 4H, Ar−CH), 7.54−7.57 (m, 2H, Ar−CH), 9.99
(s, 1H, NCH−N). 13C NMR (CDCl3, 100.61 MHz): δ 23.47, 24.10,
29.58, 59.22, 90.86, 124.90, 125.23, 128.56, 128.92, 132.18, 132.72,
144.68, 145.00, 155.89, 157.29, 160.61. 19F NMR (CDCl3, 376.461
MHz): δ −79. IR (KBr): 1739 cm−1. Anal. Calcd (%) for
C30H39N2O5SF3: C, 60.39; H, 6.59; N, 4.69. Found: C, 60.44; H,
6.66; N, 4.85.
21.05, 30.16, 56.41, 86.33, 129.22, 129.32, 130.19, 130.35, 133.99,
134.28, 136.87, 137.02, 139.42, 140.11, 159.49, 161.05, 169.95 (Ir-
CO), 178.72 (Ir-CO), 206.44 (NCN). IR (CH2Cl2): νCO 2069.30,
1977.75 cm−1. Anal. Calcd (%) for C25H26N2O4IrCl: C, 46.47; H,
4.06; N, 4.34. Found: C, 46.73; H, 4.22; N, 4.21.
Synthesis of 1,3-Dimesityl-6-methoxy-4-oxo-3,4-dihydro-
pyrimidin-2-ylidene (3a). A 10 mL vial was charged with 2a (50
mg, 0.098 mmol) and NaHMDS (17 mg, 0.093 mmol). To this
mixture was added 0.5 mL of C6D6. The resulting suspension was
stirred for 10 min and then filtered through a 0.2 μm PTFE filter into
an NMR tube to remove precipitated NaOTf. The free carbene was
Synthesis of 1,3-Dimesityl-6-methoxy-4-oxo-3,4-dihydro-
pyrimidin-1-ium-2-carbodithioate (6). A 20 mL vial was charged
with 2a (100 mg, 0.196 mmol) and NaHMDS (34 mg, 0.186 mmol).
To this mixture was added 5 mL of toluene. The resulting suspension
was stirred for 10 min and then filtered through a 0.2 μm PTFE filter
into another 20 mL vial. A large excess of carbon disulfide (0.5 mL)
was then added. An instantaneous color change from pale yellow to
red-orange was observed. The resulting solution was stirred at room
temperature for 2 h, and through the course of the reaction a brown
solid precipitated. The solid was isolated via filtration, dissolved in
dichloromethane (2 mL), and then precipitated into hexanes. The
solid was isolated via filtration, washed with hexanes (3 × 10 mL), and
dried in vacuo to afford 6 as a brown-red solid (82.2 mg, 96% yield):
1
then characterized only by H and 13C NMR (yield was >95% based
1
1
on H NMR). H NMR (C6D6, 400.13 MHz): δ 2.03 (s, 6H, Ar−
CH3), 2.08 (s, 3H, Ar−CH3), 2.19 (s, 3H, Ar−CH3), 2.29 (s, 6H, Ar−
CH3), 2.76 (s, 3H, O−CH3), 5.34 (s, 1H, CCH), 6.75 (s, 2H, Ar−
CH), 6.87 (s, 2H, Ar−CH). 13C NMR (C6D6, 100.61 MHz): δ 17.66,
18.30, 21.04, 55.34, 85.65, 129.34, 129.76, 129.87, 134.64, 134.82,
136.45, 136.85, 137.01, 137.74, 138.46, 139.96, 159.21, 160.68, 257.25
(NCN).
Deprotonation of 2b Using NaHMDS: Synthesis of 3b. A 10
mL vial was charged with 2b (58.6 mg, 0.098 mmol) and NaHMDS
(17.1 mg, 0.093 mmol). To this mixture was added 0.5 mL of C6D6.
The resulting suspension was stirred for approximately 30 s and then
filtered through a 0.2 μm PTFE filter into an NMR tube. 1H NMR was
1
mp 209−211 °C. H NMR (CDCl3, 400.13 MHz): δ 2.25 (m, 6H,
Ar−CH3), 2.34 (m, 12H, Ar−CH3), 3.97 (s, 3H, O−CH3), 5.92 (s,
1H, CCH), 6.85 (s, 4H, Ar−CH). 13C NMR (CDCl3, 100.61
MHz): δ 18.65, 18.76, 18.91, 19.43, 21.13, 30.92, 58.71, 86.02, 129.31,
130.78, 135.65, 135.89, 136.17, 136.50, 140.32, 140.99, 154.54, 158.96
(CS2 ), 161.05, 220.04 (NCN). Anal. Calcd (%) for
C25.25H28.5N2O2S2Cl2.5 (6·1.25 CH2Cl2): C, 55.67; H, 5.27; N, 5.14.
Found: C, 55.21; H, 5.08; N, 5.28.
1
collected immediately. The H NMR data suggest deprotonation of
1
the C4 position to yield a zwitterionic bent allene. Only H NMR
could be obtained on this compound. 1H NMR (C6D6, 400.13 MHz):
δ 1.10−1.39 (m, 24H, CH(CH3)2), 3.22 (sept., 3J = 8.0 Hz, 4H,
CH(CH3)2), 3.50 (s, 3H, O−CH3), 7.10−7.20 (m, 6H, Ar−CH), 9.02
(s, 1H, NCH−N).
Synthesis of 2-((tert-Butylimino)methylene)-1,3-dimesityl-6-
methoxy-3,4-dihydropyrimidin-4-one (7). A 20 mL vial was
charged with 2a (100 mg, 0.196 mmol) and NaHMDS (34 mg, 0.186
mmol). To this mixture was added 5 mL of toluene. The resulting
suspension was stirred for 10 min and then filtered through a 0.2 μm
PTFE filter into another 20 mL vial. To the resulting yellow-orange
solution was added tert-butyl isocyanide (17.8 mg, 0.215 mmol).
Immediately, the solution changed color to dark red. The red solution
was stirred at room temperature for 2 h, and the solvent was then
removed in vacuo. The dark residue was then washed with cold (−30
°C) hexanes (2 × 5 mL) and then dried in vacuo to afford the
ketenimine as a yellow solid (61.8 mg, 71% yield): mp 158−160 °C
Synthesis of (1,3-Dimesityl-6-methoxy-4-oxo-3,4-dihydro-
pyrimidin-2-ylidene)iridium(I) (1,5-cyclooctadiene) Chloride
(4a). A 20 mL vial was charged with 2a (300 mg, 0.586 mmol) and
NaHMDS (107 mg, 0.584 mmol). To this mixture was added 15 mL
of toluene. The solution was allowed to stir at room temperature for
10 min. The reaction mixture was then filtered through a 0.2 μm PTFE
syringe filter into another 20 mL vial containing [(cod)IrCl]2 (197 mg,
0.293 mmol). The resulting brick red solution was stirred for 4 h and
then passed over a plug of neutral aluminum oxide. The aluminum
oxide was then washed with 10 mL of toluene. The desired product
was then eluted off of the aluminum oxide with dichloromethane (3 ×
10 mL). All the dichloromethane fractions were combined, and the
solvent was removed in vacuo to afford the compound as a yellow solid
1
(dec). H NMR (C6D6, 400.13 MHz): δ 0.59 (s, 9H, C(CH3)3), 1.98
1
(213 mg, 52% yield): mp 202 °C (dec). H NMR (C6D6, 400.13
(s, 3H, Ar−CH3), 2.07 (s, 3H, Ar−CH3), 2.30 (s, 3H, Ar−CH3), 2.33
(s, 3H, Ar−CH3), 2.47 (s, 3H, Ar−CH3), 2.53 (s, 3H, Ar−CH3), 2.89
MHz): δ 1.23−1.30 (m, 4H, cod-CH2), 1.42−1.48 (m, 4H, cod-CH2),
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Article
(s, 3H, O−CH3), 4.56 (s, 1H, CCH), 6.65 (s, 2H, Ar−CH), 6.75 (s,
2H, Ar−CH). 13C NMR (C6D6, 100.61 MHz): δ 17.79, 17.86, 18.04,
18.35, 20.78, 20.93, 27.53, 28.63, 55.76, 59.92, 69.39, 118.48 (CC
N), 129.15, 129.22, 129.31, 132.60, 133.80, 136.37, 136.45, 137.64,
138.15, 138.42, 138.56, 162.02, 162.39, 206.67 (CCN). IR (KBr):
νCO 2025 cm−1. Anal. Calcd (%) for C28H35N3O2: C, 75.47; H, 7.92;
N, 9.43. Found: C, 75.08; H, 7.93; N, 9.39.
(16) Hudnall, T. W.; Tennyson, A. G.; Bielawski, C. W.
Organometallics 2010, 29, 4569−4578.
(17) Blake, G. A.; Moerdyk, J. P.; Bielawski, C. W. Organometallics
2012, 31, 3373−3378.
(18) Braun, M.; Frank, W.; Reiss, G. J.; Ganter, C. Organometallics
2010, 29, 4418−4420.
(19) Braun, M.; Frank, W.; Ganter, C. Organometallics 2012, 31,
1927−1934.
ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Siemeling, U.; Farber, C.; Bruhn, C.; Leibold, M.; Selent, D.;
Baumann, W.; von Hopffgarten, M.; Goedecke, C.; Frenking, G. Chem.
Sci. 2010, 1, 697−704.
■
S
(21) Hildebrandt, B.; Frank, W.; Ganter, C. Organometallics 2011, 30,
3483−3486.
(22) Makhloufi, A.; Frank, W.; Ganter, C. Organometallics 2012, 31,
2001−2008.
AUTHOR INFORMATION
Corresponding Author
*E-mail: hudnall@txstate.edu.
■
(23) Ces
361−365.
(24) For examples of monoamido-amino carbenes prepared from
anionic carbenes, see: (a) Benhamou, L.; Vujkovic, N.; Cesar, V.;
Gornitzka, H.; Lugan, N. l.; Lavigne, G. Organometallics 2010, 29,
2616−2630. (b) Cesar, V.; Tourneux, J.-C.; Vujkovic, N.; Brousses, R.;
́
ar, V.; Lugan, N.; Lavigne, G. Eur. J. Inorg. Chem. 2010,
Notes
́
The authors declare no competing financial interest.
́
ACKNOWLEDGMENTS
Lugan, N.; Lavigne, G. Chem. Commun. 2012, 48, 2349−2351.
■
(c) Biju, A. T.; Hirano, K.; Frohlich, R.; Glorius, F. Chem. Asian J.
̈
We are grateful to the Research Corporation for Science
Advancements (20092), the National Science Foundation for
MRI awards for the purchase of a Rigaku SCX-Mini XRD
(CHE-0821254) and for the upgrade of our NMR (CHE-
0946998), and Texas State University for their generous
support. We would also like to thank Dr. Vincent M. Lynch
(University of Texas at Austin) for help with crystal structure
refinement and Dr. Christopher L. Dorsey for insightful
discussions regarding the manuscript.
2009, 4, 1786−1789.
(25) For an example of an amino-acrylamido carbene prepared in the
́
coordination sphere of a transition metal see: Cesar, V.; Lugan, N.;
Lavigne, G. Chem.−Eur. J. 2010, 16, 11432−11442.
(26) Hobbs, M. G.; Forster, T. D.; Borau-Garcia, J.; Knapp, C. J.;
Tuononen, H. M.; Roesler, R. New J. Chem. 2010, 34, 1295−1308.
́
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(28) For other examples of anionic carbenes with a malonate
backbone see: Hobbs, M. G.; Knapp, C. J.; Welsh, P. T.; Borau-
Garcia, J.; Ziegler, T.; Roesler, R. Chem.−Eur. J. 2010, 16, 14520−
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