Exploring the nucleophilicity of N,N′-diamidocarbenes: Heteroallenes and related compounds as coupling reagents?

Article abstract of DOI:10.1002/poc.3004

The propensity of a stable N,N′-diamidocarbene (DAC) to react with various heteroallenes was explored. The DAC condensed with 4-nitrophenyl azide as well as 4-nitrophenyl isothiocyanate to afford the respective acyclic triazene or zwitterionic dihydropyrimidinium imidothiolate products. Treating the DAC with two equivalents of 3-nitrophenyl isocyanate afforded an iminooxazolidinone rather than the expected hydantoin. Similarly, the addition of diphenylketene to the DAC afforded a dioxolane product, whereas analogous reactions involving N-heterocyclic carbenes (NHCs) resulted in betaine adducts. Although the DAC as well as various diaminocarbenes condensed with 2,6-dimethoxyphenyl nitrile N-oxide to afford the respective nitrosoethylenes, only the six-membered DAC and NHC adducts rearranged to the corresponding nitrones at elevated temperature. Copyright

Full text of DOI:10.1002/poc.3004

Special Issue Paper
Received: 27 April 2012,
Revised: 18 June 2012,
Accepted: 21 June 2012,
Published online in Wiley Online Library:
(wileyonlinelibrary.com) DOI: 10.1002/poc.3004
Exploring the nucleophilicity of N,N0-
diamidocarbenes: Heteroallenes and related
compounds as coupling reagents†
Young-Gi Leea, Jonathan P. Moerdyka and Christopher W. Bielawskia*
The propensity of a stable N,N0-diamidocarbene (DAC) to react with various heteroallenes was explored. The DAC
condensed with 4-nitrophenyl azide as well as 4-nitrophenyl isothiocyanate to afford the respective acyclic triazene
or zwitterionic dihydropyrimidinium imidothiolate products. Treating the DAC with two equivalents of 3-nitrophenyl
isocyanate afforded an iminooxazolidinone rather than the expected hydantoin. Similarly, the addition of diphenyl-
ketene to the DAC afforded a dioxolane product, whereas analogous reactions involving N-heterocyclic carbenes (NHCs)
resulted in betaine adducts. Although the DAC as well as various diaminocarbenes condensed with 2,6-dimethoxyphenyl
nitrile N-oxide to afford the respective nitrosoethylenes, only the six-membered DAC and NHC adducts rearranged to
the corresponding nitrones at elevated temperature. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper
Keywords: diamidocarbene; N-heterocyclic carbene; dimethoxycarbene; triazene; imidothioate; oxazolidinone; nitrile oxide;
nitrone; ketene; heteroallene
Herein, we explore the nucleophilicity of a DAC (i.e., 1; Fig. 1)
INTRODUCTION
by evaluating its propensity to couple to various heteroallenes,
including azides, isothiocyanates, isocyanates, and ketenes.[50] In
As one of the most widely utilized class of carbenes, N-heterocyclic
carbenes (NHCs; e.g., SIMes and IMes; Fig. 1)[1–4] are known to be-
some cases, analogous experiments involving various NHCs (SIMes,
IMes, and/or 6Mes) were conducted in parallel for comparative
purposes. In addition, we report the use of nitrile N-oxides,[51–53]
linear functional groups that possess a heteroallene resonance
structure and frequently utilized in 1,3-dipolar cycloaddition chem-
istry,[51–60] as electrophilic coupling partners for carbenes. Beyond
enhancing our understanding of the nucleophilicity of DACs, we
sought to expand heteroallene–carbene coupling chemistry to
include six-membered systems, which have been largely absent
from previous studies.[18]
have as strong nucleophiles toward various organic and inorganic
substrates. Indeed, this property enables NHCs to function as cata-
lysts in many important transformations.[5,6] In comparison, relatively
little is known about the nucleophilic characteristics of the N,N0-
diamidocarbenes (DACs, e.g., 1; Fig. 1),[7–17] which are an emerging
class of stable carbenes. Although DACs have been shown to partic-
ipate in transformations more commonly associated with electro-
philic (e.g., methylene) or other transient (e.g., dimethoxycarbene)
carbenes – including C–H and N–H bond insertions,[7,10] [2+ 1]
cycloadditions,[15–17] and the coupling of carbon monoxide or
isonitriles[7,9,10,12,13] – the nucleophilicity of DACs, particularly
towards organic electrophiles, remains relatively unexplored.
One method for evaluating the nucleophilicity of carbenes
is to explore their abilities to couple to heteroallenes. Such
coupling reactions have been previously studied with NHCs[18]
and generally proceed via the nucleophilic attack of the carbene
on the electrophilic substrate. When the heteroallene contains a
central carbon atom (Fig. 2, top), betaine products are obtained
and often used as 1,3-dipolar cycloaddition partners in the prepa-
ration of valuable heterocyclic products (e.g., hydantoins).[19–22]
For heteroallenes containing centrally positioned, positively
charged nitrogen atoms, the NHC couples to afford delocalized
zwitterions[23,24] or 1,3-heterodienes (Fig. 2, bottom).[25–36]
These reactions are generally rapid, proceed with complete atom
economy, and give high yields of products that are attractive for
the synthesis of heteroatom rich compounds.[18,37] Moreover,
such carbene–heteroallene coupling reactions have been used
in applications that range from small molecule activation[23,24,38]
and catalysis[39–41] to the synthesis of novel polymers[25,42] and
heterocycles.[19–22,26,37,43–49]
RESULTS AND DISCUSSION
Building on our previous studies with NHCs,[27–29,32,33] initial
efforts were directed toward exploring the chemistry of DACs
and organic azides. Mixing 1 with an equimolar amount of
4-nitrophenyl azide in C6H6 ([1]0 = 0.1 M) at room temperature
* Correspondence to: Christopher W. Bielawski, Department of Chemistry and
Biochemistry, University of Texas at Austin, Austin, TX 78712, USA.
E-mail: bielawski@cm.utexas.edu
†
This article is published in the Journal of Physical Organic Chemistry as a
special issue on the 11th Latin American Conference on Physical Organic
Chemistry, edited by Prof. Eusebio Juaristi (Centro de Investigacion y de Estudios
Avanzados del Instituto Politecnico Nacional Quimica, Apartado Postal 14-740,
Mexico 07000, Mexico).
a Y.-G. Lee, J.P. Moerdyk, C.W. Bielawski
Department of Chemistry and Biochemistry, University of Texas at Austin,
Austin, TX, 78712, USA
J. Phys. Org. Chem. (2012)
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Y.-G. LEE, J. P. MOERDYK AND C. W. BIELAWSKI
the crude product from CHCl3/pentane, and the structural assign-
ment was verified via single crystal X-ray diffraction (XRD) analysis
(Fig. 3). Inspection of the solid state data revealed bond lengths
and bond angles (e.g., a wide Nimido–C–S angle of 136.3(2)ꢁ)
that were similar to analogous NHC-based imidothioates.[62,63]
As with other NHC–isothiocyanate adducts, the formation of 3
appeared to be reversible: exposure of 3 to excess elemental
sulfur for 48 h at 100 ꢁC (C7D8) afforded the isothiocyanate
starting material as well as the known[8] thiourea derivative of
1 (72% conversion).[40,42]
Figure 1. Structure of stable N-heterocyclic carbenes (SIMes, IMes, and
6Mes) and a N,N0-diamidocarbene (1). Mes = 2,4,6-trimethylphenyl
Having found that DAC 1 coupled to azides and isothiocyanates
in a manner similar to NHCs, our attention shifted to exploring the
use of isocyanates as coupling partners. Spirohydantoins are
known to form upon the addition of two equivalents of an isocy-
Figure 2. Generalized examples of the known chemistry of N-heterocyclic
carbenes (NHCs) with heteroallenes containing either centralized (top)
carbon atoms or (bottom) nitrogen atoms. X = CR2, NR, O, S; Y = CR2, NR,
O, S. Z = CR2, NR, O
anate to 1,3-diphenyl-4,5-dihydroimidazol-2-ylidene (SIPh)[19,20,22]
,
whereas isocyanurates are observed in the presence of more
electron rich NHCs, such as SIMes.[39,40] Adding two equivalents of
3-nitrophenylisocyanate to a solution of 1 in benzene immediately
resulted in the formation of a red-orange solution that became
colorless after stirring for an additional 2 h at room temperature.
Removal of the residual solvent afforded a white, crystalline solid
which, upon 1H NMR analysis (CDCl3), was determined to be a 2:1
isocyanate/DAC adduct. While the structure of the product was
initially believed to be a hydantoin (Scheme 3), XRD revealed that
the isolated compound was the 2-imino-5-oxazolindinone 4
(Fig. 4).[64]
for 1 h resulted in the near quantitative conversion to 2
(Scheme 1), as determined by 1H NMR spectroscopy. Subsequent
purification by column chromatography afforded pure 2 as
an orange solid in excellent yield (96%). Additional structural
support was obtained from mass spectrometry as well as Fourier
transform infrared (FT-IR) spectroscopy, which revealed signals at
1387 and 1339 cmꢀ1 that were diagnostic of triazene as well as
nitro groups. Moreover, these spectroscopic data were consistent
with those obtained when IMes[27,28,32] and benzimidazolylidenes[29]
were coupled to aryl azides.[61]
We postulated that the selectivity of the aforementioned
reaction to form 4 (as opposed to the isomeric 5) was driven
by sterics. To test this hypothesis, 6Mes, a stable NHC with steric
Subsequent efforts were directed toward analyzing the reaction
of 1 with isothiocyanates, substrates that typically afford
imidothioates when exposed to NHCs.[44–49] Stirring a C6H6
solution of 1 with an equimolar amount of 4-nitrophenyl
isothiocyanate at room temperature for 1 h ([1]0 = 0.13 M)
resulted in the formation of dihydropyrimidinium imidothioate
3 (Scheme 2), as evidenced by the FT-IR carbonyl stretching
frequencies observed at 1769 and 1740 cmꢀ1 and the 1:1
1
DAC:isothiocyanate stoichiometry gleaned from H NMR spec-
troscopic analysis of the crude reaction mixture. Pure 3 was
isolated in high yield (96%) as a red-orange solid by recrystallizing
Figure 3. ORTEP diagram of 3 with thermal ellipsoids drawn at 50%
probability and H-atoms omitted for clarity. Selected distances (Å)
and angles (ꢁ): C1–C25, 1.494(4); C25–S1, 1.699(3); C25–N3, 1.280(4);
N3–C25–S1, 136.3(2); N3–C25–C1, 118.5(2); C1–C25–S1, 104.83(19)
Scheme 1. The reaction of 1 with 4-nitrophenyl azide resulted in the
formation of triazene 2. Isolated yield is given. R = 4-nitrophenyl
Scheme 3. Treatment of 1 with 3-nitrophenyl isocyanate resulted in the
spiro 2-imino-4-oxazolidinone 4, rather than hydantoin 5. Isolated yield is
given. R = 3-nitrophenyl
Scheme 2. Addition of 4-nitrophenyl isothiocyanate to 1 afforded the
zwitterionic imidothioate 3. Isolated yield is given. R = 4-nitrophenyl
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J. Phys. Org. Chem. (2012)

DIAMIDOCARBENES AND HETEROALLENES
benzene and pentane. The structure of this compound was
later confirmed by XRD analysis (Fig. 5).[71]
Although Hoffmann[72] reported that the analogous dioxolane
product was obtained by coupling dimethoxycarbene with
diphenylketene, the chemistry between NHCs and ketenes has
been limited to the synthesis of cyclopentenone derivates via
the formal [4 + 1] cycloaddition of SIPh with vinyl ketenes.[73]
To broaden this chemistry, IMes, SIMes, and 6Mes were also
reacted with diphenylketene under identical conditions to those
described earlier (Scheme 5). Addition of diphenylketene to a
benzene solution of IMes resulted in the immediate precipitation
of a bright orange solid that was subsequently collected by filtra-
1
tion. Analysis of this material by H NMR spectroscopy (CDCl3)
revealed that an adduct with 1:1 IMes:ketene stoichiometry had
formed. With these spectroscopic data as well as the absence of
13C NMR signals expected for an N,N-substituted carbon within
an oxirane in hand,[15] the structure of the product was assigned
as the zwitterion 7 (93% yield). When SIMes and 6Mes were
treated with diphenylketene, the betaine adducts 8 and 9, respec-
tively, which displayed similar spectroscopic signatures as 7, were
Figure 4. ORTEP diagram of 4 with thermal ellipsoids drawn at 50%
probability and H-atoms omitted for clarity. Selected distances (Å)
and angles (ꢁ): C1–C25, 1.544(3); C25–O3, 1.199(2); C25–N3, 1.367(3);
N3–C32, 1.395(3); C32–N4, 1.256(3); C32–O4, 1.358(2); O4–C1, 1.478(2);
O4–C1–C25, 101.80(17); N4–C32–N3, 123.25(18); N4–C32–O4, 128.34
(18); N3–C32–O4, 108.41(16)
properties similar to those of DAC 1,[14] was combined with two
equivalents of 3-nitrophenylisocyanate in C6D6, which resulted in
the nearly instantaneous formation of a yellow precipitate. After
stirring for an additional 2 h at room temperature, filtration of the
reaction mixture followed by washing with toluene and chloro-
form facilitated the isolation of a pale yellow solid (44% yield)
that was identified as the isocyanate trimer on the basis of
the mass spectrometric data collected; additional structural
support was obtained by NMR spectroscopy (see the Supporting
Information). In contrast, even when 1 was stirred in the pres-
ence of a large excess of isocyanate (10 equivalents), the forma-
tion of the corresponding isocyanurate was not observed. With
these results, we believe that the propensity of 1 to form an
iminooxazolidinone upon exposure to an isocyanate, in lieu of
catalyzing an isocyanate cyclooligomerization reaction or
forming a hydantoin, was driven by electronic effects.[65,66]
Regardless, the aforementioned reaction constitutes the first
example of selectively synthesizing an iminooxazolidinone, a
pharmacologically relevant heterocyclic core,[67] instead of a
hydantoin, which is frequently obtained from the introduction
of a carbene (e.g., SIPh[19,20,22] or dimethoxycarbene[21,68,69]) to
an isocyanate.[70]
Figure 5. ORTEP diagram of 6 with thermal ellipsoids drawn at 50%
probability. H-atoms are omitted and the mesityl rings represented as
lines for clarity. Selected distances (Å) and angles (ꢁ): C1–C25, 1.549(3);
C25–O4, 1.379(3); C25–C26, 1.348(3); O4–C39, 1.358(3); C39–C40, 1.334(3);
C39–O3, 1.360(3); O3–C1, 1.445(2); O3–C1–C25, 100.54(15); C1–C25–O4,
105.73(16); C25–O4–C39, 110.99(16); O4–C39–O3, 109.38(17); C39–O3–C1,
110.48(15)
To further compare the nucleophilic reactivity of DACs to
other carbenes, 1 was treated with two equivalents of diphenyl-
ketene in C6D6 ([1]0 = 0.1 M) for 1 h (Scheme 4). 1H and 13C NMR
spectroscopic analysis of the crude reaction mixture revealed the
absence of signals diagnostic of ketones and the formation of an
adduct with a 2:1 ketene:DAC stoichiometry. On the basis of
these data, the product was assigned as the dioxolane 6
and isolated in good yield (86%) upon recrystallization from
Scheme 4. Addition of diphenylketene to 1 afforded the substituted
1,3-dioxolane 6. Isolated yield is given.
Scheme 5. Treatment of IMes, SIMes, or 6Mes with diphenylketene
afforded betaines 7–9, respectively. Isolated yields are given.
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Y.-G. LEE, J. P. MOERDYK AND C. W. BIELAWSKI
obtained (92–98% yield).[74] The structure of 9 was unambigu-
ously determined via XRD analysis (Fig. 6).[66,75]
atoms in the heterocycle and the nitroso group even though this
orientation was expected to be sterically less favored than an
orthogonal arrangement (i.e., 90ꢁ torsional angles).[78] Although
stable under ambient conditions, 10 quantitatively isomerized to
a new product upon heating to 100 ꢁC in C7D8 for 1 h, as observed
by 1H NMR spectroscopy. XRD analysis identified the thermally
rearranged compound as the nitrone[79] 11 (Fig. 7, right). The
isomerization of 10 to 11 involves the transfer of the bulky mesityl
group from the amido nitrogen to the nitroso nitrogen atom as
well as the formation of an inner salt. Given that nitroso groups
are strongly electron withdrawing,[77] we believe that the reso-
nance isomer 100 facilitated the intramolecular substitution reac-
tion (refer to the following paragraphs for additional discussion
of this transformation).
Having successfully coupled 1 with a variety of heteroallenes,
we sought to explore the chemistry between NHCs as well DAC 1
and nitrile N-oxides. Treatment of 1 with 2,6-dimethoxyphenyl
nitrile N-oxide (DMPNO) in benzene at room temperature for
10 min followed by precipitation into pentane afforded the
1,1-diamido-2-nitrosoethylene 10 (Scheme 6, 95% yield), a green
solid that displayed diagnostic IR absorbances at 1477 and
1545 cmꢀ1 (KBr).[76,77] Subsequent XRD analysis enabled unambig-
uous structure elucidation (Fig. 7, left). Acute N1–C1–C25–C26 and
N2–C1–C25–N3 dihedral angles (22.6ꢁ and ꢀ21.3ꢁ, respectively)
reflected the high degree of conjugation between the nitrogen
In parallel, IMes, SIMes, and 6Mes were also independently
treated with DMPNO under conditions otherwise identical to
those described earlier. Filtration of the resultant precipitates
followed by washing with hexanes afforded the corresponding
1,1-diamino-2-nitrosoethylene products (12–14, Scheme 7) in
high yield (91–94%), as evidenced by the diagnostic IR absor-
bances observed at 1471–1473 and 1586–1592 cmꢀ1 (KBr) among
other spectroscopic data collected (see Supporting Information).[80]
Unambiguous structural assignments were obtained for 13 and
14 via XRD analysis (see Supporting Information). Notably, the
nitroso N–O distances measured in the solid state structures of
13 and 14 (1.295 and 1.296 Å, respectively) were significantly
elongated compared with the analogous distance measured in
the solid state structure of 10 (1.240Å) as were the Ccarbenoid–Cvinyl
bonds (1.424 and 1.448 Å versus 1.386Å, respectively). Combined
with shorter Nnitroso–Cvinyl as well as Namido–Ccarbenoid distances in
13 and 14, as compared with 10, these data may reflect a stronger
donation of the heterocyclic nitrogen atoms from the carbene
derived component into the nitroso group.
Figure 6. ORTEP diagram of 9 with thermal ellipsoids drawn at 50%
probability and H-atoms omitted for clarity. Selected distances (Å)
and angles (ꢁ): C1–C23, 1.550(4); C23–O1, 1.289(4); C23–C24, 1.375(4);
C24–C23–O1, 127.3(3); C24–C23–C1, 124.7(3); C1–C23–O1, 107.5(3)
Scheme 6. The addition of 2,6-dimethoxyphenyl nitrile N-oxide to 1 resulted in the formation of the 1,1-diamido-2-nitrosoethylene 10, which
thermally rearranged to the isomeric nitrone 11. Isolated yields are given. R = 2,6-dimethoxyphenyl
Figure 7. (Left) ORTEP diagram of 10 with thermal ellipsoids drawn at 50% probability and H-atoms omitted for clarity. Selected distances (Å) and
angles (ꢁ): N1–C1, 1.4001(19); N2–C1, 1.4047(18); C1–C25, 1.387(2); C25–N3, 1.398(2); N3–O3, 1.2400(17); C1–C25–N3, 117.03(13); C25–N3–O3, 114.52
(12). (Right) ORTEP diagram of 11 with thermal ellipsoids drawn at 50% probability and H-atoms omitted for clarity. Selected distances (Å) and angles
(ꢁ): N1–C1, 1.399(3); N2–C1, 1.289(2); C1–C25, 1.486(3); C25–N3, 1.323(2); N3–O3, 1.287(2); N3–C16, 1.462(2); C1–C25–N3, 121.21(16); C25–N3–O3, 121.87
(16); O3–N3–C16, 113.61(15); C16–N3–C25, 124.45(16)
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DIAMIDOCARBENES AND HETEROALLENES
Scheme 7. (Top) Treatment of IMes or SIMes with 2,6-dimethyoxyphenyl nitrile N-oxide (DMPNO) resulted in the 1,1-diamino-2-nitrosoethylene
adducts 12 or 13, respectively, which upon heating to 100–180 ꢁC for 16 h in C7H8 or DMSO-d6 resulted in decomposition. (Bottom) The reaction of
6Mes and DMPNO afforded the 1,1-diamino-2-nitrosoethylene 14, which upon heating to 100 ꢁC in C7H8 rearranged to the nitrone 15. Isolated yields
are given. R = 2,6-dimethoxyphenyl
Unlike 10, heating 12 or 13 to elevated temperatures (100 ꢁC
in C7H8 and 180 ꢁC in DMSO-d6, respectively) for extended
periods (16 h) resulted in decomposition rather than rearrange-
isomeric nitrones at elevated temperatures. Collectively, these
observations indicate that despite being more electrophilic
than typical NHCs, the nucleophilic character of DACs is not
compromised in coupling reactions that involve electrophilic
heteroallenes. Thus, we conclude the DAC is ambiphilic and
capable of acting as both a good electrophile and a good
nucleophile.
1
ment to the corresponding nitrone, as determined by H NMR
spectroscopy.[81,82] However, heating 14 to 100 ꢁC for 1 h in
C7D8 afforded the nitrone 15 (79%), as indicated by downfield
NMR chemical shifts of the aryl peaks and other spectroscopic
data, including XRD (Supporting Information). Rearrangement
of the nitrosoethylenes derived from the six-membered carbenes,
which feature relatively wide N–C–N bond angles when compared
with their five-membered analogs, suggested to us that the
proximity of the nitroso nitrogen and the mesityl ring may facil-
itate the rearrangement. Support for this hypothesis was found
in the crystallographic data. The Nnitroso–Cipso distances in the
solid state structures of 10 (2.611 Å) and 14 (2.762 Å) were
measured to be considerably shorter than that found in 13
(2.862Å). As such, we propose that the transformation proceeds
via attack of the nitroso nitrogen atom onto the mesityl ring
followed by cleavage of the N2–Caryl bond. Regardless, 10 and
12–14 represent, to the best of our knowledge, the first examples
of coupling carbenes with a nitrile N-oxide, and the rearrange-
ment of 10 and 14 provides a new method for accessing highly
functionalized nitrones.
SUPPORTING INFORMATION
Detailed experimental procedures, X-ray crystallographic data for
3, 4, 6, 9, 10, 11, 13, 14, 15, and 16, NMR spectra, and computa-
tional energy levels and Cartesian coordinates are available free
of charge via the Internet at http://onlinelibrary.wiley.com.
Acknowledgements
We are grateful to the National Science Foundation (CHE-0645563),
the Robert A. Welch Foundation (F-1621), and the Sloan Foundation
for their generous support.
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with azides as well as isothiocyanates to afford the corresponding
acyclic triazenes and imidothioates, respectively. However, when
two equivalents of an isocyanate were added to 1, an iminooxa-
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[64] Exposure of 1 to one equivalent of 3-nitrophenyl isocyanate
afforded a moisture sensitive mixture of products that included
4 on the basis of a 1H NMR spectroscopic analysis.
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