Methylation of ylidene-triazenes: Insight and guidance for 1,3-dipolar cycloaddition reactions

Article abstract of DOI:10.1002/ejoc.201000939

Reaction of 1,3-dimesitylimidazolylidene (1) with p-functionalized phenyl azides 2 (a: H, b: OCH₃, c: NO₂) afforded the respective imidazolylidene-triazenes 3 in good yields (65-99%). Subsequent treatment with methyl iodide produced the corresponding methylated products 4 in near-quantitative yields (99%). Analysis by NOESY 1D NMR spectroscopy and single-crystal X-ray diffraction revealed that the methylation reaction was regioselective and occurred at the nitrogen atom most distal from the heterocycle. Consistent with the formation of ionic salts, the ¹H NMR signals for the imidazole protons in 4 were shifted > 0.5 ppm downfield compared to 1, indicating the accumulation of positive charge. Similarly, the λ_max values recorded for 4 exhibited a much narrower range than those for 3 (16 vs. 77 nm, respectively), suggesting that the frontier orbital energies within the former were dominated by Coulombic effects (i.e., the acquisition of positive charge) as opposed to electronic effects from the electron-donating or withdrawing groups of the aryl azide. Whereas computational analyses revealed that the observed regioselectivity of the methylation reaction may be explained by thermodynamics, kinetic factors (e.g., sterics) may also be important contributors and render one reaction pathway significantly more accessible due to a lower activation energy. Coupling of 1,3-dimesitylimidazolylidene (1) with a phenyl azide (2) afforded an ylidene-triazene 3, which was methylated with CH₃I to yield the corresponding salt 4. Crystallographic, ¹H and NOESY 1D NMR, as well as UV/Vis analyses provided insight into the structural and electronic features of 4. Subsequent computational studies revealed that 3 should engage in dipolar cycloadditions, and the absence thereof may arise from kinetic factors.

Full text of DOI:10.1002/ejoc.201000939

FULL PAPER
DOI: 10.1002/ejoc.201000939
Methylation of Ylidene-Triazenes: Insight and Guidance for 1,3-Dipolar
Cycloaddition Reactions
Andrew G. Tennyson,[a] Eric J. Moorhead,[a] Brian L. Madison,[a] Joyce A. V. Er,[a]
Vincent M. Lynch,[a] and Christopher W. Bielawski*[a]
Keywords: Nitrogen heterocycles / Dipolar cycloaddition / Click chemistry / N-Heterocyclic carbenes / Azides / Triazenes
Reaction of 1,3-dimesitylimidazolylidene (1) with p-function-
alized phenyl azides 2 (a: H, b: OCH3, c: NO2) afforded the
respective imidazolylidene-triazenes 3 in good yields (65–
99%). Subsequent treatment with methyl iodide produced
the corresponding methylated products 4 in near-quantita-
tive yields (99%). Analysis by NOESY 1D NMR spectroscopy
and single-crystal X-ray diffraction revealed that the methyl-
ation reaction was regioselective and occurred at the nitro-
gen atom most distal from the heterocycle. Consistent with
the formation of ionic salts, the 1H NMR signals for the imid-
Similarly, the λmax values recorded for 4 exhibited a much
narrower range than those for 3 (16 vs. 77 nm, respectively),
suggesting that the frontier orbital energies within the former
were dominated by Coulombic effects (i.e., the acquisition
of positive charge) as opposed to electronic effects from the
electron-donating or withdrawing groups of the aryl azide.
Whereas computational analyses revealed that the observed
regioselectivity of the methylation reaction may be explained
by thermodynamics, kinetic factors (e.g., sterics) may also be
important contributors and render one reaction pathway sig-
azole protons in 4 were shifted Ͼ 0.5 ppm downfield com- nificantly more accessible due to a lower activation energy.
pared to 1, indicating the accumulation of positive charge.
plex macromolecular architectures (i.e., ambiphilic block
Introduction
copolymers, star polymers, dendrimers, micelles, etc.).[23–26]
Modern chemistry has benefited from the synthetic
Trisubstituted triazoles have recently received attention
power and versatility of dipolar cycloaddition and cycliza-
as a potentially useful class of nitrogen containing sub-
tion reactions,[1–4] which have enabled far-reaching ad-
vances in many pursuits, from the preparation of unnatural
β-amino acids,[5] to the kinetic resolution of stereoiso-
mers,[6] to the functionalization of carbon nanotubes.[7]
Alkyne–azide 1,3-dipolar cycloadditions are an especially
practical subclass of reactions, given that molecules com-
prising terminal alkyne[8–10] and azide[11] functionalities are
synthetically accessible and often commercially available.
Huisgen first described alkyne–azide 1,3-dipolar cycload-
ditions over 40 years ago,[12] however the low yields of prod-
ucts obtained and the lack of regioselectivity limited the
practical scope of this transformation. It was not until 2002
that Sharpless[13] and Meldal[14] reported that the inclusion
of catalytic CuI facilitates formation of 1,4-disubstituted-
1,2,3-triazoles under mild conditions. Since this seminal
achievement, copper-catalyzed alkyne–azide “click” cou-
plings have revolutionized a broad swath of scientific disci-
plines, such as nucleoside and nucleotide functionaliza-
tion,[15,16] bioimaging,[17,18] drug[19,20] and gene[21] delivery,
combinatorial chemistry,[22] as well as the synthesis of com-
strates but suffer from challenging synthetic limitations.[27]
The preparation of these compounds via 1,3-dipolar cyclo-
addition chemistry requires either the alkyne or azide to be
difunctionalized. Although examples of the former have
been reported, their Cu-catalyzed cycloaddition is currently
limited to haloalkynes and proceeds by a mechanism that
is still being established. Difunctionalized azides are also
synthetically daunting targets, however a simpler analogue
is an ylidene-triazene (Y), the coupling product of an N-
heterocyclic carbene (NHC) to an azide,[28–34] which can be
viewed as a zwitterion (Z) that is suitable to engage in di-
polar cycloaddition reactions (Figure 1).
[a] Department of Chemistry and Biochemistry
The University of Texas at Austin
Austin, TX 78712, USA
Figure 1. Resonance structures of ylidene-triazenes (Y): zwitterions
Z1 and Z3 feature a formal negative charge at the N1 and N3 posi-
tions, respectively. R and RЈ = alkyl or aryl (see refs.[30–34]).
E-mail: bielawski@cm.utexas.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000939.
View this journal online at
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Despite our best efforts, attempts to effect dipolar cyclo- afforded 4 in near-quantitative yield (99%, Scheme 2).
1
additions of various alkynes with ylidene-triazenes have Upon methylation, the H NMR signals for the 4,5 posi-
been thus far unsuccessful. Given that the ability of an tions on the imidazole ring were shifted significantly down-
azide to participate in 1,3-dipolar cycloaddition reactions field (6.53–7.03 ppm in 3 vs. 7.56–7.70 in 4, CDCl3; see
depends in part on the electron density at the nitrogen Table 1), suggesting the acquisition of positive charge and
atoms,[13,14] we sought to probe the corresponding electron attendant imidazolium character. Interestingly, the values
density in ylidene-triazenes via their chemical reactivity observed for 4 occurred within a narrower range than 3,
towards alkylating agents. Herein we report the synthesis which may reflect the alteration of the factors influencing
and methylation of a series of imidazolylidene-triazenes, electron density within the imidazole ring. For example, the
provide a detailed account of their structural and electronic ylidene moiety in 3a should experience a greater electron-
properties, and offer attendant insight and guidance for en- withdrawing effect by the azide arene than in 3b (∆δ =
abling ylidene-triazenes to engage in 1,3-dipolar cycload- 0.50 ppm) due to the presence of the electron donating
ditions.
methoxy group in the latter. However, the addition of a pos-
itive charge to the molecule will have a greater effect on
the electron density within the imidazole ring than an azide
substituent (i.e., Coulombic effects Ͼ inductive effects),
which is borne out by the smaller chemical shift difference
observed between 4a and 4b (∆δ = 0.14 ppm).
Results and Discussion
Coupling of 1,3-dimesitylimidazolylidene (1) to phenyl
azide (2a) afforded imidazolylidene-triazene 3a in good
yield (65%, Scheme 1). Using similar methodology, this
coupling reaction was also performed with aryl azides bear-
ing electron-donating (OCH3, 2b) or electron-withdrawing
(NO2, 2c) para-substituents to afford the corresponding
imidazolylidene-triazene 3b or 3c, respectively, in excellent
yields (99%). Consistent with the loss of carbenoid charac-
ter at the 2 position, the 13C NMR chemical shifts for 3
(151.5–157.5 ppm, CDCl3; see Table 1) were shifted signifi-
cantly upfield from that observed in 1 (δ = 219.7 ppm,
C6D6).[35] Moreover, additional spectroscopic data recorded
for 3 were consistent with other ylidene-triazenes previously
reported in the literature.[31,32]
Scheme 2. Synthesis of N3-methylated imidazolylidene-triazenes
4a–c. i) 2.5 equiv. MeI, CH2Cl2, room temp., 16 h.
Primacy of Coulombic over inductive factors was also
apparent in the UV/Vis spectroscopic profiles of 3 vs. 4
(Table 1). For example, imidazolylidene-triazenes 3 may be
viewed as a donor–acceptor systems,[31] where the moiety
derived from the imidazolylidene functions as a donor and
the moiety derived from the azide as an acceptor. As such,
3c exhibited a significantly longer λmax than in 3a and 3b
(445, 368, and 371 nm, respectively). Although methylation
of 3c (to obtain 4c) produced a significant hypsochromic
shift in the measured λmax (∆λ = 65 nm), methylation of 3a
or 3b had a relatively minimal effect (∆λ Յ 12 nm). Collec-
tively, these results suggested to us that (i) the electron den-
sity distributions in imidazolylidene-triazenes 3 are influ-
enced by the electron-donating or -withdrawing character
of the azide, and (ii) the positive charge acquired by these
molecules upon reaction with methyl iodide dominates the
electronic topology of their alkylated derivatives 4.
Scheme 1. Synthesis of imidazolylidene-triazenes 3a–c. i) THF,
room temp., 16 h.
Table 1. Summary of spectroscopic data.[a]
δ C2 [ppm]
δ H4,5 [ppm]
λmax [nm]
3a
3b
3c
4a
4b
4c
151.5
157.5
156.9
144.6
159.3
146.3
7.03
6.53
6.69
7.70
7.56
7.71
368
371
445
367
383
380
Despite the information about the electronic properties
of 4 provided by the aforementioned NMR and UV/Vis
spectroscopic measurements, these data did not enable un-
ambiguous determination of the position of the N-methyl
group (i.e. N1 vs. N3). Therefore, NOESY experiments were
[a] NMR (1H and 13C) and UV/Vis measurements were performed
in CDCl3 and CH2Cl2, respectively.
To determine whether the N1 or N3 position featured conducted on 4c to determine which protons were proximal
greater negative polarization in the aforementioned ylidene- to those in the N-methyl substituent, and thus elucidate the
triazenes, 3 was treated with excess methyl iodide, which absolute structure of these molecules. Assuming that 4c was
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Methylation of Ylidene-triazenes
methylated at the N3 position (see Figure 2), the diagnostic
To obtain greater insight into the structure and bonding
1H NMR signals will be for the protons at: the meta-aryl within the aforementioned imidazolylidene-triazenes and
position within the mesityl rings (a), the 4,5 positions on their N3-methylated derivatives, single crystals of 3c and 4c,
the imidazole ring (b), the ortho-aryl protons on the ni- respectively, were subjected to X-ray diffraction (see Fig-
trophenyl ring (c), the N-methyl group (d), and the ortho- ures 3 and 4, respectively). In agreement with the 1H NMR
methyl substituents on the mesityl rings (e) (see Figure 2, chemical shifts for these compounds, the N4–C1–N5 angle
middle spectrum). Saturation of the transition assigned to in 3c was more obtuse than in 1 [106.67(18) vs. 101.4(2)°,
the N-methyl protons (d*, negative peak; top trace) afforded respectively],[36] reflecting its loss of carbenoid character,
two positive peaks (c and e), which indicated that the N- and was within the range of values observed for other ylid-
methyl group was in close proximity to the nitrophenyl and ene-triazenes previously reported (105.4–108.5°).[31,32] Simi-
mesityl rings. Conversely, irradiation of the signal attributed larly, the N1–N2 distance was longer in 3c [1.337(2) Å] and
to the mesityl ortho-methyl groups (e*; negative peak, bot- the N1–N2–N3 angle more acute [110.92(17)°] than the cor-
tom trace) revealed two strong positive peaks (a and b) that
were assigned to the nearest neighboring protons derived
from the imidazolylidene fragment. Collectively, the results
suggested to us that the methyl group in 4c was located at
the N3 position. In further support of this notion, irradia-
tion at d did not enhance the signal associated with the N-
mesityl methyl or aryl protons (a and e), results that would
have been expected if the methyl group was located at the
N1 position.
Figure 3. ORTEP diagram showing 50% probability thermal ellip-
soids and selected atom labels for 3c. Hydrogen atoms and solvent
molecules have been omitted for clarity. Selected bond lengths [Å]
and angles [°]: N1–C1 1.348(3), N1–N2 1.337(2), N2–N3 1.286(2),
N4–C1–N5 106.67(18), N1–N2–N3 110.92(17), N5–C1–N1–N2
24.2(2).
Figure 2. Summary of NOESY 1D NMR experiments (CDCl3,
600 MHz, mixing time = 0.8 s) used to confirm methylation at the
N3 position in 4c (see top structure, italicized lowercase letters indi-
cate labelling scheme). Saturating the transition for the N-methyl
group (d*, top spectrum) afforded strong, positive peaks at c and
e. Conversely, irradiation of the mesityl ortho-methyl substituents
(e*, bottom spectrum) afforded strong, positive peaks at a and b.
Figure 4. ORTEP diagram showing 50% probability thermal ellip-
soids and selected atom labels for 4c. Counteranion, hydrogen
atoms and solvent molecules have been omitted for clarity. Selected
bond lengths [Å] and angles [°]: N1–C1 1.372(3), N1–N2 1.283(3),
N2–N3 1.322(3), N3–C3 1.464(3), N4–C1–N5 107.2(2), N1–N2–
N3 113.6(2), N5–C1–N1–N2 8.2(2), N1–N2–N3–C3 2.6(2).
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responding features observed in the solid state structure an internal single bond (2–1–2, Figure 5); 4c can be viewed
(not shown) of 2c (1.127 Å and 173.4°),[37] consistent with as C–N=N–N, where the terminal and internal bond orders
the reduction of formal bond order (2.5 to 1.5) upon incor- are 1 and 2, respectively (1–2–1). Indeed, conversion of 3c
poration of 2c into 3c. Collectively, the metric parameters to 4c afforded an elongation of N1–C1 [1.348(3) to
for the ylidene–triazene linkage in 3c [N1–C1 1.348(3) Å, 1.372(3) Å],
a
contraction of N1–N2 [1.337(2) to
N1–N2 1.337(2) Å, N2–N3 1.286(2) Å, N1–N2–N3 1.283(3) Å], and a larger N2–N3 distance [1.286(2) to
110.92(17)°] were in good agreement with other reported 1.322(3) Å]. Therefore, we conclude that an alteration of
ylidene-triazenes that have been crystallographically charac- bond order occurs upon methylation of 3c, whereby its
terized (N1–C1 1.329–1.356 Å, N1–N2 1.328–1.370 Å, N2– alternating “double–single–double” CN3 structure (2–1–2)
N3 1.261–1.295 Å, N1–N2–N3 109.4–116.2°) and suggested is inverted upon formation of 4c, which exhibits a “single–
to us that the electronic topology within this framework double–single” CN3 structure (1–2–1).
was largely independent of the chemical identity of the vari-
ous N- and C-substituents.
Based on our experimental findings, we speculated that
selective methylation of 3 at the N3 vs. N1 position may
As shown in Figure 4, analysis of 4c by single-crystal X- reflect a thermodynamic preference beyond simple steric
ray diffraction unambiguously confirmed that the methyl considerations. To test this hypothesis, we performed RHF
group was affixed to the N3 position.[36] Although no ana- calculations on 3c and 4c using the 6-31G(*) basis set on
logues to 4c have been reported in the literature to the best all atoms.[48] The HOMO of 3c was found to be consistent
of our knowledge, there are related molecules that belong to with the 2–1–2 formulation derived from the crystal struc-
the broader class of N1,N2,N3-trisubstituted triazenes.[38–47] ture (see Figure 6). In agreement with our hypothesis, the
Despite substantial variation in the chemical identity of the total energy of the (observed) N3-methylated imidazolylid-
N-substituents, the bond lengths observed within the N3–R ene-triazene was lower than the hypothetical N1 isomer by
(where R is a variety of alkyl groups) fragments of these 6.4 kcal/mol (structure not shown). In addition being ther-
previously reported N1,N3,N3-trisubstituted triazenes did modynamically favored, formation of the former may be
not vary significantly (N1–N2 1.266–1.288 Å, N2–N3 driven by kinetic factors as well (e.g. the activation barrier
1.304–1.343 Å, N3–C3 1.435–1.466 Å) and encompassed for N1-methylation may be higher due to steric crowding
the values observed for 4c [N1–N2 1.283(3) Å, N2–N3 from the N-mesityl groups).
1.322(3) Å, N3–C3 1.464(3) Å]. Similarly, the N1–N2–N3
angle and N1–N2–N3–C3 torsion in 4c [113.6(2) and
2.6(2)°, respectively] were consistent with the corresponding
angles observed in other trisubstituted triazene systems
(111.3–114.6 and 0.1–4.0°, respectively).[38–47] Interestingly,
the fact that these structural features are so highly con-
served suggested to us that the bonding within N1,N3,N3-
trisubstituted triazenes is determined primarily by the N3–
CH3 fragment and largely independent of the chemical
identity of the other N-substituents.
Having obtained structural information for both 3c and
Figure 6. HOMO for 3c determined by RHF calculations using the
4c, we compared their metric parameters to discern the
changes in bonding that accompanied methylation.
Whereas the N4–C1–N5 angle for 3c [106.67(18)°] did not
change significantly upon conversion to 4c [107.2(2)°], the
N1–N2–N3 angle was more obtuse in the latter [110.92(17)°
for 3c vs. 113.6(2)° for 4c], possibly due to the steric effects
6-31G(*) basis set on all atoms.
Conclusions
In sum, the synthesis and methylation of a series of imid-
between N1 and C3. Interestingly, the bond lengths within azolylidene-triazenes was accomplished. Analysis by NMR
the C1–N1–N2–N3 linkage in 3c and 4c reflect the alter- and UV/Vis spectroscopy indicated that the electronic
ation of bonding that occurs upon methylation (Scheme 2). changes (such as the deshielding of imidazole ring and per-
For example, the CN3 linkage in 3c can be formally repre- turbation of the frontier orbitals) that accompanied N-
sented as C=N–N=N, featuring terminal double bonds and methylation were derived primarily from Coulombic effects.
Both NOESY NMR and single-crystal X-ray diffraction re-
vealed that the methyl group was located at the N3 position.
Interestingly, the metric parameters observed for an ylid-
ene-triazene and its methylated derivative were consistent
with their formal bond orders (i.e., C=N–N=N and C–
N=N–N, respectively). Whereas computational methods
indicated a thermodynamic preference for the N3-methyl
isomer (vs. the N1-methyl isomer), steric congestion and
other kinetic factors may contribute to the observed re-
gioselectivity as well.
Figure 5. Bond order within the CN3 fragment in: (left) an ylidene-
triazene (2–1–2, C=N–N=N) compared to (right) an N3-methylated
derivative (1–2–1, C–N=N–N).
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Methylation of Ylidene-triazenes
and 2 H, NCH), 3.73 (s, 3 H, O-CH3), 2.35 (s, 6 H, 4-CH3), 2.16
(s, 12 H, 2,6-CH3) ppm. 13C NMR (CDCl3, 75.47 MHz): δ = 157.7,
151.5, 145.2, 138.4, 135.1, 129.2, 116.5, 113.2, 55.2, 21.0, 17.9 ppm.
Collectively, our findings demonstrate that ylidene-triaz-
enes feature the requisite formal charges to engage in 1,3-
dipolar cycloaddition reactions but do not exhibit the nec-
essary distribution thereof. Decreasing the steric congestion
at the N1 position might circumvent the kinetic obstacles
to reactivity at this atom (e.g., employ imidazolylidenes
with N-methyl in lieu of N-mesityl substituents); however,
IR (KBr): ν = 1608 (w), 1557 (m), 1501 (s), 1426 (s), 1340 (s), 1294
˜
(w), 1233 (w), 1189 (w), 1109 (w), 1029 (w), 854 (w) cm–1. UV/Vis
(CH2Cl2): λmax = 371 nm, ε = 2.04ϫ104 –1 cm–1. HRMS: [M +
H]+ calcd. for C28H31N5O (453.58): 453.2529; found 453.2526.
such ylidene-triazenes are often unstable.[49] Alternatively, 3c: Yield 74 mg (99%), m.p. 279 °C. 1H NMR (CDCl3,
1
destabilizing any negative charge build-up at the N3 posi-
400.27 MHz): δ = 7.87 (d, J = 9.3 Hz, 2 H, meta-Ph-H), 7.02 (s, 4
1
H, Mes-H), 6.69 (s, 2 H, NCH), 6.58 (d, J = 9.2 Hz, 2 H, ortho-
tion via judicious choice of electron-donating substituents
Ph-H), 2.38 (s, 6 H, 4-CH3), 2.15 (s, 12 H, 2,6-CH3) ppm. 13C
may favor reactivity at the N1 position and enable ylidene-
NMR (CDCl3, 125.59 MHz): δ = 156.9, 151.2, 144.4, 139.2, 134.7,
133.6, 129.4, 124.1, 121.0, 117.7, 21.1, 17.8 ppm. UV/Vis (CH2Cl2):
triazenes to engage in dipolar cycloaddition reactions. Ef-
forts toward achieving this goal are underway and will be
described in due course.
IR (KBr): ν = 1558 (m), 1522 (s), 1502 (s), 1421 (w), 1324 (s), 1288
˜
(s), 1259 (m), 1182 (s), 1154 (s), 1102 (s), 854 (m) cm–1. λmax
=
445 nm, ε = 2.65ϫ104 –1 cm–1. HRMS: [M + H]+ calcd. for
C27H29N6O2 (469.56): 469.2352; found 469.2347.
General Procedure for the Synthesis of 4: To a solution of 3
(66 µmol) in CH2Cl2 (5 mL) was added methyl iodide (10 µL,
0.16 mmol, 2.5 equiv.) under ambient conditions, and the reaction
was allowed to stir at room temperature. After 16 h, the solvent
was removed under reduced pressure, and the residual solids were
then washed with Et2O and dried to afford the desired N3-methyl-
ated imidazolylidene-triazene 4.
Experimental Section
Materials and Methods: 1,3-Dimesitylimidazolylidene (1)[35] and
aryl azides (2)[50,51] were prepared as described previously. Solvents
were dried and degassed by a Vacuum Atmospheres Company sol-
vent purification system (model 103991) and stored over 3 Å mo-
lecular sieves in a nitrogen-filled glove box. 1H and 13C{1H} NMR
spectra were recorded using a Varian 300, 400 or 500 MHz spec-
trometer. All NOESY 1D NMR spectra were collected on a
600 MHz NMR (mixing time = 0.8 s). Chemical shifts δ (in ppm)
are referenced to tetramethylsilane using the residual solvent as an
internal standard. For 1H NMR: CDCl3, 7.24 ppm. For 13C NMR:
CDCl3, 77.0 ppm. Coupling constants (J) are expressed in Hertz
(Hz). FT-IR spectra were recorded using Perkin–Elmer Spectrum
BX system. High-resolution mass spectra (HRMS) were obtained
with a VG analytical ZAB2-E instrument (ESI or CI). All syntheses
were performed under inert atmosphere unless specified otherwise.
4a: Yield 31 mg (99%), m.p. 208 °C. 1H NMR (CDCl3,
400.27 MHz): δ = 7.70 (s, 2 H, NCH), 7.23–7.20 (m, 3 H, Ph-H),
1
7.08 (s, 4 H, Mes-H), 6.56 (d, J = 6.9 Hz, 2 H, N–Ph–H), 3.50 (s,
3 H, N-CH3), 2.40 (s, 6 H, 4-CH3), 2.10 (s, 12 H, 2,6-CH3) ppm.
13C NMR (CDCl3, 75.47 MHz): δ = 144.6, 141.8, 140.7, 133.9,
130.8, 129.7, 129.1, 127.9, 122.6, 118.9, 36.1, 21.0, 17.6 ppm. IR
(KBr): ν = 1608 (w), 1557 (m), 1501 (s), 1426 (s), 1340 (s), 1233
˜
(m), 1189 (w), 1109 (w), 1029 (w), 854 (w) cm–1. UV/Vis (CH2Cl2):
λmax = 367 nm, ε = 1.66ϫ104 –1 cm–1. HRMS: [M + H]+ calcd.
for C28H32N5 (438.59): 438.2658; found 438.2652.
General UV/Visible Spectroscopic Considerations: UV/Visible ab-
sorption spectra were recorded on a Perkin–Elmer Lambda 35
spectrometer. All room-temperature measurements were made
using matched 6Q Spectrosil quartz cuvettes (Starna) with 1 cm
path lengths and 3.0 mL of sample solution volumes. Absorption
spectra were acquired in CH2Cl2 under ambient conditions for all
complexes. Extinction coefficients (ε) were determined from Beer’s
law measurements using 10, 20, 30 and 40 µ concentrations of the
analyte.
4b: Yield 39 mg (99%), m.p. 222 °C. 1H NMR (CDCl3,
400.27 MHz): δ = 7.56 (s, 2 H, NCH), 7.06 (s, 4 H, Mes-H), 6.70
(d, 1J = 9.2 Hz, 2 H, Ph-H), 6.52 (d, 1J = 9.2 Hz, 2 H, Ph-H), 3.78
(s, 3 H, O-CH3), 3.49 (s, 3 H, N-CH3), 2.39 (s, 6 H, 4-CH3), 2.08
(s, 12 H, 2,6-CH3) ppm. 13C NMR (CDCl3, 125.59 MHz): δ =
159.3, 145.0, 140.7, 135.4, 134.1, 131.1, 129.8, 122.3, 120.7, 114.3,
55.6, 36.6, 21.1, 17.7 ppm. IR (KBr): ν = 1604 (w), 1553 (m), 1508
˜
(s), 1429 (s), 1354 (s), 1294 (w), 1254 (w), 1178 (w), 1028 (w), 832
(w) cm–1. UV/Vis (CH2Cl2): λmax = 383 nm, ε = 1.71ϫ104 –1 cm–1.
HRMS: [M + H]+ calcd. for C29H34N5O (468.61): 468.2763; found
468.2762.
General Procedure for the Synthesis of 3: To a solution of 1 (50 mg,
0.16 mmol) in THF (3 mL) was added phenyl azide 2 (0.16 mmol)
in one portion and the reaction was allowed to stir at room tem-
perature. After 16 h, the solvent was removed under reduced pres-
sure, and the residual solids were then washed with hexanes and
dried to afford the desired imidazolylidene-triazene 3.
4c: Yield 39 mg (99%), m.p. 238 °C. 1H NMR (CDCl3,
1
400.27 MHz): δ = 8.01 (d, J = 9.2 Hz, 2 H, meta-Ph-H), 7.71 (s, 2
1
H, NCH), 7.10 (s, 4 H, Mes-H), 6.88 (d, J = 9.2 Hz, 2 H, ortho-
3a: Yield 45 mg (65%), m.p. 252 °C. 1H NMR (CDCl3,
300.14 MHz): δ = 7.06–6.99 (m, 2 H, NCH and 5 H, Ph-H) 6.57
(br. s, 4 H, MesH), 2.36 (s, 6 H, 4-CH3), 2.16 (s, 12 H, 2,6-CH3)
ppm. 13C NMR (CDCl3, 75.47 MHz): δ = 151.6, 151.3, 138.5,
135.0, 134.1, 129.2, 127.9, 125.3, 121.3, 116.7, 21.0, 17.9 ppm. IR
Ph-H), 3.64 (s, 3 H, N-CH3), 2.41 (s, 6 H, 4-CH3), 2.09 (s, 12 H,
2,6-CH3) ppm. 13C NMR (CDCl3, 125.59 MHz): δ = 146.3, 145.9,
143.9, 141.3, 133.9, 130.8, 130.1, 124.7, 123.4, 119.4, 36.4, 21.2,
17.7 ppm. IR (KBr): ν = 1606 (w), 1593 (m), 1558 (m), 1522 (s),
˜
1505 (s), 1429 (s), 1390 (m), 1324 (s), 1314 (s), 1272 (m), 1182 (m),
1099 (m), 856 (m) cm–1. UV/Vis (CH2Cl2): λmax = 380 nm, ε =
1.87ϫ104 –1 cm–1. HRMS: [M + H]+ calcd. for C28H31N6O2
(483.58): 483.2503; found 483.2502.
(KBr): ν = 1530 (s), 1491 (m), 1481 (m), 1458 (m), 1382 (s), 1176
˜
(m), 1156 (m), 852 (w) cm–1. UV/Vis (CH2Cl2): λmax = 368 nm, ε
= 2.01ϫ104 –1 cm–1. HRMS: [M + H]+ calcd. for C27H30N5
(424.56): 424.2501; found 424.2498.
Supporting Information (see also the footnote on the first page of
3b: Yield 74 mg (99%), m.p. 259 °C. 1H NMR (CDCl3, this article): Crystallographic tables ORTEP diagrams for 4a and
1
400.27 MHz): δ = 6.98 (m, 4 H, Mes-H), 6.55–6.52 (m, 4 H, Ph-H
4b, H and 13C NMR spectra.
Eur. J. Org. Chem. 2010, 6277–6282
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6281

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Received: July 3, 2010
Published Online: September 24, 2010
6282
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 6277–6282