Demethylation of an allene bearing two dimethoxythioxanthene groups by oxidation via a vinyl cation intermediate

Article abstract of DOI:10.1071/CH10297

With the objective of preparing an isolable triplet carbene, we have carried out the oxidation of an allenic compound bearing two thioxanthene moieties (5). Relatively weak oxidants such as Ph₃C ⁺BF₄^- gave 8, wh

Full text of DOI:10.1071/CH10297

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 1638–1644
www.publish.csiro.au/journals/ajc
Demethylation of an Allene Bearing Two
Dimethoxythioxanthene Groups by Oxidation
via a Vinyl Cation Intermediate
A
A
A
Torahiko Yamaguchi, Shin-ichi Fuku-en, Shun Sugawara,
A
A B
,
Satoshi Kojima, and Yohsuke Yamamoto
A
Department of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan.
B
Corresponding author. Email: yyama@sci.hiroshima-u.ac.jp
With the objective of preparing an isolable triplet carbene, we have carried out the oxidation of an allenic compound
bearing two thioxanthene moieties (5). Relatively weak oxidants such as Ph3CþBF4ꢀ gave 8, which is the conjugate acid of 5,
as a result of a one-electron oxidation followed by hydrogen abstraction, whereas relatively strong oxidants such as
SbCl5 furnished a dicationic ketal (9) as a consequence of oxidation and demethylation. Computations on the supposed
dicationic intermediate suggest that the singlet state is more stable than the triplet state by 6.7 kcal molꢀ1 and that the
reason for this peculiarity is because the singlet state is essentially a vinyl cation stabilized by a coordinating methoxy
group.
Manuscript received: 10 August 2010.
Manuscript accepted: 31 October 2010.
Introduction
5, which bears two such moieties in an orthogonal arrangement
was found to be a suitable precursor for 9,90-methanetetraylbis
(1,8-dimethoxy-10-methyl-9H-thioxanthenium) dication 6,
which could be regarded as a hexacoordinate carbon species.[11c]
During the course of these studies, we hit upon the idea that 5
might also be appropriate as a precursor for a stable triplet
carbene, which could be obtained by oxidation. We envisaged
that the presence of the sulfur atoms would facilitate oxidation of
the allenic moiety, enabling us to directly carry out oxidation
with commercially available oxidizing reagents, and avoid the
preparation and use of azo compounds. The presence of the four
methoxy groups was anticipated to add kinetic stability to the
carbene.
Carbenes, in a primary sense, are neutral divalent carbon com-
pounds which assume either a singlet or triplet state ground
state.[1] Although carbenes of both states are generally too
reactive to be isolated, stabilization by the introduction of
adjacent heteroatoms has enabled the isolation of storable
singlet carbenes, some of which have enjoyed successful
application in organic synthesis.[2–5]
However, isolable triplet carbenes are much more difficult to
obtain than their singlet counterparts.[1,6,7] Therefore, the use of
diazo compounds as precursors is usually required and the triplet
carbenes are generated from them by photolysis in low tem-
perature matrices with electron spin resonance (ESR), ultravio-
let (UV), or infrared (IR) measurements used for detection.
A major breakthrough was brought forth by Tomioka et al.,
who delivered two long-living carbenes 3[8] and 4 (Scheme 1).[9]
Diaryl carbene 3, in which the central carbene is protected by
bulky bromine atoms and trifluoromethyl groups, was found to
survive nearly 1 day in benzene at room temperature. Dianthryl
carbene 4, which is further stabilized by strong resonance
effects, survived nearly 2 weeks under the same conditions.
However, this stability was still not enough to enable the ‘direct’
observation of 4 by X-ray structural analysis, and only by in situ
X-ray analysis of the triplet carbene partially generated by
irradiation of a single crystal of the precursor diazo compound
was it possible to gain solid phase parameters, although this in
itself was a remarkable achievement.[10]
Contrary to expectation, however, the oxidation of 5 did not
furnish a stable triplet carbene but instead gave a ketal as a result
of two demethylations. Interestingly, theoretical calculations
Ar
C
Ar
C
Ph
C
Br
CF3
F3C
Br
Ar
Ar
Ph
4
We have recently found thioxanthene to be a versatile
framework for the preparation of some novel compounds and
for the demonstration of unique chemistry.[11] In particular,
allenic 9,90-methanetetraylbis(1,8-dimethoxy-9H-thioxanthene)
t-Bu
3
Ar ꢀ
Scheme 1.
Ó CSIRO 2010
10.1071/CH10297
0004-9425/10/121638

RESEARCH FRONT
Demethylation of an Allene
1639
Me
O
Me
O
S
C
S
C
S
Me
Me
Me
Me
Me
Me
2ꢁ
Me
Me
Me
Me
Me
O
O
O
O
O
O
MeI
O
C
S
O
O
O
AgCHB11H5Br6
Me
S
S
Me
Me
5
6A
6B
S
C
S
C
S
C
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
O
O
O
O
O
O
–2e
O
O
O
O
O
O
Me
S
S
S
5
7A
7B
Scheme 2. Two-electron oxidation of 5.
S
S
ꢁ
ꢂ
Me
Me
Me
Me
Ph3C BF4 (x1.1)
Me
Me
Me
O
O
O
O
O
O
CH
(1)
C
O
O
Me
CH2Cl2
–
S
S
X
8
5
S
C
NOꢁSbFꢂ6 (x2)
or SbCl5 (x3)
Me
Me
O
O
O
O
5
(2)
CH2Cl2
–
2X
S
9
Scheme 3. Attempted two-electron oxidation of 5.
suggested that the intermediate that gave rise to the product was
an unexpected onium ion and not a triplet carbene. Herein, we
provide details of our findings.
structure of 8 was identified by the X-ray analysis of its
CHB11H5Brꢀ6 salt as illustrated in Fig. 1.
Since the hydrogen atom anticipated to be attached to the
central carbon atom could not be located with certainty, a
comparison with supposedly structurally similar and electro-
nically neutral 11 (Scheme 4), which could be prepared from 10
by Suzuki coupling, was carried out. The bond lengths and
angles around the central carbon of 8 were very similar to those
of 11 as shown in Fig. 2, thereby ascertaining its vinylic
structure. An alternative synthesis of cation 8 by treatment of
5 with a strong acid, such as TfOH, added support for the
structural identity of 8.[13] The 1H NMR spectrum of 8 showed a
characteristic singlet at 9.4 ppm (CDCl3), which was assigned to
the hydrogen bound to the central carbon atom.
Results and Discussion
The oxidation reaction of 5 was carried out with several oxi-
dizing reagents.[12] The reaction of 5 with 2 equivalents of
(p-BrC6H4)3NꢁþSbCl6ꢀ or Ph3CþXꢀ (Xꢀ ¼ BF4ꢀ, B(C6F5)ꢀ4 ,
CHB11H5Brꢀ6 ) in CH2Cl2 afforded cation 8, which is the con-
jugate acid of 5 (Scheme 3, Eqn 1). The product of the reaction
with Ph3Cþ was cleaner than that with aminium, which yielded
another unidentified compound as a by-product. Furthermore,
the reaction of 5 with only 1.1 equivalents of Ph3CþBF4ꢀ also
afforded 8 in high yield (94%), thus indicating that only a single
oxidation is actually required for the formation of 8. The
When a stronger oxidizing reagent such as NOþSbF6ꢀ or
SbCl5 was used, the demethylated dication 9 was obtained as the

RESEARCH FRONT
1640
T. Yamaguchi et al.
One-electron oxidation takes place to give the unstable vinyl
radical 5ꢁþ and this is converted to 8 (Scheme 5) by hydrogen
abstraction.
However, as for the reaction of 5 with the relatively strong
oxidants, based upon the redox behaviour, it is reasonable to
assume that a two-electron oxidation had occurred to give a
carbene intermediate. Coupled with the fact that the presumed
carbene intermediate 7 did not exhibit unpaired electron reac-
tivity as observed for monoradical 5ꢁþ, it was speculated that the
spin state of the intermediate was not the open-shell triplet state
but the closed-shell singlet state (Scheme 6).
S2
To verify this point, we have carried out theoretical calcula-
tions on the dicationic intermediate that arises upon depriving
two electrons from 5. Computations were carried out at the
B3PW91/6–31G(d) and UB3PW91/6–31G(d) levels of the-
ory,[14] for the triplet and the singlet states, respectively. The
optimized geometries are shown in Figs 6 and 7, respectively.
As expected from the diverse results between the one
electron and two electron oxidation reactions, the singlet state
was found to be more stable than the triplet state, and that by
6.7 kcal molꢀ1. An interesting point to note on structures is that
while the geometry of the triplet state in Fig. 6 is essentially a
D2d symmetric octahedral with the lengths of the two central
C–C bonds being identical, that of singlet-7 (Fig. 7) is com-
pletely different. There is a distinct difference in the two bond
O3
O4
O2
O1
S1
Fig. 1. ORTEP representation of the X-ray structure of 8ꢂCHB11H5Brꢀ6 .
All hydrogen atoms except that bonded to the central carbon are omitted for
clarity.
˚
lengths (1.375 and 1.462 A) involving the central carbon atom
and that the distance between this carbon atom and one flanking
˚
methoxy oxygen atom is only 1.518 A. These unique aspects
imply this carbon does not have the character of a singlet
carbene but a vinyl cation, which is stabilized by interaction
with the flanking methoxy group. This structural feature as a
whole shows likelihood for the central carbon to react with this
‘coordinating’ methoxy group to form an oxonium ion of which
the methyl group is prone to nucleophilic attack by even weak
nucleophiles for demethylation. A second round of similar
events should lead to the second demethylation, thereby
accounting for the mechanism of the formation of acetal 9.
There have been theoretical predictions that vinyl cations with a
b electron-donating group would favour the triplet state.[15] In
this case the d sulfur atom would have been expected to behave
in the same way. However, it turned out that the coordinating
property of the methoxy groups on the thioxanthene framework
heavily influenced the character of the intermediate to comple-
tely prevail over the expected donor effect of sulfur.
H
Tmp
OMe
H
Br
MeO
MeO
OMe
TmpB(OH)2
Pd(PPh3)4, Cs2CO3
DMF
S
S
11 y. 36%
10
Tmp ꢀ 2,4,6-trimethoxyphenyl
Scheme 4. Synthesis of 11.
major product (Scheme 3, Eqn 2). The structure of 9ꢂ(SbClꢀ6 )2
was determined by X-ray analysis as depicted in Fig. 3. The
structural constitution clearly showed that what had been
obtained was indeed a product of multiple oxidations. All
attempts to observe intermediate triplet carbene species by
ESR failed. This implied that either the ensuing reactions to
give non-triplet species were too fast or that triplet species were
not involved in the first place.
To assess the redox behaviour of 5, cyclic voltammetry
measurements were carried out, as shown in Fig. 4. Since 8
was anticipated to contribute to the spectrum of 5, a measure-
ment of 8 was also conducted (Fig. 5). A comparison of spectra
suggests that the oxidation waves at Epa ¼ 1.00 and 1.23 V, and
the reduction wave at Epc ¼ 1.10 V correspond to the redox
profile of 8. Since the second oxidation wave (Epa ¼ 1.23 V) has
a corresponding reduction wave (Epc ¼ 1.10 V), it could be
supposed that the two oxidation peaks each corresponded to
one electron oxidation processes, or in other words, the two-
electron oxidation was stepwise. This implied that both one-
and two-electron oxidations were viable with proper choice of
oxidizing agent as we had observed by chemical oxidation.
Based upon the experimental results, the reaction of 5
with relatively weak oxidants can be rationalized as follows.
Conclusion
We have attempted the preparation of an isolable triplet carbene
by the oxidation of an allene compound having two thio-
xanthene moieties. Although we could not fulfil our objective,
we obtained two different products depending upon the strength
of the oxidizing agents. Relatively weak oxidants such as
Ph3CþBFꢀ4 gave 8 as a result of a one-electron oxidation fol-
lowed by hydrogen abstraction, whereas relatively strong oxi-
dants such as SbCl5 furnished 9 as a consequence of what we
believe to be a two-fold two-electron oxidation process invol-
ving two ionic demethylation processes. In order to rationalize
the diverse behaviour between the supposed intermediates,
monoradical cation 5ꢁþ and dication 7, theoretical calculations
on 7 were carried out. Computations indicated that the singlet
state was favoured over the triplet state by 6.7 kcal molꢀ1. The
optimized structure of the singlet state implied that the reason
for this difference was because singlet-7 is essentially a vinyl

RESEARCH FRONT
Demethylation of an Allene
1641
ꢂ
X-ray structure of 8·CHB11H5Br6
S
C
1.43(3) Å
Me
Me
Me
O
O
H
O
CH
C
C
131.1(16)ꢃ
O
Me
R ꢀ 0.1182 (I ꢄ 2σ(I))
w ꢀ 0.4192 (all data)
GOF ꢀ 1.038
1.35(3) Å
ꢂ
R
S
X
8
X-ray structure of 11
C
Tmp
OMe
H
1.434(3) Å
MeO
H
C
C
131.7(2)ꢃ
R ꢀ 0.0430 (I ꢄ 2σ(I))
w ꢀ 0.1339 (all data)
1.337(3) Å
R
S
GOF ꢀ 1.071
11
Tmp ꢀ 2,4,6-trimethoxyphenyl
Fig. 2. The geometries of 8 and 11.
S2
O4
O2
O1
O3
S1
Fig. 3. ORTEP representation of the X-ray structure of 9ꢂ(SbClꢀ6 )2. All
hydrogen atoms are omitted for clarity.
1.5
1.0
0.5
0.0
V versus SCE
Fig. 5. Cyclic voltammogram of 8 in CH2Cl2 containing 0.1 M TBAPF6
(versus SCE).
cation stabilized by a coordinating methoxy group. Based upon
these new findings concerning diaryl carbenes, we have initiated
examinations on derivatives that bear other peri-substituents for
the unending pursuit of persistent triplet carbenes.
Experimental
General
CH2Cl2 was distilled from calcium hydride under argon. Com-
mercially available anhydrous dimethylformamide (DMF) was
used as received. Merck silica gel 60 (0.063–0.200 nm) was used
for column chromatography and Merck silica gel 60 GF254 was
used for preparative TLC. Melting points were measured using a
Yanagimoto micro-melting point apparatus and are uncorrected.
2.0
1.5
1.0
0.5
0.0
ꢂ0.5
V versus SCE
1
The H NMR spectra (400 MHz) were recorded using a JEOL
EX-400 or AL-400 spectrometer. The 1H NMR chemical shifts
Fig. 4. Cyclic voltammogram of 5 in CH2Cl2 containing 0.1 M TBAPF6
(versus SCE). The black dots indicate peaks assigned to 8 (see Fig. 5).

RESEARCH FRONT
1642
T. Yamaguchi et al.
S
S
C
S
C
S
Me
Me
Me
Me
Me
Me
ꢂ
Me
O
Me
Me
Me
O
O
O
O
O
O
ꢂe
H–R
O
O
CH
O
O
O
Me
Me
S
S
•؉
5
5
8
Scheme 5. Plausible reaction mechanism to afford 8.
S
C
S
C
S
Me
Me
Me
Me
Me
O
O
Me
Me
O
O
O
O
O
ꢂ
O
O
ꢂ2e
C
O
O
O
Me
Me
Me
S
S
S
5
7
9
Singlet or triplet
Scheme 6. Plausible reaction mechanism to afford 9.
Interatomic distances [Å], and angles [ꢃ]
S
C
S
1.381
C
C
Me
Me
Me
O
O
O
O
O
O
O
O
Me
2.421
2.420
180.0
C
C
C
C
2.422
O
O
O
O
Triplet
ꢁ6.7 kcal molꢂ1
2.422
1.381
Fig. 6. Optimized structure of triplet-7.
(d) were recorded from internal CHCl3 (d 7.26), CHDCl2
(d 5.32), or CHD2CN for H (d 1.93). Elemental analysis was
residue was washed with ether. Compound 8 (29 mg,
0.047 mmol, 94%) was obtained as a purple solid, mp 195–
1978C (decomp.). dH (400 MHz, CD2Cl2) 3.39 (s, 6H, 2CH3),
4.16 (s, 6H, 2CH3), 6.52 (d, J 8.0, 2H, ArH), 7.16–7.37 (m, 6H),
7.52 (d, J 8.0, 2H, ArH), 7.71 (t, J 8.0, 2H, ArH), 9.36 (s, 1H,
CH). dC (CDCl3, 100 MHz) 55.9 (CH3), 57.1 (CH3), 109.41
(CH), 111.07 (CH), 118.43 (CH), 122.03 (C), 124.96 (C), 127.87
(CH), 127.90 (CH), 133.81 (CH), 134.97 (CH), 139.01 (C),
139.11 (C), 146.81 (C), 156.64 (C), 157.81 (C), 159.04 (C).
m/z (HRMS-ESI) Calc. for [C31H25O4S2]þ 525.1189. Found
525.1183. Anal. Calc. for C31H25BF4O4S2 ꢂH2O: C 59.06,
H 4.32. Found: C 58.88, H 3.97%.
1
performed using a Perkin–Elmer 2400CHN elemental analyzer.
All oxidizing agents were used as received. The preparation of 5
and 10 has been described previously.[11c] 2,4,6-Trimethoxy-
phenylboronic acid was prepared according to a literature
procedure.[16]
Synthesis of 8ꢂBF42
A solution of 5 (26 mg, 0.050 mmol) and Ph3CþBF4ꢀ (18 mg,
0.055 mmol) in dry CH2Cl2 (2 mL) was stirred for 16 h at room
temperature. The solvent was removed under vacuum, and the

RESEARCH FRONT
Demethylation of an Allene
1643
Interatomic distances [Å], and angles [ꢃ]
S
1.462
O
C
O
C
C
Me
Me
Me
O
O
O
C
O
2.610
1.518
O
O
O
Me
2.823
143.4
C
C
S
O
O
O
2.550
Singlet
C
1.375
Fig. 7. Optimized structure of singlet-7.
Synthesis of 9ꢂ(SbCl62)2
111.49 (C), 112.13 (CH), 118.60 (CH), 119.43 (CH), 125.69
(CH), 126.79 (CH), 126.90 (CH), 127.26 (C), 127.88 (C), 129.12
(C), 135.28 (C), 136.17 (C), 156.31 (C), 156.74 (C), 158.44 (C),
160.36 (C). Anal. Calc. for C25H24O5S: C 68.79, H 5.54. Found:
C 68.53, H 5.71%.
To a stirred solution of 5 (11 mg, 0.021 mmol) in dry CH2Cl2
(2 mL) under Ar, SbCl5 (1.0 M in CH2Cl2, 0.07 mL, 0.07 mmol)
was added at ꢀ788C. The reaction mixture was stirred for 3 h at
room temperature. The solvent was removed under vacuum, and
dry CD3CN (,0.6 mL) was then added to the mixture to give a
dark green solution. The solution was transferred to a J. Young
NMR tube under Ar for NMR measurements. 9ꢂ(SbClꢀ6 )2.
mp43008C. dH (400 MHz, CD3CN) 2.66 (s, 6H, Me), 7.27 (d, J
8.0, 2H, ArH), 7.64 (d, J 8.0, 2H, ArH), 8.34 (t, J 8.0, 2H, ArH),
8.43 (d, J 8.0, 2H, ArH), 8.47 (d, J 8.0, 2H, ArH), 8.60 (t, J 8.0,
2H, ArH). m/z (HRMS-ESI) Calc. for [C29H28O4S2]2þ 247.0318.
Found 247.0320. Anal. Calc. for C29H18Cl12O4S2Sb2ꢂ2CH2Cl2:
C 27.92, H 1.66. Found: C 27.67, H 1.52%.
Theoretical Calculations
DFT calculations were performed using the Gaussian 98 pro-
gram package.[17] Geometry optimization of the singlet states
of 7 and 12 were carried out using the DFT/B3PW91 level of
theory with the 6–31G(d) basis set for all atoms. Geometry
optimization of the triplet states of 7 and 12 were computed at
the DFT/UB3PW91 level of theory with the 6–31G(d) basis set
for all atoms. No symmetry constraints were imposed during the
optimizations.
Preparation of a Single Crystal of 9(SbCl62
)
2
Acknowledgements
To a solution of 5 (11 mg, 0.021 mmol) in dry CH2Cl2 (0.5 mL)
under Ar, SbCl5 (0.51 M in CH2Cl2, 0.14 mL, 0.071 mmol) was
added at ꢀ788C. The resulting reddish purple solution was
settled for 3 h at ꢀ788C to afford single crystals of 9ꢂ(SbClꢀ6 )2
as dark plates.
This work was supported by two Grants-in-Aid for Scientific Research on
Priority Areas (Nos. 14340199, 17350021) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. The authors are grateful to
Dr Shiro Matsukawa of Toho University for assistance in X-ray analysis.
References
Synthesis of 11 from 10
[1] (a) For leading reviews, see: W. Sander, G. Bucher, S. Wierlacher,
Chem. Rev. 1993, 93, 1583. doi:10.1021/CR00020A009
(b) G. Bertrand, Carbene Chemistry: From Fleeting Intermediates to
Powerful Reagents 2002 (Marcel Dekker: New York, NY).
[2] (a) For recent leading reviews on singlet carbenes, see: D. Bourissou,
O. Guerret, F. P. Gabba¨ı, G. Bertrand, Chem. Rev. 2000, 100, 39.
doi:10.1021/CR940472U
A suspension of 10 (333 mg, 0.954 mmol), 2,4,6-trimethoxy-
phenylboronic acid (422 mg, 1.99 mmol), Pd(PPh3)4 (122 mg,
0.106 mmol), and Cs2CO3 (833 mg, 2.56 mmol) in dry DMF
(20 mL) was stirred for 24 h at 808C under argon. The reaction
mixture was filtered through Celite to remove any insoluble
material and washed with THF. The filtrate was concentrated
and the resulting liquid residue was purified by column chro-
matography (silica gel, n-hexane/CH2Cl2 1:3) to afford 11
(149 mg, 0.341 mmol, 36%) as a brown solid, mp 207–2098C. dH
(CDCl3, 400 MHz) 3.20 (s, 3H, Me), 3.51 (br s, 6H, Me), 3.78 (s,
3H, Me), 3.96 (s, 3H, Me), 6.03 (br s, 1H, CH), 6.87 (m, 2H),
6.86–7.12 (m, 6H); dC (CDCl3, 100 MHz, 508C) 55.2 (CH3),
55.3 (CH3), 55.6 (CH3), 57.1 (CH3), 91.3 (CH), 109.55 (CH),
(b) G. Bertrand, in Reactive Intermediate Chemistry 2004, Ch. 8,
pp. 329–373 (Eds R. A. Moss, M. S. Platz, M. Jones, Jr.) (Wiley-VCH:
Hoboken, NJ).
(c) Y. Canac, M. Soleilhavoup, S. Conejero, G. Bertrand, J. Organo-
met. Chem. 2004, 689, 3857. doi:10.1016/J.JORGANCHEM.2004.
02.005
(d) P. de Fre´mont, N. Marion, S. P. Nolan, Coord. Chem. Rev. 2009,
253, 862. doi:10.1016/J.CCR.2008.05.018

RESEARCH FRONT
1644
T. Yamaguchi et al.
(e) J. Vignolle, X. Cattoe¨n, D. Bourissou, Chem. Rev. 2009, 109, 3333.
doi:10.1021/CR800549J
(b) A. Nemirowski, P. R. Schreiner, J. Org. Chem. 2007, 72, 9533.
doi:10.1021/JO701615X
[3] (a) For recent comprehensive reviews on the use of carbenes as
ligands, see: W. A. Herrmann, Angew. Chem. Int. Ed. 2002, 41,
1290. doi:10.1002/1521-3773(20020415)41:8o1290::AID-ANIE12904
3.0.CO;2-Y
[8] (a) K. Hirai, H. Tomioka, J. Am. Chem. Soc. 1999, 121, 10213.
doi:10.1021/JA991387C
(b) T. Itoh, Y. Nakata, K. Hirai, H. Tomioka, J. Am. Chem. Soc. 2006,
128, 957. doi:10.1021/JA056575J
(b) C. M. Crudden, D. P. Allen, Coord. Chem. Rev. 2004, 248, 2247.
doi:10.1016/J.CCR.2004.05.013
[9] E. Iwamoto, K. Hirai, H. Tomioka, J. Am. Chem. Soc. 2003, 125,
14664. doi:10.1021/JA038423Z
(c) N. M. Scott, S. P. Nolan, Eur. J. Inorg. Chem. 2005, 1815. doi:10.1002/
EJIC.200500030
[10] M. Kawano, K. Hirai, H. Tomioka, Y. Ohashi, J. Am. Chem. Soc. 2007,
129, 2383. doi:10.1021/JA067306B
(d) S. P. Nolan, N-Heterocyclic Carbenes in Synthesis 2006 (Wiley-
VCH: Weinheim).
[11] (a) T. Yamaguchi, Y. Yamamoto, Y. Fujiwara, Y. Tanimoto, Org. Lett.
2005, 7, 2739. doi:10.1021/OL050961O
(e) H. Clavier, S. P. Nolan, Annu. Rep. Prog. Chem. Sect. B 2007, 103,
193. doi:10.1039/B614412H
(b) T. Yamaguchi, Y. Yamamoto, Chem. Lett. 2007, 36, 1438.
doi:10.1246/CL.2007.1438
(f) F. Glorius, N-Heterocyclic Carbenes in Transition Metal Catalysis
2007 (Springer-Verlag: Berlin-Heidelberg).
(c) T. Yamaguchi, Y. Yamamoto, D. Kinoshita, K.-y. Akiba, Y. Zhang,
C. A. Reed, D. Hashizume, F. Iwasaki, J. Am. Chem. Soc. 2008, 130,
6894. doi:10.1021/JA710423D
(d) T. Yano, T. Yamaguchi, Y. Yamamoto, Chem. Lett. 2009, 38, 794.
doi:10.1246/CL.2009.794
(g) F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122.
doi:10.1002/ANIE.200703883
(h) R. Corbera´n, E. Mas-Marza´, E. Peris, Eur. J. Inorg. Chem. 2009, 1700.
doi:10.1002/EJIC.200801095
(i) S. D´ıez-Gonza´lez, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109, 3612.
doi:10.1021/CR900074M
(j) X. Bantreil, J. Broggi, S. P. Nolan, Annu. Rep. Prog. Chem. Sect. B
2009, 105, 232. doi:10.1039/B822056P
[12] N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877. doi:10.1021/
CR940053X
[13] Compound 5 could not be protonated by the weakly acidic H2O.
However, compound 8 could be deprotonated by pyridine to
reproduce 5.
(k) O. Kuhl, Functionalised N-Heterocyclic Carbene Complexes 2010
(Wiley: Chichester).
[14] We have previously carried out DFT calculations at B3PW91/6–
31G(d) level to estimate weak interactions and have found that
experimental results are reproduced to a high degree with this method.
(cf. M. Yamashita, Y. Yamamoto, K.-Y. Akiba, D. Hashizume,
F. Iwasaki, N. Takagi, S. Nagase, J. Am. Chem. Soc. 2005, 127,
4354.) doi:10.1021/JA0438011
[4] (a) For leading reviews on the use of carbenes as organocatalysts,
see: N. Marion, S. D´ıez-Gonza´lez, S. P. Nolan, Angew. Chem. Int. Ed.
2007, 46, 2988. doi:10.1002/ANIE.200603380
(b) D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 2007, 107, 5606.
doi:10.1021/CR068372Z
[15] A. H. Winter, D. E. Falvey, J. Am. Chem. Soc. 2010, 132, 215.
doi:10.1021/JA906139M
[16] H. Chaumeil, S. Signorella, C. L. Drian, Tetrahedron 2000, 56, 9655.
doi:10.1016/S0040-4020(00)00928-5
(c) E. M. Phillips, A. Chan, K. A. Scheidt, Aldrichim Acta 2009, 42, 55.
[5] (a) For leading reviews on other applications of carbenes, see: K. M.
Hindi, M. J. Panzner, C. A. Tessier, C. L. Cannon, W. J. Youngs,
Chem. Rev. 2009, 109, 3859. doi:10.1021/CR800500U
(b) M.-L. Teyssot, A.-S. Jarrousse, M. Manin, A. Chevry, S. Roche,
F. Norre, C. Beaudoin, L. Morel, D. Boyer, R. Mahiou, A. Gauti,
Dalton Trans. 2009, 6894. doi:10.1039/B906308K
(c) L. Mercs, M. Albrecht, Chem. Soc. Rev. 2010, 39, 1903. doi:10.039/
B902238B
[6] (a) For leading reviews on triplet carbenes, see: H. Tomioka, Acc.
Chem. Res. 1997, 30, 315. doi:10.1021/AR9602157
(b) H. Tomioka, in Reactive Intermediate Chemistry 2004, Ch. 9,
pp. 375–461 (Eds R. A. Moss, M. S. Platz, M. Jones, Jr.) (Wiley-VCH:
New Jersey, NJ).
[17] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E.
Stratmann, Jr, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniel,
S. K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M.
Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J.
Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cio-
slowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.
M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C.
Gonzalez, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian 98
(Revision A.5) 1998 (Gaussian, Inc.: Pittsburgh, PA)
(c) T. Itoh, K. Hirai, H. Tomioka, Bull. Chem. Soc. Jpn. 2007, 80, 138.
doi:10.1246/BCSJ.80.138
(d) K. Hirai, T. Itoh, H. Tomioka, Chem. Rev. 2009, 109, 3275.
doi:10.1021/CR800518T
[7] (a) For factors to stabilize triplet carbenes, see: H. L. Woodcock,
D. Moran, B. R. Brooks, P. R. Schleyer, H. F. Schaefer III, J. Am.
Chem. Soc. 2007, 129, 3763. doi:10.1021/JA068899T