Azavinylidenephosphoranes: A Class of Cyclic Push–Pull Carbenes

Article abstract of DOI:10.1002/chem.201402880

The synthesis of a novel family of cyclic push–pull carbenes, namely, azavinylidene phosphoranes, is described. The methodology is based on a formal [3+2] cycloaddition between terminal alkynes and phosphine–imines followed by an oxidation/deprotonation step. Carbenes 6, obtained by simple deprotonation, exhibit typical transient carbene reactivity like the intramolecular C H insertion reaction and a pronounced ambiphilic character exemplified by [2+1] cycloaddition with electron-poor methyl acrylate. Owing to the cyclic structure, carbenes 6 also exhibit an excellent coordination ability toward transition metals. Rh^Icomplex 10 was obtained in excellent yield and was fully characterized by multinuclear NMR spectroscopy and X-ray crystallography. The corresponding Rh^I–carbonyl complex was also prepared; this indicates that carbenes 6 belong to the strongest σ-donating ligands to date. DFT calculations confirmed the high σ-donation ability of 6 and their classification as push–pull carbenes with a relatively small singlet–triplet energy gap of 23.2–24.3 kcal mol^?1.

Full text of DOI:10.1002/chem.201402880

DOI: 10.1002/chem.201402880
Full Paper
&
Carbenes
Azavinylidenephosphoranes: A Class of Cyclic Push–Pull Carbenes
Florie Lavigne,[a] Aimꢀe El Kazzi,[a] Yannick Escudiꢀ,[a] Eddy Maerten,*[a] Tsuyoshi Kato,[a]
Nathalie Saffon-Merceron,[b] VicenÅ Branchadell,[c] Fernando P. Cossꢁo,[d] and
Antoine Baceiredo*[a]
Abstract: The synthesis of a novel family of cyclic push–pull
carbenes, namely, azavinylidene phosphoranes, is described.
The methodology is based on a formal [3+2] cycloaddition
between terminal alkynes and phosphine–imines followed
by an oxidation/deprotonation step. Carbenes 6, obtained
by simple deprotonation, exhibit typical transient carbene
reactivity like the intramolecular CꢀH insertion reaction and
a pronounced ambiphilic character exemplified by [2+1] cy-
cloaddition with electron-poor methyl acrylate. Owing to the
cyclic structure, carbenes 6 also exhibit an excellent coordi-
nation ability toward transition metals. RhI complex 10 was
obtained in excellent yield and was fully characterized by
multinuclear NMR spectroscopy and X-ray crystallography.
The corresponding RhI–carbonyl complex was also prepared;
this indicates that carbenes 6 belong to the strongest s-do-
nating ligands to date. DFT calculations confirmed the high
s-donation ability of 6 and their classification as push–pull
carbenes with a relatively small singlet–triplet energy gap of
23.2–24.3 kcalmolꢀ1
.
NHCs by a sp3-hybridized carbon leading to cyclic alkyl-
(amino)carbenes (CAACs),[8] which are stronger s-donating li-
gands than NHCs. Closely related, in 2008, the groups of Kawa-
shima, Fꢂrstner, and Bertrand independently reported the syn-
thesis of carbenes bearing an exocyclic ylide function adjacent
to the carbene center, namely, amino(ylide)carbenes (N-
YHCs),[9] which are also excellent donor ligands. Moreover,
CAACs present an extraordinary rich chemistry such as small-
molecule activation (H2, NH3, CO, P4),[10] and the corresponding
transition-metal complexes demonstrated very original catalyt-
ic properties (such as a-arylation of ketones and aldehydes
with aryl chlorides at room temperature; coupling enamines
with terminal alkynes to yield allenes with loss of imines; or
hydroamination of alkynes and allenes with ammonia).[11]
These properties are the result of the particular structure of
CAACs but also the consequence of a considerably reduced
singlet–triplet (S/T) energy gap (45.1 kcalmolꢀ1) when com-
pared with NHCs (ꢁ80 kcalmolꢀ1).[12] There are only a few ex-
amples of stable cyclic carbenes presenting a high electrophilic
character. We can cite diamidocarbenes (DACs) showing an
atypical NHC reactivity such as CꢀH insertion, reversible cou-
pling with carbon monoxide, and cyclopropanation reactions
with a broad range of olefins.[13] The original N-heterocyclic
carbene (NpyrHC), featuring a bridgehead nitrogen atom, also
shows interesting electrophilic properties especially as a ligand
for gold complexes, which are active catalysts in the hydroami-
nation of alkynes and allenes with hydrazine.[14] Finally, car-
benes bearing an intracyclic ylide function, known as cyclic vi-
nylidenephosphoranes (CVPs) also display strong electron-do-
nating abilities and a small S/T energy gap, however, up to
now, their development was limited because of restricted syn-
thesis (Scheme 1).[15]
Introduction
Since the isolation of the first stable carbenes in the late
1980s,[1] carbene chemistry has become a very active research
field.[2] The development of various stabilization modes afford-
ed many useful tools like synthetic reagents,[3] organocata-
lysts,[4] or ligands for transition metals in homogeneous cataly-
sis.[5] The success in this latest application is mainly owing to
the peculiar properties of N-heterocyclic carbenes (NHCs).[6]
Indeed, their strong s-donor character and their excellent abili-
ty to bind metals afford not only robust but also very efficient
organometallic catalysts.[7] Interestingly, Bertrand et al. offered
new possibilities for structure modifications around the car-
benic center with the substitution of one amino group of
[a] Dr. F. Lavigne, Dr. A. E. Kazzi, Y. Escudiꢀ, Dr. E. Maerten, Dr. T. Kato,
Dr. A. Baceiredo
Universitꢀ de Toulouse, UPS, and CNRS
LHFA UMR 5069, 31062 Toulouse Cedex 9 (France)
Fax: (+33)561558204
E-mail: baceired@chimie.ups-tlse.fr
maerten@chimie.ups-tlse.fr
[b] Dr. N. Saffon-Merceron
Universitꢀ de Toulouse, UPS, and CNRS
ICT FR 2599, 31062 Toulouse (France)
[c] Prof. V. Branchadell
Departament de Quꢁmica
Universitat Autꢂnoma de Barcelona
08193 Bellaterra, Barcelona (Spain)
[d] Prof. F. P. Cossꢁo
Departamento de Quꢁmica Organica I
Universidad del Paꢁs Vasco–Euskal Herriko Unibertsitatea
Facultad de Quꢁmica, P.K. 1072, San Sebastian-Donostia (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402880.
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Results and Discussion
Synthesis
With this framework in mind, we designed a general synthetic
approach to CAVPs. Our methodology, inspired by seminal re-
sults of Schmidpeter and Zeiss,[18] is based on a formal [3+2]
cycloaddition between phosphine–imine 1 and terminal al-
kynes 2 to afford five-membered cyclic phosphazenes 4. An
oxidation/deprotonation step using CBr4 gives the phosphoni-
um salts 5, which are the precursors of CAVPs (Scheme 4). The
easy modification of the substituents of phosphine–imine
1 and/or acetylenic derivatives 2 should allow an easy modula-
tion of the steric and electronic properties of the correspond-
ing carbenes.
Scheme 1. Stable carbenes.
The peculiar structure of CVPs has retained our at-
tention. Indeed, CVPs are cyclic heteroallenes con-
taining a PC ylidic bond and a CC double bond,
which can be formally described as cyclic push–pull
carbenes (III; Scheme 2).
Scheme 4. Synthetic approach.
The study was initiated with phosphine–imine 1 bearing
bis(diisopropylamino) groups on the phosphorus atom that
will further bring an efficient kinetic stabilization of the carben-
ic center. Three types of acetylenic derivatives 2a–c were con-
sidered to balance the electrodeficiency of the carbenic center
by electronic or steric effects. The reaction is very substrate de-
pendent, and can be performed at room temperature with an
activated alkyne (2a), whereas xylene reflux is needed in the
case of phenylacetylene (2b), and no reaction was observed in
the case of mesitylacetylene (2c), even under drastic condi-
tions, probably for steric reasons. The first formed phospha-
zenes 3 (observed by 31P NMR spectroscopy in the case of 3a)
rearrange to the thermodynamically more stable phospha-
zenes 4, by means of a 1,3-proton shift. Derivatives 4a and 4b
were isolated in good yields of 82 and 57%, respectively, and
fully characterized by multinuclear NMR spectroscopy. Selected
spectroscopic data, in good agreement with previously de-
scribed similar heterocycles, can be found in Table 1.[19]
Scheme 2. Mesomeric structures of cyclic vinylidenephosphoranes.
Structural study revealed that the ylidic p bond is extremely
polarized owing to a very weak interaction between the car-
bene lone pair and the phosphonio group (III).[15b] As a conse-
quence, the carbene lone pair is fully available for coordination
to transition metals, which explains why this cyclic carbene
was found to be a strongly nucleophilic ligand. In addition, be-
cause of the substitution pattern of CVPs, this carbene pres-
ents
a considerably reduced S/T energy gap (32.4 kcal
molꢀ1).[15b] Similarly to the case of Enders’ carbene,[16] the intro-
duction of a phosphazene fragment at the backbone, instead
of the ylide function, should further increase the ambiphilic
character of a new family of carbenes, the cyclic azavinylidene-
phosphoranes (CAVPs).
As a part of our research program towards new push–pull
carbenes,[17] herein, we report the synthesis, reactivity, and
ligand properties of the first examples of CAVPs (Scheme 3).
Table 1. Selected spectroscopic data for 4, 5, and 6 (chemical shifts in
ppm, coupling constants in hertz).
4
5
6
P{1H}
1H
4a 74.6
4b 72.6
4a 2.99 (10.4)
4b 3.08 (10.5)
4a 31.5 (68.5)
4b 36.4 (69.5)
5a 69.7
5b 66.3
5a 9.81 (28.9)
5b 8.83 (30.6)
5a 141.8 (93.9)
5b 134.7 (101.8)
6a 103.5
6b 95.8
–
–
13C
–
6b 234.4 (93.6)
Scheme 3. Mesomeric structures of cyclic azavinylidenephosphoranes.
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The oxidation–deprotonation sequence of phosphazenes 4
can be performed either in one step with CBr4 or in two steps
using Br2 as oxidant then NEt3/tBuOK as base. Phosphonium
salts 5a and 5b display single resonances (d=69.7 and
66.3 ppm) in 31P NMR spectroscopy, and the characteristic eth-
Table 2. Calculated HOMO energy (EH, in eV), singlet-triplet energy gap
(DEST, in kcalmolꢀ1), electrophilicity (w, in eV) and maximum saturation
electronic charge (DNmax, in au), for various cyclic CAVPs and CVPs at the
B3LYP/6-31G(d) level of calculation.
R’
EHOMO
DES/T
w
DNmax
1
ylenic protons appear as doublets at low field in the H NMR
spectra (d=9.81 ppm, 2J(H,P)=28.9 Hz and d=8.83 ppm,
2J(H,P)=30.6 Hz). To optimize the purification step and to
obtain suitable crystals for an X-ray diffraction analysis, anion
metathesis was quantitatively performed with Me3SiOTf (OTf=
trifluoromethanesulfonate; Scheme 4). Both compounds 5’a,b
were obtained as colorless crystals from saturated solutions of
CH2Cl2/Et2O or CH2Cl2/THF, respectively, and the structures
were confirmed by X-ray diffraction analysis (Figures 1 and 2).
Similarly to the previously described conjugated acid of
CVP1 (see Table 2), cyclic phosphonium salts 5’a,b show signifi-
cant localized diene character with C1ꢀC2 and C3ꢀN1 bonds,
which are typically short at 1.342(7) and 1.317(7) ꢄ for 5’b, re-
6a
CO2Me
ꢀ5.03
23.2
1.82
1.07
6b
6c
6d
Ph
Mes
Me
ꢀ4.83
ꢀ4.77
ꢀ4.70
24.3
22.7
23.7
1.65
1.67
1.48
1.03
1.04
0.95
CAVP1
–
ꢀ5.01
26.2
1.53
0.94
CVP1
CVP2
–
–
ꢀ4.40
ꢀ4.70
32.4
27.9
0.91
1.23
0.71
0.84
CVP3
–
ꢀ4.58
25.5
1.34
0.92
spectively, whereas the C2ꢀC3 bond lengths are in the range
expected for a single bond.[15b]
Deprotonation of 5’ was carried out in THF at low tempera-
ture (ꢀ808C) using non-nucleophilic strong bases such as po-
tassium hexamethyldisilazane (KHMDS) or mesityl lithium
(Scheme 5).[20] The formation of the desired azavinylidenephos-
phoranes 6a and 6b was indicated by 31P NMR spectroscopy
Figure 1. Molecular structure of phosphonium salt 5’a. Thermal ellipsoids
were set at 30% probability. Hydrogen atoms, except the one on the five-
membered ring, and TfOꢀ were omitted for clarity; selected bond lengths
[ꢄ] and angles [8]: P1ꢀN1 1.690(2), P1ꢀC1 1.800(3), N1ꢀC3 1.302(4), C1ꢀC2
1.330(4), C2ꢀC3 1.516(4), C2ꢀC4 1.498(4), C3ꢀC6 1.468(4), P1ꢀN2 1.612(2),
P1ꢀN3 1.616(2); C2-C1-P1 105.9(2), N1-P1-C1 96.45(12), C1-C2-C3 112.9(2),
N1-C3-C2 115.8(2), C3-N1-P1 108.61(19).
Scheme 5. Deprotonation of phosphonium salt 5’a,b.
by single resonances at d=103.5 and 95.8 ppm, respectively,
reminiscent of the phosphorus chemical shift observed for
CVP1 (d=86.4 ppm in C6D6).[15b] Variable-temperature multinu-
clear NMR spectroscopy studies indicated that 6a was unstable
and decomposed at low temperature (ꢀ808C). However, 6b is
perfectly stable at low temperature (T<ꢀ158C), and it can be
fully characterized by NMR spectroscopy. Of special interest,
the characteristic 13C NMR spectroscopy signal of the carbenic
center appears as
a low-field doublet at d=234.4 ppm
(1J(C,P)=96.3 Hz), in the typical area of NHC carbenic centers.
Unfortunately, all our attempts to crystallize 6b were unsuc-
cessful.
Figure 2. Molecular structure of phosphonium salt 5’b. Thermal ellipsoids
were set at 30% probability. Hydrogen atoms, except the one on the five-
membered ring, and TfOꢀ were omitted for clarity; selected bond lengths
[ꢄ] and angles [8]: P1ꢀN1 1.669(4), P1ꢀC1 1.778(5), N1ꢀC3 1.317(7), C1ꢀC2
1.342(7), C2ꢀC3 1.511(7), P1ꢀN2 1.611(4), P1ꢀN3 1.618(4); C2-C1-P1 107.4(4),
N1-P1-C1 96.6(2), C1-C2-C3 110.7(4), N1-C3-C2 115.9(5), C3-N1-P1 108.4(3).
Calculations
To get a better insight into the structure and electronic nature
of CAVPs, theoretical calculations have been performed. We
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have optimized the geometries of the phosphonium salts 5’b
and carbene 6b at the B3LYP level of calculation. The results
obtained for 5’b are in excellent agreement with the experi-
mental values shown in Figure 2, with mean absolute devia-
tions of 0.014 ꢄ for interatomic distances and 0.48 for bond
angles.[21] According to the natural bond orbital (NBO) method,
the Lewis structure that gives the best description of the elec-
tronic structure of 6 is II (Scheme 3).[21] Figure 3 shows the
a good compromise between the relative stability and reactivi-
ty, with an DNmax value of approximately 1 a.u.
Carbene properties
The reactivity of stabilized carbenes depends on the singlet–
triplet energy gap, and carbene 6b, which has a relatively
small S/T gap of 24.3 kcalmolꢀ1, should exhibit an ambiphilic
character and the reactivity of transient singlet car-
benes.
The stability of carbenes 6 is strongly dependent
on the nature of their substituents. Indeed, 6a (R’=
CO2Me) decomposes at low temperature giving
a complex mixture of products, whereas 6b (R’=Ph)
is stable for days at temperatures lower than ꢀ158C.
By increasing slowly the temperature to room tem-
perature, carbene 6b undergoes a clean CꢀH inser-
tion involving the phosphorus isopropyl substituent,
which affords the fused bicyclic structure
7
Figure 3. HOMO (left) and LUMO (right) of 6b (ꢂ0.05 contours) computed at the B3LYP/
(Scheme 6). Phosphazene 7 was isolated as a colorless
6-31G(d) level of theory.
HOMO and LUMO of 6b. The HOMO is localized on the carben-
ic center whereas the LUMO corresponds to the delocalized p
system. The energy level of the HOMO and the singlet–triplet
energy gap were calculated for different CAVPs and CVPs
(Table 2). The values obtained for the HOMO level of 6a–d are
high, which means that all these carbenes are more nucleo-
philic than NHCs. As expected, the HOMO levels of CAVPs are
directly influenced by the inductive effect of the R’ group. In
contrast, the S/T energy gaps calculated for carbenes 6a–d are
very similar (ꢁ24 kcalmolꢀ1), much smaller than the value re-
Scheme 6. Reactivity of carbene 6b: CꢀH insertion and cyclopropanation re-
ported for the previously described CVP1.[15b] To understand
the origin of this difference, we calculated the S/T energy gap
of other CAVPs and CVPs. By comparing values obtained for
CAVP1 and CVP2,3, it appears than both the nitrogen atom
and the phenyl group at the backbone are needed to obtain
a carbene showing a pronounced ambiphilic character.
actions.
oil and fully characterized by multinuclear NMR spectroscopy.
A single resonance at d=66.6 ppm is observed in the 31P NMR
spectrum. In the 1H NMR spectrum, the inserted hydrogen
atom appears as a doublet signal at d=3.37 ppm (2J(H,P)=
12.0 Hz), and the corresponding carbon atom resonates at d=
61.9 ppm as a doublet with a large coupling constant (1J(C,P)=
54.7 Hz) in the 13C NMR spectrum. The bicyclic structure was
confirmed by 2D HMBC experiments, which showed a strong
3J heteronuclear chemical-shift correlation between the carbon
alpha to the phosphorus and the two methyl groups of the
four-membered ring. 13C NMR spectroscopy data are in good
agreement with azaphosphetanes previously described in the
literature.[23]
We have also computed the electrophilicity (w) and the max-
imum saturation electronic charge (DNmax) of each carbenic
species 6a–d, CAVP1, and CVP1–3 (Table 2). The latter magni-
tude DNmax is closely related to the number of electrons re-
quired to complete the octet of compounds gathered in
Table 2. Thus, for an ideal carbene with non-interacting groups
one would expect an approximate value of DNmax of approxi-
mately 2.0. In singlet C2v-symmetric carbenes CH2 and CCl2 the
computed values are 1.50 and 1.43 au, and the corresponding
electrophilicities are 3.68 and 3.88 eV, respectively.[22] Our re-
sults for compounds CVP1 and CVP2 show quite low w and
DNmax values because of the efficient electron-releasing effect
on the carbene moiety of the fused bicyclic ring and phenyl
groups. Moreover, carbene 6a, which combines a nitrogen
atom at the backbone and an electron-withdrawing ester func-
tion, exhibits the highest w and DNmax values (Table 2), in good
agreement with its experimentally observed high reactivity
(see below). Carbenes 6b,c, which incorporate two vicinal non-
coplanar aryl groups in the 1,2l5-azaphosphole ring, represent
Usually, alkenes react with singlet carbenes in a concerted
fashion to give the corresponding cyclopropane products.[2b] In
the case of carbene 6b, a clean reaction was observed with an
electron-poor olefin, methyl acrylate, at ꢀ808C, to afford the
corresponding cyclopropane 8 (Scheme 6). Since the spiro-
phosphazene 8 slowly decomposes at room temperature, it
was characterized by multinuclear NMR spectroscopy at low
temperature.
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A single resonance in the 31P NMR spectrum at d=75.7 ppm
indicates a diastereoselective cyclopropanation reaction. The
spiranic carbon atom resonates as a doublet at d=37.3 ppm
in the 13C NMR spectrum with a large coupling constant
(1J(C,P)=96.7 Hz) indicative of a direct PC connectivity. In addi-
tion, the other two three-membered ring carbon atoms CH2
and CH appear also as doublets at d=22.3 (2J(C,P)=2.8 Hz)
and 26.6 ppm (2J(C,P)=7.2 Hz), respectively. In the 1H NMR
spectrum, the diastereotopic methylene protons give a multip-
let at d=1.42 ppm and a doublet of doublet of doublets at
d=1.89 ppm (2J(H,H)=4.0 Hz, 3J(H,H)=6.4 Hz, 3J(H,P)=
13.0 Hz). The alpha proton of the ester function is located at
d=2.10 ppm (ddd, 3J(H,H)=5.6 Hz, 3J(H,H)=6.4 Hz, 3J(H,P)=
7.6 Hz). Finally, the spiranic structure was clearly confirmed by
2D HMBC experiments that showed a strong heteronuclear
chemical-shift correlation between the three-membered ring
protons and the quaternary carbon alpha to the phosphorus
atom. The instability of 8 is mainly due to the presence of the
phosphazene function, which presents a strong nucleophilic
character. Indeed, the addition of one equivalent of MeOTf, at
low temperature, smoothly affords the air-stable spirophos-
phonium salts 9, which exhibits very similar NMR spectroscopic
data to those of 8 (Scheme 6).
Figure 4. Molecular representation of rhodium complex 10. Thermal ellip-
soids were set at 30% probability. Hydrogen atoms were omitted for clarity;
selected bond lengths [ꢄ] and angles [8]: Rh1ꢀC1 2.050(2), C1ꢀC2 1.364(3),
C2ꢀC3 1.499(3), C3ꢀC4 1.486(3), C2ꢀC10 1.484(3), Rh1ꢀCl1 2.3999(1), P1ꢀN2
1.632(2), N1ꢀC3 1.301(3), P1ꢀC1 1.823(2), P1ꢀN3 1.638(2), P1ꢀN1 1.688(2);
C1-Rh1-Cl1 95.24(5), N2-P1-N3 109.16(9), N1-P1-C1 99.67(9), C3-N1-P1
105.97(13), C2-C1-P1 101.11(14), C2-C1-Rh1 127.45(15), P1-C1-Rh1 130.96(11),
C1-C2-C3 115.37(17), C10-C2-C3 120.14(17), N1-C3-C2 117.41(17).
donor versus p-acceptor properties of ligand L.[24] The corre-
sponding Rh–dicarbonyl complex 11 was easily prepared by
bubbling carbon monoxide through a solution of 10 in di-
chloromethane (Scheme 8). On monitoring the transformation
Ligand properties
Despite its thermal instability, carbene 6b appears to be an ef-
ficient new ligand for transition metals. Indeed, mixing its pre-
cursor 5’b, potassium bis(trimethylsilyl)amide (KHMDS), and
[{RhCl(cod)}2] (cod=1,5-cyclooctadiene) at ꢀ808C leads to the
corresponding RhI complex 10 in 71% yield as a thermally and
air-stable red solid (m.p. 159–1618C) (Scheme 7). In the
Scheme 8. Synthesis of RhI complex 11.
by 31P NMR spectroscopy, the gradual consumption of 10 with
the concomitant formation of 11 was observed. Complex 11
exhibits a signal at d=84.6 ppm as a doublet (2J(P,Rh)=6.7 Hz)
in the 31P NMR spectrum. The chelating carbon appears at d=
177.9 ppm as a doublet of doublets (1J(C,P)=35.8 Hz and
1J(C,Rh)=23.6 Hz) in the 13C NMR spectrum. In addition, two
characteristic resonances were observed for the two carbonyl
Scheme 7. Synthesis of RhI complex 10.
31P NMR spectrum, a signal at d=80.9 ppm appears as a dou-
blet with a small Rh–P coupling constant (1J(P,Rh)=6.2 Hz). The
formation of complex 10 can clearly be ascertained by
13C NMR spectroscopy where the carbene center moves up-
field, as is typical for this type of metal complex, to d=
1
groups at d=183.5 (d, J(C,Rh)=74.2 Hz) and 185.4 ppm (dd,
1
3J(C,P)=10.4 Hz and J(C,Rh)=52.5 Hz). The infrared spectrum
shows the characteristic CO stretching frequencies with an
average value of 2028 cmꢀ1. This value highlights the strong s-
donation ability of 6b, which is much stronger than NHCs, but
remains weaker than CVP1 probably because of the higher
electrophilic character of 6b (Scheme 9). The computed scaled
average carbonyl-stretching frequency for 11 was found to be
2025 cmꢀ1, in excellent agreement with experimental values
(see Scheme 9). The geometry of the complex of CVP1 with
Rh(CO)2Cl was also optimized and the computed average CO
stretching is 2018 cmꢀ1. Finally, the electron-donor character of
6b is also reflected in the natural atomic population analysis
1
1
195.8 ppm (dd, J(C,P)=41.2 Hz and J(C,Rh)=7.6 Hz), thereby
confirming the direct connectivity between the carbon and
rhodium atoms. The structure of 10 was unambiguously estab-
lished by X-ray diffraction analysis (Figure 4). The P1-C1-C2
angle in 10 (101.18) is slightly narrower than the azavinylidene-
phosphorane precursor 5’b (107.48). The rhodium–carbon
bond length is typical for a single bond (2.050 ꢄ), in the range
of previously reported rhodium complexes (1.94–2.10 ꢄ).[5b]
The carbonyl stretching frequencies of cis-[RhCl(CO)2L] com-
plexes are recognized to give an excellent measure of the s-
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reflux was extended for 24 h. All
the volatiles were removed under
vacuum. Extraction of phospha-
zene 4b with pentane (3ꢅ20 mL)
was performed. Evaporation of the
solvent to obtain a saturated solu-
tion of pentane was realized and
precipitation occurred at ꢀ308C.
Phosphazene 4b was obtained as
Scheme 9. Comparison of the average IR band of u(CO) in cis-[RhCl(CO)2L].[8,15b,25]
a brownish powder (12.4 g, 57%
yield). M.p. 105–1078C; 1H NMR
(300.1 MHz, 298 K, CDCl3): d=1.28
(d, 3J(H,H)=6.6 Hz, 12H; NCHCH3), 1.34 (d, 3J(H,H)=6.9 Hz, 12H;
NCHCH3), 3.08 (d, 2J(H,P)=10.5 Hz, 2H; PCH2), 3.73 (sept d,
3J(H,H)=6.9 Hz, 3J(H,P)=13.8 Hz, 4H; NCHCH3), 6.82–6.83 (m, 3H;
CHAr), 6.99–7.01 (m, 2H; CHAr), 7.20–7.25 (m, 3H; CHAr), 7.37–
7.41 ppm (m, 2H; CHAr); 13C{1H} NMR (75.1 MHz, 298 K, CDCl3): d=
of complexes 10 and 11, which shows that the charge trans-
ferred from 6b to the metal fragment is 0.40 (10) and 0.46 au
(11).
1
23.1 (s; NCHCH3), 23.3 (s; NCHCH3), 36.4 (d, J(C,P)=69.5 Hz; PCH2),
Conclusion
2
46.2 (s; NCHCH3), 103.4 (d, J(C,P)=22.3 Hz; NC=C), 121.8 (s; CHAr),
126.0 (s; CHAr), 126.7 (s; CHAr), 127.5 (s; CHAr), 128.4 (s; CHAr), 128.5
We established an efficient new methodology to access cyclic
azavinylidenephosphoranes in three steps. The first example,
6b, was fully characterized by multinuclear NMR spectroscopy,
and its chemical reactivity highlights a strong ambiphilic char-
acter exemplified by CꢀH insertion or cyclopropanation reac-
tions with electron-poor alkenes. This chemical behavior is in
good agreement with calculations that predict a small S/T
energy gap for this new family of carbenes. These cyclic push–
pull carbenes also show an excellent ability to bind transition
metals and their high s-donor aptitude was established by
analysis of IR CO stretching frequencies of the corresponding
RhI–carbonyl complex 11. In addition, the use of these car-
benes as ligands in asymmetric catalysis can be easily envis-
aged because of the presence of a potentially P-chiral center
adjacent to the carbenic center that brings the chiral informa-
tion as close as possible to the metallic center and therefore
hopefully leads to high stereoinduction. Development of new
models using this methodology to increase their thermal sta-
bility is under investigation.
3
3
(s; CHAr), 140.3 (d, J(C,P)=16.3 Hz; Cipso), 142.2 (d, J(C,P)=28.2 Hz;
ipso), 157.7 ppm (d, 2J(C,P)=12.1 Hz; N-C=C); 31P{1H} NMR
C
(121.5 MHz, 298 K, CDCl3): d=72.6 ppm; HRMS (desorption chemi-
cal ionization (DCI), CH4): m/z calcd for C27H41N3P: 438.3038
[M+H]+; found: 438.3034.
Phosphonium salt 5b
A solution of CBr4 (6.98 g, 21.0 mmol) in dichloromethane (20 mL)
was added dropwise at ꢀ808C to a solution of cyclic phosphazene
4b (9.20 g, 21.0 mmol) in dichloromethane (20 mL). After stirring
for 1 h at ꢀ808C, the solution was warmed up to room tempera-
ture. All the volatiles were removed under vacuum and the crude
was washed with diethyl ether (3ꢅ10 mL). Phosphonium salt 5b
was obtained as a brownish powder (8.2 g, 75% yield). M.p. 136–
1378C; 1H NMR (300.1 MHz, 298 K, CDCl3): d=1.36 (d, 3J(H,H)=
3
6.6 Hz, 12H; NCHCH3), 1.38 (d, J(H,H)=6.6 Hz, 12H; NCHCH3), 4.01
(sept d, 3J(H,H)=6.3 Hz, 3J(H,P)=12.9 Hz, 4H; NCHCH3), 7.29–7.32
(m, 3H; CHAr), 7.38–7.42 (m, 3H; CHAr), 7.50–7.54 (m, 2H; CHAr),
7.68–7.70 (m, 2H; CHAr), 8.83 ppm (d, 2J(H,P)=30.6 Hz; PCH);
13C{1H} NMR (75.1 MHz, 298 K, CDCl3): d=22.5 (s; NCHCH3), 22.8 (s;
2
NCHCH3), 48.5 (d, J(C,P)=3.9 Hz; NCHCH3), 128.6 (s; CHAr), 128.7 (s;
CHAr), 128.9 (s; CHAr), 130.8 (s; CHAr), 132.4 (d, 3J(C,P)=23.6 Hz;
Cipso), 133.3 (d, 3J(C,P)=28.9 Hz; Cipso), 134.1 (s; CHAr), 134.7 (d,
1J(C,P)=101.8 Hz; PCH), 156.8 (d, 2J(C,P)=34.3 Hz; NCꢀC),
Experimental Section
General procedures
2
183.5 ppm (d, J(C,P)=7.1 Hz; N=C); 31P{1H} NMR (121.5 MHz, 298 K,
CDCl3): d=66.3 ppm; HRMS (DCI, CH4): m/z calcd for C27H39N3P:
436.2882 [M]+; found: 436.2897.
All manipulations were performed under an inert atmosphere of
argon by using standard Schlenk techniques. Dry and oxygen-free
1
solvents were used. H, 13C, and 31P NMR spectra were recorded on
1
Brucker Avance 400 or Avance 300 spectrometers. H and 13C NMR
Phosphonium salt 5’b
spectroscopy chemical shifts are reported in parts per million
(ppm) relative to Me4Si as external standard. 31P NMR spectroscopy
downfield chemical shifts are expressed in ppm relative to 85%
H3PO4. IR spectra were recorded on a Varian 640-IR. Mass spectra
were recorded on a Hewlett Packard 5989A spectrometer.
Trimethylsilyltriflate (5.1 mL, 28.0 mmol) was added dropwise at
ꢀ808C to a solution of phosphonium salt 5b (13.11 g, 25.4 mmol)
in dichloromethane (60 mL). After 30 min, the solution was
warmed up to room temperature and the volatiles were removed
under vacuum. The crude was washed with diethyl ether (3ꢅ
20 mL). Then phosphonium salt 5’b was obtained as colorless crys-
tals from a saturated CH2Cl2/THF solution (7.9 g, 53% yield). M.p.
Cyclic phosphazene 4b
1
3
Bis(diisopropylamino)phosphine imine 1 (16.7 g, 49.9 mmol) was
dissolved in xylene (40 mL) and heated to reflux. Then a solution
of phenylacetylene 2b (6.6 mL, 60.1 mmol) in xylene (10 mL) was
added dropwise at the top of the condenser. The mixture was
heated for 24 h. Then a new addition of a solution of phenylacety-
lene (3.0 mL, 27.4 mmol) in xylene (5 mL) was performed and
137–1388C; H NMR (300.1 MHz, 298 K, CDCl3): d=1.29 (d, J(H,H)=
3
6.6 Hz, 12H; NCHCH3), 1.34 (d, J(H,H)=6.6 Hz, 12H; NCHCH3), 3.92
(sept, 3J(H,H)=6.6 Hz, 4H; NCHCH3), 7.20 (m, 2H; CHAr), 7.37 (m,
1H; CHAr), 7.39 (m, 2H; CHAr), 7.43 (m, 2H; CHAr), 7.50 (m, 1H; CHAr),
7.61 (m, 2H; CHAr), 7.93 ppm (d, 2J(H,P)=30.9 Hz; PCH); 13C{1H}
3
NMR (75.1 MHz, 298 K, CDCl3): d=22.2 (d, J(C,P)=1.4 Hz; NCHCH3),
Chem. Eur. J. 2014, 20, 12528 – 12536
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12533
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
3
2
22.3 (d, J(C,P)=1.2 Hz; NCHCH3), 48.3 (d, J(C,P)=4.8 Hz; NCHCH3),
120.9 (q, 1J(C,F)=318.8 Hz; CF3), 128.3 (s; CHAr), 128.6 (s; CHAr),
128.8 (s; CHAr), 130.3 (s; CHAr), 130.9 (s; CHAr), 132.3 (d, 3J(C,P)=
24.1 Hz; Cipso), 132.9 (d, 3J(C,P)=28.7 Hz; Cipso), 133.2 (d, 1J(C,P)=
102.7 Hz; PCH), 134.2 (s; CHAr), 157.2 (d, 2J(C,P)=33.4 Hz; NCꢀC),
183.2 ppm (d, 2J(C,P)=7.4 Hz; N=C); 19F NMR (282.4 MHz, 298 K,
CDCl3): d=ꢀ77.4 ppm; 29Si NMR (59.0 MHz, 298 K, CDCl3): d=
28.1 ppm; 31P{1H} NMR (121.5 MHz, 298 K, CDCl3): d=66.1 ppm;
HRMS (DCI, CH4): m/z calcd for C27H39N3P: 436.2882 [M]+; found:
436.2881.
ꢀ408C. 1H NMR (400.1 MHz, 233 K, [D8]THF): d=1.32 (m, 6H;
NCHCH3), 1.41 (d, 3J(H,H)=6.8 Hz, 12H; NCHCH3), 1.42 (m, 1H;
2
3
CH2cycle), 1.48 (m, 6H; NCHCH3), 1.89 (ddd, J(H,H)=4.0 Hz, J(H,H)=
6.4 Hz, 3J(H,P)=13.0 Hz, 1H; CH2cycle), 2.10 (ddd, 3J(H,H)=5.6 Hz,
3
3J(H,H)=6.4 Hz, J(H,P)=7.6 Hz, 1H; CHcycle), 3.66 (m, 1H; NCHCH3),
3.69 (s, 3H; OCH3), 3.99 (m, 1H; NCHCH3), 4.09 (m, 2H; NCHCH3),
7.00–7.01 (m, 3H; CHAr), 7.01–7.02 (m, 2H; CHAr), 7.20–7.21 (m, 1H;
CHAr), 7.26–7.28 (m, 2H; CHAr), 7.39–7.41 ppm (m, 2H; CHAr); 13C{1H}
NMR (100.1 MHz, 233 K, [D8]THF): d=21.3 (s; NCHCH3), 22.3 (d,
3
2J(C,P)=2.8 Hz; CH2cycle), 22.5 (s; NCHCH3), 25.1 (d, J(C,P)=3.2 Hz;
2
1
NCHCH3), 26.6 (d, J(C,P)=7.2 Hz; CHcycle), 37.3 (d, J(C,P)=96.7 Hz;
PC), 47.1 (s; NCHCH3), 47.6 (s; NCHCH3), 47.9 (s; NCHCH3), 51.7 (s;
OCH3), 109.6 (d, 2J(C,P)=36.6 Hz; NC=C), 125.8 (s; CHAr), 126.6 (s;
CHAr), 128.3 (s; CHAr), 128.9 (s; CHAr), 138.3 (d, 3J(C,P)=12.8 Hz;
Cipso), 139.9 (d, J(C,P)=24.6 Hz; Cipso), 151.9 (d, J(C,P)=8.1 Hz; Nꢀ
C), 172.4 ppm (d, 3J(C,P)=5.3 Hz; C=O); 31P{1H} NMR (161.9 MHz,
233 K, [D8]THF): d=75.7 ppm.
Azavinylidene phosphorane 6b
Phosphonium salt 5’b (100 mg, 0.17 mmol) and potassium hexam-
ethyldisilazane (35 mg, 0.17 mmol) were dissolved in [D8]THF
(0.6 mL) at ꢀ808C. Cyclic azavinylidene phosphorane 6b was ob-
tained and characterized by multinuclear NMR spectroscopy at
ꢀ808C without further purification. 1H NMR (300.1 MHz, 193 K,
[D8]THF): d=1.42 (d, 3J(H,H)=7.2 Hz, 12H; NCHCH3), 1.44 (d,
3J(H,H)=7.8 Hz, 12H; NCHCH3), 4.50 (m, 4H; NCHCH3), 7.30 (m, 3H;
CHAr), 7.45 (m, 2H; CHAr), 7.49 (m, 2H; CHAr), 7.61 ppm (m, 3H;
CHAr); 13C{1H} NMR (75.1 MHz, 193 K, [D8]THF): d=23.2 (s; NCHCH3),
42.4 (s; NCHCH3), 123.9 (s; CHAr), 125.0 (s; CHAr), 125.8 (s; CHAr),
126.8 (s; CHAr), 127.8 (s; CHAr), 128.7 (s; CHAr), 135.5 (d, 3J(C,P)=
40.4 Hz; Cipso), 142.1 (d, 3J(C,P)=56.7 Hz; Cipso), 156.6 (d, 2J(C,P)=
9.2 Hz; NC=C), 175.9 (d, 2J(C,P)=42.1 Hz; NꢀC), 234.4 ppm (d,
1J(C,P)=93.6 Hz; PC); 31P{1H} NMR (121.5 MHz, 193 K, [D8]THF): d=
95.8 ppm.
3
2
Spirophosphonium salt 9
Phosphonium salt 5’b (182 mg, 0.31 mmol) and potassium hexam-
ethyldisilazane (61.7 mg, 0.31 mmol) were dissolved in THF
(3.0 mL) at ꢀ808C. After 30 min, methylacrylate (56 mL, 0.62 mmol)
was added and the solution was kept at ꢀ608C for 48 h. One
equivalent of methyltriflate (35 mL, 0.31 mmol) was added at
ꢀ608C, then the solution was warmed up to room temperature.
All volatiles were removed under vacuum, and the crude was
washed with diethyl ether (3ꢅ2 mL). Spirophosphonium salt 9 was
obtained as a white powder (95.6 mg, 45% yield). M.p. 142–1448C;
3
1H NMR (300.1 MHz, 298 K, CDCl3): d=1.45 (d, J(H,H)=6.9 Hz, 6H;
Azaphosphetane 7
3
3
NCHCH3), 1.48 (d, J(H,H)=6.9 Hz, 6H; NCHCH3), 1.49 (d, J(H,H)=
6.6 Hz, 6H; NCHCH3), 1.50 (d, 3J(H,H)=6.6 Hz, 6H; NCHCH3), 1.52
(m, 1H; CH2cycle), 2.11 (ddd, 2J(H,H)=4.5 Hz, 3J(H,H)=6.6 Hz,
3J(H,P)=16.5 Hz, 1H; CH2cycle), 2.49 (ddd, 3J(H,H)=6.6 Hz, 3J(H,H)=
Phosphonium salt 5’b (60 mg, 0.10 mmol) and potassium hexame-
thyldisilazane (20 mg, 0.10 mmol) were dissolved in [D8]THF
(0.6 mL) at ꢀ808C. The solution was slowly warmed up to room
temperature. Azaphosphetane 7 was obtained quantitatively (indi-
cated by 31P NMR spectroscopy) and analyzed without further pu-
rification (43.5 mg). 1H NMR (300.1 MHz, 298 K, C6D6): d=0.88 (d,
3J(H,H)=6.3 Hz, 3H; NCHCH3), 1.14 (d, 3J(H,H)=6.8 Hz, 6H;
3
3
9.0 Hz, J(H,P)=16.5 Hz, 1H; CHcycle), 2.75 (d, J(H,P)=10.5 Hz, 3H;
NCH3), 3.79 (s, 3H; OCH3), 3.95 (sept d, 3J(H,H)=6.6 Hz, 2J(H,P)=
13.2 Hz, 2H; NCHCH3), 4.03 (sept d, 3J(H,H)=7.2 Hz, 2J(H,P)=
14.4 Hz, 2H; NCHCH3), 6.80–6.89 (m, 2H; CHAr), 6.94–7.05 (m, 2H;
CHAr), 7.19–7.21 (m, 3H; CHAr), 7.21–7.22 ppm (m, 3H; CHAr); 13C{1H}
3
3
NCHCH3), 1.26 (d, J(H,H)=6.3 Hz, 6H; NCHCH3), 1.28 (d, J(H,H)=
6.6 Hz, 3H; NCHCH3), 1.33 (s, 3H; NCCH3), 1.44 (s, 3H; NCCH3), 3.18
(sept, 3J(H,H)=6.8 Hz, 3J(P,H)=7.8 Hz, 1H; NCHCH3), 3.37 (d,
2J(H,P)=12.0 Hz, 1H; PCH), 3.63 (m, 2H; NCHCH3), 6.81–6.87 (m,
2H; CHAr), 7.04–7.08 (m, 3H; CHAr), 7.09–7.11 (m, 2H; CHAr), 7.18–
7.20 (m, 2H; CHAr), 7.91–7.94 ppm (m, 1H; CHAr); 13C{1H} NMR
(75.1 MHz, 298 K, C6D6): d=21.3 (d, 3J(C,P)=5.6 Hz; NCCH3), 22.4
(brs; NCHCH3), 22.9 (brs, NCHCH3), 23.4 (d, 3J(C,P)=6.6 Hz;
NCHCH3), 24.1 (d, 3J(C,P)=6.1 Hz; NCHCH3), 32.4 (d, 3J(C,P)=
3
NMR (75.1 MHz, 298 K, CDCl3): d=22.1 (d, J(C,P)=0.8 Hz; NCHCH3),
23.7 (s; NCHCH3), 23.8 (s; NCHCH3), 23.9 (s; CH2cycle), 25.2 (d,
3J(C,P)=2.2 Hz; NCHCH3), 29.4 (d, 3J(C,P)=5.7 Hz; CHcycle), 30.6 (d,
1J(C,P)=134.4 Hz; PC), 34.9 (d, 2J(C,P)=1.0 Hz; NCH3), 49.9 (d,
2
2J(C,P)=4.8 Hz; NCHCH3), 50.0 (d, J(C,P)=4.9 Hz; NCHCH3), 53.1 (s;
OCH3), 116.5 (d, 2J(C,P)=18.6 Hz; NC=C), 126.4 (s; CHAr), 126.6 (s;
3
CHAr), 126.7 (s; CHAr), 129.2 (s; CHAr), 129.6 (d, J(C,P)=8.1 Hz; Cipso),
130.2 (d, 3J(C,P)=9.5 Hz; Cipso), 131.2 (s; CHAr), 170.3 (d, 2J(C,P)=
5.6 Hz; NꢀC), 174.8 ppm (s; C=O); 31P{1H} NMR (121.5 MHz, 298 K,
CDCl3): d=72.7 ppm; HRMS (DCI, CH4): m/z calcd for C32H47N3O2P:
536.3406 [M]+; found: 536.3412.
2
2
10.3 Hz; NCCH3), 44.7 (d, J(C,P)=3.8 Hz; NCHCH3), 47.4 (d, J(C,P)=
4.9 Hz; NCHCH3), 61.9 (d, 1J(C,P)=54.7 Hz; PCH), 66.7 (d, 2J(C,P)=
2
6.7 Hz; NC(CH3)2), 110.8 (d, J(C,P)=17.3 Hz; N-C=C), 122.5 (s; CHAr),
127.4 (s; CHAr), 127.6 (s; CHAr), 127.9 (s; CHAr), 128.2 (s; CHAr), 129.6
3
3
(s; CHAr), 141.3 (d, J(C,P)=15.6 Hz; Cipso), 142.4 (d, J(C,P)=30.1 Hz;
Cipso), 162.7 ppm (d, 2J(C,P)=17.2 Hz; NꢀC); 31P{1H} NMR
(121.5 MHz, 298 K, C6D6): d=66.6 ppm.
Rhodium–cyclooctadiene complex 10
Phosphonium salt 5’b (100 mg, 0.17 mmol) and potassium hexam-
ethyldisilazane (35 mg, 0.18 mmol) were dissolved in THF (2 mL) at
ꢀ808C. After 30 min,
a solution of [{RhCl(cod)}2] (42 mg,
Spirophosphazene 8
0.085 mmol) in THF (1 mL) was added and stirred for 15 h at
ꢀ808C. Then the solution was warmed up to room temperature
and the volatile material was removed under vacuum. The crude
was washed with diethyl ether (2ꢅ2 mL) and complex 10 was ex-
tracted with dichloromethane. Red crystals of 10 were grown from
a dichloromethane/diethyl ether solution at ꢀ308C (82 mg, 71%
yield). M.p. 159–1618C; 1H NMR (300.1 MHz, 298 K, [D8]THF): d=
1.46 (d, 3J(H,H)=4.2 Hz, 12H; NCHCH3), 1.50 (d, 3J(H,H)=6.9 Hz,
Phosphonium salt 5’b (60 mg, 0.10 mmol) and potassium hexame-
thyldisilazane (20 mg, 0.10 mmol) were dissolved in [D8]THF
(0.7 mL) at ꢀ808C. After 30 min, methylacrylate (18.9 mL,
0.21 mmol) was added and the solution was kept at ꢀ608C for
48 h. Spiro derivative 8 was obtained quantitatively as indicated by
31P NMR spectroscopy. This product is not stable at room tempera-
ture and was analyzed by multinuclear NMR spectroscopy at
Chem. Eur. J. 2014, 20, 12528 – 12536
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12534
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
12H; NCHCH3), 1.73 (s, 4H; CH2), 2.18 (s, 4H; CH2), 3.34 (s, 2H; CH),
4.11 (s, 4H; NCHCH3), 4.79 (s, 2H; CH), 7.23–7.46 (m, 2H; CHAr),
7.27–7.28 (m, 2H; CHAr), 7.29–7.30 (m, 1H; CHAr), 7.39–7.42 (m, 1H;
CHAr), 7.43–7.46 (m, 2H; CHAr), 7.72–7.75 ppm (m, 2H; CHAr); 13C{1H}
NMR (75.1 MHz, 298 K, [D8]THF): d=23.8 (s; NCHCH3), 23.9 (s;
NCHCH3), 28.6 (s; CH2), 32.6 (s; CH2), 48.9 (d, 2J(C,P)=4.9 Hz;
NCHCH3), 66.7 (s; CH), 94.7 (s; CH), 126.7 (s; CHAr), 126.9 (s; CHAr),
127.5 (s; CHAr), 129.7 (s; CHAr), 130.4 (s; CHAr), 131.1 (s; CHAr), 136.4
in which the corresponding chemical potentials (m) and the hard-
nesses (h) were computed within the ground-state parabola
model[32] according to the following approximate expressions relat-
ed to the orbital energies of the Kohn–Sham frontier orbitals
[Eqs. (3) and (4)]:
m ꢁ ðeLUMO þ eHOMOÞ=2
h ꢁ eLUMOꢀeHOMO
ð3Þ
ð4Þ
3
3
(d, J(C,P)=32.3 Hz; Cipso), 141.4 (d, J(C,P)=37.9 Hz; Cipso), 156.3 (d,
2
2
2J(C,P)=30.9 Hz; NꢀC), 180.0 (dd, J(C,P)=20.9 Hz, J(C,Rh)=3.4 Hz;
NC=C), 195.8 ppm (dd, 1J(C,P)=41.2 Hz, 1J(C,Rh)=7.6 Hz; PC);
31P{1H} NMR (121.5 MHz, 298 K, [D8]THF): d=80.9 ppm (d, J(P,Rh)=
2
All calculations have been performed using the Gaussian 09 suite
of programs.[33]
6.2 Hz); HRMS (DCI, CH4): m/z calcd for C35H50N3PClRh: 681.2486
[M+H]+; found: 681.2469.
Acknowledgements
Rhodium–carbonyl complex 11
This work was supported by the CNRS and the Universitꢀ de
Toulouse (UPS). This work was granted access to the HPC re-
sources of CALMIP under the allocation 2012-P1225. Financial
support from the Spanish Ministry of Economy and Competi-
tiveness (CTQ2010-15408/BQU and CTQ2010-16959/BQU) and
Generalitat de Catalunya (2009SGR-733), UPV/EHU (UFI QOSYC
11/22), and Basque Government (T673-13) are gratefully ac-
knowledged.
Carbon monoxide was bubbled into a solution of rhodium–cyclo-
octadiene complex 10 (50 mg, 0.07 mmol) in dichloromethane
(2 mL) at room temperature. The solution turned from red to
yellow, then all the volatile material was removed under vacuum.
Complex 11 was obtained as a yellow powder (46.1 mg, 90%
yield). M.p. 130–1328C; 1H NMR (300.1 MHz, 298 K, [D8]THF): d=
1.27 (d, 3J(H,H)=6.6 Hz, 12H; NCHCH3), 1.38 (d, 3J(H,H)=6.6 Hz,
3
12H; NCHCH3), 4.22 (sept, J(H,H)=6.9 Hz, 4H; NCHCH3), 7.14–7.19
(m, 3H; CHAr), 7.21–7.24 (m, 2H; CHAr), 7.29–7.32 (m, 2H; CHAr),
7.38–7.40 ppm (m, 3H; CHAr); 13C{1H} NMR (75.1 MHz, 298 K,
[D8]THF): d=23.6 (d, 3J(C,P)=1.7 Hz; NCHCH3), 23.8 (d, 3J(C,P)=
Keywords: carbene ligands · carbenes · density functional
calculations · synthetic methods · ylides
2
2.9 Hz; NCHCH3), 48.8 (d, J(C,P)=4.9 Hz; NCHCH3), 127.2 (s; CHAr),
127.9 (s; CHAr), 128.2 (s; CHAr), 129.9 (s; CHAr), 130.4 (s; CHAr), 131.9
3
3
(s; CHAr), 135.2 (d, J(C,P)=31.4 Hz; Cipso), 140.8 (d, J(C,P)=36.4 Hz;
Cipso), 159.5 (d, 2J(C,P)=29.7 Hz; NꢀC), 177.9 (dd, 1J(C,P)=35.8 Hz,
[1] a) A. Igau, H. Grꢂtzmacher, A. Baceiredo, G. Bertrand, J. Am. Chem. Soc.
1988, 110, 6463; b) A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem.
Soc. 1991, 113, 361.
1J(C,Rh)=23.6 Hz; PC), 181.4 (dd, J(C,P)=18.5 Hz, J(C,Rh)=2.9 Hz;
2
2
1
1
[2] a) W. Kirmse, Carbene Chemistry, 2ednd edAcademic Press, New York,
1971; b) D. Bourissou, O. Guerret, F. P. Gabbai, G. Bertrand, Chem. Rev.
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N-C=C), 183.5 (d, J(C,Rh)=74.2 Hz; CO), 185.4 ppm (dd, J(C,Rh)=
52.5 Hz, 3J(C,P)=10.4 Hz; CO); 31P{1H} NMR (121.5 MHz, 298 K,
[D8]THF): d=84.6 ppm (d, 2J(P,Rh)=6.7 Hz); IR (THF): n˜ =2066,
1990 cmꢀ1 (CO).
Computational details
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and (2)]:
m2
ð1Þ
ð2Þ
w ¼
2h
m
h
DNmax ¼ ꢀ
Chem. Eur. J. 2014, 20, 12528 – 12536
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12535
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Received: April 1, 2014
Published online on August 14, 2014
Chem. Eur. J. 2014, 20, 12528 – 12536
www.chemeurj.org
12536
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim