Imidazoline-functionalized diphosphines: Models for N-heterocyclic carbene-diphosphinocarbene coupling

Full text of DOI:10.1002/anie.200461120

Communications
Hybrid Carbenes
The diphosphinoketenimine ligand coordinated in the
+
cationic complex [Mn(CO)4{(PPh2)2C C NPh}]
(1a)[6]
= =
Imidazoline-Functionalized Diphosphines:
Models for N-Heterocyclic Carbene–
Diphosphinocarbene Coupling**
reacts with propargylamine in refluxing toluene to afford 2a
(Scheme 1), which is a diphosphine complex containing an
Javier Ruiz,* Marta E. G. Mosquera, Gabriel García,
Fernando Marquínez, and Víctor Riera
N-Heterocyclic carbenes I derived from imidazol-2-ylidene,
also referred to as Arduengo carbenes, are important com-
pounds whose isolation as stable, free molecules has greatly
contributed to the development of carbene chemistry in
recent years.[1] They show no tendency to dimerize to afford
tetraazafulvalenes II, except under special circumstances,[2]
unlike the corresponding saturated N-heterocyclic carbenes.
In contrast, related diphosphinocarbenes III are not isolable,
and have been reported in the literature only as transient
species,[3,4] whereas their corresponding dimers IV are known
to be stable.[5] As far as we are aware, no example of coupling
of both types of carbenes (V in Figure 1) has been described.
Scheme 1. Proposed mechanism for the formation of complexes 2a–c.
[Mn]=[Mn(CO)4]+. 1a: R=Ph; 1b: R=p-tolyl. 2a: R=Ph, Z=NH;
2b: R=Ph, Z=O; 2c: R=p-tolyl, Z=NMe.
imidazoline functionality. This complex is formed in a cyclo-
addition process involving the ketenimine fragment of the
ligand. This result is in sharp contrast to the reaction of free
diphosphinoketenimines with propargylamines, which affords
phosphinine derivatives due to the additional involvement of
a phosphorus atom in the cycloaddition process.[7] As
proposed in Scheme 1, a likely mechanism for the formation
of 2a implies, in a first step, nucleophilic attack of the amine
to the ketenimine to afford an enediamine derivative. Then, a
5-exo-dig addition of the phenylamino moiety to the alkyne
leads to the formation of an imidazolidine cycle, which is
converted into the corresponding imidazoline residue by a 1,3
proton shift from the endocyclic methylene group to the
=
exocyclic one. This results in a change of the C C bond
position and the formation of a conjugated six-p-electron
system. The reaction of 1a with propargyl alcohol proceeds
similarly to give the corresponding diphosphine complex 2b
bearing an oxazoline functionality (Scheme 1).
Figure 1. Schematic representation of imidazol-2-ylidenes and diphos-
phinylcarbenes, and their coupling products.
We report here the metal-assisted synthesis of new diphos-
phine ligands bearing an imidazoline residue that can serve as
models of N-heterocyclic carbene–diphosphinocarbene cou-
pling. The synthesis of analogous oxazoline-functionalized
diphosphines is also described.
It is well known that the reaction of ketenimines with
amines and alcohols usually gives the corresponding amidine
and imidate derivatives, respectively.[8] However, as far as we
are aware, no reaction of organic ketenimines with prop-
argylic amines or alcohols has been reported thus far. The
reaction described here therefore represents a completely
new method for the formation of imidazoline and oxazoline
functionalities by a new cyclization process.
The spectroscopic data of 2a,b (see the Experimental
Section) are in agreement with the proposed formulation.
Moreover, their solid-state structures were confirmed by X-
ray analysis (Figure 2 and Figure 3).[9] The most important
feature in the structures of 2a and 2b is the presence of the
imidazoline and oxazoline cycle, respectively, which is copla-
[*] J. Ruiz, G. García, F. Marquínez, V. Riera
Departamento de Química Orgµnica e Inorgµnica
Facultad de Química, Universidad de Oviedo
Oviedo (Spain)
Fax: (+34)98-5103-446
E-mail: jruiz@fq.uniovi.es
M. E. G. Mosquera
Departamento de Química Inorgµnica
Edificio de Farmacia, Universidad de Alcalµ
Alcalµ de Henares (Spain)
À
nar with the PCP skeleton of the diphosphane. The C1 C2
bond lengths in 2a (1.429(4) Š) and 2b (1.385(6) Š) are, to
[**] This work was supported by the Spanish Ministerio de Ciencia y
Tecnología (PGE and FEDER funding, Project BQU2003-01011) and
by the Principado de Asturias (Project PR-01-GE-7).
varying degrees, intermediate between single and double
À
À
bonds, as are the P1 C1 and P2 C1 distances and the
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DOI: 10.1002/anie.200461120
Angew. Chem. Int. Ed. 2005, 44, 102 –105

Angewandte
Chemie
Figure 4. Resonance structures of imidazoline-functionalized diphos-
phane complexes. [Mn]=[Mn(CO)4]+.
known nucleophilic character of N-heterocyclic carbenes[10]
and the electrophilic character of transient metalladiphosphi-
nocarbenes,[11] whose coupling should lead to a net charge-
transfer from the former to the latter, thus justifying the
contribution of structure b in Figure 4. The IR spectra of 2a
and 2b in the nCO region (see the Experimental Section),
which show absorptions at values intermediate between those
of diphosphine complexes and diphosphinomethanide deriv-
atives,[12] support the above interpretation and indicate that
the new diphosphines are strongly basic. Note that these
absorptions occur at lower frequency in 2a than in 2b. This
reflects the fact that the imidazoline fragment is a better
electron donor than the oxazoline fragment, which also
Figure 2. A view of the structure of the cation of 2a. Selected bond
lengths [Š] and angles [8]: P1-C1 1.749(3), P2-C1 1.761(3), C1-C2
1.429(4), N1-C2 1.342(3), N1-C3 1.382(4), N2-C2 1.371(3), N2-C4
1.416(3), C3-C4 1.333(4); P1-Mn-P2 69.84(2), P1-C1-P2 99.23(13), N1-
C2-N2 105.4(2), N1-C2-C1 125.0(2), N2-C2-C1 129.5(2).
À
explains why the C1 C2 bond in 2b is shorter than that in 2a.
The 31P{1H} NMR spectra of 2a,b at room temperature in
CD2Cl2 show a single resonance, which suggests free rotation
À
around the C1 C2 bond. For 2a, this single signal is
maintained when the spectrum is recorded at À808C, whereas
it splits into two resonances, corresponding to the two
inequivalent phosphorus atoms, in the case of 2b. This
observation agrees with the proposal outlined above that
À
C1 C2 bond in 2b has higher multiple-bond character than in
2a.
Substituted propargylamine also reacts with coordinated
diphosphinoketenimines in a similar manner. Thus, reaction
+
= =
of [Mn(CO)4{(PPh2)2C C N-p-tolyl}] (1b) with methylpro-
pargylamine affords 2c (Scheme 1), whose spectroscopic data
are similar to those of 2a. However, the structural data
derived from the X-ray analysis (Figure 5) show appreciable
changes that deserve comment. Thus, the plane of the
imidazoline ring in 2c is orthogonal to the PCP plane, with
the methyl substituent of the nitrogen atom apparently locked
between two phenyl groups of different phosphorus atoms.
Figure 3. A view of the structure of the cation of 2b. Selected bond
lengths [Š] and angles [8]: P1-C1 1.781(4), P2-C1 1.770(4), C1-C2
1.385(6), N1-C2 1.338(6), N1-C4 1.422(5), O1-C2 1.364(5), O1-C3
1.398(6), C3-C4 1.335(7); P1-Mn-P2 71.09(4), P1-C1-P2 99.7(2), N1-C2-
O1 107.9(3), N1-C2-C1 135.4(4), O1-C2-C1 116.7(4).
À
Consequently, the C1 C2 bond in 2c (1.461(9) Š) is clearly
longer than in 2a (1.429(4) Š) and 2b (1.385(6) Š), which
suggests that the imidazolium ylide resonance form is
dominant for 2c.
different interatomic distances in the imidazoline and oxazo-
line cycles (see the legends for Figure 2 and Figure 3). These
data are consistent with a bond description in the diphosphine
ligand resulting from the contribution of two resonance
structures, as illustrated in Figure 4 for the imidazoline case
(olefinic form a and imidazolium ylide form b). Obviously, in
the ylidic form the negative charge of the diphosphinometh-
anide fragment is stabilized by charge delocalization through
the metallacycle, whereas the positive one is stabilized by p-
donation from the nitrogen lone-pairs. If we consider com-
plexes 2a and 2b as models for carbene–carbene coupling, the
structural data described here are in agreement with the well-
The structural and spectroscopic data of complexes 2a–c
discussed above allow us to conclude that the degree of
charge transfer from the five-membered heterocycle to the
À
metallacycle through the C1 C2 bond, and therefore the
basicity of the new diphosphines, can be modulated by
changing either the heteroatoms in the cycle (imidazoline or
oxazoline), the substituents at these heteroatoms, or the
À
extent of molecular motion around the C1 C2 bond.
To summarize, we have described a new cycloaddition
reaction for the formation of imidazoline and oxazoline
functionalities, which consists of the treatment of coordinated
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103

Communications
Ph + CH), 1.26 (s, 3H, CH3) ppm; 13C{1H} NMR (75.5 MHz, CH2Cl2/
=
D2O): d = 8.1 (s, CH3), 50.4 (t, 1JC,P = 47 Hz, P2C), 156.8 (s, OCN),
212.3 (br, 4 ” CO) ppm; 31P{1H} NMR (121.5 MHz, CD2Cl2): d = 8.7
(br) ppm.
ꢀ
2c·ClO4: NH(CH3)CH2C CH (13 mL, 0.16 mmol) was added to a
solution of [Mn(CO)4{(PPh2)2C C N-p-tolyl}]ClO4 (1b·ClO4; 0.10 g,
= =
0.13 mmol) in 40 mL of toluene. The mixture was then refluxed for
50 min. A solid precipitated out of the solution on cooling, which was
isolated and dried under vacuum. Crystals suitable for an X-ray
diffraction study were obtained from crystallization from a CHCl3/
hexane mixture (1:1.5). Yield: 0.09 g, 83%. Elemental analysis (%)
calcd for C41H34ClMnN2O8P2: C 58.97, H 4.10, N 3.35; found: C 59.09,
H 4.08, N 3.58; IR (CH2Cl2): n˜ = 2079 (s), 2010 (m), 1990 (vs) cmÀ1
;
1H NMR (300 MHz, CDCl3): d = 7.6–6.9 (m, 24H, Ph), 2.63 (s, 3H,
NCH3), 2.22 (s, 3H, PhCH3), 1.91 (s, 3H, CH3) ppm; 13C{1H} NMR
(75.5 MHz, CH2Cl2/D2O): d = 11.7 (s, CH3), 22.3 (s, PhCH3), 27.7 (t,
1JC,P = 45 Hz, P2C), 35.6 (s, NCH3), 213 (s, 4 ” CO) ppm; 31P{1H} NMR
(121.5 MHz, CDCl3): d = 16.1 (br) ppm.
Received: June 29, 2004
Figure 5. A view of the structure of the cation of 2c. Selected bond
lengths [Š] and angles [8]: P1-C1 1.755(7), P2-C1 1.744(7), C1-C2
1.461(9), N1-C2 1.351(8), N1-C4 1.404(9), N2-C2 1.346(8), N2-C3
1.385(9), C3-C4 1.356(9); P1-Mn-P2 69.90(7), P1-C1-P2 99.9(), N1-C2-
N2 105.9(6), N2-C2-C1 124.9(6), N1-C2-C1 129.2(6).
Keywords: carbenes · cyclizations · manganese ·
nitrogen heterocycles · phosphines
.
[1]For recent reviews see: a) A. J. Arduengo, Acc. Chem. Res. 1999,
32, 913; b) D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand,
Chem. Rev. 2000, 100, 39 – 91; c) W. A. Herrmann, Angew.
Chem. 2002, 114, 1326; Angew. Chem. Int. Ed. 2002, 41, 1290.
[2]T. A. Taton, P. Chen, Angew. Chem. 1996, 108, 1098 – 1100;
Angew. Chem. Int. Ed. Engl. 1996, 35, 1011 – 1013.
[3]a) A. Baceiredo, A. Igau, G. Bertrand, M. J. Menu, Y. Darti-
guenave, J. J. Bonnet, J. Am. Chem. Soc. 1986, 108, 7868; b) M.
Soleilhavoup, A. Baceiredo, G. Bertrand, Angew. Chem. 1993,
105, 1245; Angew. Chem. Int. Ed. Engl. 1993, 32, 1167.
[4]Their protonated phosphonium phosphinocarbene derivatives
[(R2P)(R2PH)C:]+ are known to be stable: M. Soleilhavoup, A.
Baceiredo, O. Treutler, R. Ahlrichs, M. Nieger, G. Bertrand, J.
Am. Chem. Soc. 1992, 114, 10959.
diphosphinoketenimines with propargylamines or propargyl
alcohol. The resulting functionalized diphosphine ligands can
be considered as models for the coupling of either N-
heterocyclic carbenes or N,O-heterocyclic carbenes with
diphosphinocarbenes. These models raise the experimental
challenge of coupling imidazol-2-ylidenes (Arduengo-type
carbenes) with phosphinocarbenes (Bertrand-type carbenes)
to yield new types of electron-rich olefins, which should
behave as strongly basic phosphines.
[5]D. M. Anderson, P. B. Hitchcock, M. F. Lappert, J. Organomet.
Chem. 1989, 363, C7 – C11.
Experimental Section
[6]This complex was prepared by simple substitution of iodide by
CO, in the presence of AgClO4, in the compound fac-[MnI-
All reactions and manipulations were performed under an atmos-
phere of dry nitrogen by using standard Schlenk techniques. Solvents
were distilled from appropriate drying agents under dry nitrogen
prior to use. Chemical shifts of the NMR spectra are referenced to
internal SiMe4 (1H and 13C) or external H3PO4 (31P).
= =
(CO)3{(PPh2)2C C NPh)]: J. Ruiz, V. Riera, M. Vivanco, M.
Lanfranchi, A. Tiripicchio, Organometallics 1998, 17, 3835.
[7]J. Ruiz, F. Marquínez, V. Riera, M. Vivanco, S. García-Granda,
M. R. Díaz, Chem. Eur. J. 2002, 8, 3872 – 3878.
[8]a) “The Chemistry of Ketenes, Allenes and Related Com-
pounds”: M. W. Barker, W. E. McHenry, in The Chemistry of
Functional Groups, Part 2 (Ed.: S. Patai), Interscience, New
York, 1980, chapter 17, pp. 701 – 720; b) G. Tennant, in Compre-
hensive Organic Chemistry. The Synthesis and Reactions of
Organic Compounds, Vol. 2 (Eds.: D. Barton, W. D. Ollis, I. O.
Sutherland), Pergamon, Oxford, 1979, pp. 21 – 527.
= =
2a·ClO4: [Mn(CO)4{(PPh2)2C C NPh}]ClO4 (1a·ClO4; 0.05 g,
0.066 mmol) was dissolved in 20 mL of toluene, and NH2CH2C CH
ꢀ
(10 mL, 0.13 mmol) was added to this solution. The mixture was then
refluxed for 30 min. Cooling the solution to room temperature led to
the formation of yellow crystals that were suitable for X-ray analysis.
Yield: 0.04 g, 76%. Elemental analysis (%) calcd for
C39H30ClMnN2O8P2: C 58.04, H 3.75, N 3.47; found: C 57.87, H
4.03, N 3.53; IR (CH2Cl2): n˜ = 2081(s), 2009 (sh), 1999 (vs), 1979
[9]Crystal data for 2a·ClO4 (C39H30ClMnN2O8P2): Mr = 806.98,
monoclinic, space group P21/a, a = 17.3936(5), b = 11.2150(3),
c = 19.5595(6) Š, b = 106.637(2)8, V= 3655.73(18) Š3, Z = 4,
1
(s) cmÀ1. H NMR (300 MHz, CD2Cl2): d = 8.28 (s, 1H, NH), 7.8–6.6
=
(m, 26H, Ph
+
CH), 1.60 (s, 3H, CH3) ppm; 31P{1H} NMR
(121.5 MHz, CD2Cl2): d = 3.7 (br) ppm.
1calcd = 1.466 gcmÀ3
1.5418 Š), m = 4.913 mmÀ1
,
F(000) = 1656, CuKa radiation (l =
crystal dimensions 0.25 ” 0.12 ”
ꢀ
2b·ClO4: HOCH2C CH (20 mL, 0.33 mmol) was added to a
;
solution of 1a·ClO4 (0.05 g, 0.066 mmol) in 10 mL of toluene. The
mixture was then refluxed for 15 min. A solid precipitated out of the
solution upon cooling, which was isolated and dried under vacuum.
Crystals suitable for X-ray analysis were obtained from a mixture of
CHCl3 and hexane (1:1.5). Yield: 0.03 g, 57%. Elemental analysis
(%) calcd for C39H29ClMnNO9P2: C 57.97, H 3.62, N 1.73; found: C
57.75, H 3.62, N, 1.58; IR (CH2Cl2): n˜ = 2083 (s), 2014 (s), 2001 (vs),
0.05 mm. Data collection was performed at 120(2) K on a
Nonius Kappa CCD diffractometer. The structure was solved by
direct methods and refined using full-matrix least-squares
techniques on F2. Non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were placed geometrically and left riding
on their parent atoms, except for H1 and H3, which were found
in the difference map and refined isotropically. The final cycle of
full-matrix least-squares refinement based on 6890 reflections
1985 (s) cmÀ1
.
1H NMR (300 MHz, CDCl3): d = 6.6–8.0 (m, 26H,
104
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Angew. Chem. Int. Ed. 2005, 44, 102 –105

Angewandte
Chemie
and 486 parameters converged to R1 (F2 > 2s(F2)) = 0.0426 and
wR2 (F2 > 2s(F2)) = 0.1106. Crystal data for 2b·(ClO4)·CHCl3
¯
(C40H30Cl4MnNO9P2): Mr = 927.33, triclinic, space group P1,
a = 11.1996(3),
b = 11.9095(2),
c = 16.1366(3) Š,
a =
74.9480(10), b = 79.413(2), g = 74.0010(10)8, V= 1983.34(7) Š3,
Z = 2, 1calcd = 1.553 gcmÀ3, F(000) = 944, CuKa radiation (l =
1.5418 Š), m = 6.438 mmÀ1
; crystal dimensions 0.32 ” 0.30 ”
0.10 mm. Data collection was performed at 120(2) K on a
Nonius Kappa CCD diffractometer. The structure was solved by
direct methods and refined using full-matrix least-squares
techniques on F2. All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were placed geometrically and left
riding on their parent atoms except for H3 and H1D, which were
found in the difference map and refined isotropically. The final
cycle of full-matrix least-squares refinement based on 7427
reflections and 523 parameters converged to R1 (F2 > 2s(F2)) =
0.0727 and wR2(F2 > 2s(F2)) = 0.2075. Crystal data for
2c·(ClO4)·(CHCl3)2 (C43H36Cl7MnN2O8P2): Mr = 1073.77, tri-
¯
clinic, space group P1, a = 11.6701(16), b = 11.733(2), c =
18.812(3) Š, a = 107.144(10), b = 92.229(7), g = 99.475(10)8,
V= 2417.3(6) Š3, Z = 2, 1calcd = 1.475 gcmÀ3
, F(000) = 1092,
CuKa radiation (l = 1.5418 Š), m = 6.843 mmÀ1; crystal dimen-
sions 0.17 ” 0.10 ” 0.03 mm. Data collection was performed at
200(2) K on a Nonius Kappa CCD diffractometer. The structure
was solved by direct methods and refined using full-matrix least-
squares techniques on F2. All non-hydrogen atoms were refined
anisotropically except for one oxygen atom in the counteranion,
which is disordered over two positions. Hydrogen atoms were
placed geometrically and left riding on their parent atoms,
except for H3, H7, H8, H10, and H11, which were found in the
difference map and refined isotropically. The final cycle of full-
matrix least-squares refinement based on 7386 reflections and
590 parameters converged to R1 (F2 > 2s(F2)) = 0.0868 and
wR2(F2 > 2s(F2)) = 0.2437. CCDC-241569 (2a), -241570 (2b),
and -241571 (2c) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
[10]Y. Kim, A. Streitwieser, J. Am. Chem. Soc. 2002, 124, 5757 –
5761, and references cited therein.
[11]J. Ruiz, M. E. G. Mosquera, G. García, E. Patrꢀn, V. Riera, S.
García-Granda, F. Van der Maelen, Angew. Chem. 2003, 115,
4915 – 4919; Angew. Chem. Int. Ed. 2003, 42, 4767 – 4771.
[12]IR (CH 2Cl2) data for representative tetracarbonyl complexes of
MnI are as follows: n˜[Mn(CO)4{(PPh2)2CH2}]+ = 2094 (m), 2027
(m), 2014 (s), 1999 (sh) cmÀ1; n˜[Mn(CO)4{(PPh2)2CH}] = 2071
(m), 1990 (s), 1955 (m) cmÀ1: J. Ruiz, V. Riera, M. Vivanco, S.
García-Granda, A. García-Fernꢁndez, Organometallics 1992, 11,
4077 – 4082.
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