Chiral phosphine-phosphonium ylide rhodium complexes

Article abstract of DOI:10.1021/om000885n

The phosphonium methylide of (R)-BINAP acts as a novel chiral dissymmetric chelating ligand in the stable [Rh(cod)(BINAPCH₂)]⁺ complex. A rationale for the conformation of the eight-membered metallacycle derived from chiral phosphine-phosphonium ylide ligands is proposed on the basis of DFT calculations on a model complex.

Full text of DOI:10.1021/om000885n

808
Organometallics 2001, 20, 808-810
Ch ir a l P h osp h in e-P h osp h on iu m Ylid e Rh od iu m
Com p lexes
Lydie Viau, Christine Lepetit, Ge´rard Commenges, and Remi Chauvin*
Laboratoire de Chimie de Coordination du CNRS, 205 Route de Narbonne,
31077 Toulouse Cedex 4, France
Received October 16, 2000
Sch em e 1. Decoor d in a tion of Sta bilized Ylid es of
Summary: The phosphonium methylide of (R)-BINAP
acts as a novel chiral dissymmetric chelating ligand in
the stable [Rh(cod)(BINAPCH2)]+ complex. A rationale
for the conformation of the eight-membered metallacycle
derived from chiral phosphine-phosphonium ylide
ligands is proposed on the basis of DFT calculations on
a model complex.
BINAP for Z ) CO2Ra
a
Whereas the coordination chemistry of chiral diphos-
phines with group 9 and group 10 metals has been
widely studied, owing to their role in asymmetric
catalysis,1 complexes of chiral diylides have received
little specific attention. Related to them, however, are
chiral iminophosphoranes, which can be regarded as
aza-phosphonium ylide ligands and which were shown
to give efficient asymmetric catalysts for copper-medi-
ated cyclopropanation2a,b and palladium-mediated allylic
alkylation.2c A palladium iminophosphorane-phosphine
complex derived from an achiral diphosphine has also
been reported.3a A natural prolongation is the study of
hybrid ligands containing both a η1-phosphine terminus
and a η1-C nonstabilized phosphonium ylide terminus.3b,4
S denotes a solvent molecule. 1a is the chelate form and
1b the open form.
Sch em e 2. P r ep a r a tion of Rh od iu m
P h osp h in e-P h osp h on iu m Meth ylid e Com p lex 4
A phosphine-phosphonium precursor derived from
the chiral skeleton BINAP5 is considered. Indeed, this
ligand lends itself to monoquaternization by methyl
iodide to give a chiral phosphonio-phosphine termed
as “methylbinapium” 2.7
To the best of our knowledge, no chiral version of this
hybrid type of ligand was reported until recently, when
stabilized monophosphonium ylides of BINAP were
shown to act as chelating ligands in Rh(I) and Pd(II)
complexes.8a However, the stabilized nature of the free
ylides gave rise to an equilibrium between a chelate
form (1a ) and an open form (1b) of the complex (Scheme
1). This is reminiscent of the hemilabile character of the
BINAP monoxide ligand.8b
The design of a nonstabilized free ylide is anticipated
to shift the equilibrium to the chelate form (1a ). The
ylide of methylbinapium 2 has thus been considered.
As a model, the methylphosphonium tetrafluoroborate
salt 3 was first prepared from 1,2-bis(diphenylphosphi-
no)benzene.9 Addition of 10% of DMSO was required to
dissolve 3 in a THF/DMSO mixture, and deprotonation
by n-BuLi afforded the non-salt-free ylide, which was
then reacted in situ with [Rh(cod)Cl]2. 1H and 31P NMR
analysis indicates that formation of complex 4 is quan-
titative.10 In contrast to the reaction of stabilized ylides
and diylides with Pd(cod)Cl2, the cyclooctadiene ligand
is neither attacked11a nor displaced.11b Nevertheless, due
to the presence of DMSO, chloride anions are present
in the medium and the structure of 4 could be assigned
to either an ionic or a zwitterionic form (Scheme 2).
* To whom correspondence should be addressed. Tel: 05 61 33 31
13. Fax: 05 61 55 30 03. E-mail: chauvin@lcc-toulouse.fr.
(1) Brown, J . M. In Comprehensive Asymmetric Catalysis; J acobsen,
E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: Heidelberg, Germany,
1999; Vol. 1, Chapter 5.1, and references therein.
(2) (a) Reetz, M. T.; Bohres, E.; Goddard, R. Chem. Commun. 1998,
935-936. (b) Brunel, J .-M.; Legrand, O.; Reymond, S.; Buono, G. J .
Am. Chem. Soc. 1999, 121, 5807. (c) Sauthier, M.; Fornie´s-Camer, J .;
Toupet, L.; Re´au, R. Organometallics 2000, 19, 553-562.
(3) (a) Molina, P.; Arques, A.; Garcia, A.; de Arellano, M. C. R. Eur.
J . Inorg. Chem. 1998, 1359. (b) H. Schmidbaur, Angew. Chem., Int.
Ed. Engl. 1983, 22, 907.
(4) Belluco, U.; Michelin, R. A.; Mozzon, M.; Bertani, R.; Facchin,
G.; Zanotto, L.; Pandolfo, L. J . Organomet. Chem. 1998, 557, 37.
(5) Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito, T.;
Souchi, T.; Noyori, R. J . Am. Chem. Soc. 1980, 102, 7932.
(6) Brunet, J . J .; Chauvin, R.; Donnadieu, B.; Leglaye, P. Tetrahe-
dron Lett. 1998, 39, 9179.
(7) Methyldiopium derived from DIOP was also considered, but the
corresponding ylide turned out to be unstable.
(8) (a) Ohta, T.; Fujii, T.; Kurahashi, N.; Sasayama, H.; Furukawa,
I. Sci. Eng. Rev. Doshisha University 1998, 39, 133; Chem. Abstr. 1998,
130, 139441r. (b) Gladiali, S.; Alberico, E.; Pulacchini, S.; Kollar, L. J .
Mol. Catal. A 1999, 143, 155.
(9) Bowmaker, G. A.; Herr, R.; Schmidbaur, H. Chem. Ber. 1983,
116, 3576.
10.1021/om000885n CCC: $20.00 © 2001 American Chemical Society
Publication on Web 02/01/2001

Communications
Organometallics, Vol. 20, No. 5, 2001 809
Sch em e 3. Syn th esis of th e Meth ylbin a p iu m Ylid e Rh od iu m Com p lex 6
Ch a r t 1. Mod el 7 of Com p lex 6 Used for DF T
Mod elin g
The tetrafluoroborate salt of methylbinapium 2 was
then reacted with a base (n-BuLi or LDA) in a 9/1 THF/
DMSO mixture at 0 °C.12 Ylide 5 was converted in situ
to the non-salt-free oily complex 6 by reaction with [Rh-
(cod)Cl]2 (Scheme 3). DMSO and inorganic salts could
be extracted in water. Indeed, complex 6 is stable at
20-25 °C in CDCl3, in methanol and in CH2Cl2/water
mixtures. Chloride ions could be removed by addition
of AgBF4 to a CH2Cl2 solution of the crude product. The
NMR data, however, remained the same, suggesting
that the noncoordinating nature of the chloride coun-
teranions was preexisting in a nonzwitterionic 16-
electron Rh(I) structure.
2
and J PRh coupling constants would vanish in the open
form, while the observed multiplet signals are sharp and
consistent with a well-defined P-Rh-CH2-P+ environ-
ment.8a,13b
Since no crystal structure could be obtained, struc-
tural information was extracted from geometry optimi-
zation of the model complex 7 at the B3PW91/6-31G*/
LANL2DZ(Rh) level. In 7, the BINAP skeleton of 6 was
simplified by exchanging PPh2 for PH2, the binaphthyl
for a bis(o-tolyl), and the cod ligand for two ethylene
ligands (Chart 1).
The atropoisomeric chirality is thus retained in order
to estimate the diastereoisomeric control of the R/S
chirality element of the ligand on the conformation of
the eight-membered metallacycle. The latter is regarded
as resulting from the insertion of an ylide CH2 unit into
a seven-membered metallacycle: the structure is there-
fore discussed by reference to the λ,δ nomenclature
coined for the seven-membered C2-symmetric metalla-
cycles (Chart 2).15 With an R ligand configuration,
geometry optimization led to the λ-type conformation
7a of the seven-membered cyclic sequence of atoms in
the metallacycle, from which the ylide CH2 group has
Structure 6 is supported by a clean +ESMS three-
peak fragmentation at m/z 847.2, which fits the calcu-
lated isotopic pattern for the [(BINAPCH2)Rh(cod)]+
fragment. 31P NMR data indicate a η1 coordination of
1
the ylide CH2 group: δ 25.65 (dd, J PRh ) 155 Hz,
3
2
J PP ) 5.3 Hz, PRh) and 34.18 (dd, J PRh ) 3J PP ) 5.2
+
+
Hz, P+CH2Rh). These chemical shifts and the coupling
constant values are consistent with literature data for
phosphorus atoms in similar environments.8a The ylidic
CH2 group is also characterized by shielded H and 13C
1
NMR signals: in contrast with the reported difficulty
in observing analogous signals in other Rh(I) ylide
complexes,13 the diastereotopic protons occur as resolved
multiplets, at δ 0.58 and 1.22. The coupling constants
were assigned by selective 31P decoupling experiments
2
2
3
(2J HH ) 1.6 Hz, J HRh ≈ 11.4 Hz, J HP ≈ 11 Hz, J HP
≈
+
5.5 Hz). All of the protons of the binaphthylene and the
cyclooctadiene moieties were accurately assigned by 1H,
31P, 13C, COSY 1H-1H, and COSY 1H-13C NMR analy-
sis.14 No evidence of a decoordination of the methylide
terminus (Scheme 1 for Z ) H) was observed from the
31P NMR spectrum of 6. A fast exchange on the NMR
time scale between the resonances of open and chelate
(14) Other NMR data for 6 are as follows (see atom numbering in
the Supporting Information). 1H NMR (CDCl3, 400 MHz): δ 0.58 (dddd;
1
2
2
3
+
J HH ) 1.6 Hz, J HRh ) 11.4 Hz, J HP ) 11 Hz, J HP ) 5.5 Hz, 1 H,
3
+
P+CH2Rh); 1.11-1.18 (m, 1 H, Hf); 1.18-1.25 (m, 1 H, P+CH2Rh); 1.34-
1.37 (m, 1 H, Hf ′); 1.50-1.60 (m, 1 H, Hg′); 1.60-1.75 (m, 2 H, Hh,
Hh′); 1.75-1.90 (m, 1 H, Hg); 2.37-2.50 (m, 1 H, He′); 2.75-2.82 (m, 1
H, Hd); 2.82-2.89 (m, 1 H, He); 3.25-3.37 (m, 1 H, Hc); 4.96-5.04 (m,
forms of the complex can indeed be ruled out: the J PP
(10) NMR data for spectroscopically pure complex 4 are as follows.
1H NMR (CDCl3, 200 MHz): δ 1.30-1.40 (m, 1 H, PCH2); 1.41-1.50
(m, 1 H, PCH2); 1.70-2.20 (m, 8 H, CH2(cod)); 3.43-3.53 (m, 2 H,
CH(cod)); 4.73-4.83 (m, 2 H, dCHcod); 7.24-7.72 (m, 24 H, aromatic
3
3
2 H, Ha, Hb); 5.60 (d, 1 H, J HH ) 8.5 Hz, Hi); 5.90 (d, 1 H, J HH ) 8.5
3
3
Hz, Hi′); 6.42 (dd, J HH ) 8.2 Hz, J HH ) 7.2 Hz, 1 H, Hj); 6.66 (dd,
3
3J HH ) 8.2 Hz, J HH ) 7.2 Hz, Hj′); 6.74-6.82 (m, 1 H, Hn); 6.89-6.92
2
3
3
CH). 31P{1H} NMR (CDCl3, 81 MHz): δ 28.75 (dd, J PRh ) 5 Hz,
(m, 1 H, Hm); 7.16 (dd, J HH ) 7.9 Hz, J HH ) 7.2 Hz, 1 H, Hk); 7.30
3 3
1
J P ) 26 Hz, P+Ph2CH2Rh); 33.96 (dd, J PRh ) 163 Hz, J PP ) 26 Hz,
(dd, J HH ) 7.9 Hz, J HH ) 7.2 Hz, 1 H, Hk′); 7.60-7.68 (m, 2 H Hl,
Hl′); 7.79-7.84 (m, 1 H, Hm′); 8.06-8.12 (m, 1 H, Hn′); 6.9-8.1 (m, 20
H, C6H5P, C6H5P+). 13C{1H,31P+} NMR (CDCl3, 100 MHz): δ 8.29 (dd,
+
+P
PPh2Rh). For an analogous phosphino-iminophosphorane rhodium
complex displaying similar NMR characteristics, see: Reed, R. W.;
Santarsiero, B.; Cavell, R. G. Inorg. Chem. 1996, 35, 4292.
1J CRh ) 27.3 Hz, J CP ) 26.9 Hz, CH2Rh); 27.36 (s, Cf); 31.21 (s, Cg);
2
1
2
(11) (a) Vicente, J .; Chicote, M.-T.; MacBeath, C.; Fernandez-Baeza,
J .; Bautista, D. Organometallics 1999, 18, 2677. (b) Spannenberg, A.;
Baumann, W.; Rosenthal, U. Organometallics 2000, 19, 3991.
(12) 31P NMR data for 5 (81 MHz, THF/DMSO, external lock on
C6D6): δ 26.55 (broad dd; 2J PH ) 10 and 13 Hz; (Np)Ph2PCH2); -14.03
(m; P(Np)Ph2). Np ) naphthylene substituent.
(13) (a) Werner, H.; Hofman, L.; Paul, W.; Schubert, U. Organome-
tallics 1988, 7, 1106. (b) Marder, T. B.; Fultz, W. C.; Calabrese, J . C.;
Harlow, R. L.; Milstein, D. J . Chem. Soc., Chem. Commun. 1987, 1543.
34.00 (s, Ch); 35.37 (s, Ce), 86.17 (dd, J CRh ) 7.0 Hz, J CP ) 18.5 Hz,
1 1
Cd); 89.05 (d, J CRh ) 8.0 Hz, Cc); 92.12 (d, J CRh ) 8.0 Hz, Cb); 96.73
(dd, 1J CRh ) 11.0 Hz, 2J CP ) 18.5 Hz, Ca); 120.05-143.00 (m, aromatic
C). 13C{1H,31P,31P+} NMR (CDCl3, 100 MHz): δ 8.29 (d, J CRh ) 27.3
1
Hz, CH2Rh); 27.36 (s, Cf); 31.21 (s, Cg); 34.00 (s, Ch); 35.37 (s, Ce), 86.17
1
1
1
(d, J CRh ) 7 Hz, Cd); 89.05 (d, J CRh ) 8 Hz, Cc); 92.12 (d, J CRh ) 8
1
Hz, Cb); 96.73 (d, J CRh ) 11 Hz, Ca); 120-143 (m, aromatic C).
(15) Pavlov, V. A.; Klabunovskii, E. I.; Struchkov, Voloboev, A. A.;
Yanovsky, A. I. J . Mol. Catal. 1988, 44, 217.

810 Organometallics, Vol. 20, No. 5, 2001
Communications
Ch a r t 2. Ra tion a le for th e Con for m a tion a l An a lysis of Eigh t-Mem ber ed P h osp h in e-P h osp h on iu m
Meth ylid e Meta lla cycles
ration of a third chirality element, namely the PRh-
CH2P+ axis. The corresponding M/ P chirality element
is weakly controlled by the λ/ δ ring chirality element
(Chart 2): the (λ,M) configuration is favored by c.a. 4
kcal‚mol-1 over the (λ,P) configuration (Chart 2). This
suggests a (R,λ,M)-7a U (R, λ, P)-7b flexibility of the
eight-membered chelation ring at room temperature.
1
Finally, the H, 13C, and 31P NMR chemical shifts of
the CH2P+ group in the R,λ,M structure 7a and R,λ,P
structure 7b have been calculated.18 A qualitatively
good agreement is found between calculated (gas phase)
and experimental (CDCl3) data (Supporting Informa-
tion).
In conclusion, methylbinapium methylide leads to a
stable eight-membered chelated rhodium(I) complex
with a novel type eight-membered metallacycle. In the
presence of more or less coordinating anions, the metal
would lie in the peculiar dissymmetric chiral electro-
static environment of formally â-zwitterionic organo-
metalate complexes.19 This feature should provide spe-
cific effects on the catalytic properties of such complexes,
which are currently under investigation.20
F igu r e 1. Two optimized R,λ type geometries of the model
complex 7 at the B3PW91/6-31G*/LANL2DZ level. sp2 CH
and ethylene ligands have been omitted for clarity. The
centroids X and Y of the ethylene ligands are almost in
the PfRh-CH2 plane, which lies 9.5° off the X‚‚‚Rh‚‚‚Y
plane.
been excluded (Figure 1). In this configuration, denoted
as R,λ, the ylide CH2 group is slightly tilted out of the
P-Rh‚‚‚P+ plane, anti to the CH2CH2P+ ethylene se-
quence. This corresponds to an M configuration of the
P-Rh-CH2-P+ helicoidal axis.16 A second R,λ-type
conformation, 7b, is found 3.7 kcal mol-1 above the
former, where the ylide CH2 group is switched syn to
the CH2CH2P+ ethylene sequence and corresponds to a
P configuration of the P-Rh-CH2-P+ helicoidal axis
(Figure 1).17
No R,δ-type conformation could be obtained as a
minimum. Therefore, the R/ S configuration of the
atropoisomeric axis completely controls the λ/ δ config-
uration of the fictitious seven-membered metallacycle.
The conformation of a real eight-membered metallacycle
is characterized by another feature: the M/ P configu-
Ack n ow led gm en t. We thank Dr. Miche`le Soleil-
havoup and Ms. Christina Canal for discussions. We
also thank Dr. Philippe Arnaud for computational
assistance and CALMIP (CALcul en MIdi-Pyre´ne´es,
Toulouse, France) for computational facilities.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures and comprehensive NMR characteristics
for compounds 3, 4, and 6 and detailed computational results.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM000885N
(18) The NMR spectra were calculated at the B3PW91/6-31G*/
LANL2DZ(Rh) level within the framework of the GIAO formalism
implemented in Gaussian98 (Wolinski, K.; Hinton, J . F.; Pulay, P. J .
Am. Chem. Soc. 1990, 112, 8251. Cheeseman, J . R.; Trucks, G. W.;
Keith, T. A.; Frisch, M. J . J . Chem. Phys. 1996, 104, 5497).
(19) Chauvin, R. Eur. J . Inorg. Chem. 2000, 577.
(16) See for example: Dodziuk, H.; Mirowickz, M. Tetrahedron:
Asymmetry 1990, 1, 171.
(17) Other geometrical parameters of the two minima are similar.
In particular, the H2C-Rh-PH2 bite angles, ca. 88 and 95°, indicate
an approximate square-planar crystal field around the rhodium atom
in both the conformers.
(20) Brunet, J . J .; Chauvin, R.; Commenges, G.; Donnadieu, B.;
Leglaye, P. Organometallics 1996, 15, 1752.