Ruthenium(IV) complexes featuring p,O-chelating ligands: Regioselective substitution directly from allylic alcohols

Article abstract of DOI:10.1002/anie.200907034

(Figure Presented) Branching out: A new ruthenium(IV) complex (1), containing a P,O-chelating ligand, is an efficient precatalyst for regioselective allylations starting from various allylic alcohol derivatives.

Full text of DOI:10.1002/anie.200907034

Communications
DOI: 10.1002/anie.200907034
Homogeneous Catalysis
Ruthenium(IV) Complexes Featuring P,O-Chelating Ligands:
Regioselective Substitution Directly from Allylic Alcohols**
Basker Sundararaju, Mathieu Achard, Bernard Demerseman, Loic Toupet,
Gangavaram V. M. Sharma, and Christian Bruneau*
Transition metal catalyzed regioselective C- and O-allylation
reaction is a powerful tool to access chiral branched
compounds.[1] In particular, ruthenium complexes featuring
[Ru(Cp*)] and [Ru(Cp)] (Cp* = h5-C5Me5, Cp = cyclopenta-
dienyl) moieties have received special attention in allylation
reactions, as some of them are able to promote the selective
formation of branched or linear products directly from allylic
alcohols with water as the only by-product.[2–4] Therefore,
several new ruthenium(IV) complexes have emerged as
efficient precatalysts to achieve these transformations. How-
ever, in most reports branched-type products were obtained
from branched allylic alcohol substrates, whereas the use of
linear starting substrates resulted in a loss of selectivity.[5]
Recently, monocationic [RuIV(Cp)]-based catalysts contain-
ing a five-membered pyridine-2-carboxylato N,O chelate
have been successfully used,[2] and even applied in an
enantioselective intramolecular allylation thereby avoiding
the selectivity issue (Figure 1).[2c] Dicationic [Ru(Cp*)(allyl)-
(MeCN)2]2+ complexes[3a,b] and catalytic systems generated
from [Ru(Cp*)(CH3CN)3]PF6 (A) in the presence of sulfonic
acid derivatives represent other types of catalyst precursors
used with success in regioselective C allylation (Figure 1).[5,6]
In contrast, P,O chelates have received less attention. A
notable example has been reported for [Ru3(CO)12] and
diphenylphosphino benzoic acid (DPPBA; 1a) to afford a
ruthenium(II) [Ru(p-allyl)(CO)2(k2-DPPBz)] (DPPBz =
diphenylphosphino benzoate) complex featuring a six-mem-
bered P,O chelate, which allowed linear allylation of nucleo-
philes from allyl acetates.[7] To the best of our knowledge,
anionic phosphine monosulfonate ligands[8] have never been
applied in allylation reactions with ruthenium. These ligands
have been widely used because of their water solubility
properties, and have found applications in copolymerization
processes with ethylene involving palladium and nickel as
transition metals.[9] They have also been applied to hydro-
formylations employing rhodium[10] and to palladium-cata-
lyzed Heck coupling reaction.[11]
Herein, we report the synthesis of novel ruthenium(IV)
allyl complexes bearing DPPBz and diphenylphosphinoben-
zene sulfonate (DPPBS) ligands, and their catalytic activities
in regioselective allylation reactions of various nucleophiles,
starting from both branched and linear allylic derivatives are
compared.
Reaction of [Ru(Cp*)(CH3CN)3]PF6 (A)[12] with ligand 1a
in the presence of crotyl alcohol 2a using dichloromethane as
the solvent gave the RuIV complex B, bearing the six-
membered ring P,O chelate, in an excellent 90% yield
(Scheme 1). Surprisingly, the reaction of A with o-diphenyl-
phosphinobenzenesulfonic acid ligand 1b did not afford the
analogous complex C (see Scheme 2), but led to the formation
of the allylphoshonium salt 3a, and the starting ruthenium
complex A was recovered regardless of the solvent used.
However, in the absence of A, no allylphosphonium salt 3a
was formed at room temperature, indicating that a ruthe-
nium-catalyzed allylation of 1b was involved.
We believe the difference in the reactivities between 1a
and 1b is related to the zwitterionic nature of 1b and to the
stability of the corresponding ruthenium(IV) species under
acidic conditions. Therefore, to avoid the presence of acid,
treatment of compound 1b with potassium tert-butoxide in
methanol and then successive addition of A and allyl chloride
4a afforded the expected complex C in 80% yield after
crystallization (Scheme 2).
Figure 1. Active complexes in substitution from allyl alcohol
derivatives.
[*] B. Sundararaju, Dr. M. Achard, Dr. B. Demerseman, Dr. C. Bruneau
UMR6226 CNRS—Universitꢀ de Rennes, Sciences chimiques de
Rennes, Catalyse et Organomꢀtalliques
Campus de Beaulieu, 35042 Rennes Cedex (France)
Fax: (+33)2-2323-6939
E-mail: christian.bruneau@univ-rennes1.fr
Homepage: http://scienceschimiques.univ-rennes1.fr/catalyse/
Dr. L. Toupet
UMR 6251 CNRS—Universitꢀ de Rennes, Institut de Physique de
Rennes, Campus de Beaulieu, 35042 Rennes Cedex (France)
Dr. G. V. M. Sharma
The solid-state structure of complex B clearly demon-
strates that the phosphino carboxylate coordinates as a
bidentate ligand through both the phosphorous and oxygen
atoms. The allyl moiety shows an endo configuration and a
cis relationship between the methyl substituent and the
carboxylate moiety (Figure 2).[13] As observed in most ruth-
Organic Chemistry Division III, D 211, Discovery Laboratory, Indian
Institute of Chemical Technology, Hyderabad 500 007 (India)
[**] This work was supported by a grant from CEFIPRA/IFCPAR (IFC/A/
3805-2/2008/1720) to B.S.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200907034.
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Angewandte
Chemie
(Figure 3).[15] Whereas the half-chair conformation
of the six-membered metallacycle was found to be
exo for B, the structure of C revealed an endo
conformation. Notably, the ruthenium complex C is
closely related to the active species [RuCp*(RSO3)-
(allyl)(MeCN)][RSO3] proposed by Pregosin and co-
workers when they used [A + RSO3H] as a catalyst
precursor (Figure 1).[6b]
With these complexes in hand, we performed a
comparative study of complexes B and C as preca-
talysts for the O allylation of p-methoxyphenol (5a)
in the presence of cinnamyl chloride (4b; Table 1).
Reaction with B appeared to be regioselective,
thereby favoring the branched product, and the best
result was obtained using dimethyl carbonate (DMC)
as the solvent; this reaction led to a branched/linear
Scheme 1. Reactivity of [Ru(Cp*)(CH3CN)3]PF6 (A) with 1a and 1b.
Scheme 2. Preparation of complex C.
enium(IV) complexes bearing an unsymmetrical allylic
ligand, the distance from the ruthenium center to the
substituted carbon atom of the allylic ligand is longer than
that to the terminal CH2 group, leading to a distorted p-allyl
ligand coordination.[14]
Structural information on the complex C was also
provided by the X-ray diffraction study which showed the
ruthenium atom to be coordinated by the P,O chelate
Figure 3. Thermal ellipsoids diagram (50%) of C. All hydrogen atoms
and the PF6 moiety are omitted for clarity. Selected bond lengths [ꢁ]
and angles [8]: Ru(1)–O(1) 2.136, Ru(1)–P(1) 2.421, Ru(1)–C(1) 2.224,
Ru(1)–C(2) 2.162, Ru(1)–C(3) 2.225, C(1)–C(2) 1.407, C(2)–C(3)
1.386; O(1)-Ru(1)-P(1) 77.10, C(1)-Ru(1)-C(3) 64.32.
Table 1: O Allylation of 5a with cinnamyl chloride 4b.[a]
Entries
Catalyst
Solvent
T [8C]
6b/7b[b]
Yield [%][c]
1
2
3
4
5
B
B
B
C
C
CH3CN
CH2Cl2
DMC
CH3CN
CH2Cl2
40
40
40
RT
RT
86:14
81:19
94:6
98:2
97:3
77
71
93
95
95
Figure 2. Thermal ellipsoids diagram (50%) of B·MeOH. All hydrogen
atoms, MeOH, and the PF6 moiety are omitted for clarity. Selected
bond lengths [ꢁ] and angles [8]: Ru(1)–O(2) 2.096, Ru(1)–P(1) 2.374,
Ru(1)–C(1) 2.203, Ru(1)–C(2) 2.184, Ru(1)–C(3) 2.316, C(1)–C(2)
1.427, C(2)–C(3) 1.383; O(2)-Ru(1)-P(1) 76.82, C(1)-Ru(1)-C(3) 64.51.
[a] All reactions were carried out at 0.08m concentration in specified
solvent for 16 h under inert atmosphere with 4b/5a/[Ru]/K2CO3 in
1.1:1:0.025:1.1 molar ratio. [b] Ratio determined by H NMR analysis of
1
the crude reaction mixture. [c] Yield of isolated products.
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Communications
(6b/7b) ratio of up to 94:6 (Table 1, entry 3). Notably,
purely aliphatic linear hex-2-en-1-ol (2e) also led to the
branched product with remarkable regioselectivity in favor of
9e (Table 2, entry 5).
The reaction of the branched and linear isomers of
cinnamyl alcohol 2b and vinyl benzyl alcohol 2 f proceeded
smoothly, giving a regioselectivity of up to 98:2 (Table 2,
entries 6, 7). The use of a tertiary alcohol such as dimethyl
vinyl methanol (2g) resulted in the formation of the branched
product 9g as the major compound (Scheme 3).
moderate heating was necessary to achieve full conversion
of the starting material 5a (Table 1, entries 1–3), thereby
highlighting the lower activity which results from the presence
of the electron-rich phosphine as compared with our previous
results based on analogous ruthenium precatalysts featuring
an N,O chelate.[16] The presence of the more electron-
deficient sulfonate moiety in C was expected to lead to
more reactive ruthenium species and indeed, good reactivity
and regioselectivity were obtained at room temperature using
either acetonitrile or dichloromethane (Table 1, entries 4 and
5).[6b] Moreover, the reaction was faster and full conversion of
5a occurred within six hours in dichloromethane. In the
presence of precatalyst A or C, no reaction occurred when an
allylphosphonium was treated with 5a, which tends to
demonstrate that the cationic complex C is the active species
during the allylation reactions.[17]
We then explored the reactivity of various allylic alcohols
2 in the C allylation of indole 8 (Table 2). When the indole 8
was treated with 1.2 equivalents of allyl alcohol 2c in the
presence of B at 608C, 3-allylindole (9c) was obtained in 85%
yield (Table 2, entry 1). Interestingly, C gave the same
product 9c, even at room temperature and it was isolated in
83% yield (Table 2, entry 2).
The scope of this catalytic reaction was then evaluated
starting from various allylic alcohols. For substituted allyl
alcohols the reaction was carried out at 608C with an
additional catalytic amount of 1b (6 mol%), to ensure
complete conversion within 16 hours, and in the presence of
water to avoid diallyl ether formation as a side reaction.
Therefore, reaction of but-3-en-2-ol (2d) or crotyl alcohol 2a
in the presence of an extra catalytic amount of 1b afforded
the branched 3-allylated compound 9a in good yield with
excellent regioselectivity (Table 2, entries 3 and 4). The other
Scheme 3. Regioselective allylation of 8 with tertiary alcohol 2g.
DCE=1,2-dichloroethane.
For these reactions we propose that the benefit of a
catalytic excess of ligand 1b might result from its ability to
facilitate the allyl formation through alcohol protonation, but
we cannot exclude a possible counteranion exchange for
À
complex C (PF6 to sulfonate), which would lead to the
formation of dicationic allyl ruthenium diphosphine spe-
cies.[4,18] This new catalytic system is highly regioselective for
the formation of branched products starting from either
branched or linear allylic alcohols. It represents a comple-
mentary catalytic system to previous ones, which were either
unreactive or much less regioselective, especially when
starting from linear substrates.[5,6a]
The difference of reactivity between linear and branched
starting allyl alcohols 2 was next investigated for the
diallylation of 8 in the presence of an excess amount of
alcohol. Therefore, the reaction of 8 in the presence of three
equivalents of crotyl alcohol 2a and a catalytic amount of C
resulted in the complete consumption of the indole 8 to afford
mono C-allylated compound 9a and a small amount of
diallylated product 11a (Scheme 4). However, the use of the
branched allylic alcohol 2d gave the diallylated product 11a
as the major compound with a high level of regioselectivity in
favor of the dibranched product. The double allylation of
indoles at the same position has been reported, but only
starting from allyl alcohol itself leading to only one possible
product corresponding to N-allylated 9c. For comparison, a
Table 2: Regioselective allylation of indole with allylic alcohols 2a–f.[a]
Entry Catalyst T [8C]
Alcohol 2
9/
R
Product
(yield)[c]
10[b]
1
B
C
C
60
25
60
2c
2c
–
–
H
H
9c (85%)
9c (83%)
2
3[d]
2a 97:3 Me 9a (84%)
4[d]
C
60
2d 99:1 Me 9a (88%)
5[d]
6[d]
C
C
60
60
2e 98:2 nPr 9e (91%)
2b 97:3 Ph 9b (85%)
7[d]
C
60
2 f 98:2 Ph 9b (88%)
[a] All reactions were carried out at 0.08m concentration in 1,2-dichloro-
ethane for 16 h under inert atmosphere with 2/8/[Ru] in 1.2/1/0.025
molar ratio. [b] Ratio determined by 1H NMR analysis of the crude
reaction mixture. [c] Major product and yield of isolated product.
[d] Reactions were performed with C/1b in 0.025:0.06 molar ratio.
Scheme 4. Allylation of 8 from linear and branched allyl alcohols.
Yields shown are those of isolated products.
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Angewandte
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744; b) W. Weng, Z. Shen, R. F. Jordan, J. Am. Chem. Soc. 2007,
similar reaction of 8 with allyl alcohol 2c afforded 90% of
diallylated compound with traces of mono 3-allylindole 9c.
In conclusion, we have reported the preparation and
structural characterization of new ruthenium(IV) complexes
129, 15450; c) T. Kochi, S. Noda, K. Yoshimura, K. Nosaki, J.
Am. Chem. Soc. 2007, 129, 8948; d) L. Betucci, C. Bianchini, C.
Claver, E. J. Garcia Suarez, A. Ruiz, A. Meli, W. Oberhauser,
Dalton Trans. 2007, 5590; e) S. Liu, S. Borkar, D. Newsham, H.
Yennawar, A. Sen, Organometallics 2007, 26, 210; f) A. Naka-
mura, K. Munakata, T. Kochi, K. Nozaki, J. Am. Chem. Soc.
2008, 130, 81128.
featuring P,O-chelating ligands. The cationic [Ru(Cp*)(p-
_
C3H5)(PSO2O)] C has been shown to be an active precatalyst
for regioselective allylation reactions either with allylic
chlorides or alcohols. Excellent regioselectivities for the
formation of branched products were obtained starting from
branched, but also, more gratifyingly from linear allylic
alcohols. The first example of doubly regioselective disub-
stitution of indole at the N1- and C3-positions leading to the
dibranched diallylated indole is presented.
[10] L. Bettucci, C. Bianchini, A. Meli, W. Oberhauser, J. Mol. Catal.
A 2008, 291, 57.
[11] T. Schultz, A. Pfaltz, Synthesis 2005, 1005.
[12] a) A. M. McNair, D. C. Boyd, K. R. Mann, Organometallics
1986, 5, 303; b) B. M. Trost, P. L. Fraisse, Z. T. Ball, Angew.
Chem. 2002, 114, 1101; Angew. Chem. Int. Ed. 2002, 41, 1059.
[13] Crystal data: C34H40F6O3P2Ru, Mr = 773.67, orthorhombic, space
group P212121, Z = 4, a = 9.4887(2) ꢂ, b = 10.9680(2) ꢂ, c =
32.3709(5) ꢂ, a = 908, b = 908, g = 908, U = 3368.91(11) ꢂ3, T=
150(1) K, F(000) = 1584, MoKa radiation (l = 0.71073 ꢂ), m =
0.627 mmÀ1, 26050 reflexions measured in the range 2.65 ꢀ q ꢀ
26.99, 26050 unique,(Rint=0.0296) which were used in all
calculations. The structure was refined by using full-matrix
least-squares on F2 to R1 = 0.0299, wR2 = 0.0681, S = 0.989, for
7257 reflections (> 2s) and 437 refined parameters, R1(all data)
0.0360, wR2(all data) = 0.0693, goodness-of-fit on F2 = 0.989.
CCDC 727698 (B·MeOH) contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: December 14, 2009
Published online: March 12, 2010
Keywords: allylic compounds · homogeneous catalysis ·
.
phosphorus · P,O chelate · ruthenium
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3144.45(19) ꢂ3, T= 150(2 K, F(000) = 1552, MoKa radiation
(l = 0.71073 ꢂ), m = 0.735 mmÀ1, 12814 reflexions measured in
the range 2.71 ꢀ q ꢀ 27.00, 26050 unique,(Rint=0.0396) which
were used in all calculations. The structure was refined by using
full-matrix least-squares on F2 to R1 = 0.0436, wR2 = 0.0908, S =
0.911, for 6594 reflections (> 2s) and 412 refined parameters,
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Angew. Chem. Int. Ed. 2010, 49, 2782 –2785
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2785