Arene-ruthenium chemistry and bronsted acid catalysis of a chiral phosphane-hydroxyl ligand

Article abstract of DOI:10.1021/om401215m

The phosphane-hydroxyl ligand (S_C1,R_C2)-Ph ₂PC(Ph)HC(OH)HCH₂OMe (POH) displays κ¹P, κ²P,O, κ³P,O,O′, and anionic deprotonated κ³P,O,O′ coordination modes toward the me

Full text of DOI:10.1021/om401215m

Communication
pubs.acs.org/Organometallics
Arene−Ruthenium Chemistry and Brønsted Acid Catalysis of a Chiral
Phosphane-Hydroxyl Ligand
,†
,†
†
,†
†
́
Daniel Carmona,* M. Pilar Lamata,* Pilar Pard‡o, Ricardo Rodrıguez,* Fernando J. Lahoz,
†
‡
†
́
Pilar Garcıa-Orduna, Ibon Alkorta, Jose Elguero, and Luis A. Oro
́
̃
†
́
́
́
Instituto de Sıntesis Quımica y Catal
Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain
́
isis Homogen
́
ea (ISQCH), Departamento de Quımica Inorgan
́
ica, CSIC-Universidad de
‡
́
́
Instituto de Quımica Medica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain
S
* Supporting Information
ABSTRACT: The phosphane-hydroxyl ligand (SC1,RC2)-Ph2PC(Ph)HC-
(OH)HCH2OMe (POH) displays κ1P, κ2P,O, κ3P,O,O′, and anionic
deprotonated κ3P,O,O′ coordination modes toward the metallic (η6-p-
MeC6H4iPr)Ru moiety. The hydroxyl group in [(η6-p-MeC6H4iPr)Ru-
(κ3P,O,O′-POH)][SbF6]2 (3) has strong acidic properties. This complex
catalyzes the Diels−Alder reaction between cyclopentadiene and trans-β-
nitrostyrene or methacrolein as well as the Friedel−Crafts reaction between
indole and methyl 3,3,3,-trifluoropyruvate or trans-β-nitrostyrene. Solution
studies strongly indicate that complex 3 acts as a metallic Lewis acid assisted
Brønsted-acid catalyst.
Scheme 1. Preparation of the BH3-POH Ligand
he use of weakly acidic small organic molecules as
electrophile activators through either hydrogen bond or
T
Brønsted acid catalysis constitutes an important branch of
organocatalysis that over the last years has grown at a dramatic
pace. Diols,1 (thio)ureas,1a,b,d,e,2 and phosphoric acids1a,c−e,3 are
the most representative organocatalysts of this class. The H-
bond donating ability of the catalysts is usually increased by
means of electron-withdrawing organic substituents. In
principle, this role can also be played by metallic Lewis acids,
giving rise to Lewis acid assisted Brønsted acid catalysts
(LBA).4 Brønsted acid catalysts of this type are scarce. Among
them, it has been reported that coordination of binols4,5 or a
hydroxyl−phosphane ligand derived from binol6 to SnCl4 or to
La(OTf)3, respectively, enhances the acidity of the ligand OH
groups, rendering active LBA catalysts for the enantioselective
protonation of silyl enol ethers and biomimetic cyclization of
polyprenoid. Toste et al. have successfully applied to the former
process chiral Brønsted acids obtained by activation of the OH
group of EtOH or iPrOH by coordination to gold diphosphane
complexes,7 and it has been recently reported8 that the
hydroxyl group of a water ligand participates in the redox
isomerization of allylic alcohols in aqueous medium by Ru(IV)
based complexes.
ring metallacycles (see complex 2, Scheme 2). Furthermore, the
acidic properties of the OH group would be enhanced by
coordination to metallic cations, resulting in potential LBA
catalysts.
Scheme 2. Synthesis of 1−3
In 2008, Vidal-Ferran et al. reported the regioselective and
stereospecific preparation of the BH3-protected phosphano−
hydroxyl compound (SC1,RC2)-(BH3)Ph2PC(Ph)HC(OH)-
HCH2OMe (BH3-POH) by ring opening of the corresponding
(RC1,RC2)-epoxide with potassium diphenylphosphanide
(Scheme 1).9 We anticipate that POH can act as a bidentate
κ2P,O chelate ligand through the phosphane and hydroxyl
functionalities, giving rise to stable unstrained five-membered-
Received: December 19, 2013
Published: January 24, 2014
© 2014 American Chemical Society
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Organometallics
Communication
In the present communication, we report on (i) the
preparation, characterization, and reactivity of p-cymene−
ruthenium compounds (p-cymene = η6-p-MeC6H4iPr) con-
taining a variety of coordination modes of the phosphano−-
hydroxyl ligand POH, (ii) the application of one of these
complexes, namely [(η6-p-MeC6H4iPr)Ru(κ3P,O,O′-POH)]-
[SbF6]2 (3), as a catalyst in examples of Diels−Alder (DA)
and Friedel−Crafts (FC) reactions, and (iii) NMR spectro-
scopic evidence for the Brønsted acid activation of the
electrophilic substrates involved in the catalytic reactions.
The ruthenium dimer [{(η6-p-MeC6H4iPr)RuCl}2(μ-Cl)2]
reacts with POH, rendering the mononuclear compound [(η6-
p-MeC6H4iPr)RuCl2(κ1P-POH)] (1), which in turn reacts with
AgSbF6 to afford [(η6-p-MeC6H4iPr)RuCl(κ2P,O-POH)]-
[SbF6] (2) (Scheme 2). Compound 2 was isolated as a 65/
35 ratio mixture of the two epimers at metal. Subsequent
treatment of 2 with 1 equiv of AgSbF6 gives [(η6-p-
MeC6H4iPr)Ru(κ3P,O,O′-POH)][SbF6]2 (3). Even in the
presence of good coordinating ligands such as pyridine, PPh3,
and P(OMe)3 the η3 coordination of the ligand is preferred.
The SC1,RC2 configuration of the POH ligand induces the
diastereopure formation of the R at ruthenium epimer of
complex 3.
We have calculated the acidity of compound 3, defined as its
energy difference with 4, as ΔH° = 751.7 kJ mol−1 at the
B3LYP/LANL2DZ level,11 using the facilities of the Gaussian
09 package.12 This value corresponds to a very strong acid.
According to the NIST database,13 neutral acids have values in
the range 1456 (CH3CO2H)−1278 kJ mol−1 (CF3SO3H), all of
which are much weaker than 3. However, compound 3 is a
dication which by losing a proton yields the monocation 4,
while the reported compounds are neutral molecules. To find
closer acidity values, it is necessary to search for cations that on
loss of a proton became neutral molecules:13
+
C6H7 → C6H6 (benzene) 750.5 kJ mol−1
+
H3SiOH2 → H3SiOH (silanol) 746.4 kJ mol−1
On the other hand, complex 3 catalyzes the DA reaction
between cyclopentadiene and trans-β-nitrostyrene or meth-
acrolein as well as the FC reaction between indole and methyl
3,3,3,-trifluoropyruvate or between different indoles and
substituted trans-β-nitrostyrenes (Scheme 4).
Scheme 4. Catalytic Reactions
Complexes 1−3 have been characterized by analytical and
spectroscopic means as well as by the determination of the
crystal structures of 2 and 3·Et2O (see the Supporting
Information). In particular, δ(OH) shifts downfield on going
from 1 to 3 (δ(OH) 3.84 (1), 5.96 (2), 7.95 ppm (3)),
indicating that, as a consequence of the coordination with the
metal and the increase of the formal charge of the cation,
hydrogen acidity increases in the same direction.
As expected, the molecular structure of 3 reveals that the
configuration at the metal is R.10 Due to the polarization of the
OH bond, in the crystal, the oxygen atom of a molecule of Et2O
is hydrogen-bonded to the OH group of the cation (Figure 1).
In general, good conversions were achieved and activation
was shown to be efficient even in the case of the DA reaction
involving a trans dienophile (reaction 1). Catalytic reactions
proceed with enantioselectivity indicating that complex 3, the
only possible source of chirality, plays a key role in the process.
In particular, under the conditions indicated in reaction 4 and
Table 1, around 80% ee was obtained for the reaction between
indoles and substituted trans-β-nitrostyrenes (Table 1).
To shed light on the catalytic systems, solutions containing
catalyst 3 and methacrolein were monitorized by NMR
Figure 1. Molecular representation of the cation of 3 showing the H-
bonded Et2O molecule.
a
Table 1. Catalytic Results for Reaction 4
b
c
entry
R1
R2
R3
conversn (%)
ee (%)
Accordingly, complex 3 reacts with NaHCO3 to render the
deprotonated monocation 4 (Scheme 3), which has been
isolated and spectroscopically characterized (see the Supporting
Information).
1
2
3
4
5
6
7
8
H
H
H
93
68
75
78
81
81
79
28
15
35
H
H
2-OMe
4-Me
2-Cl
4-Br
H
H
H
43
H
H
>99
78
Scheme 3. Deprotonation of Complex 3
H
H
H
Me
H
>99
94
Me
Me
H
Me
H
>99
a
Reaction conditions: catalyst 0.03 mmol (5.0 mol %), indole 0.60
b
mmol, trans-β-nitrostyrene 0.90 mmol, in 2 mL of CH2Cl2. Based on
indole. Determined by NMR. Determined by HPLC.
c
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Organometallics
Communication
spectroscopy. Figure 2 shows the evolution of a selected region
of the 1H NMR spectrum, by successive addition of
In summary, in this communication we have reported on the
richness of ruthenium coordination chemistry that the POH
ligand displays. Examples of κ1P, κ2P,O, and κ3P,O,O′ as well as
anionic deprotonated κ3P,O,O′ coordination modes have been
described (Scheme 6). From theoretical, crystallographic, and
Scheme 6. Coordination Modes of the Ligand POH
spectroscopic data, it can be concluded that complex 3 is an
active metallic Brønsted acid catalyst for DA and FC reactions
through the OH coordinated group. Brønsted acid activation of
carbonyl- and nitro-containing substrates occurs with asym-
metric induction. Optimization and further development of
enantioselective catalytic processes mediated by complex 3, as
well as other related M−OH-containing species, are currently
under investigation in our laboratory.
Figure 2. Selected region of the 1H NMR spectrum of the addition of
methacrolein to complex 3: (a) complex 3; (b−e) after the addition of
0.5, 1.6, 5.9, and 20 equiv of methacrolein, respectively. The asterisk
denotes residual CHDCl2.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving experimental
procedures and spectroscopic and analytical characterization
data of the new complexes and DA and FC catalytic results, X-
ray crystallographic data for complexes 2 and 3·Et2O, and
Cartesian coordinates of compounds 3 and 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
methacrolein to a CD2Cl2 solution of 3. All proton resonances
of 3 remain essentially unchanged except for that of the
hydroxyl group.14 This resonance undergoes a gradual
downfield displacement from 7.95 (trace a, δ value in the
absence of methacrolein) to 9.84 ppm (trace e, 20 equiv of
methacrolein added). Notably, the 1H NMR OMe singlet
appears invariably at 4.25 ppm and the 31P{1H} NMR spectrum
consists of one singlet that methacrolein addition only shifts
0.55 ppm, from 54.83 to 55.38 ppm. All these data strongly
support that, in solution, the metallic cation is an 18-electron
coordinatively saturated species with the four groups, i.e. p-
cymene, PPh2, OMe, and OH, remaining bonded to it in the
presence of excess methacrolein. Therefore, methacrolein does
not interact directly with the metal. Similar trends in the NMR
spectra were observed with progressive addition of trans-β-
nitrostyrene or methyl 3,3,3-trifluoropyruvate to complex 3
(see the Supporting Information).15
AUTHOR INFORMATION
Corresponding Author
*E-mail: dcarmona@unizar.es (D.C.); plamata@unizar.es
(M.P.L.); riromar@unizar.es (R.R.) Tel: 34-976-762027. Fax:
34-976-761187.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Ministerio de Economıa y Competitividad of
Spain (CTQ2012-32095) and Gobierno de Aragon
Consolidado: Catalizadores Organometal
vos) for financial support. This work was supported by the
■
́
́
(Grupo
icos Enantioselecti-
The spectroscopic data can be accounted for by assuming
that an equilibrium between complex 3 and adduct 5, in which
methacrolein is hydrogen-bonded to the hydroxyl group of 3, is
established in solution (Scheme 5). Such an equilibrium
́
CONSOLIDER INGENIO 2010 program under the project
́
“Factorıa de Cristalizacion
́
” (CSD2006-0015). R.R. and P.P.
acknowledge the CSIC and European Social Fund for JAE
Scheme 5. Equilibrium between Complex 3 and
Methacrolein
́
grants. I.A. and J.E. thank the Ministerio de Economıa y
Competitividad of Spain (Project CTQ2012-13129-C02-02)
and the Comunidad Auton
SOLAR2, ref S2009/PPQ-1533) for financial support. Dedi-
́
oma de Madrid (Project MADRI-
́
́
cated in memory of Marıa Pilar Garcıa Clemente.
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of two NO2 oxygen atoms accounts for this behavior.
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