Ligand-metal cooperating PC(sp3)P pincer complexes as catalysts in olefin hydroformylation

Article abstract of DOI:10.1002/chem.201303311

Ligand-metal cooperation: A new ligand-metal cooperating catalyst for the hydroformylation of olefins is described (see scheme). The mechanism of the H₂ activation and C-H bond formation of the catalyst involves an intramolecular cooperation between the structurally remote functionality and the metal center and proceeds without change of the oxidation state of the metal.

Full text of DOI:10.1002/chem.201303311

DOI: 10.1002/chem.201303311
Ligand–Metal Cooperating PCACHTNUGRTNEUNG
(sp3)P Pincer Complexes as Catalysts in Olefin
Hydroformylation
Sanaa Musa,[a] Oleg A. Filippov,[b] Natalia V. Belkova,*[b] Elena S. Shubina,[b]
Gleb A. Silantyev,[b] Lutz Ackermann,[c] and Dmitri Gelman*[a]
In recent years, metal complexes, in which both the metal
center and the ligand play active and cooperative roles, have
emerged as very promising catalysts, capable of activating
and forming chemical bonds through nonoxidative pathways
(i.e., alternative to the conventional oxidative addition/re-
ductive elimination sequence). The key mechanistic steps
usually include a reversible switching between the coordina-
tion modes of the ligands that are bound to the catalytically
active metal center (Scheme 1).
species in electron rich CACTHGNUTERNNUG
(sp3)-metalated pincer complexes,[9]
as well as on keto–enol[10] or lactam–lactim tautomerism.[11]
These systems established a basis for the design of mild cat-
alysts for nonoxidative (de)hydrogenation processes[12] and
chemoselective H/D exchange.[13]
The examples are impressive, however, they provide only
a hint of the real synthetic potential concealed in ligand–
metal cooperation. Therefore, we strongly believe that fur-
ther extension of this concept to new reaction schemes will
eventually lead to the discovery of conceptually novel catal-
ysis.
Herein, we wish to report on the development of IrIII- and
RhIII-based catalysts that are capable of promoting olefin
hydroformylation through the unprecedented metal–ligand
cooperating mechanism. In our previous work, we devel-
Scheme 1. Ligand–metal cooperation in bond activation/formation.
ACTHNUTRGNEUNG
oped the IrIII PC(sp3)P pincer complex 1, which forms mo-
lecular hydrogen by an intramolecular interaction that in-
volves both the metal center (hydride) and a polar O-based
ligand sidearm (Scheme 2).[14] The catalyst showed very high
For example, metal–amide/metal–amine interconversion,
originally described by Noyori and Ikariya,[1] led to the dis-
covery of very efficient asymmetric hydrogenation cata-
lysts.[2] Aromatization/dearomatization in heteroaromatic
PNP and PNN pincer ligands made nonoxidative activation
[3]
[4]
[5]
À
À
À
of H H, C H, and N H bonds possible and a family
of hydrogenation/dehydrogenation catalysts was established
by Milstein.[6] Nonaromatic switchable PNP pincer systems
have been proven to facilitate heterolytic bond cleavage and
catalyze transfer[7] and acceptorless dehydrogenation/hydro-
genation reactions.[8]
Scheme 2. H2 activation/formation through ligand–metal cooperation in
ACTHNUTRGNEUNG
bifunctional IrIII PC(sp3)P pincer complexes.
So far, ligand–metal cooperation relies on a dynamic in-
terplay between carbometalated and a- or b-H eliminated
activity in the acceptorless dehydrogenation of primary and
secondary alcohols. We also observed that the release of
molecular hydrogen is efficiently intercepted by the pres-
ence of unsaturated substrates.
[a] S. Musa, Prof. D. Gelman
Institute of Chemistry, The Hebrew University
Edmond Safra Campus, 91904 Jerusalem (Israel)
Fax : (+972)2-6585279
E-mail: dmitri.gelman@mail.huji.ac.il
Based on this observation, we hypothesized that the hy-
droformylation of double bonds may proceed through a re-
versed metal–ligand cooperating mechanism (Scheme 3).
The hypothetical catalytic cycle is likely to include the fol-
lowing elementary steps: i) heterolytic nonoxidative H2 acti-
vation by the species a through alkoxide ring-opening to
[b] O. A. Filippov, Prof. N. V. Belkova, Prof. E. S. Shubina,
G. A. Silantyev
A.N.Nesmeyanov Institute of Organoelement Compound RAS
28 Vavilov str., 119991 Moscow (Russia)
E-mail: nataliabelk@ineos.ac.ru
[c] Prof. Dr. L. Ackermann
Georg-August-Universitꢀt Gçttingen
Institut fꢁr Organische und Biomolekulare Chemie
Tammannstrasse 2, 37077 Gçttingen (Germany)
À
form the M H complex b; ii) migratory insertion of an
À
alkene into the Ir H bond, resulting in the formation of c;
iii) carbonylation of c, followed by migratory insertion of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303311.
À
CO into the M C bond to form e and f (iii–iv), which re-
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COMMUNICATION
Table 1. Hydroformylation of alkenes with 1 and 3.
Entry Catalyst Substrate
Lin./br./hyd.[c] Isomerization
1
2
5
4
5
6
7
8
9
10
11
12
13
14
1[a]
1[a]
1[a]
1[a]
1[a]
3[b]
3[b]
3[b]
3[b]
3[b]
3[b]
3[b]
3[b]
3[b]
styrene
0.2:1:0.3
0.1:1:0.1
0.1:1:0.1
0.13:1:0.76
–
–
–
–
–
–
–
–
–
–
–
0.4
0.6
–
2-chlorostyrene
4-chlorostyrene
4-methylstyrene
2-vinylnaphthalene 0.18:1:0.71
styrene 0.8:1:0
2-vinylnaphthalene 0.6:1:0
4-vinylbenzoic acid 0.5:1:0
4-methylstyrene
4-methoxystyrene
3-Cl-styrene
0.9:1:0
0.8:1:0
0.6:1:0.1
0.9:1:0
0.8:1:0
0.5:1:0
1-octene
trans-2-octene
vinylcyclohexane
Scheme 3. Possible mechanism for the hydroformylation of olefins by 1.
[a] Catalyst 1 (1 mol%) and alkene (3 mmol) in 1,2-DCE (10 mL) pres-
surized in Parr bomb for 24 h at 908C under 1/24 bar of H2/CO. [b] Cata-
lyst 3 (1 mol%) and alkene (3 mmol) in 1,2-DCE (10 mL) pressurized in
Parr bomb for 12 h at 908C under 7/7 bar of H2/CO. [c] Determined by
GC versus dodecane as an internal standard. Lin.=linear, br.=branched,
hyd.=hydrogenated.
Hydrogen activation (Scheme 2) was demonstrated when
Figure 1. Precatalysts used in this work.
2, showing a set of doublets at d=15.8 and 1.6 ppm (JPÀP =
13 Hz) in the 31P NMR spectrum, was gradually transformed
into 1, which has a characteristic hydride signal that appears
in DMSO as a deceptively simple triplet at d=À19.86 ppm
(JPÀH =12 Hz) and different sets of doublets around d=
26 ppm (JPÀP =10 Hz) in the 31P NMR spectrum,[14] under
7 bar of hydrogen.
leases the oxo product nonoxidatively by regeneration of
a (v).
To prove this hypothesis, a series of stoichiometric and
catalytic experiments by using the known iridium-based
complex 1 and its new rhodium analogue 3 (Figure 1), as
well as DFT optimization of some proposed intermediates
were carried out.
Indeed, hydroformylation of styrene as a model substrate
by using 1 mol% of the catalyst 1 at 908C under 14 bar of
1:1 syngas in 1,2-dichloroethane (DCE) led to the formation
of 1- and 2-phenylpropanal along with ethylbenzene in a
1/0.1/4 ratio at complete conversion. Not surprisingly, ethyl-
À
To demonstrate the participation of the sidearm in the C
H-bond-forming step, the hydroxyl protons of 1 were com-
pletely replaced with deuterium (no Ir–H/D exchange was
observed by H NMR spectroscopy) and the deuterated
1 ([D]-1) was heated in the presence of a slight excess of sty-
rene (Scheme 4). Analysis of the reaction mixture showed
À
benzene originates from protonolysis of the Ir C bond in c,
producing the sidearm-closed species a (Scheme 3). This hy-
drogenation bypath is strongly favorable for the iridium-
based catalyst, due to the hemilabile nature of the hydroxyl
sidearm and the necessity for an adequate number of vacant
coordination sites for CO migratory insertion. However,
chemoselectivity was achieved when the Parr bomb was
purged with hydrogen and pressurized with carbon monox-
ide to 24 bar (Table 1, entry 1).
Scheme 4. Stoichiometric labeling experiment: hydrogenation of styrene
by deuterated 1.
Of course, the reactivity itself does not prove the suggest-
ed nonoxidative mechanism. Moreover, conventional hydro-
formylation may proceed through concerted hydrogenolysis
of metal–acyl intermediates,[15] leading to the formation of
the aldehyde product and the catalytically active species, es-
pecially, when the reaction is catalyzed by RhI,[16] PdII,[17] or
RuII[18] complexes. Therefore, a clear-cut evidence for the
suggested metal–ligand cooperation and, in particular, for
the presence of monodeuterated ethylbenzene labeled in
both possible positions (MS) and the presence of 2 (P NMR
spectroscopy). This unequivocally indicates an intramolecu-
lar transfer of both the proton and the hydride to the start-
ing alkene under stoichiometric conditions.
A similar experiment was performed under catalytic hy-
droformylation conditions. Thus, heating the mixture of
1 (10 mol%) and styrene in C2H4Cl2/CH3OD under 24 bar
of H2/CO 1:23 resulted in the formation of the correspond-
ing linear and branched dimethyl acetals, which after hydrol-
À
the active participation of the acidic proton in the C H-
bond-forming step is required.
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N. V. Belkova, D. Gelman et al.
Figure 2. Energy profile for the Ir-catalyzed hydroformylation.
Scheme 5. Catalytic labeling experiment: hydroformylation of styrene by
deuterated 1.
mechanism that operates via monohydride intermediates
only.[22]
ysis showed about 60% isotope enrichment at the formyl
group (Scheme 5), proving that the ligand–metal cooperat-
ing mechanism is operative also under catalytic condi-
tions.[19]
Importantly, P NMR analysis of the reaction mixture indi-
cated the presence of a major organometallic species, which
was isolated and crystallographically identified as a new alk-
DFT/M06 calculations were performed to strengthen the
feasibility of the proposed catalytic cycle using 1 (complex
b in Scheme 3) as a catalyst and ethylene as a model sub-
strate (Figure 2; for more details see the Supporting Infor-
mation). Complex b, featuring a pseudo-octahedral geome-
try with a coordinated sidearm and a P-Ir-P angle of about
1008 (fac-arrangement of P-C-P ligand), is taken as the zero-
À
A
energy point. Ethylene insertion into the Ir H bond (b to c)
molecular nucleophilic attack of the hydroxyl sidearm onto
a terminal CO ligand. This complex is a resting state of the
catalyst, since it was found to be equally reactive upon load-
ing it to the reaction in isolated form. The complex may
have relevance in the field of carbon monoxide reduction
and will be studied in details.
turns out to be favorable (DEbÀc =À23.1, DGbÀc =À6.7 kcal
molÀ1) and can be assisted by coordination to the hydroxy-
methylene arm. The barrier to promote proton transfer and
release of the hydrogenated product is about 9 kcalmolÀ1,
which is still lower in energy than the starting material on
the DE scale. The activation energy for this step (DE°cÀa) is
28.0 kcalmolÀ1 (DG°cÀa =25.2 kcalmolÀ1). An estimation of
the gas-phase Gibbs free energy under standard thermody-
namic conditions (298.15 K, 1 atm) shows that all other
transformations in the hydrogenation cycle are spontaneous.
Moreover, the addition of H2 to a is very exoergic, making
the overall hydrogenation process energetically favorable.
For the hydroformylation, the CO addition to ethyl com-
plex c is very exoergic (Figure 2). CO insertion (d to e) fur-
ther lowers the energy of the system about 6 kcalmolÀ1.
Similar to the hydrogenation cycle, the release of the alde-
hyde product proceeds through proton transfer from a pend-
ant OH group to the carbon of the alkanoyl ligand. This
step is endoergic again, however, it is more energy demand-
ing (by ca. 20 kcalmolÀ1) than in the case of hydrogenation.
This is in agreement with the more rigorous conditions that
are necessary for hydroformylation to occur.
Scope and limitations studies showed that the chemo- and
regioselectivity of catalyst 1 in hydroformylation reactions
(Table 1, entries 1–5), for which hydrogenation is a very
competitive process,[20] is largely comparable with the state-
of-the-art iridium-based catalysts. However, rhodium-based
catalysts showed a higher chemoselectivity in the hydrofor-
mylation of olefins. Thus, in the presence of 3, essentially no
hydrogenation product was observed, even under 14 bar of
1:1 syngas (Table 1, entries 6–14). For example, nonisomeriz-
able substrates, such as differently substituted styrenes and
2-vinyl naphthalene (Table 1, entries 6–11), were successfully
hydroformylated to a mixture of the branched and linear al-
dehydes. Hydrogenation was also not observed in the Rh-
catalyzed hydroformylation of isomerizable alkenes
(Table 1, entries 12–14). In these cases, isomerization can
precede hydroformylation, but all double-bond regioisomers
eventually react. Thus, both 1-octene and trans-2-octene led
to the formation of 1-nonanal, 2-methyloctanal and 2-ethyl-
heptanal in comparable ratios. The highest TON of 727
demonstrated by 3 was obtained in the hydroformylation of
styrene.
In conclusion, we have described a conceptually new
mechanism for the hydroformylation of olefins. The mecha-
À
nism for the H2 activation and C H bond formation in-
volves an intramolecular cooperation between the structur-
ally remote functionality and the metal center. This is the
most important aspect of our work, because coordinational
diversity in ligand–metal cooperating catalytic systems
offers new reaction patterns in many classical reaction
schemes and renders new reactivity to many classical or-
It is worth noting that a fast isomerization is not charac-
teristic for the conventional rhodium-catalyzed hydroformy-
lation, because rhodium–phosphine–dihydride species show
very little, if any, tendency toward olefin isomerization.[21]
This observation again supports the proposed cooperative
ACHUTNGERNgNUG anoHCATUNGTRENNmUGN etallic compounds. Further synthetic and catalytic
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Ligand–Metal Cooperating Catalysts for Hydroformylation
COMMUNICATION
A. P. M. Robertson, A. Friedrich, S. Schneider, P. J. Gates, J.
Schmedt auf der Gunne, I. Manners, J. Am. Chem. Soc. 2010, 132,
13332–13345; B. Askevold, J. T. Nieto, S. Tussupbayev, M. Diefen-
bach, E. Herdtweck, M. C. Holthausen, S. Schneider, Nat. Chem.
2011, 3, 532–537.
studies, including employment of chiral complexes in this
and related transformations, are a topic of our active stud-
ies.
[9] H. D. Empsall, E. M. Hyde, R. Markham, W. S. McDonald, M. C.
Norton, B. L. Shaw, B. Weeks, J. Chem. Soc. Chem. Commun. 1977,
589–590; D. G. Gusev, A. J. Lough, Organometallics 2002, 21, 2601–
2603; A. M. Winter, K. Eichele, H.-G. Mack, W. C. Kaska, H. A.
Mayer, Organometallics 2005, 24, 1837–1844; R. J. Burford, W. E.
Piers, M. Parvez, Organometallics 2012, 31, 2949–2952.
[10] Y. Blum, Y. Shvo, J. Organomet. Chem. 1985, 282, C7–C10; Y.
Shvo, D. Czarkie, Y. Rahamim, D. F. Chodosh, J. Am. Chem. Soc.
1986, 108, 7400–7402.
Acknowledgements
This research was partially supported by the Israel Science Foundation
(ISF), the Russian Foundation for Basic Research (RFBR; projects 12-
03-31326 and 12-03-33018), and the German-Israel Science Foundation
(GIF). We thank Dr. Shmuel Cohen and Ms. Benny Bogoslavsky for
solving the X-ray structures.
[11] K.-i. Fujita, N. Tanino, R. Yamaguchi, Org. Lett. 2007, 9, 109–111.
[12] R. Kawahara, K.-i. Fujita, R. Yamaguchi, J. Am. Chem. Soc. 2012,
134, 3643–3646; R. Kawahara, K.-i. Fujita, R. Yamaguchi, Angew.
Chem. Int. Ed. 2012, 51, 12790–12794; R. Kawahara, K.-i. Fujita, R.
Yamaguchi, J. Am. Chem. Soc. 2010, 132, 15108–15111; R. Yamagu-
chi, C. Ikeda, Y. Takahashi, K.-i. Fujita, J. Am. Chem. Soc. 2009,
131, 8410–8412; Y. Shvo, I. Goldberg, D. Czerkie, D. Reshef, Z.
Stein, Organometallics 1997, 16, 133–138; D. Hollmann, S. Baehn,
A. Tillack, M. Beller, Angew. Chem. 2007, 119, 8440–8444; Angew.
Chem. Int. Ed. 2007, 46, 8291–8294; B. L. Conley, T. J. Williams,
Chem. Commun. 2010, 46, 4815–4817.
Keywords: catalysis
ligands · ligand–metal cooperation · pincer complexes
·
hydroformylation
·
noninnocent
[1] K.-J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya, R. Noyori, Angew.
Chem. 1997, 109, 297–300; Angew. Chem. Int. Ed. Engl. 1997, 36,
285–288; M. Yamakawa, H. Ito, R. Noyori, J. Am. Chem. Soc. 2000,
122, 1466–1478; K. Abdur-Rashid, S. E. Clapham, A. Hadzovic,
J. N. Harvey, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2002,
124, 15104–15118.
[2] R. Noyori, T. Okhuma, Angew. Chem. 2001, 113, 40–75; Angew.
Chem. Int. Ed. 2001, 40, 40–73; T. Ikariya, K. Murata, R. Noyori,
Org. Biomol. Chem. 2006, 4, 393–406.
[13] J. Zhou, J. F. Hartwig, Angew. Chem. 2008, 120, 5867–5871; Angew.
Chem. Int. Ed. 2008, 47, 5783–5787.
[14] S. Musa, I. Shaposhnikov, S. Cohen, D. Gelman, Angew. Chem. Int.
Ed. 2011, 50, 3533–3537.
[15] C. Kubis, D. Selent, M. Sawall, R. Ludwig, K. Neymeyr, W. Bau-
mann, R. Franke, A. Boerner, Chem. Eur. J. 2012, 18, 8780–8794;
J. C. S. da Silva, R. P. Dias, W. B. deAlmeida, W. R. Rocha, J.
Comput. Chem. 2010, 31, 1986–2000; M. Sparta, K. J. Borve, V. R.
Jensen, J. Am. Chem. Soc. 2007, 129, 8487–8499; J. Klosin, C. R.
Landis, Acc. Chem. Res. 2007, 40, 1251–1259.
[3] C. Gunanathan, B. Gnanaprakasam, M. A. Iron, L. J. W. Shimon, D.
Milstein, J. Am. Chem. Soc. 2010, 132, 14763–14765.
[4] E. Ben-Ari, G. Leitus, L. J. W. Shimon, D. Milstein, J. Am. Chem.
Soc. 2006, 128, 15390–15391; M. Feller, A. Karton, G. Leitus,
J. M. L. Martin, D. Milstein, J. Am. Chem. Soc. 2006, 128, 12400–
12401.
[16] A. M. Trzeciak, J. J. Ziolkowski, Coord. Chem. Rev. 1999, 190–192,
883–900; D. Fuchs, G. Rousseau, L. Diab, U. Gellrich, B. Breit,
Angew. Chem. Int. Ed. 2012, 51, 2178–2182; G. T. Whiteker, C. J.
Cobley, Top. Organomet. Chem. 2012, 42, 35–46.
[5] E. Khaskin, M. A. Iron, L. J. W. Shimon, J. Zhang, D. Milstein, J.
Am. Chem. Soc. 2010, 132, 8542–8543; M. Feller, Y. Diskin-Posner,
L. J. W. Shimon, E. Ben-Ari, D. Milstein, Organometallics 2012, 31,
4083–4101.
[17] R. Jennerjahn, I. Piras, R. Jackstell, R. Franke, K.-D. Wiese, M.
Beller, Chem. Eur. J. 2009, 15, 6383–6388.
[18] K. Takahashi, M. Yamashita, T. Ichihara, K. Nakano, K. Nozaki,
Angew. Chem. 2010, 122, 4590–4592; Angew. Chem. Int. Ed. 2010,
49, 4488–4490; K. Takahashi, M. Yamashita, Y. Tanaka, K. Nozaki,
Angew. Chem. Int. Ed. 2012, 51, 4383–4387.
[19] A similar isotope enrichment was observed when the Rh-based com-
pound 3 was applied as a catalyst.
[20] I. Piras, R. Jennerjahn, R. Jackstell, A. Spannenberg, R. Franke, M.
Beller, Angew. Chem. 2011, 123, 294–298; Angew. Chem. Int. Ed.
2011, 50, 280–284.
[6] J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, J. Am. Chem. Soc.
2005, 127, 10840–10841; E. Balaraman, C. Gunanathan, J. Zhang,
L. J. W. Shimon, D. Milstein, Nat. Chem. 2011, 3, 609–614; B. Gna-
naprakasam, E. Balaraman, Y. Ben-David, D. Milstein, Angew.
Chem. 2011, 123, 12448–12452; Angew. Chem. Int. Ed. 2011, 50,
12240–12244, S12240/12241-S12240/12249; C. Gunanathan, D. Mil-
stein, Acc. Chem. Res. 2011, 44, 588–602.
[7] Z. E. Clarke, P. T. Maragh, T. P. Dasgupta, D. G. Gusev, A. J. Lough,
K. Abdur-Rashid, Organometallics 2006, 25, 4113–4117.
[8] S. Bꢀhn, D. Hollmann, A. Tillack, M. Beller, Adv. Synth. Catal.
2008, 350, 2099–2103; A. Boddien, B. Loges, H. Junge, M. Beller,
ChemSusChem 2008, 1, 751–758; S. Bꢀhn, S. Imm, L. Neubert, M.
Zhang, H. Neumann, M. Beller, ChemCatChem 2011, 3, 1853–1864;
M. Bertoli, A. Choualeb, D. G. Gusev, A. J. Lough, Q. Major, B.
Moore, Dalton Trans. 2011, 40, 8941–8949; M. Bertoli, A. Choualeb,
A. J. Lough, B. Moore, D. Spasyuk, D. G. Gusev, Organometallics
2011, 30, 3479–3482; D. Spasyuk, S. Smith, D. G. Gusev, Angew.
Chem. Int. Ed. 2012, 51, 2772–2775; A. Staubitz, M. E. Sloan,
[21] R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2134–
2143.
[22] M. D. Fryzuk, P. A. MacNeil, Organometallics 1983, 2, 682–684;
M. D. Fryzuk, P. A. MacNeil, Organometallics 1983, 2, 355–356.
Received: August 23, 2013
Published online: November 7, 2013
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