Metalloenzyme-inspired catalysis: Selective oxidation of primary alcohols with an iridium-aminyl-radical complex

Article abstract of DOI:10.1002/anie.200605170

(Chemical Equation Presented) A little goes a long way: An iridium-nitrogen-radical complex is a highly active and selective catalyst for the dehydrogenation (oxidation) of primary alcohols to aldehydes in the presence of the oxidant benzoquinone (see simplified scheme). With only 0.01 mol% of the complex, turnover frequencies of up to 150 000 s^-1 are reached.

Full text of DOI:10.1002/anie.200605170

Angewandte
Chemie
DOI: 10.1002/anie.200605170
Catalytic Oxidation
Metalloenzyme-Inspired Catalysis: Selective Oxidation of Primary
Alcohols with an Iridium–Aminyl-Radical Complex**
Martin Königsmann, Nicola Donati, Daniel Stein, Hartmut Schönberg, Jeffrey Harmer,
Anandaram Sreekanth, and Hansjörg Grützmacher*
Complexes of redox-active metals and oxygen-centered
radicals (tyrosinyl (TyrOC), OCꢀ, or O2Cꢀ) have highly specific
functions in biological systems.[1–5] The copper-dependent
metalloenzyme galactose oxidase (GOase)[6] specifically
dehydrogenates (oxidizes) primary alcohols to the corre-
sponding aldehydes at very high turn over frequencies (TOF)
of greater than 2.5 ” 106 hꢀ1. The simplified catalytic cycle
shown in Scheme 1highlights the roles of the Cu 2+ center and
the coordinated TyrOC radical in this process. The catalytic
cycle starts with the coordination of the alcohol substrate
R-CH2OH to the Cu2+ center, in proximity to the TyrOC
radical (Scheme 1a). The transfer of an a-hydrogen atom
from the alcohol to the TyrOC radical via the transition state
TS (b) leads to a coordinated ketyl radical R-HCC-O (c),
which is intramolecularly oxidized by the Cu2+ center to give
=
the aldehyde R-CH O (d). The resulting copper(I)–tyrosine
complex is subsequently oxidized by molecular O2, thereby
turning over the catalytic cycle (e).
The catalytic reaction corresponds to a transfer hydro-
=
=
genation, R-CH2OH + O X!R-CH O + HO-XH, in which
an O2 molecule (X = O) acts as the H2 acceptor. Transfer
hydrogenations using ketones (X = CR2) as the H2 scavengers
are catalyzed with high efficiency by diamagnetic organome-
tallic complexes. The proposed mechanisms do not involve
redox reactions.[7] Numerous copper complexes[8–13] and a
wide range of other transition-metal complexes catalyze the
oxidation of alcohols to carbonyl compounds, and some of
these may even be used in combination with O2 or H2O2 as the
primary oxidants.[14] However, improvements of the activity
and chemoselectivity remain desirable.[15] Herein, we report
that iridium–aminyl-radical complexes catalyze the dehydro-
genation of alcohols to aldehydes, with 1,4-benzoquinone
(BQ) as the H2 acceptor; primary non-activated alcohols, in
Scheme 1. Top: Simplified catalytic cycle for the GOase-catalyzed dehy-
drogenation of primary alcohols, highlighting the roles of the Cu2+
center and the coordinated TyrOC radical. Bottom: Aminyl-radical com-
plexes I (with bis(2-pyridylmethyl)-2-aminoethylamine as ligand and
R1 =tBu) and II (with trop2NH as ligand; see Scheme 2 for the formula
of trop).
particular, can be converted with very high activity and
selectivity.
Calculations predict that complexes of late transition
metals with the TyrOC radical may be efficient models for
GOase.[16] Recently, Tanaka et al. proposed the rutheni-
um(II)–aminyl-radical complex I (Scheme 1) as a key inter-
mediate in the electrocatalytic dehydrogenation of alcohols to
aldehydes.[17] We reported that rhodium–aminyl-radical com-
plexes such as II (Scheme 1), in which approximately 60% of
the spin population of the unpaired electron is located on the
aminyl nitrogen atom,[18,19] are capable of abstracting hydro-
gen atoms from a variety of substrates.
[*] Dr. M. Königsmann, Dr. N. Donati, Dr. D. Stein, Dr. H. Schönberg,
Prof. Dr. H. Grützmacher
Laboratory of Inorganic Chemistry
Department of Chemistry and Applied Biosciences
ETH Zürich, 8093 Zürich (Switzerland)
Fax: (+41)446-331-418
The iridium complex that we use in the catalytic
dehydrogenation of alcohols can be synthesized in two
simple steps (Scheme 2): 1) the reaction of the tetradentate
ligand trop2dach[20] (1) with [Ir2(m-Cl)2(coe)4] (trop2dach =
N,N-bis(5-H-dibenzo[a,d]cycloheptene-5-yl)-1,2-diamino-
cyclohexane, coe = cyclooctene) in the presence of AgOTf
E-mail: gruetzmacher@inorg.chem.ethz.ch
Dr. J. Harmer, Dr. A. Sreekanth
Laboratory of Physical Chemistry
Department of Chemistry and Applied Biosciences
ETH Zürich, 8093 Zürich (Switzerland)
E-mail: harmer@phys.chem.ethz.ch
(OTfꢀ = CF3SO3 ) to give orange-red [Ir(trop2dach)]OTf (2);
ꢀ
2) the deprotonation of this remarkably acidic complex
(pK1a(dmso) = 10.5, pK2a(dmso) = 19.6) with KOtBu in THF or
chlorobenzene to give first the red neutral amino amido
complex [Ir(trop2dachꢀH)] (3) and then the deep burgundy
[**] This work was supported by the Swiss National Science Foundation
and the ETH Zürich.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 1. a) Structure of (R,R)-[K(thf)([18]crown-6)][Ir(trop2dachꢀ2H)].
Only one of the two crystallographically independent molecules is
shown. The cation is represented as a ball-and-stick model. The
hydrogen atoms and two THF molecules in the crystal lattice are
omitted for clarity. Selected bond lengths [Š] and angles [8]: Mole-
ꢀ
ꢀ
ꢀ
ꢀ
cule 1: Ir N1 1.988(3), Ir N2 1.961(3), Ir ct1 1.996(4), Ir ct2
ꢀ
ꢀ
2.002(4), C4 C5 1.445(6), C25 C26 1.444(6); sum of angles at N1:
348.9, sum of angles at N2: 356.4, dihedral angle between the N1-Ir-
ꢀ
ꢀ
ct1 and N2-Ir-ct2 planes: 6.0. Molecule 2: IrA N1A 1.970(3), IrA N2A
ꢀ
ꢀ
ꢀ
1.962(3), IrA ct1A 1.995(4), IrA ct2A 2.001(4), C4A C5A 1.440(6),
ꢀ
C25A C26A 1.442(6); sum of angles at N1A: 352.4, sum of angles at
N2A: 353.2, dihedral angle between the N1A-IrA-ct1A and N2A-IrA-
ct2A planes: 10.3. b) The Q-band HYSCORE spectrum (35.3 GHz) of 5
measured at 20 K at the echo maximum of the field-swept EPR
spectrum. The cross peaks are assigned to single quantum transitions
from the a and b electron spin manifolds of the nitrogen atoms N1
and N2. Inset: Proton region of the Davies ENDOR spectrum of 5. The
two sets of peaks centered about the proton Larmor frequency nI are
assigned to two pairs of protons, Ha1/Ha2 and Hb1/Hb2. The peak at nI
originates from many protons in the vicinity of the unpaired electron.
Scheme 2. Preparation of 4 and proposed catalytic cycle for the
dehydrogenation of primary alcohols. BQ or Fc+ can be used as the
oxidant.
anionic diamido complex [Ir(trop2dachꢀ2H)]ꢀ (4;
trop2dachꢀH and trop2dachꢀ2H denote the singly and
doubly deprotonated forms of the ligand).
Complex 4 was characterized as the crown-ether adduct
(R,R)-[K(thf)([18]crown-6)][Ir(trop2dachꢀ2H)] by single-
crystal X-ray diffraction (Figure 1a).[21] The rigidity and
steric encumbrance imposed by the trop2dach ligand are
clearly evident. The structure closely resembles those of
analogous rhodium complexes.[19,22] Notably, the [K(thf)-
([18]crown-6)]+ ion has a contact (3.3 Š) to one of the
benzo groups of the [Ir(trop2dachꢀ2H)]ꢀ complex, and the
coordination geometries of the two nitrogen centers are
slightly pyramidal (Figure 1a).
which gives some insight into its electronic structure. Hyper-
fine sublevel correlation (HYSCORE) and electron–nuclear
double resonance (ENDOR) spectra (Figure 1b) show sig-
nals from two strongly coupled nitrogen nuclei (N1and N2
with hyperfine coupling constants of A = (ꢀ2, ꢀ2, 37) MHz)
and four protons (Ha1 and Ha2 with A = (29, 31, 38) MHz, and
Hb1 and Hb2 with A = (14, 15, 20) MHz; see the Supporting
Information).[24]
The electronic structure of 5 is very similar to that of the
analogous rhodium complex, which we proposed to be
essentially equivalent to that of 4 after a one-electron
oxidation.[19] However, as the fit between the experimental
EPR spectroscopic parameters and those calculated by
density functional theory (DFT) for a model analogous to
that proposed for the rhodium complex is rather poor, we
refrain from making a definite assignment of the electronic
structure of 5. We expect that the proton couplings originate
Adding an oxidant (ferrocenium (Fc+) or BQ) to a THFor
chlorobenzene solution of 4 gives the transient radical 5 and
the semiquinone radical anion (SQCꢀ; Scheme 2), which was
unambiguously identified by its five-line electron paramag-
netic resonance (EPR) spectrum.[23] The radical 5 is rather
unstable, and attempts to isolate it always resulted in the
formation of the amino amido complex 3. Upon oxidation of 4
with FcOTf, 5 can be detected by pulse EPR spectroscopy,
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from either the benzyl protons of the trop residue and/or the
b and g protons of the cyclohexane ring. The small shift of the
g values in the EPR spectrum of 5 (g1 = 1.974, g2 = 1.993, g3 =
2.028) relative to that of the free electron (ge = 2.0023)
indicates that the unpaired electron is predominantly delo-
calized over the organic ligand and is not localized at the
metal center (see the Supporting Information).
2 equiv BQ, the corresponding lactones are obtained in
excellent yields at TOFs greater than 60000 hꢀ1. We assume
that, in these reactions, the primary alcohol function is also
selectively converted: an intramolecular cyclization to a
semiacetal is followed by a rapid noncatalytic dehydrogen-
ation by BQ. The hydroquinone (HQ) produced in the
reaction precipitates and can be nearly quantitatively re-
oxidized to the H2 acceptor BQ.[25,26]
In the presence of the diamido complex 4 and with O2 as
the H2 acceptor, benzyl alcohol is quantitatively converted to
benzaldehyde in a stoichiometric reaction. The metal-con-
taining product of this reaction is probably an iridium(III)–
hydroxide complex, from which 4 cannot be regenerated.
However, if the storable, stable precursor 2 and the H2
acceptor BQ are used, a highly active and selective catalyst
system is obtained for the dehydrogenation (oxidation) of
primary alcohols to the corresponding aldehydes (Table 1).[25]
With only 0.01mol% of 2 and 0.01–0.1 mol% of KOtBu, in
chlorobenzene at 80–1008C, benzyl alcohol is nearly com-
pletely converted to benzaldehyde in 3 min; the correspond-
ing TOF is greater than 150000 hꢀ1 (entry 1). Quantitative
conversions are also achieved at room temperature, although
with lower catalyst activity (entries 2, 4, 9). Methylthioether
(entry 3) or heterocyclic substrates (entries 4, 8, 9) are
converted in good to excellent yields, and allylic alcohols
are also smoothly converted (data not reported herein). Non-
activated aliphatic alcohols such as methanol, ethanol, and
particularly octanol are very rapidly dehydrogenated to the
corresponding aldehydes (entries 5–7). Most remarkably, the
highly selective dehydrogenation of 1,3-butanediol gives 3-
hydroxybutanal exclusively. To our knowledge, no other
transition-metal complex or GOase model system show
comparable activity and chemoselectivity. In the dehydrogen-
ation of the dihydroxy compounds 1,4-pentanediol (entry 11)
and 1,2-di(hydroxymethylene)cyclohexane (entry 12) with
We propose the catalytic cycle shown in Scheme 2, which
is supported by the following observations: 1) A mixture of
BQ, SQCꢀ, and HQ2ꢀ alone does not lead to dehydrogenation.
2) No reaction is observed in the absence of the base KOtBu.
However, 1equiv KO tBu with respect to 2 is sufficient to start
the catalysis; with 2 equiv KOtBu, excellent results are
obtained. 3) The formation of the deep violet SQCꢀ radical
anion is detected in all the reactions. 4) In a dehydrogenation
of benzyl alcohol, the amino amido complex 3 was recovered
in addition to benzaldehyde at the end of the reaction.
Ambiguity remains with respect to the intermediates I-1
and I-2 (Scheme 2), for which we have no experimental
evidence. When the substrate coordinates to the iridium
center in close proximity to the aminyl nitrogen atom in I-1,
the ketyl-radical complex I-2 could be generated through an
intramolecular hydrogen-atom transfer reaction. This step is
the rate-limiting step in GOase-catalyzed reactions (isotope
effect: kH/kD ꢁ 22). When the deuterated substrate C6D5-
CD2OH is used in catalytic runs with [Ir(trop2dachꢀ2H)]ꢀ,
we find a much smaller isotope effect (kH/kD ꢁ 2). We assume
that the coordinated ketyl radical in I-2 is intermolecularly
oxidized by SQCꢀ,[27] while in the GOase-catalyzed reactions
with Cu2+ as the oxidant, this step proceeds intramolecularly
(Scheme 1d).
Although we do not know the exact structure nor the fate
of the radical 5 under the catalytic conditions, the EPR
Table 1: Catalytic dehydrogenation of alcohols to aldehydes.[a]
Entry
Substrate
Product[b]
Yield [%]
t
Entry
7
Substrate
Product[b]
Yield [%]
t
1
94
3 min
>98
10 min
2
3
4
>98
70
16 h[c]
1 h
8
9
>98
>98
>98
12 h
2 h[c]
1 h
>98
3 h[c]
10
5
6
64
94
4 h
4 h
11
12
>98
>98
10 min[d]
5 min
[a] With BQ as the H2 scavenger, 0.01 mol% 2, and 0.03 mol% KOtBu in chlorobenzene at 808C. [b] The identity of each product was verified by gas
1
GC, MS, and H NMR spectroscopy through comparison with a reference sample. [c] In chlorobenzene at 258C. [d] In chlorobenzene/THF (3:1) at
1008C.
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Communications
[7] a) J. S. M. Samec, J.-E. Bäckvall, P. G. Andersson, P. Brandt,
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[12] B. A. Jazdewski, W. B. Tolman, Coord. Chem. Rev. 2000, 200–
202, 633.
spectroscopic data indicate that an iridium–nitrogen-radical
complex plays an important role in the catalytic transfer
dehydrogenation of the alcohols. The efficiency of this
reaction is comparable to that of the transfer hydrogenation
of carbonyl compounds promoted by diamagnetic transition-
metal complexes. The reactions reported herein complement
this chemistry and should be of equally high synthetic value.
[13] I. E. Markó, P. R. Giles, M. Tsukazaki, S. M. Brown, C. J. Urch,
Science 1996, 274, 2044.
Experimental Section
2: [Ir2(m2-Cl)2(coe)4] (300 mg, 0.34 mmol) was dissolved in THF
(25 mL), and trop2dach (1; 331mg, 0.68 mmol) and a few drops of
CH3CN were added. Subsequently, AgOTf (172 mg, 0.68 mmol) was
added to this mixture. After stirring for 1h at room temperature, the
mixture was filtered and concentrated in vacuo. Red crystals were
grown from THF/hexane (1:1). Yield: 259 mg (309 mmol, 91%). M.p.
[14] Reviews: a) S. V. Ley, J. Norman, W. P. Griffith, S. P. Marsden,
Synthesis 1994, 639; b) R. A. Sheldon, I. W. C. E. Arends, A.
Dijksman, Catal. Today 2000, 57, 157; c) R. A. Sheldon,
I. W. C. E. Arends, G.-J. ten Brink, A. Dijksman, Acc. Chem.
Res. 2002, 35, 774; d) R. Irie, T. Katsuki, Chem. Rec. 2004, 4, 96;
e) S. S. Stahl, Angew. Chem. 2004, 116, 3480; Angew. Chem. Int.
Ed. 2004, 43, 3400.
[15] The most active homogeneous catalyst in an Oppenauer
oxidation yet described is [IrCp*(NHC)(MeCN)2]2+ (Cp* =
pentamethylcyclopentadienyl; NHC = N-heterocyclic carbine),
which reaches turnover numbers (TON) of approximately 7000:
F. Hanasaka, K. Fujita, R. Yamaguchi, Organometallics 2005, 24,
3422.
[16] L. Guidoni, K. Spiegel, M. Zumstein, U. Röthlisberger, Angew.
Chem. 2004, 116, 3348; Angew. Chem. Int. Ed. 2004, 43, 3286.
[17] Y. Mityazato, T. Wada, K. Tanaka, Bull. Chem. Soc. Jpn. 2006,
79, 745.
[18] T. Büttner, J. Geier, G. Frison, J. Harmer, C. Calle, A. Schweiger,
H. Grützmacher, Science 2005, 307, 235.
[19] P. Maire, M. Königsmann, A. Sreekanth, J. Harmer, A.
Schweiger, H. Grützmacher, J. Am. Chem. Soc. 2006, 128, 6578.
[20] P. Maire, F. Breher, H. Schönberg, H. Grützmacher, Organo-
metallics 2005, 24, 3207.
[21] Crystal data for 4: C52H64IrKN2O7·C4H8O; crystal size: 0.31”
0.23 ” 0.21mm 3, monoclinic, space group P21, a = 11.7856(7), b =
24.750(1), c = 17.254(1) Š, b = 95.342(1)8, V= 5010.89 Š3, Z = 4,
m = 2.80 mmꢀ1, T= 200 K, 69595 reflections, 24616 independent,
1
(decomp.): 208–2128C. H NMR (300.1MHz, [D 8]THF): d = 4.02 (d,
3JH,H = 12.0 Hz, 2H, CHolefin), 4.75 (d, 3JH,H = 11.7 Hz, 2H, CHolefin),
5.53 (s, 2H, CHbenzyl), 5.92 (s br, 2H, NH), 7.41–7.72 ppm (m, 16H,
CHar). 13C NMR (62.9 MHz, [D8]THF): d = 53.0 (s, 2C, CHbenzyl), 60.4
(s, 2C, CHolefin), 67.5 ppm (s, 2C, CHolefin). UV/Vis (THF): lmax
=
486 nm.
3: The addition of 1equiv KO tBu to 2 in [D8]THF led to a deep
red solution of the neutral amino amido complex 3, which was
1
characterized by NMR spectroscopy. H NMR (300.1MHz): signals
for four inequivalent olefinic protons: d = 2.66 (d, 3JH,H = 8.5 Hz), 3.34
(d, 3JH,H = 8.3 Hz), 3.46 (d, 3JH,H = 8.5 Hz), 4.09 ppm (d, JH,H
=
3
8.5 Hz); signal for NH: d = 5.74 ppm (d, JH,H = 12.5 Hz). 13C NMR
(62.9 MHz): signals for four inequivalent olefinic carbon nuclei: d =
47.8, 51.4, 54.8, 62.9 ppm. UV/Vis (THF): lmax = 504 nm.
3
4: To a solution of 2 (100 mg, 0.12 mmol) in THF, KOtBu (29 mg,
0.26 mmol) and [18]crown-6 (32 mg, 0.12 mmol) were added. The
color of the reaction mixture changed to dark red. After layering the
mixture with n-hexane, red crystals of 4 grew overnight. Yield: 101 mg
(0.11 mmol, 94%). M.p. (decomp.): 186–1898C. 1H NMR
(300.1MHz, [D 8]THF): d = 2.63 (d, 3JH,H = 8.3 Hz, 2H, CHolefin),
2.91(d, 3JH,H = 8.5 Hz, 2H, CHolefin), 5.32 ppm (s, 2H, CHbenzyl).
13C NMR (62.9 MHz, [D8]THF): d = 49.8 (s, 2C, CHolefin), 52.5 (s, 2C,
CHolefin), 65.8 ppm (s, 2C, CHbenzyl). UV/Vis (THF): lmax = 524 nm.
Catalyses: The catalyst precursor 2 (6 mmol) was dissolved in
chlorobenzene (25 mL) at the temperature given in Table 1. Sub-
sequently, KOtBu (6–18 mmol) was added, and after 5 min stirring, the
substrate (60 mmol) was added. After another 5 min, freshly sub-
limed BQ (8.6 g, 80 mmol) was added to the reaction mixture. The
progress of the reaction was monitored by GC and 1H NMR
spectroscopy. Further details are given in the Supporting Information.
Rint = 0.0304, 1225 parameters, 14 restraints, R1 = 0.0302 (for
22911 reflections with I > 2s(I)), wR2 = 0.0685 (for all data),
GooF = 1.032 (on F2), max./min. residual electron density:
1.611/ꢀ0.502 eŠꢀ3. CCDC-622200 (4) contains the supplemen-
tary crystallographic 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.
[22] P. Maire, F. Breher, H. Grützmacher, Angew. Chem. 2005, 117,
6483; Angew. Chem. Int. Ed. 2005, 44, 6325.
[23] EPR (X-band, 298 K): g = 2.005, a(1H) = 6.7 MHz. J. C. Chip-
pendale, E. Warhurst, J. Chem. Soc. Faraday Trans. 1968, 64,
2332.
Received: December 21, 2006
Published online: March 30, 2007
[24] A. Schweiger, G. Jeschke, Principles of Pulse Electron Para-
magnetic Resonance, Oxford University Press, Oxford, 2001.
[25] For the use of quinones as electron-transfer mediators in the
ruthenium-catalyzed aerobic oxidation of secondary alcohols,
see: G. Csjernyik, A. H. Éll, L. Fadini, B. Pugin, J.-E. Bäckvall, J.
Org. Chem. 2002, 67, 1657.
Keywords: alcohols · homogeneous catalysis · iridium ·
oxidation · radicals
.
[26] B. B. F. Mirjalili, M. A. Zolfigol, A. Bamoniri, A. Zarie, Bull.
Korean Chem. Soc. 2003, 24, 400.
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ꢀ
ꢀ
=
[27] Reduction potentials: PhCH O/PhCH-OC : ꢀ1.67 V; BQ/SQC :
ꢀ0.54 V, SQCꢀ/HQ2ꢀ: ꢀ1.4 V (vs. the saturated calomel electrode
´
(SCE), aprotic solvents): a) V. P. Vanysek in CRC Handbook of
Chemistry and Physics, 87th ed., CRC, Boca Raton, 2006, pp. 8 –
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