Iron achieves noble metal reactivity and selectivity: Highly reactive and enantioselective iron complexes as catalysts in the hydrosilylation of ketones

Article abstract of DOI:10.1021/ja512986m

Chiral iron alkyl and iron alkoxide complexes bearing boxmi pincers as stereodirecting ligands have been employed as catalysts for enantioselective hydrosilylation reactions with unprecedented activity and selectivity (TOF = 240 h^-1 at -40 °C, ee up to 99% for alkyl aryl ketones), which match the performance of previously established noble-metal-based catalysts. This shows the potential of earth-abundant metals such as iron for replacing platinum-metals without any drawbacks for the reaction design.

Full text of DOI:10.1021/ja512986m

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Communication
Iron Achieves Noble Metal Reactivity and Selectivity:
Highly Reactive and Enantioselective Iron Complexes
as Catalysts in the Hydrosilylation of Ketones
Tim Bleith, Hubert Wadepohl, and Lutz H. Gade
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja512986m • Publication Date (Web): 08 Feb 2015
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Iron Achieves Noble Metal Reactivity and Selectivity: Highly Reac-
tive and Enantioselective Iron Complexes as Catalysts in the Hy-
drosilylation of Ketones
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Tim Bleith, Hubert Wadepohl, Lutz H. Gade*
Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
The boxmi-iron(II) acetato complexes (either prepared in situ
or isolated beforehand) were found to catalyze hydrosilylation of
the test substrate 1a at 65 °C with full conversion and a selectivity
of 83 % ee (Table 1, entry 1). However at 5 mol% catalyst load-
ing, a reaction time of 72 h proved to be necessary due to a long
induction period to generate the active catalyst. In order to over-
come this, suitable pre-catalysts were developed which gave rise
to significantly greater overall reaction rates.
ABSTRACT: Chiral iron alkyl and iron alkoxide complexes
bearing boxmi pincers as stereodirecting ligands have been
employed as catalysts for enantioselective hydrosilylation
reactions with unprecedented activity and selectivity (TOF: 240 h-
1 at -40 °C, ee up to 99 % for alkyl-aryl-ketones) which match the
performance of previously established noble metal based
catalysts. This shows the potential of earth-abundant metals such
as iron to replace platinum-metals without any drawbacks for the
reaction design.
Table 1: Precursor Screening for the Hydrosilylation of 4-
Phenylacetophenone
Enantioselective, transition metal-catalyzed reductions of ke-
tones are valuable tools for the synthesis of chiral alcohols. Typi-
cally, these reactions are catalyzed by platinum-group metals such
as Ru, Rh, Ir, or Pt which suffer from common drawbacks in that
they are quite scarce and frequently toxic. Recently, more envi-
ronmentally benign catalyst systems based on earth-abundant
metals1–7 such as Fe have been the focus of research studies.8–18 In
general, these iron-based molecular catalysts did not display the
selectivity nor the reactivity of the systems they aim to replace,
and the quest has thus been for catalysts which match – or even
exceed – the performance of the latter.19 Nevertheless, the recent
breakthrough in iron-catalyzed asymmetric transfer hydrogenation
by Morris and coworkers has fueled the search for further molecu-
lar catalysts which are competitive with their noble metals ana-
logues.20–23
entry
X
conv. (%)a
> 95b
ee (%)a
83 (S)
1
2
3
4
5
OAc
CH2TMS (3)
(S)-OCHPhCH3 (4)
(S)-OCHPhCH3 (4)
(S)-OCHPhCH3 (4)
> 95
94.5 (S)
99.0 (S)
98.0 (S)
94.0 (S)
> 95
> 95c
> 95d
aDetermined by HPLC analysis. breaction temperature 65 °C for 72 h.
cSilane added at -78 °C, cold bath removed immediately, quenched after
1 h. dreaction carried out at room temperature and quenched after 2 h.
In the hydrosilylation of ketones, optimized rhodium catalysts
may display high activities down to -60 °C and achieve enantiose-
lectivities of greater than 90 % ee for a wide range of sub-
strates.24–26 On the other hand, Fe-complexes which have been
studied as catalysts for this reaction usually require higher tem-
peratures up to 65 °C and longer reaction times to give a product
with generally lower selectivity.27–36 Only for sterically very
demanding substrates, ee-values for the resulting secondary alco-
hols of > 95 % were obtained.28
Using the iron(II) neosyl complexes 3, which were readily
available by reaction of FeCl2(py4) with LiCH2TMS and boxmiH
(Scheme 1), instead of the actetate resulted in a dramatic increase
in activity and allowed us to reduce the reaction temperature.
Under the conditions indicated in Table 1 an significantly in-
creased enantioselectivity of 94.5 % ee was observed for the
reduction of the test substrate 1a (Table 1, entry 2).
Chirik, Tilley, and Glorius have shown, that the choice of cata-
lyst precursor ([(N-donor-ligand)Fe(CH2TMS)2], [Fe(N(TMS)2)2],
or [(NHC)2FeMe2]) has an enormous impact upon catalyst activi-
ty.37–40 In this communication we present the first iron-based
precatalyst that achieves both activity and selectivity which are
comparable to noble metals in catalytic hydrosilylation reactions.
Scheme 1: Synthesis of Fe(boxmi) Neosyl and Alkoxido
Complexes
Based on previous studies in our group29 and Nishiyama’s
lab,27,30 we explored the iron-catalyzed enantioselective hydrosi-
lylation
of aryl-alkyl
ketones.
We
employed
the
bis(oxazolinylmethylidene)isoindoline (boxmi)-ligand, a mono-
anionic NNN-pincer, that already showed excellent chiral induc-
tion in Nozaki-Hiyama-Kishi reactions as well as a wide range of
reactions of electrophiles with β-ketoesters and oxindoles.41–46
That the combination of the tridentate chiral boxmi ligand and
an alkyl co-ligand system gave rise to a highly reactive and selec-
tive catalytic species, contrasts with previous results for Fe(II)
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Page 2 of 5
alkyl complexes bearing chiral box or pybox ligands which only
ble 2, entry 8 and SI). Although the reaction took place with high
displayed modest selectivity.38
selectivity in Et2O, this solvent was not further used, due to poor
solubility of the catalyst and the ketone at lower temperatures
(Table 2, entry 9). The same could be observed using hexane or
(EtO)2MeSiH as solvents (see SI). Finally, chlorinated solvents
were also found to provide suitable reaction conditions (Table 2,
entry 10).
1
2
3
4
5
6
7
8
It seemed likely to us that in a first reaction step of the highly
reactive iron alkyl precatalyst an alkoxido iron complex was
formed. Since this was also potentially a key intermediate in the
catalytic cycle of the hydrosilylation, it was thought to be an
active catalyst in its own right. Indeed, the excellent selectivity
obtained with the alkyl pre-catalyst was superceded upon using
the (S)-1-phenyl-1-ethanolato complex 4, which had been ob-
tained by clean conversion alcoholytic transformation of the alkyl
complex 3 into the alkoxido derivative 4 (Table 1, entry 3, and
Scheme 1). The observed enantioselectivity of 99 % ee for 1a
matches the results of the most selective rhodium catalysts report-
ed to date. Even at room temperature, the test substrate was re-
duced with 94 % ee (Table 1, entry 5).
Catalyst activity was further probed for the hydrosilylation of
acetophenone with reduced catalyst loadings (0.1 mol%). Turn
over numbers (TON) of over 500 were achieved under these
conditions. On the other hand, for the standard 5 mol% catalyst
loading employed in this study TOFs of at least 240 h-1 were
observed at -40 °C. The reaction protocol is directly applicable to
conversions on a gram-scale. Reducing 1.0 g of 1a with the ap-
propriate amount of (EtO)2MeSiH in presence of 2.3 mol% 4
yielded the corresponding alcohol in 94 % isolated yield and
98.4 % ee.
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Variation of the substituent pattern on the boxmi ligand showed
that alterations in the backbone (R’) did not significantly alter the
selectivity of the reaction (Table 2, entry 1). As expected, varia-
tion of the chiral substituent R on the oxazoline rings had a signif-
icant impact on the enantioselectivity. Whereas higher selectivity
in the test reaction was observed for the isopropyl-substituted
derivative, the enantioselectivity dropped dramatically upon use
of the benzyl-substituted boxmi ligand, and no conversion with
the tert-butyl-substituted derivatives (Table 2, entries 2, 3, and
SI). In general, the phenyl-substituted ligand proved to be the
most efficient for the substrates in this work (vide infra and SI).
Although the complexes employed in the catalyst screening
were prepared in situ, the Fe boxmi compounds in question were
found to be readily isolable and were therefore used as such in all
further studies. The molecular structure of a pyridine adduct of
[Fe((R)-Hboxmi-Ph)(CH2TMS)] (3) was determined by X-ray
diffraction and is shown in Figure 1. The isolated complex pos-
sesses a trigonal-bipyramidal coordination sphere with the pincer
ligand coordinating in the classical meridional fashion to the iron
atom.
Table 2: Optimization of Reaction Conditions for the Hy-
drosilylation of Ketones
entry [Fe]
R
Silane
Phb (EtO)2MeSiH
iPrc
(EtO)2MeSiH
Bnd (EtO)2MeSiH
solvent
tol
conv. (%)a
> 95
> 95
> 95
> 95
90
ee (%)a
94.2 (R)
98.7 (R)
33 (R)
99.0 (S)
93.4
1
2
3
3
3
4
4
4
4
4
4
4
tol
3
tol
4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
(EtO)2MeSiH
PMHSd
iPr3SiH
nBuSiH3
tol
5
tol
6
tol
<5
n.d.e
Figure 1: Molecular structure of 3(py) at 50% probability ellipsoids;
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Fe-C(34): 2.093(4); Fe-N(1): 2.208; Fe-N(2): 2.133(3); Fe-
N(3): 2.205(3); Fe-N(4): 2.163(3); N(1)-Fe-N(3): 164.47(11); N(2)-Fe-
C(34): 127.76(13); N(2)-Fe-N(4): 120.69(12); N(4)-Fe-C(34): 111.54(13).
7
tol
83
57
8
(EtO)2MeSiH
(EtO)2MeSiH
thf
> 95
> 95
> 95
92.3
99.2f
9
Et2O
As indicated above, the alkoxido complex [Fe((R)-Hboxmi-
Ph)((S)-OCHCH3Ph)] (4) could be prepared by reacting 3 with
(S)-1-phenyl-1-ethanol at ambient temperature. Its pyridine adduct
was characterized by single-crystal X-ray diffraction and the
molecular structure of 4(py) is displayed in Figure 2. Both of the
isolated pyridine adducts, 3(py) and 4(py), were found to be
catalytically active and displayed the same selectivity as the in
situ generated catalysts 3 and 4, respectively. All iron complexes
employed in this study were determined to be in a high-spin state.
10
(EtO)2MeSiH CH2Cl2
93.8
aDetermined by HPLC analysis. b(S)-Phboxmi-Ph employed. c(S)-
Phboxmi-iPr
used.
c(S)-Meboxmi-Bn
employed.
dPMHS
=
e
f
Poly(methylhydrosiloxane). not determined. precipitation observed upon
cooling to -78 °C
.
Silane screening showed, that by replacing (EtO)2MeSiH with
comparatively inexpensive PMHS (poly(methylhydrosiloxane))
the selectivity and conversion were slightly lower (Table 2, entry
4) whereas the use of iPr3SiH yielded almost no conversion (Table
2, entry 5) which we attribute to the high steric demand of that
reagent and its tendency to react in radical pathways. nBuSiH3 was
able to reduce 4-phenylacetophenone with good conversions, but
only with a moderate ee (Table 2, entry 7). These findings suggest
that the cleavage of the Si-H bond is a heterolytic rather than a
homolytic step and the silane is involved in the reaction step
determining selectivity.
With the reaction conditions optimized, the substrate scope of
this reaction was then investigated. Reduction of para-substituted
acetophenone derivatives (1a-f) showed, that para-substituents on
the phenyl ring had almost no effect on the selectivity (Scheme 2,
1st and 2nd row) and they were reduced with ee’s usually greater
than 98 %. Cyclic derivatives such as α-tetralone (1m) were
converted completely with an ee-value of 99 %.
Upon variation of the solvent, it was observed that coordinating
solvents such as thf or MeCN resulted in lower selectivities (Ta-
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Journal of the American Chemical Society
an excellent level of greater than 95 %, and dodecanophenone (1l)
1
2
3
4
is fully converted at 99 % ee. Since 1-acetylnaphtone (1o) was
only converted partially at standard conditions (see SI), but with a
high ee-value of 97 %), prolonged reaction times or a larger ex-
cess of silane was necessary for sterically extremely demanding
substrates.
5
As observed previously for iron-catalyzed hydrosilylations, the
reduction of dialkylketones (1r) only occurs with low levels of
enantioselectivity.11,27–29 Diarylketones, on the other hand, could
be reduced to the corresponding alcohol with significant selectivi-
ty. Phenyl-4’-methoxyphenyl-ketone (1q) and phenyl-
2’,3’,4’,5’,6’-pentafluorophenyl-ketone (1p) were fully converted
with ee-s of 31 % and 81 %, respectively. This demonstrates that
the stereodiscrimination is not exclusively based on the steric
properties of the substituents on the carbonyl group, but directed
by the electronic structure of the two aryl rings. This also readily
explains the high selectivity for arylalkylketones bearing α-
unbranched alkyl moieties.
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9
10
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13
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Figure 2: Molecular structure of 4(py) at 50% probability ellipsoids;
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hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Fe-O(3) 1.8925(17); Fe-N(4) 2.132(2); Fe-N(1) 2.107(2);
Fe-N(2) 2.177(2); Fe-N(3) 2.1786(19); O(3)-Fe-N(4) 103.19(8); O(3)-
Fe-N(1) 133.08(8); N(1)-Fe-N(4) 123.68(8); N(2)-Fe-N(3) 168.22(7).
It seems likely to us, that the drastic change in catalytic activity
of the complexes employed in this work is due to precatalyst
activation, which may be incomplete for the acetate complexes
studied previously. Taking the results of Chirik et al. and
Nishiyama et al. into account,27,38 this seems to be a more general
pattern in iron-catalyzed hydrosilylation reactions.
Scheme 2: Substrate Scope of the Iron-catalyzed Hydrosi-
lylation of Ketones
Unlike noble metals, iron preferably undergoes one electron
transfers (or no changes in formal oxidation state) instead of two-
electron processes such as oxidative addition or reductive elimina-
tion. Therefore, mechanisms such as the Ojima mechanism47 for
Rh catalysts are very unlikely for this catalyst system.48 We
probed the presence of radicals during the reaction by addition of
0.1 or 1 eq. (referring to the ketone at 5 mol% catalyst loading) of
radical
traps.
Neither
triphenylmethane
nor
9,10-
dihydroanthracene had a significant impact on the reactivity ob-
served which indicates that radical processes are unlikely. Also
treatment of 4 with 1 eq. of (EtO)2MeSiH yields the silylether of
the alkoxide previously bound to Fe in compound 4.
Scheme 3: Mechanistic Proposal for the Iron-catalyzed
Hydrosilylation of Ketones
From our results and in analogy to the mechanism proposed for
Cu(I)-catalyzed hydrosilylation,49–52 we propose a mechanism for
the iron-catalyzed hydrosilylation as shown in scheme 3. After
precatalyst activation, σ-bond metathesis takes place between the
alkoxido complex and the silane, in which the silylether is formed
(I). The remaining hydridocomplex undergoes rapid coordination
of the ketone (II) and subsequent insertion of the ketone into the
Fe-H-bond (III). This step determines the stereoselectivity of the
reaction and reforms the alkoxido complex. Numerous attempts to
synthesize and isolate a hydrido complex remained unsuccessful.
We attribute to its high thermal instability which is common for
iron(II) high-spin hydrido complexes. This is illustrated by the
fact that, to the best of our knowledge, only one such high-spin
complex has been characterized by X-ray diffraction, whereas
low-spin iron(II) hydride complexes are more common.53,54
a4 eq. of (EtO)2MeSiH were added
.
We further investigated the reduction of arylalkylketones (1j-l)
with larger alkyl substituents (Scheme 2, 3rd row). Whereas isobu-
tyrophenone (1i) only partially reacted and the selectivity was also
decreased considerably to 73 % ee, tert-butylphenylketone did not
react at all. The latter two substrates appear to be too sterically
encumbered to allow efficient reduction. Notably, the ee-values
for arylalkylketones with long unbranched alkyl chains stayed at
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(20)
Mikhailine, A. A.; Maishan, M. I.; Morris, R. H. Org. Lett.
In summary, we have presented the first iron-catalyzed hydrosi-
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and allows catalytic transformations at low temperatures. This
makes it a promising system for the further development of the
field and, potentially, an important step towards applicability of
iron complexes as catalysts in synthesis.36 Further investigations
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sion of the scope of iron boxmi complexes as catalysts are under-
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