Amine-Activated Iron Catalysis: Air- and Moisture-Stable Alkene and Alkyne Hydrofunctionalization

Article abstract of DOI:10.1002/adsc.201600570

A simple alkylamine [(iPr)₂NEt] has been used to activate an air- and moisture-stable iron(II) pre-catalyst for alkene and alkyne hydrofunctionalization reactions. This amine activation has enabled the highly operationally simple hydrosilylatio

Full text of DOI:10.1002/adsc.201600570

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DOI: 10.1002/adsc.201600570
Amine-Activated Iron Catalysis: Air- and Moisture-Stable Alkene
and Alkyne Hydrofunctionalization
Amy J. Challinor,a Marc Calin,a Gary S. Nichol,a Neil B. Carter,b
and Stephen P. Thomasa,*
a
EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, EH9 3FJ, U.K.
E-mail: stephen.thomas@ed.ac.uk or G.S.Nichol@ed.ac.uk (for crystallographic information)
Syngenta, Jealottꢀs Hill Research Centre, Bracknell, Berkshire, RG42 6EX, U.K.
b
Received: May 31, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600570.
Abstract: A simple alkylamine [(iPr)2NEt] has been
used to activate an air- and moisture-stable iron(II)
pre-catalyst for alkene and alkyne hydrofunctionali-
zation reactions. This amine activation has enabled
the highly operationally simple hydrosilylation and
hydroboration of alkenes and alkynes using just
0.25–2 mol% iron catalyst and 1–25 mol% amine.
Significantly, these reactions proceed in equal yield
under both air and inert reaction conditions.
the anti-Markovnikov hydrosilylation of olefins with
exceptional turnover frequency and turnover number
(Figure 1, [a]).[6,10–12] However, the Fe(0) (pre)-catalyst
requires strictly air- and moisture-free environments
for handling and use, thus limiting possible adoption,
particularly on an industrial scale.[10]
The in situ activation of more robust Fe(II/III) pre-
cursors has emerged as a method to avoid the need to
prepare and handle low oxidation-state species.[10,13]
Strongly reducing organometallic reagents such as
Grignard reagents,[13g,14] NaHBEt3
and activated
[13f,15,16]
Keywords: catalysis; hydrosilylation; iron; sustaina-
ble chemistry
magnesium[13d] are commonly used as activators
Following Speierꢀs seminal development of the homo-
geneous platinum-catalyzed hydrosilylation of al-
kenes,[1] this transformation has become one of the
largest industrial processes in current operation, due
to the wide range of applications that use hydrosilyla-
tion products, for example, silicon rubbers, molding
products, release coatings, and pressure-sensitive ad-
hesives.[2,3] Although powerful, the current state-of-
the-art catalytic processes rely on precious metal com-
plexes (typically Pt or Rh), and are susceptible to un-
wanted side reactions such as dehydrogenative silyla-
tion and over-hydrosilylation to the tertiary silane.[2,4,5]
More significantly, the homogeneous nature of these
processes facilitates the loss of the precious metal by
incorporation into the (polymeric) product.[6,7] The
majority of hydrosilylation products are dispersive
polymers that make metal recovery and recycling ex-
tremely difficult. This is exemplified by the annual
loss of 5.6 tons of Pt in hydrosilylation processes.[6]
Consequently, investigations into sustainable alter-
natives are paramount to future hydrosilylation proce-
dures.[8,9] Chirik and co-workers have developed
Figure 1. In situ activation methods, including this work. [a]
Using organometallic reagents for pre-catalyst activation.
a series of formally iron(0) complexes which catalyze [b] Amine activation.
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(Figure 1, [a]). These reagents are inherently pyro- therefore hindered any possible activation. We thus
phoric, unstable to both air and moisture, can be in- proposed that the use of a more weakly coordinating
compatible with substrate functionality, and negate counter-ion could facilitate amine activation. The tri-
the inherent environmental and practical benefits of fluoromethanesulfonate (triflate) counter-ion is signif-
using an iron (pre-)catalyst.
icantly more labile than the halide counterparts, but
We sought to develop an iron-catalyzed hydrosilyla- yet imparts stability in the solid state.[19] Complexa-
tion reaction that made use of inexpensive, readily tion of iron(II) triflate with the 2,6-bis[1-(2,6-diethyl-
available, and, importantly, easily handled reagents phenylimino)ethyl]pyridine (EtBIP) ligand gave
(particularly the activator and catalyst) that could be EtBIPFe(OTf)2 (1c), an analogous pre-catalyst to the
applied on a large-scale without the need for air- and iron(II) halides, a purple mono-ligated high-spin
moisture-free techniques and reaction environments Fe(II) complex. The single crystal X-ray structure re-
(Figure 1, [b]). Simple tertiary amines satisfy all of vealed a coordinated aqua ligand,[19] and so an acces-
these requirements, and offer a novel mode of activa- sible site for possible amine coordination (Figure 2).
tion for iron catalysts. The use of amines to reduce Using the triflate pre-catalyst 1c in the presence of
metal pre-catalysts has been reported previously, but both (i-Pr)2NEt and NEt3 gave the linear hydrosilyla-
this is not known for the production of low oxidation- tion product 3a, with complete control of regioselec-
state iron species at room temperature.[17] In fact, the tivity and without any deleterious side-reactions such
in situ activation of iron pre-catalysts has relied on re- as dehydrogenative silylation,[20] over-hydrosilylation
duction by organometallic reagents with a (conjugate (to the tertiary silane product), hydrogenation or
acid) pKa of above 36.[18]
alkene isomerization (entries 5 and 6). This promising
Initial investigations focused on using simple result represents the first use of an amine to activate
amines to activate bis(imino)pyridine (BIP) Fe(II) an iron (pre)-catalyst directly at room temperature, to
pre-catalysts[9,11] for the hydrosilylation of 1-octene the best of our knowledge, and greatly simplifies the
(2a). The iron(II) chloride (1a) and bromide (1b) practicality of low oxidation-state iron catalysis. Sig-
complexes showed no catalytic activity when using nificantly, it was found that the presence of air had
either triethylamine or diisopropylethylamine [(i- a negligible effect on catalyst activation and activity,
Pr)2NEt] as the activator (Table 1, entries 1–4). We and thus the use of Schlenk/air-free techniques were
postulated that the strong iron-halide bond prevented
coordination of the amine to the pre-catalyst and
Table 1. Initial investigations and reaction optimization.
En-
try
Catalyst
(mol%)
Amine
(mol%)
Reaction
Time [h]
Yield 3a
[mol%][a]
1
2
3
4
5
6
1a (7)
1a (7)
1b (7)
1b (7)
1c (7)
1c (7)
1c (7)
1c (7)
1c (7)
1c (7)
1c (2)
(i-Pr)2NEt (120)
NEt3 (120)
(i-Pr)2NEt (120)
NEt3 (120)
(i-Pr)2NEt (120)
NEt3 (120)
(i-Pr)2NEt (120)
NEt3 (120)
24
24
24
24
24
24
24
24
1
–
–
–
–
13
45
95
43
87
80
95
7[b]
8[b]
9[b]
10[b]
11[b]
Figure 2. Molecular structure of EtBIPFe(OTf)2 (1c). 50%
probability of ellipsoids; hydrogen atoms and solvent mole-
cules omitted for clarity; black=C, blue=N, red=O,
white=H, yellow=S, green=F. CCDC 1452177 contains
the supplementary 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.
(i-Pr)2NEt (120)
(i-Pr)2NEt (25)
(i-Pr)2NEt (25)
1
1
[a]
Yields determined by 1H NMR spectroscopy, using
a 1,3,5-trimethoxybenzene internal standard (20 mol%).
No solvent added.
[b]
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not necessary.[21] The use of organometallic reagents
as activators for pre-catalyst 1c led to reduced prod-
uct yields compared to those using amine activation
(Table SI.1, Supporting Information).[21]
In a bid to increase the industrial relevance of this
system, solvent-free conditions were investigated, and
again the linear silane 3a was produced (entries 7 and
8).[7,22] Solvent-free reactions offer significant econom-
ic and environmental advantages by decreasing waste
and avoiding the use of organic solvents. As (i-
Pr)2NEt gave an excellent yield of product 3a under
these conditions, subsequent investigations were car-
ried out using this amine. A reaction time of 1 hour
or less was found to be sufficient for this reaction
(entry 9). Reducing the loading of (i-Pr)2NEt used to
25 mol% gave equal reactivity to that using stoichio-
metric amine (entry 10). Even when the catalyst load-
ing was reduced to 2 mol% an excellent yield and per-
fect regioselectivity were still achieved (entry 11).
Control experiments showed that both pre-catalyst 1c
and (i-Pr)2NEt were required for successful hydrosily-
lation (Table SI.2, Supporting Information).
The need for amine coordination to initiate pre-cat-
alyst activation was confirmed by the addition of pyri-
dine to a standard reaction [1c (2 mol%), (i-Pr)2NEt
(25 mol%), pyridine (50 mol%)] resulting in greatly
reduced catalyst activity (8% 3a compared to 95%
under standard reaction conditions, Scheme 1, [a]).
In order to further understand the mechanism of
activation a series of stoichiometric reactions and
NMR studies were carried out (see the Supporting In-
formation for full details). Amines are well known to
reduce 2nd and 3rd row transition metal species by co-
ordination and b-hydride elimination.[17a,b,h] A limited
number of examples have also been proposed for iron
species.[13b,17d,e] Thus we initially focused on the obser-
vation of b-hydride elimination products from the
amine activator; imine (or enamine, following tauto-
merization) or aldehyde, following aqueous work-up.
1H NMR and GC-MS studies of the reaction of
amine and pre-catalyst 1c, in the absence and pres-
ence of reactants, showed no observable formation of
products arising from b-hydride elimination. In fact,
the addition of (i-Pr)2NEt to pre-catalyst 1c resulted
solely in ligand exchange to generate the ligated
Scheme 1. Summary of mechanistic investigations; [a] Pyri-
dine addition hindered reactivity. [b] Displacement of tri-
flate counter-ions by amine. [c] Attempts to recover amine
by-products were unsuccessful, whilst primary amines do
not activate this system. [d] Addition of radical traps hinders
reactivity. [e] Catalyst is still active upon completion of hy-
drosilylation; see the Supporting Information for further in-
formation.
Fe(II) bis-amine complex and free triflate, as con- not the mechanism of pre-catalyst activation, but
firmed by 19F NMR (Supporting Information, Fig- a novel pathway is operating.
ure SI.1). N,N-Dimethylbenzylamine was found to be
Although no reaction was observed for Fe(OTf)2 or
an effective activator and offered the possibility of EtBIPFe(OTF)2 without amine, the addition of tetra-
generating more stable b-hydride elimination prod- butylammonium triflate in place of amine confirmed
ucts. Once again, no imine, enamine or aldehyde were that the triflate counter ion itself was not acting as
observed using this amine activator (Scheme 1 [c] and the activator once dissociated (Supporting Informa-
Supporting Information, Table SI.3, Scheme SI.1).[17] tion, Table SI.5).[21] The use of triflate salts with ben-
Furthermore, 2,6-lutidene, which has no available b- zylamine, an amine which displaces triflate from the
hydrogens, was able to active pre-catalyst 1c, albeit pre-catalyst but does not initiate catalysis, also gave
with low activity (Supporting Information, Table no hydrosilylation (Supporting Information, Table
SI.4). Thus, we propose that b-hydride elimination is SI.6).
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As b-hydride elimination was not detected, and
To determine if catalyst decomposition was occur-
free triflate was not active, we next considered a radi- ring under these reaction conditions, an equivalent of
cal-type activation of the pre-catalyst. The exclusion 1-octene and phenylsilane were added to a standard
of light and oxygen was not detrimental to activation reaction with 4-phenylbutene after 30 min. Both hy-
or catalyst activity (Supporting Information, Scheme drosilylation products 3a and 3d were obtained in
SI.2). Tertiary amines have been shown to participate near-equal yield to that of the isolated hydrosilylation
in single-electron transfer (SET) reactions, thus offer- reactions (88% and 82% vs. 95% and 91%, respec-
ing a mechanism for pre-catalyst activation.[23] How- tively), demonstrating that this catalyst is still active
ever, under no conditions could an amine radical after hydrosilylation is complete (Scheme 1, [e]).[21]
cation be observed by EPR spectroscopy, although Further mechanistic investigations are ongoing.
the short lifetime of trialkylamine radical cations may
The substrate scope and limitations of this novel
hinder detection. When 2,2,6,6-tetramethylpiperidin- hydrosilylation method were investigated. In all cases
1-yl)oxyl (TEMPO) was added to standard reaction the highly efficient anti-Markovnikov hydrosilylation
conditions, catalytic activity decreased with increasing of terminal alkenes, to give linear silanes, was ob-
stoichiometry of TEMPO until no activity was ob- served with complete control of regioselectivity and
served when the loading of TEMPO equalled that of without any dehydrogenative silylation or over-hydro-
amine (Scheme 1, [d]). Catalytic activity could be re- silylation observed (Table 2). Aliphatic and aromatic
gained by adding excess amine to a ꢂstalledꢀ reaction alkenes underwent successful hydrosilylation to give
(Supporting Information, Table SI.7). The use of an the anti-Markovnikov silane (Table 2, 3a–d). Chemo-
alternative radical trap, galvinoxyl, gave similar re- selective hydrosilylation of the terminal alkene of vi-
sults. Therefore a radical activation mechanism is sug- nylcyclohexene (2c) was achieved without multiple
gested.
hydrosilylation or alkene isomerization (3c). Styrene
Table 2. Reaction scope.
[a]
Using EtBIPFe(OTf)2 (4 mol%), 2 hour reaction time.
[b]
Using EtBIPFe(OTf)2 (3 mol%).
[c]
Using EtBIPFe(OTf)2 (4 mol%), 18 hour reaction.
[d]
Recovered starting material.
[e]
1
Using pinacolborane (120 mol%), EtBIPFe(OTf)2 (3 mol%), (i-Pr)2NEt (10 mol%). Yields determined by H NMR spec-
troscopy using a 1,3,5-trimethoxybenzene internal standard (20 mol%), isolated yields are shown in parentheses.
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derivatives, including those bearing electron-donating
and electron-withdrawing substituents, underwent suc-
cessful hydrosilylation, again with complete control of
regioselectivity (3e–3g). The chemoselective hydrosi-
lylation of alkenes containing carbonyl functionalities
was achieved without detriment to the regioselectivity
for the linear silane, and with no reaction at either
the ester or ketone groups observed (3h–i).[24] 1-
Bromo-4-(3-butenyl)benzene (2j) underwent chemo-
and regioselective hydrosilylation without any proto-
dehalogenation observed.[25] High reactivity was ob-
served using terminal alkynes to give the (Z)-vinylsi-
lanes, again with excellent control of regio- and ste-
reoselectivity, and, significantly, without hydrosilyla-
tion of the product alkene (3k–l).[13h] N-Phenyl-4-vi-
nylbenzaldimine (2m) underwent the chemoselective
hydrosilylation of the alkene in a moderate yield and
Scheme 2. Gram-scale hydrosilylation.
without reduction of the imine, to give product 3m. pheres were found for gram-scale reactions. This is
However, substrates bearing very strongly coordinat- a unique achievement in the field of low oxidation-
ing functionalities (e.g., nitrile and nitro groups) in- state iron catalysis and one which paves the way for
hibited catalyst activity giving only recovered starting future exploitation by both the expert and non-expert
material (Table SI.8; Supporting Information).[21] This alike. The pre-catalyst was found to be resistant to de-
method was highly selective for terminal alkenes and composition both in use and during storage ꢂon the
alkynes, with internal alkenes giving significantly benchꢀ, as aged pre-catalyst was found to still offer
lesser yields (Supporting Information, Table SI.9). significant activity, again demonstrating the possibility
However, norbornene, containing a strained internal of both this pre-catalyst and activation method to be
alkene, underwent successful hydrosilylation in good used by the widest possible user base (Supporting In-
yield (3n).
formation, Scheme SI.3).[21]
In order to demonstrate complimentary reactivity
In summary, the highly operationally simple hydro-
to current hydrosilylation manifolds and increase the functionalization of alkenes and alkynes has been de-
industrial applicability of the hydrosilylation products veloped using a bench-stable iron(II) pre-catalyst and
prepared, the hydrosilylation of allyltriethoxysilane in situ amine activator. Excellent reactivity, chemose-
was carried out in good yield and regioselectivity lectivity and complete control of regioselectivity has
using phenylsilane (3o). Unfortunately, the use of been achieved for a series of functionalized and un-
other silanes resulted in low reaction yield and cata- functionalized alkenes and alkynes and, significantly,
lyst activity. Amine activation was also found to be the reaction was found to be unaffected by the pres-
successful for the iron-catalyzed hydroboration of al- ence of air. A catalyst loading of just 0.25 mol% was
kenes. Using pre-catalyst 1c and pinacolborane (H- shown to be highly active in the hydrosilylation of 1-
BPin) directly, the hydroboration of 1-octene was ach- octene on gram-scale in only 10 min, under air, thus
ieved, to give the linear boronic ester (4a).
enabling the adoption of this catalytic manifold for
1-Octene (2a) is a highly industrially relevant hy- more sustainable large-scale hydrosilylation applica-
drosilylation example, thus the limits of both catalyst tions.
and amine loading were investigated with this sub-
strate (Supporting Information, Table SI.10). Success-
ful hydrosilylation was achieved using just 0.5 mol% Experimental Section
pre-catalyst 1c and 1.2 mol% (i-Pr)2NEt without any
significant decrease to catalytic activity, regioselectivi- Preparation of EtBIPFe(OTf)2 (1c)
ty or product yield (Supporting Information, Table
SI.10, entry 1). In addition, an increase of (i-Pr)2NEt
Iron(II) trifluoromethanesulfonate and 2,6-bis-[1-(2,6- dieth-
ylphenylimino)ethyl]pyridine were stirred in THF for 16 h.
to 80 mol% had no effect on the hydrosilylation yield,
allowing the amine to become the reaction solvent for
solid substrates (Supporting Information, Table SI.10,
entry 2). Importantly, and of industrial and practical
relevance, the gram-scale hydrosilylation of 1-octene
was complete within just 10 min, using only
0.25 mol% of catalyst 1c under an atmosphere of air
(Scheme 2). Equal reactivity in air and inert atmos-
The solvent was then removed under vacuum. Diethyl ether
was added and this mixture was stirred for a further 24 h.
The solvent was removed to provide an amorphous pink/
purple solid. See the Supporting Information for further de-
tails.
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General Procedure for the Hydrosilylation of Olefins
In a vial equipped with a magnetic stir bar, diisopropylethyl-
amine (25 mol%) was added to a solution of alkene
(100 mol%, 0.5 mmol), phenylsilane (120 mol%), and
EtBIPFe(OTf)2 (1c, 2 mol%). The mixture was stirred at
room temperature for 1 h. Diethyl ether was subsequently
added. The mixture was passed through a short silica plug to
remove the iron species, and the solvent removed under
vacuum to give the crude product. 1,3,5-Trimethoxybenzene
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Details on all materials and methods, along with charac-
terization data are available in the Supporting Information.
Acknowledgements
A. J.C. and S.P.T. thank Syngenta for the provision of a stu-
dentship and the University of Edinburgh for supporting
a studentship. S.P.T. thanks the Royal Society for a University
Research Fellowship. We thank Prof. Tehshik Yoon and Dr
Stephen Moggach for useful discussions. We thank John
Walton for useful discussions and performing EPR investiga-
tions. We thank Neil Robertson for useful discussions. We
thank both the NMR and Mass Spectrometry departments at
Edinburgh University.
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Adv. Synth. Catal. 0000, 000, 0 – 0
6
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ÞÞ
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COMMUNICATIONS
Amine-Activated Iron Catalysis: Air- and Moisture-Stable
Alkene and Alkyne Hydrofunctionalization
7
Adv. Synth. Catal. 2016, 358, 1 – 7
Amy J. Challinor, Marc Calin, Gary S. Nichol,
Neil B. Carter, Stephen P. Thomas*
Adv. Synth. Catal. 0000, 000, 0 – 0
7
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!