Regiodivergent hydrosilylation, hydrogenation, [2π + 2π]-cycloaddition and C-H borylation using counterion activated earth-abundant metal catalysis

Article abstract of DOI:10.1039/c8sc05391j

The widespread adoption of earth-abundant metal catalysis lags behind that of the second- and third-row transition metals due to the often challenging practical requirements needed to generate the active low oxidation-state catalysts. Here we report the development of a single endogenous activation protocol across five reaction classes using both iron- and cobalt pre-catalysts. This simple catalytic manifold uses commercially available, bench-stable iron- or cobalt tetrafluoroborate salts to perform regiodivergent alkene and alkyne hydrosilylation, 1,3-diene hydrosilylation, hydrogenation, [2π + 2π]-cycloaddition and C-H borylation. The activation protocol proceeds by fluoride dissociation from the counterion, in situ formation of a hydridic activator and generation of a low oxidation-state catalyst.

Full text of DOI:10.1039/c8sc05391j

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DOI: 10.1039/C8SC05391J
Regiodivergent Hydrosilylation, Hydrogenation, [2π+2π]-
Cycloaddition and C–H Borylation using Counterion Activated
Earth-abundant Metal Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Riaz Agahi,1† Amy J. Challinor,1† Joanne Dunne,1 Jamie H. Docherty,1 Neil B. Carter2 and Stephen P.
Thomas1*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The widespread adoption of Earth-abundant metal catalysis lags behind that of the second- and third-row transition metals
due to the often challenging practical requirements needed to generate the active low oxidation-state catalysts. Here we
report the development of a single endogenous activation protocol across five reaction classes using both iron- and cobalt
pre-catalysts. This simple catalytic manifold uses commercially available, bench-stable iron- or cobalt tetrafluoroborate salts
to perform regiodivergent alkene and alkyne hydrosilylation, 1,3-diene hydrosilylation, hydrogenation, [2π+2π]-
cycloaddition and C–H borylation. The activation protocol proceeds by fluoride dissociation from the counterion, in situ
formation of a hydridic activator and generation of a low oxidation-state catalyst.
catalyst activation. Iron- and cobalt carboxylate salts have been
shown to act as pre-catalysts for alkene hydrosilylation that do not
require an external activator (Scheme 1, b).8 Similarly, Huang showed
that a tridentate PNN-cobalt(II) dichloride pre-catalyst could be
activated thermally for alkene hydrosilylation.5b However, these are
limited to a single reaction class and the carboxylate counterions are
pre-catalyst specific.
Introduction
The ubiquity of catalytic protocols using precious metals such as
platinum, palladium and rhodium can be ascribed to the highly
robust and reliable nature of these methods and widespread
commercial availability of the catalyst precursors. Operational
simplicity has made reaction screening and optimisation routine
using these metals. However, beyond high value applications the low
natural abundance, volatile cost and toxicity of these metals remains
problematic. Earth-abundant transition metals offer an alternative,
sustainable platform for catalysis, particularly for bulk and dispersive
technologies, but are yet to achieve the widespread adoption even
though excellent catalytic activity has been achieved in exemplar
industrial reactions.1 Thus, the operational simplicity of Earth-
abundant-metal-catalysed reactions must be addressed to enable
the widespread use and development of these powerful
methodologies.
a) Exogenous Pre-catalyst Activation
H
Fe or Co Pre-catalyst
+
H
[Si]
R
[Si]
+ Activator
R
typically: NaHBEt3, EtMgBr or NaOtBu
b) Endogenous Pre-catalyst Activation (olefin hydrosilylation pre-catalysts)
R
P(iPr)2
Cl
N
O
OPiv
Co(
Fe(
)
2
NC
Co
O
Co
O
N
Cl
N
or
+
O
N
OPiv
)
N
2
R
ligand
The global silicone industry is forecast to be worth $18.3 billion
in 2021 and finds applications in areas as diverse as soft materials,
cosmetics and food additives.2 Alkene and alkyne hydrosilylation
reactions underpin this industry and the homogenous nature of
these processes results in the loss of over 5 tonnes of platinum
annually.3 Thus a transition to Earth-abundant metal catalysis would
be beneficial both environmentally and economically. Seminal
studies using isolated, low oxidation-state iron- and cobalt pre-
catalysts have shown the potential of these metals for alkene
hydrosilylation,4 and in situ activation of metal(II/III) pre-catalysts
using organometallic reagents1b,5 has decreased the operational
barrier to use. Methods have also been developed using bench-
stable reductants such as alkoxide reagents6 or amines7 (Scheme 1,
a). However, an additonal, external reagent is still required for pre-
Carboxylate activation
c) This Work: Endogenous Activation by [BF4] Counter-ion
Thermal activation
H
[Si]
R
H
BF
BF
[Co( 4)2•6(H2O)]
[Fe( 4)2•6(H2O)]
H
[Si]
+
R
EtBIP
(1 mol%)
EtBIP
(2 mol%)
R
[Si]
15 examples
up to 98:2 (B/L)
20 examples
up to 1:99 (B/L)
• Air/H2O tolerant • Fe and Co • F Activation
• Hydrosilylation • Hydrogenation • [22 • C(sp2)-H borylation
Scheme 1 Overview of prior alkene hydrosilylation reactions using iron- and cobalt
catalysts. a) Pre-catalyst activation using additives. b) Catalyst activity facilitated by
carboxylate ligands or thermal activation. c) This work: catalyst activation through use of
weakly coordinating tetrafluoroborate counterions.
The alkoxide activation of iron- and cobalt pre-catalysts (Scheme 1,
a) was proposed to proceed by reaction of the alkoxide and silane to
form a hydridic silcon ‘ate’ complex which reduces the pre-catalyst
by a hydride transfer.6a In contrast, the carboxylate and thermal
activation methods were proposed to proceed by a σ-bond
metathesis reaction between the metal carboxylate and silane
(Scheme 1, b).8 We postulated that using a counterion which is
known to dissociate a nucleophile would allow an activation method
that combined the operational simplicity of carboxylate activation
a. EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road,
Edinburgh, EH9 3FJ, U.K. Fax: (+44)-(0)-131-650-6543
b. Syngenta, Jealott's Hill International Research Centre, Bracknell, Berkshire, RG42
6EX, U.K.
† These authors contributed equally.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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and the broad scope of alkoxide activation. The tetrafluoroborate h-1. In the cobalt-catalysed system, using only 110 ppm cobalt with
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counterion is known to dissociate to BF3 and fluoride.9 The fluoride phenylsilane gave the branched silane 3a with an overall turnover
DOI: 10.1039/C8SC05391J
could react with the silane to give a hydridic silicon ‘ate’ complex,10 number of 5940, and turnover frequency of 2970 h-1. As all reaction
and activate a pre-catalyst by hydride transfer (Scheme 1, c).11
components are air- and moisture stable the reactions can be set up
without the need for specialist equipment. Therefore the iron- and
cobalt catalysed hydrosilylation of 1-octene 1a was carried out in air
with only a limited loss of catalyst activity and regioselectivity
(Scheme 2, 2a and 3a). Alkyl- and aryl substituted alkenes underwent
hydrosilylation in excellent yield and regioselectivity for both the
iron- 2e-p and cobalt-catalysed 3d-l systems. The iron-catalysed
system was found to tolerate electron-withdrawing substituents 2k
and substituted alkenes such as β-pinene 2m, norbornene 2n, α-
methylstyrene 2o and limonene 2p without detriment to yield or
regioselectivity. The lower oxophilicity of cobalt was exemplified by
chemoselective alkene hydrosilylation in the presence of ketone 3i,
ester 3j, epoxy 3k and amido 3l functionalities. Divergent
diastereoselectivity was observed for the hydrosilylation of terminal
alkynes6a with cobalt catalysis preferentially giving the (E)-
alkenylsilane 3m and the iron-catalysed system giving the (Z)-
alkenylsilane 2q.
Results and discussion
In order to establish the utility of the tetrafluoroborate
counterion for activation of iron(II) pre-catalysts we selected alkene
hydrosilylation as
a model reaction. Baseline reactivity was
determined by the use of iron pre-catalysts bearing strongly
coordinating chloride anions [EtBIPFeCl2] (Table 1, Entry 1) and
weakly coordinating triflate anions [EtBIPFe(OTf)2] (Entry 2). Both
reactions showed no catalytic activity. The tetrafluoroborate pre-
catalyst, formed in situ by the reaction of the commercially available
hydrate salt Fe(BF4)2•6H2O with bisiminopyridine ligand (EtBIP),
showed excellent catalytic activity without an external activator,
giving the linear silane product 2a in excellent yield and
regioselectivity (Table 1, entry 3). Lower catalyst loadings showed
good, but reduced, reactivity in the production of 2a (Table 1, entries
4-5). Using iron(II) tetrafluoroborate, with less hindered
bisiminopyridine ligands (HBIP, MeBIP, MesBIP) led to low reactivity,
with only the N-mesityl ligand giving good yield, with similar
linear:branched selectivity to that observed with EtBIP, while the
more hindered N-isopropyl ligand gave only trace reactivity (Table 1,
entries 6-9). Control reactions showed the need for both iron and
ligand to achieve catalysis (see Supporting Information, Table SI 1.1).
Table 1 Reaction optimisation for iron- and cobalt-catalysed hydrosilylation using
tetrafluoroborate pre-catalyst activation.
RBIP (n mol%)
[FeII] or [CoII] (n mol%)
H
SiH2Ph
n-C6H13
1a
SiH2Ph
H
+
+
-C H
6 13
n
n-C6H13
THF, r.t, 4 h
PhSiH3
2a
3a
HBIP
Ar = Ph
MeBIP
MesBIP
EtBIP
iPrBIP
Ar = 2,6-Me-Ph
Ar = 2,4,6-Me-Ph
Ar = 2,6-Et-Ph
Ar = 2,6-iPr-Ph
Having established the tetrafluoroborate activation for iron-
catalysed hydrosilylation, we next attempted to apply the same
protocol to cobalt-catalysed alkene hydrosilylation. Using
commercially available cobalt(II) tetrafluoroborate and the N-
dimethylphenyl bisiminopyridine ligand gave the linear silane 3a in
excellent yield and good regioselectivity (Table 1, entry 11); the
opposite to that observed under iron catalysis. Variation of the ligand
N-aryl substituent showed that all cobalt catalysed systems
selectively gave the branched silane, with the more hindered
catalysts giving reduced yields of silane 3a (Table 1, entries 10-14).
Notably, and to the best of our knowledge, this is the first example
of a regiodivergent alkene hydrosilylation where high levels of
regioselectivity are observed using an identical ligand and the same
reaction conditions, but only varying the metal used.5b Use of a lower
catalyst loading of 1 mol% gave the branched silane product in
excellent yield and selectivity (Table 1, entry 15).
N
N
N
Ar
Ar
Entry
[M]
Loading
(mol%)
2
Ligand
Yield (%)
(2a:3a)
0
1
2
FeCl2
EtBIP
EtBIP
EtBIP
EtBIP
EtBIP
HBIP
MeBIP
MesBIP
iPrBIP
HBIP
MeBIP
MesBIP
EtBIP
iPrBIP
EtBIP
Fe(OTf)2
2
2
0
3
Fe(BF4)2•6H2O
Fe(BF4)2•6H2O
Fe(BF4)2•6H2O
Fe(BF4)2•6H2O
Fe(BF4)2•6H2O
Fe(BF4)2•6H2O
Fe(BF4)2•6H2O
Co(BF4)2•6H2O
Co(BF4)2•6H2O
Co(BF4)2•6H2O
Co(BF4)2•6H2O
Co(BF4)2•6H2O
Co(BF4)2•6H2O
87 (93:7)
67
4
0.5
1
5
82
6
2
0
7
2
trace
8
2
78 (>95:5)
trace
9
2
10
11
12
13
14
15
2
84 (86:14)
72 (4:96)
68 (3:97)
82 (5:95)
31 (16:84)
90 (8:92)
2
2
Having established optimal conditions for olefin hydrosilylation,
we set out to investigate the scope and limitations of these reactions
(Scheme 2). A variety of alkyl- and alkoxysilane reagents were
successfully used for alkene hydrosilylation with both iron- (anti-
Markovnikov) and cobalt (Markovnikov) pre-catalysts providing the
linear- 2a-p and branched alkyl silanes 3a-l in excellent yield and
regioselectivity, respectively. Iron-catalysed hydrosilylation of 1-
octene with a tertiary silane (triethoxysilane) gave the linear silane
2d with a turnover number of 425 and a turnover frequency of 5100
2
2
1
Reaction conditions: 1-Octene (1.00 equiv.), phenylsilane (1.10 equiv.) and metal
tetrafluoroborate (n mol%), THF (1 M), r.t., 1 h. Yields determined by 1H NMR
spectroscopy of the crude reaction mixture using 1,3,5-trimethoxybenzene as an
internal standard.
2 | J. Name., 2012, 00, 1-3
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EtBIP (2 mol%)
EtBIP (2 mol%)
b) Hydrosilylation: Markovnikov
a) Hydrosilylation: anti-Markovnikov
View Article Online
R
Fe(BF4)2•6H2O (2 mol%)
Co(BF4)2•6H2O (2 mol%)
DOI: 10.1039/C8SC05391J
H
[Si]
1a-t
+
[Si]
H
THF, r.t, 4 h
THF, r.t, 4 h
R
R
[Si]
H
2a-t
3a-o
2a: [Si] = SiPhH2, (87%) (7:93 B:L)
3a: [Si] = SiPhH2, >95% (86%) (92:8 B:L)
under air: 90% (95:5 B:L)
H
[Si]
under air: 84% (23:77)
[Si]
H
2b: [Si] = Si(C6H13)H2 96% (22:78 B:L)
[Co] 110 ppm 3.4 g product
TON = 5940, TOF = 2970 h-1
n-C6H13
n-C6H13
2c: [Si] = SiPh(Me)H (74%) 72% (1:99 B:L)
3b: [Si] = Si(C6H13)H2 (88%) (98:2 B:L)
3c: [Si] = SiPh(Me)H (50%) 66% (1:99 B:L)
2a-d
3a-c
2d: [Si] = Si(OEt)3 (85%) (2:98 B:L)
7.5 g product TON = 425, TOF = 5100 h-1
SiPhH2
F3C
F3C
H
SiPhH2
H
SiPhH2
H
H
SiPhH2
H
SiPhH2
H
(EtO)3Si
Si(OEt)3
2
2
2
2
tBu
2e: 65% (60%)
2f: >95% (72%) (7:93 B:L)
2g: (77%) (13:87 B:L)
3d: 86% (78%) (>98:2 B:L)
3e: (72%) (>98:2 B:L)
3f: 88% (83%) (55:45 B:L)
H
H
SiPhH2
H
H
O
SiPhH2
SiPhH2
SiPhH2
SiPhH2
SiPhH2
H
(EtO)3Si
H
2
tBu
MeO
MeO
3g: (83%) (~1:1 B:L)
2h: 86% (85%) (>2:98 B:L)
2i: (79%) (>2:98 B:L)
2j: (78%) (2:>98 B:L)
3h: 95% (79%) (>98:2 B:L)
3i: 89% (72%) (90:10 B:L)
SiPhH2
H
O
O
H
O
SiPhH2
SiPhH2
SiPhH2
H
H
O
SiPhH2
N
SiPhH2
H
H
2
MeO
MeO
8
O
2
8
F
2l: 82% (75%) (6:94 B:L)
3l: (42%) (93:7 B:L)
2m: 82% (82%)
3k: 85% (67%) (95:5 B:L)
2k: 38% (32%) (>2:98 B:L)
3j: 82% (78%) (94:6 B:L)
H
H
H
H
H
SiPhH2
nPr
SiPhH2
SiPhH2
H
SiPhH2
5
H
nPr
SiPhH2
SiPhH2
2n: 82% (58%)
2o: 82% (73%)
2p: 81% (71%)†
c) Iterative
3m: (63%) (E/Z 83:17)
3n: 68% (60%)
3o: 72% (68%) (>98:2, 1,4/4,1)
SiPhH2
H
SiPhH2
nPr
H
H
H
H
H
H
hydrosilylation-hydrosilylation
Si
Ph
Si
5
nPr
Ph
5
5
5
Ph
H
2t: (87%) (7:93 B:L)
2q: (79%) (E/Z 27:73)
2r: 42% (33%)
2s: (87%) (7:93 B:L)
Scheme 2 Scope for iron- and cobalt-catalysed hydrosilylation reactions enabled by tetrafluoroborate activation. a) Reaction conditions: olefin, PhSiH3 (1.1 eq.), EtBIP (2 mol%) and
Fe(BF4)2•6H2O (2 mol%), THF, r.t., 4h. †MesBIP (2 mol%) used. b) Reaction conditions: olefin, PhSiH3 (1.1 eq.), EtBIP (2 mol%) and Co(BF4)2•6H2O (2 mol%), THF, r.t., 4h. c) Reaction
conditions: alkene (1 equiv.), PhSiH3 (1 equiv.) Fe(BF4)2•6H2O (2 mol%), EtBIP (2 mol%), THF, r.t., 30 min then a second alkene (1 equiv) added, 3h. Yields determined by 1H NMR
spectroscopy of the crude reaction mixture using 1,3,5-trimethoxybenzene as an internal standard, isolated yields in parenthesis.
Both the iron and cobalt catalysts gave the (E)-alkenylsilane, the ligands in combination with Co(BF4)2•6H2O effectively catalysed the
product of syn addition, in the hydrosilylation of 4-octyne 2r and 3n, hydrosilylation of 1-octene 1a with phenylsilane to generate linear
respectively. Hydrosilylation of the 1,3-diene myrcene proceeded silane 2a in excellent yield as a single exclusive regioisomer (Scheme
with 1,4-selectivity 3o.
3, a). Adamantyl isocyanide as a ligand additionally proved effective
for both iron- and cobalt-catalysed alkene hydrosilylation to
generate 2u and 2v respectively (Scheme 3, b).13 The bidentate
iminopyridine ligand, used previously by Ritter for 1,4-hydrosilylation
of 1,3-dienes,14 was effective using Fe(BF4)2•6H2O to give allylic
silane 2w (Scheme 3, c).
The potential of the developed tetrafluoroborate activated
hydrosilylations for polymer crosslinking and late-stage
functionalisation was demonstrated by iterative homo- 2s and
hetero- 2t bis-hydrosilylations using two alkenes. Beyond showing
that the tetrafluoroborate activation strategy can be applied across
a number of unique ligand classes and metal salts, these results also
As we presumed the active hydrosilylation catalyst was a low
demonstrate how, as a tool for reaction screening, this method can oxidation-state metal species, we next explored the use of the
provide a facile method for uncovering new and contrasting tetrafluoroborate activation as a general platform to access these
reactivity.
species. We postulated that mixing the tetrafluoroborate pre-
catalysts with a substoichiometric amount of silane reagent would
give a generic low oxidation-state catalyst that would be applicable
to transformations beyond hydrosilylation. This would negate the
need to isolate a catalyst with limited stability or use pyrophoric
reagents in low oxidation-state iron- and cobalt catalysis.6a
To assess the generality of tetrafluoroborate iron- and cobalt-
salts for hydrosilylation we opted to perform alkene hydrosilylation
reactions with ligands distinct from bisiminopyridine. We initially
selected Xantphos and dppf as these had been used previously by Ge
for cobalt-catalysed alkene hydrosilylation.12 Using both of these
This journal is © The Royal Society of Chemistry 20xx
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Alternative ligands
Another example of a reaction which has been catalysed by low
View Article Online
Ligand (n mol%)
[Fe/Co(BF4 2•6H2O] (n mol%)
DOI: 10.1039/C8SC05391J
R'
R
oxidation-state species is the intramolecular [2π+2π]-cycloaddition
)
R'
R
H
[Si]
H
of 1,6-dienes to give [3.2.0] bicyclic systems.17 In this case, N-benzyl-
+
[Si]
THF
N,N-diallylamine
7a
was
converted
into
N-benzyl-3-
a)
b)
c)
NC
PPh2
PPh2
azabicyclo[3.2.0]heptane 8a in good yield and N-4-fluorophenyl-N,N-
diallyl amine 7b cyclised to give bicyclic 8b in excellent yield (Scheme
3, e). A number of procedures have been developed for cobalt-
catalysed C–H borylation, and cobalt(I) boryl complexes have been
proposed to be the key catalytic intermediate.18 In order to apply
tetrafluoroborate activation to a range of mechanistically distinct
reactions, C–H borylation of 2-methylfuran 9 was carried out using a
cobalt-terpyridine tetrafluoroborate pre-catalyst, to give the boronic
ester 10 in 67% yield (Scheme 3, f).
N
Fe
(3 mol%)
(5 mol%)
O
N
Ar
PPh3
Metal:
Product:
PPh3
)2•6H2O (1 mol%)
Ar = 2,6-diisopropylphenyl
Co(BF4)2•6H2O
(3 mol%)
Co(BF4
H
Fe(BF4
)
2•6H2
O
Fe(BF4
)
2•6H2O (5 mol%)
(3 mol%)
H
H
(EtO)3Si
H
SiMe2Ph
SiMe2Ph
SiH2Ph
n-C6H13
2a
2u
24%
2v
2w
82% (>99:1 L:B)
98% (>99:1 L:B)
51% (24:76, 1,4/4,1)
73% (>99:1 L:B)
Xantphos
dppf
Alternative reactions
d) Hydrogenation
[Fe/Co(BF4)2•6H2O] (0.5 mol%)
H
MesBIP (0.5 mol%)
H
The facile activation observed using this protocol was thought to
result from pre-catalyst reduction by an in situ formed hydridic
silicon ‘ate’; formed by reaction of the silane reagent with fluoride
dissociated from the counterion. This putative ‘ate’ complex then
transfers hydride to the pre-catalyst, facilitating reductive
elimination of dihydrogen (Scheme 4, a).4b Silane reagents have been
shown to be hydridic reagents in the presence of suitable
nucleophiles, such as fluoride, and this has been applied in the
reduction of carbonyls.10 To examine whether similar reactivity could
be obtained in this case, n-butylammonium tetrafluoroborate
(TBABF4) was reacted with phenylsilane in the presence of 4-
fluorobenzaldehyde 10 to give the primary alcohol reduction product
11 in excellent yield (Scheme 4, b). This underlines the possibility of
pre-catalyst reduction by a hydridic silicon ‘ate’ complex6a formed by
reaction of fluoride and phenylsilane.11 Although the exact nature of
the active catalyst is not known, the reaction was tested in the
presence of radical inhibitors and was unaffected (Scheme 4, c and
Table SI 10), suggesting that reduction occurs by a two-electron
mechanism.4b
R2
PhSiH3 (5 mol%), H2 (20 bar)
R1
R1
R2
6a-e
THF, r.t., 7 h
5a-e
H
H
H
H
H
H
H
(EtO)3Si
H
H
H
91% (72%), 23%‡
60%§
85%§
6a
e) [2+2] Cycloaddition
6b 87%
6c
6d 72%
6e
EtBIPCo(BF4
)2 (10 mol%),
H
H
PhSiH3 (20 mol%)
R
N
R
N
80 °C, 24 h
7a-b
8a-b
H
H
H
N
F
N
H
8a 59%
8b >95%
f) C-H Borylation
Ar
ArterpyCo(BF4
) =
2
ArterpyCo(BF4
)
2 (5 mol%)
PhSiH3 (10 mol%)
LiOMe (1 equiv.)
B2pin2 (1 equiv.)
O
N
O
H
N
Co
N
80 °C, 24 h
Bpin
10 67%
F4
B
BF4
9
Ar = (4-NMe2)Ph
Scheme 3 Application of tetrafluoroborate activation to other ligand classes for alkene
hydrosilylation (top). Reaction conditions: a) Xantphos or dppf (1 mol%), Co(BF4)2•6H2O
(1 mol%), 1-octene (1 mmol), phenylsilane (1.1 mmol), THF (2 M), r.t., 4 h. b) Adamantyl
isocyanide (9 mol% [Co] or
6 mol% [Fe]), metal tetrafluoroborate (3 mol%), α-
Conclusions
methylstyrene ([Co], 1 mmol) or styrene ([Fe], 1 mmol), phenyldimethylsilane (1.3
mmol), THF (2 M), 80 °C, 3 h. c) iPrIP (5 mol%), Fe(BF4)2•6H2O (5 mol%), myrcene (1 mmol),
triethoxysilane (1.2 mmol), THF (2 M), r.t., 16 h. d) Alkene (1 equiv.), Co(BF4)2·6H2O (0.5
mol%), MesBIP (1 mol%), PhSiH3 (5 mol%), H2 (20 bar), r.t., 7h. ‡Alkene (1 equiv.),
Fe(BF4)2•6H2O (2 mol%), MesBIP (2 mol%), PhSiH3 (5 mol%), H2 (20 bar), r.t., 7h.
§Co(BF4)2•6H2O (1 mol%) and MesBIP (1 mol%). e) 1,6-diene (1 equiv.), EtBIPCo(BF4)2•6H2O
A procedure for the regiodivergent hydrosilylation of olefins, a highly
valuable industrial reaction, has been developed using iron- and
cobalt tetrafluoroborate catalysts without the need for an external
activator or the use of isolated low oxidation-state complexes. This
(10 mol%), PhSiH3 (20 mol%), 80 °C, 24 h. f) Arene (15 equiv.), (4-NMe2-Ph-terpy)Co(BF4)2 has been used as an activation platform to access the low oxidation-
(5 mol%), PhSiH3 (20 mol%), LiOMe (1 equiv.), B2pin2 (1 equiv.), 80 °C, 24 h.
state catalysts in a range of iron- and cobalt-catalysed reactions
including hydrosilylation, 1,3-diene hydrosilylation, alkene
hydrogenation, [2π+2π]-cycloaddition and C–H borylation. The
The first reaction tested was hydrogenation of alkenes, an
industrially important transformation.15,16 Using 0.5 mol% of the
developed tetrafluoroborate activation represents
a versatile
iron- or cobalt tetrafluoroborate salts and the MesBIP ligand in
combination with substoichiometric phenylsilane gave an active
catalyst for alkene reduction in both cases (Scheme 3, d). 4-Phenyl-
1-butene underwent hydrogenation to the alkene 6a in good yield
under cobalt catalysis and reduced yield with the analogous iron
system. The cobalt-catalysed hydrogenation was found to be
successful for 1,1-disubstituted alkenes 6b and 6c, 1,2-disubstituted
alkenes 6d and allyl silane 6e (Scheme 3, d).
platform for activation, and serves as a generic strategy for accessing
low oxidation-state reactivity with both iron and cobalt. It is hoped
that this work will streamline the discovery of new reactivity,
development of novel synthetic methodology and, ultimately, in the
replacement of precious metals with their Earth-abundant
counterparts.
4 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
a)
2008, 41, 1500–1511. f) A. Fürstner, ACS Cent.VSiecwi.A,rt2ic0le1O6n,lin2e,
F
H
[Si]
BF3
H
H
778–789.
LM
OH
LM0/I
DOI: 10.1039/C8SC05391J
H [Si]
LM(BF4)2

2
Global Silicones Market by Market, Product and Country, 4th
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[Si]
F
H2
observed
[LMBF4]
11B = -1 ppm
3
4
b)
O
PhSiH3
TBABF4
via:
F
H
H
H
Ph Si
THF, r.t., 72 h
F
F
H
10
11
86%
5
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H
c)
n-C6H13
SiH2Ph
EtBIP (2 mol%)
[M(BF4)2•6H2O] (2 mol%)
Radical Trap
n-C6H13
2a
1a
+
(2 mol%)
THF, r.t., 1 h
+
PhSiH3
SiH2Ph
H
n-C6H13
3a
Radical Trap
3a 4a
: )
Entry
M
Yield (%) (
6
O
N
72 (92:8)
68 (6:94)
1
2
Fe
Co
3
4
Fe
Co
86 (94:6)
69 (3:97)
7
8
tBu
O
tBu
O
5
6
Fe
Co
72 (90:10)
35 (14:86)
tBu
tBu
Scheme
4 Proposed activation mechanism and mechanistic studies. a) Metal
tetrafluoroborate pre-catalyst activation strategy from reaction with silane reagents to
generate a low oxidation-state active catalyst. b) Interaction of tetrabutylammonium
tetrafluoroborate and phenylsilane for the reduction of 4-fluorobenzaldehyde,
suggestive of hydride formation. c) Attempted radical inhibition experiments with radical
trapping reagents.
9
Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
RA and SPT thank the Royal Society for funding a PhD studentship.
AJC and SPT thank Syngenta for part funding a PhD studentship. SPT
thanks the Royal Society for a University Research Fellowship. JD
thanks M. Shaver for useful discussions. All thank the University of
Edinburgh and the School of Chemistry Technical Staff for generous
support.
Notes and references
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6 | J. Name., 2012, 00, 1-3
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Low oxidation-state iron- and cobalt catalysis has been enabled using tetrafluoroborate activationView Article Online
across 5 reaction classes and seven ligand frameworks using a single catalysis protocol. DOI: 10.1039/C8SC05391J
H
[Si]
R
H
Fe BF
4)2•6(H2O)
Co BF
(
(
4)2•6(H2O)
H
[Si]
+
R
EtBIP
(1 mol%)
EtBIP
(2 mol%)
R
[Si]
• Hydrosilylation • Hydrogenation • [22-Cycloaddition • C(sp2)-H borylation