Iron-catalysed chemo-, regio-, and stereoselective hydrosilylation of alkenes and alkynes using a bench-stable iron(II) pre-catalyst

Article abstract of DOI:10.1002/adsc.201300827

The chemo-, regio-, and stereoselective iron-catalysed hydrosilylation of alkenes and alkynes with excellent functional group tolerance is reported (34 examples, 41-96% yield). The catalyst and reagents are commercially available and easy to handle, with the active iron catalyst being generated in situ, thus providing a simple and practical methodology for iron-catalysed hydrosilylation. The silane products can be oxidised to the anti-Markovnikov product of olefin hydration, and the one-pot iron-catalysed hydrosilylation-oxidation of olefins to give silane(di)ols directly is also reported. The iron pre-catalyst was used at loadings as low as 0.07 mol%, and displayed catalyst turnover frequencies (TOF) approaching 60,000 molh^-1. Initial mechanistic studies indicate an iron(I) active catalyst.

Full text of DOI:10.1002/adsc.201300827

UPDATES
DOI: 10.1002/adsc.201300827
Iron-Catalysed Chemo-, Regio-, and Stereoselective
Hydrosilylation of Alkenes and Alkynes using a Bench-Stable
Iron(II) Pre-Catalyst
Mark D. Greenhalgh,a Dominik J. Frank,a and Stephen P. Thomasa,*
a
School of Chemistry, University of Edinburgh, Joseph Black Building, West Mains Road, Edinburgh EH9 3JJ, U.K.
Fax :ACHTUNGTRENNUNG(+44)-(0)-131-650-6543; e-mail: Stephen.thomas@ed.ac.uk
Received: September 11, 2013; Revised: November 12, 2013; Published online: February 2, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300827.
Abstract: The chemo-, regio-, and stereoselective
iron-catalysed hydrosilylation of alkenes and al-
kynes with excellent functional group tolerance is
reported (34 examples, 41–96% yield). The catalyst
and reagents are commercially available and easy
to handle, with the active iron catalyst being gener-
ated in situ, thus providing a simple and practical
methodology for iron-catalysed hydrosilylation. The
silane products can be oxidised to the anti-Markov-
nikov product of olefin hydration, and the one-pot
iron-catalysed hydrosilylation–oxidation of olefins
to give silane(di)ols directly is also reported. The
iron pre-catalyst was used at loadings as low as
0.07 mol%, and displayed catalyst turnover frequen-
cies (TOF) approaching 60,000 molhÀ1. Initial
mechanistic studies indicate an iron(I) active cata-
lyst.
tion of silyl groups which could undergo further syn-
thetic transformations (Figure 1).
The hydrosilylation of alkenes and alkynes can pro-
duce alkyl-, allyl-, and vinylsilanes,[6] which are used
in fine chemical synthesis for stereospecific oxida-
tion[7] and cross-coupling reactions,[8] amongst other
applications (Figure 1).[9] The cross-linking of silicone
polymers by hydrosilylation represents one of the
largest applications of homogeneous catalysis on an
industrial scale.[6] To date, hydrosilylation has been
dominated by precious metal catalysts such as plati-
num, palladium, ruthenium and rhodium, however
these catalysts can suffer from competitive alkene iso-
merisation, dehydrosilylation reactions, and incompat-
ibility with amino-substituted olefins.[10]
Chirik has reported that bis
bis(dinitrogen) complexes,[11] and terpyridine and bis-
AHCTUNGTRENNUNG
(imino)pyridine iron(II) dialkyl complexes,[12] are pre-
ACHTUNGERTN(NUNG imino)pyridine iron(0)
catalysts for the hydrosilylation of alkenes and al-
kynes under mild conditions. Despite the high activity,
only limited functional group tolerance has been dis-
closed, and the pre-catalysts are air- and moisture-
sensitive.[5e] Ritter reported an iminopyridine iron(0)
complex as a (pre-)catalyst for the regio- and stereo-
Keywords: catalysis; hydrosilylation; iron; olefins;
synthetic methods
Iron offers significant advantages as a catalyst over
precious metals due to its low toxicity, low cost, natu-
ral abundance and sustainable long-term commercial
availability.[1] Although iron catalysis is well estab-
lished for cross-coupling[2] and redox reactions,[3] the
hydrofunctionalisation of olefins has been less devel-
oped. In part, this can be attributed to the procedural-
ly challenging reaction conditions and catalyst synthe-
ses required, in comparison to the simplicity of iron-
catalysed cross-coupling reactions. Building upon our
work,[4] and previous work of others,[5] in which low-
valent iron catalysts were generated in situ, we sought
to develop an operationally simple iron-catalysed hy-
drosilylation of olefins, in which all reagents were in-
expensive, widely available and bench-stable. Further-
Figure 1. Selected synthetic transformations of alkyl- and vi-
more, we sought to use this method for the introduc- nylsilanes.
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Iron-Catalysed Chemo-, Regio-, and Stereoselective Hydrosilylation
Table 1. Development and optimisation of iron-catalysed hydrosilylation of alkenes.[a]
Entry
R1
Ligand
Silane (equiv.)
Yield [%][b]
4/7
5[c]
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Bu
Bu
Bu
3a
3a
3a
3b
3b
3b
3b
3b
3c
3b
3b
3b
3b
3b
3b
PhSiH3 2a (1)
PhSiH3 2a (1)
PhSiH3 2a (1)
PhSiH3 2a (1.1)
PhSiH3 2a (1.1)
PhSiH3 2a (1)
PhSiH3 2a (1.1)
PhSiH3 2a (0.5)
PhSiH3 2a (1.2)
Ph2SiH2 2b (1)
Et2SiH2 2c (1)
Me2PhSiH 2d (1)
BnMe2SiH 2e (1)
>95
–
5
>95
42
60
>95
–
10
>95
>95
14
–
–
–
–
2[d]
3[e]
4[f]
5[g]
6
12
40
–
>95
90
–
7
8
9
10
11
12
13
14
15
–
N/A
N/A
N/A
N/A
78
85
86
A
[a]
Conditions: 0.7 mmol of 1/6, FeCl2 (1 mol%), ligand (1 mol%), EtMgBr (2 mol%), silane (0.5–1.2 equiv.), THF (0.25M),
1 h, room temperature.
[b]
[c]
[d]
[e]
[f]
Yield determined by 1H NMR using 1,3,5-trimethoxybenzene as internal standard.
Based on silane.
EtMgBr (1 mol%).
EtMgBr (5 mol%).
PhMgBr (3 mol%) in place of EtMgBr.
NaBEt3H (3 mol%) in place of EtMgBr.
[g]
selective 1,4-hydrosilylation of terminal 1,3-dienes to gen- and phosphine-based ligands in combination
give allylsilanes.[13] The iron(0) (pre-)catalyst was ele- with FeCl2 (1 mol%) and ethylmagnesium bromide
gantly synthesised in situ by a ligand-initiated reduc- (2 mol%) as in situ activator.[18] The bis-
tive elimination of an iron(II) complex, however the
ACHTUNGTREN(NUGN imino)pyridine class of ligands 3a–c, gave silylation
iron(II) precursor itself was still relatively unstable. products in quantitative yield within 1 hour at room
Only limited substrate scope was disclosed and reac- temperature (Table 1). Minimal activity was observed
tion times of up to 24 h and catalyst loadings of for all other ligand/salt combinations.[18] No activity
5 mol% were required. Other notable recent exam- was observed in the absence of ligand, and high
ples of iron-catalysed hydrosilylation of olefins have purity FeCl2 (99.99%) in combination with 3b showed
been reported by Enthaler,[14] Plietker,[15] and Nakaza- equal catalytic activity.[18] The hydrosilylation of sty-
wa,[16] however high temperatures and/or long reac- rene with phenylsilane using FeCl2 (1 mol%) and the
tion times were required in each case.
2,6-diisopropylaryl-substituted
bisACHTNGUTERNNU(G imino)pyridine
The addition of a Grignard reagent to a bench- ligand 3a (1 mol%) gave the linear hydrosilylation
stable iron(II) salt gives a low-valent, active iron spe- product 4a in quantitative yield and with perfect re-
cies,[17] which we speculated might operate as a hydro- gioselectivity (Table 1, entry 1). Significantly reduced
functionalisation catalyst.[4d] This would overcome the activity was observed when either 1 mol% or more
practical constraints of using an unstable iron than 3 mol% EtMgBr were used (entries 2 and 3).
ACHTUNGTRENNUNG(pre-)catalyst and only require a catalytic amount of Phenylmagnesium bromide (3 mol%) was also an ef-
Grignard reagent for activation.
fective activating agent, however sodium triethylboro-
Initial studies focussed on the hydrosilylation of hydride[16b] (3 mol%) gave only a moderate yield and
styrene 1 with phenylsilane 2a (1 equiv.), using nitro- a mixture of both mono- and bis-silylation products
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UPDATES
Mark D. Greenhalgh et al.
Table 2. Iron-catalysed hydrosilylation of olefins: Scope, and functional group tolerance.[a]
[a]
Conditions: olefin (0.7 mmol), FeCl2 (1 mol%), 3b (1 mol%), EtMgBr (2 mol%), PhSiH3 (1.1 equiv.), THF (0.25M), 1 h,
room temperature.
[b]
[c]
[d]
[e]
[f]
Yield determined by 1H NMR using 1,3,5-trimethoxybenzene as internal standard, isolated yield given in parentheses.
FeCl2 (3 mol%), 3b (3 mol%), EtMgBr (6 mol%) used.
FeCl2 (5 mol%), 3b (5 mol%), EtMgBr (10 mol%) used.
Olefin added after addition of EtMgBr and PhSiH3.
14% alcohol product isolated.
[g]
[h]
NMR yield not determined.
Conditions: olefin (0.7 mmol), [3bFeCl2] (1 mol%), n-BuLi (2 mol%), PhSiH3 (1.1 equiv.), toluene (0.25M), 1 h, room
temperature.
Reaction in neat cyclooctene.
10% a-isomer [C6H5CACTHNUGTRNE(UNG SiPhH2)=CHCH3] produced.
[i]
[j]
4a and 5a (entries 4 and 5). Thus, 2 mol% EtMgBr nary silane 4d (entries 10–12). The hydrosilylation of
was used for all further experiments. Variation of the 1-hexene 6 with tertiary silanes, benzyldimethylsilane
2,6-aryl substituents of the bisACHTNUTRGNE(NUG imino)pyridine ligand 2e and the industrially representative silicone polymer
from isopropyl 3a to ethyl 3b gave a more active cata- cross-linkers, heptamethyltrisiloxane 2f and pentame-
lyst, however some further reaction of the secondary thyldisiloxane 2g, gave quaternary silanes 7e, 7f and
silane product 4a, to give the tertiary silane (bis-silyla- 7g in excellent yield and with perfect regioselectivity
tion product) 5a, was observed (entry 6). The mono- (entries 13–15).
silylation product 4a was produced exclusively when
The applicability of the developed iron-catalysed
1.1 equiv. of phenylsilane was used, whilst the bis-sily- hydrosilylation was investigated using a range of func-
lation product 5a was quantitatively prepared using tionalised alkenes and alkynes under the optimised
0.5 equiv. of phenylsilane (entries 7 and 8). The 2,6-di- conditions of FeCl2 (1 mol%), 3b (1 mol%), EtMgBr
methyl ligand 3c showed decreased selectivity with re- (2 mol%) and phenylsilane (1.1 equiv.) (Table 2). 4-
spect to multiple silylations, even with excess silane Phenylbutene 8a was hydrosilylated to give linear
(entry 9). Secondary silanes (Ph2SiH2 2b and Et2SiH2 silane product 9a in quantitative yield as a single re-
2c) gave the linear silane products 4b and 4c in quan- gioisomer, demonstrating equal catalytic activity and
titative yield with complete regioselectivity; however regioselectivity for aliphatic alkenes. Styrene deriva-
tertiary silane 2d gave only small quantities of quater- tives 8b–i bearing electron-donating and electron-
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UPDATES
Iron-Catalysed Chemo-, Regio-, and Stereoselective Hydrosilylation
withdrawing groups gave linear silane products 9b–
i in excellent yield with perfect regioselectivity. The
differing electronic nature of each substrate had no
influence on activity or regioselectivity. Significantly,
a primary amine 8d was tolerated,[19] and aryl fluoride
8e and chloride 8f functionalities were conserved,
with no addition into the aryl-halide bond.[17b,20] The
ester functionalised styrene derivatives 8h and 8i gave
Scheme 1. Gram-scale, ꢁsolvent-freeꢂ hydrosilylation of sty-
rene.
linear silane products 9h and 9i, respectively, with no (Table 2), the terminal alkyne 10d gave (Z)-vinylsi-
[21]
ester hydrosilylation,[22] or lane 11d arising from a formal trans-addition of the
À
C O bond cleavage,
À
Grignard addition observed. An aliphatic ester 8j was Si H bond. The formation of the thermodynamically
also tolerated. Iron-catalysed hydrosilylation of keto- unfavoured (Z)-vinylsilane product has been observed
nes[3a,b,23] and imines[24] has been well documented; with other transition metal catalysts,[6] and may be ex-
however ketone 8k underwent chemoselective hydro- plained using the mechanisms proposed by Crab-
silylation at the alkene, with no ketone reduction. Ali- tree[28] and Ojima[29] in which silylmetallation is fol-
phatic ketone 8l underwent chemoselective hydrosily- lowed by E/Z isomerisation of the intermediate silyl-
lation at the alkene, albeit with lower selectivity – 4:1 vinylene complex through either a zwitterionic car-
in favour of alkene hydrosilylation. This result is com- bene[29] or a metallocyclopropene[28] intermediate.
plementary to that using bis
G
The synthetic utility of the silane products was then
dialkyl complexes, which have been shown to be che- investigated. Selected alkyl- and vinylsilane products
moselective for the hydrosilylation of ketones at were directly oxidised using hydrogen peroxide[30] to
room temperature.[23e] Imine and imino ester function- give alcohols 12a–c and ketone 13a in excellent yield
alities were also tolerated and gave silanes 9m and 9n and with no over-oxidation observed (Scheme 2, route
in excellent yield and regioselectivity, however cya- A). A one-pot hydrosilylation–oxidation protocol was
nostyrene 8o gave only a moderate yield of silane 9o. also developed (Scheme 2, route B), where in the
À
A heteroaromatic substrate, 2-vinylquinoline 8p, presence of the iron catalyst selective Si H bond
gave the linear silylation product 9p in quantitative (over Si C bond) oxidation occurred, to give
À
yield and with complete regioselectivity, however 4-vi- silane(di)ol products 14a and 14b in excellent yield.
nylpyridine 8q reacted in only 26% yield due to com- Alkylsilanols have numerous applications in materials
peting polymerisation. 4-Vinylcyclohexene 8r, was hy- chemistry,[6,31] and vinylsilanol products, such as 14b,
drosilylated selectively at the terminal alkene, leaving can be used as substrates in cross-coupling,[32] Mizoro-
the internal alkene intact. The conjugated diene 1- ki–Heck,[33] and carbonyl addition reactions.[34]
phenyl-trans-butadiene 8s reacted selectively at the
A large body of work has focussed on the develop-
terminal alkene to give a 1:1 regioisomeric mixture of ment and mechanistic understanding of polymeri-
the linear and branched silane products 9s-l and 9s-b. sation reactions catalysed by iron bisACTHNUTRGNEUNG(imino)pyridine
To demonstrate increased industrial applicability, complexes.[35] Chirik has also identified and character-
the methodology was developed to operate in toluene, ised iron(II) dialkyl[5d,12] and iron(0) bis(dinitrogen)
ACTHNUTRGNEUNG
or ꢁsolvent-freeꢂ conditions.[25,26] A suspension of pre- and bis(silane) complexes[11] which are effective pre-
formed
bis
(imino)pyridine
iron(II)
complex catalysts for hydrosilylation, however the in situ ob-
[3bFeCl2][27] (1 mol%) in either toluene or neat servation of the true catalytic species has proved chal-
alkene, was activated by addition of n-BuLi (2 mol%)
to give an active catalyst which was used for the hy-
drosilylation of 1,1- and 1,2-disubstituted alkenes 8t–
v. Silanes 9t–v were obtained in excellent yield within
1 hour, and in the case of 9t and 9u with complete re-
gioselective control for the linear silane product.
Using ꢁsolvent-freeꢂ reaction conditions, the gram-
scale hydrosilylation of styrene using diethylsilane
was complete within just 90 seconds using only
0.07 mol% iron catalyst, corresponding to a catalyst
turnover frequency (TOF) approaching 60,000 molhÀ1
(Scheme 1).
Alkynes were also suitable substrates, giving vinyl-
silanes with high stereoselectivity, and without multi-
ple hydrosilylation. Interestingly, whilst internal al-
kynes 10a–c gave (E)-vinylsilanes 11a–c stereoselec-
tively in excellent yield (syn-addition of Si H) ketones (route A) and silane(di)ols (route B).
Scheme 2. Oxidation of alkyl- and vinylsilanes to alcohols,
À
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UPDATES
Mark D. Greenhalgh et al.
tional groups were tolerated, without deleterious side-
reactions. Primary, secondary and tertiary silanes
were used for the hydrosilylation of terminal, 1,1- and
1,2-disubstituted alkenes, and internal and terminal
alkyl- and arylalkynes. The methodology was extend-
ed to operate under ꢁsolvent-freeꢂ conditions, improv-
ing industrial applicability and ease of product isola-
tion. The silane products were oxidised selectively to
give alcohol, ketone or silane(di)ol products. Prelimi-
nary mechanistic studies indicate the formation of an
iron(I) catalyst. The use of an in situ generated iron
catalyst simplifies the operational requirements of
these hydrosilylation reactions, compared to those
using highly active but unstable iron (pre-)catalysts,
and should allow the non-expert to fully utilise this
synthetic methodology.
Scheme 3. Mechanistic investigations on the iron-catalysed
hydrosilylation of alkenes
Experimental Section
lenging.[36] We sought to provide some insight by cal-
culating the average oxidation state of iron attained General Iron-Catalysed Hydrosilylation Procedure
under catalytic conditions by activation of the iron(II)
An olefin (0.7 mmol) was added to a solution of iron(II)
pre-catalyst using tolylmagnesium bromide (Tol-
chloride (0.9 mg, 0.007 mmol) and 2,6-bis-[1-(2,6-diethylphe-
MgBr).[4d,17a,37a–e] Using the formation of bitolyl 15 as
nylimino)ethyl]pyridine 3b (3.0 mg, 0.007 mmol) in anhy-
a measure of the reduction of iron, maximum catalyt-
ic activity was calculated to correspond to an average
oxidation state of iron(I) (Scheme 3, A).[18,37] This is
in keeping with our previously reported mechanistic
studies on iron-catalysed hydroboration.[4d]
The mechanism was further probed using diphe-
nyl(silane-d2) 2b-d2 for the deuterosilylation of sty-
rene (Scheme 3, B).[18] The expected addition product
4b-d1, with a single deuterium at the benzylic position
was formed as the major product, however double
deuterium incorporation in the benzylic position 4b-
d2, along with the non-deuterated product 4b were
also formed. H-transfer from the benzylic methylene
to silicon was observed in both hydrosilylation prod-
ucts (4b-dn) and recovered diphenylsilane (2b-d1 and
2b). No deuterium incorporation in the homobenzylic
position (alpha to silicon) was observed. Although re-
drous tetrahydrofuran (3 mL) at room temperature under
an atmosphere of nitrogen. Ethylmagnesium bromide (1.5M
in Et2O, 10 mL, 2 drops, 0.015 mmol) was added, followed by
a silane (0.77 mmol) and the reaction mixture was stirred at
room temperature for 1 hour. Aqueous sulfate buffer
(10 mL) was added and the aqueous phase extracted with
diethyl ether (3ꢃ20 mL). The combined organic extracts
were washed with water and brine, dried (MgSO4) and con-
centrated under vacuum. 1,3,5-Trimethoxybenzene (23.5 mg,
0.14 mmol) was added as an internal standard, and a yield
for the reaction determined by 1H NMR. Isolated yields
were determined after purification by silica chromatography.
Acknowledgements
We thank the University of Edinburgh and the School of
versible hydrometallation cannot be ruled out, previ- Chemistry, University of Edinburgh, for financial support
ous work[4b] would not support a hydrometallation
step with the regioselectivity observed in this process.
(S.P.T., Fellowship; M.D.G., studentship). D.J.F. thanks
EPSRC for funding (EP/I036281/1).
The H/D-scrambling between the benzylic position
and silicon may be explained by the formation of iron
carbenoid intermediates (on- or off-cycle); however References
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