Conversion of alkanes to linear alkylsilanes using an iridium-iron-catalysed tandem dehydrogenation-isomerization-hydrosilylation

Article abstract of DOI:10.1038/nchem.2417

The conversion of inexpensive, saturated hydrocarbon feedstocks into value-added speciality chemicals using regiospecific, catalytic functionalization of alkanes is a major goal of organometallic chemistry. Linear alkylsilanes represent one such speciality chemical - they have a wide range of applications, including release coatings, silicone rubbers and moulding products. Direct, selective, functionalization of alkanes at primary C-H bonds is difficult and, to date, methods for catalytically converting alkanes into linear alkylsilanes are unknown. Here, we report a well-defined, dual-catalyst system for one-pot, two-step alkane silylations. The system comprises a pincer-ligated Ir catalyst for alkane dehydrogenation and an Fe catalyst that effects a subsequent tandem olefin isomerization-hydrosilylation. This method exhibits exclusive regioselectivity for the production of terminally functionalized alkylsilanes. This dual-catalyst strategy has also been applied to regioselective alkane borylations to form linear alkylboronate esters.

Full text of DOI:10.1038/nchem.2417

ARTICLES
PUBLISHED ONLINE: 21 DECEMBER 2015 | DOI: 10.1038/NCHEM.2417
Conversion of alkanes to linear alkylsilanes
using an iridium–iron-catalysed tandem
dehydrogenation–isomerization–hydrosilylation
Xiangqing Jia and Zheng Huang
*
The conversion of inexpensive, saturated hydrocarbon feedstocks into value-added speciality chemicals using regiospecific,
catalytic functionalization of alkanes is a major goal of organometallic chemistry. Linear alkylsilanes represent one such
speciality chemical—they have a wide range of applications, including release coatings, silicone rubbers and moulding
products. Direct, selective, functionalization of alkanes at primary C–H bonds is difficult and, to date, methods for
catalytically converting alkanes into linear alkylsilanes are unknown. Here, we report a well-defined, dual-catalyst system
for one-pot, two-step alkane silylations. The system comprises a pincer-ligated Ir catalyst for alkane dehydrogenation and
an Fe catalyst that effects a subsequent tandem olefin isomerization–hydrosilylation. This method exhibits exclusive
regioselectivity for the production of terminally functionalized alkylsilanes. This dual-catalyst strategy has also been
applied to regioselective alkane borylations to form linear alkylboronate esters.
lkanes are abundant and low-cost feedstocks. The selective are far more abundant than α-olefins. Thus, the direct production
conversions of these compounds into more useful products, of linear alkylsilanes from alkanes is a process with enormous
A
such as α-olefins1 and linear alkylsilanes2,3, are tremendously potential applicability. However, examples of the conversion of
attractive reactions. However, alkanes are among the least chemi- alkanes into alkylsilanes are extremely rare22–24. Tilley showed that
cally reactive organic molecules, as they consist only of strong, a Sc complex catalyses dehydrosilylation of methane with diphenyl-
nonpolar C−C and Csp3−H bonds. In Csp3−H bonds, the silane to form diphenylmethylsilane24. To date, no catalyst systems
primary C−H bonds are stronger than the secondary and tertiary have been reported for silylation of abundant, simple alkanes
C−H bonds (bond dissociation energies of primary, secondary larger than methane. Here, we report terminal, regioselective silyla-
and tertiary C−H bonds are 101.1, 98.6 and 96.5 kcal mol−1, tions of alkanes via a one-pot, two-step cascade method comprising
respectively)4 and hence the selective functionalization of a Ir-catalysed alkane dehydrogenation and Fe-catalysed tandem
primary C−H bond at the terminal position of an alkane represents olefin isomerization–hydrosilylation. Moreover, the strategy was
one of the ultimate challenges in this field5–7. In fact, catalytic successfully applied to alkane borylations.
carbene8,9 or oxo10 transfer reactions involving reactive radicals
occur predominantly at the internal positions, and enzymatic trans- Results and discussion
formations of linear alkanes with carbon numbers greater than two Design of tandem dehydrogenation–isomerization–hydrosilylation
afford regioisomeric mixtures11. Although stoichiometric reactions for regioselective alkane silylations. Our method for alkane
with some transition-metal complexes do occur regioselectively at silylations uses two well-defined molecular catalysts, one for
the terminal positions of alkanes12,13, there are very few catalytic alkane dehydrogenation and the other for α-olefin hydrosilylation.
methods for selective functionalization of primary C−H bonds. The basic catalytic process is outlined in Fig. 1a. The initial step
Photocatalytic carbonylations of linear alkanes give linear aldehydes involves transfer dehydrogenation of linear alkanes catalysed by a
as the major products, but secondary products are also produced14. PSCOP-type (PSCOP = C6H3-2-(SP(iPr)2)-6-(OP(iPr)2)) Ir complex 1
Pincer Ir-catalysed transfer dehydrogenations of linear alkanes gen- of a hybrid phosphinothious/phosphinite pincer ligand developed
erate α-olefins selectively at low conversions, but the terminal olefins by our group. On activation with sodium t-butoxide (NaOtBu),
isomerize rapidly into internal olefins under the reaction 1 is remarkably active for the dehydrogenation of alkanes with
conditions15. A major breakthrough was achieved by Hartwig, t-butylethylene (TBE) as the hydrogen acceptor25. However,
who reported that Cp*Rh(η4-C6Me6) catalyses alkane borylations similar to early reports1,15,26,27
regiospecifically to form linear alkylboronate esters16.
thermodynamically more stable internal olefins as the major
, the reaction provides the
Linear alkylsilanes are widely used as raw materials in the indus- products owing to facile olefin isomerization. For instance,
trial manufacture of release coatings, silicone rubbers, moulding 1-octene was one of the major products at the early stage (5 min),
implants, aerospace lubricants and silicone adhesives2. Current but accounted for only 2% of the total octenes when the reaction
technology for the synthesis of linear alkylsilanes relies on Pt- reached full conversion after 1 h at 200 °C (Fig. 1b). To produce
catalysed anti-Markovnikov hydrosilylation of linear α-olefins3,17–19
,
linear alkylsilanes from the mixture of olefin isomers, the dual-
and, worldwide, the silicone industry consumes a significant catalyst system must meet the following requirements (Fig. 1a):
amount of Pt (∼180,000 troy ounces in 2007)20. Linear α-olefins (1) the catalytic hydrosilylation of the α-olefin must occur faster
are commonly manufactured by ethylene oligomerization, which with high terminal selectivity compared to the reaction of internal
suffers from the formation of a wide range of products21. Alkanes olefins, because the terminal olefin constitutes <5% of the total
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Science, 345 Lingling Road, Shanghai
200032, China. e-mail: huangzh@sioc.ac.cn
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2417
ARTICLES
a
[Si]
Table 1 | Iron-catalysed tandem olefin isomerization–
hydrosilylation.
R˝
R´
Ineffective for
x mol% [Fe]
[Fe]
and/or y mol% 1
2(x+y) mol% NaHBEt3
nC6H13
internal olefins
H[Si]
SiMe(OSiMe3)2
+
(Me3SiO)2MeSiH
[Fe]
n-hexane
r.t., 12 h
6a
R˝
R´
4
R
R
Fast olefin
isomerization
Internal olefins
1 (mol%)
[Fe]
[Fe] (mol%)
Entry
Yield (%) of 6a*
Chemo- and
regioselective
α-olefin
[Fe]
_
Transfer
dehydrogenation
H[Si]
[Ir]
[Ir]H2
1
0.5
0
0
5
5
5
5
5
5
0
hydrosilylation
2
3
4
5
6
7
2a
2b
2c
2d
2c
2d
13
4
[Si]
R
0
tBu
tBu
0
86
86
82
82
0.2 mol% 1
0.3 mol% NaOtBu
0
b
n-octane
Internal
octenes
0.5
0.5
+
+
1-octene
tBu
–
tBu
5 min
1 h
38 mM
10 mM
76 mM
200 °C
TBE
490 mM
500 mM
Fe precatalysts:
2b, Ar = 2,6-iPr2-C6H3
2c, Ar = 2,6-Et2-C6H3
2d, Ar = 2,6-Me2-C6H3
N
N
S
O
tBu2P
N
N
N
Fe
Fe
Ar
iPr2P
PiPr2
Ar
Ir
Cl
Cl
Br
Br
H
Cl
1
2a
Reaction conditions: [Fe] (x mol%),
(0.2 mmol), 4 (0.2 mmol) in n-hexane (1 ml) at room temperature, 12 h. *Determined by GC
using mesitylene as an internal standard.
1
(y mol%), NaHBEt3 (2(x+y) mol%), trans-3-octene
Figure 1 | Design of tandem dehydrogenation–isomerization–hydrosilylation
for conversion of alkanes to linear alkylsilanes. a, Ir-catalysed alkane
dehydrogenation (red) gives internal olefins as the major products. The Fe
catalyst is ineffective for internal olefin hydrosilylation, but highly efficient
for olefin isomerization (blue) and regioselective α-olefin hydrosilylation
(red-brown). Therefore, the internal olefin products from alkane
dehydrogenation can be selectively converted into linear alkylsilanes.
b, A (PSCOP)IrHCl precatalyst was used for alkane dehydrogenations in this
work. The Ir-catalysed dehydrogenation is regioselective for the formation of
α-olefin at the early stage of the reaction, but gives internal olefins as the
major products at high conversions.
with NaHBEt3 (10 mol%) as the activator, reactions of trans-3-
octene with hydrosilane 4 gave the primary alkylsilane 6a in 13%
and 4% yield, respectively (entries 2 and 3). Gratifyingly,
substitutions of 2b by either complex 2c or 2d containing ethyl or
methyl substituents at the 2,6-positions of the N-aryl groups led
to a high yield of 6a (86% each, entries 4 and 5). As the dual-
catalyst system contains the Ir dehydrogenation catalyst, we
assessed whether the Fe catalyst can tolerate the Ir species. The
olefin products; (2) at least one of the two catalysts is capable of reaction using a combination of the Fe complex (2c or 2d, 5 mol%
catalysing fast olefin isomerization between the terminal and each) and 1 (0.5 mol%) provided 6a in 82% yield (entries 6 and
internal olefins; and (3) although the dehydrogenation reaction 7), indicating that the Fe catalysts remained active for olefin
occurs with the Ir catalyst itself, the second hydrosilylation isomerization–hydrosilylation under these reaction conditions.
catalyst should be compatible with the Ir dehydrogenation catalyst Notably, no branched alkylsilane was detected by gas chromatography
because of the presence of both catalysts in the subsequent olefin (GC) for all Fe-catalysed reactions. Reactions with other
isomerization–hydrosilylation reactions.
transition-metal catalysts, such as bis(imino)pyridine-ligated Co
complex35 and Ir and Rh complexes of bis(phosphine) ligands,
Development of Fe-catalysed tandem olefin isomerization– have also been investigated. None of these catalysts is effective for
hydrosilylation. To date, methods for the conversion of internal the conversion of internal olefins into linear alkylsilanes
olefins into linear alkylsilanes remain to be developed28,29
.
(Supplementary Table 1).
Therefore, our initial study focused on developing a system that
can catalyse tandem olefin isomerization and selective α-olefin Conversion of alkanes to linear alkylsilanes via alkane
hydrosilylation. We first tested whether the Ir dehydrogenation dehydrogenation and isomerization–hydrosilylation. Having
precatalyst 1 is itself active for olefin isomerization–hydrosilylation. identified a catalyst for selective conversion of internal olefins into
With 1 (0.5 mol%) and NaHBEt3 as the activator (1 mol%), the linear alkylsilanes, we next explored the one-pot conversion of
reaction of trans-3-octene with bis(trimethylsiloxy)methylsilane alkanes into primary alkylsilanes via tandem alkane
((Me3SiO)2MeSiH) 4 in n-hexane at room temperature did not dehydrogenation and olefin isomerization–hydrosilylation. In
form the primary alkylsilane 6a after 12 h (Table 1, entry 1), but most C−H bond activations, the alkane is present as the solvent
gave 6% of branched alkylsilanes resulting from the or in excess. Thus, we conducted reactions of simple alkanes in
hydrosilylation of trans-3-octene. Most trans-3-octene (93%) was neat substrates. The reaction of n-octane with TBE (0.25 M) in the
recovered, indicating that the Ir complex is inactive for olefin presence of the Ir catalyst formed in situ from the combination of
isomerization under these conditions. Recently, Fe complexes of 1 (1 mol% relative to TBE) and NaOtBu (1.2 mol%) at 200 °C for
tridentate bipyridyl-based phosphine (2a)30 and bis(imino) 10 min, followed by addition of the Fe precatalyst 2c (10 mol%),
pyridine (2b–d)31–34 ligands have proved effective for anti- NaHBEt3 (20 mol%) and tertiary hydrosilane 4 (1 equiv. relative to
Markovnikov α-olefin hydrofunctionalizations. When (PNN)FeCl2 2a TBE), provided the desired product n-octylsilane 6a in 77% yield
and (iPrPDI)FeBr2 2b (5 mol% each) were used as the precatalysts (67% isolated yield) after 12 h at room temperature (Table 2, entry 1).
2
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2417
Table 2 | Catalytic silylations of various alkanes via alkane dehydrogenation and isomerization–hydrosilylation.
ARTICLES
1 equiv. TBE
1 mol% 1
1.2 mol% NaOtBu
1 equiv. silane
10 mol% [Fe]
20 mol% NaHBEt3
6, [Si] = SiMe(OSiMe3)2
7, [Si] = SiHEt2
R
[Si]
R
Neat or p-xylene
t, 200 °C
r.t., 12 h
3
Entry
Alkane
t (min)
Solvent
[Fe]
2c
Silane
Product
Yield (%)*
n-octane
None
1
10
77 (67)
6a
(Me3SiO)2MeSiH 4
( )6
SiMe(OSiMe3)2
n-octane
n-decane
None
None
4
4
2
3
30
10
63
2c
2c
6a
71 (63)
6b
6c
6d
( )8
( )4
SiMe(OSiMe3)2
None
None
4
4
4
4
5
30
71 (60)
76 (63)
n-hexane
2c
2c
2c
SiMe(OSiMe3)2
30
( )4
( )2
( )4
SiMe(OSiMe3)2
SiMe(OSiMe3)2
6†
Me3Si
300
6e
54
p-xylene
Me3Si
7†
8‡
Me3Si
tBu
300
300
76 (63)
45 (15)
4
4
p-xylene
p-xylene
2d
2d
6e
tBu
Ph
SiMe(OSiMe3)2
SiMe(OSiMe3)2
6f
6g
6h
9†
300
300
2d
2d
53 (35)
60
4
4
p-xylene
p-xylene
Ph
10†
SiMe(OSiMe3)2
MeO
MeO
F
F
11†
300
6i
27
4
p-xylene
2d
SiMe(OSiMe3)2
n-octane
n-decane
n-hexane
None
None
None
Et2SiH2 5
12
13
14
10
10
30
7a
7b
7c
83 (76)
74 (68)
73 (55)
2c
2c
2c
( )6
( )8
SiHEt2
5
5
SiHEt2
SiHEt2
( )4
None
5
5
15
30
2c
2c
81 (74)
73 (64)
7d
7e
( )4
( )4
( )2
SiHEt2
SiHEt2
16†
Me3Si
300
p-xylene
Me3Si
Reaction conditions: 1 (1 mol%), NaOtBu (1.2 mol%), TBE (0.5 mmol) in alkane (2 ml) at 200 °C for allotted time, then added [Fe] (10 mol%), 4 or 5 (0.5 mmol), NaHBEt3 (20 mol%), room temperature for 12 h.
*Determined by GC using mesitylene as an internal standard. Numbers in parentheses are isolated yields. †Alkane (1.5 mmol), p-xylene (2 ml). ‡Alkane (2.5 mmol), p-xylene (2 ml).
Increasing the reaction time of the dehydrogenation step from 10 to respectively. In the case of 2-methylheptane, it is worth noting
30 min strikingly led to a lower yield of 6a (entry 2, 63%). that the silylation occurred exclusively at the least hindered
Monitoring the transfer dehydrogenation reaction by GC and gas methyl group despite the presence of two other methyl groups.
chromatography-mass spectrometry (GC-MS) revealed that ∼95%
For complex substrates less abundant than simple n-alkanes,
of TBE had been converted into 2,2-dimethylbutane (the practical silylations should avoid using alkanes in large excess.
hydrogenation product) after 10 min; heating for a longer time Significantly, the reaction could be performed in a p-xylene solution
(30 min) resulted in the formation of trace amounts of octadienes, without a large excess of substrates. Presumably due to greater steric
as detected by GC-MS. We thus hypothesized that the diene hindrance of the substrates, the transfer dehydrogenations of
species may have a detrimental effect on the Fe-catalysed 1-(trimethylsilyl)butane and 2,2-dimethylpentane in p-xylene
hydrosilylation, presumably through chelation to the Fe centre. were slower than those of neat n-alkanes (entries 1–5). It should
Indeed, an independent experiment showed that addition of also be noted that in these cases the less sterically bulky Fe precata-
(E)-deca-1,3-diene did inhibit Fe-catalysed olefin isomerization– lyst 2d is superior to 2c for the isomerization–hydrosilylation of the
hydrosilylation (Supplementary Section 2 ‘Procedures for Table 1’). resulting olefins (entries 6 versus 7). With alkane:TBE:silane ratios
The results suggest that careful control of the reaction time for the of 3:1:1 or 5:1:1, the reactions of 1-(trimethylsilyl)butane and
dehydrogenation step is crucial to the success of alkane silylations. 2,2-dimethylpentane over a 5 h time period for dehydrogenation
This protocol was used for regiospecific silylation of other linear and a subsequent 2d-catalysed isomerization–hydrosilylation pro-
alkanes as well as branched alkanes. The reaction of n-decane pro- vided the desired products 6e and 6f in 76% and 45%, respectively
vided n-decylsilane 6b in 71% yield (entry 3). Compared to the reac- (entries 7 and 8).
tions with n-octane and n-decane, the dehydrogenations of
The catalytic dehydrogenation of n-propylbenzene forms
n-hexane and 2-methylheptane required a longer reaction time to β-methylstyrene as the predominant product. This internal olefin
obtain the corresponding olefins in high yields. Following a is reluctant to undergo isomerization as it is thermodynamically
30 min period of dehydrogenation, hydrosilylations with 4 provided much more stable than the corresponding terminal olefin. The
the desired quaternary silanes 6c and 6d in 71% and 76% yields, silylation of n-propylbenzene is therefore likely to be more difficult
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2417
ARTICLES
than the reactions with simple alkanes. However, the reaction of reaction16. In contrast, the reaction of 2-methylheptane using
n-propylbenzene using our method proceeded smoothly, furnishing our method occurred exclusively at the least hindered methyl
the linear product exclusively in 53% yield (entry 9). It is note- group, furnishing 8d in excellent yield (entry 4). Similarly,
worthy that aromatic groups can be tolerated as silylation of the aro- 2,4-dimethylhexane was selectively borylated at the least hindered
matic Csp2−H bonds does not occur under our reaction conditions. primary C−H bond, although three different methyl groups exist
In addition, the derivatives of n-propylbenzene containing methoxy in the structure. This substrate exhibits low reactivity in
and fluoro substituents, 1-(4-methoxyphenyl)propane (entry 10) dehydrogenation due to steric effects, thus resulting in a low yield
and 1-(3-fluorophenyl)propane (entry 11), underwent regioselective of the borylation product (entry 6). The borylation of
silylation at the terminal positions of alkyl chains, although the n-propylbenzene furnished α-, β- and γ-substituted products in a
latter gave a low yield of the silylation product.
ratio of 1:1.7:1.4 (entry 7). An independent experiment showed
The dual-catalyst system can also efficiently catalyse alkane silyl- the Fe-catalysed hydroboration of β-methylstyrene, the major
ations with secondary hydrosilanes to form tertiary silane products. dehydrogenation product, formed three isomers in a similar ratio
Using the conditions used for silylations with (Me3SiO)2MeSiH 4, (1:1.4:1.4). As a comparison, the silylation of n-propylbenzene
reactions of alkanes with diethylsilane 5 occurred smoothly to gave the terminal silylated product with excellent regioselectivity
afford diethyl alkylsilanes 7a–7e. Most of the products can be iso- (Table 2, entry 9). The results indicate the olefin isomerization–
lated in good yields with high purity. All transformations of hydrofunctionalization step controls the site selectivity of
alkanes with silanes are remarkable in their regiospecificity for the alkane functionalizations.
functionalizations of primary C−H bonds (entries 12–16).
Conclusion
In summary, we report a well-defined, dual-catalyst system that con-
verts linear alkane feedstocks to higher-value linear alkylsilanes, in
an unprecedented catalytic transformation. This catalyst system
comprises an Ir pincer transfer dehydrogenation catalyst working
with an Fe catalyst capable of regioselective hydrosilylation of
internal olefins to terminal silanes. The Fe catalyst operates via
rapid isomerization of the internal olefins produced by the Ir cata-
lyst to the much more reactive terminal olefins, followed by anti-
Markovnikov hydrosilylation to yield terminal silanes. A similar
dual-catalyst strategy can successfully be applied to the formation
of terminal boronate esters from alkanes by combining Ir-catalysed
transfer dehydrogenation with catalytic olefin isomerization–hydro-
boration. These transformations generally illustrate the potential for
combining alkane dehydrogenations with regioselective olefin
isomerization–hydrofunctionalizations to produce terminally
functionalized alkanes, which are useful for a variety of applications
and as precursors to other value-added chemicals.
Conversion of alkanes to linear alkylboronate esters via alkane
dehydrogenation and isomerization–hydroboration. Employing
the same strategy, but with a different Fe catalyst for olefin
hydroboration, we developed
a method for transforming
n-alkanes into the linear alkylboronate esters that are vital
intermediates in organic synthesis36–38. A catalyst system based on
the combination of the dehydrogenation precatalyst 1 and (PNN)
FeCl2 2a as the precatalyst for tandem olefin isomerization–
hydroboration proved active for alkane borylations. The reactions
of simple n-alkanes using pinacolborane (HBPin) as the
hydroboration reagent afforded the linear alkylboronate esters
8a–c in high yields (Table 3, entries 1–3). A silyl group-
containing alkylboronate 8e was obtained in 92% yield (entry 5).
Hartwig et al. showed that the two different methyl groups in
2-methylheptane both undergo borylation in the Rh-catalysed
Table3 | Catalytic borylations of various alkanes via alkane
dehydrogenation and isomerization–hydroboration.
Received 4 August 2015; accepted 11 November 2015;
published online 21 December 2015
1 equiv. TBE
1 mol% 1
1.2 mol% NaOtBu
1 equiv. HBPin
10 mol% 2a
O
20 mol% NaHBEt3
R
B
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R
O
Neat or p-xylene
t, 200 °C
r.t., 12 h
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5†
6‡
7†
300
300
300
Me3Si
( )2
p-xylene
p-xylene
p-xylene
8e 92
Me3Si
BPin
31
8f
BPin
α
γ
8g 76
Ph
Ph
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β
α:β:γ = 1:1.7:1.4
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for allotted time; then added 2a (10 mol%), HBPin (0.25 mmol), NaHBEt3 (20 mol%), room
temperature for 12 h. *Determined by GC using mesitylene as an internal standard. †Alkane
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4
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2417
ARTICLES
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Acknowledgements
The authors acknowledge financial support from the National Basic Research Program of
China (2015CB856600), the National Natural Science Foundation of China (21422209,
21432011, 21421091) and the Science and Technology Commission of Shanghai
Municipality (13JC1406900). The authors thank Y. Tang for suggestions and comments.
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Author contributions
Z.H. directed the project and wrote the manuscript. X.J. carried out the reactions.
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Additional information
Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online
at www.nature.com/reprints. Correspondence and requests for materials should be
addressed to Z.H.
Competing financial interests
The authors declare no competing financial interests.
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