E-H (E = R3Si or H) bond activation by B(C6F 5)3 and heteroarenes; Competitive dehydrosilylation, hydrosilylation and hydrogenation

Article abstract of DOI:10.1039/c3cc47372d

In the presence of B(C₆F₅)₃ five-membered heteroarenes undergo dehydrosilylation and hydrosilylation with silanes. The former, favoured on addition of a weak base, produces H₂ as a by-product making the process catalytic in B(C₆F₅) ₃ but also enabling competitive heteroarene hydrogenation. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc47372d

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E–H (E = R3Si or H) bond activation by B(C6F5)3
and heteroarenes; competitive dehydrosilylation,
hydrosilylation and hydrogenation†
Cite this: Chem. Commun., 2014,
50, 5270
Received 26th September 2013,
Accepted 27th November 2013
Liam D. Curless, Ewan R. Clark, Jay J. Dunsford and Michael J. Ingleson*
DOI: 10.1039/c3cc47372d
www.rsc.org/chemcomm
In the presence of B(C6F5)3 five-membered heteroarenes undergo (FLP) mediated hydrogenation as an additional, potentially competi-
dehydrosilylation and hydrosilylation with silanes. The former, tive, reaction pathway (Scheme 1, bottom).2
favoured on addition of a weak base, produces H2 as a by-product
We were interested in determining how I reacts with nucleophilic
making the process catalytic in B(C6F5)3 but also enabling competi- heteroarenes, particularly as related silicon cations have been recently
tive heteroarene hydrogenation.
demonstrated to exclusively dehydrosilylate arenes.7–9 Whilst B(C6F5)3
reacts with highly nucleophilic arenes such as N-alkyl-indoles, this
The activation of H2 and silanes by boron Lewis acids and a occurs only extremely slowly.10 As many heteroarenes actually have
nucleophile is developing into a powerful metal-free approach to lower nucleophilicities than R3SiH11 compound I will form in their
hydrogenate, hydrosilylate and dehydrosilylate a range of substrates.1,2 presence. Nucleophilic attack on I by a heteroarene will initially
B(C6F5)3, and its derivatives, are the Lewis acids of choice combining generate [R3Si–arenium][HB(C6F5)3], II, with multiple outcomes then
considerable electrophilicity with sufficient bulk to ‘frustrate’ Lewis possible. Herein we report a study into these competing pathways
adduct formation.2 B(C6F5)3 activates R3Si–H via species I (Scheme 1),3 which include: (i) dehydrosilylation by the direct reaction of
with subsequent transfer of R3Si+ to a nucleophile.4 To date the [HB(C6F5)3]ꢀ with arenium cation II (Scheme 2, left), or by base
combination of I with a nucleophile forms products from either catalysis where a Lewis base deprotonates the arenium cation II
hydrosilylation (e.g., with ketones) or dehydrosilylation (e.g., with before dehydrocoupling with [HB(C6F5)3]ꢀ (Scheme 2, right).2
alcohols).4 However, with substrates such as heteroarenes and (ii) Hydrosilylation by hydride transfer from [HB(C6F5)3]ꢀ to
heteroatom substituted alkenes these outcomes are not necessarily [R3Si–arenium]+ (Scheme 2, red), and (iii) hydrogenation. These
mutually exclusive. Indeed, Oestreich et al., have shown that both the processes are all catalytic in B(C6F5)3, but turnover is limited by
hydrosilylation and the dehydrosilylation of enolizable carbonyl com- competing deactivation pathways that have also been elucidated.
pounds is possible with a related silicon cation.5,6 Furthermore, the
Studies commenced with 2-methylthiophene (2-MT) and
generation of H2 from dehydrosilylation permits frustrated Lewis pair Ph3SiH. 2-MT is less nucleophilic than Ph3SiH and does not
react with B(C6F5)3. The combination of equimolar Ph3SiH,
B(C6F5)3 and 2-MT produced 2-Me-5-(Ph3Si)-thiophene,
2
(Table 1, entry 1). However, aliphatic 2-MT derived species were
Scheme 1 Dehydro-/hydro-silylation and hydrogenation with B(C6F5)3.
School of Chemistry, University of Manchester, Manchester, M13 9PL, UK.
E-mail: Michael.ingleson@manchester.ac.uk
† Electronic supplementary information (ESI) available: Experimental procedures
and characterisation. See DOI: 10.1039/c3cc47372d
Scheme 2 Dehydro-/hydro- (red) silylation catalysed by B(C6F5)3.
5270 | Chem. Commun., 2014, 50, 5270--5272
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Table 1 Stoichiometric and catalytic electrophilic silylation of 2-MT
where one new resonance moved progressively upfield to a limiting
d ꢀ5 ppm). We surmised that aliphatic sulfides, 3, were forming
R2S - B(C6F5)3 species retarding the catalysis. Indeed, equimolar
tetrahydrothiophene and B(C6F5)3 produced 11B and 19F NMR spectra
comparable to those at the end of the catalytic runs.12 Importantly,
this mixture was inactive in silylation (entry 8), thus 3 may be an
effective catalyst poison.
It was noteworthy that the overall conversion in reactions with
Cl2-py (e.g., entries 3 and 4) would be greater than 100% based on
Ph3SiH if all the 2-MT derived aliphatic products were from the
double hydrosilylation of 2-MT. As H2 is the by-product from dehydro-
silylation this results in competitive hydrogenation thus products
from; (i) hydrosilylation and hydrogenation of 2-MT and (ii) the
hydrogenation of 2-MT to 2-methyl-tetrahydrothiophene (2-Me-THT)
dominate.12 Related alkene and heteroarene hydrogenation by FLPs
has been reported.13,14 To determine what components in the reac-
tion mixture are activating H2 equimolar B(C6F5)3/2-MT was placed
Entry Base B(C6F5)3/base (mol%) Time (h) Temp. (1C) 2a (%) 3a (%)
1
2
3
4
—
100/0
42
72
24
24
24
36
24
24
20
20
20
60
60
60
60
60
34
39
51
56
42
51
46
0
31
10
33
34
18
27
32
0
tBu2-py 100/100
Cl2-py 100/100
Cl2-py 20/20
Cl2-py 5/5
5
6
Cl2-py 5/100
Cl2-py 5/5
Cl2-py 100/100
7b
8c
a
Yields based on conversion of 2-MT by 1H NMR spectroscopy, remaining
b
c
material is 2-MT. With 1.5 equivalents of Ph3SiH. In the presence of
1 eq. of tetrahydrothiophene.
observed and unreacted 2-MT remained despite consumption of all under D2 (4 atm.) in the absence of Cl2-py. At 20 1C no reduction
Ph3SiH, indicating a non-stoichiometric reaction. B(C6F5)3 remained occurred but deuterium incorporation into the alpha position of 2-MT
the dominant borane species (by 11B and 19F NMR spectroscopy), was observed indicating reversible activation of dihydrogen. On
therefore an additional 4 equivalents of Ph3SiH and 2-MT were heating to 60 1C aliphatic resonances were now also observed
added. This produced further equivalents of 2 indicating a catalytic in the 2H NMR spectrum indicating 2-MT reduction to partially
process. Throughout, the aliphatic region of the 1H NMR spectrum deuterated isotopomers of 2-Me-THT (eqn (1)).12 B(C6F5)3/2-MT is a
was complex but contained three doublets corresponding to three rare example of a FLP in which an aromatic carbon nucleophile
2-Me groups (collectively termed 3) each representing a different (2-MT) is activating dihydrogen.14,15 The reduction of 2-MT with
substituted tetrahydrothiophene derived from hydrosilylation.12 At B(C6F5)3 in the presence of Cl2-py was more facile, with 66%
no point were vinylic resonances of substituted dihydrothiophene
intermediates observed.
The ability of base to increase the proportion of 2 formed by
facilitating the deprotonation of the arenium cation was next inves-
tigated. Addition of 2,6-ditertbutylpyridine (tBu2-py, entry 2) increased
(1)
the ratio of 2 relative to 3. However, to be catalytic the resultant
conversion of 2-MT to 2-Me-THT at only 20 1C (16 h, 4 atm. H2,
[H(amine)][HB(C6F5)3] has to evolve H2, a reaction requiring a weakly
eqn (2)) indicating that Cl2-py/B(C6F5)3 is more effective for 2-MT
nucleophilic amine to be energetically favoured.2 This precludes
hydrogenation, analogous to the high reduction activity of FLPs with
tBu2-py and 2,6-lutidine, the latter the optimal base in stoichiometric
other weak bases.16 Complete reduction of 2-MT at 20 1C is retarded
Sila-Friedel–Crafts reactions.9 As the steric bulk of the base strongly
affects the barrier to deprotonation of silylated arenium cations
isosteric bases to 2,6-lutidine were explored.8,9 Using 2,6-dichloro-
pyridine (Cl2-py) as a suitably weak base, the amount of 2 produced
(relative to 3) increased (entry 3). Replacing CH2Cl2 with benzene
resulted in no silylation (24 h, 20 1C). In contrast, the silylation of
carbonyl moieties with B(C6F5)3/R3SiH is more rapid in non-polar
solvents than in CH2Cl2 which obviated ionic intermediates.3a The
necessity for polar solvents for heteroarene silylation implies the
formation of unobserved ionic species, for example II (Scheme 1).
The silylation of 2-MT with various silanes using B(C6F5)3/Cl2-py
was also explored, but Ph3SiH produced the highest amount of 2
relative to 3, with less dehydrosilylation observed on decreasing
silane steric bulk.12
by coordination of 2-Me-THT to B(C6F5)3.12 The necessity for Cl2-py for
2-MT reduction at 20 1C is consistent with the complete absence of
2-Me-THT in base free reactions (Table 1, entry 1, by NMR spectro-
scopy).12 As previous reactions were performed in a closed system the
H2 concentration increases as dehydrosilylation proceeds enabling
competitive hydrogenation. Silylation with B(C6F5)3/Cl2-py performed
in a tube sealed under vacuum (to minimise build up of dissolved H2)
produced no 2-Me-THT, but whilst there was a relative increase in 2,
aliphatic species (from hydrosilylation) were still present.12
(2)
Catalytic loadings of B(C6F5)3/Cl2-py required heating for reason-
able reaction times (entries 4 and 5) and led to similar ratios of 2:3.
The product distribution in the silylation of other hetero-
Attempts with excess Cl2-py did not significantly improve the selectivity arenes using Ph3SiH/B(C6F5)3/Cl2-py was also explored. Thiophene,
for 2 (entry 6). Full consumption of 2-MT was not achieved even at 2,20-bithiophene and thieno-[3,2,b]-thiophene all resulted in no
longer times and using 1.5 eq. of Ph3SiH (entry 5 vs. 7), suggesting reaction at 20 1C presumably due to reduced arene nucleophilicity
catalyst deactivation. During catalysis one new boron containing relative to 2-MT. 2-tBu-thiophene, 2-BT, was amenable to stoichio-
species gradually increased in intensity (by 11B NMR spectroscopy metric and catalytic electrophilic silylation which occurs with
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Table 2 Electrophilic functionalisation of select heteroarenes
proton transfer to another molecule of N-TIPS-indole, as observed
in electrophilic borylations,17 and finally reduction to N-TIPS-
indoline by hydride transfer (Scheme 3).
In conclusion, R3Si–H–B(C6F5)3, I, still forms in the presence of
activated heteroarenes, which for the first time are shown to be
viable nucleophiles towards I. Catalytic silylation pathways are
demonstrated, but the competitive activation of Si–H and H–H
bonds by boron Lewis acids/weak nucleophiles leads to multiple
products. Furthermore, the formation of aliphatic R2S species from
thiophene hydrosilylation/hydrogenation inhibits catalyst turnover
by coordination to B(C6F5)3. Finally, the hydrogenation of both 2-MT
and N-TIPS-indole with only B(C6F5)3/H2 confirms both these hetero-
arenes are carbon nucleophiles capable of activating H2 in a FLP.
This suggests that many other arenes will be viable as carbon
nucleophiles for H2 cleavage in a FLP.
B(C6F5)3/
Cl2-py
Y–H (mol%) t (h)
Sila-FCa Reda
Entry Substrate
T (1C) (%)
(%)
1
2
3
4
5
6
7
8
9
2-BT
2-BT
Si–H 100/100 18
Si–H 5/5
N-TIPS pyrrole Si–H 100/100 48
N-TIPS-indole Si–H 100/100 24
N-TIPS-indole H–H 100/100 16
20
60
20
20
20
20
20
20
54
70
42
59
—
—
30
—
32
17
45
24
19b
80b
80c
21b
16b
35b
2-BT
H–H 100/100 24
N-TIPS-indole Si–H 100/0
N-TIPS-indole H–H 100/0
N-TIPS-indole H–H 100/0
24
24
24 + 24 20 + 60 —
We thank the Royal Society (M. J. I.), the Leverhulme Trust
(E. R. C), the European Research Council under FP7 (J. J. D.)
and the University of Manchester (L. D. C.) for support.
a
Conversion by consumption of the substrate and growth of products as
determined by 1H NMR spectroscopy, unreacted starting material also
b
present. Combined conversion to the indoline and protonated indoline.
c
Acid induced tBu migration results in multiple reduction products.
Notes and references
1 W. E. Piers, A. J. V. Marwitz and L. G. Mercier, Inorg. Chem., 2011,
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2 G. Erker and D. W. Stephan, Frustrated Lewis Pairs, Uncovering and
Understanding, Springer-Verlag, Berlin, 2013.
concomitant hydrogenation (Table 2, entries 1 and 2). Electrophilic
silylation via I could be extended to 5-membered N-heterocycles. Whilst
N-TIPS protected pyrrole and indole were amenable to silylation,
hydrogenation was again competitive (entries 3 and 4), although
no hydrosilylation was observed in either case. Hydrogenation
products were confirmed by independent reduction under 4 atm. H2
(e.g., entry 5). Catalytic (in B(C6F5)3) reductions were limited as (i) the
hydrogenation of N-TIPS-indole produces a better Brønsted base,
N-TIPS-indoline, that cleaves H2 with B(C6F5)3 to form [N-H-N-TIPS-
indolinium][HB(C6F5)3] thus sequestering B(C6F5)3 and preventing turn-
over, (ii) the catalytic hydrogenation of tBu-thiophene was retarded by
coordination of 2-tBu-tetrahydrothiophene to B(C6F5)3 (entry 6).12
The dehydrosilylation of N-TIPS-indole without Cl2-py led to
increased proportions of the reduction product, N-TIPS-indoline,
(entry 7 vs. 4) analogous to 2-MT reactivity. Furthermore, in the
absence of Cl2-py the FLP hydrogenation of N-TIPS-indole with
B(C6F5)3 also proceeds confirming that N-TIPS indole is also a viable
carbon nucleophile for FLP H2 activation (entries 8 and 9).12 It is
noteworthy that there is less reduction of N-TIPS-indole at 20 1C
under H2 than there is during silylation (entry 7 vs. 8) thus another
reduction mechanism must be operating in silylation. Reduction
presumably proceeds by silylation of N-TIPS-indole followed by
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Scheme 3 Reduction of N-TIPS-indole by competing mechanisms.
5272 | Chem. Commun., 2014, 50, 5270--5272
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