Cyclohexa-1,3-diene-based dihydrogen and hydrosilane surrogates in B(C6F5)3-catalysed transfer processes

Article abstract of DOI:10.1039/c7cc06195a

The cyclohexa-1,3-diene motif is introduced as an equally effective alternative to the cyclohexa-1,4-diene platform in B(C₆F₅)₃-catalysed transfer processes. The transfer hydrogenation of alkenes is realised with α-terpinene and the related transfer hydrosilylation is achieved with 5-trimethylsilyl-substituted cyclohexa-1,3-diene. Both yields and substrate scope are comparable with the prior systems.

Full text of DOI:10.1039/c7cc06195a

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Cyclohexa‐1,3‐diene‐based dihydrogen and hydrosilane
surrogates in B(C6F5)3‐catalysed transfer processes†
Received 00th January 20xx,
Accepted 00th January 20xx
Weiming Yuan,‡ Patrizio Orecchia‡ and Martin Oestreich*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The cyclohexa‐1,3‐diene motif is introduced as an equally effective
alternative to the cyclohexa‐1,4‐diene platform in B(C6F5)3‐
catalysed transfer processes. The transfer hydrogenation of
alkenes is realised with α‐terpinene and the related transfer
hydrosilylation is achieved with 5‐trimethylsilyl‐substituted
cyclohexa‐1,3‐diene. Both yields and substrate scope are
comparable with the prior systems.
previous work
present work
R
R
R
El
El
El
B(C6F5)3
B(C6F5)3
+
H
H
[H–B(CC6F5)3]–
I
III
El = Me3Si and H
III
El = Me3Si and H
common inntermediate
The (formal) transfer of a gaseous molecule from one
compound to another is an old technique in synthetic
chemistry. Transfer hydrogenation in all its facets is probably
the classic example.1 The area of transfer reactions recently
gained new momentum when Morandi and co‐workers as well
as ourselves disclosed novel approaches. Morandi’s transition‐
metal‐catalysed shuttle catalysis2 enabled potentially
reversible transfer hydrocyanation3 (HCN) and transfer
hydrochlorocarbonylation4 (CO and HCl). Our contributions
make use of adequately substituted cyclohexa‐1,4‐dienes5
that, e.g., release hydrosilanes6,7 (R3SiH and even SiH4) and
transfer dihydrogen8,9 (H2) by treatment with the strong Lewis
acid tris(pentafluorophenyl)borane [B(C6F5)3]. These ionic
transfer reactions are initiated by hydride abstraction from
++
R3
R1
SiMe3
R3
R1
H
H
R3
El = Me3Si
B(C F )
El = H
H
B(C F )


R1
R2
6
5 3
6 5 3
R2
R2
R
R
V (El = Me3Si)
IVV
V (El = H)
–
–
involving formation of Me3Si–H
no formation of free H–H
Scheme 1 Cyclohexa‐1,4‐dienes (known) and cyclohexa‐1,3‐dienes (planned) as
dihydrogen and hydrosilane surrogates iin B(C6F5)3‐catalysed transfer processes
to alkenes. El = electrofuge.
The success with the B(C6F5)3‐promoted degradation of
cyclohexa‐1,4‐dienes5 led us to the question whether
cyclohexa‐1,3‐dienes with the double bonds in conjugation
would also be suitable hydrosilane and dihydrogen surrogates.
The conversion of cyclohexa‐1,3‐diene to benzene and
dihydrogen is exothermic but just a few reports employ
substituted cyclohexa‐1,3‐dienes as a dihydrogen sources.13
We envisioned that B(C6F5)3 could also abstract hydride from
cyclohexa‐1,3‐dienes to arrive at the Wheland complex as the
cyclohexa‐1,4‐dienes I 8b,10
low‐energy10 or high‐energy8b Wheland complexes with the
borohydride as counteranion ( II, Scheme 1, left). The fate
,
either forming silicon‐stabilised
I
→
of intermediates II depends on the electrofuge: II (El = Me3Si)
recombines to yield the hydrosilane and the arene prior to
B(C6F5)3‐mediated Si–H bond activation11 followed by alkene
common intermediate (III
→ II, Scheme 1, right). We describe
here transfer hydrogenation as well as hydrosilylation of
alkenes using cyclohexa‐1,3‐diene‐based surrogates.
hydrosilylation12 for El = Me3Si).10 The situation
(II → V
changes for II (El = H). It directly protonates alkene IV, and the
resulting carbenium ion is then reduced by the borohydride (II
To test the feasibility of our plan, we chose 1,1‐diarylalkene
as the model substrate (1 → 3, Table 1). The parent
→
V
for El = H).8b
1
surrogate, cyclohexa‐1,3‐diene (2a), was fully consumed but
the yield was low (entry 1). That outcome made us remember
the problems arising from hetero‐ and homodimerisation of
the reactants in our previous study with cyclohexa‐1,4‐diene
Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 115, 10623
Berlin, Germany. E‐mail: martin.oestreich@tu‐berlin.de
(
4a).8b We therefore subjected methyl‐substituted cyclohexa‐
†
Electronic Supplementary Information (ESI) available. See DOI:
10.1039/x0xx00000x
These authors contributed equally.
1,4‐dienes 2b and 2c to the standard setup and found little
improvement (entries 2 and 3). These results are not fully
‡
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Entry
1
2
3
4
5
6
7
8
Cyclohexa‐1,3‐diene 2
Solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
1,2‐F2C6H4
1,2‐Cl2C6H4
n‐Hexane
CH2Cl2
Yield of 3 (%)b
consistent with what was observed with methyl‐substituted
cyclohexa‐1,4‐dienes 4b and 4c where yields were
substantially higher.8b The more electron‐donating methoxy
group in 2d suppressed the reaction completely because of
interaction with the Lewis‐acid catalyst (entry 4); this is in
agreement with 4d in the cyclohexa‐1,4‐diene series. We then
examined the commercially available terpenes α‐phellandrene
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DOI: 10.1039/C7CC06195A
2a
2b
2c
2d
2e
2f
2f
2f
2f
2f
2f
2f
2f
2f
2f
30
35
52
n.r.
33
86
74
84
78
67
85
(
2e) and α‐terpinene (2f). The yield with 2e was in the same
range as before (entry 5) but cheap 2f afforded excellent 86%
yield (entry 6). Screening of various solvents did not bring
about higher yields (entries 7–10). Also, an increased amount
of reagent 2f (2.0 vs 1.3 equiv.) did not change the yield (entry
9
10
11c
12d
13f
14g
15h
CH2Cl2
CH2Cl2
CH2Cl2
1,2‐F2C6H4
92 (88)e
89
84
45
11).
A slightly better yield was obtained at higher
concentration, furnishing 88% isolated yield (entry 12). The
reactions were routinely run inside a glovebox but worked
without any relevant loss in yield outside the glovebox, even
with exposure to air (entries 13 and 14). When exposed to
moisture, proton catalysis could be also operative as a result of
Lewis adduct formation between B(C6F5)3 and water.14 We had
shown before that this transfer process is catalysed by
Brønsted acids, and Tf2NH had emerged as broadly applicable
to cyclohexa‐1,4‐dienes 4a
converted into in only 45% yield (entry 15).
– . However, α‐terpinene (2f)
c 8c
a Unless otherwise noted, all reactions were performed inside a glovebox with 0.1
mmol of 1, 1.3 equiv. of 2 and 10 mol% of B(C6F5)3 in CH2Cl2 (0.3 mL) at room
temperature. b Determined by 1H NMR spectroscopy using CH2Br2 as the internal
standard. 2.0 equiv. of 2f used. 0.1 mL CH2Cl2 (1.0 M) used. Isolated yield
after purification by flash chromatography on silica gel. Performed under inert
atmosphere outside the glovebox. Performed exposed to air and, hence,
moisture. 10 mol% of Tf2NH (10 mol%) used as catalyst in 1,2‐F2C6H4 at 80 °C.
n.r. = no reaction.
1
3
Table 1 Optimisation of the transfer hydrogenation of alkenes with different cyclohexa‐
1,3‐dienes (with published results8b for related cyclohexa‐1,4‐dienes at the bottom)a
c
d
e
f
g
B(C6F5)3
H
H
h
R
R
H
H
(10 mol%)
+
+
Ar
Ar
Ar
solvent
r.t.
Ar
With the optimised reaction conditions in hand, we
assessed the substrate scope (Scheme 2). 1,2‐
1
2a–f
3
16 h
(Ar = 4-MeC6H4) (1.3 equiv.)
(Ar = 4-MeC6H4)
Diphenylethylene reacted with comparable yield (
electron‐donating as well as electron‐withdrawing groups
were tolerated ( 15 17); the halogenated substrates
were less reactive, and the reaction had to be performed in
1,2‐F2C6H4 at 50 °C. A thien‐2‐yl group was also compatible (
18). Besides of 1,1‐diarylalkenes, 1‐alkyl‐1‐arylalkenes
participated equally well (10 11 19 20), and even a
trisubstituted alkene was converted in moderate yield (12
21, grey box). A 1,1‐dialkylalkene was also shown to react in
5 → 14), and
iPr
Me
R
H
6–
8
→
–
H
H
iPr
H
H
H
H
H
9
H
Me
2c
Me
H
Me
2f
→
2a
2b (R = Me)
2e
/
→
/
2d (R = OMe)
→
good yield (13
→
22).
2 | Chem. Commun., 2017, 53, 1‐4
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Entry
Solvent
Temperature (°C)
Yield of 24 (%)
82
86
85
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iPr
iPr
DOI: 10.1039/C7CC06195A
B(C6F5)3
1
2
3
4b
5
6
7
8
Toluene
Toluene
Toluene
Toluene
CH2Cl2
n‐Pentane
1,2‐Cl2C6H4
1,2‐F2C6H4
r.t.
50
R3
R1
R3
R1
(10 mol%)
+
+
80
50
r.t.
r.t.
r.t.
r.t.
CH2Cl2
r.t.
R2
5–13
R2
52
65 (5)c
24 (53)c
66 (33)c
82 (16)c
Me
2f
Me
16 h
14–22
15 (from 6): 98%
(X = OMe)
16 (from 7): 84%b
Me
Me
a
Unless otherwise noted, all reactions were performed with 0.1 mmol of 5, 1.3
equiv. of 23 and 5.0 mol% of B(C6F5)3 in the indicated solvent (0.1 mL) at room
temperature. Performed exposed to air and, hence, moisture. Yield of
recovered alkene in parentheses.
(X = F)
b
c
b
X
X
17 (from 8): 57%
14 (from 5): 92%
(X = Br)
Me
Me
Me
Monitoring the transfer hydrosilylation by 1H NMR spectroscopy
exhibited that Me3SiH is rapidly liberated, i.e., within a few minutes.
Subsequent addition of the alkene to the reaction mixture then
revealed that the hydrosilylation is markedly slower (see the
Electronic Supplementary Informaꢀon for details†). These findings
agree with the two‐step reaction sequence established for the
transfer hydrosilylation with 3‐silylated cyclohexa‐1,4‐dienes6b,d
(cf. Scheme 1, left).
Me
S
Me
18 (from 9): 41%
19 (from 10): 90%
20 (from 11): 90%c
Me
21 (from 12): 36%b c
22 (from 13): 70%b
,
Scheme 2 Substrate scope of the transfer hydrogenation of alkenes.a Unless
otherwise noted, all reactions were performed with 0.2 mmol of the indicated
alkene, 1.3 equiv. of 2f and 10 mol% of B(C6F5)3 in CH2Cl2 (0.2 mL) at room
a
We then applied the above protocol to different types of
alkenes with mono‐, di‐ and trisubstituted double bonds
(Scheme 3). 1,1‐Diarylalkenes with substitution in the para
position reacted cleanly. No reaction was seen with methoxy
b
c
temperature. The solvent was 1,2‐F2C6H4 and the temperature was 50 °C.
Determined by 1H NMR spectroscopy using CH2Br2 as the internal standard.
groups as substituents (as in
quantitative yield in the related transfer hydrogenation (
15, Scheme 2), and we attribute its failure to react to silicon’s
strong affinity to oxygen. The thien‐2‐yl group was again
6); this alkene had reacted in
We had recently established ionic transfer hydrosilylation
as a safe way to engage highly flammable hydrosilanes such as
Me3SiH or SiH4 in alkene hydrosilylation reactions.6 To
complement our earlier method with 3‐silylated cyclohexa‐1,4‐
dienes as surrogates,6 we anticipated that 5‐silylated
cyclohexa‐1,3‐dienes could also serve as hydrosilane sources.
To verify this, we prepared 5‐trimethylsilyl‐substituted
cyclohexa‐1,3‐diene 23 according to a procedure by Sarlah and
co‐workers.15 We were then pleased to see that the B(C6F5)3‐
catalysed transfer hydrosilylation of 1,1‐diphenylethylene with
6
→
tolerated (
styrenes reacted cleanly in the transfer hydrosilylation (25
39 40) as did an α‐olefin (27 41). Likewise, both exocyclic
1,1‐ and endocyclic 1,2‐disubstituted alkenes showed good to
excellent reactivity 28 42 and 29 30 43 44).
Trisubstituted 1‐methylcyclohexene reacted with good cis
9
→
38). Unlike in the transfer hydrogenation,
/
26
→
/
→
(
→
/
→
/
selectivity6b
(31 → 45). As observed with the cyclohexa‐1,4‐
23 proceeded smoothly in toluene at room temperature (5 →
diene‐based surrogate,6b indenes with or without substitution
24, Table 2, entry 1). The reaction temperature had only little
o
at C2 transformed into the corresponding indanes with the
effect, and yields did not improve significantly at 50 C and
trimethylsilyl group in the homobenzylic position (32
46 47). Finally, an indole‐containing alkene underwent the
transfer hydrosilylation as well (34 48).
/33 →
80 °C, respectively (entries 2 and 3). Running the reaction
under air led to a decrease in yield (entry 4). Other commonly
used solvents were screened but yet again without success
(entries 5‒8).
/
→
Table 2 Optimisation of the transfer hydrosilylation of alkenes with a 5‐silylated
cyclohexa‐1,3‐dienea
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2
For recent overviews of transition‐metal‐catalVyisewedArtticrlaenOsnfliener
processes, see: (a) B. N. Bhawal and BD. OMI:o1r0a.1n0d3i9,/CCh7CeCm0.6–1E9u5rA.
J., 2017, 23, DOI: 10.1002/chem.201605325; (b) B. N. Bhawal
B(C6F5)3
R3
R1
R3
R1
SiMe3
R2
SiMe3
(5.0 mol%)
+
+
and B. Morandi, ACS Catal., 2016, 6, 7528–7535.
toluene
50 °C
23 h
R2
3
4
X. Fang, P. Yu and B. Morandi, Science, 2016, 351, 832–836.
X. Fang, B. Cacherat and B. Morandi, Nat. Chem., 2017,
DOI: 10.1038/nchem.2798.
1, 5–7, 9
23
24
9
,
and 25–34
and 35–48
5
6
For a recent overview, see: S. Keess and M. Oestreich, Chem.
Sci. 2017, , 4688–4695.
SiMe3
8
35 (from 1): 88% (X = Me)
24 (from 5): 86% (X = H)
36 (from 6): n.r. (X = OMe)
37 (from 7): 80% (X = F)
X
For a review of transfer hydrosilylation, see: (a) M. Oestreich,
Angew. Chem., Int. Ed., 2016, 55, 494–499; (b) A. Simonneau
and M. Oestreich, Angew. Chem., Int. Ed., 2013, 52, 11905–
11907; (c) A. Simonneau, J. Friebel and M. Oestreich, Eur. J.
Org. Chem., 2014, 2077–2083; (d) S. Keess, A. Simonneau
and M. Oestreich, Organometallics, 2015, 34, 790–799; (e) A.
X
SiMe3
SiMe3
SiMe3
Simonneau and M. Oestreich, Nat. Chem., 2015, 7, 816–822;
Me
for the related transfer hydrogermylation, see: (f) S. Keess
and M. Oestreich, Org. Lett., 2017, 19, 1898–1901.
S
38 (from 9): 45%
39 (from 25): 98%
40 (from 26): 87%
SiMe3
7
8
For other approaches to transfer hydrosilylation, see: (a) S.
Amrein, A. Timmermann and A. Studer, Org. Lett., 2001, 3,
2357–2360; (b) C. Chauvier, P. Thuéry and T. Cantat, Angew.
Chem., Int. Ed., 2016, 55, 14096–14100.
SiMe3
SiMe3
n
nHex
(a) I. Chatterjee and M. Oestreich, Angew. Chem., Int. Ed.,
2015, 54, 1965–1968; (b) I. Chatterjee, Z.‐W. Qu, S. Grimme
and M. Oestreich, Angew. Chem., Int. Ed., 2015, 54, 12158–
12162; (c) I. Chatterjee and M. Oestreich, Org. Lett., 2016,
18, 2463–2466.
For another example of an alkene transfer hydrogenation
using cyclohexa‐1,4‐diene, see: B. Michelet, C. Bour and V.
Gandon, Chem.–Eur. J., 2014, 20, 14488–14492.
41 (from 27): 67%b
42 (from 28): 96%b
43 (from 29): 85% (n = 1)
44 (from 30): 80% (n = 2)
Me
SiMe3
Me
9
SiMe3
R
SiMe3
N
Me
45 (from 31): 89% 46 (from 32): 93% (R = H)
d.r. = 90:10c
47 (from 33): 70% (R = Me)
48 (from 34): 88%
10 For the quantum‐chemical analysis of the transfer
hydrosilylation of alkenes, see: K. Sakata and H. Fujimoto,
Organometallics, 2015, 34, 236–241.
11 (a) S. Rendler and M. Oestreich, Angew. Chem., Int. Ed.,
2008, 47, 5997–6000; (b) K. Sakata and H. Fujimoto, J. Org.
Chem., 2013, 78, 12505–12512. For a comprehensive review
of B(C6F5)3‐catalysed Si–H and H–H bond activation, see: (c)
M. Oestreich, J. Hermeke and J. Mohr, Chem. Soc. Rev., 2015,
44, 2202–2220.
Scheme 3 Substrate scope of the transfer hydrosilylation of alkenes.a Unless
a
otherwise noted, all reactions were performed with 0.1 mmol of the indicated
b
alkene, 1.3 equiv. of 23 and 5.0 mol% of B(C6F5)3 in toluene (0.1 mL) at 50 °C.
The solvent was CH2Cl2 and the temperature was 40 °C. c Determined by 1H NMR
spectroscopy.
Herein, we disclosed the B(C6F5)3‐catalysed transfer
hydrogenation and hydrosilylation of alkenes employing
cyclohexa‐1,3‐diene‐based dihydrogen (H2) and trimethylsilane
(Me3SiH) surrogates. The present work complements the prior
art with cyclohexa‐1,4‐dienes and extends the range of
reagents for these transfer reactions. While the substrate
scope is largely the same as before,6,8 the advantage lies in the
availability of α‐terpinene for the transfer hydrogenation.
Conversely, Sarlah’s 5‐trimethylsilyl‐substituted cyclohexa‐1,3‐
diene is not yet accessible in useful quantities compared to the
12 M. Rubin, T. Schwier and V. Gevorgyan, J. Org. Chem., 2002,
67, 1936–1940
13 Examples include: (a) E. N. Frankel, J. Org. Chem. 1972, 37
1549–1552; (b) D. C. Tabor, F. H. White, L. W. Collier, IV and
S. A. Evans, Jr., J. Org. Chem. 1983, 48, 1638 1643; (c) H.
Shima and T. Yamaguchi, J. Catal. 1984, 90, 160 164; (d) N.
,
‒
‒
A. Eddy, J. J. Richardson and G. Fenteany, Eur. J. Org. Chem.,
2013, 5041–5044.
14 A. A. Danopoulos, J. R. Galsworthy, M. L. H. Green, S.
Cafferkey, L. H. Doerrer and M. B. Hursthouse, Chem.
Commun., 1998, 2529–2530.
easy‐to‐make
1,4‐regioisomer.
Nevertheless,
both
15 M. Okumura, S. M. N. Huynh, J. Pospech and D. Sarlah,
Angew. Chem., Int. Ed., 2016, 55, 15910–15914.
transformations are rare examples of cyclohexa‐1,3‐dienes as
transfer reagents.
This research was supported by the Cluster of Excellence
Unifying Concepts in Catalysis of the Deutsche
Forschungsgemeinschaft (EXC 314/2) and the Berlin Graduate
School of Natural Sciences and Engineering (predoctoral
fellowship to P.O., 2016–2019). M.O. is indebted to the
Einstein Foundation (Berlin) for an endowed professorship.
There are no conflicts to declare
References
1
For a review, see: D. Wang and D. Astruc, Chem. Rev., 2015,
115, 6621–6686.
Transition‐metal‐free transfer hydrogenation and transfer
hydrosilylation of alkenes are achieved with cyclohexa‐1,3‐
diene‐based surrogates.
4 | Chem. Commun., 2017, 53, 1‐4
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