B(C6F5)3-Catalyzed Transfer of Dihydrogen from One Unsaturated Hydrocarbon to Another

Article abstract of DOI:10.1002/anie.201504941

A transition-metal-free transfer hydrogenation of 1,1-disubstituted alkenes with cyclohexa-1,4-dienes as the formal source of dihydrogen is reported. The process is initiated by B(C₆F₅)₃-mediated hydride abstraction from the dihydrogen surrogate, forming a Bronsted acidic Wheland complex and [HB(C₆F₅)₃]^-. A sequence of proton and hydride transfers onto the alkene substrate then yields the alkane. Although several carbenium ion intermediates are involved, competing reaction channels, such as dihydrogen release and cationic dimerization of reactants, are largely suppressed by the use of a cyclohexa-1,4-diene with methyl groups at the C1 and C5 as well as at the C3 position, the site of hydride abstraction. The alkene concentration is another crucial factor. The various reaction pathways were computationally analyzed, leading to a mechanistic picture that is in full agreement with the experimental observations.

Full text of DOI:10.1002/anie.201504941

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201504941
German Edition: DOI: 10.1002/ange.201504941
Transfer Hydrogenation
B(C F ) -Catalyzed Transfer of Dihydrogen from One Unsaturated
6
5 3
Hydrocarbon to Another
Indranil Chatterjee, Zheng-Wang Qu, Stefan Grimme,* and Martin Oestreich*
Abstract: A transition-metal-free transfer hydrogenation of
1
,1-disubstituted alkenes with cyclohexa-1,4-dienes as the
formal source of dihydrogen is reported. The process is
initiated by B(C F ) -mediated hydride abstraction from the
6
5 3
dihydrogen surrogate, forming a Brønsted acidic Wheland
À
complex and [HB(C F ) ] . A sequence of proton and hydride
6
5 3
transfers onto the alkene substrate then yields the alkane.
Although several carbenium ion intermediates are involved,
competing reaction channels, such as dihydrogen release and
cationic dimerization of reactants, are largely suppressed by the
use of a cyclohexa-1,4-diene with methyl groups at the C1 and
C5 as well as at the C3 position, the site of hydride abstraction.
The alkene concentration is another crucial factor. The various
reaction pathways were computationally analyzed, leading to
a mechanistic picture that is in full agreement with the
experimental observations.
Scheme 1. B(C F ) -catalyzed transfer hydrogenation of imines and 1,1-
6
5 3
disubstituted alkenes using cyclohexa-1,4-dienes as the dihydrogen
T
he heterolytic splitting of dihydrogen by the strong Lewis
source. PG=protective group.
[1–3]
acid B(C F ) continues to attract ample attention.
The
6
5 3
HÀH bond activation usually leads to the formation of
À
[
HB(C F ) ] and an onium ion that is derived either directly
unsaturated hydrocarbon to another unsaturated hydrocar-
bon in a process that is catalyzed by a main-group Lewis acid
6
5 3
from the substrate or from an additional Lewis base,
rendering it an FLP-type process (FLP = frustrated Lewis
pair). These systems are applicable to the reduction, namely
the transition-metal-free hydrogenation, of a broad range of
substrates.
achieved when cyclohexa-1,4-dienes are employed as dihy-
drogen surrogates (Scheme 1, top).
abstract a hydride from adequately substituted cyclohexa-1,4-
dienes,
[14,15]
(Scheme 1, bottom).
However, the involvement of car-
[
3]
benium ions at different stages of this process is likely to
provoke cationic oligomerization of both reaction partners. It
must be mentioned that Stephan and co-workers recently
reported one example of a B(C F ) -catalyzed alkene hydro-
[
4–9]
We recently found that the same net result is
6
5 3
[10]
B(C F ) is able to
genation that required assistance by Et O as a Lewis basic
2
6
5
3
[8,9,16,17]
component.
The same laboratory also elaborated
[
10–12]
and the resulting Wheland intermediate then acts
a transfer hydrogenation of alkenes that is catalyzed by
a Lewis acidic phosphonium ion [(C F ) PF] , where hydride
+
as a Brønsted acid to fulfill the role of the aforementioned
6
5 3
onium ion. We thus accomplished the transfer hydrogenation
of imines (Scheme 1, top).
The application of this approach to alkenes is a worthwhile
goal as it would allow for the transfer of dihydrogen from one
and proton were generated in a preceding cross-dehydrogen-
[10,13]
[18]
ative SiÀO or SiÀN coupling.
Herein, we disclose the
transfer hydrogenation of 1,1-disubstituted alkenes catalyzed
[8,9]
by B(C F ) alone.
Undesired reactions of the formed
6
5 3
carbenium ions were prevented by the use of a 1,3,5-
trisubstituted cyclohexa-1,4-diene as the dihydrogen source.
We chose 1,1-diphenylethylene (1) as the model sub-
[
*] Dr. I. Chatterjee, Prof. Dr. M. Oestreich
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
[
8]
strate,
and 1,5-dimethylcyclohexa-1,4-diene (2a) was
selected as the reductant as it had performed best in the
[10]
Homepage: http://www.organometallics.tu-berlin.de
transfer hydrogenation of imines (see Scheme 1, top).
Dr. Z.-W. Qu, Prof. Dr. S. Grimme
However, the application of this procedure to 1 resulted in
little conversion (Table 1, entry 1). The situation changed in
halogenated solvents such as CH Cl (Table 1, entry 2; for the
solvent screen, see the Supporting Information). A gradual
decrease in the temperature to room temperature and
a change in concentration from 0.24m to 0.4m furnished
alkane 3 with near-quantitative conversion; the use of 1,2-
Mulliken Center for Theoretical Chemistry
Institut für Physikalische und Theoretische Chemie
Rheinische Friedrich-Wilhelms-Universität Bonn
Beringstrasse 4, 53115 Bonn (Germany)
E-mail: grimme@thch.uni-bonn.de
2
2
Homepage: http://www.thch.uni-bonn.de/tc
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201504941.
F C H brought about further improvement (Table 1,
2
6
4
entries 3–5). It is noteworthy that unsubstituted cyclohexa-
1
2158
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Table 1: Screening of cyclohexa-1,4-dienes as the dihydrogen source and
identification/optimization of the reaction conditions.
was isolated in 33% yield and fully characterized. The
undesired pathway is more pronounced with unsubstituted
b, which is devoid of methyl groups at the C1 and C5
2
positions, than with 2a, which suggests that the more
stabilized Wheland intermediate is less prone to attack by
the alkene at the remaining electrophilic C3 position.
We therefore tested 2c, which features more strongly
electron-donating methoxy groups in the C1 and C5 positions,
[10]
but no reaction was seen (Table 1, entry 7). The idea was
then to suppress the nucleophilic attack of the alkene at the
C3 carbon atom by installation of another methyl group in
this position. B(C F ) was still able to abstract the hydride
6
5 3
Entry Cyclohexa-1,4-diene Solvent
T
Conv. Yield
from that methine carbon atom in 2d. With 2d, we indeed
achieved excellent yields of 3 at full conversion of 1 in either
[a]
[b]
[
%]
[%]
[
c]
CH Cl or 1,2-F C H (Table 1, entries 8 and 9). None of the
1
2
3
4
5
6
7
8
9
0
2a
2a
2a
2a
2a
2b
2c
2d
2d
2d
toluene
1258C 13
808C 54
–
–
2 2 2 6 4
[
[
[
c]
c]
d]
CH Cl
previously observed adducts were obtained. Lowering the
catalyst loading to 5.0 mol% did not make a difference
(Table 1, entry 10). The reactions were routinely maintained
at room temperature for eight hours, but following the
2
2
2
2
CH Cl
RT
RT
RT
RT
RT
RT
RT
RT
66
88
90
100
n.r.
–
2
CH Cl
61
64
38
–
2
[
[
[
d]
d]
d]
1,2-F C H
2
6
4
4
4
1,2-F C H
2
6
1
progress by H NMR spectroscopy showed that full conver-
1,2-F C H
2
6
[
d]
sion was reached within less than one hour.
CH Cl
100
100
100
86
97
97
2
2
[
[
d]
We decided to proceed with the optimized procedure
(5.0 mol% B(C F ) and 1.3 equiv 2d at RT) and tested
1,2-F C H
2
6
4
4
d,e,f]
1
1,2-F C H
2 6
6
5 3
various 1,1-disubstituted alkenes (Table 2). 1,1-Diarylalkenes
–11 reacted smoothly, independent of the electronic proper-
ties of the arene, which could even be a sulfur-containing
heterocycle (Table 2, entries 2–6). The situation changed
when one of the aryl groups was replaced by an alkyl group
[
a] Determined by GLC analysis with respect to the substrate. [b] Yield of
isolated product after flash column chromatography on silica gel.
c] Alkene (0.24m) in the indicated solvent. [d] Alkene (0.4m) in the
7
[
indicated solvent. [e] Using 5.0 mol% of catalyst. [f] Reaction time: 1 h.
n.r.=no reaction.
(Table 2, entries 7–9). A small alkyl group as in a-methyl-
styrene (12) resulted in significant dimerization whereas
isopropyl and cyclohexyl substituents as in 13 and 14 were not
detrimental, furnishing the corresponding alkanes in excellent
yields. The same trend was seen with the 1,1-dialkylalkenes 15
and 16 as dimerization was again observed for these alkene
substrates with primary alkyl substituents (Table 2, entries 10
and 11). Monosubstituted, 1,2-disubstituted, trisubstituted as
well as exocyclic alkenes did not participate.
1
,4-diene (2b), which had been completely unreactive in the
[
10]
imine hydrogenation,
entry 6).
afforded full conversion (Table 1,
Although the conversions were high, the yields of isolated
products were substantially lower (Table 1, entries 5 and 6). A
closer look at these reactions revealed that significant
amounts of both alkene 1 and reductant 2a/2b are consumed
in the formation of adducts 4–6 (Scheme 2). The high-energy
Wheland complex is expected to protonate 1, but instead of
acting as a strong Brønsted acid, it is nucleophilically attacked
by 1 (Scheme 2, bottom left). Reduction of the resulting
Table 2: Transfer hydrogenation of 1,1-disubstituted alkenes.
À
carbenium ion with [HB(C F ) ] yields 4 as the primary
6
5 3
adduct. The double bonds in 4 are subject to further hydro-
genation to afford 5 and 6. An experiment on larger scale with
2
b showed that 4 and 6 were formed in trace amounts, while 5
1
2
[a]
Entry
Alkene
R
R
Alkane
Yield [%]
1
2
3
4
5
6
7
8
9
1
7
8
Ph
Ph
Ph
4-FC H
4-MeC H
Ph
Ph
Ph
Ph
Me
Cy
Ph
3
17
18
19
20
21
22
23
24
25
26
97
94
87
80
99
88
4-OMeC H
4-BrC H
4-FC H
4-MeC H
thien-2-yl
Me
iPr
Cy
nHept
Cy
6
4
6
4
9
6
4
6
4
10
11
12
13
14
15
16
6
4
6
4
[
b]
b]
33
99
95
53
92
[
[
[
b]
b]
1
1
0
1
[
a] Yield of isolated product after flash column chromatography on silica
1
Scheme 2. Undesired pathways: the Wheland complex as an electro-
phile or a Brønsted acid.
gel. [b] Determined by H NMR spectroscopy with 1,3,5-trimethoxyben-
zene as the internal standard, which was added after the reaction.
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1
À1
Monitoring the transfer hydrogenation by H NMR
spectroscopy using a slight excess of 2d (1.3 equiv) revealed
that the reaction was finished within one hour. Importantly,
dihydrogen is present from the beginning of the reaction.
Treatment of 1,3,5-trimethylcyclohexa-1,4-diene (2d) with
B(C F ) at room temperature in the absence of an alkene
gonic by 4.5 and 12.2 kcalmol , respectively, over moderate
°
À1
barriers (DG ) of 12.4 and 14.7 kcalmol for 2d and 2a,
respectively. These numbers confirm that the additional
methyl group at the C3 position renders 2d a better hydride
donor than 2a. Furthermore, this endergonic step will keep
the concentrations of the separated ion pair intermediates
consisting of Wheland complex 2d or 2a and the borate
anion [HB(C F ) ] relatively low. When no alkene substrate
is present, intermolecular proton/hydride recombination
between 2d or 2a and [HB(C F ) ] easily occurs with
a low Gibbs free energy barrier of 5.9 or 2.4 kcalmol and is
highly exergonic by 16.5 or 22.4 kcalmol to release dihy-
drogen along with the arene (mesitylene or meta-xylene).
Hence, the initial hydride transfer of this dehydrogenation is
rate-limiting.
6
5 3
+
+
quantitatively yielded mesitylene and dihydrogen within the
same time. However, any released dihydrogen will not
contribute to the transfer hydrogenation as B(C F ) alone
À
6
5 3
6
5 3
[8]
+
+
À
cannot catalyze this alkene hydrogenation, even under
a high pressure of dihydrogen (100 bar), at room temperature
in 1,2-F C H . Furthermore, reactions with equimolar
amounts of 1 and 2d did not reach full conversion of alkene
, which clearly indicates that the dihydrogen release is
6 5
3
À1
À1
2
6
4
[20]
1
a minor pathway.
To gain deeper mechanistic insight, the 1,1-disubstituted
alkene 1 and two cyclohexa-1,4-dienes, 2a and 2d, were
chosen as the model systems for our detailed theoretical
study. State-of-the-art dispersion-corrected DFT calcula-
When alkene substrate 1 is present at relatively high
[20]
+
+
concentration, the Wheland complexes 2d and 2a are
also able to react competitively with 1 at the less hindered
alkene terminus in an undesired or a desired way: either as an
[19]
+
+
tions
at the PW6B95-D3/def2-QZVP + COSMO-RS-
electrophile at the C3 position (2 !4 ) or as a Brønsted acid,
[21] + +
(
CH Cl ) level of theory using the TPSS-D3/def2-TZVP +
releasing a proton from the C6 position
(2 !3 ). The
2
2
À1
COSMO(CH Cl ) optimized geometries and thermal correc-
former pathway is endergonic by 15.7 or 8.4 kcalmol and
proceeds over sizable barriers of 19.1 and 12.4 kcalmol to
2
2
À1
tions were performed to explore the Gibbs free energies (DG)
of the various reaction paths (Scheme 3). As shown for 1,3,5-
trimethylcyclohexa-1,4-diene (2d), the catalyst B(C F )
+
+
form the carbenium ions 4d and 4a , respectively. These
intermediates are eventually transformed into the experi-
mentally observed adducts 4–6 after hydride transfer and
further hydrogenation (see Scheme 2). The Wheland complex
6
5 3
selectively abstracts a hydride from the C3 position [italicized
[11c]
numbers for 1,5-dimethylcyclohexa-1,4-diene (2a)].
Hy-
+
dride transfer from the C6 carbon atom is kinetically negli-
2d with a methyl group at the C3 position is less electrophilic
À1
+
À1
gible owing to a ꢀ 10 kcalmol higher Gibbs free energy
than 2a , with a 6.7 kcalmol higher Gibbs free energy
barrier for the addition to 1. This is mainly a result of
barrier. The initial hydride transfers to B(C F ) are ender-
6
5 3
+
increased steric congestion at the electrophilic site in 2d .
Conversely, the intermolecular proton transfer from 2d or
a to 1 is exergonic by 10.9 or 16.8 kcalmol and proceeds
over 10.0 or 6.1 kcalmol lower Gibbs free energy barriers to
yield intermediate 3 along with mesitylene or meta-xylene.
The electron-donating methyl group at the C3 position in 2d
reduces its reactivity relative to that of 2a as reflected by
a 2.8 kcalmol higher barrier for the proton transfer to 1. The
+
+
À1
2
À1
+
[21]
+
+
À1
+
+
desired pathway (2 !3 ) is not only kinetically favored over
+
+
the undesired CÀC bond formation (2 !4 ), but the barrier
+
+
for alkene protonation by 2d or 2a is also 3.3 or
À1
8
.4 kcalmol lower than that for the preceding hydride
transfer and thus not rate-limiting.
[20,22]
There are also two competing reaction channels for the
+
carbenium ion intermediate 3 : either reduction by
À
5 3
+
[
HB(C F ) ] (3 !3) or nucleophilic attack by another
6
+ +
(3 !27 ). The hydride transfer from
+ À1
molecule of
1
À
[
HB(C F ) ] to 3 is highly exergonic by 27.0 kcalmol ,
6
5 3
À1
with a low Gibbs free energy barrier of 3.4 kcalmol , and
furnishes the desired alkane 3 together with the regenerated
Scheme 3. Catalytic cycle of the B(C F ) -catalyzed transfer hydrogena-
6
5 3
B(C F ) catalyst. On the other hand, dimerization through
tion of alkenes with competing pathways (see the Supporting Informa-
tion for calculated structures of relevant intermediates and transition
states). For each reaction step, the Gibbs free reaction energies and
barriers (labeled with an asterisk in parentheses) were computed at
the PW6B95-D3 level of theory for 1,3,5-trimethylcyclohexa-1,4-diene
6
5 3
nucleophilic attack by the alkene terminus of 1 is almost
À1
thermoneutral with a 7.3 kcalmol higher Gibbs free energy
+
barrier. The formation of 27 is therefore kinetically less
competitive mainly owing to severe steric hindrance around
(
2d) as the dihydrogen source, B(C F ) as the catalyst, and 1,1-
6 5 3
+
the carbenium carbon atom in 3 . However, for alkenes with
diphenylethylene (1) as the alkene substrate. For comparison, the
corresponding Gibbs free energies are also given for the reaction with
smaller substituents at the internal carbon atom, such as 12
and 15, this pathway becomes prevalent (see Table 2, entries 7
and 10).
1
,5-dimethylcyclohexa-1,4-diene (2a) as the dihydrogen source (itali-
cized values).
1
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We have herein disclosed a rare case of an alkene transfer
[8] For B(C F ) -catalyzed, ether-assisted alkene hydrogenation,
6 5 3
[14]
see: a) L. J. Hounjet, C. Bannwarth, C. N. Garon, C. B. Caputo,
S. Grimme, D. W. Stephan, Angew. Chem. Int. Ed. 2013, 52,
hydrogenation catalyzed by a main-group element.
The
present approach relies on hydride abstraction from cyclo-
hexa-1,4-dienes by the boron Lewis acid B(C F ) , and the
7
492 – 7495; Angew. Chem. 2013, 125, 7640 – 7643 (Et O; 1,1-
2
6
5 3
diphenylethylene); b) D. J. Scott, M. J. Fuchter, A. E. Ashley,
Angew. Chem. Int. Ed. 2014, 53, 10218 – 10222; Angew. Chem.
2014, 126, 10382 – 10386 (THF; 1,2,3,4,5-pentamethylcyclo-
penta-1,3-diene).
whole process passes through cationic intermediates until
À
terminated by hydride transfer from [HB(C F ) ] . Problems
6
5 3
arising from cationic hetero- and homodimerization of the
reactants were solved by using a cyclohexa-1,4-diene with
substitution at the site of hydride abstraction and by adjusting
the alkene concentration. The mechanism, including the
experimentally observed competing reaction pathways, was
analyzed by state-of-the-art quantum-chemical calculations.
[9] For seminal work on FLP-type alkene hydrogenation, see: a) L.
Greb, P. OÇa-Burgos, B. Schirmer, S. Grimme, D. W. Stephan, J.
Paradies, Angew. Chem. Int. Ed. 2012, 51, 10164 – 10168; Angew.
Chem. 2012, 124, 10311 – 10315; b) L. Greb, S. Tussing, B.
Schirmer, P. OÇa-Burgos, K. Kaupmees, M. Lðkov, I. Leito, S.
Grimme, J. Paradies, Chem. Sci. 2013, 4, 2788 – 2796.
[
10] I. Chatterjee, M. Oestreich, Angew. Chem. Int. Ed. 2015, 54,
1
965 – 1968; Angew. Chem. 2015, 127, 1988 – 1991.
Acknowledgements
[11] For B(C F ) -mediated hydride abstraction from cyclohexa-2,5-
6
5 3
dienyl-substituted silanes as an entry into the related transfer
hydrosilylation, see: a) A. Simonneau, M. Oestreich, Angew.
Chem. Int. Ed. 2013, 52, 11905 – 11907; Angew. Chem. 2013, 125,
This research was supported by the Deutsche Forschungsge-
meinschaft (EXC 314/2 “Unifying Concepts in Catalysis” to
I.C. and M.O. as well as SFB 813 to Z.-W.Q. and S.G.). M.O. is
indebted to the Einstein Foundation (Berlin) for an endowed
professorship.
1
2121 – 12124; b) S. Keess, A. Simonneau, M. Oestreich, Orga-
nometallics 2015, 34, 790 – 799; for a computational analysis of its
reaction mechanism, see: c) K. Sakata, H. Fujimoto, Organo-
metallics 2015, 34, 236 – 241.
[
12] B(C F ) also mediates hydride abstraction from 1,4-dihydro-
6
5 3
Keywords: alkenes · boron · density functional calculations ·
hydrogenation · Lewis acids
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Stephan, C. M. Crudden, Chem. Eur. J. 2010, 16, 4895 – 4902;
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Angew. Chem. 2015, 127, 12326–12330
1
423.
[
[
13] B(C F ) is also able to abstract hydrides from amines, and
6
5 3
a Meerwein – Pondorf – Verley-type reduction of imines was
developed on this basis; see: J. M. Farrell, Z. M. Heiden, D. W.
Stephan, Organometallics 2011, 30, 4497 – 4500.
[
[
1] For general reviews, see: a) M. Oestreich, J. Hermeke, J. Mohr,
Chem. Soc. Rev. 2015, 44, 2202 – 2220; b) W. E. Piers, A. J. V.
Marwitz, L. G. Mercier, Inorg. Chem. 2011, 50, 12252 – 12262.
2] For reviews on mainly FLP-, but also B(C F ) -catalyzed hydro-
14] Gandon and co-workers reported a gallium(III)-assisted transfer
hydrogenation of alkenes with cyclohexa-1,4-diene as the
dihydrogen source. The mechanism and the role of the main-
group metal remain to be clarified; see: B. Michelet, C. Bour, V.
Gandon, Chem. Eur. J. 2014, 20, 14488 – 14492.
15] For the use of a Hantzsch 1,4-dihydropyridine in the Pd/C-
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Tetrahedron Lett. 2009, 50, 1026 – 1028.
6
5 3
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2
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[
[
[
[
[
3] For authoritative reviews, see: a) D. W. Stephan, G. Erker,
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1
27, 6498 – 6541; b) D. W. Stephan, G. Erker, Angew. Chem. Int.
16] For an alternative, mechanistically different approach to alkene
Ed. 2010, 49, 46 – 76; Angew. Chem. 2010, 122, 50 – 81.
hydrogenation catalyzed by HB(C F ) , see: Y. Wang, W. Chen,
4] For B(C F ) -catalyzed imine hydrogenation, see: a) P. A. Chase,
6
5 2
6
5 3
Z. Lu, Z. H. Li, H. Wang, Angew. Chem. Int. Ed. 2013, 52, 7496 –
499; Angew. Chem. 2013, 125, 7644 – 7647.
T. Jurca, D. W. Stephan, Chem. Commun. 2008, 1701 – 1703;
b) D. Chen, J. Klankermayer, Chem. Commun. 2008, 2130 –
7
17] For selected examples of FLP-type hydrogenations of electron-
deficient alkenes, see: a) J. S. Reddy, B.-H. Xu, T. Mahdi, R.
Frçhlich, G. Kehr, D. W. Stephan, G. Erker, Organometallics
2
5
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[
[
5] For B(C F ) -catalyzed oxime ether hydrogenation, see: J. Mohr,
6 5 3
M. Oestreich, Angew. Chem. Int. Ed. 2014, 53, 13278 – 13281;
Angew. Chem. 2014, 126, 13494 – 13497.
2
6] For B(C F ) -catalyzed partial hydrogenation of nitrogen-con-
c) L. Greb, C.-G. Daniliuc, K. Bergander, J. Paradies, Angew.
Chem. Int. Ed. 2013, 52, 5876 – 5879; Angew. Chem. 2013, 125,
5989 – 5992.
6
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1
978.
[
7] For B(C F ) -catalyzed ketone and aldehyde hydrogenation, see:
6
5 3
a) T. Mahdi, D. W. Stephan, J. Am. Chem. Soc. 2014, 136, 15809 –
5812; b) D. J. Scott, M. J. Fuchter, A. E. Ashley, J. Am. Chem.
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1
Angew. Chem. Int. Ed. 2015, 54, 12158 –12162
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 12161

.
Angewandte
Communications
+
Ahlrichs, J. Chem. Phys. 2003, 119, 12753 – 12762; h) A. Schae-
fer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100, 5829 – 5835;
i) TURBOMOLE V6.4 2012, a development of the University of
Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989 –
group at the C3 position of 2d as well as 14.9 and 19.8 kcal
+
À1
mol for the methyl groups at the C1/C5 positions of 2a and
2d , respectively. These cannot compete with the desired
exergonic proton transfer to 1 from the C6 carbon atom, which
directly reestablishes aromaticity.
+
2
007, TURBOMOLE GmbH, since 2007; available from
www.turbomole.com.
[22] However, the introduction of more strongly electron-donating
substituents at the C1 and C5 (as well as C3) positions lent that
much stabilization to the Wheland complex that the proton
transfer eventually became rate-limiting or was even thwarted.
This situation was experimentally observed with the 1,5-dime-
thoxy-substituted cyclohexa-1,4-diene 2c (see Table 1, entry 7).
[
[
20] In practice, a higher alkene concentration enhances the proton
transfer from the common Wheland intermediate onto the
alkene substrate, thereby helping to suppress the undesired
dihydrogen release.
21] The possible proton transfer from the methyl groups of Wheland
+
+
complexes 2d and 2a to 1 was also considered computation-
ally. At the highest PW6B95-D3 level of theory, such proton-
transfer reactions leading to non-aromatic intermediates (not
Received: June 1, 2015
Published online: August 26, 2015
À1
shown) are highly endergonic: 15.8 kcalmol for the methyl
1
2162 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 12158 –12162