Catalyst-free dehydrocoupling of amines, alcohols, and thiols with pinacol borane and 9-borabicyclononane (9-BBN)

Article abstract of DOI:10.1039/c6cc06096j

Contrary to recent reports, the dehydrocoupling of pinacol borane and 9-borabicyclononane with a variety of amines, alcohols and thiols can be achieved under mild conditions without catalyst. This process involves the formation of Lewis acid-base adducts

Full text of DOI:10.1039/c6cc06096j

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Catalyst-Free Dehydrocoupling of Amines, Alcohols, and Thiols
with Pinacol Borane and 9-Borabicyclononane (9-BBN)
Erik A. Romero, Jesse L. Peltier, Rodolphe Jazzar and Guy Bertrand*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Scheme 1. Proposed catalyst-free dehydrocoupling pathway through Lewis acid-
base adducts F.
Contrary to recent reports, the dehydrocoupling of pinacol borane
and 9-borabicyclononane with a variety of amines, alcohols and
thiols can be achieved under mild conditions without catalyst. This
process involves the formation of Lewis acid-base adducts
featuring a hydridic B–H in close proximity to an acidic Nu–H.
Nu
H
R
H
Nu
Nu
R
R
H
+
B
B
H
B
-H2
In 2013 Nolan and coworkers1 described the
dehydrocoupling reaction of pinacol and catechol boranes with
Nu = RN, O, S
F
se adducts would feature hydridic B–H bonds in close
proximity to acidic Nu–H bonds, and therefore should release
H2 without the need for a catalyst.
thiols catalyzed by the ruthenium complex
A (Fig. 1). More
recently, the catalytic dehydrogenative coupling of amines
and/or alcohols with 9-borabicyclononane (9-BBN) and/or
pinacol borane have been reported using catalysts based on
Indeed, although numerous catalysts have been reported
to promote the dehydrogenation of several amine-borane
adducts, including ammonia-borane (NH3-BH3), it has also
been shown that catalyst-free dehydrogenative processes can
occur under thermal conditions.6 Herein we report the facile
catalyst-free dehydrocoupling of amines, alcohols, and thiols
with pinacol borane and 9-BBN.7
osmium B 2
aluminum E 5
metal-ligand cooperativity to activate pinacol borane. The
other catalysts ( ) are proposed to proceed through metal
,
alkali earth metals C 3
,
alkali metals D 4
, and
.
The osmium complex
B is postulated to utilize
C-E
amido, alkoxide or sulfide intermediates, which readily react
with the Lewis acidic hydridoboranes. Subsequent hydride
transfer to the corresponding metal yields the highly reactive
metal hydride that can deprotonate another equivalent of Nu–
H (E = RN, O, S) to close the cycle. Interestingly, these results
seem to exclude that pinacol borane and 9-BBN are sufficiently
We began our investigation using a stoichiometric mixture
of aniline and pinacol borane8 (0.4 mmol) in 0.4 mL of C6D6
(Table 1, Entry 1). After 4 h at room temperature, 60%
conversion into 1a was observed, with no byproducts
detected. The influence of the solvent (Entries 1-4) and of the
concentration (Entries 5-8) on the rate of the reaction were
tested. The best results were found in the absence of solvent.9
In this case, bubbling of H2 stopped after 15 minutes at room
temperature, and after another 10 minutes the solution
solidified. Dissolving this solid in acetonitrile led to nearly
quantitative formation of 1a, the only impurities being traces
of pinBOH and aniline.
Lewis acidic to form adducts of type F shown in Scheme 1. The-
Fig. 1. Reported catalysts for the dehydrocoupling of amines, alcohols and thiols
with pinacol borane, catechol borane and/or 9-BBN.
Nolan et al.
Ph
Yus et al.
iPr3P
Wilson et al. Panda et al. Roesky et al.
Ar
Ar
N
Al
N
N
M
N
O
H
H
NPh2
N
Si
Si
Os
N
N
Cl
Ru
H
i
M
PPh3
Pr3P
Cl
Ar
Ar
PPh3
The scope of the amine dehydrocoupling reaction with
pinacol borane was studied in the absence of solvent, except
when amines are solid at room temperature; in these cases
0.151 mL of acetonitrile was used with 0.8 mmol of both
reactants (Fig. 2). This reaction can be applied to both aliphatic
and aromatic amines with varying degrees of steric and
electronic contributions. Due to the high selectivity of the
dehydrogenative coupling reaction, products 1a-e and 1g-r
were obtained in good purity (as can be seen from the NMR
M = Li, Na, K
M = Mg, Ca
A
B
C
D
E
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Fig. 2. Substrate scope for the dehydrocoupling of pinacol borane and 9-BBN
with amines at room temperature, with conversions measured by 1H NMR
(referencing residual starting amine), and reaction times. a120 oC and 1 mol%
NEt3.
Table 1. Solvent and Concentration Effects on Amine Dehydrocoupling with pinacol
boranea
H
N
BPin
NH2
O
O
BPin
BPin
BPin
O
RT
B
HN
HN
HN
HN
+
H
B
O
Br
- H2
Entry Time (h) Solvent Conc. (M) Conversion (%)b
CF3
1c: 97%
6 h
1
2
3
4
5
6
7
8
4
4
4
C6D6
CDCl3
THF-d8
CD3CN
CD3CN
CD3CN
CD3CN
-
0.8
0.8
0.8
0.8
0.8
1.3
2.8
-
60
22
30
75
29
49
85
94
1a: 96%
0.5 h
1b: 97%
2 h
1d: 92%
12 h
4
BPin
BPin
NH
BPin
NH
BPin
HN
N
H
0.5
0.5
0.5
0.5
F
F
1e: 97%
6 h
1f: 97%a
24 h
1g: 96%
24 h
1h: 97%
0.25 h
aThe reactions were carried out in a J-Young NMR tube at room temperature
under an argon atmosphere using a 1:1 mixture (0.40 mmol) of aniline and
pinacol borane. bConversion measured by 1H NMR referencing residual starting
aniline.
BPin
BPin
NH
BPin
BPin
HN
N
4
N
H
O
spectra included in the Supporting Information) by simply
removing all volatiles under vacuum.
1i: 96%
0.1 h
1j: 96%
2 h
1k: 95%
1 h
1l: 93%
0.5 h
Interestingly, among all the amines that were considered,
2,6-diisopropylaniline was the only one that appeared to be
o
unreactive even after 24 hours at 120 C. This prompted us to
BPin
N
BPin
N
investigate the mechanism of the dehydrogenative coupling.
Monitoring by 1H NMR spectroscopy the reaction of various
amines with pinacol borane does not show the presence of
any intermediates. However, in the case of allylamine, the
addition of pinacol borane immediately produced a white solid
with no hydrogen evolution. The physical state of both starting
materials (liquids) suggested that the white solid obtained was
the Lewis acid-base adduct Fg, which was confirmed by a
single crystal X-ray diffraction study (Fig. 3). Upon dissolution
of the solid in acetonitrile, 1H, 13C and 11B NMR analysis initially
showed exclusively starting allylamine and free pinacol borane,
in agreement with a weak B…N interaction. Monitoring the
reaction by 1H NMR, showed complete formation of the
dehydrocoupled product 1g after 24 h (Fig. 4).10
1m: 95%
4 h
1n: 96%
0.5 h
9BBN
9BBN
9BBN
9BBN
NH
HN
N
4
N
H
O
1o: 97%
1p: 98%
1 h
1q: 97%
1 h
1r: 97%
24 h
1 h
Fig. 3. Solid-state structure of Fg at 50% probability thermal ellipsoids (C: grey, N:
blue, B: yellow, O: red). Selected bond length: N–B = 1.61(7) Å.
These observations confirm that the first step for
dehydrocoupling is the reversible formation of Fg. To further
substantiate this hypothesis, pinacol borane was added to
fluorene, which has a comparable Brønsted acidity to that of
amines, but (a) lacks the heteroatom coordination ability. No
reaction occurred regardless of time and temperature.
Similarly, hydrochloric acid failed to react with pinacol borane
to form chloropinacolborane. These results highlight the
intermediacy of the Lewis adduct in the dehydrocoupling
reaction. On the other hand, the stability of Fg in the solid
state led to the question of whether H2 elimination is an intra-
or intermolecular process. The latter could explain why the
dehydrogenative coupling of the very bulky Dipp–NH2 with
pinacolborane did not occur even under elevated
temperatures. Indeed, we found that addition of traces of
triethylamine11 allowed for this coupling, the borylated
product 1f being quantitatively formed after 24 hours at 120
oC (Fig. 2). These results suggest that, at least with this
sterically hindered substrate, the H2 elimination is an intermol-
Fig. 4
(bottom), 12 h (middle), and 24 h (top).
.
1H NMR spectra of allylamine and pinacol borane in acetonitrile at t0
2 | J. Name., 2012, 00, 1-3
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Fig. 6. Substrate scope for the dehydrocoupling of pinacol borane and 9-BBN
with thiols with conversions measured by 1H NMR (referencing residual starting
thiol). a1 mol% NEt3.
ecular process.
To extend the scope of the non-catalyzed dehydrocoupling
reaction, we then used a broad range of alcohols and phenols
as substrates. Under the standard experimental conditions
used for amines, we observed quantitative formation of the
desired coupling products 2a-k and 2m-w within minutes (Fig.
5). Note that here also, traces of triethylamine had to be
added when the bulky 2,6-di-tert-butylphenol was used (2l).
As demonstrated by Roesky and others, the
dehydrocoupling of thiols is more challenging than that of
amines and alcohols.5 We found that no reaction occurred
between thiols with pinacol borane even at 120 oC for 48
hours. However, addition of 1 mol% of triethylamine allowed a
complete conversion into product 3a and 3b after 48 h and 96
h, respectively. With 9-BBN, the reaction proceeds at room
temperature with dodecane thiol and at 60 oC for 2,6-
dimethylthiophenol, without the need for Et3N.
BPin
9BBN
S
S
BPin
9BBN
S
S
10
10
3a: 95%a
3b: 95%a
3c: 95%
16 h, RT
3d: 95%
24 h, 60 o
C
48 h, 120 oC
96 h, 120 oC
Herein we have disclosed the first extensive investigation into
the non-catalyzed dehydrocoupling of amines, alcohols, and
thiols with both pinacol borane and 9-BBN. Among the
possible applications of this process is the synthesis of BN
containing polymers.12 In addition, novel uses for these
dehydrocoupled products are currently being developed by us
and others. For example, Fernandez and coworkers13 have
shown amino- and thio- pinacol boranes to act as convenient
sources of E– with both ynones and Michael acceptors yielding
cis-1,4-addition products.
Fig. 5. Substrate scope for the dehydrocoupling of pinacol and 9-BBN with
alcohols and phenols at room temperature with isolated yields. a24 h at 120 oC
with 1 mol% NEt3.
BPin
BPin
BPin
BPin
Acknowledgements
O
O
O
O
tBu
Thanks are due to the DOE (DE-FG02-13ER16370) for financial
support of this work.
2a: 98%
2b: 96%
2c: 97%
2d: 97%
Notes and references
1
2
3
4
5
6
J. A. Fernandez-Salas, S. Manzini and S. P. Nolan, Chem.
Commun. 2013, 49, 5829.
T. Bolaño, M. A. Esteruelas, M. P. Gay, E. Oñate, I. M. Pastor
and M. Yus, Organometallics 2015, 34, 3902.
D. J. Liptrot, M. S. Hill, M. F. Mahon and A. S. Wilson, Angew.
Chem. Int. Ed. 2015, 54, 13362.
BPin
BPin
BPin
BPin
O
O
O
O
O
O
O
O
Cl
Cl
O
A. Harinath, S. Anga and T. K. Panda, RSC Adv. 2016,
35648.
6,
2e: 99%
2f: 99%
2g: 95%
2h: 99%
Z. Yang, M. Zhong, X. Ma, K. Nijesh, S. De, P. Parameswaran
and H. W. Roesky, J. Am. Chem. Soc. 2016, 138, 2548.
For reviews, see: (a) C. W. Hamilton, R. T. Baker, A. Staubitz
and I. Manners, Chem. Soc. Rev. 2009, 38, 279; (b) U. B.
Demirci and P. Miele, Energy Environ. Sci. 2009, 2, 627; (c) A.
BPin
BPin
BPin
BPin
O
O
O
O
tBu
tBu
Br
2i: 98%
Staubitz, A. P. M. Robertson and I. Manners, Chem. Rev.
2010, 110, 4079; (d) G. Alcaraz and S. Sabo-Etienne, Angew.
Chem. Int. Ed. 2010, 49, 7170; (e) M. Yadav and Q. Xu, Energy
2j: 95%
2k: 98%
2l: 98%a
Environ. Sci. 2012,
Energy Environ. Sci. 2012, 5
5
, 9698; (f) Z. G. Huang and T. Autrey,
, 9257; (g) F. H. Stephens, V. Pons
BPin
BPin
BPin
BPin
O
O
O
O
and R. T. Baker, Dalton Trans. 2007, 2613.
Cl
7
Rare examples of non-catalyzed dehydrogenative coupling of
pinacol borane with NuH bonds have been reported: (a) S.
M. Preshlock, D. L. Plattner, P. E. Maligres, S. W. Krska, R. E.
Maleczka Jr. and M. R. Smith III, Angew. Chem. Int. Ed. 2013,
52, 12915; (b) S. S. Barnes, C. M. Vogels, A. Decken and S. A.
Westcott, Dalton Trans. 2011, 40, 4707; (c) C. M. Vogels, P. E.
O’Connor, T. E. Phillips, K. J. Watson, M. P. Shaver, P. G.
Hayes and S. A. Westcott, Can. J. Chem. 2001, 79, 1898.
Reagents were purchased from commercial suppliers and
used without further purification or drying. Brand new
glassware and magnetic stir bars were used to eliminate
potential traces of metal.
2m: 99%
2n: 99%
2o: 98%
2p: 96%
BPin
BPin
BPin
O
O
O
CF3
2q: 95%
9BBN
2r: 95%
9BBN
2s: 96%
9BBN
8
9
9BBN
O
O
O
O
Detailed kinetic studies showing the role of the
concentration can be found in Supporting information, Fig.
S1.
2t: 96%
2u: 94%
2v: 97%
2w: 95%
10 The increased reaction time can be attributed to the
marginal solubility of Fg
.
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11 Et3N has also been postulated to enhance the hydridic
character of boranes: P. Eisenberger, A. M. Bailey and C. M.
Crudden, J. Am. Chem. Soc. 2012, 134, 17384.
12 (a) T. Lorenz, A. Lik, F. A. Plamper and H. Helten, Angew.
Chem. Int. Ed. 2016, 55, 7236. (b) A. Ledoux, P. Larini, C.
Boisson, V. Monteil, J. Raynaud and E. Lacote, Angew. Chem.
Int. Ed. 2015, 54, 15744.
13 (a) C. Solé and E. Fernández, Angew. Chem. Int. Ed. 2013, 52
11351-11355; (b) M. G. Civit, X. Sanz, C. M. Vogels, C. Bo, S.
A. Wescott and E. Fernández, Adv. Synth. Catal. 2015, 357
,
,
3098.
4 | J. Name., 2012, 00, 1-3
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