Tandem ammonia borane dehydrogenation/alkene hydrogenation mediated by [Pd(NHC)(PR3)] (NHC = N-heterocyclic carbene) catalysts

Article abstract of DOI:10.1039/c2cc38145a

[Pd(NHC)(PR₃)] complexes were shown to be active catalysts in the dehydrogenation of ammonia borane and the subsequent hydrogenation of unsaturated compounds at very low catalyst loadings (0.05 mol% for some substrates). The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cc38145a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Tandem ammonia borane dehydrogenation/alkene
hydrogenation mediated by [Pd(NHC)(PR3)]
(NHC = N-heterocyclic carbene) catalysts†
Cite this: Chem. Commun., 2013,
49, 1005
Received 11th November 2012,
Accepted 12th December 2012
a
ab
Caroline E. Hartmann, Vaclav Jurcık, Olivier Songisb and Catherine S. J. Cazin*ab
´
ˇ´
DOI: 10.1039/c2cc38145a
www.rsc.org/chemcomm
[Pd(NHC)(PR3)] complexes were shown to be active catalysts in the
dehydrogenation of ammonia borane and the subsequent hydro-
genation of unsaturated compounds at very low catalyst loadings
(0.05 mol% for some substrates).
Catalytic hydrogenation of C–C multiple bonds is one of the
most important processes in organic chemistry.1 Associated
with this process is the use of hydrogen gas whose physical
properties complicate its safe, efficient and economical
storage.2 As the need for an easily transportable source of
hydrogen has attracted considerable attention in the context
of novel environmentally benign energy sources,3 we hypothe-
Scheme 1 Formation of the dihydride complex 4.
sised that an opportunity existed to employ one of these [Pd(NHC)(PR3)] complexes could react with NH3BH3 to, in a
‘‘hydrogen storage materials’’4 as a readily available hydrogen first instance, generate H2 and subsequently, in a tandem
source. We reasoned that such a material could be used as a process, use this H2 to perform hydrogenation of C–C multiple
chemical reagent in catalysis.
bonds.
In initial reactions, [Pd(IPr)(PCy3)] 1 (IPr = N,N0-bis-(2,6-di-
The level of research activity to identify a material that could
deliver hydrogen on demand has been high in recent years.2,5,6 iso-propylphenyl)imidazol-2-ylidene) was mixed with morpholine-
In this context and in efforts to look ahead where alternatives borane 2 in C6D6 and resulted in an immediate gas release. The
to petroleum-based fuels will be required, ammonia borane identity of the gas as H2 was confirmed by 1H NMR analysis of
(H3N-BH3, AB) appears attractive in view of its high thermal the reaction headspace. Hydrogen release was accompanied by
stability and hydrogen content (19.6% by weight).7 Recent the formation of morpholinoborane 3. When the experiment
reports have focused on several aspects of AB as a hydrogen was performed in a sealed NMR-tube, quantitative formation of
source and the means to control its H2 release.8 A small the dihydride complex, [Pd(H)2(IPr)(PCy3)] 4 was observed after
number of studies have reported application of AB-congeners several days (Scheme 1).11
in hydrogenation reactions catalyzed by heterogeneous Pd/C
and Ra–Ni9 and limited homogeneous hydrogenation of [Pd(IPr)(PCy3)] 1 to mediate the catalytic release of dihydrogen
ketones using Ru.10
from NH3BH3. As a negligible amount of gas was released at
These results encouraged us to test the capability of
We have recently described activation of H2 by [Pd(NHC)(PCy3)] room temperature, solutions of NH3BH3 in various solvents
(NHC = N-heterocyclic carbene)11 and applications of these were heated to 50 1C in the presence of 1 (1 mol%). A closed
systems in the catalytic hydrogenation of alkenes.12,13 flask equipped with a pressure sensor permitted the moni-
This initial reactivity encouraged us to explore whether these toring, in a quantitative manner, of the hydrogen volume
generated as a function of time and solvent. Reactions in
a
¨
Institute of Chemical Research of Catalonia (ICIQ), Av. Paısos Catalans 16,
hexane did not result in any gas release while there was slow
H2 release in THF and toluene. Optimum results were obtained
when reactions were performed in isopropanol.14
Since [Pd(NHC)(PR3)] complexes are known to be highly
efficient catalysts for hydrogenation reactions,12 we reasoned
that a tandem reaction could possibly be established if the
43007 Tarragona, Spain
b EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST,
UK. E-mail: cc111@st-andrews.ac.uk; Fax: +44 (0)1334 463808
† Electronic supplementary information (ESI) available: Experimental details,
kinetic profiles, dehydrogenation experiments and NMR spectra. See DOI:
10.1039/c2cc38145a
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1005--1007 1005

View Article Online
Communication
ChemComm
Table 1 Solvent and catalyst screeninga
Table 2 Substrate scope of hydrogenation using NH3BH3 as a hydrogen sourcea
Entry Catalyst
Loading (mol%) Solvent
Conv.b (%)
1
2
3
4
5
6
7
8
[Pd(IPr)(PCy3)] 1
1
1
1
1
0.05
0.05
—
0.05
0.05
0.05
THF
Toluene o5
o5
Entry Substrate
1
Product
Loading (mol%) Yieldb (%)
[Pd(IPr)(PCy3)] 1
[Pd(IPr)(PCy3)] 1
[Pd(IPr)(PCy3)] 1
[Pd(IPr)(PCy3)] 1
[Pd(SIPr)(PCy3)] 5
Without Pd
CH2Cl2
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
o5c
95
88d
99
0.05
0.05
0.05
>99
88
N.R.
o5
o5
25
60
N.R.
[Pd(IPr)(PPh3)] 6
[Pd(SIPr)(PPh3)] 7
[Pd(IPr)2] 8
2
3
9
10
11
12
[Pd(IPr)(PAd2nBu)] 9 0.05
>99
Pd/C 10
0.05
a
Reaction conditions: trans-stilbene (0.5 mmol), catalyst, NH3BH3
b
4
5
6
0.05
0.05
0.25
>99c
78
(1 eq.), solvent (1 mL), 50 1C, 16 h. Conversion, determined by GC,
average of at least two runs. Reaction carried out at 40 1C. No
c
d
reaction occurred at rt.
reaction medium contained an alkene. The hydrogenation of
trans-stilbene was selected as a model reaction. Table 1 pre-
sents some of the most salient results of this model tandem
NH3BH3 dehydrogenation/alkene hydrogenation sequence.15
As was observed for the NH3BH3 dehydrogenation reaction,
alcohol solvents lead to highest conversions (Table 1,
entries 1–4). Control reactions (Table 1, entries 7 and 12)
without palladium and with palladium on charcoal led to no
conversion. A series of mixed NHC/phosphine were then tested
(Table 1, entries 5, 6 and 8–11). The complex [Pd(SIPr)(PCy3)] 5
(SIPr = N,N0-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene),
was proven to be the most effective at lower catalyst loadings
(Table 1, entries 5 and 6). Although [Pd(SIPr)(PCy3)] 5 performs
best, the trend emerging from Table 1 clearly shows that bulky
electron-rich mixed systems are capable of achieving significant
conversions (Table 1, entries 10 and 11) while less electron-rich
complexes (Table 1, entries 8 and 9) fail in the tandem
transformation.
78c
7
0.25
98
8
9
0.25
0.25
97c
79c
10
11
12
1
>99c,d
80c
0.5
0.5
69c
13
14
0.05
0.25
88
75c,e
As NH3BH3 can liberate up to 3 equivalent of H2, the amount
of NH3BH3 necessary for an efficient transformation was varied
to gauge the effectiveness of the NH3BH3 dehydrogenation
reaction. When the reaction was performed with 1 equivalent
of ammonia-borane and 6-fold excess of 4-allylanisole, a 53%
yield of the hydrogenated product was obtained which corre-
sponds to 3 equivalents of H2 from one molecule of ammonium
a
Reaction conditions: substrate (0.5 mmol), [Pd(SIPr)(PCy3)] 5, NH3-BH3
i
b
(0.5 eq.), PrOH (1 mL), 50 1C, 16 h. Isolated yield, average of two
reactions. Yield determined by 1H NMR using 1,3,5-trimethoxybenzene
c
d
or 4-bromoanisole as internal standard. Using MeOD as solvent.
e
Using 0.33 eq. NH3BH3.
borane. This is in agreement with the expected transformation case of methylcinnamate, the CQC bond was selectively
of NH3BH3 to the corresponding ammonium tetraalkoxyborate reduced (Table 2, entry 6), while with chalcone and cyclo-
and 3 equivalent of dihydrogen.2b Further NMR experiments hexenone quantitative yields of completely hydrogenated products
aiming to clarify the fate of AB indicate that NH3BH3 is directly were obtained (Table 2, entries 7 and 8). Selective reduction of
converted to borate.2b Kinetic profiling of the oxidation of 1,4-cyclooctadiene to cyclooctene was achieved in 79% yield in
borane to borate corresponds to the conversion of 4-allylanisole isopropanol (Table 2, entry 9), while in methanol complete
to 4-propylanisole.12
reduction of the diene to cyclooctane was achieved (Table 2,
To examine the substrate scope of this tandem reaction, a entry 10). Finally, we were interested to test our system for the
number of alkenes were subjected to the optimised reaction semi-reduction of internal alkynes. Diphenylethyne was semi-
conditions. A range of olefins could be hydrogenated efficiently reduced to cis-stilbene in high yield using a catalyst loading of
at 0.05 mol% catalyst loading (Table 2, entries 1–5), whereas only 0.05 mol% (Table 2, entry 13) while dimethylhexynediol
with less reactive substrates 0.25–1 mol% of 5 was used. In the was semi-reduced using 0.33 eq. of NH3BH3 (Table 2, entry 14).
c
1006 Chem. Commun., 2013, 49, 1005--1007
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
(d) C. W. Yoon and L. G. Sneddon, J. Am. Chem. Soc., 2006,
128, 13992; (e) F. Cheng, H. Ma, Y. Li and J. Chen, Inorg. Chem.,
2007, 46, 788; ( f ) M. T. Nguyen, V. S. Nguyen, M. H. Matus,
G. Gopakumar and D. A. Dixon, J. Phys. Chem. A, 2007, 111, 679;
(g) P. V. Ramachandran and P. D. Gagare, Inorg. Chem., 2007,
46, 7810.
6 (a) T. J. Clark, C. A. Russell and I. Manners, J. Am. Chem. Soc., 2006,
128, 9582; (b) M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey
and K. L. Goldberg, J. Am. Chem. Soc., 2006, 128, 12048;
(c) R. J. Keaton, J. M. Blacquiere and R. T. Baker, J. Am. Chem.
Soc., 2007, 129, 1844; (d) T. J. Clark, G. R. Whittell and I. Manners,
Inorg. Chem., 2007, 46, 7522; (e) P. M. Zimmerman, A. Paul, Z. Zhang
and C. B. Musgrave, Angew. Chem., Int. Ed., 2009, 48, 2201;
( f ) G. Alcaraz, L. Vendier and S. Sabo-Etienne, Angew. Chem.,
Int. Ed., 2010, 49, 918.
A highly efficient [Pd(NHC)(PR3)] catalytic system for the
tandem dehydrogenation of AB coupled with the hydrogenation
of alkenes and alkynes has been developed which proves
effective for a wide array of alkenes. The optimised protocol
makes use of alcohol solvents and very low catalyst loadings
(0.05–0.25 mol%). The efficient hydrogenation of alkene/alkyne
substrates using solid NH3BH3 in place of H2 presents
an improvement in terms of safety and logistics with the
removal of the risks inherent to the transport and manipulation
of an explosive compressed gas. Further studies aimed at
testing the generality of this tandem reaction and demon-
strating the usefulness of AB as a convenient H2 delivery agent
in chemical transformations are presently ongoing in our
laboratories.
We thank the EPSRC and the Royal Society (University
Research Fellowship to CSJC), the German Federal Excellence
Initiative (FYS grant to C.H.), the Karlsruhe House of Young
Scientists (KHYS) from the Karlsruhe Institute of Technology
(grant to C.H.) and the Deutscher Akademischer Austauschdienst
(German Academic Exchange Service, grant to C.H.) for support.
7 (a) US DOE ‘‘Basic Research Needs for the Hydrogen Economy’’
(http://www.sc.doe.gov/bes/reports/files/NHE_rpt.pdf);
(b) D. A. Dixon and M. Gutowski, J. Phys. Chem. A, 2005, 109, 5129.
8 (a) J. S. Wang and R. A. Geanangel, Inorg. Chim. Acta, 1988, 148, 185;
(b) M. E. Blumh, M. G. Bradley, R. Butterick, U. Kusari and
L. G. Sneddon, J. Am. Chem. Soc., 2006, 128, 7748; (c) G. Wolf,
J. Baumann, F. Baitalow and F. P. Hoffmann, Thermochim. Acta,
2000, 343, 19; (d) F. Baitalow, J. Baumann, G. Wolf, K. Jaenicke-
Rossler and G. Leitner, Thermochim. Acta, 2002, 391, 159; (e) C. A.
Jaska, K. Temple, A. J. Lough and I. Manners, J. Am. Chem. Soc.,
2003, 125, 9424; ( f ) F. H. Stephens, R. T. Baker, M. H. Matus,
D. J. Grant and D. A. Dixon, Angew. Chem., Int. Ed., 2007, 46, 746.
´
9 (a) M. Couturier, J. L. Tucker, B. M. Andresen, P. Dube and J. T.
Negri, Org. Lett., 2001, 3, 465; (b) M. Couturier, B. M. Andresen,
Notes and references
´
J. L. Tucker, P. Dube, S. J. Brenek and J. T. Negri, Tetrahedron Lett.,
1 J. G. de Vries and C. J. Elsevier, Handbook of Homogeneous
Hydrogenation, Wiley-VCH, Weinheim, 2007.
2 For recent reviews on hydrogen storage materials for automotive
2001, 42, 2763; (c) M. Couturier, B. M. Andresen, J. L. Tucker,
´
P. Dube, S. Breneck and J. T. Negri, Tetrahedron Lett., 2001, 42,
2285.
applications, see: (a) J. Yang, A. Sudik, C. Wolverton and D. J. Siegel, 10 N. Blaquiere, S. Diallo-Garcia, S. I. Gorelsky, D. A. Black and
Chem. Soc. Rev., 2010, 39, 656; (b) C. W. Hamilton, R. T. Baker, K. Fagnou, J. Am. Chem. Soc., 2008, 130, 14034.
ˇ´
A. Staubitz and I. Manners, Chem. Soc. Rev., 2009, 38, 279; 11 S. Fantasia, J. D. Egbert, V. Jurcık, C. S. J. Cazin, H. Jacobsen,
´
(c) A. W. C. van der Berg and C. O. Arean, Chem. Commun., 2008,
L. Cavallo, D. M. Heinekey and S. P. Nolan, Angew. Chem., Int. Ed.,
2009, 48, 5182.
668; (d) T. B. Marder, Angew. Chem., Int. Ed., 2007, 46, 8116.
ˇ´
a recent discussion on this topic see: T. W. Graham, 12 V. Jurcık, S. P. Nolan and C. S. J. Cazin, Chem.–Eur. J., 2009, 15,
3 For
C.-W. Tsang, X. Chen, R. Guo, W. Jia, S.-M. Lu, C. Sui-Seng, 2509.
C. B. Ewart, A. Lough, D. Amoroso and K. Abdur-Rashid, Angew. 13 For a recent review on NHC-based catalysts in hydrogenation and
Chem., Int. Ed., 2010, 49, 8708.
transfer hydrogenation, see N-Heterocyclic Carbene Complexes in
Additions to Multiple bonds, A. A. Danopoulos, in N-Heterocyclic
Carbenes in Transition Metal Catalysis and Organocatalysis,
ed. C. S. J. Cazin, Springer, London, 2011, vol. 32, p. 24.
4 (a) R. Coontz and B. Hanson, Science, 2004, 305, 957; (b) G. W.
Crabtree and M. S. Dresselhaus, MRS Bull., 2008, 33, 421;
(c) I. P. Jain, P. Jain and A. Jain, J. Alloys Compd., 2010, 503, 303.
5 (a) M. Chandra and Q. J. Xu, J. Power Sources, 2006, 156, 190; 14 For details on the dehydrogenation of NH3BH3 using 1, see ESI,†
(b) M. Chandra and Q. J. Xu, J. Power Sources, 2006, 159, 855; section 2.
(c) Q. J. Xu and M. Chandra, J. Power Sources, 2006, 163, 364; 15 For the full optimization study, see ESI†.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1005--1007 1007