Transfer hydrogenation of unfunctionalised alkenes using N-heterocyclic carbene ruthenium catalyst precursors

Article abstract of DOI:10.1039/c1cc12923f

Transfer hydrogenation of unfunctionalised and aliphatic alkenes in iPrOH/KOH is efficiently catalysed by an olefin-tethered N-heterocyclic carbene ruthenium complex, which also catalyses double bond migration as a competitive and considerably faster process.

Full text of DOI:10.1039/c1cc12923f

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COMMUNICATION
Transfer hydrogenation of unfunctionalised alkenes using N-heterocyclic
carbene ruthenium catalyst precursors
Sabine Horn and Martin Albrecht*
Received 18th May 2011, Accepted 17th June 2011
DOI: 10.1039/c1cc12923f
Transfer hydrogenation of unfunctionalised and aliphatic
alkenes in iPrOH/KOH is efficiently catalysed by an olefin-
tethered N-heterocyclic carbene ruthenium complex, which also
catalyses double bond migration as a competitive and considerably
faster process.
Fig. 1 Catalyst precursors for the transfer hydrogenation of olefins.
Homogeneous hydrogenation of olefins is classically performed
by direct hydrogenation using molecular H2.1 In contrast,
comprised predominantly of isomeric dodecene mixtures (76%)
transfer hydrogenation of olefins from an immobilised hydrogen
and dodecane (24%). Prolonged heating gave only a slight
source2 is rare,3,4 and involves in most cases a heterogeneous
increase of hydrogenated product (32% dodecane after 24 h,
catalytic phase.4 The low abundance of transfer hydrogenation
Table 1). The activity of complex 4 was similar to that of 3 after
for olefin reduction appears rather surprising when considering
5 h (37% hydrogenated product, 63% dodecenes), yet hydro-
that the catalytically active species for direct hydrogenation and
genation continued and reached 79% after 24 h. Significant
transfer hydrogenation are closely related. Both methods require
higher transfer activity was observed for the olefin-functionalised
a metal-dihydride species, or a monohydride complex when a
NHC complex 1, which induced consumption of all starting
ligand or extraneous auxiliary is assisting the H2 or hydrogen-
material within 30 min and complete hydrogenation within 24 h.
donor activation.1,2 A distinct number of complexes are indeed
These initial studies suggested that coordinative lability of
known to catalyse hydrogenations both via transfer or direct
one ligand (site) is important to induce catalytic activity. This
hydrogenation.5 However, these systems are often limited
hypothesis is supported by the results from catalytic runs
to polarised substrates,6 or display only low activity towards
performed with complex 4 after activation with one mol equiv.
olefins.7
AgBF4 in order to abstract one chloride ligand from the
precursor.9,10z Transfer hydrogenation under these conditions
We have recently observed that ruthenium complexes
comprising a chelating N-heterocyclic carbene (NHC) ligand
was significantly accelerated, reaching 81% dodecane formation
are catalyst precursors both for the direct hydrogenation of
olefins8 as well as for the transfer hydrogenation of ketones
after 5 h (cf. 61% with 1). The increased catalytic activity
is also reflected by the higher turnover frequency at 50%
and polarised CQC bonds,9 indicating that the catalytically
active species may be accessible either via H2 activation or via
iPrOH activation, and that this species is able to hydrogenate
both styrene (established for direct hydrogenation) or ketones
(via hydrogen transfer). Here we report on a combination
of these concepts and demonstrate the effective transfer hydro-
genation of unactivated and aliphatic olefins.
Complexes 1–48 (Fig. 1) were evaluated as catalyst precursor
(1 mol% loading) in the transfer hydrogenation of 1-dodecene as
a model substrate of an unfunctionalised and unactivated alkene,
using classical2 hydrogen transfer conditions (KOH as base,
iPrOH as solvent and hydrogen source, Table 1).w The four
catalyst precursors displayed strongly diverging behaviour.
Complex 2 was completely inactive. When using complex 3, all
starting material was consumed within 5 h and the solution was
conversion, TOF50 = 12 hꢀ1 for 1 and 19 hꢀ1 for activated
4. However, the catalyst robustness deteriorated and the
hydrogenation ceased after ca. 90% conversion, while complex
1 showed prolonged activity and reached full conversion.
Table 1 Catalytic transfer hydrogenation of 1-dodecenea
% Yield (dodecane/dodecenes)b
0.5 h
2 h
5 h
24 h
Complex
1
2
3
4
100 (9/91) 100 (25/75) 100 (61/39) 100 (100/0)
0
50 (11/39) 87 (23/64) 100 (24/76) 100 (32/68)
67 (7/60) 95 (16/79) 100 (37/63) 100 (79/21)
(0/0) 0 (0/0) 0 (0/0) 0 (0/0)
4 + BF4 95 (20/75) 100 (42/58) 100 (81/19) 100 (89/11)
a
General conditions: 1-dodecene (2.0 mmol), KOH (0.2 mmol),
complex (20 mmol; S/B/C 100 : 10 : 1), and 3,5-dimethylanisole
b
(80 mL, internal standard) in iPrOH (10 mL), 80 1C (reflux). conversion
determined by 1H NMR spectroscopy and GC-MS, dodecenes are
mixtures of isomers.
School of Chemistry & Chemical Biology, University College Dublin,
Belfield, Dublin 4, Ireland. E-mail: martin.albrecht@ucd.ie;
Fax: +353 1716 2501; Tel: +353 1716 2504
c
8802 Chem. Commun., 2011, 47, 8802–8804
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Catalyst robustness may be enhanced by the presence of the
hemilabile olefin group, though it remains limited. Upon
reducing the loading of complex 1 from 1 mol% to 0.1 mol%,
activity is preserved for the first 5 h (51% hydrogenated product
formed), though drops substantially after that time and resulted
in a mere 59% conversion to dodecane after 24 h, corresponding
to a total turnover number of 590. Efficient hydrogenation
is evidently compromised by a latent catalyst instability,
presumably associated with slow hydrogenation of the olefin
wingtip group of the NHC ligand.
50 mol equiv. coe confirmed the absence of any discrimination
of these two substrates, and both cyclooctane and dodecane were
produced at essentially identical rates as in independent runs
using pure substrates. Apparently, ruthenium coordination by
cis-olefins (cf. coe) is not significantly different from coordination
by dodecene,12 which may occur through a cis-, trans-, or
terminal olefin due to the high isomerisation activity of
the ruthenium catalyst. In line, internal linear olefins were
equally susceptible to transfer hydrogenation (entry 8). Dienes
were converted less cleanly. For example, hydrogenation
of a,o-octadiene produced 54% monohydrogenated octene
and 15% fully hydrogenated octane (84 turnovers) after 24 h
(entry 9), isomerisation was again considerably faster (31%
conversion after 10 min).
The scope and limitation of this transfer hydrogenation was
examined by using complex 1 as catalyst precursor for the
hydrogenation of different olefin substrates (Table 2). Styrene
was converted to ethylbenzene in moderate 30% after 24 h.
Di-arylated olefins were less reactive and both cis- and trans-
stilbene were hydrogenatated to diphenylethane only in trace
amounts (entries 3, 4). The main process with cis-stilbene was the
expected isomerisation to the thermodynamically more stable
trans-isomer (42% after 0.5 h).11 Allylbenzene, and in particular
b-methylstyrene were significantly faster converted, providing
48% and 92% propylbenzene, respectively (entries 5, 6).y
Cyclooctene (coe) was used as a substrate to probe the preference
for terminal vs. internal olefins (entry 7). Hydrogenation rates
to cyclooctane were comparable to those of dodecene. A
competition experiment using 50 mol equiv. dodecene and
Limited reactivity was observed in the transfer hydrogenation
of alkynes. Phenylacetylene, and terminal or internal aliphatic
alkynes were converted only in minor quantities (entries 10–12).
Interestingly, in all cases semi-hydrogenation occurred, perhaps
due to tighter bonding of alkynes as opposed to alkenes, and the
corresponding olefins were the only observed products.13
The catalytic species remains active upon repetitive addition
of 1-dodecene. Addition of new substrate (100 mol equiv.)
after 2, 4, and 6 h indicated full consumption of the substrate
within the subsequent 2 h (Fig. 2). However, the relative ratio
of isomerised vs. hydrogenated product gradually increased
from 3 : 2 after converting the first batch to 3 : 1 after the third
batch, and 18 h reaction time were required to achieve the
initial 3 : 2 ratio after adding the forth batch of substrate
(172 TONs). The continuous decrease of transfer hydrogenation
activity (total TONs are 41, 74, and 78 after 2, 4, and 6 h,
respectively) indicates a limited stability of the catalytically
active species, thus corroborating the results obtained from
catalytic runs with lowered catalyst loading (vide supra).
Further mechanistic insights were obtained from experiments
in perdeuterated isopropanol (iPrOD–D8) as solvent. The
formed dodecane contained deuterium in both the terminal
and internal positions (2 : 7 integral ratio in the 2H NMR
spectrum). This ratio suggests rapid isomerisation, presumably
via a p-allylic mechanism rather than a 2,1-alkene insertion/
b-H elimination process, in which a ruthenium-bound
hydride rapidly undergoes H/D exchange with iPrOD–D8.14
Complementary analyses by HRMS revealed a multitude of
dodecane isotopes, ranging from monodeuterated dodecane to
dodecane–D19.
Table 2 Transfer hydrogenation of different olefins using complex 1a
Entry
Substrate
Product
Conversion (24 h)
100%
1
2
30%
3
7%
4
5
6
7%
48%
92%
Catalytic runs in the presence of mercury provided ambivalent
results.15 Addition of a large excess (350 mol equiv.) of Hg0 to
the catalytic mixture after 10 min reaction time, when the
reaction mixture comprised 74% 1-dodecene, 23% isomerised
dodecenes, and 3% hydrogenated dodecane,16 did not stop the
consumption of the starting material, yet decelerated transfer
hydrogenation substantially. After 5 h, only 23% dodecane
was formed and 34% after 24 h (cf. 61% and 100%,
respectively, in the absence of mercury). Hydrogen transfer
was thus ongoing, albeit much slower. Isomerisation was also
affected, yet not inhibited, by the presence of mercury, and
almost 5 h were required for the complete isomerisation of
1-dodecene to internal olefins. While mass transport limitations
may be effective, we cannot rule out the presence of different
mechanisms for the olefin transfer hydrogenation. For example,
7
100%
8
9
84%
15%b
10
17%
11
12
a
9%
21%c
b
General conditions identical to Table 1. 54% monohydrogenated
c
octene formed as mixture of isomers. and other isomers of octene.
c
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z A procedure was used as described above, except for adding AgBF4
(3.9 mg, 20 mmol) to the light-protected solution of complex 4 and
stirring for 2 min before adding the base and the substrate.
y Since for allylbenzene, isomerisation was again a competing and
much faster process than hydrogenation (after 10 min, approximately
50% conversion to b-methylstyrene and 6% hydrogenated product
was observed), we would have expected similar rates for the transfer
hydrogenation of allylbenzene and b-methylstyrene, however, the
former was reproducibly converted at slower rates.
1 Handbook of Homogeneous Hydrogenation, ed. J. G. de Vries and
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J. A. Peters, in ref. 1, vol. 2, p. 585; S. Gladiali and E. Alberico,
Fig. 2 Composition of reaction mixtures of multi-batch experiments
with complex 1 before introducing additional batches of substrate
after 2, 4, and 6 h (4 batches added in total) revealing full conversion
of each batch, yet gradual decrease of hydrogenation activity.
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mercury, thus rationalising the slower product formation.
Previous experiments under identical reaction conditions using
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In conclusion, transfer hydrogenation of unfunctionalised
alkenes was accomplished using NHC ruthenium complexes.
The substitution pattern at the NHC ligand plays a critical
role, and highest activity as well as sufficient robustness was
achieved with a potentially hemilabile olefin as chelating
wingtip group. Olefin isomerisation is a significantly faster
process and presumably facilitates the hydrogenation of
internal alkenes via double bond migration to terminal positions.
A limitation of the NHC ruthenium complexes constitutes the
stability of the catalytically active species, which prevents
conversions at low catalyst loading and restricts catalytic
activity upon repetitive substrate addition. Appropriate
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The authors thank Dr C. Gandolfi for preliminary
measurements, Prof S. Gladiali for stimulating discussions,
and the European Research Council (ERC-StG 208561) and
COST Action D40 for financial support.
Notes and references
w Representative catalytic procedure: A 25 mL oven-dried Schlenk-
tube was charged under N2 with anhydrous iPrOH (10 mL). The
solvent was degassed via 3 freeze-pump-thaw cycles and the ruthenium
complex (20 mmol) was added and dissolved by sonication (10 min,
40 1C). Then KOH (0.1 mL, 2M in H2O, 0.2 mmol) was introduced
and the mixture pre-heated to 90 1C for 10 min before the substrate
(2.0 mmol) and 3,5-dimethylanisole (80 mL, 0.6 mmol as internal
standard) were added. Aliquots (0.2 mL) were taken at fixed times,
quenched with pentane (1 mL), and filtered through a short pad of
silica. The silica was washed with Et2O (2 mL) and the combined
organic filtrates were analysed by GC-MS and, after careful evaporation,
by 1H NMR spectroscopy.
c
8804 Chem. Commun., 2011, 47, 8802–8804
This journal is The Royal Society of Chemistry 2011