Transfer hydrogenation of carbonyl compounds and alkenes catalyzed by ruthenium(ii)-n-heterocycle carbene complexes

Article abstract of DOI:10.1135/cccc20071037

The Ru(II)-N-heterocycle carbene complexes [RuCl₂(η ⁶-p-cymene)L] (L = 1-butyl-3-methylimidazol-2-ylidene) and [RuCl(η⁶-p-cymene)L(pta)]Cl (pta = 1,3,5-triaza-7- phosphaadamantane) showed excellent catalytic activities (with turnover frequencies up to 1116 h^-1) in the hydrogen transfer reduction of cinnamaldehyde and several ketones using propan-2-ol/KOH as a H-donor. Similar hydrogenations of trans-stilbene and cyclohexene were characterized by low conversions. The hydrogenation of 4-phenylbut-3-en-2-one and cinnamaldehyde proceeded with moderate selectivities of the formation of the saturated alcohol or of the of C=C hydrogenation (giving saturated ketone or saturated aldehyde). In the case of cinnamaldehyde, the unsaturated alcohol is initially formed; however, subsequent redox isomerization to the saturated aldehyde with the same catalyst diminishes its yield. The hydrogen transfer from formate to 4-phenylbut-3-en-2-one in an aqueous-organic two-phase mixture was also demonstrated.

Full text of DOI:10.1135/cccc20071037

Ru(II)–N-Heterocycle Carbene Complexes
1037
TRANSFER HYDROGENATION OF CARBONYL COMPOUNDS AND
ALKENES CATALYZED BY RUTHENIUM(II)–N-HETEROCYCLE
CARBENE COMPLEXES
a
b,
Marianna FEKETE and Ferenc JOÓ *
a
Research Group of Homogeneous Catalysis, Hungarian Academy of Sciences,
Debrecen, H-4010 Hungary; e-mail: fmariann@delfin.unideb.hu
Institute of Physical Chemistry, University of Debrecen,
b
Debrecen 10, P.O. Box 7, H-4010 Hungary; e-mail: joofer49@yahoo.com
Received April 16, 2007
Accepted Jun e 26, 2007
Dedicated to Professor Štefan Toma on the occasion of his 70th birthday in recognition of his
outstanding contributions to organic synthesis and transition metal catalysis.
6
Th e Ru(II)–N-h eterocycle carben e com plexes [RuCl (η -p-cym en e)L] (L = 1-butyl-3-m eth ylim id-
2
6
azol-2-yliden e) an d [RuCl(η -p-cym en e)L(pta)]Cl (pta = 1,3,5-triaza-7-ph osph aadam an tan e)
–
1
sh owed excellen t catalytic activities (with turn over frequen cies up to 1116 h ) in th e h y-
drogen tran sfer reduction of cin n am aldeh yde an d several keton es usin g propan -2-ol/KOH as
a H-don or. Sim ilar h ydrogen ation s of trans-stilben e an d cycloh exen e were ch aracterized by
low con version s. Th e h ydrogen ation of 4-ph en ylbut-3-en -2-on e an d cin n am aldeh yde pro-
ceeded with m oderate selectivities of th e form ation of th e saturated alcoh ol or of th e of C=C
h ydrogen ation (givin g saturated keton e or saturated aldeh yde). In th e case of cin n am -
aldeh yde, th e un saturated alcoh ol is in itially form ed; h owever, subsequen t redox isom eriz-
ation to th e saturated aldeh yde with th e sam e catalyst dim in ish es its yield. Th e h ydrogen
tran sfer from form ate to 4-ph en ylbut-3-en -2-on e in an aqueous-organ ic two-ph ase m ixture
was also dem on strated.
Keyw ord s: Aldeh ydes; Carben e com plexes; Hydrogen tran sfer; Keton es; Ruth en ium ; Cata-
lytic h ydrogen ation ; Reduction s.
Hydrogen ation of un saturated substrates by h ydrogen tran sfer from suit-
1
able H-don ors (DH ) is a valuable altern ative to th e use of dih ydrogen . Th e
2
m ost often em ployed H-don ors are propan -2-ol, form ic acid (or its azeo-
trope with trieth ylam in e) an d aqueous solution s of form ates. All th ese re-
agen ts are liquids, easy to h an dle; th e reaction con dition s are gen erally
m ild. Several excellen t reviews2 on th is topic are available, describin g a
wide variety of reaction s perform ed with h eterogen eous or h om ogen eous
catalysts. Am on g th e latter, ruth en ium (II) com plexes stan d out with th eir
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 8, pp. 1037–1045
©
2007 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc20071037

1
038
Fekete, Joó:
catalytic activity an d selectivity. In certain cases, proch iral substrates, such
as acetoph en on e were h ydrogen ated with 100% con version an d 99% e.e.
by h ydrogen tran sfer from aqueous form ate in th e presen ce of Ru(II) com -
plexes with approriate ch iral ligan ds, successful exam ples bein g th e m on o-
tosylated N,N′-diph en yleth ylen ediam in e an d th e m on otosylated cyclo-
2
a,3
h exan e-1,2-diam in e
.
Very im portan tly, th e h ydrogen tran sfer reduction s are n ot sim ple repro-
duction s of h ydrogen ation s usin g H ; in several cases differen t selectivities
2
h ave been foun d. For exam ple, un saturated aldeh ydes, such as cin n am -
aldeh yde are h ydrogen ated at th eir C=C bon ds un der catalysis with [RuCl2-
(
PPh ) ]. Con versely, aldeh ydes are easily h ydrogen ated to alcoh ols in th e
3 3
presen ce of th e sam e catalyst by a biph asic h ydrogen tran sfer from aqueous
4
form ate , an d cin n am aldeh yde was selectively reduced to cin n am yl alcoh ol
with aqueous Na form ate wh en th e water-soluble an alog of th e above cata-
lyst, [{RuCl (mtppm s) } ] was used (mtppm s = Na-3-(diph en ylph osph an yl)-
2
2 2
5
ben zen e-1-sulfon ate) . Th e differen ces between h ydrogen ation s with H2
an d DH origin ate from th e form ation of differen t h ydride species in th e re-
2
action s of th e sam e catalyst precursor an d H or DH an d due to differen t
2
2
m ech an ism s of th e two reaction types5
c–5e
.
N-h eterocycle carben es (NHC) are n ow widely used in h om ogen eous ca-
6
talysis . Usually, th e tran sition m etal com plexes of th ese ligan ds are robust
an d with stan d decom position un der catalytic con dition s better th an com -
plexes of tertiary ph osph in es. As part of our earlier studies, we h ave syn th e-
7
6
sized th e h alf-san dwich Ru–NHC com plex, [RuCl (η -p-cym en e)L] (1) (L =
2
1
-butyl-3-m eth ylim idazol-2-yliden e) from th e easily available [{RuCl -
2
6
(
η -p-cym en e)} ] by tran sm etallation with th e silver carben e com plex ob-
2
tain ed from 1-butyl-3-m eth ylim idazolium ch loride an d Ag O. Reaction of 1
2
with th e water-soluble 1,3,5-triaza-7-ph osph aadam an tan e (pta) afforded
6
[
RuCl(η -p-cym en e)L(pta)]Cl (2), con tain in g both NHC an d tertiary ph os-
ph in e ligan ds. Com plexes 1 an d 2 sh owed m oderate activity in th e h ydro-
7
gen ation of alken es, aldeh ydes an d keton es (10 bar H , 80 °C, turn over
2
–
1
frequen cies, TOF, in th e ran ge 39–139 h ) as well as in th e oligom eriz-
8
ation of alkyn es. In terestin gly, th ese com plexes also catalyzed th e redox
isom erization of allylic alcoh ols9 to th e correspon din g oxo com poun ds
(
e.g. oct-1-en -3-ol to octan -3-on e) with TOF up to 125 h –1
.
Of th e possible m ech an ism s of th e redox isom erization of allylic alco-
3
10
3
11,12
h ols, both th e η -allyl an d th e η -oxo-allyl
m ech an ism s con sider in i-
tial deh ydrogen ation of th e allylic alcoh ol with th e con com itan t form ation
of a h ydrido com plex; th e h ydride ligan d is tran sferred to th e C=C bon d in
th e n ext steps of th e reaction , th ereby closin g th e catalytic cycle. In oth er
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 8, pp. 1037–1045

Ru(II)–N-Heterocycle Carbene Complexes
1039
words, such redox isom erization s are in tern al tran sfer h ydrogen ation s. Th e
pron oun ced ability of 1 an d 2 to catalyze th e isom erization of allylic alco-
h ols prom pted us also to in vestigate th eir catalytic properties in m ore
widely em ployed in term olecular h ydrogen tran sfer reaction s, an d th e first
results of th ese studies are described h erein .
EXPERIMENTAL
Th e products were iden tified an d th e catalytic reaction s were followed by 1H NMR spectros-
copy (Bruker Avan ce 360) or by gas ch rom atograph y (HP5890 Series II, Ch rom pack WCOT
Fused Silica CP WAX 52 CB 30 m × 0.32 m m , FID, carrier gas argon ).
6
6
[
RuCl (η -p-cym en e)L] (1) an d [RuCl(η -p-cym en e)L(pta)]Cl (2) (Sch em e 1) were prepared
2
7
as described earlier . All oth er reagen ts were obtain ed from Aldrich , Fluka an d Merck an d
used as received after purity ch eck by GC an d H NMR.
1
SCHEME 1
6
6
Structures of [RuCl (η -p-cym en e)L] (1) an d of [RuCl(η -p-cym en e)L(pta)]Cl (2) (L = 1-butyl-
2
3
-m eth ylim idazol-2-yliden e; pta = 1,3,5-triaza-7-ph osph aadam an tan e)
All m an ipulation s were run un der oxygen -free argon atm osph ere. A represen tative exam -
ple of a catalytic tran sfer h ydrogen ation is as follows: 1 (4.5 m g, 0.01 m m ol) an d ben zyl-
iden eaceton e (296 m g, 2.0 m m ol) were placed in to a Sch len k tube closed with a rubber
septum . Th e tube was th en evacuated an d filled with argon several tim es. Propan -2-ol (10 m l),
con tain in g KOH (56 m g, 1 m m ol) was in jected th rough th e septum . Th e tube was im m ersed
in to a th erm ostatted bath (80 °C) an d th e reaction m ixture was stirred vigorously for th e de-
sired reaction (or sam plin g) tim e. Sam ples were filtered th rough a sm all silica gel plug an d
an alyzed by gas ch rom atograph y. In th e case of th e aqueous-organ ic biph asic tran sfer h y-
drogen ation of ben zyliden eaceton e, th e reaction m ixture was extracted twice with CHCl3
(
1.5 m l), th e com bin ed extracts were dried over an h ydrous MgSO4 an d an alyzed by GC.
RESULTS AND DISCUSSION
In th e presen ce of KOH, both 1 an d 2 catalyzed th e h ydrogen tran sfer
from propan -2-ol to keton es (acetoph en on e, ben zoph en on e, ben zyliden e-
aceton e), cin n am aldeh yde, stilben e an d cycloh exen e. Th e tim e course of
th e reaction of acetoph en on e is sh own in Fig. 1. Th e lin ear ph ase of th e
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 8, pp. 1037–1045

1
040
Fekete, Joó:
con version -vs-tim e fun ction (between 2 an d 7 h ) is ch aracterized by a TOF
–
1
of 19 h . Un der th e experim en tal con dition s used, th e h igh est con version
was 85% (total n um ber of turn overs, TON = 120), in dicatin g th e equilib-
rium position of th e reaction acetoph en on e + propan -2-ol
2-ph en yl-
eth an -1-ol + aceton e in th e closed Sch len k tube. As seen from th e data in
Table I (n ote th at experim en tal con dition s are som ewh at differen t from
th ose in Fig. 1), both catalysts were h igh ly active even in th e case of
sterically m ore h in dered ben zoph en on e, leadin g to close-to-quan titative
con version s. On th e oth er h an d, trans-stilben e an d cycloh exen e un derwen t
con version s in th e 7–4% ran ge with catalyst 1, an d cycloh exen e sh owed
h ardly an y con version (1%) with catalyst 2. Th ese low con version s are n ot
un expected as propan -2-ol is gen erally m ore efficien t in tran sfer h ydrogen -
1
ation s of aldeh ydes an d keton es th an in th ose of alken es or alkyn es .
Th e possible products of h ydrogen ation of ben zyliden eaceton e an d
cin n am aldeh yde are sh own in Sch em e 2. With both catalysts, th e tran sfer
h ydrogen ation of th e un saturated keton e proceeded with 100% con version ;
h owever, 1 an d 2 sh owed differen t selectivities. Wh ile with 1 th e m ajor
product (82%) was th e saturated alcoh ol (4-ph en ylbutan -2-ol), with 2 th e
reaction afforded m ain ly th e saturated keton e (4-ph en ylbutan -2-on e). Con -
versely, in th e h ydrogen ation of cin n am aldeh yde 1 was foun d less active
th an 2; n everth eless, th e saturated aldeh yde was th e m ajor product with
both com plexes.
FIG. 1
Tim e course of tran sfer h ydrogen ation of acetoph en on e to 2-ph en yleth an -1-ol with
6
propan -2-ol/KOH catalyzed with [RuCl (η -p-cym en e)L] (1). Con dition s: 0.0095 m m ol cata-
2
lyst, 1.334 m m ol substrate, 2 m l propan -2-ol, 1 m m ol KOH, 80 °C
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 8, pp. 1037–1045

Ru(II)–N-Heterocycle Carbene Complexes
1041
SCHEME 2
Possible products of h ydrogen tran sfer reduction of cin n am aldeh yde (R = H) an d ben zyl-
iden eaceton e (R = CH3)
Th e catalytic tran sfer h ydrogen ation of 4-ph en ylbut-3-en -2-on e with
propan -2-ol with 1 as th e catalyst is a fast reaction . Wh en run with a rela-
tively sm all [substrate]/[catalyst] ratios ([S]/[C] ≤ 100), th e substrate fully re-
acted in a few m in utes an d on ly th e h ydrogen ation of th e in term ediate
TABLE I
Reduction product yields (in %) of various substrates by h ydrogen tran sfer from propan -2-ol
6
6
catalyzed with [RuCl (η -p-cym en e)L] (1) an d [RuCl(η -p-cym en e)L(pta)]Cl (2) (L = 1-butyl-
2
3
-m eth ylim idazol-2-yliden e; pta = 1,3,5-triaza-7-ph osph aadam an tan e)
Catalyst
Substrate
Product(s)a
1
2
Acetoph en on e
2-ph en yleth an -1-ol
diph en ylm eth an ol
4-ph en yl-3-buten -2-ol
95
98
96
97
Ben zoph en on e
trans-4-Ph en ylbut-3-en -2-on e
0
17
82
0
77
23
(
ben zyliden eaceton e)
4
4
-ph en ylbutan -2-on e
-ph en ylbutan -2-ol
trans-Cin n am aldeh yde
cin n am yl alcoh ol
17
39
2
2
77
23
3
3
-ph en ylpropan al
-ph en ylpropan -1-ol
trans-Stilben e
1,2-diph en yleth an e
cycloh exan e
7
4
7
1
Cycloh exen e
Con dition s: 0.02 m m ol catalyst, 2.0 m m ol substrate, 10 m l propan -2-ol, 1 m m ol KOH,
8
0 °C, 4 h .
a
Wh en product yields do n ot sum up to 100%, th e rest is un reacted substrate.
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 8, pp. 1037–1045

1
042
Fekete, Joó:
4
-ph en ylbutan -2-on e to 4-ph en ylbutan -2-ol could be followed. However, as
sh own in Fig. 2, at h igh substrate loadin gs th e un saturated alcoh ol in term e-
diate, 4-ph en ylbut-3-en -2-ol was also observed in th e first few m in utes of
th e reaction . Un der th e con dition s in Fig. 2, th e tran sfer h ydrogen ation of
4
7
-ph en ylbut-3-en -2-on e was over in 30 m in , i.e. th e catalyst sh owed TOF =
–
1
04 h
Th e tran sfer h ydrogen ation of cin n am aldeh yde catalyzed by 1 sh owed
.
som e peculiar features (Fig. 3). Th e total con version of th e substrate
reach ed 93% (TOF = 1116 h –1) with in th e first 10 m in an d did n ot ch an ge
sign ifican tly in th e n ext 50 m in . At th e sam e tim e, th e am oun t of th e to-
tally reduced product, 3-ph en ylpropan -1-ol, slowly decreased from 5 to 3%.
Th ese two ph en om en a refer to th e possibility of th e reverse h ydrogen tran s-
fer, i.e. from 3-ph en ylpropan -1-ol to aceton e (th e product of th e deh ydro-
gen ation of propan -2-ol). An obvious an d sign ifican t ch an ge in th e product
distribution accom pan ied th e tran sform ation of cin n am yl alcoh ol to di-
h ydrocin n am aldeh yde (3-ph en ylpropan al). It seem s th at th e catalyst is also
active in th e redox isom erization (th e in tern al h ydrogen tran sfer) of
9
cin n am yl alcoh ol, as can be expected from our earlier fin din gs . From th e
data in Fig. 3 it can be con cluded th at in th e very fast first ph ase of th e re-
action , cin n am yl alcoh ol an d dih ydrocin n am aldeh yde are form ed in de-
FIG. 2
Tim e course of tran sfer h ydrogen ation of 4-ph en ylbut-3-en -2-on e to 4-ph en ylbut-3-en -2-ol
(
ꢀ), 4-ph en ylbutan -2-on e (᭿) an d 4-ph en ylbutan -2-ol (᭹) with propan -2-ol/KOH catalyzed
6
with [RuCl (η -p-cym en e)L] (1). Con dition s: 0.01 m m ol catalyst, 4.0 m m ol substrate, 10 m l
2
propan -2-ol, 2 m m ol KOH, 80 °C
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 8, pp. 1037–1045

Ru(II)–N-Heterocycle Carbene Complexes
1043
FIG. 3
Tim e course of tran sfer h ydrogen ation of cin n am aldeh yde (ᮀ total con version ) to 3-ph en yl-
propan al (᭿), cin n am yl alcoh ol (᭹) an d 3-ph en ylpropan -1-ol (ꢀ) with propan -2-ol/KOH cata-
6
lyzed with [RuCl (η -p-cym en e)L] (1). Con dition s: 0.01 m m ol catalyst, 2.0 m m ol substrate, 10
2
m l propan -2-ol, 1 m m ol KOH, 80 °C
FIG. 4
Tim e course of th e tran sfer h ydrogen ation of 4-ph en ylbut-3-en -2-on e to 4-ph en ylbutan -2-on e
6
(
(
᭿) an d 4-ph en ylbut-3-en -2-ol (ꢀ) with HCO Na/H O catalyzed with [RuCl (η -p- cym en e)L]
2
2
2
1). Con dition s: 0.01 m m ol catalyst, 2.0 m m ol substrate, 1.0 m m ol HCO Na, 3 m l 0.1 M ph os-
2
ph ate buffer (pH 7.0), 80 °C
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 8, pp. 1037–1045

1
044
Fekete, Joó:
pen den tly an d at about th e sam e rate (after 10 m in , th eir con cen tration s
were 43 an d 45%, respectively) an d it is on ly th e alm ost total lack of furth er
reactivity of th e saturated aldeh yde, wh ich m akes th e redox isom erization
of cin n am yl alcoh ol observable.
An attem pt was also m ade to exploit th e water solubility of 1 an d run th e
catalytic tran sfer h ydrogen ation of 4-ph en ylbut-3-en -2-on e in an aqueous-
organ ic system usin g an aqueous solution of Na form ate as th e H-don or. At
th e reaction tem perature, th e substrate m elts an d form s a separate organ ic
ph ase. Figure 4 sh ows th at un der such con dition s, a slow reaction (TOF =
–
1
1
8 h ) took place affordin g 4-ph en ylbutan -2-on e as th e m ajor product ac-
com pan ied by on ly a sm all am oun t of th e un saturated alcoh ol. Th is selec-
tivity is opposite to th at observed with propan -2-ol/KOH as th e H-don or
(
Table I), wh ile th e low reaction rate m ay be (partly) due to th e m ass tran s-
fer lim itation s in th e two-ph ase m ixture. Th e un reacted substrate an d th e
products can be separated from th e catalyst-con tain in g aqueous ph ase by
extraction ; h owever, th e possibility of recyclin g th e catalyst an d th e ways
to im prove th e catalytic efficien cy of h ydrogen tran sfer n eed furth er in ves-
tigation s.
CONCLUSIONS
Th e Ru(II)–N-h eterocycle carben e com plexes 1 an d 2 sh owed excellen t
–
1
catalytic activities (with turn over frequen cies up to 1116 h ) in th e h ydro-
gen tran sfer reduction of cin n am aldeh yde an d several keton es usin g
propan -2-ol/KOH as th e H-don or. Sim ilar h ydrogen ation s of trans-stilben e
an d cycloh exen e were ch aracterized by low con version s. Th e h ydrogen -
ation of an un saturated keton e (4-ph en ylbut-3-en -2-on e) an d un saturated
aldeh yde (cin n am aldeh yde) sh owed on ly m oderate selectivities in th e for-
m ation of th e saturated alcoh ol or th e product of C=C h ydrogen ation (satu-
rated keton e or saturated aldeh yde) wh ile th e yield of th e product of
selective C=O h ydrogen ation (un saturated alcoh ol) n ever exceeded 20%.
Th e possibility of h ydrogen tran sfer from form ate to 4-ph en ylbut-3-en -
2
-on e in an aqueous-organ ic two-ph ase m ixture was also dem on strated.
Financial support of the Hungarian National Research Fund (OTKA T043365) and a loan of
hydrated RuCl3 from Johnson Matthey plc are gratefully acknowledged. This work is a part of the
research within the Marie Curie Research Training Network MRTN-CT-2003-503864 (AQUACHEM).
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