G Model
CATTOD-9135; No. of Pages6
ARTICLE IN PRESS
2
I. Szatmári et al. / Catalysis Today xxx (2014) xxx–xxx
aqueous phase may eliminate substrate inhibition even in cases
when the aldehyde concentration in the organic phase is high (or
when the neat substrate is applied without any added organic sol-
vent) [45]. Solubility of inorganic salts in the aqueous phase can
be beneficial, too. For example, significant rate increasing effects of
various cations were observed in aqueous–organic biphasic hydro-
genation of aldehydes with several Ru(II)-mtppts catalysts [25,26].
Finally, mutual solubility of water in the organic phase and vice
versa should also be considered.
Scheme 1. Hydrogenation of trans-cinnamaldehyde.
In case of substrates with very low aqueous solubility a straight-
forward way to speed up the reaction is the use of co-solvents.
Nevertheless, a co-solvent can always increase leaching of the
water-soluble catalyst into the organic phase. For example, Mon-
flier et al. investigated the hydrogenation of water-insoluble
aldehydes in the presence of various co-solvents (with Ru(II)-
mtppts catalysts) with beneficial effects on the reaction rate,
however, the amount of co-solvents had to be kept below 5%
(w/w) of the aqueous phase due to increased leaching [51]. In
other cases, the reaction was found faster in the co-solvent alone
such as cinnamaldehyde and 3-(1,3-benzodioxol-5-yil)-2-methyl-
propenal (the saturated aldehyde is the precursor of the fragrance
Helional®) was accelerated by addition of ethylene glycol to the
mixture of the aldehyde and water [20]. Furthermore, the high-
est rate was observed by running the reaction in ethylene glycol
(in what the Rh(I)-(l)-cysteine or Rh(I)-(s)-captopril were soluble
under conditions of the reaction).
of aldehydes (and ketones) in water/2-propanol 10/3 (v/v) mix-
tures with Na2CO3 as base and [{RhCl(COD)}2] + 15 mtppts catalyst.
High conversions were obtained in 2 h at 80 ◦C (e.g. benzaldehyde
98%, 2-thiophenecarboxaldehyde 72%) [11]. Obviously, in this sys-
tem 2-propanol had the dual role of H-donor and cosolvent. Under
the applied conditions, the reaction mixtures were homogeneous
and the product was isolated by extraction with diethyl ether. No
unsaturated aldehydes were studied therefore no data are available
from this work on the selectivity of the catalyst in basic aqueous
2-propanol.
In our earlier investigations, we have already used HCOONa
as base in transfer hydrogenation of unsaturated aldehydes and
ketones from 2-propanol catalyzed by chiral Rh(I)-, Ru(II) and Ir(I)-
aminoacidate complexes [52,53]. It was established that presence
of water in the H-donor solvent up to 4% (v/v) was neither benefi-
cial nor detrimental on the reaction rate and selectivity. Based on
our experience in aqueous–organic transfer hydrogenation of alde-
hydes catalyzed by [{RuCl2(mtppms)2}2] + n mtppms we initiated
a study of transfer hydrogenation of aldehydes from aq. HCOONa
was whether both formate and 2-propanol act as H-donors and
whether the water/2-propanol ratio effects the selectivity in the
case of unsaturated aldehydes. These investigations led to the dis-
covery of an exceedingly fast transfer hydrogenation of aldehydes
under mild condition as described in the following.
of the 2-propanol–water mixtures even in those cases when water-
soluble catalysts were applied. Notable exceptions are the transfer
hydrogenations of ketones studied by Williams et al. [39,40] and
by Ajjou and Pinet [11] where up to 51% (v/v) water could be
applied beneficially. Xiao applied a water-insoluble catalyst in
aqueous–organic biphasic system for transfer hydrogenation of
ketones from formate and observed an accelerating effect of water
(on water reaction) [41].
In contrast to 2-propanol, the most suitable solvent for formic
acid/formate salts is water and the insolubility of many of the
hydes and ketones were successfully hydrogenated by H-transfer
Rh(I)- and Ir(I)-complexes are also known to act as catalysts in
such reactions [48,49], in fact, Ir(I)-complexes with monotosy-
lated ethylenediamine ligands [9,10] showed outstanding catalytic
activities up to turnover frequencies, TOF = 3.0 × 105 h−1 (TOF = mol
reacted aldehyde × (mol catalyst)−1 × h−1) [10].
Interestingly, in the first biphasic transfer hydrogenation of
aldehydes both the [RuCl2(PPh3)3] catalyst and the substrates were
dissolved in the same (organic) phase and the aqueous phase served
only as a reservoir of the H-donor (Na-formate) [42,43]. Conse-
quently, a phase transfer catalyst (Aliquat 336) had to be used in
order to attain reasonably high reaction rates. In addition, due to
substrate inhibition [42], the aldehyde concentration in the organic
phase had to be kept low. For substrate-catalyst separation and
catalyst recycling a better arrangement is to dissolve the cata-
lyst together with HCOONa in the aqueous phase and contact it
with an organic phase of the aldehyde (neat or dissolved in a suit-
biphasic system, we achieved 100% selective transfer hydrogena-
tion of unsaturated aldehydes to unsaturated alcohols at 80 ◦C by
using [{RuCl2(mtppms)2}2] + nmtppms or [RuCl2(pta)4] catalysts
and 5 M aq. HCOONa as H-donor [44–47].
Catalytic hydrogenations and transfer hydrogenations in aque-
ous–organic biphasic systems can be influenced by several factors
which may be belong to one of the following groups: (1) effects
connected to the presence of water, (2) effects of phase transfer
of the catalyst by promoting hydrolysis (formation of hydroxo-
complexes) [50]; preferring heterolytic activation of H2 [50];
allowing formation of several hydrido- and molecular hydrogen
complexes from the same catalyst precursor depending on the
pH of the aqueous phase and on H2 pressure [23,24,50]; protona-
tion/deprotonation equilibria, etc. Concerning phase transfer and
solubility effects the chemical reaction may proceed either in the
catalyst-containing bulk aqueous phase, or at the interphase of the
two bulk phases. The most important factor is perhaps the transfer
of substrates to the interphase and their dissolution into the aque-
ous phase and this can limit the overall rate of the hydrogenation
process. Another rate-decreasing factor is the lower solubility of
H2 in water, compared to the usual organic solvents. On the other
hand, limited solubility of the substrate (e.g. an aldehyde) in the
2. Experimental
Aldehydes (Aldrich) and other reagents and solvents were
commercially available and used as received. The water-soluble
phosphine ligand mtppms [31] and [{RuCl2(mtppms)2}2] [31] were
prepared by published procedures.
All reactions and manipulations were carried out under argon
atmosphere. Reaction mixtures were analyzed by gas chro-
matography (HP5890 Series II; Chrompack WCOT Fused Silica
30 m × 32 mm CP WAX52CB; FID; carrier gas: argon). The products
Please cite this article in press as: I. Szatmári, et al., Unexpectedly fast catalytic transfer hydrogenation of aldehydes by formate in