M. Pandrala et al. / Journal of Catalysis 378 (2019) 283–288
285
Fig. 2. Plot showing the TOF against the initial solution pH for the reduction of 3-
methoxybenzaldehyde, using Ir-Phen and 4.5 eq of HCOONa in water and ethanol
mixture at 100 °C and S/C 1000:1. The initial pH value was determined by varying
the HCOOH/NaOH amounts.
Once the reaction conditions were optimized several struc-
turally diverse aldehydes were examined. The results on the TH
of aromatic aldehydes with Ir-phen at S/C 1000:1 and 500:1 are
shown in Table 1. Impressively, most of the aldehydes reduced to
the corresponding alcohols within 30 min, giving excellent
yields. For example, the reduction of 3-methoxy- and
3-cyanobenzaldehydes were obtained with over 99% yield
in 15 min (Table
1 entries 1 and 3). In addition, other
Fig. 1. TOF of Ir-catalysts were calculated for the reduction of 3-methoxyben-
zaldehyde using 4.5 eq of HCOONa in water and ethanol mixture at 100 °C and S/C
1000:1.
meta-substituted aldehydes, such as 3-halobenzaldehydes and
3-trifluoromethylbenzaldehyde, were readily reduced to the
corresponding alcohols. (Table 1, entry 4–7).
Ir-Me2bpy did not demonstrate an effective conversion rate. The
catalysts, Ir-phen and Ir-Me2phen, showed similar activity and
better conversion rates than Ir-Me4phen when reducing 3-
methoxybenzaldehyde (Fig. 1). However, Ir-phen showed superior
activity in reducing 4-methoxybenzaldehyde, and it was observed
that the catalyst activity was decreased as the methyl groups on
the ligand increased (Fig. S1). Therefore, further optimization was
performed using Ir-phen. We observed that aldehydes used in this
study were immiscible (not soluble) in pure water, as was the case
in previous studies [8,13]. Therefore, we decided to use a homoge-
nous mixture of water and ethanol.
Remarkable benefits were observed while conducting the reac-
tion in aqueous ethanol. For example, adding ethanol not only
aided to dissolve the aldehyde completely, but it also facilitated
the rate of the reaction, resulting in observed higher conversion
rates when 70% ethanol in water was used. (Tabel S1). Further-
more, using this solvent mixture in the reaction helped remove
to the byproduct (Na2CO3) by filtration as it precipitates upon reac-
tion completion (Fig. S2).
Reduction of the carboxylic group containing aldehydes, such as
3- or 4-carboxybenzaldehydes, were unsuccessful in conditions
reported earlier [8], as the initial pH of the reaction solution was
slightly acidic, and the catalyst was only effective at neutral or
slight basic pH giving high TOF [8]. The solution pH was found to
play a crucial role in reducing these carboxy benzaldehydes, with
acidic conditions favoring the reduction. Subsequently, these alde-
hydes were reduced using Tang’s catalyst while the catalyst was
effective at acidic pH [13]. Nevertheless, Tang’s conditions were
not suitable to reduce 2-carboxybenzaldehyde as the reduced pro-
duct cyclized under these conditions [13] In contrast, we found
that using Ir-phen 2-, 3-, and 4-carboxybenzaldehydes were suc-
cessfully reduced in acidic, neutral, and basic conditions with
excellent yields (Table 1, entry 9, 24 and 25). Of a particular note,
2-carboxybenzaldehyde was reduced using excess sodium formate
(9.0 eq) to keep the solution pH as basic as possible. To the best of
our knowledge, these are the first iridium(III) based catalysts
reported to have reduced all 2-, 3-, and 4-carboxybenzaldehydes
efficiently.
We examined the effectiveness of Ir-phen by carrying out the
TH reaction at higher substrate/catalyst (S/C) ratios in 70% EtOH
in water. For the reduction of 3-methoxybenzaldehyde, at an S/C
ratio of 1000:1, Ir-phen led to a Turnover frequency (TOF) of
4725 hꢁ1 (Fig. S3). It was observed that the catalyst TOF is also sub-
strate dependent. For example, using similar conditions, Ir-phen
led to a TOF of 5558 hꢁ1 and 1830 hꢁ1 for the reduction of 2-
Furthermore, we found that using Ir-phen catalyst, 3-, and 4-
hydroxybenzaldehydes reduced to the corresponding alcohols in
excellent yields, which was challenging by catalysts developed
by others [8,13]. (Table 1, entry 8 and 20). Although reduction of
4-hydroxybenzaldehyde was found to be difficult, it was success-
fully obtained using higher catalyst loading and prolonged reaction
time.
trifluoromethylbenzaldehyde
and
4-methoxybenzaldehyde,
With our methodology the reduction of aldehyde to alcohol
could be obtained even in the presence of a variety of other func-
tionalities. For example, with the reduction of 3- or 4-
cyanobenzaldehyde, the cyano group remained intact with no
major side-products observed (Table 1, entry 3 and 19). Also,
trifluoromethyl- and methoxy- groups containing aldehydes
underwent the reduction to yield the desired products in more
than 90% yields (Table 1, entry 7, 16, 30, 35 and 36). With the
exception of 3-pyrrole carboxaldehyde, hetero aryl aldehydes,
fused aryl aldehydes, and fused hetero aryl aldehydes were suc-
cessfully reduced in very good to excellent yields (Table 1,
entry13–15 and 31–33). Although 3-pyrrole carboxaldehyde was
respectively (Fig. S4). Particularly, aromatic aldehydes that contain
electron-donating groups at para- position reduced relatively
slower than orth- and meta- substituted aromatic aldehydes.
Although earlier reported catalysts showed higher TOF than Ir-
phen, those catalysts were only effective at a narrow pH of the
reaction [8,13,15]. We found that using our catalyst and reaction
conditions the reductive capabilities of Ir-phen were retained
through a broad pH range (Fig. 2). Moreover, the Ir-phen catalyst
reduced a variety of aldehydes that were difficult to reduce using
previously reported catalysts as the pH of the reaction solution
restricted the reduction (Table 1, entry 8, 9, 20, 24 and 25).