Transfer hydrogenation of aromatic and linear aldehydes catalyzed using Cp*Ir(pyridinesulfonamide)Cl complexes under base-free conditions

Article abstract of DOI:10.1016/j.jorganchem.2017.05.004

Cp*Ir(pyridinesulfonamide)Cl (Cp*?=?pentamethylcyclopentadienyl) precatalysts 1–7 are active for the transfer hydrogenation of aryl, alkyl, and heterocyclic aldehydes. Catalysis is conducted under base-free conditions in air without dried or degassed substrates and solvents. These reductions occur rapidly in moderate to high conversion (39–100%). Benzaldehyde derivatives are reduced to alcohols within 30?min?at 85?°C using 1?mol% iridium precatalyst; reduction also occurs at lower temperatures and loadings (60?°C, 0.50?mol% precatalyst). Benzaldehyde derivatives that possess electron-rich and electron-poor substituents in the para position, including base-sensitive 4-hydroxybenzaldehyde, are readily reduced. Aryl aldehydes containing electron-poor groups are reduced faster than substrates possessing electron-rich moieties. Reduction of the positional isomers of methoxybenzaldehyde and isopropylbenzaldehyde shows highest reduction for the ortho isomer, followed by the meta isomer. Heterocyclic substrates, including biomass derived 5-hydroxymethylfurfural and 2-furfural, were reduced selectively to the alcohol. Decyl aldehyde was reduced to the linear alcohol; importantly self-condensation was not observed. Competition studies demonstrated selective reduction of aldehydes over ketones and a mercury poisoning experiment supports a homogeneous catalyzed pathway.

Full text of DOI:10.1016/j.jorganchem.2017.05.004

Accepted Manuscript
Transfer hydrogenation of aromatic and linear aldehydes catalyzed using
Cp*Ir(pyridinesulfonamide)Cl complexes under base-free conditions
Tanya M. Townsend, Christopher Kirby, Andrew Ruff, Abby R. O'Connor
PII:
S0022-328X(17)30290-5
10.1016/j.jorganchem.2017.05.004
JOM 19930
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 7 March 2017
Revised Date: 29 April 2017
Accepted Date: 2 May 2017
Please cite this article as: T.M. Townsend, C. Kirby, A. Ruff, A.R. O'Connor, Transfer hydrogenation of
aromatic and linear aldehydes catalyzed using Cp*Ir(pyridinesulfonamide)Cl complexes under base-free
conditions, Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.05.004.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Transfer Hydrogenation of Aromatic and Linear Aldehydes Catalyzed Using
Cp*Ir(pyridinesulfonamide)Cl Complexes Under Base-Free Conditions
a
a,1
a
a*
Tanya M. Townsend , Christopher Kirby , Andrew Ruff , Abby R. O’Connor
a
Department of Chemistry, The College of New Jersey, 2000 Pennington Rd. Ewing, NJ 08628
Corresponding author: oconnora@tcnj.edu
Current Address
1
Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa
Barbara, CA 93106-9510
Abstract
Cp*Ir(pyridinesulfonamide)Cl (Cp* = pentamethylcyclopentadienyl) precatalysts 1-7 are active
for the transfer hydrogenation of aryl, alkyl, and heterocyclic aldehydes. Catalysis is conducted
under base-free conditions in air without dried or degassed substrates and solvents. These
reductions occur rapidly in moderate to high conversion (39-100%). Benzaldehyde derivatives
are reduced to alcohols within 30 min at 85 °C using 1 mol% iridium precatalyst; reduction also
occurs at lower temperatures and loadings (60 °C, 0.50 mol% precatalyst). Benzaldehyde
derivatives that possess electron-rich and electron-poor substituents in the para position,
including base-sensitive 4-hydroxybenzaldehyde, are readily reduced. Aryl aldehydes containing
electron-poor groups are reduced faster than substrates possessing electron-rich moieties.
Reduction of the positional isomers of methoxybenzaldehyde and isopropylbenzaldehyde shows
highest reduction for the ortho isomer, followed by the meta isomer. Heterocyclic substrates,
including biomass derived 5-hydroxymethylfurfural and 2-furfural, were reduced selectively to
the alcohol. Decyl aldehyde was reduced to the linear alcohol; importantly self-condensation was
not observed. Competition studies demonstrated selective reduction of aldehydes over ketones
and a mercury poisoning experiment supports a homogeneous catalyzed pathway.
Key Words
Iridium, transfer hydrogenation, aldehydes, base-free, catalysis
1
. Introduction
Hydrogenation is a well-utilized step in the synthesis of pharmaceuticals, flavors, and
fragrances and although a multitude of catalysts and precatalysts for this reaction exist,
challenges include chemoselectivity and high-
pressures of hydrogen.[1] A safer, cheaper, and
greener alternative to hydrogenation is transfer
hydrogenation. Common hydrogen donors used in
transfer hydrogenation include formic acid, aqueous
sodium formate, azeoptropic mixtures of formic
acid/triethylamine,
and
2-propanol.[2]
Some
substrates reduced under transfer hydrogenation
conditions include alkenes, alkynes, imines, ketones,
and aldehydes.[2–6] A limited number of complexes
exist for the transfer hydrogenation of aldehydes. A
major constraint in many transfer hydrogenation
systems is the addition of base, which can cause side
Figure 1. Recently reported complexes A-D
for transfer hydrogenation of aldehydes.

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reactions including the aldol condensation of aliphatic aldehydes; thus limiting the substrate
scope.
Several Ru complexes catalyze transfer hydrogenation of aldehydes (Figure 1). Ru-based
complex A, which is utilized in refluxing 2-propanol/KOH, catalyzes the reduction of
benzaldehyde.[7] Another Ru complex containing a sulfonated phosphine ligand catalyzes
transfer hydrogenation of benzaldehyde derivatives in aqueous sodium formate and 2-propanol
solutions.[8] Nolan’s complex C also catalyzes the transfer hydrogenation of benzaldehyde in 2-
propanol using 2.5 mol% of potassium hexamethylsilazane.[9] Complex B, possessing either a
Ru or Os metal center, is active for transfer hydrogenation with 1 mol% sodium isopropoxide in
refluxing 2-propanol; aliphatic aldehydes are reduced in under 30 min.[10,11] Building upon this
work, Barrata more recently demonstrated that Ru complexes containing CNN pincer ligands
catalyze transfer hydrogenation of a variety of aldehydes in 2-propanol with potassium carbonate
2 2
as the base.[11] Additionally, Singh reported that [Ru(p-cymene)Cl ] catalyzes the transfer
hydrogenation of aromatic and heteroaromatic aldehydes in an 2-propanol/potassium acetate
solution in air.[12]
Non-noble metal complexes for transfer hydrogenation of aldehydes are also known and
include Fe-based pincer complexes. Iron PONOP (PONOP = 2,6-bis(phosphinito)pyridine)
complexes catalyze the transfer hydrogenation of aromatic and aliphatic aldehydes with sodium
formate in methanol[6] and iron PNNP (PNNP = (R,R)-{PPh (o-C H )CH=NC H N=CH(o-
2
6
4
6
10
C H )PPh }) complexes catalyze reduction of benzaldehyde to benzyl alcohol (94%) with 4
6
4
2
mol% potassium tert-butoxide in 2-propanol.[13] Hanson’s Co-based complex D (Figure 1)
catalyzes the transfer hydrogenation of aromatic and α,β-unsaturated aldehydes in 3:1 2-
propanol:THF.[14]
Of the aforementioned complexes, only A and [Ru(p-cymene)Cl ] performed transfer
2
2
hydrogenation in air; all of the other complexes catalyze transfer hydrogenation under inert
atmosphere. Additionally, D was the only complex capable of transfer hydrogenation without
base. Most substrate scopes are limited to aryl aldehydes, which cannot undergo self-
condensation in the presence of base.
Several iridium complexes catalyze transfer
hydrogenation of aldehydes (Figure 2). Complex E
catalyzes the transfer hydrogenation of benzaldehyde in
9
8% conversion in 7 h in neat glycerol; however, a molar
equivalent of KOH is required.[15] Sarkar’s complex F
catalyzes transfer hydrogenation of benzaldehyde and
acetophenone; nearly complete conversion of benzaldehyde
is observed in 3 h with 2-propanol and 20 mol% KOH.[16]
In addition to requiring KOH as an additive, transfer
hydrogenation using complexes E and F requires air-free
conditions. One example of an Ir catalyst utilized in air is
Xiao’s complex G, which also catalyzes reductive
amination of aromatic and aliphatic ketones and aldehydes,
and transfer hydrogenation of imines, ketones, aldehydes,
β-keto ethers, α- and β-keto esters.[17–21] Crabtree’s Ir
2 3
complex H, in refluxing 2-propanol and K CO , reduces p-
nitrobenzaldehyde in 98% conversion of substrate.[22]
Transfer hydrogenation of benzaldehyde derivatives and
Figure 2. Recently reported Ir complexes
E-I for transfer hydrogenation.

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2 2
α,β-unsaturated aldehydes has also been achieved using (Cp*IrCl ) in conjunction with
monotosylated ethylenediamine ligand (I) in a sodium formate and water solution.[23] Aliphatic
aldehydes are not reduced, likely due to competing aldol condensation. The basic or acidic
reaction conditions of these systems can limit the substrate scope to aldehydes that do not
contain acidic protons or pH sensitive functional groups.
An attractive reaction is the selective
reduction of biomass-derived aldehydes to
alcohols to form monomers for polymerization
applications.[24–26] Glucose derived 5-
hydroxymethylfurfural (HMF) can be reduced
using heterogeneous catalysts and H to either Figure 3. Reduction of HMF to BHMF or DMF.
2
2
2
,5-bis(hydroxymethyl)furan (BHMF)[27] or
,5-dimethylfuran (DMF)[28] (Figure 3). Of the few catalysts that have been reported for
transfer hydrogenation of HMF, most are heterogeneous.[29,30] No examples of this reduction
under base-free transfer hydrogenation conditions using homogeneous complexes are known
based upon our searches.
Herein we describe the application of previously reported Cp*Ir(pyridinesulfonamide)Cl
precatalysts[31] for transfer hydrogenation of aromatic, aliphatic, and heterocyclic aldehydes in
air without additives or rigorously dried or degassed solvents and reagents. The electronics of the
precatalysts were varied from electron-donating to withdrawing groups on the ligand. The
electron rich precatalysts reduced aldehydes to alcohols in higher conversion than electron poor
precatalysts. The substrate scope includes a number of para substituted aryl aldehyde
derivatives, as well as positional isomers of methoxybenzaldehyde and isopropylbenzaldehyde,
and 2-chloro-5-nitrobenzaldehyde. Other substrates include furfural derivatives, linear
aldehydes, and 4-acetylbenzaldehyde. Under these base-free conditions, aryl aldehydes, 2-
furfural, and HMF do not undergo aldol condensation with acetone and self-condensation of
decanal is also not observed. Competition studies demonstrate selective and faster reduction of
aldehydes in the presence of ketones. In contrast to our previous findings on reduction of α,β-
unsaturated ketones, α,β-unsaturated aldehydes are not reduced. Only a minimal amount of
aldehyde reduction is observed and the alkene is not reduced.
3
. Results and Discussion
Figure 4. Precatalysts screened in base-free transfer hydrogenation of aldehydes.

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The Cp*Ir(pyridinesulfonamide)Cl precatalysts 1-7 (Figure 4) were prepared using a
literature
method.[31]
Catalytic
screening
was
conducted
to
evaluate
Cp*Ir(pyridinesulfonamide)Cl precatalysts for transfer hydrogenation of aldehydes in 2-propanol
under base-free conditions. All reactions, unless noted, were performed in air without base or
dried and degassed reagents at 85 °C. In all cases, only the alcohol product and unreacted starting
1
material (in some cases) were observed, as confirmed by GCMS and H NMR spectroscopy.
Precatalysts 1-7 catalyze the reduction of benzaldehyde to benzyl alcohol in 30 min at 85
°
C (Table 1). The conversion of substrate is directly correlated to the electronics of the
precatalyst. Precatalysts 7, 6, and 2, possessing electron-withdrawing substituents on the
sulfonamide moiety, exhibit lower conversion, 87, 81, and 72%, respectively (entries 5-7).
Precatalysts containing electron-donating substituents on the sulfonamide exhibit higher
conversions to benzyl alcohol because electron-rich precatalysts are expected to more easily
reduce the substrates. This trend was also observed for the reduction of 4-nitroacetophenone
using these precatalysts.[31] This ligand effect is indicative of a reaction pathway that
cooperatively involves both the metal and ligand. Control experiments were conducted to
determine if 1 mol% (Cp*IrCl
benzaldehyde (entries 9 and 10). Only 12% conversion to benzyl alcohol was observed with the
Cp*IrCl ) dimer; no reduction was observed with ligand. A 15% conversion to benzyl alcohol
2 2
) dimer or 2 mol% ligand catalyze transfer hydrogenation of
(
2
2
was observed with (Cp*IrCl ) dimer in conjunction with the ligand (entry 8). These results
2
2
support that the precatalyst is responsible for the high reactivity observed and in situ formation
of the precatalyst does not readily occur from dimer and ligand. No reduction was observed in
trials without iridium (entry 11).
a
Table 1. Transfer hydrogenation of benzaldehyde
Entry Precatalyst
% Conversion
100±1
100±1
100±1
100±1
87±2
81±1
72±3
15±3
12
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
5
4
3
7
6
2
(Cp*IrCl ) /2L
(Cp*IrCl2)2
2 equiv L
No Catalyst
3
1
1
1
1
2
2
0
1
2
3
4
5
6
0
0
b
c
d
e
f
100±1
10±1
98±2
66±1
92±1

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a 1
H NMR used to determine conversion. Reported as an average of 3 trials, with error measured between the three
trials. 1,4-Dimethoxybenzene used as internal standard. A 1.0 M solution of substrate in 2-propanol. L= N-(2-
b
c
d
(
°
pyridin-2-yl)ethyl)benzenesulfonamide. Elemental Hg added. Reaction run at 60 °C, 30 min. Reaction run at 60
e f
C, 6 h. 0.50 mol% precatalyst. Reaction run for 1 h, 0.50 mol% precatalyst.
In all trials, decarbonylation of benzaldehyde to benzene was not observed, which has
been reported for other iridium and rhodium precatalysts.[22,32,33] A mercury poisoning
experiment was performed using precatalyst 3 and benzaldehyde (entry 12). Full reduction
occurred in 30 min, supporting that the active catalyst is homogeneous, and likely not a
heterogeneous or colloidal iridium species. Transfer hydrogenation is also catalyzed at lower
temperatures; nearly complete conversion is observed after 6 h (entries 13 and 14). Lower
loadings for reduction of benzaldehyde can be used, 66% conversion with 0.50 mol% of
precatalyst 1 after 30 min and 92% conversion after 1 h (entries 15 and 16) are observed.
Aryl aldehyde substrates were screened to evaluate the substrate scope. Electron-poor
precatalyst 6 was used in order to show appreciable changes in turnover number within 30 min
trials (Table 2). Substrates that possess electron-withdrawing substituents in the para position,
such as nitro or trifluoromethyl, exhibited the highest conversion: 97% and 96%, respectively
(entries 1 and 2). Substrates containing electron-donating groups in the para position, such as
hydroxy and methoxy, exhibited lower conversions in the same reaction time: 40% and 39%,
respectively (entries 6 and 8). In addition, reduction of the nitro group of p-nitrobenzaldehyde is
not observed. Electron-withdrawing groups in the para position remove electron density from
the ring and weaken the carbonyl bond, making it more susceptible to reduction. Furthermore, a
base sensitive substrate, 4-hydroxybenzaldehyde, was completely reduced to the alcohol product
in 20 h (entry 7). Base-sensitive substrates are often not reduced under previously reported
transfer hydrogenation conditions due to basic additives.[23]
a
Table 2. Reduction of Aromatic Aldehydes
Entry
R
NO
2
CF3
Cl
H
% Conversion
97±2
1
2
3
4
5
6
96±4
96±1
81±1
76±1
40±1
98±1
39±1
CH
3
OH
OH
OMe
b
7
8
a 1
H NMR spectroscopy used to determine conversion. Reported as an average of 3 trials, with error measured
between the three trials. 1,4-Dimethoxybenzene used as an internal standard. A 1.0 M solution of substrate in 2-
b
propanol. Reaction for 20 h.
The ortho, meta, and para isomers of methoxybenzaldehyde were screened for transfer
hydrogenation using precatalyst 2 to examine substituent positional effects on conversion. The

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ortho isomer exhibited complete conversion after 30 min, while the meta and para isomers were
only partially converted to product (45%) in the same time (Table 3, entries 1-3). This indicates
that inductive effects are more significant than resonance effects in these reductions. A similar
trend was also observed for methoxyacetophenone reductions using these precatalysts.[31]
Another series of isomers was evaluated. Ortho, meta, and para isopropylbenzaldehyde were
screened for transfer hydrogenation (entries 4-6). This substrate contains a bulkier substituent,
and displayed the same trend as methoxybenzaldehyde; the ortho isomer had the highest
conversion while para had the lowest conversion: 97 vs. 83%, respectively. These results
indicate the most sterically hindered isomer results in the highest conversion to alcohol product
and inductive effects are less significant for isopropylbenzaldehyde.
Chemoselectivity was examined using precatalyst 7 with p-nitroacetophenone and p-
nitrobenzaldehyde as substrates in the same reaction vessel for 1 h (Scheme 1). p-
Nitrobenzaldehyde was selectively reduced in the presence of p-nitroacetophenone (100% vs.
5
%, respectively), which is consistent with the reduction times necessary for equivalent
conversions of the aldehydes and ketones to the respective alcohol products. 4-
Acetylbenzaldehyde also showed selective reduction of the aldehyde before the ketone moiety,
with full reduction of the aldehyde moiety within 30 min (Table 4).
a
Table 3. Reduction of ortho, meta, and para methoxybenzaldehyde and isopropylbenzaldehyde
Entry R
% Conversion
100±1
46±1
1
2
3
4
5
6
2-OMe
3-OMe
4-OMe
2-iPr
3-iPr
4-iPr
44±1
97±1
91±1
83±1
a 1
H NMR spectroscopy used to determine conversion. Reported as an average of 3 trials, with error measured
between the three trials. 1,4-Dimethoxybenzene used as an internal standard. A 1.0 M solution of substrate in 2-
propanol.
a
Scheme 1. Competition between aryl aldehyde and ketone.
O
O
OH
1
mol% 7
+
H
+
OH
o
iPrOH, 85 C, 1h
O N
2
O N
2
O N
2
O N
2
5
±1%
100±1%

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a
Table 4. Reduction of 4-acetylbenzaldehyde
Entry t (h) % Conversion
1
2
3
4
0.5
3
6
81±1, 19±1
54±1, 46±1
42±1, 58±1
26±1, 74±1
12
a 1
H NMR spectroscopy used to determine conversion. Reported as an average of 3 trials, with error measured
between the three trials. 1,4-Dimethoxybenzene used as an internal standard. A 1.0 M solution of substrate in 2-
propanol.
a
Table 5. Substrate scope
Entry Reactant
Product
t (h)
% Conversion
1
0.5
100±1
O
O
2
N
2
O N
H
OH
Cl
Cl
2
3
0.5
0.5
95±1
O
O
O
H
OH
100±1
O
O
O
HO
H
HO
OH
4
5
6
7
0.5
3
6
53±1
78±1
84±1
O
OH
8
H
8
12
100±1
a 1
H NMR spectroscopy used to determine conversion. Reported as an average of 3 trials, with error measured
between the three trials. 1,4-Dimethoxybenzene used as an internal standard. A 1.0 M solution of substrate in 2-
propanol.
The substrate scope was expanded to include polysubstituted aryl, aliphatic, and
heterocyclic aldehydes. The complete reduction of 2-chloro-5-nitrobenzaldehyde was observed
in 30 min with precatalyst 3 (Table 5, entry 1). Only aldehyde reduction was observed and
reduction of the nitro group was not observed. 2-Furfural and 5-hydroxymethylfurfural (HMF)
were reduced to 95% furfuryl alcohol and 100% 2,5-bis(hydroxymethyl)furan (BHMF), in 30
min respectively (entries 2 and 3). There have not been any previously reported homogeneous
catalysts capable of selective transfer hydrogenation of HMF to BHMF without base or

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formation of side products. Decyl aldehyde was reduced to the alcohol product in 12 h, without
condensation products.
Scheme 2. Proposed Catalytic Cycle for Transfer Hydrogenation.
N
Ir
Cl
N
O
S
O
R
Cl-
+
OH
OH
R'
R'
N
Ir
N
O
R
S
O
+
+
N
Ir
H
N
Ir
O
R'
N
H
O
R'
O
NH
O
S
O
O
R
S
R
O
+
+
R'
R'
+
N
Ir
H
H
H
H
N
Ir
O
No Substrate
H
Ir
Ir
NH
O
NH
O
Ligand Dissociates
O
O
O
S
S
R
R
A proposed catalytic cycle is shown in Scheme 2. Preliminary mechanistic studies
indicate the dissociation of the chloride from the precatalyst to generate the active catalyst for
transfer hydrogenation. It is likely the mechanism is cooperative catalysis between the iridium
center and the sulfonamide moiety of the ligand as supported by the catalysis data. Initial
1
findings obtained by H NMR spectroscopy confirm the presence of protonated ligand and a
metal hydride intermediate, although the exact nature of this complex is still being investigated.

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Without substrate, the hydride intermediate decomposes to the known cationic Cp* iridium
trihydride dimer, which is not active for transfer hydrogenation catalysis under these conditions.
Attempts to synthesize the proposed intermediates in Scheme 2 are currently underway, in
addition to DFT calculations.
3
. Conclusions
Transfer hydrogenation was catalyzed by precatalysts 1-7 for a variety of aryl, alkyl, and
heterocyclic aldehydes in 2-propanol at 85 °C under base-free conditions. Electron-donating
groups present on the ligand of the precatalysts and substrates possessing electron-withdrawing
moieties were found to have the highest conversion to alcohol product. Reduction of the ortho
isomers of both methoxybenzaldehyde and isopropylbenzaldehyde was observed in higher
conversions than the meta and para isomers. Decyl aldehyde was fully reduced after 12 h,
without self-condensation. Biomass-derived HMF was fully reduced to BHMF in 30 min, a
conversion not previously achieved via homogeneous transfer hydrogenation. Competition
studies with p-nitroacetophenone and p-nitrobenzaldehyde showed selective reduction of
aldehydes over ketones. The reduction of 4-acetylbenzaldehyde also demonstrated full reduction
of the aldehyde moiety before the ketone. Control experiments indicate that the iridium complex
is responsible for catalysis and a mercury poisoning experiment supports a homogeneous
mechanism. There is clearly a ligand effect observed for these catalysts, indicating both the
ligand and metal play a cooperative role in the catalysis. Preliminary mechanistic studies support
the dissociation of chloride to form a cationic iridium catalyst, which is active for transfer
hydrogenation. Spectroscopic data supports cooperative catalysis involving an iridium hydride
and protonated ligand. Current work is focused on isolating relevant intermediates in the
proposed catalytic cycle along with kinetic studies and DFT calculations to support the proposed
catalytic cycle.
4
4
. Experimental Procedure
.1. Materials
Solvents and materials involved in the synthesis of the ligands and all substrates were
purchased from Aldrich or Alfa Aesar and used as received. IrCl
3 2
*H O was purchased from
Pressure Chemical. The (Cp*IrCl dimer was prepared according to a literature procedure[34]
2 2
)
and stored at room temperature in air. The products were examined using NMR spectroscopic
methods. A Bruker Biospin Ascend 400 MHz Nuclear Magnetic Resonance Spectrometer and a
Bruker Biospin Ultra Shield 400 MHz Nuclear Magnetic Resonance Spectrometer were used to
1
3
1
19
1
1
collect C{ H}, F{ H}, and H NMR spectra. Chemical shifts are referenced to residual CHCl
3
1
13
13
13
(
δ 7.24 for 1H), CH(D)Cl (δ 5.32 for H), CDCl (δ 77.0 for C), and CD Cl (δ 54.0 for
C). All NMR spectra were collected at 27°C. GCMS was carried out using an Agilent 6890 gas
2 3 2 2
1
3
chromatograph/5873 quadropole mass spectrometer system with a 7683B autoinjector. All
glassware was dried for at least 1 h at 150 ºC before use.
4
.2. General Procedure to synthesize pyridinesulfonamide ligands
A literature procedure was used to prepare the ligands.[31,35,36]
4
.3. General Procedure to synthesize iridium precatalysts 1-7
A literature procedure was used to prepare the precatalysts.[31]

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4
.4. Catalytic Transfer Hydrogenation
The precatalyst (0.005 mmol) was weighed into three different 1 dram vials. If the
substrate was liquid, a standard solution containing the substrate (0.500 mmol), and the internal
standard 1,4-dimethoxybenzene (0.050 mmol) was prepared using 2-propanol as the solvent. If
the substrate was solid it was weighed directly into the vial with the precatalyst and the standard
solution was prepared from 1,4-dimethoxybenzene (0.050 mmol) using 2-propanol as the
solvent. A 500 µL aliquot of the standard solution was added via syringe to each of the vials
containing precatalyst. The vials were then heated to 85 °C in an OptiMag-ST aluminum heating
plate containing wells fit for the vials for the time indicated. The product yield was determined
1
using H NMR spectroscopy (sample spectra in Supporting Information). Each run was
conducted in triplicate and the values are reported as an average with standard deviation in the
tree trials.
Acknowledgements
This work was supported by the donors of the American Chemical Society Petroleum Research
Fund (UNI#3 53605). NMR data (#1125993) were collected on instruments purchased through
MRI grants from the National Science Foundation. We acknowledge The College of New Jersey
for start-up funding.
Appendix A. Supplementary data: Representative NMR spectra for conversion
determination.
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Highlights
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Selective transfer hydrogenation of aldehydes under base-free conditions in air
Electron-poor aryl aldehydes are reduced faster than electron-rich substrates
Iridium precatalysts bearing electron-donating substituents on the sulfonamide moiety of the
ligand leads to more product in less time
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Competition experiments show selective reduction of aldehydes in the presence of ketones
Mercury poisoning experiments support a homogeneous species as the active catalyst