Efficient and Selective Iron-Complex-Catalyzed Hydroboration of Aldehydes

Article abstract of DOI:10.1021/acscatal.7b03785

An imine-coupled [Fe-N₂S₂]₂ complex, prepared from a readily available benzothiazolidine ligand, catalyzes selectively the hydroboration of aliphatic and aromatic aldehydes at low catalyst loadings (0.1 mol %) using pinaco

Full text of DOI:10.1021/acscatal.7b03785

Subscriber access provided by READING UNIV
Letter
Efficient and selective iron complex-catalyzed hydroboration of aldehydes
Uttam Kumar Das, Carolyn S. Higman, Bulat Gabidullin, Jason E Hein, and R. Tom Baker
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03785 • Publication Date (Web): 02 Jan 2018
Downloaded from http://pubs.acs.org on January 3, 2018
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 7
ACS Catalysis
1
2
3
4
5
6
7
8
9
Efficient and selective iron complex-catalyzed hydroboration
of aldehydes
Uttam K. Das,† Carolyn S. Higman,‡ Bulat Gabidullin,† Jason E. Hein*,‡ and R. Tom Baker*,†
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
† Department of Chemistry and Biomolecular Sciences and Centre for Catalysis Research and Innovation, University of
Ottawa, Ottawa, Ontario K1N 6N5, Canada
‡ Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
ABSTRACT: An imine-coupled [Fe-N2S2]2 complex, prepared from a readily-available benzothiazolidine ligand, catalyzes selec-
tively the hydroboration of aliphatic and aromatic aldehydes at low catalyst loadings (0.1 mol %) using pinacolborane. Both mono-
and disubstituted aromatic and aliphatic aldehydes are hydroborated selectively in the presence of ketones, nitriles, alkenes, amines,
and halides. Reaction of the [Fe-N2S2]2 complex with CO and preliminary reaction progress kinetic studies point to a complex
mechanism.
Keywords. Iron, aldehyde hydroboration, pinacolborane, selectivity, reaction progress kinetic analysis
Catalytic hydroboration of carbonyl compounds using
pinacolborane (HBpin) represents an attractive strategy in
organic synthesis1 not only for synthesizing widely used
borate ester intermediates,2,3 but also for conversion into
the corresponding alcohols.4 Since the Bpin group can
serve as a versatile directing and protecting group, this
catalytic process enjoys further advancement in synthetic
chemistry.5-8 Intrigued by recent reports from C. Gunana-
than and co-workers of selective aldehyde (vs. ketone)
hydroboration catalysed by a simple Ru precursor,9 we
disclose herein an efficient and strictly selective iron-
based catalyst containing an imine-coupled, redox-active
N2S2 ligand.
containing the anionic thioether-imine-thiolate [SMeNS]
ligand.36 During the course of this study, we observed that
treatment of the low-coordinate iron complex,
[Fe{N(SiMe3)2}2] with two eq. of [SMeNHS] in THF at
room temperature afforded the title complex, 1 as a purple
solid in 92% yield (Scheme 1). Complex 1 was character-
1
ized by H NMR and UV-vis spectroscopy, ESI-MS, and
1
single-crystal X-ray diffraction. The H NMR spectrum
shows broadened and shifted resonances spanning the
range from
ic complex.
 88.03 to 14.42, indicative of a paramagnet-
To date, hydroboration catalysts for carbonyl com-
pounds based on transition metals (Ti,10-14 Mo,15 Fe,16 Ru,9
Co,17 and Cu18), main group metals (Li,19 Mg,20-22 Ca,23
Al,24,25 Ga,26 Zn,27-30 Ge and Sn31), and main group ele-
ments (P),32 are known. Recently, rare-earth metal cata-
lysts,33,34 and a silica-supported Zr catalyst35 have also
been reported. Selective hydroboration of aldehyde over
ketone, however, has only recently been reported using
Fe(acac)3,16 aluminum monohydride,25 diazaphosphine,32
and [(p-cymene)RuCl2]29 catalysts. In these cases, ketones
can also be hydroborated by increasing either the catalyst
loading, reaction temperature or reaction time.
Herein we report the synthesis of a paramagnetic,
imine-coupled Fe(II) complex, [Fe(N2S2)]2
1 and its ap-
plication to selective hydroboration catalysis of various
aliphatic and aromatic aldehydes at low catalyst loading
(0.1 mol %) at room temperature using HBpin. In this
unique example of iron-catalysed carbonyl hydroboration,
the reaction tolerates a variety of reducible functional
groups, including nitriles, amines, alkenes, halides, and
ketones even at elevated temperatures.
Scheme 1. Synthesis of [Fe(N2S2)]2, 1
Single crystals of for X-ray diffraction were grown
1
from a saturated THF solution. Interestingly, the solid-
state structure of shows formation of a thiolate-bridged
1
dimer containing imine-coupled N2S2 ligands (Figure S7
in the Supporting Information). One of the two thiolates
in each monomer binds to an iron centre in the other, link-
ing the two. Moreover, the imine groups of two tridentate
SMeNS ligands have been transformed into a diamido unit,
We recently reported an easily-prepared benzothiazoli-
dine ligand, [SMeNHS] containing a potentially labile thi-
oether donor, that undergoes facile ring-opening to afford
a series of mono-, di- and trinuclear Fe(II) complexes
ACS Paragon Plus Environment

ACS Catalysis
forming the redox-active (N2S2)2- ligand with two uncoor-
Page 2 of 7
1
2
3
4
5
6
7
8
dinated thioether groups.
mol %) at 60 C for 16 h, less than 10% of the corresponding
borate ester was detected, indicating that precatalyst 1’ is se-
lective toward aldehydes (vs. ketones). A brief screening of
solvents showed that both tetrahydrofuran and acetonitrile
gave comparable yields of the hydroboration product (Table 1,
entries 8 and 9). Without precatalyst 1’ less than 5% of the
hydroboration product was formed in two hours (entry 10).
While 1 is dimeric in the solid-state, in solution (CD2Cl2,
THF-d8 and C6D6; Figures S1-S3) it exists solely as a mono-
mer that retains the square pyramidal coordination about iron
(Scheme 1). The monomeric structure in solution is confirmed
1
by a broad paramagnetic singlet (3H) at  88.03 in the H
NMR spectrum due to coordination of one of the thioether
groups to the paramagnetic iron centre; the methyl resonance
of the uncoordinated S-Me is observed at  2.21. The dimeric
structure of 1 was not observed in any solvent. The identity of
1’ [Fe(N2S3)] was further confirmed by electrospray mass
spectrometry (Figures S5-S6). The solution magnetic moment
of 1’ at room temperature (measured by Evans’ method37) is
2.7 μB, consistent with two unpaired spins. Detailed analysis of
the magnetic properties and electronic interactions between
the high-spin, square pyramidal iron centre and the redox ac-
tive (N2S2)2- ligand of 1’ will be published elsewhere.
With the optimized reaction conditions in hand, the scope of
the hydroboration was examined with a variety of aldehydes.
Precatalyst 1’ showed remarkable efficiency with excellent
selectivity in hydroboration reactions of both aliphatic and
aromatic aldehydes (Table 2). Cyclic aliphatic aldehyde, elec-
tron-rich 1-pentanal and 2-(methylthio) benzaldehyde all af-
forded high yields of their corresponding borate esters (entries
1, 2 and 15). The ,-unsaturated aldehyde yielded a single
product (entry 3). Aromatic aldehydes containing electron-
withdrawing as well as reducible functional groups showed
excellent tolerance (entries 7-12, 14) yielding > 99% of the
hydroboration products under these conditions. Interestingly,
electron-donating aromatic aldehydes gave low to moderate
yields (entries 5 and 13), in contrast to 1-pentanal, suggesting
that both steric and electronic factors are at play in this system.
Significantly, terephthalaldehyde was cleanly converted into
the bis(borate ester) derivative with 2 eq. of HBpin (entry 16).
However, when 1 eq. of HBpin was treated with equimolar
terephthalaldehyde at room temperature, as well as heating at
50 C for 4 h, only the monohydroborated product was ob-
served, indicating insignificant activation by the para-
CH2OBpin group. Importantly, 4-acetylbenzaldehyde under-
went selective hydroboration only at the aldehyde group (entry
17) demonstrating the synthetic utility of this system. As ke-
tone substrates are not hydroborated even at elevated tempera-
tures, this is a rare example of exclusive aldehyde selectivity
over ketone when compared, for example, to Fe(acac)3-
catalyzed chemoselective hydroboration of aldehydes over
ketones in which poor selectivity was achieved with 3-
acetylbenzaldehyde.16 To compare the efficiency of 1’ with a
closely related derivative, we prepared and fully characterized
the diamagnetic phosphite complex, [Fe(N2S2)P(OMe)3], 2
(Scheme 2).38 Treatment of 1 eq. of benzaldehyde with 1 eq.
of HBpin catalyzed by 0.1 mol % of 2 afforded 86% of the
corresponding borate ester in 30 minutes (Table 1, entry 11).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
We commenced our catalytic study using complex 1’ as a
catalyst for the reduction of carbonyl compounds. While poor
results were obtained using H2 or Et3SiH, treatment of 1 eq. of
benzaldehyde, with 1 eq. of HBpin and 10 mol % of 1’ in
C6D6 at room temperature gave the hydroboration product in
quantitative yield in one hour (Table 1, entry 1) as observed
by both 1H and 11B NMR spectroscopy. While the purple solu-
tion of 1’ was unchanged upon addition of benzaldehyde, once
HBpin was added, the colour changed quickly to light beige or
colorless, showing that complex 1’ acts as a precatalyst. Re-
ducing the catalyst loading to 0.1 mol % still gave excellent
yields (> 80%) within 15 minutes (entry 2) that were essential-
ly quantitative after 30 minutes (entry 3) at room temperature.
In contrast, use of alternate Fe(II) precatalysts (i.e., FeCl2,
Fe(OTf)2, and Fe{N(SiMe3)2}2) gave much lower yields of
the hydroboration product along with dark precipitates (entries
4-6), underscoring the uniqueness of 1’.
A similar test with the [SMeNHS] ligand alone (entry 7)
failed to give more than 5% hydroboration product. Surpris-
ingly, reaction of equimolar acetophenone and HBpin using
0.1 mol % of 1’ at room temperature afforded less than 5% of
the hydroboration product. Even with high catalyst loading (10
Table 1. Optimization of reaction conditionsa
Entry
Catalyst (mol %)
Solvent
Time (h)
Yield (%)b
1
1’ (10)
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
THF-d8
CD3CN
C6D6
C6D6
1
> 99
> 80
> 99
20
2
1’ (0.1)
0.25
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2
3
1’ (0.1)
4
Fe{N(TMS)2}2 (0.5)
FeCl2 (0.5)
Fe(OTf)2 (0.5)
[SMeNHS] ligand (1)
1’ (0.1)
5
35
Scheme 2. Synthesis of [Fe(N2S2)P(OMe)3], 2
Table 2. Scope of hydroboration of aldehydes
6
51
7
< 5
95
8
9
1’ (0.1)
> 99
< 5
86
10
None
11
2 (0.1)
0.5
aReaction conditions: benzaldehyde (0.436 mmol), HBpin (0.436 mmol),
solvent (0.3 ml), catalyst loading relative to benzaldehyde. bYields were
determined by 1H NMR spectroscopy using mesitylene as an internal stand-
ard.
Entry Aldehyde
Product
Yield (%)a
2
ACS Paragon Plus Environment

Page 3 of 7
ACS Catalysis
In order to gain insight about potential catalytic intermedi-
1
2
3
4
5
6
7
8
ates or resting states in this hydroboration system, catalytic
hydroboration of 4-(trifluoromethyl) benzaldehyde was scaled
up using 1 mol % of 1’, and of benzaldehyde using 2.5 mol %
of 2. The remaining brown liquid of the catalytic species, ob-
tained from catalytic hydroboration using 1 mol % of 1’,39
showed a mix of diamagnetic and paramagnetic resonances in
1
2
3
> 99
> 99
85
1
the H NMR spectrum due to unidentified species with no
evidence of precatalyst 1’ (Figure S38). Nonetheless, this mix-
ture further efficiently converted a portion of fresh sub-
strate/HBpin mixture into the borate ester product, confirming
4
5
> 99
70
9
1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
the presence of active catalyst. Likewise, the H NMR spec-
trum of the remaining green solid resulting from catalytic hy-
droboration using 2.5 mol % of diamagnetic species 2 was
also inconclusive, showing a mix of resonances from both
diamagnetic and paramagnetic species (Figure S39). The
31P{1H} NMR spectrum taken during the reaction displays two
singlets due to free P(OMe)3 and an unidentified species along
with a broad singlet due to precatalyst 2 (Figure S40). The
stoichiometric reaction of 2 with excess HBpin at room tem-
perature also provided little insight into the catalyst resting
state, affording small amounts of borohydride, observed pre-
viously in reactions of catecholborane and phosphorus
ligands40 (Figures S41-S42).
6
> 99
7
8
> 99
> 99 (87)
To gain further mechanistic insight into this efficient iron
catalysis, we performed kinetic studies to identify the resting
state and substrate dependence of the catalytic process. We
initially carried out analysis using the Reaction Progress Ki-
netic Analysis (RPKA) technique.41,42 Using this technique,
the kinetic order of the aldehyde and HBpin can be solved by
carrying out a series of experiments in which the initial con-
centrations of each component are varied.
9b
10
11
12
39
> 99
> 99
> 99
Using less reactive 4-methylbenzaldehyde, we were able to
validate that the observed reaction rate is indeed sensitive to
both the initial concentration of aldehyde and HBpin. Increas-
ing initial concentration of either aldehyde or HBpin results in
a commensurate increase in the observed rate of reaction, sug-
gesting that the catalytic system bears a positive order in both
components. However, extracting a meaningful integer value
for the order in each component was not straightforward, as
both the percent conversion and shape of the reaction progress
curve changed dramatically depending on if the reaction was
performed with equimolar (blue) or excess (red and yellow)
starting materials (Figure 1).
13
26
14
15
> 99
> 99
450
400
350
300
250
200
150
100
50
16c
> 99
17
> 99
aConditions: Aldehyde (0.436 mmol) was added to 1’ (0.1 mol %) in C6D6
(0.3 ml) followed by HBpin (0.436 mmol) at room temperature. All reac-
tions afforded a single product and yields were calculated vs. mesitylene
internal standard (0.058 mmol), average of at least 2 runs. bCD3CN (0.4 ml)
was used. c2 equiv. of HBpin were used. Isolated yield in parentheses using
1 mol % of 1’.
0
0
5
10
15
20
time (min)
3
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 7
Figure 1. Effect of concentration on rate of formation of product
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
in the hydroboration of 4-methylbenzaldehyde by precatalyst 1’
([1’] = 0.44 mM, 23 °C, C6H6; blue [CHO] = [HBpin] = 450 mM;
red [CHO] = 450 mM, [HBpin] = 675 mM; yellow [CHO] = 675
mM, [HBpin] = 450 mM).
5
6
7
8
When utilizing the RPKA method to extract information,
analysis of the data is simplified because the difference in
initial concentration of the two components (designated the
reaction excess) is maintained for each experiment. However,
for hydroboration using Fe precatalyst 1’, analysis by ReactIR
revealed that in a typical experiment, the rate of consumption
of HBpin was greater than that for the aldehyde (Figure 2).
This results in incomplete reduction of the aldehyde at short
reaction times, unless more than one equivalent of HBpin is
present.
max conv.
0.65
0.86
0.92
0.91
Cycle 1
Cycle 2
Cycle 3
Cycle 4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0
5
10
time (min)
15
20
Figure 3. Comparing process efficiency for the first substrate
addition and subsequent additions. See also Figure 4, and SI, Fig-
ure S44 for concentration profiles.
Analysis of the reaction revealed no obvious by-products to
account for the extra consumption of HBpin.43 Due to this
observation, we hypothesized that HBpin was being utilized to
activate the initial iron catalyst. Consistent with this, a dra-
matic colour change was observed upon addition of substrates
to a solution of 1’ (from deep purple to either beige or colour-
less). Kinetic evidence for catalyst activation could be gleaned
These observations revealed conclusively that the iron cata-
lyst needs to undergo irreversible activation through reaction
with HBpin to form the active complex. Furthermore, once
activated, this catalyst is both exceptionally active (capable of
producing ca. 40 mmol product/min) and very long-lived. In
our studies, we demonstrated that a TON of ca. 5200 was easi-
ly achieved; note that this is by no means the limit of the activ-
ity. Importantly, reaction of the iron complex with HBpin in
the absence of aldehyde appears to be detrimental. In experi-
ments where the order of reagent addition was reversed, that is
HBpin was added before aldehyde (Figure 4, cycle 5), we see
a net decrease in the rate of reduction in the subsequent exper-
iment (Figure 4, cycle 6).
450
[aldehyde]
400
350
300
250
200
150
100
50
[HBpin]
450
400
350
300
250
200
150
100
50
0
0
10
20
30
time (min)
Figure 2. Unequal rates of consumption of aldehyde and HBpin
in the hydroboration of p-methylbenzaldehyde by precatalyst 1’
([1’] = 0.44 mM, 23 °C, C6H6, [CHO] = [HBpin] = 450 mM).
by carrying out a series of reductions where aliquots of both
aldehyde and HBpin are added, allowing the reaction to pro-
ceed to completion between each subsequent addition. These
experiments revealed several key features. First, conversion to
product is lowest after the first equimolar addition of aldehyde
and HBpin, and increases in all subsequent doses (~65% con-
version cf. ~90% for latter cycles; Figure 3). In addition, the
rate of reaction appears to increase after the first aliquot addi-
tion. This is observed from the reaction profile for the concen-
tration of HBpin for a series of sequential additions (Figure 3,
cycle 1 vs. 2-4).
0
0
100
200
300
400
time (min)
Figure 4. Concentration of HBpin throughout the multi-dose ex-
periment. The experiment was initiated by the addition of HBpin
(0.45 mmol), and aldehyde (0.45 mmol) to a solution of 1’ (0.44
µmol) in C6H6 at 23 °C. Cycles (2-4, 6) were triggered by the
addition of HBpin (0.45 mmol), followed by aldehyde (0.45
mmol), ca. 15 s later. In Cycle 5, the order of addition of the rea-
gents was reversed.
As no reaction of precatalyst 1’ was observed with alde-
hydes, we investigated its reaction with the more reactive car-
bonyl moiety, carbon monoxide. Treatment of complex 1 with
CO in acetonitrile afforded an iron complex, [Fe(3-SNS)(2-
1
SNS)CO], 3 (Scheme 3) which was characterized by IR, H
NMR and X-ray crystallography. The IR spectrum (Figure
S13) of 3 in the solid state shows a strong and sharp CO
stretching vibration at 1952 cm-1, confirming the addition of a
CO ligand to the iron centre. The X-ray data show that car-
4
ACS Paragon Plus Environment

Page 5 of 7
ACS Catalysis
bonylation of 1’ cleaves the diamido C-C bond, generating a
catalysed hydroboration system will therefore be attractive for
synthetic, medicinal and fine chemical catalysis.
1
2
3
4
5
6
7
pseudooctahedral iron centre occupied by one CO and two
1
SNS ligands (Figure S14). Interestingly, the H NMR spec-
AUTHOR INFORMATION
trum of diamagnetic complex 3 dissolved in CD2Cl2 shows
also the paramagnetic resonances of complex 1’, indicating
that CO addition to iron is reversible in solution.
Corresponding Authors
*Email for R. Tom Baker: rbaker@uottawa.ca and for Jason E.
Hein: jhein@chem.ubc.ca
Notes
8
The authors declare no competing financial interest.
9
ASSOCIATED CONTENT
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org. Detailed experimental procedures,
characterization of the Fe(II) complexes, crystallographic details
1
of Fe(II) complexes, and copies of H, 11B, and 31P{1H} NMR
spectra.
Scheme 3. Synthesis of [Fe(3-SNS)(2-SNS)CO], 3
ACKNOWLEDGMENT
We thank the NSERC (2014-RGPIN-05841 to R.T.B. and 2016-
RGPIN-04613 to J.E.H.) for generous financial support and the
University of Ottawa, the University of British Columbia, Canada
Foundation for Innovation, and Ontario Ministry of Economic
Development and Innovation for essential infrastructure. Donation
of process analytical equipment (ReactIR and EasyMax) from
Mettler Toledo Autochem (to J.E.H.) is gratefully acknowledged.
Additionally, we thank Merck Research Labs for postdoctoral
research support (to C.S.H.) Glenn Facey and Eric Ye are thanked
for assistance with NMR experiments.
Metal-catalyzed hydroboration of carbonyl compounds typ-
ically proceeds through either Lewis acid substrate activation
or B-H bond activation.4 In the latter case formation of an iron
hydride could be accompanied by boryl transfer to either the
nitrogen or sulphur (Scheme 4a).44-46 In light of the reversible
reaction of precatalyst 1’ with CO, however, one may also
have to consider reaction pathways that involve bifunctional
Fe SNS catalysts (Scheme 4b).
REFERENCES
(1) Arai, N.; Ohkuma, T. In Modern Reduction Methods; Wiley-
VCH Verlag GmbH & Co. KGaA: 2008, p 159-181.
(2) Washburn, R. M.; Levens, E.; Albright, C. F.; Billig, F. A. In
Metal-Organic Compounds; Adv. Chem. Ser. 1959, 23, 129-157.
(3) Roesky, H. W.; Atwood, D. A. Group 13 Chemistry III:
Industrial Applications; Springer-Verlag: Berlin, 2003; Vol.
105, p 1-40.
(4) Chong, C. C.; Kinjo, R. ACS Catal. 2015, 5, 3238-3259.
(5) Murphy, J. M.; Lawrence, J. D.; Kawamura, K.; Incarvito, C.;
Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 13684-13685.
(6) Hartwig, J. F. Acc. Chem. Res. 2012, 45, 864-873.
(7) Preshlock, S. M.; Plattner, D. L.; Maligres, P. E.; Krska, S. W.;
Maleczka, R. E.; Smith, M. R. Angew. Chem. Int. Ed. 2013, 52,
12915-12919.
(8) Demory, E.; Devaraj, K.; Orthaber, A.; Gates, P. J.; Pilarski, L.
T. Angew. Chem. Int. Ed. 2015, 54, 11765-11769.
(9) Kaithal, A.; Chatterjee, B.; Gunanathan, C. Org. Lett. 2015, 17,
4790-4793.
(10) Lindsley, C. W.; DiMare, M. Tetrahedron Lett. 1994, 35, 5141-
5144.
(11) Giffels, G.; Dreisbach, C.; Kragl, U.; Weigerding, M.;
Waldmann, H.; Wandrey, C. Angew. Chem. Int. Ed. 1995, 34,
2005-2006.
(12) Almqvist, F.; Torstensson, L.; Gudmundsson, A.; Frejd, T.
Angew. Chem. Int. Ed. 1997, 36, 376-377.
(13) Sarvary, I.; Almqvist, F.; Frejd, T. Chem. Eur. J. 2001, 7, 2158-
2166.
(14) Oluyadi, A. A.; Ma, S.; Muhoro, C. N. Organometallics 2013,
32, 70-78.
Scheme 4. Potential B-H bond activation pathways for
aldehyde hydroboration catalysis using 1’ (a, top) and 3 (b,
bottom)
Finally, our observations highlighting the robust and effi-
cient nature of the iron catalyst led us to push the limits of
stability for this system. Thus, we could demonstrate that this
catalyst is tolerant of many common species that would typi-
cally deactivate such a metal catalyst. This includes running
the reaction with crude aldehyde (contaminated with 5% 4-
methylbenzoic acid) and even performing the reduction in
open air (see SI, Figures S45–S46). Further kinetic studies are
ongoing with catalysts 1 and 2 and their stable redox partners,
i.e., (1’)+ and (1’)-.
In summary, we have prepared and characterized a five-
coordinate, paramagnetic imine-coupled iron complex,
[Fe(N2S2)]2 that demonstrates excellent efficiency and selec-
tivity in hydroboration catalysis of various aldehydes. The key
advantages of this process are its exclusive aldehyde selectivi-
ty over ketone, wide reducible functional group tolerance,
mild reaction conditions and catalyst lifetime. This simple iron
(15) Khalimon, A. Y.; Farha, P.; Kuzmina, L. G.; Nikonov, G. I.
Chem. Commun. 2012, 48, 455-457.
(16) Tamang, S. R.; Findlater, M. J. Org. Chem. 2017, 82, 12857-
12862.
(17) Guo, J.; Chen, J.; Lu, Z. Chem. Commun. 2015, 51, 5725-5727.
(18) Bagherzadeh, S.; Mankad, N. P. Chem. Commun. 2016, 52,
3844-3846.
5
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 7
(19) Mukherjee, D.; Osseili, H.; Spaniol, T. P.; Okuda, J. J. Am.
Chem. Soc. 2016, 138, 10790-10793.
(20) Arrowsmith, M.; Hadlington, T. J.; Hill, M. S.; Kociok-Kohn,
G. Chem. Commun. 2012, 48, 4567-4569.
(21) Mukherjee, D.; Ellern, A.; Sadow, A. D. Chem. Sci. 2014, 5,
959-964.
1
2
3
4
5
6
7
8
(22) Lampland, N. L.; Hovey, M.; Mukherjee, D.; Sadow, A. D. ACS
Catal. 2015, 5, 4219-4226.
(23) Yadav, S.; Pahar, S.; Sen, S. S. Chem. Commun. 2017, 53,
4562-4564.
(24) Yang, Z.; Zhong, M.; Ma, X.; Nijesh, K.; De, S.; Parameswaran,
P.; Roesky, H. W. J. Am. Chem. Soc. 2016, 138, 2548-2551.
(25) Jakhar, V. K.; Barman, M. K.; Nembenna, S. Org. Lett. 2016,
18, 4710-4713.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(26) Blake, A. J.; Cunningham, A.; Ford, A.; Teat, S. J.; Woodward,
S. Chem. Eur. J. 2000, 6, 3586-3594.
(27) Roh, S.-G.; Park, Y.-C.; Park, D.-K.; Kim, T.-J.; Jeong, J. H.
Polyhedron 2001, 20, 1961-1965.
(28) Locatelli, M.; Cozzi, P. G. Angew. Chem. Int. Ed. 2003, 42,
4928-4930.
(29) Roh, S. G.; Yoon, J. U.; Jeong, J. H. Polyhedron 2004, 23,
2063-2067.
(30) Lummis, P. A.; Momeni, M. R.; Lui, M. W.; McDonald, R.;
Ferguson, M. J.; Miskolzie, M.; Brown, A.; Rivard, E. Angew.
Chem. Int. Ed. 2014, 53, 9347-9351.
(31) Hadlington, T. J.; Hermann, M.; Frenking, G.; Jones, C. J. Am.
Chem. Soc. 2014, 136, 3028-3031.
(32) Chong, C. C.; Hirao, H.; Kinjo, R. Angew. Chem. Int. Ed. 2015,
54, 190-194.
(33) Weidner, V. L.; Barger, C. J.; Delferro, M.; Lohr, T. L.; Marks,
T. J. ACS Catal. 2017, 7, 1244-1247.
(34) Chen, S.; Yan, D.; Xue, M.; Hong, Y.; Yao, Y.; Shen, Q. Org.
Lett. 2017, 19, 3382-3385.
(35) Eedugurala, N.; Wang, Z.; Chaudhary, U.; Nelson, N.; Kandel,
K.; Kobayashi, T.; Slowing, I. I.; Pruski, M.; Sadow, A. D. ACS
Catal. 2015, 5, 7399-7414.
(36) Das, U. K.; Daifuku, S. L.; Gorelsky, S. I.; Korobkov, I.;
Neidig, M. L.; Le Roy, J. J.; Murugesu, M.; Baker, R. T. Inorg.
Chem. 2016, 55, 987-997.
(37) Evans, D. F. J. Chem. Soc. 1959, 2003-2005.
(38) See experimental section and Supporting Information for X-ray
structure and selected bond lengths & angles of 2.
(39) See Supporting Inofrmation for detailed procedure of catalytic
hydroboration
of
4-(trifluoromethyl)benzaldeyde
using
precatalyst 1.
(40) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T.;
Calabrese, J. C. Inorg. Chem. 1993, 32, 2175-2182.
(41) Blackmond, D. G. Angew. Chem. Int. Ed. 2005, 44, 4302-4320.
(42) Blackmond, D. G. J. Am. Chem. Soc. 2015, 137, 10852-10866.
(43) The data reveal consumption of a quantity of HBpin in excess of
the equimolar quantity needed solely for stoichiometric catalyst
activation or reaction with aldehyde. This does point to an as-
yet-unidentified pathway for the consumption of HBpin.
Products arising from dehydrocoupling (B2pin2) or diborylation
of substrate were not observed.
(44) Stahl, T.; Müther, K.; Ohki, Y.; Tatsumi, K.; Oestreich, M. J.
Am. Chem. Soc. 2013, 135, 10978-10981.
(45) Xu, Y.; Rettenmeier, C. A.; Plundrich, G. T.; Wadepohl, H.;
Enders, M.; Gade, L. H. Organometallics 2015, 34, 5113-5118.
(46) Song, H.; Ye, K.; Geng, P.; Han, X.; Liao, R.; Tung, C.-H.;
Wang, W. ACS Catal. 2017, 7709-7717.
6
ACS Paragon Plus Environment

Page 7 of 7
ACS Catalysis
1
2
SYNOPSIS
3
4
5
An imine-coupled [Fe-N2S2]2 complex, catalyzes selectively the hydroboration of aliphatic and aromatic
aldehydes at low catalyst loadings (0.1 mol %) using pinacolborane.
6
7
8
9
Table of Contents
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
7
ACS Paragon Plus Environment