Manganese catalyzed N-alkylation of anilines with alcohols: Ligand enabled selectivity

Article abstract of DOI:10.1039/c8ob01886c

Ligand enabled Earth-abundant manganese catalyzed N-alkylation of amines with alcohols via a hydrogen auto-transfer strategy is reported. The choice of the ligand plays a significant role in the alcohol reactivity (aliphatic or aromatic) toward N-alkylation reactions.

Full text of DOI:10.1039/c8ob01886c

Organic &
Biomolecular Chemistry
View Article Online
View Journal | View Issue
PAPER
Manganese catalyzed N-alkylation of anilines with
alcohols: ligand enabled selectivity†
Cite this: Org. Biomol. Chem., 2018,
16, 8175
Vinod G. Landge, Akash Mondal, Vinit Kumar, Avanashiappn Nandakumar and
Ekambaram Balaraman
*
Received 3rd August 2018,
Accepted 8th October 2018
Ligand enabled Earth-abundant manganese catalyzed N-alkylation of amines with alcohols via a hydro-
gen auto-transfer strategy is reported. The choice of the ligand plays a significant role in the alcohol reac-
tivity (aliphatic or aromatic) toward N-alkylation reactions.
DOI: 10.1039/c8ob01886c
rsc.li/obc
last few decades, this concept had taken an elegant leap from
both synthesis and application points of view.8–12
Introduction
Amines constitute an important class of compounds and find
Manganese is the third Earth-abundant metal in the
manifold applications in the synthesis of pharmaceuticals, Earth’s crust, cheap, and biorelevant.13 In recent times, the
agrochemicals, polymers, dyes, and fine chemicals.1–3 In this use of manganese as a benign alternative to precious metal
regard, the development of sustainable methods for the for- catalysts in homogeneous catalysis is interesting and has
mation of the C–N bond is in high demand. Traditionally, the received much attention. The research group of Milstein used
C–N bond formation has been carried out using the reaction a Mn–PNP pincer complex for the coupling of alcohols and
of amines with alkyl halides. However, this process has serious amines to form aldimines.14 Noteworthy progress has been
limitations such as over-alkylation, and the formation of non- made using electron-rich phosphine based Mn-complexes by
selective byproducts. An alternative method for the synthesis various groups.15 The pioneering work by Beller and co-
of amines is the reduction of amides by utilizing stoichio- workers demonstrated that a well-defined Mn-complex stabil-
metric amounts of reactive reagents which produce copious ized with an electron-rich tridentate phosphine ligand is an
waste. Also, the Buchwald–Hartwig reaction,4 the Ullmann efficient catalyst for the N-alkylation of amines using alcohol
reaction5 and hydroamination6 impose a serious challenge to as an alkylating reagent (Scheme 1).15b,c
N-alkylation, as these reactions use hazardous, prefunctiona-
lized chemicals which leads to low atom- and step-economy.
Notably, the known Mn-catalytic systems consist of elec-
tron-rich phosphine ligands. Despite the widespread appli-
Of late, the hydrogen auto-transfer (HA) or borrowing cations and tremendous success of phosphine ligands in
hydrogen (BH) strategy is one of the chemical reactions in the homogeneous catalysis,16,17 the identification of an alternative
chemist’s tool-box for the preparation of amines.7 ligand system to electron-rich phosphines is in high demand
Mechanistically, HA includes three principal steps: (1) de- in academia and industry. Here, we report a manganese cata-
hydrogenation of an alcohol to a reactive carbonyl compound, lytic system in combination with a simple and efficient nitro-
(2) condensation of the amine with the carbonyl derivative to
yield an imine, (3) in situ hydrogenation of the imine using the
hydrogen generated in the initial dehydrogenation step selec-
tively leading to the N-alkylated amine. This domino reaction
seldom requires an oxidant in the initial step, and an external
hydrogen source for the final hydrogenation step. Thus, it can
circumvent the generation of metal salt waste to yield the
desired N-alkylated product with the formation of water. In the
Organic Chemistry Division, Dr. Homi Bhabha Road, CSIR-National Chemical
Laboratory (CSIR-NCL), Pune – 411008, India. E-mail: eb.raman@ncl.res.in
†Electronic supplementary information (ESI) available: Details of experimental
procedures, characterization of compounds and copy of NMR data. See DOI:
10.1039/c8ob01886c
Scheme 1 Selected phosphine based Mn-complexes for (de)hydrogen-
ation reactions.
This journal is © The Royal Society of Chemistry 2018
Org. Biomol. Chem., 2018, 16, 8175–8180 | 8175

View Article Online
Paper
Organic & Biomolecular Chemistry
gen based ligand for selective N-alkylation of amines with tion and gave 4a in 74% and 80% yields, respectively (Table 1,
alcohols.
entries 5 and 6). Notably, the salen based ligand L5 also
reacted well and gave 4a in 91% yield.
Under the optimal conditions, other manganese salts
(MnCl2, Mn(OAc)2, MnBr2, and Mn2(CO)8) failed to give 4a. By
lowering the oil-bath temperature from 140 to 100 °C, 60%
yield of 4a was obtained (Table 1, entry 9). The control experi-
ments revealed that the catalyst, ligand, and base are impor-
tant for this transformation (Table 1, entries 10–12). The
efficiency of the reaction was significantly affected in the
absence of the base, and KOtBu showed significant improve-
ment in the yield of 4a (Table 1, entries 12–14). The reaction
was carried out with a catalytic amount of KOtBu under
optimal conditions; however, the N-alkylated product 4a was
observed in lower yield (30%). The effect of other bases was
examined under our Mn-catalyzed conditions (see the ESI†).
Further screening of the solvent did not influence the
N-alkylation of amine (Table 1, entry 15).
After establishing the optimized reaction conditions
(Table 1), we set out to prove its versatility in the N-alkylation
of various substituted alcohols and amines. The developed
synthetic methodology is general and has a broad substrate
scope (Tables 2 and 3). Regardless of the positions of the elec-
tron-donating and the electron-withdrawing substituents on
anilines, the reaction proceeded smoothly under manganese
catalysis. Aniline derivatives possessing the para substituents,
such as methoxy, and chloro substituents were converted into
the corresponding products in moderate to good yields (3b in
Results and discussion
Initially, we began our investigation of the manganese cata-
lyzed N-alkylation reaction using m-toluidine 1a, and benzyl
alcohol 2a as the benchmark substrates. To our delight, the
N-alkylated product 4a was obtained in 65% yield, by employ-
ing commercially available MnBr(CO)5 (5 mol%) as the Mn-
source, dppb (5 mol%) as the ligand, and KOtBu (1.1 equiv.) as
the base, in toluene (Table 1, entry 1). Next, we examined the
effect of a nitrogen based ligand such as 1,10-phenanthroline
under the same reaction conditions. However, the required
product (4a) was obtained in poor yield (Table 1, entry 2).
Indeed, the commercially available tridentate ligand L1 was
found to be beneficial for this transformation and gave 92%
isolated yield of 4a (Table 1, entry 3). By changing tridentate to
NON-ligand L2, a moderate yield of 4a was observed (Table 1,
entry 4). Gratifyingly, sulfur containing tridentate SNS-ligands
L3 and L4 were also effective in this C–N bond forming reac-
Table 1 Optimization of the reaction conditionsa,b
Entry
Reaction conditionsa
Yield of 4a b (%)
Table 2 Scope of aminesa,b
1
2
3
4
dppb used as ligand
1,10-Phenanthroline used as ligand
L1 used as ligand
65
18
92 (58)c
70
L2 used as ligand
5
6
L3 used as ligand
L4 used as ligand
74
80
7
8
9
L5 used as ligand
MnCl2 used as [Mn] source
At 100 °C
91
Trace
60
10
11
12
13
14
15
Without [Mn] cat.
Without L1
Without KOtBu
KOH instead of KOtBu
LiOtBu instead of KOtBu
1,4-Dioxane used as solvent
n.d.
20
Trace
40
51
75
a Reaction conditions: Aniline 1a (0.5 mmol), alcohol 2a (0.55 mmol),
MnBr(CO)5 (5 mol%), ligand (5 mol%), KOtBu (0.55 mmol), 1 mL
toluene, 140 °C (oil-bath temperature), 18 h. b Isolated yields. L1 = N1- a Reaction conditions: Aniline 1 (0.5 mmol), alcohol 2a (0.55 mmol),
(3-(Dimethylamino)propyl)-N3,N3-dimethylpropane-1,3-diamine; L2
=
MnBr(CO)5 (5 mol%), ligand (5 mol%), KOtBu (0.55 mmol), 1 mL
2,2′-Oxybis(N,N-dimethylethanamine); L3
=
Bis(2-(methylthio)ethyl) toluene, 140 °C (oil-bath temperature), 18 h. b Isolated yield. c Ligand
amine; L4 = Bis(2 (isopropylthio)ethyl)amine. c Under open air atm.
L5 was used.
8176 | Org. Biomol. Chem., 2018, 16, 8175–8180
This journal is © The Royal Society of Chemistry 2018

View Article Online
Organic & Biomolecular Chemistry
Paper
Table 3 Scope of alcoholsa,b
source gave a poor yield of the products. This experiment
result highlights the ligand enabled selectivity in the alcohol
reactivity towards N-alkylation reactions. Remarkably, (R)-
(+)-β-citronellol underwent N-alkylation selectively to give 4i in
75% yield with the retention of the double bond and the chir-
ality under our Mn-catalysis (Mn/L5).
The choice of the ligand plays a critical role in the alcohol
reactivity (aliphatic vs. aromatic) towards the present C–N
bond formation by the Mn-catalyst. To demonstrate this
phenomenon, 4-amino phenyl ethyl alcohol (5a) was selected
as a model substrate (Table 4). Thus, the reaction of 5a with
benzyl alcohols (2) in the presence of the L1 ligand under stan-
dard reaction conditions led to the N-alkylated products in
moderate to good yields (63%–85% yield). Interestingly, the
alcoholic motif in 5a remains intact. Various para-substituted
benzyl alcohols such as 4-Me, 4-Cl, 4-Br, and 4-Ph substituted
alcohols underwent the N-alkylation reaction under optimized
conditions to give the corresponding products in 63–85%
yields. The ortho- and meta-substituted benzyl alcohols are
also suitable substrates for this N-alkylation process and
offered the expected products in moderate to good yields
(Table 4, products 6g–6i). Notably, 3,4-dimethoxy benzyl
alcohol and furfuryl alcohol underwent the N-alkylation reac-
a Reaction conditions: Aniline 1a (0.5 mmol), alcohol 2 (0.55 mmol),
MnBr(CO)5 (5 mol%), ligand (5 mol%), KOtBu (0.55 mmol), 1 mL
toluene, 140 °C (oil-bath temperature), 18 h. b Isolated yield. c Ligand
L5 was used.
67%, and 3c in 65% yields). The substituents on anilines such tion and afforded 6j in 76% and 6k in 63% isolated yields,
as trifluoromethyl, fluoro, chloro and phenyl substituents were respectively.
tolerated and provided the corresponding C–N bonded pro-
To highlight the synthetic utility of this procedure and the
ducts (3d, and 3d–3g) in moderate yields. The disubstituted tri- switching of N-alkylation under different ligand systems a
fluoromethyl (1h), and dimethoxy (1i) anilines underwent the sequential N-alkylation of diamine such as m-phenylene-
N-alkylation reaction and afforded the expected products 3h diamine 7a was carried out using L1 and L5 ligands
and 3l in 95% and 45% isolated yields, respectively. The under optimal conditions (Scheme 2). The reaction proceeded
heterocyclic anilines such as 2-aminopyridine, 8-aminoquino-
line, and 6-aminoquinoline were well tolerated under Mn-cata-
lysis in the presence of the L5 ligand to give the expected pro-
ducts 3j in 85%, 3k in 72%, and 3l in 75% yields. However, the
Table 4 Scope of aminesa,b
ligand L1 under optimized reaction conditions gave a poor
yield of 3j (35% yield). This is because the amine 1j provides a
stronger coordination site than L1 to the Mn center.
Unfortunately, aliphatic amines such as 1-pentyl amine and
octylamine were ineffective and did not yield the desired
N-alkylated products under optimal conditions.
Next, the scope of alcohol substrates in Mn-catalyzed C–N
bond forming reactions was tested, and the results are sum-
marized in Table 3. Benzyl alcohols containing various substi-
tuents such as 4-Me and 4-Cl had undergone N-alkylation
under the optimized conditions to offer the desired products
(4b–c) in moderate to good yields. The ortho-substituted meth-
oxybenzyl alcohol reaction proceeded smoothly and gave the
product 4d in 78% isolated yield. Gratifyingly, the heterocyclic
furfuryl alcohol was well tolerated and afforded the N-alkylated
product 4e in 74% yield. Notably, aliphatic alcohols were also
susceptible to the N-alkylation process under optimized con-
ditions in the presence of ligand L5. Thus, various aliphatic
alcohols such as n-butanol, octanol, and decanol underwent
a Reaction conditions: 5a (0.5 mmol), alcohol 2 (0.55 mmol), MnBr
N-alkylation smoothly to give the corresponding products 4f
(42%), 4g (58%), and 4h (57%) in moderate yields. Notably,
under similar reaction conditions, ligand L1 and the Mn
(CO)5 (5 mol%), ligand L1 (5 mol%), KOtBu (0.55 mmol), 1 mL
toluene, 140 °C (oil-bath temperature), 24 h. b Yield of the isolated
product.
This journal is © The Royal Society of Chemistry 2018
Org. Biomol. Chem., 2018, 16, 8175–8180 | 8177

View Article Online
Paper
Organic & Biomolecular Chemistry
tate ligand is crucial to obtain the N-alkylated product in a
higher yield. Interestingly, the salen ligand also showed a
similar reactivity to the tridentate ligand. At this moment, it is
challenging to mention in which fashion the ligand coordi-
nates to the Mn-center and how the alcohol gets activated (the
alcohol reactivity). Indeed, the isolation of the Mn complex
with the salen ligand and detailed mechanistic investigation
are in progress in our laboratory.
Scheme 2 Sequential N-alkylation of diamine.
Experimental section
General information
efficiently with benzyl alcohol and afforded the selective
mono-N-alkylated product 7b in 65% isolated yield using All catalytic experiments were carried out using standard
ligand L1. Then applying the L5 ligand yielded the unsym- Schlenk techniques. All solvents were of reagent grade or
metrical N-alkylated product 7c in 60% yield using octanol as better. Deuterated solvents were used as received. Toluene was
the alkylating reagent. Similarly, (R)-(+)-β-citronellol (2i) under- refluxed over sodium/benzophenone followed by distillation
went selective sequential N-alkylation and gave 7e in good under an argon atmosphere and stored over sodium. Metal
yield.
complexes and other chemicals used in catalysis reactions
Interestingly, the salen-based Mn-complex is very efficient were used without additional purification. Thin layer chrom-
in the present N-alkylation of amines with alcohols. Indeed, atography (TLC) was performed using silica gel precoated glass
the isolation of the salen–Mn complex was unsuccessful. plates, which were visualized with UV light at 254 nm or under
Notably, performing the N-alkylation reaction in the presence iodine. Column chromatography was performed with SiO2
of Bu4NBr (a salt known to stabilize nano-particles)18 did not (Silicycle Siliaflash F60 (230–400 mesh). 1H NMR (400 or
enhance the either the rate of the reaction or the yield of the 500 MHz) and 13C{1H} NMR (100 MHz) spectra were recorded
product 4a. Indeed, a low yield of 4a (65%) was observed. using an NMR spectrometer. Deuterated chloroform was used
Moreover, the addition of 200 equiv. of mercury to the catalytic as the solvent, and the chemical shift values (δ) are reported in
reaction mixture slightly reduced the reaction yield (84%), parts per million relative to the residual signals of this solvent
though it could not quench the reaction completely. All these [δ 7.26 for 1H (chloroform-d) and δ 77.2 for 13C{1H} (chloro-
experimental findings showed that the present Mn-catalyst is form-d)]. Abbreviations used in the NMR follow-up experi-
homogeneous in nature and less likely to transform into ments: br, broad; s, singlet; d, doublet; t, triplet; q, quartet;
another active catalytic species (manganese nanoparticles). and m, multiplet. GC analysis was carried out using a HP-5
Performing the reaction in the presence of radical scavenger column (30 m, 0.25 mm, 0.25μ). Mass spectra were obtained
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO; 2 equiv.) did not on a GCMS-QP 5000 instrument with an ionization voltage of
affect the yield of the product. This result indicates that the 70 eV. High resolution mass spectra (HRMS) were obtained by
radical reaction pathway could be ruled out. Indeed, in the fast atom bombardment (FAB) using a double focusing mag-
absence of an amine coupling partner the formation of an netic sector mass spectrometer and an electron impact (EI)
aldehyde product and H2 gas was observed by GC. This result ionization technique (magnetic sector–electric sector double
showed that the N-alkylation proceeds via a dehydrogenative focusing mass analyzer). HPLC analysis was performed on an
coupling strategy. The inductively coupled plasma atomic Agilent Technologies 1260 Infinity with a UV detector.
emission spectroscopy analysis confirmed the absence of
General procedure for the manganese-catalyzed N-alkylation of
anilines
other metallic impurities. Notably, time-dependent experi-
ments for the N-alkylation of 1a using 2a showed that the
present Mn/L1 or Mn/L5 catalytic system is stable during the
catalysis.19
Into an oven-dried 10 mL screw-capped vial, MnBr(CO)5
(0.025 mmol, 5 mol%), ligand L1 or L5 (0.025 mmol, 5 mol%),
alcohol 2 (0.55 mmol, 1.1 equivalents), amine 1 (0.5 mmol),
KOtBu (0.55 mmol, 1.1 equivalents), and toluene (1 mL) were
added under a gentle stream of argon. The reaction mixture
was kept under stirring at 140 °C (oil-bath temperature) for
Conclusions
In summary, we have reported ligand system enabled manga- 18 h. Then, the reaction mixture was diluted with water (4 mL)
nese catalyzed N-alkylation of anilines with alcohols under and extracted with dichloromethane (3 × 5 mL). The resultant
phosphine ligand-free conditions. The sequential N-alkylation organic layer was dried over anhydrous Na2SO4 and the solvent
of diamines with benzyl and aliphatic alcohol by employing was evaporated under reduced pressure. The crude mixture
different ligand systems has been shown. This unified strategy was purified by silica gel column chromatography
has a broad substrate scope and functional group tolerance. (230–400 mesh size) using petroleum-ether/ethyl acetate as an
We observed that the presence of the –NH group in the triden- eluting system.
8178 | Org. Biomol. Chem., 2018, 16, 8175–8180
This journal is © The Royal Society of Chemistry 2018

View Article Online
Organic & Biomolecular Chemistry
Paper
9 (a) D. Morton and D. J. Cole-Hamilton, J. Chem. Soc., Chem.
Commun., 1987, 248–249; (b) T. Zweifel, J.-V. Naubron and
H. Grützmacher, Angew. Chem., Int. Ed., 2009, 48, 559.
10 (a) D. Balcells, A. Nova, E. Clot, D. Gnanamgari,
R. H. Crabtree and O. Eisenstein, Organometallics, 2008, 27,
2529; (b) B. Blank, M. Madalska and R. Kempe, Adv. Synth.
Catal., 2008, 350, 749; (c) A. Prades, R. Corberan,
M. Poyatos and E. Peris, Chem. – Eur. J., 2008, 14, 11474;
(d) D. Gnanamgari, E. L. O. Sauer, N. D. Schley, C. Butler,
C. D. Incarvito and R. H. Crabtree, Organometallics, 2009,
28, 321; (e) B. Blank, S. Michlik and R. Kempe, Adv. Synth.
Catal., 2009, 351, 2903; (f) R. Kawahara, K. Fujita and
R. Yamaguchi, J. Am. Chem. Soc., 2010, 132, 15108;
(g) S. Michlik and R. Kempe, Chem. – Eur. J., 2010, 16,
13193; (h) O. Saidi, A. J. R. Blacker, M. M. Farah,
S. P. Marsden and J. M. J. Williams, Chem. Commun., 2010,
46, 1541; (i) R. Kawahara, K. Fujita and R. Yamaguchi, Adv.
Synth. Catal., 2011, 353, 1161; ( j) I. Cumpstey, S. Agrawal,
E. K. Borbas and B. Martin-Matute, Chem. Commun., 2011,
47, 7827; (k) S. Michlik, T. Hille and R. Kempe, Adv. Synth.
Catal., 2012, 354, 847; (l) S. Agrawal, M. Lenormand and
B. Martin-Matute, Org. Lett., 2012, 14, 1456; (m) J.-Q. Li and
P. G. Andersson, Chem. Commun., 2013, 49, 6131.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research is supported by the SERB (EMR/2015/000030)
and CSIR-INPROTICS P&A (HCP0011-A). VGL thanks the CSIR
and AM thanks the UGC for the fellowships.
Notes and references
1 G. Guillena, D. J. Ramon and M. Yus, Chem. Rev., 2010,
110, 1611.
2 (a) Pharmaceuticals: Classes, Therapeutic Agents, Areas of
Application, ed. J. L. McGuire, Wiley-VCH, 2000, vol. 1–4;
(b) Z. J. Rappoport and F. Liebman, The Chemistry of
Hydroxylamines, Oximes and Hydroxamic Acids, Wiley,
New York, 2009, p. 609.
3 S. A. Lawrence, Amine: Synthesis Properties and Applications,
Cambridge University, Cambridge, 2004.
4 (a) J. Magano and J. R. Dunetz, Chem. Rev., 2011, 111, 2177; 11 (a) T. Yan, B. L. Feringa and K. Barta, Nat. Commun., 2014,
(b) A. S. Kumar and R. Nagarajan, Org. Lett., 2011, 13, 1398;
(c) R. R. Suresh and K. C. K. Swamy, Tetrahedron Lett., 2009,
50, 6004; (d) B. Lü, P. Li, C. Fu, L. Xue, Z. Lin and S. Ma,
Adv. Synth. Catal., 2011, 353, 100.
5, 5602; (b) T. Yan, B. L. Feringa and K. Barta, ACS Catal.,
2016, 6, 381; (c) A. J. Rawlings, L. J. Diorazio and M. Wills,
Org. Lett., 2015, 17, 1086; (d) H.-J. Pan, T. W. Ng and
Y.
Zhao,
Chem.
Commun.,
2015,
51,
11907;
5 E. Sperotto, G. P. M. van Klink, G. van Koten and J. G. de
Vries, Dalton Trans., 2010, 39, 10338.
6 L. Huang, M. Arndt, K. Gooßen, H. Heydt and L. J. Gooßen,
Chem. Rev., 2015, 115, 2596.
7 (a) M. H. S. A. Hamid, P. A. Slatford and J. M. J. Williams,
Adv. Synth. Catal., 2007, 349, 1555; (b) T. D. Nixon,
M. K. Whittlesey and J. M. J. Williams, Dalton Trans.,
2009, 753; (c) K.-I. Shimizu, Catal. Sci. Technol., 2015
5, 1412; (d) A. Nandakumar, S. P. Midya, V. G. Landge
and E. Balaraman, Angew. Chem., Int. Ed., 2015, 54,
11022.
(e) B. Emayavaramban, M. Roy and B. Sundararaju, Chem. –
Eur. J., 2016, 22, 3952; (f) M. Bala, P. K. Verma, U. Sharma,
N. Kumar and B. Singh, Green Chem., 2013, 15, 1687.
12 (a) S. Rösler, M. Ertl, T. Irrgang and R. Kempe, Angew.
Chem., Int. Ed., 2015, 54, 15046; (b) G. Zhang, Z. Yin and
S. Zheng, Org. Lett., 2016, 18, 300; (c) Z. Yin, H. Zeng,
J. Wu, S. Zheng and G. Zhang, ACS Catal., 2016, 6, 6546;
(d) M. Mastalir, G. Tomsu, E. Pittenauer, G. Allmaier and
K. Kirchner, Org. Lett., 2016, 18, 3462; (e) A. Quintard and
J. Rodriguez, ChemSusChem, 2016, 9, 28.
13 D. A. Valyaev, G. Lavigne and N. Lugan, Coord. Chem. Rev.,
2016, 308, 191.
8 (a) D. Hollmann, A. Tillack, D. Michalik, R. Jackstell and
M. Beller, Chem. – Asian J., 2007, 2, 403; (b) M. H. S. 14 A. Mukherjee, A. Nerush, G. Leitus, L. J. Shimon, Y. Ben
A. Hamid and J. M. J. Williams, Chem. Commun., 2007, 725;
(c) M. H. S. A. Hamid and J. M. J. Williams, Tetrahedron
David, N. A. E. Jalapa and D. Milstein, J. Am. Chem. Soc.,
2016, 138, 4298.
Lett., 2007, 48, 8263; (d) C. Gunanathan and D. Milstein, 15 For Mn-catalyzed transfer hydrogenation, see: (a) M. Peña-
Angew. Chem., Int. Ed., 2008, 47, 8661; (e) M. H. S.
A. Hamid, C. L. Allen, G. W. Lamb, A. C. H. Maxwell,
C. Maytum, A. J. A. Watson and J. M. J. Williams, J. Am.
Chem. Soc., 2009, 131, 1766; (f) S. Imm, S. B-hn,
L. Neubert, H. Neumann and M. Beller, Angew. Chem., Int.
Ed., 2010, 49, 8126; (g) M. Zhang, S. Imm, S. Bähn,
H. Neumann and M. Beller, Angew. Chem., Int. Ed., 2011,
50, 11197; (h) F. E. Fernandez, M. C. Puerta and P. Valerga,
Organometallics, 2012, 31, 6868; (i) J. J. A. Celaje, X. Zhang,
F. Zhang, L. Kam and J. R. Herron, ACS Catal., 2017, 7,
López, P. Piehl, S. Elangovan, H. Neumann and M. Beller,
Angew. Chem., Int. Ed., 2016, 55, 14967; (b) S. Elangovan,
J. Neumann, S. Jean-Baptiste, K. Junge, C. Darcel and
M. Beller, Nat. Commun., 2016, 7, 12641; (c) J. Neumann,
S. Elangovan, A. Spannenberg, K. Junge and M. Beller,
Chem. – Eur. J., 2017, 23, 5410; (d) A. B. Voisine, D. Wang,
V. Dorcet, T. Roisnel, C. Darcel and J. B. Jean-Baptiste
Sortais, J. Catal., 2017, 347, 57; (e) D. H. Nguyen, X. Trivelli,
F. Capet, J.-F. Paul, F. Dumeignil and R. M. Gauvin, ACS
Catal., 2017, 7, 2022.
1136; ( j) L. Tang, H. Sun, Y. Li, Z. Zha and Z. Wang, Green 16 For Mn-catalyzed dehydrogenation, see: (a) M. Andérez-
Chem., 2012, 14, 3423.
Fernández, L. K. Vogt, S. Fischer, W. Zhou, H. Jiao,
This journal is © The Royal Society of Chemistry 2018
Org. Biomol. Chem., 2018, 16, 8175–8180 | 8179

View Article Online
Paper
Organic & Biomolecular Chemistry
M. Garbe, S. Elangovan, K. Junge, H. Junge, R. Ludwig and
L. J. W. Shimon, G. Leitus, L. Avram, Y. Ben-David and
D. Milstein, Chem. – Eur. J., 2017, 23, 5934; (d) R. van
Putten, E. A. Uslamin, M. Garbe, C. Liu, A. Gonzalez-de-
Castro, M. Lutz, K. Junge, E. J. M. Hensen, M. Beller,
L. Lefort and E. A. Pidko, Angew. Chem., Int. Ed., 2017, 56,
7531; (e) F. Bertini, M. Glatz, N. Gorgas, B. Stöger,
M. Peruzzini, L. F. Veiros, K. Kirchner and L. Gonsalvi,
Chem. Sci., 2017, 8, 5024; (f) A. Dubey, L. Nencini,
R. R. Fayzullin, C. Nervi and J. R. Khusnutdinova, ACS
Catal., 2017, 7, 3864; (g) V. Papa, J. R. Cabrero-Antonino,
E. Alberico, A. Spanneberg, K. Junge, H. Junge and
M. Beller, Chem. Sci., 2017, 8, 3576.
M. Beller, Angew. Chem., Int. Ed., 2017, 56, 559;
(b) M. Mastalir, M. Glatz, E. Pittenauer, G. Allmaier and
K. Kirchner, J. Am. Chem. Soc., 2016, 138, 15543; (c) N. Deibl
and R. Kempe, Angew. Chem., Int. Ed., 2017, 56, 1663;
(d) F. Kallmeier, B. Dudziec, T. Irrgang and R. Kempe,
Angew. Chem., Int. Ed., 2017, 56, 7261; (e) S. Chakraborty,
U. Gellrich, Y. Diskin-Posner, G. Leitus, L. Avram and
D. Milstein, Angew. Chem., Int. Ed., 2017, 56, 4229.
17 For Mn-catalyzed hydrogenation, see: (a) F. Kallmeier,
T. Irrgang, T. Dietel and R. Kempe, Angew. Chem., Int. Ed.,
2016, 55, 11806; (b) S. Elangovan, M. Garbe, H. Jiao,
A. Spannenberg, K. Junge and M. Beller, Angew. Chem., Int. 18 V. Artero and M. Fontecave, Chem. Soc. Rev., 2013, 42, 2338.
Ed., 2016, 55, 15364; (c) N. A. Espinosa-Jalapa, A. Nerush, 19 See ESI.†
8180 | Org. Biomol. Chem., 2018, 16, 8175–8180
This journal is © The Royal Society of Chemistry 2018