N-Substituted Hydrazones by Manganese-Catalyzed Coupling of Alcohols with Hydrazine: Borrowing Hydrogen and Acceptorless Dehydrogenation in One System

Article abstract of DOI:10.1002/anie.201712593

An unprecedented one-step synthesis of N-substituted hydrazones by coupling of alcohols with hydrazine is reported. This partial hydrogen-borrowing reaction is catalyzed by a new manganese pincer complex under mild reaction conditions, thus liberating water and dihydrogen as the only byproducts. Mechanistic insight, based on the observation of intermediates, is provided.

Full text of DOI:10.1002/anie.201712593

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Accepted Article
Title: N-Substituted Hydrazones by Manganese Catalyzed Coupling of
Alcohols with Hydrazine; 'Borrowing Hydrogen' and Acceptorless
Dehydrogenation in One System
Authors: David Milstein, Uttam Kumar Das, Yehoshoa Ben-David, and
Yael Diskin-Posner
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712593
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10.1002/anie.201712593
Angewandte Chemie International Edition
COMMUNICATION
N-Substituted Hydrazones by Manganese Catalyzed Coupling of
Alcohols with Hydrazine; ‘Borrowing Hydrogen’ and
Acceptorless Dehydrogenation in One System
Uttam Kumar Das, Yehoshoa Ben-David, Yael Diskin-Posner, and David Milstein*
Abstract: An unprecedented one-step synthesis of N-substituted
other reports on Mn catalyzed (de)hydrogenation reactions
appeared.13 Hydrogenation of multiple C-heteroatom bonds and
esters using Mn pincer complexes was reported by several groups,
including ours.14 Recently, Beller reported C-alkylation of ketones,15a
and N-methylation of amines15b catalyzed by Mn-PNP pincer
complexes via ‘borrowing hydrogen’ strategy 16. Substituted amine
formation via ‘borrowing hydrogen’ is of much current interest.17, 18
hydrazones by coupling of alcohols with hydrazine is reported. This
“partial hydrogen borrowing” reaction is catalyzed by
a new
manganese pincer complex under mild reaction conditions, liberating
water and dihydrogen as the only byproducts. Mechanistic insight,
based on intermediate observation, is provided.
N-substituted hydrazones are constituents of several natural
products,1 such as Leucoagaricone,2 Schaefferal A, Schaefferal B3
and NG-0614 were found in mushrooms and used as chromogens
and enhancers of nerve growth. In addition, N-substituted
hydrazones are important building blocks for the development of
new antifungal,5a antimicrobial, anti-inflammatory5b and anticancer5c,
d drugs for treatment of various diseases.5
In general, treatment of N-substituted hydrazines with
aldehydes/ketones has been employed for the synthesis of N-
substituted hydrazones.6 Hydrogenation7a or reduction of azines in
the presence of strong reducing agents in heterogeneous systems7c
can be an alternative path to the –NH-N=CH- functionality, but
selectivity is a major issue.7 Interestingly, N-substituted hydrazones
can be formed in modest yields by reaction of Grignard reagents
with N2O, although stoichiometric waste is generated.8 Considering
the important uses of this class of compounds in the pharmaceutical
industry and in synthetic organic chemistry, effective catalytic waste-
free procedures are desirable.
Scheme 1. Reactions of alcohols with hydrazine.
To our knowledge, there is no report on dehydrogenative
coupling of alcohols and hydrazine to form N-substituted hydrazones.
Herein we report such a reaction, which produces N-substituted
hydrazones under mild conditions. Moreover, this reaction is
catalyzed by an earth abundant, new manganese pincer complex.
Mechanistic insight is provided based on intermediate observation.
In this reaction, both acceptorless dehydrogenative coupling of
alcohols and a hydrogen borrowing process take place. We are not
aware of other reports of a partial ‘hydrogen borrowing’ process.
Dehydrogenative coupling of alcohols and hydrazine would be
an alternative route to the synthesis of hydrazones. Although the
direct use of N2H4 in catalysis is challenging, ruthenium and iridium
catalyzed dehydrogenative coupling of alcohols with hydrazines (not
leading to N-substituted hydrazones) are known (Scheme 1). In
pioneering work, C. J. Li reported alcohol deoxygenation by Ru-
catalyzed reaction of primary alcohols with hydrazine in presence of
base (Scheme 1, a).9 We reported dehydrogenative coupling of
alcohols and hydrazine, and deoxygenation of alcohols, catalyzed by
Ru-PNP10a and Mn-PNP10b pincer complexes, respectively (Scheme
1,a& b).
t
Treatment of our previously reported Bu-PNN ligand19 with
one equivalent of Mn(CO)5Br at room temperature in THF led to
formation of the new complex Mn(tBu-PNN)(CO)2Br( 1) in 85% yield
(Scheme 2). The two CO ligands of 1 give rise to bands at 1841 and
1916 cm-1 in the IR spectrum in a 1:1 ratio, indicating a nearly 90o C-
Mn-C angle (87.5°). Single crystals of 1 suitable for X-ray diffraction
were obtained by slow diffusion of pentane into a saturated solution
of THF at -30 °C. The structure of 1 exhibits an octahedral geometry
with a meridional arrangement of the tBu-PNN ligand. The CO
ligands are orthogonal to each other, in line with the IR spectrum
(Scheme 2, SI).
The application of earth-abundant base-metal complexes in
homogeneous catalysis is of much current interest.11 Manganese is
the third most abundant metal on earth’s crust. We reported the
dehydrogenative coupling of alcohols and amines to give imines,
catalyzed by a pincer Mn-PNPtBu complex.12 Subsequently, several
[*]
Dr. U. K. Das, Y. Ben-David, Prof. D. Milstein
Department of Organic Chemistry, Weizmann Institute of Institution
Rehovot, Israel, 76100
E-mail: david.milstein@weizmann.ac.il
Scheme 2. Synthesis and molecular structure of Mn(tBu-
PNN)(CO)2Br (1) . Thermal ellipsoids are at 50% probability level.
Hydrogen atoms are omitted for clarity. See Supporting Information
(SI) for experimental details.
Dr. Y. Diskin-Posner
Chemical Research Support, Weizmann Institute of Science
Rehovot, Israel, 76100
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201712593
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COMMUNICATION
Table 1. Acceptorless coupling of alcohols and hydrazine catalyzed
by 1.a
Scheme 3. Coupling of benzyl alcohol and hydrazine catalyzed by
Mn(tBu-PNN)(CO)2Br (1).
Entry
1
Alcohol
Product
Conv.(%)b
(t)
Yield
(%)c
81d
84(12h)
99(24h)
90(24h)
Complex 1 catalyzes in the presence of a catalytic amount of
tBuOK the dehydrogenative coupling of alcohols and hydrazine to
form N-substituted hydrazones with liberation of H2 and water
(Scheme 3). Thus, reaction of 0.5 mmol of benzyl alcohol, complex 1
(3 mol%) and tBuOK (5 mol%) in 1 mL of a 1M N2H4 solution in THF
at 110°C in a closed vessel for 12 h resulted in formation of the N-
substituted hydrazone (1-benzyl-2-benzylidenehydrazine) in 81%
yield (Table 1, entry 1, first row). Performing the reaction in an open
system under reflux, only 60% of the alcohol converted to the N-
substituted hydrazone and toluene was formed as a side product
after 24 h, whereas in a closed system for 24 h the product was
obtained in 92% yield (Table 1, entry 1, second row). Analysis of the
gas phase by GC indicated the formation of H2. A small amount of
toluene (3%) was also observed by GC-MS as a result of alcohol
deoxygenation via a benzylidenehydrazine intermediate.9, 10b
92
73e
2
99(24h)
99(24h)
86
90
3
4
95(24h)
99(24h)
81
81
5
6
7
8
97(24h)
85(24h)
85
74
Next, the scope of this unprecedented catalytic reaction was
probed with various aliphatic and aromatic alcohols. Reaction of
hydrazine with 4-methylbenzyl alcohol for 24
h afforded the
corresponding N-substituted hydrazone in 86% yield (Table 1, entry
2). Dehydrogenative coupling of 4-methoxybenzyl alcohol and 3-
methoxybenzyl alcohol with hydrazine led to 90% and 81% yields of
the corresponding hydrazones, respectively (Table 1, entries 3 and
4). The hydrazone product obtained by reaction of hydrazine with 4-
methoxylbenzyl alcohol was structurally characterized by single
crystal X-ray crystallography (see SI). Reaction of hydrazine with
benzylic alcohols bearing electron withdrawing groups in the para
positions (p-F, p-Cl, p-CF3) afforded the corresponding products in
good yields (Table 1, entries 5-7). The reaction also resulted in 79%
yield of the corresponding N-substituted hydrazone by coupling of
3,4-dimethoxy benzyl alcohol and hydrazine (Table 1, entry 8).
The reaction is not limited to benzyl alcohols. Hydrazine reacts
with 1-hexanol to yield 77% of 1-hexyl-2- hexylidenehydrazine
(Table 1, entry 9). Similarly, 1-heptyl-2-heptylidenehydrazine was
obtained in 65% yield of after 24 h reaction of hydrazine and 1-
heptanol (Table 1, entry 10). The products were selectively formed
as pure E-isomers, except for the aliphatic N-substituted hydrazones,
which were formed as a 4:1 mixture of E and Z isomers (see SI).
The N-substituted hydrazones are not very stable in air and convert
slowly to azines by aerial oxidation.
88(24h)
79
9
1-hexanol
1-octanol
99(36h)
90(36h)
77
65
10
aConditions: alcohol (0.5 mmol), N2H4 1M in THF (1 mL), 1 (0.015
mmol) and KOtBu (0.025 mmol), heated at 110 °C in a closed vessel
for the indicated time. bConversion determined by GC or NMR
c
analysis using toluene or m-xylene as internal standards. Yields of
isolated products. d Average yields of two consecutive runs after 12h.
eYield of reaction catalyzed by pure complex 2 (3 mol %, see below)
in absence of any added base, determined by GC and 1H NMR
analysis using m-xylene as internal standard.
Employing the freshly prepared 2 (3 mol %) as a catalyst in
the dehydrogenative coupling of benzyl alcohol and hydrazine at
110 °C resulted in the N-substituted hydrazone in 73% yield of after
24 h in absence of any added base (Table 1, entry 1, third row).
However, complex 2 is somewhat unstable in solution (in absence of
substrates) and slowly dissociates the ligand.
To gain insight into the mechanism of this reaction, complex 1
t
was reacted with 1.1 equiv. BuOK in pentane at room temperature,
forming at -30 °C deep blue crystals of the dearomatized complex
Mn(tBu-PNN*)(CO)2 (2). The structure of 2 was confirmed by X-ray
diffraction (Scheme 4, SI). Complex 2 exhibits a distorted trigonal
bipyramidal geometry with two adjacent carbonyl groups (C21-Mn-
C20 = 87.0°). The C1-C2 bond (1.368Å) is shorter by 0.112Å than
C6-C7 (1.470Å) bond, clearly indicating a double bond character.
Significantly, treatment of 2 with benzyl alcohol (2.1 equiv.) in
THF at room temperature resulted in instant formation of a new
complex which exhibits in the 31P{1H} NMR spectrum one sharp
signal at δ = 118 ppm, identified as the alkoxo complex 3 (Scheme
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10.1002/anie.201712593
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COMMUNICATION
4). Complex
3
is formed by O-H activation via metal-ligand
This in-situ formed hydrido complex also catalyzes the reaction
between hydrazine and benzyl alcohol to form the desired product.
The catalytic cycle likely involves deprotonation of complex 1
cooperation resulting in aromatization of the pincer ligand, in analogy
to the previously reported formation of an alkoxy complex by
reaction of a dearomatized PNP pincer Mn complex with benzyl
alcohol.12 The 1H NMR spectrum of 3 in solution indicates the
presence of a benzyl alcohol molecule H-bonded to the alkoxy ligand
(see SI). Crystal stucture of 3 shows a neutral octahedral complex
bearing a PNN pincer ligand, two mutually cis CO ligands and an
alkoxy group (Scheme 4, see SI). Heating of 3 at 110 °C for 5h
resulted in formation of the hydrido complex 4 (Scheme 5), which
exhibits a Mn-H peak in the 1H NMR spectrum at -2.3 ppm and a
sharp 31P{1H} NMR signal at δ = 129 ppm. GC analysis showed
quantitative formation of benzaldehyde after heating of 3, in line with
the mechanism shown in Scheme 5. It is possible that aldehyde
formation from the coordinatively saturated 3 involves a non-
classical alkoxide dissociation, assisted by the H-bonded alcohol
molecule, followed by hydride elimination from the dissociated
alkoxide, as invoked for other coordinatively saturated alkoxy
complexes.20
t
by BuOK to produce the active catalyst 2 which catalyzes alcohol
dehydrogenation via metal-ligand cooperation (left cycle, Scheme
5).21 The formed aldehyde reacts with excess hydrazine to produce
hydrazone,22 which undergoes hydrogenation of the C=N bond to
form the N-substituted hydrazine via a ‘borrowing hydrogen’ process
(right cycle, Scheme 5). The latter reacts with an additional aldehyde
molecule to form the N-substituted hydrazone (Scheme 5).
To gain more mechanistic insight, we reacted freshly prepared
benzylidenehydrazine with benzyl alcohol in the presence of catalyst
t
1 (3 mol%) and BuOK (5 mol%) in THF, in absence of hydrazine,
resulting
in
90%
yield
of
the
desired
1-benzyl-2-
benzylidenehydrazine. This experiment indicates the hydrogen
borrowing mechanism (Scheme 6, i). Under the same reaction
conditions, 1-benzylidene-2-(4-chlorobenzyl) hydrazine was obtained
in 89% yield by coupling of benzyl alcohol and 4-chlorobenzyl
hydrazine with the liberation of dihydrogen (Scheme 6, ii), indicating
a dehydrogenative coupling process. No hydrogenated product was
obtained when benzalazine was used as substrate under these
reaction conditions, indicating that it is not an intermediate in the
catalytic reaction (Scheme 6, iii).
Scheme 4. Synthesis of complexes 2 and 3 and their molecular
structure. Thermal ellipsoids are drawn at 50% probability level.
Hydrogen atoms except for the vinylic C-H are omitted for clarity.
See SI for experimental details.
Scheme 6. Mechanistic experiments.
In conclusion, we have demonstrated the unprecedented
formation of N-substituted hydrazones by coupling of alcohols and
hydrazine. Moreover, the reaction is catalyzed by a base-metal
catalyst. The reaction proceeds under homogeneous conditions,
catalyzed by the new pincer catalyst Mn(tBu-PNN)(CO)2Br (1) in the
t
presence of catalytic amounts of BuOK, as well as by the new
dearomatized complex Mn(tBu-PNN*)(CO)2 (2), with no added base.
Uniquely, the reaction involves both acceptorless dehydrogenative
coupling and ‘hydrogen borrowing’ in one system. A proposed
mechanism is provided, supported by control experiments and
observation of intermediates, formed by metal ligand cooperation.
Acknowledgements
This research was supported by the European Research Council
(ERC AdG 692775). D. M. holds the Israel Matz Professorial Chair of
Organic Chemistry. U. K. D is thankful to The Science & Engineering
Research Board (SERB), DST, and Govt. of India for the SERB
Overseas Postdoctoral Fellowship.
Scheme 5. Proposed mechanism.
In a separate reaction, treatment of complex 1 with an
equimolar amount of NaBHEt3 at RT resulted in formation of
1
complex 4 as observed by H and 31P NMR spectroscopy (see SI).
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Keywords: manganese catalyst• partial hydrogen borrowing• N-
substituted hydrazone• acceptorless alcohol dehydrogenation•
hydrazine
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CCDC 1580693 (complex 1) 1580695 (complex 2) 1589535 (complex
3) and 1579450 (hydrazine derivative) contain the supplementary
crystallographic data for this paper. These data are provided free of
charge by The Cambridge Crystallographic Data Centre.
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COMMUNICATION
Uttam Kumar Das, Yehoshoa Ben-
David, Yael Diskin-Posner, and David
Milstein*
Page No. – Page No.
N-Substituted
Manganese Catalyzed Coupling of
Alcohols with Hydrazine;
‘ Borrowing Hydrogen ’ and
Hydrazones
by
Acceptorless Dehydrogenation in
One System
An unprecedented one-step synthesis of N-substituted hydrazones by coupling of alcohols with hydrazine is reported. This “partial
hydrogen borrowing” and acceptorless dehydrogenation reaction is catalyzed by a new manganese pincer complex under mild
reaction conditions, liberating water and dihydrogen as the only byproducts.
This article is protected by copyright. All rights reserved.