C-Alkylation of Secondary Alcohols by Primary Alcohols through Manganese-Catalyzed Double Hydrogen Autotransfer

Article abstract of DOI:10.1002/cssc.201801660

A new Mn-catalyzed alkylation of secondary alcohols with non-activated alcohols is presented. The use of a stable and well-defined manganese pincer complex, stabilized by a PNN ligand, together with a catalytic amount of base enabled the conversion of renewable alcohol feedstocks to a broad range of higher-value alcohols in good yields with water as the sole byproduct. The strategy eliminates the need for exogenous and detrimental alkyl halides as well as the use of noble metal catalysts, making the C-alkylation through double hydrogen autotransfer a highly sustainable and environmentally benign process. Mechanistic investigations support a hydrogen autotransfer mechanism in which a non-innocent ligand plays a crucial role.

Full text of DOI:10.1002/cssc.201801660

DOI: 10.1002/cssc.201801660
Communications
C-Alkylation of Secondary Alcohols by Primary Alcohols
through Manganese-Catalyzed Double Hydrogen
Autotransfer
Osama El-Sepelgy,*[a] Esteban Matador+,[a] Aleksandra Brzozowska+,[a] and
Magnus Rueping*[a, b]
A new Mn-catalyzed alkylation of secondary alcohols with non-
activated alcohols is presented. The use of a stable and well-
defined manganese pincer complex, stabilized by a PNN
ligand, together with a catalytic amount of base enabled the
conversion of renewable alcohol feedstocks to a broad range
of higher-value alcohols in good yields with water as the sole
byproduct. The strategy eliminates the need for exogenous
and detrimental alkyl halides as well as the use of noble metal
catalysts, making the C-alkylation through double hydrogen
autotransfer a highly sustainable and environmentally benign
process. Mechanistic investigations support a hydrogen auto-
transfer mechanism in which a non-innocent ligand plays a
crucial role.
So far, the vast majority of these approaches rely on the use
of more expensive noble-metal catalysts, such as Ir,[5] Ru,[6] Rh[7]
and Pd.[8] Replacement of the precious-metal catalysts by
earth-abundant low-toxicity base-metal catalysts is a topic of
current interest.[9] Processes relying on the use of Fe[10] Ni[11]
and Cu[12] catalysts have been disclosed. More recently, Co cat-
alysis has been successfully used. However, higher catalyst
loading (5 mol%) along with superstoichiometric amounts of a
more sensitive and expensive base [potassium bis(trimethylsily-
l)amide (KHMDS)] may be a drawback.[13]
Despite all these efforts, the alkylation of secondary alcohols
with primary alcohols often leads to mixtures of the desired b-
alkylated alcohols along with the corresponding undesired ke-
tones, presenting a crucial selectivity issue. Therefore, the de-
velopment of a base-metal catalyst for the selective alkylation
of secondary alcohols would be highly desirable.
b-Alkylation of alcohols is one of the most fundamental
carbon-carbon bond-forming reactions. The conventional route
requires three chemical steps (i.e., stoichiometric oxidation, al-
kylation with alkyl halides, and stoichiometric reduction),
making the overall process environmentally unfriendly.[1] With
aim to avoid the use of mutagenic and waste-forming re-
agents,[2] modern processes have emerged, which are based
on the hydrogen autotransfer strategy[3] utilizing biomass-de-
rived alcohols as alkylating agents.[4] Hence, a strategy that
comprises the dehydrogenation of both of the primary and
secondary alcohols followed by base-catalyzed aldol condensa-
tion to produce a,b-unsaturated ketones and subsequent
double hydrogenation can provide b-alkylated alcohols. Thus,
stoichiometric oxidation and reduction reagents as well as
alkyl halides can be substituted by abundantly available alco-
hols.
Based on our recent experience in Mn catalysis and its appli-
cation in the reduction of alkynes and organic carbonates,[14]
we questioned if highly reactive Mn catalysts may be suitable
for the efficient hydrogenation of the intermediate a,b-unsatu-
rated ketone, resulting exclusively in the desired alkylated alco-
hols. However, at the outset of this work, the alkylation of sec-
ondary alcohols using/from primary alcohols by Mn catalysis
was not known to the best of our knowledge. During the prep-
aration of this Communication a related parallel study was re-
ported by Yu and co-workers.[15] Here, we demonstrate that
the Mn–PNN pincer complex Mn-1 is a remarkably active and
selective catalyst for the alkylation reaction of alcohols
(Scheme 1).[16,17]
The cross-coupling between 1-phenylethanol (1a) and
benzyl alcohol (2a) was selected as a model reaction (Table 1).
Initially, we screened the catalytic properties of different Mn
complexes (Mn-1 to Mn-4) in combination with 10 mol% of
Cs2CO3 in tert-amyl alcohol (TAA) as solvent. The use of
[a] Dr. O. El-Sepelgy, E. Matador,+ A. Brzozowska,+ Prof. Dr. M. Rueping
Institute of Organic Chemistry
RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail: osama.elsepelgy@rwth-aachen.de
magnus.rueping@rwth-aachen.de
[b] Prof. Dr. M. Rueping
KAUST Catalysis Center (KCC)
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955-6900 (Saudi Arabia)
[+] These authors contributed equally to this work.
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/cssc.201801660.
This article is part of a Special Issue on “Sustainable Organic Synthesis”.
A link to the issue will appear here once it is compiled.
Scheme 1. Mn-catalyzed double hydrogen autotransfer.
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(Scheme 2a). The 1-phenylethanol derivatives 1b–1d bearing
electron-donating substituents in ortho, meta, and para posi-
tions gave the desired products 3b–3d in good yields. Similar-
ly, the electron-deficient substrates 1e and 1 f furnished the
upgraded alcohols 3e and 3 f in good yields. Furthermore, the
alkylation of the naphthyl substrate 1g afforded the expected
product in 71% yield. Importantly, the substrate scope could
be extended to the use of the less active aliphatic secondary
alcohols 1h–1l; the corresponding alcohols 3h–3l were ob-
tained in good-to-moderate yields. Noteworthy, even the steri-
cally demanding alcohol 1k was tolerated in this Mn-catalyzed
reaction.
Table 1. Optimization of the reaction conditions.[a]
Entry Catalyst
Base
Solvent
Conv. Yield of 3a/3a’
([mol%]) ([mol%])
[%]
[%]
After successfully varying the secondary alcohols, we
became interested in studying the scope of the b-alkylation of
1
2
3
4
5
6
7
Mn-1 (3) Cs2CO3 (10)
Mn-2 (3) Cs2CO3 (10)
Mn-3 (3) Cs2CO3 (10)
Mn-4 (3) Cs2CO3 (10)
Mn-1 (3) Cs2CO3 (10)
Mn-1 (3) Cs2CO3 (10)
Mn-1 (3) Cs2CO3 (10)
TAA
TAA
TAA
TAA
1,4-dioxane
2-Me-THF
toluene
57
49/8
42/14
69/30
55/34
20/3
30/4
50/22
50/8
65/8
76/5
81/5
57/17
92/8
60
>99
90
62
60
73
77
90
93
88
85
>99
>99
>99
1-phenylethanol (1a) with different primary alcohols
2
(Scheme 2b). The alkylation of 1a with the electron-rich benzyl
alcohols 2b–2 f as well as the electron-poor substrates 2g–2i
resulted in the secondary alcohols 4b–4i in good yields. Also,
the alcohol 4j containing a naphthyl group was obtained in
77% yield. Additionally, various aliphatic primary alcohols
could also be used as a coupling partner to afford the alcohols
4k–4m.
Encouraged by these promising results, we decided to inves-
tigate the more challenging coupling reaction between satu-
rated aliphatic primary and secondary alcohols (Scheme 2c).
Indeed, our catalytic system also proved to be suitable to
couple the branched cyclohexanemethanol and unbranched 1-
octanol with 1-cyclohexylethanol and the products 5a and 5b
were isolated in good yields. Furthermore, 1-cyclopenylethanol
and 1-cyclopropylethanol were alkylated with different linear
and non-linear alcohols to produce the alcohols 5c–5e in
good yields.
8
Mn-1 (3) KHMDS (10) toluene
9
Mn-1 (3) KH (10)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
10
11
12
13
14
15[b]
Mn-1 (3) KOH (10)
Mn-1 (3) KOtBu (10)
Mn-1 (3) KOtBu (5)
Mn-1 (3) KOtBu (25)
Mn-1 (1) KOtBu (25)
Mn-1 (1) KOtBu (25)
92/8
83/13
[a] Reaction conditions: 1a (0.5 mmol), 2a (0.55 mmol), [Mn] and base in
0.5 mL of solvent at 1358C in a glass tube under argon for 20 h. Conver-
sions and yields were determined by the H1 NMR analysis of the crude re-
action mixture using mesitylene as an internal standard. TAA=tert-amyl
alcohol. [b] A drop of mercury was added.
3 mol% of our PNN complex Mn-1 led to promising results,
providing the desired product 3a in 49% yield along with 8%
of the corresponding ketone 3a’ (Table 1, entry 1). The applica-
tion of the di-tert-butyl complex Mn-2 resulted in similar dehy-
drogenation activity and slightly lower hydrogenation activity
(Table 1, entry 2).[16f] In the presence of PNP complexes, such as
Mn-3 and Mn-4, excellent conversion was observed. However,
the inefficient hydrogenation of the unsaturated intermediate
provided considerable amount of the undesired ketone 3a’
(Table 1, entries 3 and 4). Thus, we decided to further optimize
the model reaction using Mn-1 in combination with different
bases and solvents. 1,4-Dioxane and 2-Me-THF proved to be
unsuitable for this reaction. However, performing the reaction
in toluene resulted in better results (Table 1, entries 5–7). Addi-
tionally, we tested various bases such as KHMDS, KH, KOH, and
KOtBu (Table 1, entries 8–13). From these experiments the reac-
tion proceeded best when 25 mol% of KOtBu were applied
and the desired product was obtained in 92% yield (Table 1,
entry 13). Interestingly, we could reduce the catalyst loading to
1 mol% while still obtaining excellent yield (Table 1, entry 14).
Finally, full conversion was still observed upon adding Hg to
the reaction mixture, proving the homogenous nature of the
Mn catalyst (Table 1, entry 15).
To gain insight into the reaction mechanism, we performed
deuterium-labeling experiments (Scheme 3). When 1-phenyle-
thanol-a-d1 [D1]1a was reacted with benzyl alcohol-a,a-d2
[D2]2a, a very strong kinetic isotope effect was observed and
3a’’ was obtained in 31% yield with 86% deuteration in the a-
position and 60% deuterium incorporation at C3. No deutera-
tion occurred at the OH moiety and only 10% deuterium incor-
poration occurred at the C2 position (Scheme 3a). Similarly,
the reaction between [D1]1a and 2a gave the alkylated prod-
uct with deuterium incorporations of 32% at C1, 15% at C3,
and >3% at C2 (Scheme 3b). The presence of only 40% deu-
terium incorporation at C3 in the reaction between 1a and
[D2]2a indicates the reversibility of the primary alcohol dehy-
drogenation process and supports the hydrogen autotransfer
pathway (Scheme 3c).
Interestingly, the high deuterium content at C1 and C3 and
the low deuterium incorporation at C2 are not in alignment
with both the classical dihydride mechanism and proposed
amidate-assisted pathway.[16e] The presented deuterium experi-
ments support a monohydride mechanism and highlight the
involvement of both the metal and the non-innocent ligand in
the transfer-hydrogenation pathways.[18–20]
The metal monohydride can be formed by the b-hydride
elimination of the Mn alkoxide (inner sphere pathway). Alterna-
tively, the alcohol can be (de)hydrogenated through the outer
Next, the substrate scope of the b-alkylation of different sec-
ondary alcohols 1 with benzyl alcohol 2a was conducted
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Scheme 3. Deuterium labeling experiments.
sphere of the metal without coordination of the alcohol to the
metal center.[20] Subsequently, we investigated the progress of
the model reaction between 1a and 2a as a function of time
(see the Supporting Information for details). The investigation
indicates that the dehydrogenation of the alcohol substrates is
most likely the rate-limiting step. The proposed reaction mech-
anism is shown in Scheme 4. Initially, the precatalyst Mn-1
reacts with the base to form the 16e species A. This active
species can reversibly react with an alcohol to form the corre-
sponding ketone and the hydrogenated catalyst
B
(Scheme 4a). The alkylation reaction starts with the dehydro-
genation of both alcohols to form carbonyl compounds and
the hydrogenated catalyst B. Then, the base-catalyzed aldol
condensation leads to the irreversible formation of the a,b-un-
saturated ketone intermediate. Based on the experimental re-
sults and the reaction profile, it is suggested that the hydroge-
nation of the C=C double bond of the a,b-unsaturated ketone
Scheme 2. Scope of Mn-catalyzed C-alkylation of secondary alcohols by pri-
mary alcohols. Reaction conditions: 1 (0.5 mmol), 2 (0.55 mmol), Mn-1
(0.005 mmol) and KOtBu (0.125 mmol) in toluene (0.5 mL) were stirred at
1358C for 20 h in a glass tube under an inert atmosphere. Yields after
column chromatography. [a] Mn-1 (0.015 mmol). [b] Mn-1 (0.01 mmol).
[c] NMR yield.
Scheme 4. Proposed reaction mechanism.
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In conclusion, we report here the synthesis of C-alkylated
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Acknowledgements
E.M. acknowledges Ministerio of Economꢀa y Competitividad and
Universidad de Sevilla for predoctoral fellowships.
Conflict of interest
The authors declare no conflict of interest.
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Keywords: alcohols · alkylation · base metals · hydrogen
autotransfer · manganese catalysis
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Manuscript received: July 22, 2018
Revised manuscript received: August 14, 2018
Version of record online: && &&, 0000
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COMMUNICATIONS
O. El-Sepelgy,* E. Matador, A. Brzozowska,
M. Rueping*
&& – &&
C-Alkylation of Secondary Alcohols by
Primary Alcohols through Manganese-
Catalyzed Double Hydrogen
Autotransfer
Alcohol upgrading: A well-defined Mn
pincer complex, stabilized by a PNN
ligand, catalyzes the b-alkylation of sec-
ondary alcohols with primary alcohols
without the need for further stochio-
metric reagents. Almost equimolar
ratios of substrates are sufficient to ach-
ieve good yields of the corresponding
alkylated alcohols and water is liberated
as the only byproduct in this sustaina-
ble reaction.
ChemSusChem 2018, 11, 1 – 5
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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