Iron-Catalyzed β-Alkylation of Alcohols

Article abstract of DOI:10.1021/acs.orglett.9b03171

β-Branched alkylated alcohols have been prepared in good yields using a double-hydrogen autotransfer strategy in the presence of our diaminocyclopentadienone iron tricarbonyl complex Fe1. The alkylation of some 2-arylethanol derivatives was successfully addressed with benzylic alcohols and methanol as alkylating reagents under mild conditions. Deuterium labeling experiments suggested that both alcohols (2-arylethanol and either methanol or benzyl alcohol) served as hydrogen donors in this cascade process.

Full text of DOI:10.1021/acs.orglett.9b03171

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron-Catalyzed β‑Alkylation of Alcohols
́
Leo Bettoni, Sylvain Gaillard, and Jean-Luc Renaud*
́
Normandie Univ., LCMT, ENSICAEN, UNICAEN, CNRS, 6 boulevard du Marechal Juin, 14000 Caen, France
S
* Supporting Information
ABSTRACT: β-Branched alkylated alcohols have been pre-
pared in good yields using a double-hydrogen autotransfer
strategy in the presence of our diaminocyclopentadienone iron
tricarbonyl complex Fe1. The alkylation of some 2-arylethanol
derivatives was successfully addressed with benzylic alcohols
and methanol as alkylating reagents under mild conditions.
Deuterium labeling experiments suggested that both alcohols
(2-arylethanol and either methanol or benzyl alcohol) served as
hydrogen donors in this cascade process.
Scheme 1. Previous Work in Methylation and Benzylation
of 2-Arylethanol
he β-alkylation of alcohols belongs to the challenging
Tfamily of carbon−carbon bond-forming reactions. The
preparation of β-alkylated alcohols usually involves a three-
chemical-step process, i.e., first the oxidation of the alcohols,
then alkylation of the ketone or aldehyde with alkyl halides or
pseudohalides via the deprotonation with a strong base, and
finally the reduction of the alkylated ketones/aldehydes. While
this procedure is often used in laboratories, such an approach is
associated with the formation of wastes and the handling of
hazardous chemicals.1 In the past decade, in order to develop
more sustainable strategies, the borrowing hydrogen strategy
or hydrogen autotransfer processes have been designed for the
construction of C−C and C−N bonds.2,3 The main advantages
of this technology are the use of easily available alcohols as
alkylating agents and the generation of water as a unique side
product. Not only platinum, but also Earth-abundant, metal-
based complexes have been reported for these methods.2,3
Among the various C−C bond formation reactions, the
Guerbet reaction led to the dimerization of primary alcohols
via such β-C(sp3) alkylation.4 However, the cross-dehydrogen-
ative coupling reaction between two different primary alcohols
remains underdeveloped (Scheme 1).5,6 This reaction could be
an exciting alternative to the hydroformylation/hydrogenation
sequence of β,β-disubstituted alkenes. The favorable outcome
of such a cross-dehydrogenative coupling reaction relies on the
different reactivities of the two alcohols. Benzylation of 2-
arylethanols was initially documented by Ramon in 2012 in the
presence of a recyclable impregnated iridium oxide on
magnetite at 110 °C for 4 days5b and later on by Manojveer
and Johnson using the well-known ruthenium complex
RuCl2(PPh3)3 as catalyst in refluxing toluene for 24 h (Scheme
1).5a β- Methylation of 2-arylethanols using the hydrogen
autotransfer technology has also been elaborated, although the
dehydrogenation of methanol is higher in energy compared to
ethanol and higher alcohols. Methylation could be carried out
in homogeneous catalysis either via a dual ruthenium catalysis,
namely with the two ruthenium complexes Ru−Macho and
Shvo’s complex,6a or in the presence of a single ruthenium
complex (Ru−Macho−BH3)6b at elevated temperatures
(Scheme 1). Heterogeneous catalysis could also be applied,
and the iridium nanocluster,6c as well as platinum on carbon,6d
have been engaged in this hydrogen autotransfer reaction.
While the establishment of economically viable processes
advocated the replacement of noble-metal based complexes by
nonprecious ones, reports on such cascade reactions involving
borrowing hydrogen process are still scarcely mentioned in the
literature.7,8 Following our current research on the hydrogen
autotransfer technology,9 we detail in this work the β-
alkylation of alcohols catalyzed by the diaminocyclopentadie-
none iron tricarbonyl complex Fe1, enabling the synthesis of β-
branched benzylated or methylated alcohols from methanol,
Received: September 6, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03171
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
Scheme 2. Layout of the Iron-Catalyzed Double Hydrogen
Autotransfer Alkylation
Scheme 3. Fe-Catalyzed β-Benzylation of Alcohols
a
Table 1. Optimization of the Reaction Conditions.
b
entry
Fe
base
temp. (°C)
time (h)
conv. (%)
1
2
3
4
5
6
7
8
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe3
Fe4
K3PO4
K3PO4
K3PO4
90
90
24
48
48
40
40
40
40
40
40
40
40
40
28
32
49
26
19
49
32
110
110
110
110
110
110
110
110
110
110
Na2CO3
K2CO3
Cs2CO3
NaOMe
NaOtBu
KOtBu
NaOtBu
NaOtBu
NaOtBu
c
91 (76)
9
82
71
5
10
11
12
a
General conditions: 2-arylethanol (0.5 mmol), Fe1 (2 mol %),
NaOH (10 mol %), NaOtBu (1 equiv), benzyl type alcohol (2.5
mmol), BuOH (1 mL) at 110 °C for 40 h. Yield was based on
purified product.
3
b
t
a
General conditions: 2-phenylethanol (0.5 mmol), Fe (2 mol %),
NaOH (10 mol %), base (1 equiv), benzyl alcohol (2.5 mmol),
b
tBuOH (1 mL). Conversions were determined by 1H NMR analysis
of the crude mixture. Yield based on purified product.
c
iron−carbonyl complexes (Fe314 and Fe415) provided almost
no alkylation product (entries 11 and 12, Table 1).
benzyl alcohols, and 2-arylethanols under mild reaction
conditions (Scheme 2).
Initially, for the optimization of the reaction conditions,
alkylation of phenylethanol (1 equiv) with benzyl alcohol was
chosen as the model reaction (Tables 1 and S1).
With these optimized conditions in hand, a survey of the
scope of this unprecedented iron-catalyzed benzylation was
initiated first with several 2-arylethanol compounds and benzyl
alcohol (Scheme 3). 2-Naphthylethanol (a more sterically
hindered aromatic system) did not prevent the reactivity, and
the corresponding alcohol 1d was isolated in 65% yield
(Scheme 3). Various electron-withdrawing groups (such as
CF3 and F) within the aromatic fragment and heterocyclic
structures (such as unprotected indole, thiophene, or thiazole)
were tolerated, and the corresponding 2-benzylated alcohols
1b,c and 1e−g were isolated in moderate to good yields (38−
76%, Scheme 3). Various substituted benzyl alcohols were then
introduced as alkylating reagent, and the corresponding
substituted propanol derivatives 1h−n were isolated in 29−
74% yield (Scheme 3). Again, electron-donating and electron-
withdrawing substituents, and heterocyclic fragments as well,
were tolerated. No alkylation of the nitrogen atom in pyridine,
indole, and thiazole was noticed.
To emphasize the synthetic viability of our protocol, the
benzylation of 2-phenylethanol was performed on a 5 mmol
scale. The corresponding 2,3-diphenylpropanol 1a was isolated
in 75% yield. Despite many efforts, alkylation of aliphatic
alcohols remains unsuccessful.
Finally, to strengthen the interest for this protocol and
extend the scope of alkylating agent, methylation of 2-
arylethanol was then considered. Methylation catalyzed by
A perusal of the literature revealed four main modes of
activation for the iron−carbonyl complexes: (i) a photolytic
activation under UV irradiation,10a (ii) the use of Me3NO,11,12
(iii) Hieber’s protocol in basic conditions,9a,b,10b and (iv)
thermal activation.9d,13 The first three methods can be used
with complexes Fe1, Fe3, and Fe4, while Fe2 is thermally
activated at 70 °C. A short examination of the temperature, in
the presence of Fe1 as catalyst, benzylic alcohol (5 equiv), and
NaOtBu (1 equiv) as base in tert-butyl alcohol (1 mL) showed
that the β-benzylation of 2-arylethanol 1a was achieved at 110
°C with good conversion (Table 1) and good isolated yield
(76%, entry 8, Table 1). Any modifications of these conditions
impeded the yields (Tables 1 and S1). Decreasing the amount
of benzyl alcohol led to lower conversions (entries 1−3, Table
S1). No reaction occurred without iron complex or base. A
noticeable reduction of the conversion in 1a was noted when
other bases or a substoichiometric amount of NaOtBu was
utilized (entries 3−9, Table 1). Gratifyingly, complex Fe2 can
also be introduced without any preactivation (NaOH or
Me3NO was not required),9d,13 although the conversion was
lowered to 71% in 1a (entry 10, Table 1). Finally, other related
B
DOI: 10.1021/acs.orglett.9b03171
Org. Lett. XXXX, XXX, XXX−XXX

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Figure 1. Proposed mechanism.
a,b
Scheme 4. Fe-Catalyzed β-Methylation of Alcohols
Scheme 5. Mechanistic Studies, Deuterium-Labeling
Experiment
metal base complexes using the autotransfer hydrogen strategy
has already been demonstrated for the alkylation of
ketones,9a,16 amines,9b,16d,17 indoles,9c,16d and oxindoles.16d
However, the β-methylation of alcohols with methanol as
alkylating reagent remained underexplored.8 A variety of 2-
arylethanols were engaged in this methylation after a rapid
optimization of the reaction conditions (Scheme 4 and Table
S2). Electron-donating groups (compounds 2b−c, 2e−h, 2n),
electron-withdrawing substituents (compounds 2d, 2i, 2l−n),
and heterocyclic aromatics (compounds 2f, 2j−k) were
permitted, and the corresponding alcohols were obtained in
43−77% yield (Scheme 4). Ortho-substituted phenylethanol
furnished the methylated alcohols 2e and 2m in lower yields,
due to possible destabilizing steric interactions during the
reduction step of the enal intermediate (intermediate V, Figure
1). Gratifyingly, pyridine, unprotected indole, and thiazole
were not alkylated on the nitrogen atom, and the 2-
arylpropanols were obtained in 59−71% yield (Scheme 4).
a
General conditions: 2-arylethanol (0.5 mmol), Fe1 (2 mol %),
NaOH (10 mol %), NaOtBu (1 equiv), MeOH (0.5 mL), BuOH (1
mL) at 110 °C for 40 h. Yield was based on isolated product.
t
b
C
DOI: 10.1021/acs.orglett.9b03171
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As above with the benzylation reaction, the methylation of
2-phenylethanol was achieved on a larger scale (5 mmol), and
the corresponding 2-phenylpropanol 2a was isolated in 71%
yield.
To gain more information on the mechanism and propose
an acceptable mechanism, deuterium-labeling experiments
were carried out (Scheme 5).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jean-luc.renaud@ensicaen.fr.
ORCID
Sylvain Gaillard: 0000-0003-3402-2518
Jean-Luc Renaud: 0000-0001-8757-9622
In deuterated methanol, the CD3-labeled 2-phenylethanol 2a
was isolated in 68% yield (Scheme 5) and a deuterium
incorporation ratio of D/H = 84/16. In other words, a 53/47
mixture of CD3 and CD2H was observed by 1H NMR
spectroscopy analysis (Scheme 5 and Scheme S1). Deuterium
was also introduced in the α- and β-alcohol position. This
chemical yield is as good as the nondeuterated methylation
(Scheme 5). These experiments highlighted that both
deuterated methanol and 2-phenylethanol are a source of
hydride in the overall process.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the
“Ministere de la Recherche et des Nouvelles Technologies”,
̀
́
́
Normandie Universite, CNRS, “Region Normandie”, and the
LABEX SynOrg (ANR-11-LABX-0029).
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
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