Selective Ketone Formations via Cobalt-Catalyzed β-Alkylation of Secondary Alcohols with Primary Alcohols

Article abstract of DOI:10.1021/acs.orglett.9b02727

A homogeneous cobalt-catalyzed β-alkylation of secondary alcohols with primary alcohols to selectively synthesize ketones via acceptorless dehydrogenative coupling is reported for the first time. Notably, this transformation is environmentally benign and atom economical with water and hydrogen gas as the only byproducts.

Full text of DOI:10.1021/acs.orglett.9b02727

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Selective Ketone Formations via Cobalt-Catalyzed β‑Alkylation of
Secondary Alcohols with Primary Alcohols
Bedraj Pandey,†,§ Shi Xu,†,§ and Keying Ding*,†,‡
†Department of Chemistry, Middle Tennessee State University, Murfreesboro, Tennessee 37132, United States
‡Molecular Biosciences Program, Middle Tennessee State University, Murfreesboro, Tennessee 37132, United States
S
* Supporting Information
ABSTRACT: A homogeneous cobalt-catalyzed β-alkylation
of secondary alcohols with primary alcohols to selectively
synthesize ketones via acceptorless dehydrogenative coupling
is reported for the first time. Notably, this transformation is
environmentally benign and atom economical with water and
hydrogen gas as the only byproducts.
omogeneous transition-metal-catalyzed carbon−carbon
ketone synthesis to date. Instead, there is just one example of
Hbond formations are among the paramount methods for alcohol formations via the BH process by a PNP−Co catalyst10
value-added products.1 In the conventional β-alkylation of
(Scheme 1a). Methods for Co-catalyzed α-alkylation of
secondary alcohols to synthesize ketones or alcohols, a
multistep process is required, e.g., stoichiometric oxidation
and alkylation with toxic and mutagenic alkyl halides,
generating copious wastes.2 Thus, it is highly desirable to
develop alternative methods that are environmentally friendly
and atom and process efficient, using readily available and less
toxic substrates, e.g., alcohols. One such prominent synthetic
strategy is acceptorless dehydrogenative coupling (ADC).3,4 In
a typical ADC pathway to ketones, primary and secondary
alcohols are first dehydrogenated to aldehydes and ketones,
respectively, with the catalyst taking the hydrogen atoms. The
electrophilic aldehydes are attacked by the enolates formed via
removal of the α-C−H of the ketones by a base, leading to the
ketone products with loss of water. Finally, hydrogen gas is
liberated from the hydrogenated catalyst. Alternatively, the
hydrogenated catalyst can reduce the formed ketones,
affording the alcohol products. This process is known as
borrowing hydrogen (BH).3,4 Offering great advantages over
conventional methods, ADC and BH have recently attracted
enormous interests.
Scheme 1. Cobalt-Catalyzed Couplings of Secondary
Alcohols or Ketones with Primary Alcohols
ketones with primary alcohols to generate ketones are also
known15 (Scheme 1b). As the ketone reactants are normally
obtained from stoichiometric oxidation of the secondary
alcohols, it is more desirable to directly utilize secondary
alcohols for the ketone formations (Scheme 1c). However, as
H2 is one of the byproducts, the formed ketones could be
further hydrogenated to alcohols, imposing a challenge in
product selectivity.10 Herein, we report the first systematic
study of homogeneous Co-catalyzed β-alkylation of secondary
alcohols with primary alcohols to selectively synthesize ketones
via ADC. Notably, this reaction is environmentally benign and
atom efficient with water and hydrogen gas as the only
byproducts.
Currently, precious metal catalysts based on Rh,5 Ir,6 Ru,7
and Pd8 dominate the β-alkylation of secondary alcohols with
primary alcohols. With increasing concerns on sustainability
and environment, less toxic and earth-abundant base metal
analogues such as Fe,9 Co,10 Mn,11 Ni,12 and Cu13 are
becoming more appealing and have witnessed rapid recent
developments. However, examples of homogeneous base metal
catalyzed such transformations for ketone synthesis are rare.
In the Cu case, high catalyst and base loadings of 10% and
50% were mandatory, respectively.13b In the Mn version,
unfortunately, very limited substrates were reported with
moderate to good yields.11c Transition-metal-free examples are
also available.14 To the best of our knowledge, no
homogeneous Co catalyst has been reported for β-alkylation
of secondary alcohols with primary alcohols for selective
We have recently developed a new family of base transition-
metal complexes supported by a iPrPPPNHPyMe tetradentate
Received: August 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02727
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ligand.16 The Co complex 1a is an efficient catalyst for
dehydrogenation of secondary alcohols to ketones16 and
dehydrogenative coupling of primary alcohols to esters.17 We
speculate that 1a is a potentially efficient catalyst to mediate
the β-alkylation of secondary alcohols with primary alcohols to
synthesize ketones.
Table 2. β-Alkylation of Secondary Alcohols with Primary
Alcohols to Ketones
a,b
Initially, benzyl alcohol 2a and 1-phenylethanol 2b were
chosen as the standard substrates. Different base additives were
examined, and KOtBu turned out to be more suitable. Other
reaction parameters were also optimized. Gratifyingly, the
optimal results with 90% yield of 3-phenylpropiophenone 2c
were obtained using 2.5 mol % of 1a and 7.5 mol % of KOtBu
in toluene at 125 °C for 24 h under argon flow (Table 1, entry
a
Table 1. Optimization of Reaction Conditions
b
entry
cat.
base
solvent
toluene
temp (°C)
yield (%)
1
2
3
1a
1a
1a
1a
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
105
125
140
125
125
125
125
125
125
125
125
31
90 (86)
84
0
<1
0
c
toluene
toluene
toluene
toluene
toluene
1,4-dioxane
toluene
toluene
toluene
toluene
d
4
5
6
7
8
9
10
11
1a
1a
1a
1a
1a
1a
KOtBu
NaOtBu
KHMDS
KOH
0
78
59
<1
0
K2CO3
a
General conditions: 1a (2.5 mol %), KOtBu (7.5 mol %), 2a (0.25
mmol), 2b (0.3 mmol), and toluene (1.5 mL) for 24 h under an argon
a
General conditions: catalyst (2.5 mol %), base (7.5 mol %), 2a (0.25
mmol), 2b (0.3 mmol), and solvent (1.5 mL) for 24 h under an argon
b
c
flow atmosphere. Isolated yield. 1a (5 mol %) and KOtBu (15 mol
b
1
d
%) were used. 1a (5 mol %) and KOtBu (60 mol %) were used.
flow atmosphere. Yields were determined by H NMR with 1,3,5-
c
trimethoxybenzene as an internal standard. 1 mmol scale, isolated
d
yield. KOtBu (5 mol %) was used.
Next, we investigated the scope of secondary alcohols. Para-
substituted aromatic secondary alcohols reacted with 2a
delivering the corresponding ketones in good to excellent
yields (Table 2, 19c−22c). Meta-substituted 1-(3-
methoxyphenyl)ethanol 24b showed a diminished activity
(Table 2, 24c). Aliphatic secondary alcohols were also
amenable to this method, albeit under harsher conditions
(Table 2, 26c, 27c). It is noteworthy that the reaction of n-
hexanol 28a and 2-hexanol 28b proceeded smoothly, giving an
80% yield (Table 2, 28c). To the best of our knowledge, β-
alkylation of aliphatic secondary alcohols with primary alcohols
to ketones mediated by homogeneous base transition-metal
catalyst has not been disclosed before. Unfortunately,
heterocyclic substrates were not tolerated. Couplings of
different secondary alcohols, e.g., 2b and cyclohexanol 29b,
were incompatible with this method.
We then sought a mechanistic understanding of this
reaction. First, two derivatives of 1a (1b and 1c, Figure 1)
were investigated. Compound 1b bearing a dearomatized
pyridine arm shows comparable activity to 1a in an 81% yield,
indicating 1b is also a precatalyst. To test if metal ligand
cooperativity (MLC) from the N−H linker plays a role, a
iPrPPPNMePyMe complex 1c was tested as the precatalyst. A
66% yield of 2c and 22% yield of 2d, a further hydrogenated
product from 2c, were recorded, suggesting MLC may not
have a crucial effect.
2). It is noteworthy that the selectivity is excellent with a 2c/
1,3-diphenylpropanol 2d ratio of 100:0, despite the fact that 2d
is known as the major side product of such transformation in
the literature.7e,12b,13b Interestingly, when 5 mol % of KOtBu
was utilized, which was exactly the amount of base used for
activation of 2.5 mol % of 1a, no 2c product was observed,
indicating the crucial role of base besides precatalyst activation
(Table 1, entry 4). Further, control experiments showed both
1a and base were essential for this reaction (Table 1, entries 5
and 6). Mercury tests demonstrated a homogeneous catalytic
process. H2 was confirmed by GC from the gas phase,
suggesting an ADC pathway (see the SI).
Having established the optimized reactions, we next
explored the substrate scope and efficiency of the reaction.
We first investigated the scope of primary alcohols. Aromatic
primary alcohols with electron-donating groups like −Me,
−iPr, and −OMe at the para position furnished the
corresponding ketones in excellent 90−93% yields (Table 2,
3c−5c). Primary alcohols with electron-withdrawing groups at
the para position also reacted smoothly (Table 2, 6c, 7c).
Notably, meta-substituted substrates were transformed to the
corresponding ketones as well (Table 2, 8c−11c). Sterically
hindered 2-methyl benzyl alcohol 12a also proceeded in a
good yield (Table 2, 12c). Aliphatic primary alcohols afforded
the desired ketones in 73−80% yields (Table 2, 16c−18c).
B
DOI: 10.1021/acs.orglett.9b02727
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02727.
Experimental details, additional figures, and other results
(PDF)
Figure 1. Derivatives 1b and 1c of compound 1a examined.
Our prior work showed that 1a was capable of mediating
dehydrogenation of primary and secondary alcohols to esters
(via aldehydes)17 and ketones,16 respectively. In the current
study, when benzaldehyde 2e reacted with acetophenone 2f in
the presence of 7.5 mol % of KOtBu (Scheme 2a), a 77% yield
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: keying.ding@mtsu.edu.
ORCID
Keying Ding: 0000-0002-7367-129X
Author Contributions
§B.P. and S.X. contributed equally.
Scheme 2. Control Experiments
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We appreciate supports from National Science Foundation.
(CHE-1465051 and MRI CHE-1626549) We also thank
FRCAC, URECA, and Clean Energy Fee Funds of MTSU.
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DOI: 10.1021/acs.orglett.9b02727
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