One-Pot Conversion of Allylic Alcohols to α-Methyl Ketones via Iron-Catalyzed Isomerization-Methylation

Article abstract of DOI:10.1021/acs.orglett.9b02900

A one-pot iron-catalyzed conversion of allylic alcohols to α-methyl ketones has been developed. This isomerization-methylation strategy utilized a (cyclopentadienone)iron(0) carbonyl complex as precatalyst and methanol as the C1 source. A diverse range of allylic alcohols undergoes isomerization-methylation to form α-methyl ketones in good isolated yields (up to 84% isolated yield).

Full text of DOI:10.1021/acs.orglett.9b02900

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
One-Pot Conversion of Allylic Alcohols to α‑Methyl Ketones via
Iron-Catalyzed Isomerization−Methylation
Daniel E. Latham,† Kurt Polidano,† Jonathan M. J. Williams,‡ and Louis C. Morrill*,†
†Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, U.K.
‡Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, U.K.
S
* Supporting Information
ABSTRACT: A one-pot iron-catalyzed conversion of allylic
alcohols to α-methyl ketones has been developed. This
isomerization−methylation strategy utilized a (cyclopentad-
ienone)iron(0) carbonyl complex as precatalyst and methanol
as the C1 source. A diverse range of allylic alcohols undergoes
isomerization−methylation to form α-methyl ketones in good
isolated yields (up to 84% isolated yield).
llylic alcohols are privileged motifs in synthetic chemistry
due to their widespread availability and diverse reactivity
ketones to date (Scheme 1B).10 This interesting process
involves alkoxide generation using n-BuLi followed by
rhodium-promoted allylic alkoxide isomerization and subse-
quent alkylation using excess methyl iodide (10 equiv).
Despite the benefits of a one-pot procedure, the disadvantages
of this method include the use of a precious metal catalyst, a
pyrophoric base, and superstoichiometric quantities of a toxic
methylating agent. In an effort to address these drawbacks, we
have utilized a (cyclopentadienone)iron(0) carbonyl complex
(2 mol %)11 for the one-pot isomerization−methylation of
allylic alcohols to α-methyl ketones (Scheme 1C). This process
employs a catalyst based on an earth-abundant transition metal, a
carbonate base, and methanol as a C1 building block.
A
profile.1 An important transformation of allylic alcohols is the
redox isomerization to form synthetically useful enolizable
carbonyl compounds,2 which can be performed using metal
catalysts3 or Brønsted base catalysts (Scheme 1A).4 The
Scheme 1. Context and Outline of Iron-Catalyzed Strategy
The isomerization−methylation of 1-phenylprop-2-en-1-ol 1
was selected as the model system for optimization studies
(Table 1).12 It was determined that (cyclopentadienone)-
iron(0) carbonyl complex 2 (2 mol %),13 Me3NO (4 mol %),14
and K2CO3 (2 equiv) in MeOH ([1] = 0.5 M) at 130 °C for
24 h, facilitated the isomerization−methylation of 1, which gave
product 3 in 88% NMR yield and 76% isolated yield (entry 1).
Control experiments confirmed that no product was formed in
the absence of iron precatalyst 2 or K2CO3 (entries 2 and 3).
A selection of structurally related (cyclopentadienone)iron
carbonyl precatalysts 4−8 did not enable the formation of
α-methyl ketone 3 (entry 4).15 Substituting K2CO3 for NaOH or
KOt-Bu as base lowered the observed NMR yield of 3 (entries 5
and 6). Altering the concentration (entries 7 and 8), temperature
(entries 9 and 10), reaction time (entry 11), or catalyst loading
(entry 12), all reduced the efficiency of the isomerization−
methylation of 1. Gratifyingly, the quantity of K2CO3 could be
lowered to 10 mol % without significant reduction in conversion
(entry 13), which increased atom economy.16
subsequent incorporation of methyl groups via α-C(sp3)-
methylation can significantly impact the pharmacological prop-
erties of a molecule.5 Many commonly employed methylation
protocols utilize hazardous reagents such as methyl iodide,
diazomethane, or dimethyl sulfate.6 However, recent advances
in borrowing hydrogen catalysis7 have enabled methanol to be
employed as an attractive alternative for ketone α-C(sp3)-
methylation,8 using catalysts based on both precious metals
and more abundant 3d transition metals.9
With optimized reaction conditions in hand (Table 1, entry 1),
the scope of the iron-catalyzed isomerization−methylation protocol
Motherwell and co-workers have reported the only direct
one-pot conversion of secondary allylic alcohols to α-methyl
Received: August 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02900
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Scope of the Fe-Catalyzed Isomerization-Methylation Protocol*
*
Reactions were performed using allylic alcohol (0.5 mmol) and reagent grade MeOH. Isolated yields after chromatographic purification are
a
b
reported unless stated otherwise. Ten mmol of allylic alcohol starting material. As determined by 1H NMR analysis of the crude reaction mixture
with 1,3,5-trimethylbenzene as the internal standard.
was explored (Scheme 2A/B).17 A variety of secondary allylic
alcohols were tolerated, which provided access to α-methyl ketones
in good yields (products 3−25). The isomerization−methylation
procedure was performed on a 10 mmol scale to access 1.15 g
of product 3 (78% isolated yield). Within the aryl motif, various
alkyl or aryl substitution was accommodated at the 3- and
4-positions in addition to the sterically encumbered 1-naphthyl
motif. Electron-releasing (4-OMe, 4-OBn, and 4-OPh) and
electron-withdrawing (4-CF3) aryl substituents were accommo-
dated. However, the presence of a nitrile functionality within the
allylic alcohol produced a complex mixture of products,
presumably due to competing CN reduction. Halide substitution
was tolerated within the allylic alcohol, introducing an additional
functional handle into ketones 15 and 16. When 4-F aryl
substitution was explored, α-methyl ketone 10 was obtained in
44% isolated yield, which was presumably formed via nucleo-
philic aromatic substitution of the corresponding 4-F substituted
aryl ketone intermediate. Selective isomerization−methylation
occurs in the presence of a benzyl alcohol functionality, with
product 17 formed in 73% isolated yield. Pyridyl and thiophene
motifs were tolerated within the secondary allylic alcohols
(products 18−20). The aryl motif could be replaced by cyclohexyl,
benzyl, and homobenzyl groups to access dialkyl ketones 21−23
in good yields. Additional alkene substitution within the allylic
alcohols was also examined. 1,2-Disubstitution within the alkene
was tolerated, which gave ketones 24 and 25. However, allylic
alcohols containing 1,1-disubstituted or trisubstituted alkenes
were unreactive, which was attributed toward increased steric
congestion preventing alcohol dehydrogenation. Finally, 25%
conversion to β-C(sp3)-methylated alcohol 26 was observed
when cinnamyl alcohol was utilized as the substrate.
Upon evaluating the effect of electron-withdrawing aryl
substituents on the isomerization−methylation process, it was
found that 3,5-(CF3)2 aryl substitution resulted in the forma-
tion of secondary alcohol 27 in 49% isolated yield (Scheme 2C).
The accumulative inductive effect of the two trifluoromethyl
groups increased the electrophilicity of the carbonyl such that
the intermediate α-methyl ketone underwent subsequent
transfer hydrogenation. This one-pot transformation repre-
sented a formal Markovnikov hydromethylation of the allylic
alcohol starting material.18 Furthermore, the model reaction
was also performed using ethanol as solvent, which gave 46%
conversion to α-ethylated ketone 25 (Scheme 3D).
To gain mechanistic insight, the plausibility of various
possible intermediates was investigated (Scheme 3A). Enone
28 and propiophenone 29 would be formed via redox
isomerization of allylic alcohol 1, whereas secondary alcohol
30 would be generated upon transfer hydrogenation of 29.
B
DOI: 10.1021/acs.orglett.9b02900
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Optimization of Model Reaction*
Scheme 3. Mechanistic Experiments
a
entry
variation from “standard” conditions
none
no [Fe] precatalyst
yield (%)
1
2
88 (76)
<2
3
no K2CO3
<2
4
[Fe] precatalysts 4-8 (2 mol %) instead of 2
<5
5
NaOH (2 equiv) instead of K2CO3
47
6
KOt-Bu (2 equiv) instead of K2CO3
57
7
8
9
10
11
[1] = 0.25 M
[1] = 1 M
120 °C
140 °C
4 h
73
80
32
85
61
37
78
a
Determined by 1H NMR analysis of the crude reaction mixture with
1,3,5-trimethylbenzene as the internal standard.
Scheme 4. Plausible Mechanism
b
12
13
[Fe] precatalyst 2 (1 mol %)
K2CO3 (0.1 equiv)
*
Performed using 0.5 mmol of 1 and reagent grade MeOH. [1] = 0.5 M.
a
1
Determined by H NMR analysis of the crude reaction mixture with
1,3,5-trimethylbenzene as the internal standard. Isolated yield given in
b
parentheses. Me3NO (2 mol %).
Furthermore, β-hydroxy ketone 31, methyl ether 32, diketone 33,
and enone 34 were also possible intermediates. As such,
compounds 28−34 were individually subjected to the opti-
mized reaction conditions, and each gave conversion to
α-methyl ketone 3, which indicated that all compounds are
plausible reaction intermediates. Alcohol 30 and diketone 33
only gave 6% and 26% conversion to 3 after 24 h, respectively,
which indicated that these species would retard product
formation if they are indeed formed during the course of the
reaction. The reaction progress with time was monitored for
the isomerization−methylation of allylic alcohol 1.12 Product 3
was initially formed slowly, with only 10% conversion to
3 observed after 2 h. Beyond 2 h, the rate of formation of 3
increased, with 60% conversion observed after 4 h in addition
to 5% of ketone 29. Conversion to 3 reached a maximum of
93% after 16 h after which time small quantities of the corre-
sponding alcohol was observed. The initial slow formation of
product 3 in addition to the observation of ketone 29 over
the first 2 h of reaction suggested an initial isomerization of
allylic alcohol 1 to ketone 3, followed by α-methylation.
Further mechanistic information was provided by employing
CD3OD as solvent (Scheme 3B). Significant deuterium incor-
poration was observed at the α- and β-positions within product
35. The 54% D incorporation at the β-position indicated
the involvement of an iron hydride species in the reaction
mechanism, which would be formed upon dehydrogenation
of allylic alcohol 1 or methanol. As such, the proposed mecha-
nism proceeds via initial Me3NO-promoted CO decoordination
of precatalyst 2 to form 36 (Scheme 4), which promotes
dehydrogenation of allylic alcohol 1 and methanol to form
enone 28 and formaldehyde, respectively. Hydrogenation of 28
by iron−hydrogen complex 37 gives ketone 29, which can
undergo a subsequent aldol condensation reaction to pro-
duce enone 34. Reduction of enone 34 by 37 gives α-methyl
ketone 3 with regeneration of 36.
To conclude, an operationally simple and efficient one-pot
conversion of allylic alcohols to α-methyl ketones has been
developed using an iron-catalyzed isomerization−methylation
approach. A variety of secondary allylic alcohols was converted
to α-methyl ketones in good isolated yields (up to 84% isolated
yield). The process employs a catalyst based on an earth-abundant
transition metal, a carbonate base, and methanol as a C1 building
block, which improves upon the existing synthetic method.10
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b02900.
C
DOI: 10.1021/acs.orglett.9b02900
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Optimization data, experimental procedures, character-
Letter
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ization of new compounds, and spectral data (PDF)
AUTHOR INFORMATION
■
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Corresponding Author
*E-mail: morrilllc@cardiff.ac.uk.
ORCID
Louis C. Morrill: 0000-0002-6453-7531
Notes
The authors declare no competing financial interest.
Information about the data that underpins the results
presented in this article, including how to access them, can
be found in the Cardiff University data catalogue at 10.17035/
d.2019.0084111418 (accessed September 14, 2019).
ACKNOWLEDGMENTS
■
́
Voisine, A.; Pallova, L.; Bastin, S.; Cesar, V.; Sortais, J.-B. Chem.
We gratefully acknowledge the School of Chemistry, Cardiff
University for generous support and the EPSRC-funded Bath/
Bristol/Cardiff Catalysis Centre for Doctoral Training (D.E.L.
and K.P., EP/L016443/1).
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