Ir-catalysed formation of C-F bonds. From allylic alcohols to α-fluoroketones

Article abstract of DOI:10.1039/c1cc12653a

A novel iridium-catalysed tandem isomerisation/C-F bond formation from allylic alcohols and Selectfluor to prepare α-fluorinated ketones as single constitutional isomers is reported.

Full text of DOI:10.1039/c1cc12653a

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Ir-catalysed formation of CꢀF bonds. From allylic alcohols to
a-fluoroketonesw
´
´
Nanna Ahlsten and Belen Martın-Matute*
Received 5th May 2011, Accepted 31st May 2011
DOI: 10.1039/c1cc12653a
A novel iridium-catalysed tandem isomerisation/C–F bond forma-
tion from allylic alcohols and Selectfluors to prepare a-fluorinated
ketones as single constitutional isomers is reported.
Catalysts such as RhCl(PPh3)3, [Rh(COD)Cl]2, [Rh(COD)-
(MeCN)2]BF4, [Z5-(Ph5C5)Ru(CO)2Cl] or Cp RuCl(PPh3)2 are
known to isomerise allylic alcohols into ketones.14,16 Attempts
to obtain fluorination by combining them with reagents
such as N-fluorobenzenesulfonimide (NFSI) or Selectfluors
(SelectF) were unproductive due to deactivation of the catalyst.
To overcome this, we evaluated complexes of a higher oxidation
state, such as [RhCp*Cl2]2 and [IrCp*Cl2]2 (Table 1). When
combined with NFSI, these two complexes catalysed the
isomerisation/fluorination of 1-octen-3-ol (1a) in 43 and
70% yield, respectively (Table 1, entries 1 and 2). Unexpectedly,
the product was a mixture of isomeric a-fluoroketones 2a
and 2a0. In an attempt to explain the formation of 2a0, we
performed a control experiment and confirmed that octan-3-
one (3a) was unreactive under the reaction conditions used in
Table 1, entry 2. Therefore, this fluorination (entry 2) did not
proceed from base- (the reactions were run in the presence of
Na2CO3) or Lewis acid-catalysed enolisation of 3a, but rather
via an alternative (unselective) mechanism to that of
Scheme 1.17 When SelectF was used as the fluorine source
and water was used as a co-solvent, only the desired constitu-
tional isomer (2a) was formed in 40% yield, together with 60%
of non-fluorinated ketone (3a) (Table 1, entry 3). Under the
The introduction of fluorine into organic compounds has become
a widely applied strategy to modulate the steric, electronic and
lipophilic properties of a molecule. In this way, fluorine atoms can
be used to modify the pharmacokinetic properties of pharma-
ceuticals.1 Electrophilic fluorination of carbonyl compounds,2–5
including asymmetric versions,2b–d,6 has emerged as a powerful
tool for aliphatic C–F bond formation.7 Still, few methods for
highly regioselective fluorination of unsymmetrical ketones have
been reported, and sufficient steric differentiation at the a-carbons
is usually a requirement for obtaining high selectivity. Alter-
natively, enones,8 epoxides9 or other non-carbonyl precursors10,11
can be used to introduce a handle for regiocontrol.
Allylic alcohols can be isomerised into carbonyl compounds
in the presence of a transition metal catalyst.12 The isomerisa-
tion may occur via formation of transition metal enolates,13
and, if electrophiles are present in the reaction media,
a-functionalised carbonyl compounds are formed regio-
specifically (Scheme 1). Although previously reported for
aldehydes and imines,14,15 heteroatomic electrophiles have
never been used in this fashion. To some extent, the reason
for this has been the incompatibility of many transition metal
complexes with strongly electrophilic reagents.
Table 1 Catalyst and fluorine source screeninga
Here, we present a novel one-pot method to synthesise
a-fluoro ketones in a regiospecific manner by combining an
iridium-catalysed isomerisation of allylic alcohols with an
electrophilic fluorination.
Entry [M]
F source Solvent
THF
THF
Conv.b (%) 2a/2a0/3ab (%)
1c
2c
3
[RhCp*Cl2]2 NFSI
[IrCp*Cl2]2 NFSI
47
100
28/15/4
52/18/16d
40/—/60
—/—/100
—/—/—
[IrCp*Cl2]2 SelectF THF/H2Oe 100
[IrCp*Cl2]2 NFPY THF/H2Oe 100
Scheme 1 Tandem isomerisation/a-functionalisation of allylic alcohols.
4
5
[IrCp*Cl2]2 NFSI
THF/H2Oe
—
a
Unless otherwise noted, the reactions were run with 2.5 mol% of
metal dimer (5 mol% metal), allylic alcohol 1a (0.2 mmol), fluorine
source (0.2 mmol) and Na2CO3 (0.1 mmol) in the solvent indicated
Department of Organic Chemistry, Arrhenius Laboratory,
Stockholm University, SE 106 91 Stockholm, Sweden.
E-mail: belen@organ.su.se; Fax: +46 (0)8 15 4908;
Tel: +46 (0)8 16 2477
w Electronic supplementary information (ESI) available: Experimental
details and characterisation data of all compounds. See DOI: 10.1039/
c1cc12653a
(1 mL), and analysed after 2 h. Determined by 1H NMR with respect
b
c
to consumption of 1a. After 18 h. 14% 1-octene-3-one formed.
d
e
THF/H2O 2 : 1.
c
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Chem. Commun., 2011, 47, 8331–8333 8331

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Table 2 Isomerisation/fluorination of 1-octen-3-ol (1a) in THF/H2O
mixturesa
Table 3 Isomerisation/fluorination of allylic alcohols 1a–ha
Time/h 2a–hg/3a–hb (%)
Entry Allylic alcohol Product
Entry
Solvent
T/1C
Conv.b (%)
2a/2a0/3ab (%)
1c
2d
3
THF/H2O (5 : 1)
THF/H2O (5 : 1)
THF/buffere (5 : 1)
THF/buffere (5 : 1)
THF/buffere (5 : 1)
THF/buffere (5 : 1)
30
30
30
30
50
10
100
100
100
100
100
100
82/0/18
67/0/33
82/0/18
84/0/16
70/0/30
82/0/18
1c
1
4
82/13
78/19
4f
5
2c,d
6
a
1a (0.2 mmol) was added to a mixture of [IrCp*Cl2]2 (1 mol%) and
SelectF (0.25 mmol) in the solvent mixture indicated (1 mL), and
b
analysed after 1 h. Determined by 1H NMR with respect to con-
3
2
8
1
2
91 (77)/5
92 (74)/5
69/15
c
sumption of 1a. With Na2CO3 (10 mol%). At pH 3 after 15 min.
f
K/NaPO4 buffer (pH 7) was used. With 2.5 equiv. SelectF.
d
e
2ꢀ
4
same reaction conditions, NFSI or N-fluoropyridinium tetra-
fluoroborate (NFPY) failed to give any fluorinated products
(Table 1, entries 4 and 5). Only monofluorinated products
were obtained (i.e. no difluorinated ketones), under any of the
reaction conditions.
5
Since SelectF in THF/water mixtures successfully produced
only one constitutional isomer of the product (i.e. 2a0 was not
formed), further screening on this system was performed
(Table 2). The best results were obtained using a THF/water
mixture of 5 : 1. Less water failed to dissolve SelectF, and more
water increased by-product (3a) formation. The catalyst
loading could be lowered to 1 mol% (Table 2, entry 1). At
acidic pH, higher amounts of unwanted ketone 3a are
obtained and the reaction is complete in a shorter time
(15 min) (Table 2, entry 2). The best selectivities (2a/3a ratio)
are obtained in the presence of Na2CO3 or at neutral condi-
tions (using a phosphate buffer, pH 7) (Table 2, entries 1 and
3 vs. 2). Increasing the amount of SelectF had little effect on
the product distribution (Table 2, entry 4). At higher tempera-
tures than 30 1C, more of 3a was formed (Table 2, entry 5).
Neither did the amount of 2a increase when the temperature
was lowered to 10 1C, due to decreased solubility of SelectF
(Table 2, entry 6). The fluorination did not proceed in the
absence of the iridium catalyst. The reactions are operationally
simple, run at, or close to room temperature, and under an
atmosphere of air.
6d
78 (67)/15
7e
15
15
60/11
8d,f
82 (67)/8
a
Unless otherwise noted, reactions were run with [IrCp*Cl2]2
(1 mol%), allylic alcohol 1a–h (1 mmol), SelectF (1.25 mmol) in
THF (5 mL)/deionised H2O (1 mL) at 30 1C. Determined by 1H
NMR with 1,4-dimethoxybenzene or fluorobenzene as internal
b
c
standard. Isolated yields in parentheses. Phosphate buffer pH 7.
d
e
f
2 equiv. SelectF. 3 mol% of [IrCp*Cl2]2. THF/H2O = 10 : 1.
Formed as single constitutional isomers.
g
2c was the only fluorinated product produced (88%), and 3a
remained unconverted (see ESIw). This demonstrates that the
reaction occurs via an Ir-catalysed isomerisation/fluorination
sequence from allylic alcohols and that non-fluorinated
ketones are not reaction intermediates (i.e. Scheme 1).
When deuterated allylic alcohol 1f-d1 (96% D) was sub-
jected to the reaction conditions, 2f-d1 (95% D) was formed
via a 1,3-hydrogen (deuterium) shift, with the deuterium label
exclusively at the benzylic position (Scheme 2A). The high
conservation of deuterium in 2f-d1 (95% D) confirms that
there is no hydride exchange with water and also excludes a
mechanism via iridium dihydrides.14 The crossover experiment
shown in Scheme 2B established that the 1,3-hydrogen shift
occurs intramolecularly, since no traces of deuterium were
detected upon analysis of 2i.18z
We examined a variety of allylic alcohols (Table 3) and
observed that for substrates requiring longer reaction times
basic conditions promoted catalyst decomposition. Thus, we
used a phosphate buffer (pH 7) or deionised water in combi-
nation with THF. All allylic alcohols tested (1a–h) afforded
a-fluorinated ketones (2a–h) as single isomers. Non-fluorinated
ketones (3a–3h) were formed as by-products (5–19%). Both
terminal (Table 3, entries 1–4) and 1,2-disubstituted alkenes
(Table 3, entries 5 and 6) afforded a-fluoroketones in good
yields. Also 1g underwent isomerisation/fluorination to give 2g
with a fluorine substituent on a tetrasubstituted carbon
(Table 3, entry 7). Aromatic allylic alcohols (1h) can also be
fluorinated using this procedure in high yields, and fluorination
of the aromatic ring was not detected (Table 3, entry 8).
The C–F bond is formed exclusively at the alkenylic carbon
of the starting allylic alcohol. To further support this, 1c
(1 equiv.) was treated with [IrCp*Cl2]2 (1 mol%) and SelectF
(4 equiv.) in the presence of 3-octanone (3a, 1 equiv.). After 2 h,
We propose the mechanism outlined in Scheme 3. Step (i)
involves an oxidation of the allylic alcohol to a,b-unsaturated
ketone I with concomitant formation of an [IrꢀH] species.
Based on the deuterium labelling results, in step (ii) hydride
addition to the double bond occurs before the unsaturated
ketone leaves the coordination sphere of the metal centre
forming intermediate II. The enol(ate) may bind to the Ir in
c
8332 Chem. Commun., 2011, 47, 8331–8333
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Scheme 3 Proposed catalytic cycle.
a Z1-mode through the oxygen atom (IIa) or through the
methylenic carbon (IIc), or in a Z3-mode as an oxaallyl
(IIb).15b,19 In step (iii), the C–F bond is formed upon reaction
with the electrophilic SelectF. Step (iii) can occur directly
from II, or via formation of the free enol.y
11 a-Fluoroketones can also be prepared by fluorination of silyl enol
´
ethers: E. Be
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In conclusion, we have shown that a-fluorinated ketones
can be prepared as single constitutional isomers by combining
a tandem iridium-catalysed isomerisation of allylic alcohols
with an electrophilic fluorination. The choice of the Ir complex
was essential to successfully combine both steps. The reaction
is easy to perform and is run under an atmosphere of air. To
the best of our knowledge, this is the first report on the use of
iridium catalysts to form C–F bonds,6 and it is also the first
example on the construction of a C–heteroatom bond in
the transition metal-catalysed coupled isomerisation/bond-
forming reactions from allylic alcohols. The reaction works
for substrates with different degrees of substitution and C–F
bonds on a tetrasubstituted C can also be formed. We are
currently investigating the participation of the Ir center in the
C–F bond formation, and whether Ir–F species are involved in
the mechanism.
´
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´
visy,
´
´
´
Financial support from the Swedish Research council
(vetenskapsradet), the Knut and Alice Wallenberg Founda-
tion and the Berzelii center EXSELENT is gratefully
acknowledged.
´
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Notes and references
z We have confirmed that 1f-d1 and 1i convert at similar rates.
y In a control experiment using KF (1 equiv.) instead of SelectF, we
ruled out the formation of the C–F bond via nucleophilic attack of
fluoride to the enolate intermediate II.20
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8331–8333 8333