One-pot synthesis of α,α-difluoroimines from alkynes through tandem catalytic diboration/fluorination/imination reaction

Article abstract of DOI:10.1016/j.tetlet.2007.03.155

Tandem catalytic diboration/fluorination/imination of arylacetylenes leads to the formation of α,α-difluoroimines, where the adjacent C{double bond, long}N and C-F₂ bonds are formed simultaneously. The convenient one-pot protocol involves a Pt(0)-catalyzed diboration of terminal or internal arylalkynes followed by electrophilic fluorination with Selectfluor in the presence of primary amines and a dehydrating agent. A plausible mechanism for the three consecutive steps (diboration/fluorination/imination) is suggested in accordance with the electronic properties of the substrates. Alkynes/catalytic diboration/alkenyl diboronate esters/Selectfluor/electrophilic fluorination/α,α-difluoroimines.

Full text of DOI:10.1016/j.tetlet.2007.03.155

Tetrahedron Letters 48 (2007) 3841–3845
One-pot synthesis of a,a-difluoroimines from alkynes through
tandem catalytic diboration/fluorination/imination reaction
Jes u´ s Ram ´ı rez and Elena Fern a´ ndez*
Dept. Qu ı´ mica F ı´ sica i Inorg a` nica, Universitat Rovira i Virgili, 43007 Tarragona, Spain
Received 19 February 2007; revised 26 March 2007; accepted 28 March 2007
Available online 3 April 2007
Abstract—Tandem catalytic diboration/fluorination/imination of arylacetylenes leads to the formation of a,a-difluoroimines, where
the adjacent C@N and C–F bonds are formed simultaneously. The convenient one-pot protocol involves a Pt(0)-catalyzed dibora-
2
tion of terminal or internal arylalkynes followed by electrophilic fluorination with Selectfluor in the presence of primary amines and
a dehydrating agent. A plausible mechanism for the three consecutive steps (diboration/fluorination/imination) is suggested in
accordance with the electronic properties of the substrates. Alkynes/catalytic diboration/alkenyl diboronate esters/Selectfluor/elec-
trophilic fluorination/a,a-difluoroimines.
Ó 2007 Elsevier Ltd. All rights reserved.
The selective transfer of a fluorine atom into an organic
molecule has become a considerable challenge for chem-
ists interested in biological and medicinal applications.
O
CF R'
2
1
O
B
R
O
B
R
O
O F+
LnPt(0)
The formation of the C–F bond adjacent to the carbonyl
or imine functionality increases the electrophilicity of
R
R'
R'
O
O
O
O
O
2
3
these functional groups, improving their bioactivity.
B B
CFHR'
The direct synthesis of a-fluoroketones often involves
R
the fluorofunctionalization of carbonyl-stabilized carba-
nions, as in the electrophilic fluorination of metal eno-
4
Scheme 1.
lates, silyl enol ethers, and enol acetates. On the other
hand, fluorine ion displacement of a halide from a-halo-
carbonyl compounds, or the reaction of diazo deriva-
tives with HF, have been shown to be alternative
fluorine source may react with alkenyl-metal com-
9
10 11
pounds (M = Sn, Si, B ) to provide alkenyl fluorides
and other structurally diverse compounds such as diflu-
5
routes toward a-fluoroketones synthesis. In this regard,
9–11
orinated amides, alcohols and ethers (Scheme 2).
we have recently described a more direct synthesis of
a-fluoroketones and a,a-difluoroketones through a
tandem catalytic diboration of alkynes followed by
Taking into account that the putative carbocationic
intermediate can be quenched with water to produce
difluoromethyl alcohol derivatives, we realized that an
alkenyl diboronate ester could directly be functionalized
toward the desired adjacent C@O and C–F bonds.
6
electrophilic fluorination protocol. In that study, we
determined the most favorable reaction conditions for
selectively obtaining the a,a-difluoroketones versus a-
fluoroketones with 2 equiv of easily-handled and cost-
7
8
effective N–F reagents, such as Selectfluor (Scheme 1).
H
R
M
R'
(M= Sn, Si, B)
Our working hypothesis was based on the previous liter-
ature reports which demonstrated that an electrophilic
F+
HO
F
RO
F
R''OCHN
F
H
R
F
Keywords: Alkynes; Catalytic diboration; Alkenyl diboronate esters;
Selectfluor; Electrophilic fluorination; a,a-Difluoroimines.
Corresponding author. Tel.: +34 977558046; fax: +34 977559563;
e-mail: mariaelena.fernandez@urv.net
H
R
F
H
R
F
H
R
F
R'
R'
R'
R'
*
Scheme 2.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.155

3842
J. Ram ı´ rez, E. Fern a´ ndez / Tetrahedron Letters 48 (2007) 3841–3845
During our continuous study of the tandem diboration–
functionalization reaction, we became interested in the
direct fluorination toward the a,a-difluoroimines syn-
thesis, as these compounds are considered interesting
synthetic intermediates to fluorinated azaheterocyclic
of NaHCO base were added to the crude preparation of
3
cis-1,2-bis(4,4,5,5,-tetramethyl-1,3,2-dioxaborolan-2-yl)
styrene. After an additional 15 h at room temperature,
an NMR spectra of an aliquot showed the formation
of the expected 2,2-difluoroacetophenone, with com-
1
2
compounds and b-fluorinated amines. To the best of
our knowledge, two previous reports have described
the direct electrophilic fluorination of imines.1 More
recently, a milder and efficient procedure for synthesiz-
ing and isolating either a-fluoroimines or a,a-difluoro-
imines via the direct electrophilic fluorination of
ketimines using N-fluorobenzenesulfonimide (NFSI),
plete conversion of the substrate. The in situ addition
0
of equimolecular amounts of R NH and Montmoril-
2
16
3
lonite K-10 as dehydrating agent, allowed the a,a-
difluoroimine formation, after 15 h of reaction (Table
1
19
1). The products could be characterized by H and
F
NMR and GC–MS and the % yield was calculated using
perfluoronaphthalene as an internal standard.
1
4
has been reported. However, all these attempts have
the imine functionality as part of the substrate, and
ketones derivatives from hydrolysis cannot be avoided.
In this context we were aimed to explore an alternative
synthetic method, in which the imine functionality could
be formed simultaneously to the C–F formation. In this
Letter, we report the first tandem approach for difluoro-
functionalizing imines from alkynes substrates in a one-
pot reaction.
The direct formation of gem-difluorinated imines in this
one-pot reaction protocol is particularly attractive, since
it opens the perspective toward the in situ synthesis of
the imine functionality when primary amine is present
in the reaction media, thus avoiding any hydrolysis
byproducts. The dehydrating capability of Montmoril-
lonite K-10 (MK-10) was particularly shown for termi-
nal a,a-difluoroimine formation (Table 1, entries 1–5).
However, for internal a,a-difluoroimines, the use of
MK-10 did not completely convert the a,a-difluoro-
ketones into a,a-difluoroimine, even under refluxing
In a typical experiment of the tandem catalytic dibora-
tion/fluorination/imination sequence involving conven-
tional reflux heating (Scheme 3), phenylacetylene was
first added to an acetonitrile solution that contained
1
7
conditions (Table 1, entries 6 and 7). Therefore, TiCl4
was required to obtain a total imine transformation
(Table 1, entries 8–10). The percentage of internal a,a-
difluoroimine formation is superior to the terminal
a,a-difluoroimines, which is in agreement with the fact
that terminal alkenyldiboronates intermediates proved
to be less reactive than internal alkenyldiboronates
3
mol % of Pt(0) catalytic precursor Pt(PPh ) , (or alter-
3 4
1
5
natively Pt(NHC) catalytic system), with an equimolar
amount of the diborating reagent bis(pinacolato)di-
boron. After the mixture was stirred at reflux tempera-
ture (82 °C) for 15 h, 2 equiv of Selectfluor and 2.2 equiv
Cl
1)
N
N
F 2BF4
R'
O
Pt(0) complex
O B
O
B O
N
NaHCO3
Ph
R
R
Ph
R
Ph
O
O
O
O
2
)
R'NH2
B B
F F
R = Ph, H
Montmorillonite K-10
Scheme 3.
a,b
Table 1. Tandem catalytic diboration/fluoriation/imination of alkynes
0
c
c
Entry
Ar
R
Amine R NH
2
Activating and dehydrating agent
a,a-Difluoroketone (%)
a,a-Difluoroimine (%)
i
1
2
3
4
5
6
7
8
9
C
C
p-(CF
6
6
H
H
5
H
H
H
H
PrNH
2
MK-10
MK-10
MK-10
MK-10
MK-10
MK-10
1
—
—
—
—
18
8
—
—
—
27
33
26
40
19
27
37
45
43
42
n
5
BuNH
BuNH
BuNH
2
2
2
n
n
3
)-C
6
H
4
p-(OMe)-C
6
H
4
C
C
C
C
C
C
6
6
6
6
6
6
H
5
H
5
H
5
H
5
H
5
H
5
H
CyNH
2
n
n
n
i
Ph
Ph
Ph
Ph
Ph
BuNH
BuNH
BuNH
2
2
2
d
MK-10
TiCl
TiCl
TiCl
4
4
4
PrNH
CyNH
2
2
1
0
a
Standard conditions: for catalytic diboration: substrate/Pt catalyst (Pt(PPh
3
)
4
/diborane (bis(pinacolato)diboron) = 1/0.03/1; solvent: CH
base = 2/1/2.2; solvent: CH CN; T: rt; t: 15 h. For imination reaction:
0.6 equiv; solvent: CH CN; T:
3
CN; T:
8
2 °C; t: 15 h. For fluorination reaction: Selectfluor/substrate/NaHCO
3
3
0
2
R NH /substrate = 1/1; activating and dehydrating reagents: Montmorillonite K-10 (MK-10) 100 mg or TiCl
4
3
2
5 °C; t: 15 h.
b
c
Quantitative conversion from the substrate.
The reaction was monitored by H NMR and F NMR, in the presence of perfluoronaphthalene as an internal standard.
Temperature for the imination step: 82 °C.
1
19
d

J. Ram ı´ rez, E. Fern a´ ndez / Tetrahedron Letters 48 (2007) 3841–3845
3843
6
toward the electrophilic fluorination step. Aliphatic ter-
minal alkynes did not provide any fluorinated com-
pounds, which is in agreement with the tendency
observed by Olah et al., where the fluorination of alke-
nyl boronic acids and trifluoroboronates worked well
only with compounds leading to benzylic carbocations.
O
B
O
B
H
O
O
1
1
X
H
X
In contrast to the efficiency of the cis-1,2-bis(4,4,5,5,-tet-
ramethyl-1,3,2-dioxaborolan-2-yl) styrene derivatives,
the corresponding cis-1,2-bis(1,3,2-benzodioxaborolan-
X=OMe δ= 6.21 ppm
X=H δ= 6.29 ppm
X=CF3 δ= 6.34 ppm
X=OMe δ= 3.00 ppm
X=H
δ= 3.08 ppm
X=CF3 δ= 3.20 ppm
2
-yl) styrene was not fluorinated. A similar effect has
previously been observed in the electrophilic fluorina-
Figure 1.
tion of (E)-(2-phenylethenyl)catecholboronate and (E)-
(
2-phenylethenyl)trifluoroboronate.11 The hexylenegly-
colato alkenyl diboronate ester derivative, was also a
suitable intermediate for the fluorination, with similar
values to the pinacol derivative. This agrees with the fact
that both the hexyleneglycolato and pinacolato alkenyl
boronate esters, have previously been easily iodo- and
1,3,2-dioxaborolan-2-yl) p-CF -styrene (Fig. 1). The dif-
3
ferences in the chemical shift values could be due to the
inductive effects of the para-substituents. In addition,
the substituted alkenyl diboronate esters seem to react
more efficiently than the corresponding substituted
phenylacetylenes toward the electrophilic N–F fluorinat-
ing reagent, Selectfluor.
1
8
chloro-deboronated in the past.
Our current mechanistic understanding of this transfor-
mation suggests that the N–F reagents could react with
alkenyl diboronate ester intermediates through an elec-
trophilic attack. To confirm this, we investigated how
far the electronic nature of the intermediate substituted
alkenyl diboronate esters can affect the electrophilic
fluorination process. The electronic properties of the
substituents in the para position of phenylacetylene
influence the electrophilic fluorination pathway. Appar-
ently, the more electron rich the alkenyl diborated ester
is, the more reactive toward the electrophilic fluorina-
tion it will be (Table 1, entries 2–4). This is in agreement
with the indirect measurement of the chemical shift in
1
The H NMR is also consistent with the fact that the ter-
minal vinyldiboranes are more nucleophilic than the aryl
acetylenes (Fig. 1).
These results provide some insights into the mechanism
of the fluorination process, especially when some
authors had reported radical mechanisms in related fluo-
rination processes. A plausible mechanism for the
overall tandem sequence is shown in Scheme 4. The cat-
1
9
2
0
alytic cycle for the diboration of alkynes, was first pro-
2
1
posed by Miyaura et al. through the oxidative addition
of diboron to Pt(0), followed by the insertion of the
alkyne into the Pt–B bond and finally the reductive elim-
ination of the alkenyldiboronate esters to regenerate the
Pt(0) catalytic system. Evidence for a Pt atom contain-
ing two cis boryl and phosphine ligands as an intermedi-
ate in the cycle has been generated by a single crystal
1
the H NMR of the alkenyldiboronate ester formed
in situ during the catalytic diboration of the para-substi-
1
tuted phenylacetylenes. The H NMR values for the
alkenyl moiety increase from 6.21 ppm in cis-1,2-bis-
(
4,4,5,5,-tetramethyl-1,3,2-dioxaborolan-2-yl) p-MeO-
2
2
styrene to 6.34 ppm in cis-1,2-bis(4,4,5,5,-tetramethyl-
structure. The catalytic diboration reaction seems to
O
O
O
O
O
O
O
O
Ar
B B
B Pt
B
Ar
Pt
Pt(0)
Step A: diboration
O
O
O
O
B
B
+
Ar
O
O
B
Pt
B
O
O
O
B
O
B
O
O
Ar
Step B: fluorination
NaHCO3
Step C: imination
F+
O
B
O
B
O
O
B
F
O
B
O
B
F
R'
O
O
O
O
O
B
O
O
O
-
F
R'NH2
N
F
H
F
Cl
Ar
+
Ar
H
F
Ar
Ar
Ar
H
Ar
N
N
F 2BF4
+
-
-
O
MK-10
F
O
O
O
C
or TiCl4
F
C
O
O
O
O
O
B B
CO2
O
Scheme 4.

3844
J. Ram ı´ rez, E. Fern a´ ndez / Tetrahedron Letters 48 (2007) 3841–3845
be significantly accelerated by unsaturated Pt(0) com-
plexes having donating phosphine ligands and to be
References and notes
2
3
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