Iron-catalyzed cross-coupling reactions between arylzinc reagents and alkyl halides bearing β-fluorines

Article abstract of DOI:10.1021/om2005904

We report the first example of iron-catalyzed cross-coupling of α-halo-β,β-difluoroethylene-containing compounds with arylzinc reagents using TMEDA and dppp as coligands. The reaction affords a wide range of functional group tolerant gem-difluoromethylenated compounds in moderate to good yields. The facile dehalodefluorination of α-halo-β,β- difluoroethylene-containing compounds upon treatment of reductive metal reagents was mostly inhibited. Mechanistic studies indicated that the cross-coupling reaction could involve a single-electron-transfer process.

Full text of DOI:10.1021/om2005904

ARTICLE
pubs.acs.org/Organometallics
Iron-Catalyzed Cross-Coupling Reactions between Arylzinc Reagents
and Alkyl Halides Bearing β-Fluorines
Xiaowei Lin,† Feng Zheng,† and Feng-Ling Qing*,†,‡
†Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu,
Shanghai 200032, China
‡College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai 201620,
China
S Supporting Information
b
ABSTRACT: We report the first example of iron-catalyzed cross-coupling of
α-halo-β,β-difluoroethylene-containing compounds with arylzinc reagents
using TMEDA and dppp as coligands. The reaction affords a wide range of
functional group tolerant gem-difluoromethylenated compounds in moderate to
good yields. The facile dehalodefluorination of α-halo-β,β-difluoroethylene-
containing compounds upon treatment of reductive metal reagents was mostly
inhibited. Mechanistic studies indicated that the cross-coupling reaction could involve a single-electron-transfer process.
Scheme 1. Reductive Dehalodefluorination (Conventional
Route) and Our Approach to Cross-Coupling Reaction of
Vicinal Halo Difluorides
ransition-metal-catalyzed reactions are widely used for
carbonꢀcarbon bond formation. The past few years have seen
T
huge advances in methodologies for transition-metal-catalyzed
coupling of sp3-hybridized carbon electrophiles (C(sp3)ꢀX, X d
I, Br, Cl) with arylmetal nucleophiles (C(sp2)ꢀM).1 However,
to the best of our knowledge, the cross-coupling of alkyl halides
bearing β-fluorine(s) with arylmetal reagents has not been
reported yet. We reasoned that two problems might make the
coupling of alkyl halides bearing β-fluorine(s) challenging: (1)
the vicinal halo fluorides mostly undergo dehalodefluorination to
form fluoroalkenes upon treatment with reductive metal reagents
(Scheme 1a);2 (2) the strongly electron-withdrawing β-fluori-
nated group may stabilize the formed transition-metal complex,
which undergoes reductive elimination slowly even under harsh
conditions.3 In contrast to carbanions, it is well-known that
carbon radicals are not subject to β-elimination of functional
groups (e.g., OR, NR2, F).4 In addition, an alkyl radical could also
be inductively destabilized by β-fluorine substitutions, which
result in a more active species.5 Therefore, we envisioned that
compounds with vicinal halo fluorides moieties such as α-halo-
β,β-difluoroethylene derivatives may readily react with reductive
arylmetal reagents to give the corresponding coupling products,
if the reaction occurs through a β-fluoroalkyl radical intermediate
(Scheme 1b).
β-difluoroethylene-containing compounds with arylzinc reagents.
Herein, we report that a FeCl2/TMEDA/dppp combination is an
efficient catalyst system for the coupling of α-halo-β,β-difluoroethy-
lene-containing compounds with arylzinc reagents. The resulted gem-
difluoromethylenated compounds have wide applications in phar-
maceuticals, agrochemicals, and materials due to their special physical
and chemical properties.14
’ RESULTS AND DISCUSSION
To probe the hypothesis, we chose the reaction outlined in
Table 1 to screen the catalyst system. Nakamura and co-workers
demonstrated that FeCl3/N,N,N0,N0-tetramethylethylenedia-
mine (TMEDA)9a,d,e and FeCl2/TMEDA9d can be used effec-
tively in the coupling of C(sp2)ꢀzinc reagents with alkyl halides.
Accordingly, the coupling of 3,3-difluoro-4-iodo-6-phenylhexane
Recently, nickel,6 iron,7ꢀ10 cobalt,11 and palladium12 com-
plexes have been used as efficient catalysts for the coupling of
alkyl electrophiles with nucleophilic metal reagents. Among
them, iron salts are particularly attractive catalysts, owing to their
low cost, environmental friendliness, and lack of toxicity. From a
mechanistic point of view, although the detailed mechanisms con-
cerned with the iron-catalyzed coupling reactions were still unclear,
extensive studies indicated that single-electron-transfer (SET) pro-
cesses may be involved in these reactions.7e,8a,8b,8d,8e,8h,8f,13 With this
notion in mind, we studied the iron-catalyzed coupling of α-halo-β,
Special Issue: Fluorine in Organometallic Chemistry
Received: July 4, 2011
Published: September 07, 2011
r
2011 American Chemical Society
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|

Organometallics
Table 1. Optimization of the Iron-Catalyzed Cross-Coupling of Phenylzinc Reagent with 3,3-Difluoro-4-halo-6-phenylhexane (1)a
ARTICLE
entry
RX
Fe salt
ligandb
TMEDA
conversn (%)c
yield 3a/4a (%)c
1
1a
1a
1a0
1a00
1a
1a
1a0
1a00
1a
1a
1a
1a
1a
1a
FeCl3
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl3
100
100
90
68/12
72/8
58/10
25d,e /7
18/6
78/5
76/4
70d,e/5
50/5
55/7
70/6
73/8
0g
2
TMEDA
3
TMEDA
4
5f
TMEDA
50
dppp
60
6
TMEDA/dppp
TMEDA/dppp
TMEDA/dppp
TMEDA/dppe
TMEDA/dppb
TMEDA/dpppen
TMEDA/dppp
TMEDA/dppp
TMEDA/dppp
100
100
90
7
8
9
100
100
100
100
0
10
11
12
13
14h
FeCl2
100
0/8
a Reactions were carried out on a 0.2 mmol scale. TMEDA = N,N,N0,N0-tetramethylethylenediamine; Ph2P(CH2)nPPh2 = dppe (n = 2), dppp (n = 3),
dppb (n = 4), dpppen (n = 5). b ZnCl2 TMEDA16 was used as the source of TMEDA ligand. Diphosphine ligand (10 mol %) was used. c Yield and
3
conversion relative to benzotrifluoride as an internal standard were determined by 19F NMR. d Phenylzinc reagent (3 equiv) was used. e The reaction
time was 24 h. f ZnCl2 instead of ZnCl2 TMEDA was used. g 1a was recovered completely. h TMEDA instead of ZnCl2 TMEDA was used.
3
3
(1a) (for the preparation of compounds 1aꢀi, 1a0, and 1a00 see
the Supporting Information) with phenylzinc reagent 2a using
FeCl3/TMEDA or FeCl2/TMEDA as catalyst proceeded smoothly
to afford the desired product 3a together with a small amount of the
deiododefluorination product 4a15 (entries 1 and 2). The FeCl2/
TMEDA catalysts were more effective than FeCl3/TMEDA
(entries 1 and 2), in terms of both conversion to the desired
product and relative amounts of side products. However, when
the less reactive β,β-difluorinated bromide 1a0 and chloride 1a00
were employed in the coupling with 2a using FeCl2/TMEDA as
catalyst, the yields and selectivities with respect to side product 4a
were much lower than those obtained with 1a (entries 3 and 4).
Very recently, Nakamura’s group9c and Bedford’s group9b reported
independently that iron diphosphine catalyst systems, such as
FeCl2/1,3-bis(diphenylphosphino)propane (dppp) and FeCl2/
1,2-bis(diphenylphosphino)benzene, showed excellent activity
in the cross-coupling reactions of arylzinc reagents with alkyl
halides, which had not been achieved by using iron TMEDA
catalysts. Unfortunately, when we attempted the coupling of the
relatively reactive 1a with 2a using FeCl2/dppp catalysts, the
yield and selectivity obtained were very poor (entry 5). Since
both of the iron/TMEDA and iron/diphosphine catalysts could
be utilized effectively in the coupling of arylzinc reagents with
nonfluorinated alkyl halides, we were interested to see whether
the combination of these two kinds of iron-based catalysts (iron/
TMEDA/diphosphine) could be used to fabricate the desired
α-aryl-β,β-difluoroethylene-containing compounds. It was found
that 1a coupled with 2a smoothly in the presence of the catalyst
combination FeCl2/TMEDA/dppp to give 3a in an improved
yield and with a better selectivity than those obtained with
FeCl2/TMEDA as catalysts (entry 6). Much to our delight,
when the catalyst combination FeCl2/TMEDA/dppp was
applied to the coupling of the less reactive β,β-difluorinated
bromide 1a0 with 2a, the desired product 3a was obtained with
much better yield and selectivity (entry 7). The β,β-difluorinated
chloride 1a00 also took part in the coupling reaction using FeCl2/
TMEDA/dppp with a slightly lower reaction rate (entry 8), and
the yield and selectivity of product 3a improved significantly in
comparison with those observed using FeCl2/TMEDA. It
seemed that the diphosphine ligands enhanced the catalytic
activity of the iron/TMEDA species in the coupling. Thus, a
series of diphosphine ligands were then examined. As can be
seen, increasing the chain length of the alkyl spacer in the
biphosphines Ph2P(CH2)nPPh2 (n = 2ꢀ5) led to a general
increase in the yield of the desired coupled product (entry 6,
entries 9ꢀ11), although dppp (n = 3) showed activity higher
than anticipated from the trend. A similar activity trend of the
biphosphines was observed by Bedford et al. in the iron-catalyzed
coupling of alkyl halides with aryl Grignard reagents.8h The
coupling reaction of 1a with 2a using FeCl3/TMEDA/dppp as
catalyst also afforded the desired product 3a in yield and
selectivity slightly lower than those with FeCl2/TMEDA/dppp
but higher than those with FeCl3/TMEDA (entry 12). No
coupled product 3a was detected when the iron catalyst was
omitted, and the starting material 1a remained unchanged
(entry 13). The coupling of 1a with phenylmagnesium bromide
in the absence of ZnCl2 failed to give the coupled product 3a
(entry 14). This result showed that transmetalation from mag-
nesium to zinc was essential for the coupling reaction.
With the optimized conditions in hand (Table 1, entry 6), the
scopes of α-halo-β,β-difluoroethylene-containing electrophiles
and arylzinc reagents were then examined, as depicted in Table 2.
The secondary β,β-difluorinated iodide 1a, bromide 1a0, and
chloride 1a00 coupled with phenylzinc reagent 2a smoothly to
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Organometallics
ARTICLE
Table 2. Iron-Catalyzed Cross-Coupling of Arylzinc Reagents with α-Halo-β,β-difluoroethylene-Containing Compoundsa
a Reactions were performed using 1 (0.2 mmol), ArMgBr (1 M in THF, 3.0 equiv), ZnCl2 TMEDA (1.5 equiv), FeCl2 (10 mol %), and dppp (10 mol %)
3
in toluene/THF (1.8 mL). Isolated yield. b Phenylzinc reagent (3 equiv) was used. The reaction time was 24 h. c The conversion of 1a00 and 1h was 90%.
d Yield relative to benzotrifluoride as an internal standard was determinded by 19F NMR. e 3-MeO-PhMgBr (1 M in THF/toluene) was used.
f 4-Cl-PhMgBr (0.8 M in diethyl ether, 6 equiv) was used, and an additional volume of THF (0.6 mL) was added into the reaction mixture.
afford the corresponding product 3a in good yields. Functional
groups such as an ester (3b), terminal alkene (3e), thiomethyl
(3f), and acetal (3g) were all compatible with the reaction
conditions. Of particular note is that thiomethyl (3f), which is
often regarded to poison the transition-metal catalysts,17 had
little influence on the course of the reaction and a satisfactory
77% yield of 3f was obtained. In addition, a primary alkyl iodide
(1g) was also found to be a suitable substrate for the cross-
coupling. Functionalized aryl zinc reagents with p- and m-methoxy
groups and p-fluoro and p-chloro groups werecompetent coupling
partners to give the desired products 3jꢀm, respectively, in
moderate to good yields. Increasing the steric bulk of the α-
halo-β,β-difluoride is deleterious to the reaction. For example,
reaction of ethyl 2,2-difluoro-3-iodo-4-cyclohexanylbutanoate
(1h) gave slightly reduced conversion to the coupled product
3h. No cross-coupling product and the deiododefluorination side
product were observed when the more hindered ethyl 2,2-
difluoro-3-iodo-4,4-dimethylpentanoate (1i) was used. The start-
ing material 1i decomposed completely. Similarly, the reaction of
the o-methoxyphenylzinc reagent 2f with 1a failed to give any of
the coupled product; the deiododefluorination side product 4a
was formed in 26% yield. Compound 1a decomposed partially and
recovered in 40% yield. Compounds 1a,i were stable to FeCl2/
TMEDA/dppp in the absence of arylzinc reagents.
To gain some understanding of the mechanism, 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO, 1 equiv), a typical radical
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Organometallics
ARTICLE
Scheme 2
of PhMgBr (0.6 mL of a 1.0 M THF solution, 0.6 mmol) and ZnCl2
3
TMEDA (75.8 mg, 0.3 mmol) was stirred for 1 h. Then the resulting
suspension was added to a mixture of (4,4-difluoro-3-iodohexyl)benzene
(1a; 65.0 mg, 0.2 mmol), FeCl2 (2.8 mg, 10 mol %), and 1,3-bis-
(diphenylphosphino)propane (8.8 mg, 10 mol %) in toluene (1.2 mL)
at 0 °C in one port. The reaction mixture was stirred at that temperature
for 15 min after completion of the addition of phenylzinc reagent and
stirred at room temperature for 12 h. A saturated aqueous solution of
NH4Cl (1.0 mL) was added to quench the reaction. The organic layer was
separated, and the aqueous phase was extracted with ether (2 ꢁ 2 mL).
The combined organic layers were concentrated in vacuo, and the residue
was purified by column chromatography to give (4,4-difluorohexane-1,3-diyl)
dibenzene (3a; 42.8 mg, 78% yield).
’ ASSOCIATED CONTENT
S
Supporting Information. Text and figures giving full
b
experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: flq@mail.sioc.ac.cn.
’ ACKNOWLEDGMENT
We thank Professor Qilong Shen and Professor Xingang
Zhang of the Shanghai Institute of Organic Chemistry for
insightful discussions. The National Natural Science Foundation
of China (Nos. 21072018, 20632008) is greatly acknowledged
for funding of this work.
scavenger, was added to the reaction system of 1a and 2a. The
reaction was inhibited completely (Scheme 2). Furthermore, the
δ,ε-unsaturated secondary alkyl iodide 1o was designed and
synthesized as a radical clock. Subjection of 1o to the standard
coupling conditions with 2a afforded the 5-exo-trig cyclization
product 3o in 87% yield (dr = 1.3:118); however, the cyclization
was completely inhibited by TEMPO (Scheme 2), which suggested
that a SET process occurred. A highly simplified representation of a
possible radical-based coupling mechanism is shown in Scheme 2.
The transfer of a single electron from the active iron species to the
β,β-difluorinated iodide 1o generated the radical anion intermediate
II which might not be free but rather might be associated with the
iron center.8d Radical anion intermediate II underwent cyclization
to the radicalꢀiron complex III. Transmetalation with the in situ
formed phenylzincꢀTMEDA generated the radicalꢀironꢀaryl
complex IV, which then underwent one-electron reductive elimina-
tion to give the product and active iron species.7e,8a,8b,8d,8e,8h,8f,13
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’ CONCLUSIONS
In summary, we have demonstrated an effective catalyst
system composed of FeCl2, TMEDA, and dppp for the cross-
coupling of arylzinc reagents with α-halo-β,β-difluoroethylene-
containing compounds. To the best of our knowledge, this is the
first example of iron-catalyzed arylation of vicinal halo difluor-
ides. The facile dehalodefluorination of vicinal halo difluorides
upon treatment of reductive metal reagents was mostly inhibited.
The cross-coupling reaction could involve a radical pathway.
’ EXPERIMENTAL SECTION
General Procedure for the Iron-Catalyzed Cross-Coupling
Reaction of α-Halo-β,β-difluoroethylene-Containing Com-
pounds with Arylzinc Reagents. In a dry reaction vessel, a mixture
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ARTICLE
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