Catalytic C-F bond activation of perfluoroarenes by tricoordinated gold(I) complexes

Article abstract of DOI:10.1002/adsc.201100843

We report the first example of gold catalyzing C-F bond activation for perfluoroarenes in the presence of silanes. Tricoordinated gold(I) complexes supported by Xantphos-type ligands, such as Xantphos and ^tBuXantphos ligands, exhibit efficacy in the hydrodefluorination (HDF) of various types of perfluoroarenes. For [^tBuXantphosAu(AuCl₂)], the highest turnover number is up to 1000 in the HDF of pentafluoronitrobenzene with diphenylsilane. An examination of functional group tolerance shows the orthogonality of this gold(I) catalytic protocol to ketone, ester, carboxylate, alkynyl, alkenyl and amide groups, suggesting its potential application in chemoselective C-F activations. Mechanistic studies show that the equilibrium between tetracoordinated [L₂Au]⁺ and [LAu]⁺ is important for the reactivity of gold catalysts, which is dependent on the sterically bulky group of Xantphos-type ligands. Furthermore, computational studies for the possible reaction pathways suggest that direct oxidative addition of C-F bonds by gold(I) cation might be the key step during these catalytic reactions. Copyright

Full text of DOI:10.1002/adsc.201100843

FULL PAPERS
DOI: 10.1002/adsc.201100843
Catalytic C-F Bond Activation of Perfluoroarenes by
Tricoordinated Gold(I) Complexes
Jin-Hui Zhan,a Hongbin Lv,a Yi Yu,a and Jun-Long Zhanga,*
a
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and
Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, Peopleꢀs Republic of
China
Fax : (+86)-10-6276-7034; e-mail: zhangjunlong@pku.edu.cn
Received: October 27, 2011; Revised: March 6, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201100843.
À
Abstract: We report the first example of gold cata- chemoselective C F activations. Mechanistic studies
lyzing C F bond activation for perfluoroarenes in show that the equilibrium between tetracoordinated
À
the presence of silanes. Tricoordinated gold(I) com- [L2Au]+ and [LAu]+ is important for the reactivity of
plexes supported by Xantphos-type ligands, such as gold catalysts, which is dependent on the sterically
Xantphos and BuXantphos ligands, exhibit efficacy bulky group of Xantphos-type ligands. Furthermore,
t
in the hydrodefluorination (HDF) of various types of computational studies for the possible reaction path-
perfluoroarenes. For [tBuXantphosAu
highest turnover number is up to 1000 in the HDF of bonds by gold(I) cation might be the key step during
pentafluoronitrobenzene with diphenylsilane. An ex- these catalytic reactions.
amination of functional group tolerance shows the
(AuCl2)], the ways suggest that direct oxidative addition of C F
À
orthogonality of this gold(I) catalytic protocol to
ketone, ester, carboxylate, alkynyl, alkenyl and Keywords: catalysis; C F bond activation; DFT cal-
amide groups, suggesting its potential application in culations; gold complexes; hydrodefluorination
À
Introduction
Transition metal-catalyzed hydrodefluorination
À
À
(HDF), replacing C F with C H bonds, facilitates the
À
Fluoroarenes are versatile components of many syn- understanding of C F bond activation and provides
thetic biologically active compounds and functional access to further atom transfer reactions.[7] The forma-
materials.[1] To access these fluorinated building tion of metal fluorides often plays crucial roles during
[8–11]
À
À
blocks, activation and further functionalization of C
C F bond activating process.
For gold chemistry,
F bonds catalyzed by transition metals has become an gold fluorides have proven to be important intermedi-
important approach.[2] However, for the strong bond ates in the F atom transfer to alkynes or alkanes.[12,13]
À
À
dissociation energies (BDEs) of C F bonds, C F acti- Recently, Toste, Gouverneur, Sadighi, Nevado, Miller,
vation is one of the most challenging issues in organic Hammond, Xu and their co-workers reported gold(I)-
chemistry. Although transition metals such as nickel, catalyzed C F bond formation, for example, hydro-
À
palladium, or platinum exhibit catalytic reactivity fluorination of alkynes and alkoxyhalogenation of hy-
toward C F bond activation,
exploring new metal droxydifluoroynones.[12,14] In contrast, using gold cata-
[3,4]
À
À
catalysts is still a promising topic in this field. As iso- lysts in C F bond activation has been less explored.
A
complexes have been demonstrated to be efficient in gold(I) complexes toward catalytic HDF, especially
[5]
À
C H bond activation in recent years, but they have using silanes as hydrogen sources, might be a starting
À
rarely been investigated in C F bond activation. point to investigate the possibility of gold catalyzing
Thus, exploring the possibility of gold(I) complexes to C F bond activation. Herein, we firstly report that
catalyze C F bond activation is of importance not tricoordinated gold(I) complexes supported by Xant-
[15]
À
À
only for extending the scope of reaction types in gold phos-type ligands are active catalysts for the hydrode-
catalysis but also for integrating recent advances in fluorination of various types of fluoroarenes with
gold chemistry to organofluorine chemistry.[6]
silane (Scheme 1). In addition, computational studies
À
indicate that a direct oxidative addition in C F bonds
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Table 1. Solvent effect on HDF of PFNB.[a]
FULL PAPERS
Entry Solvent
Ligand [%] Yield [%][b] a/b
TON[c]
1
2
3
4
5
6
7
8
toluene
THF
acetonitrile –
ethanol
DCM
DCE
DCM
DCE
–
–
0
0
0
0
4
5
30
41
–
–
–
–
0
0
0
0
Scheme 1. Gold(I) complexes-catalyzed HDF of fluoroar-
enes.
–
–
–
2
2
54/46 4
58/42 5
50/50 30
67/33 41
by the gold(I) cation might be the key step during the
catalytic process.
[a]
The reactions were carried out with XantphosAuCl
(0.002 mmol, 1.6 mg); Xanthphos (0.004 mmol, 2.3 mg),
silane (0.4 mmol) and PFNB (0.2 mmol, 42.6 mg) in DCE
(4 mL) at 808C under an N2 atmosphere for 24 h.
Yields were determined by integration of 19F NMR reso-
nances in the product mixture (versus internal benzotri-
fluoride) after work-up.
Results and Discussion
Optimizing the Reaction Conditions
[b]
[c]
In this work, we began our study with [Xant-
ACHTUNGTRENNUNGphosAuCl] as catalyst in the presence of triethylsilane.
Pentafluoronitrobenzene (PFNB) was chosen as sub-
strate to optimize the reaction conditions.
TON was calculated based on yield. TON=molar ratio
of catalyst.
The solvent effect was investigated using toluene,
THF, acetonitrile, ethanol, dichloromethane and 1,2-
dichloroethane (DCE) at 808C for 8–12 h. As shown
We also investigated the effect of counter ions by
in Table 1, only in dichloromethane (DCM) and di- using AgOTf (OTf=CF3SO3, trifluoromethanesulfo-
chloroethane (DCE) were the 2,3,5,6-tetrafluoronitro- nate), CsOH, acetic acid and trifluoroacetic acid
benezene or 3,4,5,6-tetrafluoronitrobenezene products (TFA) as additives to replace chloride. As shown in
obtained in yields of 4% and 5% (entries 5 and 6), Table 2, the addition of a catalytic amount of acetic
whereas no conversion of PFNB was detected in tolu- acid (2 mol% based on PFNB) promoted complete
ene, THF, acetonitrile and ethanol (entries 1-4).
conversion of PNFB, with higher regioselectivity (the
We also observed the formation of black precipi- ratio of p- and o-HDF products increased up to 86:7)
tates during the reaction process due to the reduction (Table 2, entry 3).
of gold(I) to gold(0), resulting in low reactivity. Inter-
The reactivity of various gold(I) complexes toward
estingly, addition of another 2 equivalents of Xant- HDF was examined and the results for ligand effects
phos ligand (based on [XantphosAuCl]) was effective are shown in Table 3. When [(NHC)AuCl] complexes
to prevent formation of the black precipitate, and the (NHC=N-heterocyclic carbene, IMes, IPr and ICy)
yields of HDF products increased to 30% or 41%, re- were used as catalysts,[16] the reaction did not proceed
spectively, in DCM or DCE (entries 7 and 8).
(entries 1-.3). Monophosphane ligands such as PPh3,
Silanes such as trimethoxysilane, dimethylphenylsi- bulky PCy3 and [(t-Bu)2P(o-biphenyl)] in the support-
lane and diphenylsilane were used as hydrogen sour- ed gold(I) complexes were not effective (<5% yield,
ces. As shown in the Supporting Information, entries 4–6).[17] Other than Xanthphos ligand, biden-
Table S1, diphenylsilane showed higher reactivity tate arylphosphanes such as dppe [1,2-bis(diphenyl-
giving 85% yield (Supporting Information, Table S1, phosphino)ethylene] and BINAP [2,2’-bis(diphenyl-
entry 4), compared with other silanes (Supporting In- phosphino)-1,1’-binaphthyl] were used and less than
formation, Table S1, entries 1–3), although the regio- 5% yields of HDF products were obtained (entries 7
selectivity was relatively low (55:44 for p-:o-HDF and 8). Interestingly, only tricoordinated complexes,
products). In this reaction, diphenylsilane was con- where two phosphine atoms in Xantphos-type ligands
verted to monofluorodiphenylsilane, difluorodiphe- chelate one gold atom, showed 100% conversion of
nylsilane and some diphenylsilane remained intact. PFNB (entries 9 and 10); whereas linear bicoor-
Additionally, it is worthy to note that no reduced
nitro product was detected when using PFNB as sub-
strate.
N
2ACHTUNGTRENN(UNG dppm)Cl2],
G
ACHTUNGTRENNUNG
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Catalytic C-F Bond Activation of Perfluoroarenes by Tricoordinated Gold(I) Complexes
Table 2. Additive effect on HDF of PFNB[a]
.
duced to give the benzyl alcohol in 100% yield;
whereas 9, with an acetyl substituent, gave the HDF
product in 100% yield and no acetyl-reduced product
was detected. The carboxylate group in 10 and the
ester group in 11 had little effect on the reactivity of
HDF, but a lower regioselectivity was observed for 10
(the ratio of p-:o-H products was 66:34) than for 11.
The alkynyl-substituted perfluorobenzene 13 also
gave the HDF product in 80% yield, indicating that
the alkynyl group remained intact during the reaction
process. For 14 and 15, in spite of the strong electron-
withdrawing groups, the yields of HDF products de-
creased to 30% and 20%, respectively, owing to the
double bonds between the perfluorophenyl and the
ester/or dicyanide groups. Using perfluorophenyl
amides as substrates (16 and 17), the yields of HDF
products decreased to 8–10%, due to the strong elec-
tron-donating N,N’-diethylamino group. At last, we
extended the substrates to heterocycles and polyfluor-
oarenes such as 18–23. The use of pentafluoropyridine
18 gave a 100% yield and the ratio of p-H:o-H prod-
uct was 68:32. Perfluorobiphenyl 19 showed a lower
yield (10%) than perfluoronaphthalene 20 (45%),
which might be due to less conjugation with electron-
withdrawing polyfluoro group. Similarly, the Schiff
base with C2 symmetry 21 only gave a product with
one p-F replaced by p-H in moderate yield (18%).
For the less fluorinated nitrobenzenes 22 and 23, the
Entry
Additive
Yield [%][b]
a/b/c
TON[c]
1
2
3
4
AgOTf
CsOH
acetic acid
TFA
5
5
100
60
100/–/–
100/–/–
86/7/7
5
5
100
60
73/27/–
[a]
The reactions were performed with the procedure as
shown in Table 1 (entry 8). H2SiPh2 was used as hydrogen
source and different additives were added.
Yields were determined by integration of 19F NMR in the
product mixture (versus internal benzotrifluoride) after
work-up.
[b]
[c]
TON was calculated based on the yield. TON=molar
ratio of catalyst.
For Xantphos-type ligands, when the phenyl group reactivity went down compared to perfluoronitroben-
was replaced by the tert-butyl group, the reactivity of zene. From these results, we deduce that the electron-
[tBuXantphosAu
(AuCl2)] toward the HDF reaction is ic effect of perfluoroarenes played critical roles in the
ACHTUNGTRENNUNG
much higher than that of [XantphosAuCl] (TON: reactivity of HDF and the reaction position of per-
1000 vs. 200, entries 12 and 11), indicating the impor- fluoroarenes was more favourable to p-F than o-F.
tance of steric and electronic effects of the phosphine More importantly, most of substrates with higher re-
ligands. A control experiment was carried out in the activity (up to 100% yields) such as 1, 2, 9, 10, 18 and
absence of gold catalysts and no hydrodefluorinated 22 showed HDF of o-F products, indicating that the
product was obtained (entry 16). These results sug- regioselectivity also appeared to be governed by the
t
gested that using BuXantphos as ligand and a tricoor- electronic properties of the substrates. Most impor-
dinated gold complex is the best catalyst towards the tantly, through a functional group tolerance study, the
HDF of PFNB under these reaction conditions.
orthogonality of this gold-catalyzed protocol to
ketone, ester, carboxylate, alkynyl, alkenyl and amide
groups suggested its potential applications in chemo-
À
Scope of Substrates
selective C F bond activations.
The scope of substrates was examined using 1%
[tBuXantphosAu
ACHTUNGTRENNUNG
tronic effect of substrates, we chose compounds 1–8 Such tricoordinated [XantphosAuCl] complexes based
with electron-deficient to electron-rich substituents. on the semi-flexible spacer of 9,9-dimethylxanthene
Along with increasing electron-withdrawing ability, have been revealed to be effective in the dehydrogen-
the reactivity increased. For the electron-donating ative silylation of alcohols by Ito, Le Floch and
methoxy group, no HDF product was observed. Next, others.[19–21] These rigid phosphines were regarded to
we examined the functional group tolerance of per- be able to control and stabilize the geometry of gold
ACHTUNGTRENNUNGfluoroarenes (9–17). Except for pentafluorobenzalde- complexes. More importantly, this rigid chelating
t
hyde 12, the substrates 9–11 and 13–17 could be con- effect of Xantphos and BuXantphos ligands is crucial
verted to HDF products in yields of 8–100%. For pen- for s-bond metathesis, the first step to activate an
[19–21]
À
tafluorobenzaldehyde 12, the formyl group was re- Au Cl bond.
In this work, we found that this
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Table 3. Screening gold catalysts for the hydrodefluorination of pentafluoronitrobenzene by [Au].[a]
Entry
Catalysts
Yield [%][b]
a/b/c
TON[c]
1
G
0
–
0
2
G
0
–
0
3
G
0
–
0
4
G
0
–
0
5
6
G
<5
0
100/–/–
–
<5
0
ACHTUNGTRENNUNG
7
G
E
0
–
0
8
9
N
N
<5
100
100
20
100
<5
<5
<5
–
100/–/–
86/7/7
73/12/15
86/7/7
73/12/15
100/–/–
100/–/–
100/–/–
–
<5
100
100
200[d]
1000[d]
<5
<5
<5
–
10
11
12
13
14
15
16
ACHTUNGRTEN(NUNG AuCl2)]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a]
The reactions were performed with the procedure as shown in Table 2 (entry 3) but using different catalysts.
Yields were determined by integration of 19F NMR resonances in the product mixture (versus internal benzotrifluoride)
after work-up.
TON was calculated based on yield. TON=molar ratio of catalyst.
Catalyst (0.1 mol%), free ligand (0.2 mol%).
[b]
[c]
[d]
“[XantphosAuCl] or [tBuXantphosAu
ACTHNUTRGNEU(NG AuCl2)]+si- ing a rhodium hydride intermediate, other metal hy-
À
lanes” protocol was also effective for C F bond acti- drides could also be considered as the crucial active
vation. Thus, it is intriguing to consider the reactions intermediates in the HDF of C F bonds, such as Zr,
À
between [XantphosAuCl] or [tBuXantphosAu
and silanes.
A
we examined the reactivity of the binuclear “Au2H”
species generated by the treatment of [Xant-
AHCTUNGTRENpGNUN hosAuCl] with H2SiPh2 toward PFNB (see Support-
1. Reactions between Binuclear “Au2H” Species and
PFNB
ing Information, Figure S1 and Figure S2). We could
not observe the hydrodefluorinated product, but
rather a tetracoordinated cationic gold complex
Since Milstein and Aizenberg reported that the hy- [(XantACTHNUTRGNEUNG
phos)2Au]+.[19] Similarly, in the absence of
drodefluorination of C6F6 and C6F5H in the presence PFNB, the cationic tetracoordinated gold complex
of silane was catalyzed by Rh-silyl complexes involv-
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Catalytic C-F Bond Activation of Perfluoroarenes by Tricoordinated Gold(I) Complexes
ACTHNUTRGNEUNG
Scheme 2. Scope of perfluoroarenes in HDF catalyzed by [tBuXantphos(AuCl)2] with diphenylsilane.[a,b]
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1
Figure 1. 31P and H NMR monitoring of [LAu-(m-H)-AuL]+ intermediate on mixing with 2 equivalents of acetic acid at
1
room temperature under nitrogen in CD2Cl2. a) 31P NMR. b) H NMR. i, for [XantphosAuCl]; ii, for [LAu-(m-H)-AuL]+
formed by reaction of [XantphosAuCl] and H2SiPh2; iii, after adding 2 equivalents of acetic acid to the [LAu-(m-H)-AuL]+
intermediate.
[(XantACHTUNGTRENNUNG
phos)2Au]+ formed, suggesting this binuclear
“Au2H” species was not reactive toward PFNB.
2. Factors for the Formation of [L2Au]+
Since we observed the formation of [(Xant-
ACHTUNGTRENNUNG
phos)2Au]+, we carried out experiments for investigat-
ing the factors likely to affect the formation of
[L2Au]+. We then considered the reagents in optimi-
zation of the reactions.
(i) Addition of acetic acid: We found that the addi-
tion of acetic acid to the solution of [XantphosAuCl]
and H2SiPh2 promoted the decomposition of “Au2H”
([LAu-(m-H)-AuL]+ (33.1 and 8.61 ppm in the 31P and
Figure 2. 31P NMR monitoring of XantphosAuCl after
mixing with 1 equivalent of Xantphos ligand in the presence
of H2SiPh2 under nitrogen in CD2Cl2. i, for [XantphosAuCl];
ii, after addition of 1 equivalent of Xantphos ligand.
AHCTUNGTREG(NNUN iii) Ligand type: Using the sterically bulky ligand
1H NMR spectra, respectively) in Figure 1.[19] The tBuXantphos, no [(tBuXantphos)2Au]+ complex was
control experiment showed that acetic acid had no re- observed, according to 31P NMR spectra (Figure 3, a)
action with [XantphosAuCl] (Supporting Information, and ESI-MS (Supporting Information, Figure S7).
Figure S4). Thus, we assumed that acetic acid reacted This might be due to the sterically demanding tert-
with the “Au2H” species and promoted the formation butyl groups in the BuXantphos ligand.[19] Different
t
of [(Xantphos)2Au]+.
from [XantphosAuCl], [(tBuXantphos)Au
ACTHNGUTERNN(UG AuCl2)]
(ii) Amount of ligand: In this process, we noted tends to form bicoordinated [LAu]+ rather than tetra-
that the solution colour changed from pale yellow to coordinated [L2Au]+.
dark brown. After centrifugation, we obtained black
precipitates and a pale yellow supernatant. Transmis- of ligands to the solution of [(tBuXantphos)Au-
sion electron microscopy (TEM) of the black precipi- (AuCl2)] and [XantphosAuCl] in CD2Cl2. As shown in
tates revealed that they were gold(0) nanoparticles Figure 3, [XantphosAuCl] converted to an [L2Au]+
To better demonstrate this, we added 2 equivalents
AHCTUNGTRENNUNG
with an average size of ca. 200 nm (Supporting Infor- type complex and [(tBuXantphos)Au
ACHTNUTRGNE(UNG AuCl2)] existed
mation, Figure S3). The pale yellow supernatant ex- as an [LAu]+ type complex.
hibited a similar 31P NMR spectrum to that of the cat-
ionic tetracoordinated gold complex [(Xantpho-
s)2Au]+.[23] When extra Xantphos ligand was added to 3. Steric Effect of Ligands on the Reactivity
the solution of [XantphosAuCl] and H2SiPh2
(Figure 2), [XantphosAuCl] was converted to [(Xant- To compare the reactivity of [tBuXantphosAu
ACTHNUTRGNEN(UG AuCl2)]
phos)2Au]+ completely, according to 31P NMR spec- and [XantphosAuCl], we carried out time course ex-
tra.[23] Thus, addition of 1 or 2 equivalents of free periments. The formation of HDF product was moni-
ligand could stabilize the gold catalysts and prevent tored by 19F NMR and the time course plot of the
the formation of unreactive gold(0) precipitate.
TON is shown in Figure 4. We found that
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Catalytic C-F Bond Activation of Perfluoroarenes by Tricoordinated Gold(I) Complexes
Figure 3. 31P NMR spectra and structural transformation for two gold catalysts after mixing with free ligand under nitrogen
in CD2Cl2. a) [tBuXantphosAu
ACTHNUTRGENUN(G AuCl2)]; b) XantphosAuCl.
[tBuXantphosAu
tivity than [XantphosAuCl].
ACHTUNGTRENNUNG
To further explore the possible intermediates in- hard for the substrate to access to metal center. How-
volved in the catalytic process in situ, we monitored ever, the reactivity of tetracoordinated cationic gold
the process of the HDF reaction in CD2Cl2 using complex [(Xantphos)2Au]+ toward HDF (Supporting
31P NMR to detect the changes of the gold catalysts. Information, Figure S6) showed that [(Xant-
À
We found that [(Xantphos)2Au]+ (7.2 ppm) was the
phos)2Au]+ could be the precursor for catalytic C F
ACHTUNGTRENNUNG
main species during the whole process. At the end of bond activation. Thus, we speculated that there might
the reaction, the free ligand appeared at À15.5 ppm be an equilibrium between [L2Au]+ and [LAu]+
in the 31P NMR spectrum, indicating that gold atoms (Figure 3, b).
were
released
from
[(Xantphos)2Au]+.
For
Taken together, through mechanistic studies, we
[tBuXantphosAu
ACHTUNGTRENNUNG(AuCl2)] as catalyst, there was also found that (i) [LAu-(m-H)-AuL] is not active toward
no change for the phosphine signal in the 31P NMR the HDF reaction; (ii) addition of HOAc or/and extra
spectra. Thus, we proposed that [(Xantphos)2Au]+ or ligand is crucial to break the Au Cl bond and form
À
[tBuXantphosAu]+ was an important intermediate for the active precursor [L2Au]+ or [LAu]+ – [L2Au]+ for
the HDF reaction. According to the X-ray crystal the Xantphos ligand, and [LAu]+ the for bulky
structure determination by Lagunas,[23] the Au(I) tBuXantphos ligand; (iii) the steric difference between
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Jin-Hui Zhan et al.
FULL PAPERS
association of the substrate (through highly coordinat-
À
ed Si F bonding) with the silane.
Combining our experimental results with previously
reported mechanisms for transition metal-catalyzed
hydrodefluorination,[10,27,28] we mainly considered two
possible mechanisms: [LAu(I)H] mediating hydride
+
À
transfer or [LAu] activating C F bonds, as shown in
Scheme 3.
We firstly investigate [LAu(I)H] mediated hydride
transfer to replace the F atom of pentafluoropyridine.
However, tricoordinated gold hydride [LAu(I)H] was
not obtained by DFT calculation; instead, we opti-
mized a linear coordinated gold hydride complex
(Figure 5). This species is unstable and rapidly decom-
poses to a linear coordinated gold hydride, which re-
sults in the formation of [LAu-(m-H)-AuL] according
to the work of Jean, Mꢃzailles and Le Floch.[21] Thus,
we speculated that the pathway for this gold(I) medi-
ated hydride transfer to pentafluoropyridine in our
catalytic protocol based on the following reasons: (i)
experimental results show no reactivity of [LAu-(m-
À
Figure 4. Time course plot of the catalytic HDF of penta-
fluoropyridine (DCE, 808C, N2) by 0.5 mol% gold(I) com-
plexes showing changes in the TONs for [tBuXantphos
ACHUTNGRNENUG u-
ACHTUNGTRENNUNG(AuCl2)] vs. [XantphosAuCl].
the ligands is related to the different reactivity of H)-AuL] toward C F bond activation; (ii) monophos-
[XantphosAuCl] and [tBuXantphosAu
N
phane coordinated gold catalysts might exhibit reac-
tivity if the linear coordinated gold hydride complexes
À
are active toward C F bond activation; (iii) theoreti-
Computational Studies
cal studies for an [Au-H] intermediate show more co-
valent bond than ionic bond property between Au
À
To understand how gold(I) activates the C F bond of and H, which retards the hydride transfer from
perfluoroarene, we carried out computational studies [LAu(I)H]. Therefore, we ruled out the gold hydride
using the B3PW91 functional.[24] The single point en- as an active intermediate for the transfer of hydride
ergies were calculated by using the MP2 method[25] to replace the F atom of the C F bond.
À
À
and the solvent effect was incorporated via a polarized
Then, we considered the possibility of C F bond
continuum model (PCM) based on gas-phase opti- activation by [LAu]+ via the following pathways: (i)
mized geometries.[26] Pentafluoropyridine was chosen phosphine-assisted pathway (Path a, Scheme 3), in
À
as a model in the computational studies on the C F which phosphine might play a role in accepting a fluo-
bond activation of perfluoroarenes.[11,27] Although rine atom and (ii) a direct oxidative addition reaction
À
pentafluoropyridine may have a different reactivity or (Path b). The structures and energies of the two C F
selectivity from perfluorobenzenes, it exhibited simi- activation pathways are shown in Figure 6. The phos-
À
lar reactivity and selectivity to other pentafluoroaro- phine-assisted C F activation process (Path a) occurs
matic substrates (para-selectivity) in this work. Steric through a four-centered transition state (TS1, DE=
À1
À
bulk at the phosphine seems to be important in this 31.4 kcalmol ), which involves elongation of the C4
work and we only focus on the role of the phosphine F1 bond to 1.92 ꢂ. As following, the negative charge
atom in the catalytic process. Therefore, to save com- F1 (À0.60 e) is trapped by the positively charge P1
puting time, t-Bu/phenyl groups were substituted by (1.14 e) to form a phosphine-fluorine intermediate
methyl groups in the 9 position of the Xantphos (IM1, DE=16.9 kcalmolÀ1). Au is approximately co-
À À À
ligand were replaced by two hydrogen atoms. planar with P1, C1 and P2 (Au P1 C1 P2ꢀ1808) in
According to a reviewerꢀs suggestion, we tried to IM1, and this structure involves a five-coordinate P1,
À
À
explore the possibility of pre-association of the sub- which is supported by elongating the P1 Au and P1
strate (through highly coordinated Si F bonding) C1 bonds (2.42 and 1.97 ꢂ). IM1 is not stable in
À
with the silane. However, we could not find any energy, it either goes back to reform the reactants or
À
highly coordinated Si F bonding complex between goes forward to react with HSiMe3 through a higher
substrate and silane through calculations, because it is energy barrier (TS2, DE=40.6 kcalmolÀ1) to form the
not stable in the binary system just including penta- HDF product of pentafluoropyridine. In contrast, the
fluoropyridine and silane. The distance between Si most accessible oxidative addition process (Path b)
À
and F atoms is more than 5 ꢂ in the calculated com- corresponds to direct C F activation through TS3
plex of pyridine and silane (see Supporting Informa- (DE=30.4 kcalmol ). The C4 F1 bond is elongated
À1
À
tion). We therefore ruled out the possibility for pre- to 1.70 ꢂ, while the distances of Au···C4 and Au···F1
8
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Catalytic C-F Bond Activation of Perfluoroarenes by Tricoordinated Gold(I) Complexes
Scheme 3. Proposed mechanisms for HDF catalyzed by tricoordinated gold complexes.
À
À
À
are close to 2.17 and 2.18 ꢂ, respectively. The Au C4, formation of Si F1 and C4 H1 bonds. It is worthy to
À
À
À
À
Au F1 and Au P bonds in TS3 are coplanar (P1
note that highly coordinated Si F bonding is observed
À
À
Au F1 C1=1808), and the substrate plane is perpen- in TS2 and TS4, which is important to transfer hy-
dicular to the Au F1 C4 plane. The process gives dride from silane to C4 after C F bond breaking. The
À
À
À
À
a more stable intermediate (IM2, DE=À10.6 kcal C F bond activation is the overall rate-determining
molÀ1), which features a four-coordinate gold
AHCTUNGTRENNUNG
complex. When silane is added, the HDF of penta- state is lower in the oxidative addition process than
fluoropyridine will be finished through a low energy that in the phosphine-assisted process (Pathway a).
barrier (TS4, DE=10.4 kcalmolÀ1), since fluorosilane Therefore, [LAu]+ catalysis of C F activation might
À
formation provides a thermodynamic sink.[29] Syner- proceed by direct oxidative addition.
gistic interactions are observed in TS4, involving the
cleavage of Si H1, Au F1 and Au C4 bonds and the hydrodefluorination of pentafluoropyridine according
Finally, we tried to understand the selectivity of the
À
À
À
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Jin-Hui Zhan et al.
FULL PAPERS
to DFT calculation results. Through comparison of
the energiesÀo1 f transition states at para (TS3, DE=
30.4 kcalmol ) and ortho positions (TS3O, DE=
34.6 kcalmolÀ1) of pentafluoropyridine (Figure 7), ac-
À
tivation of the para C F bond is more favourable
À
than that of the ortho C F bond, in accordance with
the experimental results (Scheme 2). However, the
À
calculations of their C F bond energies with BSSE
correction[30] show that the para C F strength for pen-
À
tafluoropyridine is only ~0.5 kcalmolÀ1 higher than
that of the ortho C F bond, indicating the low differ-
ence between para and ortho C F bond energies. On
the other side, the charge distribution calculated by
NBO (natural bond orbital) charge analysis shows
that the charges for para and ortho carbons in TS3
and TS3O are 0.01 e and 0.26 e, respectively. A similar
charge trend is found for para and ortho carbons in
À
À
Figure 5. View of the gold hydride intermediate [LAu(I)H].
À
À
Selected bond lengths (ꢂ): Au H1=1.61, Au P2=2.34,
Au···P1=3.92.
Figure 6. Computed reaction profile for HDF of C5F5N with [LAu]+ to give the para-position hydrogenation product through
pathways a and b. Energies (kcalmolÀ1), including zero point energy and solvent corrections (PCM method, dichloroethane),
are quoted relative to isolated reactants. Selected key distances (ꢂ) and angles (8) are shown.
10
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Catalytic C-F Bond Activation of Perfluoroarenes by Tricoordinated Gold(I) Complexes
bond activation and functionalization of fluorocar-
bons.
Experimental Section
General Procedure for Gold-Catalyzed
Hydrodefluorination of Fluoroarenes
Under a nitrogen atmosphere, to a Schlenk tube containing
0.002 mmol catalyst, were added via syringe 0.004 mmol
ligand, 0.4 mmol silane and 4 mL 1,2-dichloroethane. The
solution was stirred at room temperature for 0.5 h, then
0.2 mmol substrate was added and the reaction mixture was
heated at 808C under nitrogen for 24 h. A small portion
(0.2 mL) of reaction mixture was taken from the Schlenk
tube with a syringe and passed through a short column of
silica with ethyl acetate as an eluent to give a sample for
GC/MS analysis. After the reaction was complete, the reac-
tion mixture was directly placed in an NMR tube and the
yields of the products were determined by integration of 19F
signals in the 19F NMR spectrum (with internal benzotri-
fluoride).
À
Figure 7. View of transition states for C F bond cleavage in
para and ortho positions (TS3 and TS3O, respectively). Se-
À
lected bond lengths (ꢂ) and angles (8): in TS3, Au Cp=
À
À
À
À
2.17, Au F1=2.18, Cp F1=1.70, F1 Au Cp=46.0; in
À
À
À
À
À
TS3O, Au Co=2.17, Au F1=2.17, Co F1=1.75, F1 Au
Co=47.2. Computed natural atomic charges (e): in TS3, Cp:
0.01, F1: À0.42, Au: 0.42; in TS3O, Co: 0.26, F1: À0.45, Au:
0.41.
isolated pentafluoropyridine (0.36 e and 0.59 e). This
indicates that the para carbon atom is more accessible
to the gold atom than the ortho one. Thus, the elec-
tronic effect might play an important role in the de-
Acknowledgements
This project was supported by National Key Basic Research
Support Foundation of China (NKBRSFC) (2010CB912302)
and the National Scientific Foundation of China (grant
no.20971007).
À
creasing para C F bond activation.
Conclusions
In summary, we report the first example of gold cata-
lyzing C F bond activation in the presence of silanes
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À
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14 Catalytic C-F Bond Activation of Perfluoroarenes by
Tricoordinated Gold(I) Complexes
Adv. Synth. Catal. 2012, 354, 1 – 14
Jin-Hui Zhan, Hongbin Lv, Yi Yu, Jun-Long Zhang*
14
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