Weakly nucleophilic potassium aryltrifluoroborates in palladium-catalyzed Suzuki-Miyaura reactions? Relative reactivity of K[4-RC6F4BF3] and the role of silver-assistance in acceleration of transmetallation

Article abstract of DOI:10.3762/bjoc.11.68

Small differences in the reactivity of weakly nucleophilic potassium aryltrifluoroborates are revealed in the silver-assisted Pd-catalyzed cross-coupling of K[4-RC₆F₄BF₃] (R = H, Bu, MeO, EtO, PrO, iPrO, BuO, t-BuO, CH₂=CHCH₂O, PhCH₂O, PhCH₂CH₂O, PhO, F, pyrazol-1-yl, pyrrol-1-yl, and indol-1-yl) with ArX (4-BrC₆H₄CH₃, 4-IC₆H₄F and 3-IC₆H₄F). An assumed role of silver(I) compounds Ag_mY (Y = O, NO₃, SO₄, BF₄, F) consists in polarization of the Pd-X bond in neutral complex ArPdL_nX with the generation of the related transition state or formation of [ArPdL_n][XAg_mY] with a highly electrophilic cation and subsequent transmetallation with the weakly nucleophilic borate. Efficiency of Ag_mY as a polarizing agent decreases in order Ag₂O > AgNO₃ ≈ Ag₂SO₄ > Ag[BF₄] > AgF. No clear correlation between the reactivity of K[4-RC₆F₄BF₃] and substituent electron parameters, σ_I and σ_R °, of the aryl group 4-RC₆F₄ was found.

Full text of DOI:10.3762/bjoc.11.68

Weakly nucleophilic potassium aryltrifluoroborates in
palladium-catalyzed Suzuki–Miyaura reactions:
relative reactivity of K[4-RC6F4BF3] and the role of
silver-assistance in acceleration of transmetallation
Vadim V. Bardin1,2, Anton Yu. Shabalin3 and Nicolay Yu. Adonin*2,3
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 608–616.
1N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, SB
RAS, Acad. Lavrentjev Ave. 9, Novosibirsk, 630090, Russian
Federation, 2Novosibirsk State University, Pirogova str., 2,
Novosibirsk, 630090, Russian Federation and 3G.K. Boreskov
Institute of Catalysis, SB RAS, Acad. Lavrentjev Ave. 5, Novosibirsk,
630090, Russian Federation
doi:10.3762/bjoc.11.68
Received: 23 January 2015
Accepted: 09 April 2015
Published: 04 May 2015
Associate Editor: C. Stephenson
Email:
Nicolay Yu. Adonin* - adonin@catalysis.ru
© 2015 Bardin et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
potassium polyfluoroaryltrifluoroborate; silver(I) acceleration;
Suzuki–Miyaura reaction
Abstract
Small differences in the reactivity of weakly nucleophilic potassium aryltrifluoroborates are revealed in the silver-assisted
Pd-catalyzed cross-coupling of K[4-RC6F4BF3] (R = H, Bu, MeO, EtO, PrO, iPrO, BuO, t-BuO, CH2=CHCH2O, PhCH2O,
PhCH2CH2O, PhO, F, pyrazol-1-yl, pyrrol-1-yl, and indol-1-yl) with ArX (4-BrC6H4CH3, 4-IC6H4F and 3-IC6H4F). An assumed
role of silver(I) compounds AgmY (Y = O, NO3, SO4, BF4, F) consists in polarization of the Pd–X bond in neutral complex
ArPdLnX with the generation of the related transition state or formation of [ArPdLn][XAgmY] with a highly electrophilic cation and
subsequent transmetallation with the weakly nucleophilic borate. Efficiency of AgmY as a polarizing agent decreases in order Ag2O
> AgNO3 ≈ Ag2SO4 > Ag[BF4] > AgF. No clear correlation between the reactivity of K[4-RC6F4BF3] and substituent electron
parameters, σI and σR°, of the aryl group 4-RC6F4 was found.
Introduction
The palladium-catalyzed reaction of organoboron compounds the desired reagent depends on the specific requirements in each
with C-electrophiles (Suzuki–Miyaura reaction) is one of the particular case [1-3]. Organoboron reagents containing an elec-
most intensively studied processes of the carbon–carbon bond tron-poor organic moiety exhibit a low reactivity under the
formation. Organoboronic acids, their esters and organotri- usual cross-coupling conditions [3-9] and the target products are
fluoroborates are partners in these reactions and the choice of formed in low yield and/or are contaminated with byproducts.
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Beilstein J. Org. Chem. 2015, 11, 608–616.
Reactions of weakly nucleophilic organoboron reagents (alkyl- The effect of the counterion Y in silver(I) compounds AgmY on
and cyclopropylboronic acids and esters [10-14], alken-1- the rate of the cross-coupling was slightly investigated. For
ylboronic acids and esters [15,16], some arylboronic acids [17- example, the rates of the CO insertion into cationic methylpalla-
19], K[CF2=CFBF3] [19]) often are accelerated by the addition dium complex [MePd(PMe3)2][XAgmY] generated from
of stoichiometric amounts of Ag2O. Initially this phenomenon MePd(PMe3)2Cl, Ag[BF4] or Ag[PF6] in acetone are equal to
was reported by Kishi et al. [16] for the cross-coupling of 23 ∙ 10−5 and 24 ∙ 10−5 s−1. The use of AgOTf (11 ∙ 10−5 s−1)
alkenylboronic acids with alkenyl iodides in the presence of or AgNO3 (5.9 ∙ 10−5 s–1) causes a decrease in the reactivity
Ag2O and elucidated by formation of AgOH which acts like towards the CO insertion [28,33]. The similar effect of
aqueous KOH or TlOH. Korenaga et al. [20] have studied anions Y = BF4, OTf, PF6 was observed in the formation
Pd-catalyzed cross-coupling of C6F5B(OH)2 with aryl halides of complexes [RPd(PMe3 )2 (solv.)][XAgm Y] and
in the presence of Ag2O and assumed the generation of [RPd(PMe3 )(solv.)2 ][XAgm Y] [34].
a hydroxy–palladium complex with higher ability to transmetal-
lation under the action of the corresponding organoboron In continuation of systematic research of the Suzuki–Miyaura
reagent than ArPdL2X. For example, complex trans- cross-coupling reaction of weakly nucleophilic organotrifluoro-
C6F5Pd(PEt3)2OH is formed in the reaction of trans- borates we report here the study of the relative reactivity of
C6F5Pd(PEt3)2I with Ag2O in toluene–water and undergoes K[4-RC6F4BF3] in the Pd-catalyzed reactions with some aryl
transmetallation with 4-MeOC6H4B(OH)2 [21]. The subse- bromides and iodides in the presence of Ag2O. The borates
quent reductive elimination leads to the corresponding polyfluo- were chosen as model organoboron reagents because of tuning
robiphenyl. In contrary, the reaction of trans-C6F5Pd(PEt3)2I electronic properties of 4-RC6F4 groups with varied R = H, Bu,
with 2,4,6-C6F3H2B(OH)2 and Ag2O leads to an unsymmet- MeO, EtO, PrO, iPrO, BuO, t-BuO, CH2=CHCH2O,
rical diarylpalladium complex trans-(C6F5)Pd(PEt3)2(2,4,6- PhCH2O, Ph(CH2)2O, PhO, pyrazol-1-yl (Prz), pyrrol-1-yl
C6F3H2) in 92% yield. The latter is thermally stable and does (Pyr), indol-1-yl (Ind), imidazol-1-yl (Im), and benzamidazol-1-
not produce the cross-coupling product even upon heating in yl (Bim) and the equal steric requirements near reaction site
toluene at 100 °C for 24 h [21]. The authors suggested that C1–BF3. The relative reactivity was estimated (a) from the yield
acceleration of these reactions by silver(I) oxide results in the of biphenyl 4-RC6F4–Ar vs C6F5–Ar and (b) from the rate of
generation of a hydroxy–palladium complex with a higher consumption of K[4-RC6F4BF3] and compared with the
ability to transmetallation, to coordinate to the organoboronic substituent electron parameters (SEP) of the 4-RC6F4 group.
acid with the three-coordinated boron atom to ArPdL2OH and The efficiency of some other silver(I) compounds (AgNO3,
subsequent transmetallation (Scheme 1).
Ag2SO4, Ag[BF4], and AgF) in the cross-coupling was exam-
ined too.
However, some experimental facts indicate the multilateral role
of silver(I) compounds AgmY in the acceleration of the Results
Suzuki–Miyaura cross-coupling. Beside generation of All reactions were carried out under the previously elaborated
ArPdLnOH, AgmY may act as a Lewis acid producing complex optimal conditions [35]. The cross-coupling of K[4-RC6F4BF3]
[RPdLn][XAgmY] with a highly electrophilic cation. The reac- (1a–r) with 1-fluoro-3-iodobenzene (2) produces pentafluoro-
tions often are performed in polar aprotic solvents (MeCN, biphenyls 4-(3'-FC6H4)C6F4R in 80–99% yield (Scheme 2 and
THF, acetone) and the complexes exist in solutions as solvates Table 1). Pentafluorobiphenyls 4-(4'-FC6H4)C6F4R were
[RPdLn(solv.)][XAgmY]. Some complexes are much more obtained from K[4-RC6F4BF3] and 1-fluoro-4-iodobenzene (3).
stable and can be isolated [22,23], although for preparative aims Notably borates K[4-RC6F4BF3] with R = H (1b), AllylO (1c)
they usually are generated in situ. This approach has been and azolides (1n–r) (see Table 1, entries 2, 3, 14–18) give much
successfully used for arylation of alkenes, alkynes, insertion of lower yields of the corresponding biphenyls than K[C6F5BF3]
CO species [23-31], polyfluoroarenes and thiophenes [32].
(1a) (Table 1, entry 1) [19].
Scheme 1: The assumed silver(I) oxide assisted transmetallation with organoboronic acids.
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Beilstein J. Org. Chem. 2015, 11, 608–616.
Scheme 2: Cross-coupling of K[4-RC6F4BF3] (1a–r) with 3-IC6H4F (2) and 4-IC6H4F (3).
Potassium 2,3,5,6-tetrafluoropyridyltrifluoroborate (6) exhibits
extremely low reactivity toward both 2 and 3 gives the corres-
ponding 4-(3'-fluorophenyl)- (7) and 4-(4'-fluorophenyl)- (8)
-2,3,5,6-tetrafluoropyridines with yields no more than 5%
(19F NMR) (Scheme 3). It should be noted that 2,3,5,6-tetra-
fluoropyridine was not observed in the reaction mixtures in both
cases.
Table 1: Preparation of biphenyls 4a–r and 5a–r.
Entry
K[4-RC6F4BF3]
Isolated yield (%)
4
5
1
1a (R = F)
99
99
2
1b (R = H)
26
24
3
1c (R = AllylO)
1d (R = MeO)
1e (R = EtO)
1f (R = PrO)
1g (R = Bu)
34
18
4
90
94
5
98
97
The use of the electron-rich C-electrophile, 4-IC6H4CH3 (9),
instead of 3 leads to biphenyls 4-(4'-CH3C6H4)C6F4R
(10c–f,h–p) in 60–80% preparative yields (see Table 2, entries
2–14) while 4-CH3C6H4C6F5 (10a) was isolated in 93% yield
(see Table 2, entry 1) [19]. Although aryl bromides are less
reactive than iodides, the substitution of 4-IC6H4CH3 (9) for
4-BrC6H4CH3 (11) does not affect the yields of the corres-
ponding biphenyls (Scheme 4).
6
82
96
7
89
50
8
1h (R = PhCH2O)
1i (R = BuO)
1j (R = iPrO)
1k (R = t-BuO)
1l (R = Ph(CH2)2O)
1m (R = PhO)
1n (R = Ind)
1o (R = Pyr)
1p (R = Prz)
1q (R = Im)
86
73
9
87
99
10
11
12
13
14
15
16
17
18
82
97
98
99
79
71
98
96
91
42
The obtained results show a similar or slightly reduced reactiv-
ity of the majority of K[4-RC6F4BF3] (R ≠ F) with respect to
salt 1a. Exceptions are borates with R = H (1b), CH2=CHCH2O
(1c) and azolides (1n–r), which produce cross-coupling prod-
ucts in low to moderate yields because of the side reactions.
Highly tolerant borate 6 also gives the product in a low yield,
but its conversion is low too. Hence, the data based on an
isolated yield of biphenyls 4a–r, 5a–r, or 10a,c–f,h–p are not a
84
20
47
35a
12b, d
20b, f
10b, c
20b, e
1r (R = Bim)
aByproduct 2,3,5,6-C6F4HPrz (3%); bphosphine XPhos was used
instead of PPh3; cbyproduct 2,3,5,6-C6F4HIm (7%); dbyproduct
2,3,5,6-C6F4HIm (7%); ebyproduct 2,3,5,6-C6F4HBim (5%); fbyproduct
2,3,5,6-C6F4HBim (14%).
Scheme 3: Attempted synthesis of 7 (3’-F) and 8 (4’-F) by cross-coupling reaction.
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Beilstein J. Org. Chem. 2015, 11, 608–616.
Scheme 4: Synthesis of biphenyls 10a,c–f,h–p.
Table 2: Preparation of biphenyls 10a,c–f,h–p.
Entry
K[4-RC6F4BF3]
Isolated yield of 10 (%) Entry
K[4-RC6F4BF3]
Isolated yield of 10 (%)
1
2
3
4
5
6
7
1a (R = F)
93
8
1j (R = iPrO)
71
62
1c (R = AllylO)
1d (R = MeO)
1e (R = EtO)
1f (R = PrO)
1h (R = PhCH2O)
1i (R = BuO)
25a
80
9
1k (R = t-BuO)
10
11
12
13
14
1l (R = Ph(CH2)2O)
1m (R = PhO)
1n (R = Ind)
93
65
91
73
99
70
83
43b
97
1o (R = Pyr)
1p (R = Prz)
aByproduct 2,3,5,6-C6F4HOCH2CH=CH2 (18%); bbyproduct 2,3,5,6-C6F4HOCH2Ph (28%).
convenient measure for the quantitative estimation of the rela- Discussion
tive reactivity. We hoped to get more accurate data by the The general concept of the Pd-catalyzed Suzuki–Miyaura (SM)
concurrent cross-coupling of equimolar mixtures of reaction applied to the cross-coupling of K[4-RC6F4BF3] with
K[C6F5BF3] (1a) and K[4-RC6F4BF3] (1b–p) with 11. The ArX is presented in Scheme 7.
reactions were carried out over a short period (5–15 min) and
the product ratios were determined by analyzing the crude reac- The first step is the generation of neutral complex ArPdX by the
tion mixtures with 19F NMR spectroscopy. The relative reactiv- oxidative addition of ArX to Pd(0) species. This step precedes
ity was determined as Crel = CR / CF where CR and CF are the the subsequent transformation of ArPdX and does not influence
yield (in mmol) of 4-(4'-CH3C6H4)C6F4R from salts 1b–p and the reactivity of organoboron partner K[4-RC6F4BF3] as well as
4'-CH3C6H4C6F5 from 1a, respectively (Scheme 5, Table 3).
its behavior in transmetallation and/or reductive elimination
steps. For the further consideration we need to clarify the nature
The above experiments were performed using silver(I) oxide. of the active organoboron partner. Despite of many publica-
For better understanding the role of Ag+ we estimated the rela- tions [2,6,7,36,37] in this field it is not yet fully understood. For
tive efficiency of some other silver(I) compounds under iden- illustration, we refer to two examples. Molander argues for
tical conditions (Scheme 6) (Table 4).
direct transmetallation of ArPdBr by aryltrifluoroborates in
aprotic anhydrous THF in the presence of trialkylamine as a
The presented data demonstrate clearly that AgNO3, Ag2SO4, base or in alcohols [4]. Alternatively, the Pd-catalyzed cross-
Ag[BF4], and AgF salts are less appropriate promotors for the coupling of K[ArBF3] in aqueous THF is proved to proceed
palladium catalyzed cross-coupling than Ag2O. The significant through stepwise hydrolysis to produce more reactive
contribution of the side reactions (hydrodeboration and homo- [ArBFn(OH)3-n]− or ArB(OH)2 [35,38]. However, special
coupling) results in decreasing yields of 10a and hinders isola- experiments showed that K[C6F5BF3] retards toward K2CO3
tion of the desired product (see Table 4).
[39] as well as K2CO3 in a mixture with catalytic amounts of
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Beilstein J. Org. Chem. 2015, 11, 608–616.
Scheme 5: Pd-catalyzed cross-coupling of 1a and salts 1b–p (1:1) with 11 (the results are presented in Table 3 and outlined in the Discussion
section).
soluble acidic impurities formed in the reaction and does not
Table 3: Relative reactivity Crel of K[4-RC6F4BF3] in the cross coupling
with 11.a,b
participate in the other steps [19,35].
Entry
K[4-RC6F4BF3]
CR
CF
Crel
The boron atom in K[4-RC6F4BF3] is coordinately saturated
and transmetallation via transition state A (Scheme 1) is impos-
sible. An alternative pathway is the polarization of the palla-
dium–X bond by Ag+ and the generation of complex
[ArPdLn][XAgmY] with a highly electrophilic cation which is
attacked by carbon atom C-1 of K[C6F5BF3] (1a). In toluene
salt 1a is consumed within 3 h to give 2,3,4,4’,5,6-hexafluoro-
biphenyl (5a) in 70–92% yield [19], while in the polar coordi-
nating solvents (DME, DMF) the Ag2O-assisted cross-coupling
of 1a with 3 leads to formation of 5a at unsatisfactory conver-
sions (22–38%) and low yields (10–22%). This indicates that
the strong solvation of the Pd atom with DME or DMF reduces
electrophilicity of [ArPdLn][X] as compared with the elec-
trophilicity in the case of weakly coordinating toluene.
1
1c (R = AllylO)
1g (R = Bu)
13
27
22
22
25
22
23
26
23
31
20
26
22
26
26
22
40
32
29
32
28
28
29
25
33
21
27
22
25
25
0.59
0.68
0.68
0.76
0.78
0.79
0.82
0.90
0.92
0.94
0.95
0.96
1.00
1.04
1.04
2
3
1f (R = PrO)
4
1d (R = MeO)
1h (R = PhCH2O)
1b (R = H)
5
6
7
1i (R = BuO)
1k (R = t-BuO)
1j (R = iPrO)
1e (R = EtO)
1m (R = PhO)
1n (R = Ind)
8
9
10
11
12
13
14
15
1l (R = Ph(CH2)2O)
1o (R = Pyr)
1p (R = Prz)
aConditions: Pd(OAc)2, P(t-Bu)3, Ag2O, K2CO3 in toluene, 100 °C,
5–15 min; bCrel = CR/CF, where CR = 4-(4'-CH3C6H4)C6F4R (mmol)/
K[4-RC6F4BF3] (0.20 mmol) and CF = 4'-CH3C6H4C6F5 (mmol)/
K[C6F5BF3] (0.20 mmol).
K[C6F5BF3] (1a) and AgmY are insoluble in toluene and, very
likely, the transmetallation proceeds on the surface of the
silver(I) compounds. Both the aryltrifluoroborate anion and
ArPdLnX can be adsorbed on the surface of solid AgmY due to
the interaction with Ag atoms (acidic Lewis centers). This
Pd(OAc)2 in refluxing MeOH [19]. Resistance of K[C6F5BF3] phenomenon causes an increase in the electrophilicity of the Pd
(1a) towards K2CO3 and Ag2O in toluene (without palladium atom and bond rearrangement through transition state B to give
catalyst and phosphine ligand) is confirmed by stirring the the diarylpalladium species (Scheme 8). Perhaps, transforma-
corresponding suspensions at 100 °C for 10 min. These facts tions presented in Scheme 8 are parallel, i.e., the transmetalla-
make it possible to reject any transformation of the [BF3]− tion proceeds simultaneously with polarization of ArPdLnX.
moiety before the transmetallation of the palladium-containing
intermediate generated in the previous step of the catalytic cycle There is a popular notion that the transmetallation gives both
by K[4-RC6F4BF3]. Potassium carbonate scavenges toluene- cis- and trans-ArPdLnAr'. First of them is low stable and under-
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Beilstein J. Org. Chem. 2015, 11, 608–616.
Scheme 6: The cross-coupling of 1a with 11 in the presence of different silver(I) compounds.
prepared complexes trans-C6F5PdL2(2,4,6-C6F3H2) (L = PEt3,
PMe2Ph, PMePh2) do not undergo any changes even after
heating in toluene at 100 °C for 24 h [21]. These facts lead to
assume either instability of trans-(4-RC6F4)PdL2Ar towards
isomerisation to cis-isomer or the direct formation of reactive
complex cis-(4-RC6F4)PdL2Ar during the transmetallation with
closely related borates K[4-RC6F4BF3] (Scheme 8).
Table 4: Cross-coupling of 1a with 11 in the presence of different
silver(I) compounds.
Entry Conversion of 1a ( %) AgmY
Yield (%)a,b
10a 12 13
1
2
3
4
5
69
79
75
40
19
Ag2O
99
1
0
AgNO3 20
Ag2SO4 20
Ag[BF4] 40
27
28
60
43
27
26
0
At the next step, cis-(4-RC6F4)PdL2Ar undergoes reductive
elimination which includes the carbon–palladium bond cleavage
in both 4-RC6F4-Pd and Pd-Ar moieties. If the substituent Ar is
the same for all cis-diarylpalladium, the rate of the intramolec-
ular transformation from cis-(4-RC6F4)PdL2Ar to 4-RC6F4Ar
should depend on the specific property of the 4-RC6F4 group. It
may depend on the substituent electron parameters (SEP), σI
and σR°, of substituent R or 4-RC6F4 groups. We compared the
relative rates of consumption Crel (Table 3) of K[4-RC6F4BF3]
with σI (R) and σR° (R) (Table 5) and did not find any correla-
tion between the obtained values.
AgF
51
3
aYields were determined by 19F NMR; bcalculated on reacted 1a.
SEP of the 4-RC6F4 groups are not reported as yet except
inductive constants σI (C6F5) [43,45,46] and σI (2,3,5,6-C6F4H)
[43], and resonance constants σR° (C6F5) [45]. We bridge this
gap using a series of biphenyls 4a–r and 5a–r and determine
SEP of 4-RC6F4 groups using the Taft's method [42]. The
19F NMR spectra were measured in CHCl3 (non-polar weakly
coordinating solvent) and in toluene (solvent for the present
research). The calculated SEP values in both solvents are
closely related to one another to indicate no specific intermolec-
ular interaction biphenyl–solvent (Table 6) and obtained results
are in agreement with the data by Sheppard for CCl3F or
Scheme 7: General concept of Pd-catalyzed Suzuki–Miyaura reaction.
goes easy reductive elimination to biphenyl. trans-ArPdLnAr' benzene [45]. When R = H, Bu, or alkoxy group, inductive
can react only after transformation to the corresponding cis- constants σI (4-RC6F4) consist of 0.16–0.18. The 4-RC6F4
isomer [40,41]. Complexes with polyfluorinated aryl groups are groups with R = F, pyrazol-1-yl, pyrrol-1-yl, indol-1-yl,
highly stable and do not isomerize to the cis-isomer as well as imidazol-1-yl and benzimidazol-1-yl possess a higher electron-
do not form the cross-coupling products. Thus, independently withdrawing effect (σI = 0.21 − 0.27) which achieves maximum
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Beilstein J. Org. Chem. 2015, 11, 608–616.
Scheme 8: Assumed silver(I)-assisted transmetallation of weakly nucleophilic aryitrifluoroborates.
Table 5: The substituent electron parameters of some R in RC6H4F (in CDCl3) [42].
R
H
Bu [43]
OCH3
Prz [44]
Pyr [44]
F
Im [44]
σI
0
0
0.01
0.29
0.300
0.354
0.45
0.513
σR°
−0.18
−0.56
−0.061
−0.210
−0.39
−0.155
Unfortunately, the search for possible dependence between the
relative rates of consumption of K[4-RC6F4BF3] and SEP of the
respective 4-RC6F4 group does not show certain correlation.
Table 6: The substituent electron parameters (SEP) of 4-RC6F4
groups.
R
σI
σR°
In toluene In CHCl3 In toluene In CHCl3
Conclusion
1. The relative reactivities of K[4-RC6F4BF3] in the Ag(I)-
assisted Pd-catalyzed cross-coupling reactions with 3-IC6H4F,
4-IC6H4F, 4-IC6H4CH3 or 4-BrC6H4CH3 in the presence of
P(t-Bu)3 or PPh3 (toluene, 100 °C, 8 h) differ negligibly from
each other. The same results show the competitive cross-
coupling of mixtures of K[4-RC6F4BF3] and K[C6F5BF3] (1:1,
mol) with 4-BrC6H4CH3 at a low conversion of borates
(<40%). K[2,3,5,6-C5NF4BF3] is extremely weakly nucle-
ophilic and conversion in the cross-coupling product is no more
than 5% in 8 h.
4-PhCH2O
4-iPrO
4-PrO
0.160
0.161
0.163
0.163
0.164
0.165
0.165
0.166
0.168
0.172
0.010
0.009
0.008
0.011
0.008
0.008
0.013
0.010
0.009
0.008
0.014
4-t-BuO
4-BuO
4-EtO
0.165
0.165
0.167
0.158
0.015
0.011
0.011
0.015
4-Bu
4-AllylO
4-MeO
4-PhCH2CH2O 0.170
4-Ha
0.179
0.201
0.214
0.218
0.220
0.235
0.260
0.270
0.193
0.211
0.253
0.240
0.235
0.250
0.029
0.023
0,037
0.030
0.025
0.034
4-PhO
4-Prz
0.021
0.033
0.027
0.022
0.032
0.036
0.040
2. It is very likely that transmetallation of ArPdLnX with
K[4-RC6F4BF3] proceeds on the surface of the silver(I) com-
pounds (K[4-RC6F4BF3] and AgmY are insoluble in toluene).
Neutral complex ArPdLnX is adsorbed on the acidic center
(Ag+) of the solid surface to form highly reactive complex
[ArPdLn···X···AgmY] which facilitates exchange of group BF3
in K[4-RC6F4BF3] by ArPdLn and thus accelerates the forma-
tion of (4-RC6F4)PdLnAr.
4-Pyr
4-Fb
4-Ind
4-Im
4-Bim
4-CF3C6F4
4-C5NF4
2-C5NF4
0.293
0.300
0.346
0.248
0.044
0.056
0.064
0.070
aσI = 0.33 (water, 25 °C) [43]; bσI = 0.31 (water, 25 °C) [43,46], 0.25
(CCl3F) [45]; σR° = 0.02 (CCl3F) [45].
3. Verification of the assumed correlation between reactivity
(Crel) of K[4-RC6F4BF3] and the substituent electron parame-
ters (SEP) (σI and σR°) as the measure of the electron-with-
for the 2,3,5,6-tetrafluoropyridyl group (σI = 0.35). Resonance drawing properties of 4-RC6F4 gives an unclear result. Inter-
constants σR° of all tetrafluoroaryl groups are insignificant and vals of change of both Crel (0.6–1.0) and σI (0.16–0.27) are too
reflect the substantial non-coplanarity of aryl moieties [45].
narrow and small experimental errors of measurements may
614

Beilstein J. Org. Chem. 2015, 11, 608–616.
19.Frohn, H.-J.; Adonin, N. Yu.; Bardin, V. V.; Starichenko, V. F.
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corrupt or mask electronic effect of 4-RC6F4 with respect to
C6F5 group.
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Supporting Information
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Supporting Information File 1
Full experimental details and compound characterization
data.
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