Towards iron-catalysed suzuki biaryl cross-coupling: Unusual reactivity of 2-halobenzyl halides

Article abstract of DOI:10.1055/s-0034-1380135

The reaction of 2-halobenzyl halides with the borate anion Li[(Ph)(t-Bu)Bpin] leads not only to the expected arylation at the benzyl position, but also to some Suzuki biaryl cross-coupling. Preliminary mechanistic investigations hint towards the intermediacy of benzyl iron intermediates that can either: (a) directly cross-couple with the aryl boron reagent to give observed monoarylated species, or (b) undergo oxidative addition of the aryl halide to generate the diarylated species on reaction with the boron-based nucleophile.

Full text of DOI:10.1055/s-0034-1380135

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2015, 47, 1761–1765
paper
1761
R. B. Bedford et al.
Paper
Synthesis
Towards Iron-Catalysed Suzuki Biaryl Cross-Coupling: Unusual
Reactivity of 2-Halobenzyl Halides
Robin B. Bedford*
X1
Ph
Ph
Timothy Gallagher
[Fe] (5 mol%)
MgBr2 (20 mol%)
X2
Ph
X2
Dominic R. Pye
William Savage
+
+
THF, r.t., 3 h
O
tBu
Ph
B
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
r.bedford@bristol.ac.uk
Li
O
Received: 08.12.2014
Accepted: 07.01.2015
Published online: 12.02.2015
DOI: 10.1055/s-0034-1380135; Art ID: ss-2014-c0739-st
Grignard gives only a low yield of the cross-coupled product
when [Fe(acac)3] is used as the catalyst.6 Subsequently,
Nakamura showed that high yields could be obtained in the
coupling of aryl chlorides with aryl Grignard reagents if an
N-heterocyclic carbene/iron fluoride catalyst mixture is
employed.7 Recently, Chua and Duong showed that the di-
meric alkoxide [Fe2(Ot-Bu)6] can be used in place of iron
fluorides.8 Meanwhile Knochel and co-workers established
that arylcuprates can be used as nucleophiles in iron-cata-
lysed biaryl bond formation.9
Abstract The reaction of 2-halobenzyl halides with the borate anion
Li[(Ph)(t-Bu)Bpin] leads not only to the expected arylation at the benzyl
position, but also to some Suzuki biaryl cross-coupling. Preliminary
mechanistic investigations hint towards the intermediacy of benzyl iron
intermediates that can either: (a) directly cross-couple with the aryl bo-
ron reagent to give observed monoarylated species, or (b) undergo oxi-
dative addition of the aryl halide to generate the diarylated species on
reaction with the boron-based nucleophile.
For iron to be seen to be truly competitive with palladi-
um requires the development of an iron-catalysed variant
of the Suzuki biaryl bond-forming reaction. There is one re-
port in the literature that describes examples of such a pro-
cess, conducted at very high pressure (15 kbar),10 however,
the extreme conditions employed render this interesting,
but ultimately impractical. To date, the majority of more
practical iron-catalysed Suzuki reactions occur between al-
kyl, benzyl and allyl halides and arylboron nucleophiles de-
rived from either tetraorganoborates (Scheme 2, a)11 or the
alkyl aryl pinacol boronate esters 1 (Scheme 2, b)12 formed
by reaction of aryl pinacol boronate esters with tert-butyl-
lithium or ethyl Grignard.
Key words iron, cross-coupling, aryl, Suzuki reaction, organoborate
While the earliest reported iron-catalysed cross-cou-
pling reaction dates back 70 years,1 for much of the inter-
vening period there was a lull in activity in the field, with
only sporadic reports appearing before the early 2000s, no-
tably from the groups of Kochi,2 Molander3 and Cahiez.4
Since then there has been a renaissance in the field, with
many excellent reports appearing over the last decade or
so.5 In many cases the activity, selectivity and coupling-
partner tolerance obtained in iron-catalysed cross-cou-
plings make such processes comparable to or better than
palladium-catalysed counterparts. However, one area
where this is not the case is biaryl bond formation (Scheme
1) where iron lags behind palladium in terms of both appli-
cability and generality.
(a) tetraorganoborate nucleophiles:
[Fe]
R1 R2
R1
X
+
R2 BR3
Zn salt
(b) aryl alkyl pinacol boronate ester nucelophiles:
[Fe]
O
O
Ar1 Ar2
Ar1
X
Ar2
M
Ar
[Fe]
+
R1
X
+
R1 Ar
B
Mg salt
Scheme 1 Iron-catalysed biaryl cross-coupling
Alk
Scheme 2 Iron-catalysed Suzuki cross-coupling reactions with organo-
Fürstner found that while 2-halo-N-heterocyclic sub-
strates couple with aryl Grignard reagents, the equivalent
reaction between an activated aryl chloride and phenyl
borate reagents
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1761–1765

1762
R. B. Bedford et al.
Paper
Synthesis
While the former procedure can be used to couple 2-
halo-N-heterocycles,11a to the best of our knowledge there
are no reports that describe the Suzuki cross-coupling of
aryl halides with arylboron-based nucleophiles under mild
conditions. Indeed, when 2-halobenzyl halides are used as
substrates in the reactions outlined in Scheme 2 (a), cou-
pling occurs exclusively at the benzylic position.11a There-
fore, we were surprised to find that the coupling of 2-bro-
mobenzyl bromide (2a) with the pinacol borate anion 1a
led not only to coupling of the benzyl group to give the an-
ticipated product 3a, but also the aryl bromide function,
furnishing the diarylated product 4, albeit in low yield
(Scheme 3, a). By contrast, no coupling was observed when
4-tolyl bromide was subjected to the same reaction condi-
tions.13
(a) olefin-assisted C–Cl
activation via haptotropic
migration
(b) benzyl-assisted C–Br
activation?
(Jacobi von Wangelin):
[Fe]
[Fe]
Br
Cl
(c) benzyl bromide
assisted C–Br activation?
(d) 2-aryl-assisted benzyl
halide?
Br
X
[Fe]
[Fe]
Br
Figure 1 Possible modes of coordination-directed C–X functionalisa-
tion
(a)
pears that the rates of production of the two species are in-
dependent of one another. This was further reflected in the
fairly consistent ratio of mono- versus diarylated product
obtained on changing the ratio of 2a:1a in the reaction
(Scheme 4).
[Fe(acac)3] (5 mol%)
MgBr2 (20 mol%)
Br
+
3 Li[PhtBuBpin]
THF, r.t., 3 h
Br
2a
1a
Ph
Ph
+
Ph
+
Br
+
Br
Ph
Ph
not observed
3a (41%)
5 (4%)
6
4 (21%)
(b)
[Fe(acac)3] (5 mol%)
MgBr2 (20 mol%)
Br
+
1.5 1a
no reaction
THF, r.t., 3 h
Scheme 3 (a) Iron-catalysed Suzuki arylation of 2a with 1a. Conver-
sions into 3a, 4, 5 and 6 determined by GC (dodecane internal stan-
dard). t-BuBpin produced as co-product. (b) Attempted coupling of 4-
bromotoluene.
Figure 2 Conversion into 3a () and 4 () during first ten minutes of
the reaction of 1a with 2a
[Fe(acac)3]
Our first thought to try and explain the unusual reac-
tion was that if 3a forms first, the newly introduced arene
substituent may then coordinate to the iron centre by an
ηn-arene interaction, which may in turn direct C–Br bond
activation. This suggestion is reminiscent of the one provid-
ed by Jacobi von Wangelin for the iron-catalysed coupling of
aryl14 or alkyl15 Grignard reagents with chlorostyrenes (Fig-
ure 1, a), where initial coordination of the iron to an alkene
is proposed to be followed by a haptotrophic shift and at-
tack at the C–X bond. Figure 1 (b) shows the equivalent pro-
cess that may occur here.
If this were the case then it would be anticipated that
the concentration of 3a would build up early in the reaction
and there would be a lag in the production of 4. However,
the profile of product distribution against time (Figure 2)
does not seem to support this hypothesis. Instead, it ap-
x 1a + 2a
4 + 3a
x = 0.5: 10% 24% (ratio = 0.42)
x = 1.0: 16% 45% (ratio = 0.36)
x = 1.5: 23% 70% (ratio = 0.33)
Scheme 4 Effect of varying 1a:2a on the ratio of mono- 3a and di-
arylated 4 products. Conditions as per Scheme 3, conversions based on
amount of benzyl halide converted into products, determined by GC
analysis (dodecane standard).
Furthermore, subjecting isolated 3a and its aryl chloride
counterpart, 3b, to the same reaction conditions in the
presence of four different iron catalysts (Table 1) gave little
or none of the diarylated product 4. In some cases, using 3a
as substrate, modest amounts the hydrodehalogenated spe-
cies 5 were obtained. Taken together, the data strongly sug-
gests that 3a is not an intermediate in the formation of 4.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1761–1765

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R. B. Bedford et al.
Paper
Synthesis
Table 1 Reaction of 1a with 2-XC6H4CH2Ph [X = Br (3a) or Cl (3b)] with
pears not to be the case: none of the substrates shown in
Figure 3, which contain a range of potentially directing do-
nor groups, are arylated by 1a under the standard reaction
conditions.
Various Iron Catalystsa
Entry
Substrate [Fe]
Yield (%) of 4 Yield (%) of 5
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3b
3b
3b
3b
[Fe(acac)3]
1
1
0
3
0
0
0
1
16
8
O
NMe2
OMe
[FeCl2(dppe)]
[FeCl2(dpbz)2]
[FeCl2(dppp)]
[Fe(acac)3]
O
N
1
Br
Br
Br
Br
44
2
O
Ph
O
NMe2
O
[FeCl2(dppe)]
[FeCl2(dpbz)2]
[FeCl2(dppp)]
0
O
1
Br
Br
6
Figure 3 Substrates that show no biaryl bond formation with 1a (con-
a Conditions as per Scheme 3; dppe: 1,2-bis(diphenylphosphino)ethane;
dpbz: 1,2-bis(diphenylphosphino)benzene; dppp: 1,3-bis(diphenylphosphi-
no)propane.
ditions as per Scheme 3, analysed by GC-MS)
Secondly, if aryl coupling does indeed precede benzylic
coupling, one might expect to observe the formation of 2-
phenylbenzyl halides, such as 6. These, however, were nev-
er seen, even when the amount of borate used in the reac-
tion was restricted to sub-stoichiometric amounts (0.5
equivalent with respect to 2a, Scheme 4). It is possible that
the rate of coupling at the benzylic position is accelerated
by the introduction of the 2-aryl function, perhaps by ηn-
coordination of the iron to the 2-aryl group (Figure 1, d).
An alternative tentative mechanism that accounts for
many of the experimental observations is summarised in
Scheme 5. In this scenario, a low-valent iron centre16 reacts
with the benzyl halide residue to yield the iron benzyl in-
termediate 7.17 This intermediate could then undergo reac-
tion with 1a to generate the simple cross-coupled product
3. Alternatively the intermediate 7 could undergo either
mono- or binuclear oxidative addition of the 2–X′-aryl
function to generate the metallacyclobutene intermediate 8
or a dinuclear intermediate 9. Subsequent reactions with 1a
would liberate the diarylated product 4.
If the arene introduced into the benzylic position is not
responsible for directing the aromatic C–Br bond activation,
then could coordination of the benzyl bromide perform a
similar role (Figure 1, c)? If this were the case, then replac-
ing the benzyl bromide with other benzyl halides may also
facilitate biaryl bond formation. It is clear from the results
in Table 2 that replacing the benzyl bromide (entries 1 and
3) with a benzyl chloride function (entries 2 and 4) leads to
greater conversion to the diarylated product 4a, however,
no activity is seen with a benzyl fluoride based substrate
(entry 6). With regards coupling of the aryl halide, it ap-
pears the order is Br > Cl > I.
Table 2 Coupling of 2 with 1a Catalysed by [Fe(acac)3]a
[Fe(acac)3]
X2
Ph
Ph
+ 1a
+
X1
X1
Ph
2
4
3
low-valent
X1
1a
Entry
X1
X2
Br (2a)
Yield (%) of 4
Yield (%) of 3
[Fe]
Fe
Ph
– X1–
(3a, X1 = Br) 41
(3a) 12
X2
X2
1
2
3
4
5
6
Br
Br
Cl
Cl
I
21
41
17
27
9
X2
7
3
Cl (2b)
Br (2c)
Cl (2d)
Cl (2e)
F (2f)
(3b, X1 = Cl) 36
(3b) 34
(3c, X1 = I) 0
(3a) 0
8
[Fe]
X2
Br
0
1a
Ph
or
a Conditions as per Scheme 3.
4
Ph
While these data are consistent with coordination of the
benzyl halide directing the oxidative addition of the aryl
halide, there are a couple of inconsistencies that suggest
such a pathway may not be operative.
Firstly, it might be imagined that substituting the benzyl
bromide for other potentially coordination-directing
groups would facilitate biaryl coupling. However, this ap-
9
X2 [Fe] [Fe]
Scheme 5 Tentative mechanism for the iron-catalysed mono- and di-
arylation of 2-halobenzyl halides
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1761–1765

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R. B. Bedford et al.
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Synthesis
In summary, we have shown that the reaction of 2-halo-
benzyl halides with the boronate anion 1a leads not only to
the anticipated arylation of the benzyl position, but also to
a unique iron-catalysed Suzuki biaryl bond formation un-
der mild conditions. This does not seem to occur by the
benzylic aryl group directing the second C–X bond cleav-
age. Instead our current hypothesis is that either the benzyl
halide acts as a directing group or, perhaps more likely, a
benzyl ligand is formed on iron, then the aryl C–X bond un-
dergoes oxidative addition in either a mono- or dimetallic
process. Probing this mechanism more deeply should allow
us to understand the reaction and thus maximise its scope,
and these studies are ongoing.
13C NMR (100 MHz, CDCl3): δ = 51.2, 99.7, 129.0, 130.2, 130.3, 139.9,
140.0.
NMR data consistent with previously reported data.18
2-Bromobenzyl Fluoride (2f)
Prepared according to a synthesis of benzyl fluoride.19
A solution of 2-bromobenzyl alcohol (1.87 g, 10 mmol) in anhydrous
CH2Cl2 (10 ml) was added to a Deoxo-Fluor® solution (50% in THF,
4.73 mL, 11 mmol) in CH2Cl2 (10 mL) at –78 °C and the resultant mix-
ture stirred for 3 h. The mixture was quenched by a slow addition of
sat. aq NaHCO3 (25 mL) over 10 min and extracted with CH2Cl2 (3 × 15
mL), dried (Na2SO4) and concentrated in vacuo to afford a yellow oil,
which was purified by flash chromatography (hexane–EtOAc); yield:
1.06 g (56%).
1H NMR (400 MHz, CDCl3): δ = 5.49 (d, J = 47.2 Hz, 2 H, ArCH2), 7.23 (t,
J = 7.9 Hz, 1 H, ArH), 7.37 (t, J = 7.6 Hz, 1 H, ArH), 7.49 (d, J = 7.6 Hz, 1
H, ArH), 7.58 (d, J = 7.9 Hz, 1 H, ArH).
All reactions involving air- or moisture-sensitive reagents or products
were carried out under an atmosphere of N2 using standard Schlenk
line techniques. Solvents were dried and purified using Anhydrous
Engineering double alumina columns and alumina-copper catalyst
drying columns. GC analysis was conducted using an Agilent Technol-
ogies 7820A, using calibration curves obtained from a minimum of
five different concentrations of genuine samples of the products, with
dodecane as an internal standard. GC-MS analysis was performed on
a Varian Saturn 2100T. Substrates 2a, c and d were purchased and
used as received, as was intermediate 3b.
Preparation of Reference Samples of Products
2-Benzyl-1-bromobenzene (3a)20
Benzene (20 mL) was added slowly to 2-bromobenzyl alcohol (8.0 g,
42 mmol) and FeCl3 (3.2 g, 20 mmol), then the mixture was heated at
reflux for 12 h. Once cooled to r.t., the solvent was removed under re-
duced pressure and the residual solid purified by column chromatog-
raphy (eluent: pentane) affording 3a as a white solid; yield: 4.67 g
(55%).
1H NMR (400 MHz, CDCl3): δ = 7.59 (dd, J = 8.0, 1.2 Hz, 2 H, ArH), 7.32
(t, J = 7.6 Hz, 2 H, ArH), 7.26–7.21 (m, 4 H, ArH), 7.16 (dd, J = 7.6, 1.7
Hz, 1 H, ArH), 7.10 (td, J = 8.0, 1.7 Hz, 1 H, ArH), 4.14 (s, 2 H, ArCH2).
Preparation of Substrates
THF Solution of Li[(Ph)(t-Bu)Bpin] (1a)
Anhydrous THF (6 mL) was added to 4,4,5,5-tetramethyl-2-phenyl-
1,3,2-dioxaborolane (0.408 g, 2.0 mmol) and the solution stirred and
cooled to –40 °C. t-BuLi (1.10 mL, 1.7 M in pentane, 1.87 mmol) was
added dropwise at –40 °C and then the mixture stirred at this tem-
perature for 30 min, then at 0 °C for a further 30 min before allowing
to warm to r.t. The solution of 1a was used without purification in the
catalytic reactions.
13C NMR (100 MHz, CDCl3): δ = 41.6, 124.9, 126.2, 127.4, 127.8, 128.5,
129.0, 131.0, 132.8, 139.9, 140.4.
2-Benzyl-1,1′-biphenyl (4)
NiCl2 (13.1 mg, 0.101 mmol), Cy3P (57.7 mg, 0.202 mmol) and 3a (0.5
g, 2.02 mmol) were stirred for 10 min in anhydrous THF (4 mL). A
solution of phenylmagnesium chloride (1.52 mL, 3.04 mmol, 2.0 M) in
THF was added dropwise at 0 °C with vigorous stirring and the resul-
tant mixture stirred for a further 2 h. The reaction was quenched with
sat. aq NH4Cl (20 mL) at 0 °C, extracted with Et2O (3 × 10 mL), dried
(MgSO4) and the solvent removed under reduced pressure to afford 4;
yield: 74.3 mg (15%); white solid.
1H NMR (400 MHz, CDCl3): δ = 7.21–7.42 (m, 12 H, ArH), 7.00 (d, J =
7.4 Hz, 2 H, ArH), 3.98 (s, 2 H, ArCH2).
13C NMR (100 MHz, CDCl3): δ = 39.18, 125.91, 126.30, 127.61, 128.17,
128.37, 129.00, 129.44, 130.26, 130.44, 138.34, 141.59, 141.79,
142.40.
2-Bromobenzyl Chloride (2b)
A solution of 2-bromobenzyl alcohol (0.94 g, 5 mmol) in THF (3 mL)
was cooled to 0 °C. SOCl2 (1.81 mL, 25 mmol) was added dropwise
with vigorous stirring. The mixture was stirred for a further 3 h, then
warmed to r.t. and concentrated in vacuo affording 2b; yield: 0.97 g
(95%); white solid.
1H NMR (400 MHz, CDCl3): δ = 4.70 (s, 2 H, ArCH2), 7.18 (t, J = 7.6 Hz, 1
H, ArH), 7.32 (t, J = 7.6 Hz, 1 H, ArH), 7.47 (d, J = 7.6 Hz, 1 H, ArH), 7.58
(d, J = 7.6 Hz, 1 H, ArH).
13C NMR (100 MHz, CDCl3): δ = 46.3, 124.3, 128.0, 130.2, 131.0, 133.3,
136.8.
NMR data consistent with previously reported data.21
NMR data consistent with previously reported data.18
Catalytic Reactions; General Procedure
A mixture of the appropriate iron catalyst (0.025 mmol), Et2O·MgBr2
(1.0 mL, 0.1 M) in THF and the appropriate 2-halobenzyl halide (0.5
mmol) was stirred for 5 min. The borate anion solution 1a in THF (5
mL, 1.5 mmol, 0.33 M) was added dropwise over 5 min with vigorous
stirring. The reaction mixture was allowed to stir for 3 h before the
addition of dodecane as an internal standard (110 μL, 0.50 mmol). An
aliquot (~0.5 mL) was taken and passed through a plug of silica gel be-
fore being diluted with THF (0.5 mL) and analysed by GC.
2-Iodobenzyl Chloride (2e)
Prepared in the same manner as described for 2b, using 2-iodobenzyl
alcohol (1.17 g, 5 mmol), which afforded 2e; yield: 1.12 g (89%);
white solid.
1H NMR (400 MHz, CDCl3): δ = 7.84 (d, J = 7.9 Hz, 1 H), 7.46 (d, J = 7.7
Hz, 1 H), 7.34 (t, J = 7.6 Hz, 1 H), 6.99 (t, J = 7.7 Hz, 1 H), 4.66 (s, 2 H).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1761–1765

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R. B. Bedford et al.
Paper
Synthesis
(12) (a) Hatakeyama, T.; Hashimoto, T.; Kondo, Y.; Fujiwara, Y.; Seike,
H.; Takaya, H.; Tamada, Y.; Ono, T.; Nakamura, M. J. Am. Chem.
Soc. 2010, 132, 10674. (b) Hashimoto, T.; Hatakeyama, T.;
Nakamura, M. J. Org. Chem. 2012, 77, 1168. (c) Bedford, R. B.;
Brenner, P. B.; Carter, E.; Carvell, T. W.; Cogswell, P. M.;
Gallagher, T.; Harvey, J. N.; Murphy, D. M.; Neeve, E. C.; Nunn, J.;
Pye, D. R. Chem. Eur. J. 2014, 20, 7935.
Acknowledgment
We thank the EPSRC for funding from the Bristol Chemical Synthesis
Centre for Doctoral Training (D.R.P.).
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