Iron phosphine catalyzed cross-coupling of tetraorganoborates and related group 13 nucleophiles with alkyl halides

Article abstract of DOI:10.1021/om500518r

Iron phosphine complexes prove to be good precatalysts for the cross-coupling of alkyl, benzyl, and allyl halides with not only aryl triorganoborate salts but also related aluminum-, gallium-, indium-, and thallium-based nucleophiles. Mechanistic studies revealed that while Fe(I) can be accessed on catalytically relevant time scales, lower average oxidation states are not formed fast enough to be relevant to catalysis. EPR spectroscopic studies reveal the presence of bis(diphosphine)iron(I) complexes in representative catalytic reactions and related processes with a range of group 13 nucleophiles. Isolated examples were studied by M?ssbauer spectroscopy and single-crystal X-ray structural analysis, while the electronic structure was probed by dispersion-corrected B3LYP DFT calculations. An EPR study on an iron system with a bulky diphosphine ligand revealed the presence of an S = ¹/₂ species consistent with the formation of a mono(diphosphine)iron(I) species with inequivalent phosphine donor environments. DFT analysis of model complexes allowed us to rule out a T-shaped Fe(I) structure, as this is predicted to be high spin.

Full text of DOI:10.1021/om500518r

Article
pubs.acs.org/Organometallics
Iron Phosphine Catalyzed Cross-Coupling of Tetraorganoborates and
Related Group 13 Nucleophiles with Alkyl Halides
Robin B. Bedford,*,† Peter B. Brenner,† Emma Carter,‡ Jamie Clifton,† Paul M. Cogswell,†
Nicholas J. Gower,† Mairi F. Haddow,† Jeremy N. Harvey,† Jeffrey A. Kehl,§ Damien M. Murphy,‡
Emily C. Neeve,† Michael L. Neidig,§ Joshua Nunn,† Benjamin E. R. Snyder,§ and Joseph Taylor†
†School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
‡School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.
§Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: Iron phosphine complexes prove to be good precatalysts for the cross-coupling of alkyl, benzyl, and allyl halides
with not only aryl triorganoborate salts but also related aluminum-, gallium-, indium-, and thallium-based nucleophiles.
Mechanistic studies revealed that while Fe(I) can be accessed on catalytically relevant time scales, lower average oxidation states
are not formed fast enough to be relevant to catalysis. EPR spectroscopic studies reveal the presence of bis(diphosphine)iron(I)
complexes in representative catalytic reactions and related processes with a range of group 13 nucleophiles. Isolated examples
were studied by Mossbauer spectroscopy and single-crystal X-ray structural analysis, while the electronic structure was probed by
̈
dispersion-corrected B3LYP DFT calculations. An EPR study on an iron system with a bulky diphosphine ligand revealed the
presence of an S = 1/2 species consistent with the formation of a mono(diphosphine)iron(I) species with inequivalent phosphine
donor environments. DFT analysis of model complexes allowed us to rule out a T-shaped Fe(I) structure, as this is predicted to
be high spin.
The majority of the reports published to date focus on the
use of Grignard reagents in iron-catalyzed cross-coupling, but
following on from our reported use of iron phosphine catalysts
in Grignard cross-coupling,10 such complexes have been found
to be applicable not only with Grignards11 but also with a range
of softer nucleophiles, including diorganozincs,12 tetraorgano-
borates,12c.13 diorgano(pinacolato)borates,14 tetraorganoalumi-
nates,12c.15 and tetraorganoindates.12c
We now report the optimization and use of iron-phosphine
catalysts for the cross-coupling of alkyl halides and related
substrates with a range of tetraorganoborate substrates,
including for the first time those based on 9-BBN, as well as
related organo-group 13 nucleophiles.
INTRODUCTION
■
Palladium-catalyzed cross-coupling reactions are ubiquitous in
organic synthesis, but there is a growing need to replace
expensive, toxic, environmentally deleterious metals with more
benign, sustainable metals in a range of catalytic processes.
With its high natural abundance and low cost and toxicity, iron
makes an ideal potential substitute for palladium.
While iron-catalyzed cross-coupling (Scheme 1) can be
traced back 70 years, with intermittent reports in the
Scheme 1. Iron-Catalyzed Cross-Coupling
RESULTS AND DISCUSSION
Iron Phosphine Complexes for the Coupling of BR4
Nucleophiles. We previously found that the preformed
■
−
intervening decades,1,2 it is only comparatively recently that
the field has attracted serious and sustained attention,3
triggered in particular by groundbreaking reports in 2002−
Special Issue: Catalytic and Organometallic Chemistry of Earth-
Abundant Metals
2004 from the groups of Furstner,4 Nakamura,5 and Hayashi.6
̈
In the past decade there has been a significant number of
papers published on the cross-coupling of alkyl, aryl, and vinyl
halides and related substrates.7−9
Received: May 15, 2014
© XXXX American Chemical Society
A
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iron(II) and iron(I) complexes 1−3 can be used as effective
precatalysts in the coupling of benzyl or 2-heteroaryl halides
with tetraarylborate salts.12c,13a We were interested to see
whether appropriate modifications in the structure of the
phosphine ligands would lead to enhanced catalyst perform-
ance. Accordingly, we prepared the ligands 4−7.
Table 1. Ligand Influence in a Representative Suzuki
Reaction
a
b
entry
ligand
ligand:Fe
yield of 9a, %
c
1
dpbz
dpbz
4
1:2
99
96
93
92
45
46
54
50
55
90
91
25
35
48
40
2
1:1
1:2
1:1
1:2
1:2
1:2
1:1
3
4
4
5
5a
6
5b
7
dppe
dppe
dppe
6
8
d
9
1:1
10
11
12
13
14
15
16
17
1:2
1:1
6
e
dmpe
depe
dppp
dppp
7a
1:2
1:2
1:2
1:1
1:2
1:2
fg
88 ,
fh
83 ,
7b
18
none
31
a
b
1
All reactions run at 85 °C for 4 h. Spectroscopic yield based on H
c
NMR (1,3,5-(MeO)3C6H3 internal standard). Complex 1 used as
precatalyst. [FeCl2(dppe] used as precatalyst. [FeCl2(dmpe)2] used
as precatalyst. Zn(4-tolyl)2 used as additive. Combined yield of 9a
(82%) and 4-tolyl analogue 9b (6%). Combined yield of 9a (75%)
d
e
The crystal structures of four of the ligands are shown in
Figure 1, while the performance of iron catalysts based on these
f
g
h
and 4-tolyl analogue 9b (8%).
employed here; indeed, the alkyl-substituted ligands dmpe and
depe showed activity comparable to or worse than that of iron
chloride in the absence of phosphine. In contrast, the sterically
encumbered analogues of dppe and dppp, ligands 6 and 7,
performed reasonably well. The good results obtained with
dpbz, coupled with its commercial availability, led us to focus
on this ligand for the remainder of the studies.
The outcome of the reactions summarized in Scheme 2
shows that essentially only one of the aryl groups is transferred
Scheme 2. Extent of Aryl Transfer
from the tetraphenylborate ion during the reaction with 8a
catalyzed by complex 1. This of course wastes three aryl
residues. A second issue is the scarcity of commercially available
tetraarylborate salts. While we previously found that
aryltrialkylborate salts can be exploited in place of tetraarylbor-
ates,13a they tended to give poorer yields. Therefore, we
decided to reinvestigate the use of mixed aryltrialkylborate and
diaryldialkylborate salts. These are easily accessed from
commercially available precursors and the appropriate
aryllithium or Grignard reagents.
While in most cases the borate salts were produced as
required and used without characterization, their formation was
confirmed by 11B NMR spectroscopy. Thus, the spectra of the
product mixtures formed in the representative reactions of
Figure 1. Single-crystal X-ray structures of ligands 4, 6, and 7a,b.
Hydrogen atoms are omitted for clarity, and thermal ellipsoids are set
at the 50% probability level.
ligands and the commercially available ligands dpbz, dppe,
dmpe, depe, and dppp16 in the representative cross-coupling
reaction of the benzyl bromide 8a with sodium tetraphenylbo-
rate are summarized in Table 1.
The mixed alkyl/aryl bis(phosphine) ligand 4 showed a
slightly poorer performance in comparison with dpbz, while the
mixed amino-phosphine analogues 5a,b performed significantly
worse. The commercially available ligands dppe, dppp, dmpe,
and depe also gave disappointing results under the conditions
B
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B(sBu)3, 9-BBN-iPr, or 9-BBN-Ph with 4-tolylmagnesium
chloride (Scheme 3) showed sharp singlets at δ −12.5,
−16.0, and −15.1 ppm corresponding to the borates 10a−c,
respectively.17
with MgCl[9-BBN(iPr)4-tolyl], provided it was used in slight
excess (entry 15).
Table 3 summarizes the coupling of a range of alkyl halides
with MgX[9-BBN(iPr)Ar] or MgX[sBu3BPh]. As can be seen,
the reactions progressed reasonably well with benzyl and
secondary alkyl halide substrates, but lower activity was
observed with a representative primary alkyl halide (entry 20).
Disappointingly low activity was seen with allyl bromide
substrates (entries 24 and 25), irrespective of which boron-
containing nucleophile was used. In contrast, we have recently
shown that iron catalysis can give good results in the coupling
of allyl halides when arylboron pinacol esters, activated with
alkyllithium reagents, are used as the nucleophile.14c Substrates
with sensitive functional groups (ester, cyano) were tolerated,
as were aryl bromide residues on benzyl halide substrates,
confirming the preference for benzyl halides over aryl halides
observed previously.12a,c,13a In contrast, no product was
obtained in the coupling of a nitro-substituted benzyl bromide
(entry 16). Typically the 9-BBN-derived reagents show
marginally enhanced performance, but the requirement for an
extra step in the synthesis of these borates from commercially
available MeO-9-BBN detracts slightly from their appeal in
comparison with the reagents prepared in one step from tri-sec-
butylborane.
Scheme 3. Formation of Di- and Trialkylborates
Table 2 summarizes the results of the coupling of the
arylborates with the representative benzyl bromide 8a catalyzed
a
Table 2. Survey of Aryl Triorganoborates as Nucleophiles
Cross-Coupling of Heavier Group 13 Nucleophiles.
Nakamura and co-workers demonstrated that complex 1 is an
effective precatalyst for the coupling of tetraarylaluminates with
representative primary and secondary alkyl halides,15 while we
found that the dppe-containing complexes 2 and 3a can also be
exploited.12c These latter complexes also show activity in the
representative coupling of a tetraarylindate with a benzyl
bromide.12c We therefore decided to compare relative activities
of group 13 aryl nucleophiles in the representative cross-
coupling reactions with 8a, catalyzed by complex 1, and these
results are summarized in Table 4. In all cases, except with
thallium, we used the appropriate tetraarylmetalate, prepared
immediately prior to use. To the best of our knowledge, the
equivalent tetraarylthallates are not known beyond systems
with perhalogenated aryls and these show a far lower
propensity to undergo transmetalation.18 Accordingly, we
used triphenylthallium instead and for comparison we also
examined the use of the neutral nucleophiles M(4-tolyl)3 (M =
Al, In).
All of the group 13 nucleophiles tested showed activity. With
aluminum and indium there was no need to add cocatalytic zinc
for good performance when 1−1.2 equiv of the nucleophile was
used. With boron-, gallium-, and thallium-based nucleophiles
poorer activity was seen in the absence of zinc. On comparison
of entries 2 and 3, it is apparent that more than one aryl group
can be transferred from the aluminum center. This hypothesis
is further supported by the observation that a neutral
triarylaluminum is also a competent nucleophilic coupling
partner (entry 4). Similar results were observed for indium,
although in this case a zinc additive proved beneficial when less
than 1 equiv of the tetraarylindate was used (entry 10).
Table 5 summarizes the results of the coupling of a
representative range of electrophiles with MgCl[M(4-tolyl)4]
(M = Al, In). As can be seen, in most cases good to excellent
conversions to the desired cross-coupled products were
observed. In the majority of cases, the indate performed as
well or better than the aluminate. A range of benzyl bromides
and chlorides were successfully coupled, including those with
sensitive functional groups such as cyano and ester residues as
b
yield, %
entry
M[R3BAr]
Li[Et3BPh]
9a or 9b
11
1
5
54
15
32
52
73
18
78
56
51
58
25
93
71
85
30
77
34
15
51
23
16
13
65
21
24
49
32
46
7
2
Li[Bu3BPh]
3
Li[s-Bu3BPh]
4
MgCl[Et3B(4-tolyl)]
MgCl[Bu3B(4-tolyl)]
MgCl[sBu3B(4-tolyl)]
as above no Zn salt
5
6
7
c
8
as above with ZnCl2
MgCl[sBu3BPh]
9
d
d
10
11
12
13
14
15
16
17
MgBr[9-BBN(Me)Ph]
d
Li[9-BBN(Bu)Ph]
MgBr[9-BBN(sBu)Ph]
d
MgBr[9-BBN(Ph)2]
e
MgCl[9-BBN(iPr)4-tolyl]
MgCl[9-BBN(iPr)4-tolyl]
as above no Zn salt
20
15
50
18
ef
,
c
as above with ZnCl2
a
b
1
All reactions run at 85 °C for 4 h. Spectroscopic yield based on H
NMR (1,3,5-(MeO)3C6H3 internal standard). 20 mol %. Formed in
c
d
e
situ from Ph-9-BBN and appropriate RMgX or RLi (1:1). Formed in
f
situ from iPr-9-BBN and appropriate 4-tolylMgCl. 1.2 equiv of borate
used.
by complex 1. The survey of borates formed from commercially
available trialkylboranes (entries 1−9) revealed that an increase
in the steric bulk of the alkyl group resulted in more selective
cross-coupling. As observed before with tetraarylborate
salts,12c,13a the reaction requires substoichiometric quantities
of either an organozinc reagent or zinc chloride (compare
entries 6−8 and 15−17); in the absence of zinc additives,
homocoupling of the electrophile predominated. With 9-BBN-
based nucleophiles, the best activity was seen with the
diarylborate salts; however, an acceptable yield was obtained
C
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a
Table 3. Coupling of Aryltrialkylborates
a
b
1
All reactions run at 85 °C for 4 h. Spectroscopic yield based on H NMR (1,3,5-(MeO)3C6H3 internal standard), isolated yield in parentheses.
c
[FeCl2(dppp)] + 1 equiv of dppp used in place of 1.
D
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Table 4. Coupling of Group 13 MR3 and [MR4]−
Nucleophiles
Table 5. Coupling of a Representative Range of
Electrophiles with MgCl[M(4-tolyl)4] (M = Al, In)
a
a
b
entry
nucleophile
Na[BPh4]
Nu:8a
additive
ZnPh2
yield, %
1
1.2
1.2
0.6
1.2
1.2
0.5
0.5
1.2
1.0
0.5
1.0
1.2
99
99
86
69
58
39
54
94
91
86
69
50
90
2
MgCl[Al(4-tolyl)4]
MgCl[Al(4-tolyl)4]
Al(4-tolyl)3
3
4
5
MgCl[Ga(4-tolyl)4]
MgCl[Ga(4-tolyl)4]
MgCl[Ga(4-tolyl)4]
MgCl[GaPh4]
MgCl[In(4-tolyl)4]
MgCl[In(4-tolyl)4]
In(4-tolyl)3
6
7
Zn(4-tolyl)2
ZnPh2
8
9
10
11
12
13
Zn(4-tolyl)2
TlPh3
TlPh3
1.2
ZnPh2
a
b
1
All reactions run at 85 °C for 4 h. Spectroscopic yield based on H
NMR (1,3,5-(MeO)3C6H3 internal standard).
well as aromatic bromides and chlorides. Primary and
secondary alkyl bromides could also be coupled with the
indate to give reasonable yields of the desired products. The
indate and aluminate nucleophiles gave better yields in the
couplings with allyl halides (entries 18−20) in comparison with
the equivalent borates (Table 3, entries 24 and 25).19 2-
Bromopyridine could also be coupled; however, the yield was
low (entry 21). This result is in contrast with that for substrate
8z, where no substitution of the pyridyl chloride residue was
observed (entry 13), and presumably reflects the greater
reactivity of the pyridyl bromide.20
Mechanistic Considerations. There is some debate in the
literature as to what the lowest oxidation state is for the iron
center in the catalytic cycle when using arylmetal nucleophiles
as coupling partners, with suggestions ranging from Fe(−II)
through to Fe(II). An Fe(−II)/Fe(0) manifold has been
proposed,7y on the basis of comparative stoichiometric studies
of isolated organoiron species in a variety of oxidation states
and their performance as (pre)catalysts. However, as we have
previously suggested,21 the differences in activity can be better
explained by the presence or absence of Cp/Cp* ligands in the
complexes, rather than their oxidation state.22 No compelling
evidence currently exists that oxidation states as low as Fe(−II)
are relevant in the catalytic cycle with aryl nucleophiles,
although it is apparent that Fe(−II) complexes with labile
ligands are excellent precatalysts. Similarly it has been shown
that an Fe(−I) species with labile anthracene ligands can serve
as a competent precatalyst.7am
The formation of catalytically active zerovalent iron nano-
particles in certain Grignard cross-coupling reactions7n
demonstrates that oxidation states lower than Fe(II) can be
accessed under catalytically relevant conditions. However, these
nanoparticles may be a resting state for higher-valent active
intermediates, since they react with excess alkyl halides in the
absence of nucleophilic coupling partners to give homogeneous
solutions.14c There has also been a claim that a homoleptic
Fe(0) “ate” complex can be isolated under appropriate
conditions;7y however, this was based on repeating a crystal
structure analysis originally undertaken by Shilov,23 which had
previously been shown to have been solved in the incorrect
a
b
1
All reactions run at 85 °C for 4 h. Spectroscopic yield based on H
NMR (1,3,5-(MeO)3C6H3 internal standard), isolated yield in
parentheses.
space group and the compound was more likely an Fe(II)
hydride.24 Interestingly, EPR spectra of freshly formed
suspensions of iron nanoparticles produced from ArMgX
1
show the presence of a soluble S = /2 species consistent
with the formation of a low-spin Fe(I), suggesting that
homogeneous species below the Fe(II) oxidation state can
indeed be accessed.14c Such Fe(I) species have been proposed
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as possible catalytic intermediates in the coupling of aryl
Grignards and related nucleophiles by Kochi,2b,25 Norrby,26
us,27,12c,14c and others.28,7y
With regard to the Fe(II) oxidation state, iron(II) mesityl
complexes have been isolated with both chelating diamine29,30
and diphosphine ligands,7bj and it has been proposed that such
complexes represent the lowest oxidation state species in the
catalytic cycle.29 It is important to recognize, though, that the
use of such sterically encumbered nucleophiles may represent a
special rather than a general case in iron-catalyzed cross-
coupling, since bulky aryl ligands on the metal may severely
inhibit reduction below Fe(II).31
It is crucially important to distinguish between thermody-
namically accessible and kinetically relevant oxidation states. For
a proposed intermediate to be kinetically viable, it must be
produced on reaction with the nucleophile on a time scale
commensurate with the rate of catalytic turnover and similarly
must react with the electrophile at an appropriate rate. Here we
focus our attention on the former consideration.32
The formation of 9b from 8a and MgCl[M(4-tolyl)4] (M =
Al, In), with complex 1 acting as precatalyst, is shown in Figure
2. The reactions both reach 50% completion, corresponding to
10 turnovers of the cycle, within 30 s.
Figure 3. Reduction of 1 in the presence of 20 equiv of MgCl[M(4-
tolyl)4] (M = Al, In) at 85 °C.
first sampling point (30 s) was consistent with the reduction of
the bulk of the iron to Fe(I). While it is apparent that the
amount of bitolyl increases beyond these points, suggesting that
lower oxidation state iron species are thermodynamically
accessible with both nucleophiles, the rate at which these
reductions occur in comparison with the rate of catalysis
indicates that oxidation states below Fe(I) are unlikely to be
kinetically relevant to cross-coupling.
We previously obtained similar results in iron-catalyzed
Negishi reactions, with evidence pointing toward Fe(I) being
the lowest kinetically relevant oxidation state when complex 1
was used as the precatalyst. Furthermore, we isolated the Fe(I)
complexes 12a−c from the reactions of 1 with either Ar2Zn or
Grignard reagents. While 12a,c both proved to be catalytically
competent, the rate of catalysis with 12a is too slow for it to be
an on-cycle intermediate, while 12c shows a performance very
similar to that of the precatalyst 1, consistent with it being on-
cycle or in equilibrium with an on-cycle intermediate. Similarly,
we showed that [FeCl2(dppe)] reacts with benzyl Grignards or
the boronate Li[PhB(pinacolato)(tBu)] to give the Fe(I)
complexes 3a,b.12c,14c Kinetic and spectroscopic data for a
Suzuki reaction catalyzed by [FeCl2(dppe)] was found to be
consistent with the intermediacy of an iron(I) species with a
single chelating phosphine ligand, with the five-coordinate
iron(I) species 3b acting as an off-cycle resting state.14c
Figure 2. Reaction progress for the formation of 9b in the coupling of
8a with either MgCl[In(4-tolyl)4] or MgCl[Al(4-tolyl)4] at 85 °C,
catalyzed by 1.
Reduction of the precatalyst by the arylmetal nucleophile is
accompanied by the formation of 4,4′-bitolyl (Scheme 4); thus,
Scheme 4. Correlation of Average Oxidation State with
Biaryl Formation
the amount of bitolyl formed acts as a proxy for the average
oxidation state of the bulk of the iron. Monitoring the rate at
which the biaryl is produced provides an indication of whether
a given oxidation state is accessed fast enough to be viable in
catalysis.
Figure 3 shows the production of 4,4′-bitolyl in the reactions
of complex 1 under the same conditions as the catalytic
reactions, but without the electrophile 8a. With both Al- and
In-based nucleophiles, the amount of bitolyl produced at the
We have not previously structurally characterized Fe(I)
complexes with both dppe and an aryl ligand. Accordingly, the
complex 3c was prepared by the reaction of the 4-tolyl
Grignard with the isolated iron(I) halide complex 3a. The
previously reported complex 12a could also be conveniently
prepared from 12b by this method. The single-crystal X-ray
structures of 3c and the related phenyl complex are shown in
Figures 4 and 5, respectively, while the EPR spectrum of 3c and
its simulated spectrum are shown in Figure 6.33
F
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Figure 6. EPR spectrum of complex 3c (140 K): (a) experimental; (b)
simulated. The spectrum also shows the presence of a small amount of
residual 3a, as evidenced by the weak resonance at ∼305 mT.
correction, see the Supporting Information for full details)
analysis of the ground-state structure of 3c gave a geometry and
electronic structure consistent with low-spin Fe(I) character
(Figure 7), with the low-spin state preferred by 14.2 kcal/
Figure 4. Single-crystal X-ray structure of 3c. Hydrogen atoms and
solvate are omitted for clarity, and thermal ellipsoids are set at the 50%
probability level.
Figure 7. Calculated SOMO (isovalue 0.05 (electron/bohr3)1/2) of
low-spin complex 3c, with the ligand residues shown in wireframe.34
mol.34 The Mulliken spin density corresponding to the
unpaired electron is located primarily on the Fe atom
(87.2%), with only very small contributions from the ligating
P and C atoms and the other ligand atoms.
Figure 5. Single-crystal X-ray structure of 3d. Hydrogen atoms are
omitted for clarity, and thermal ellipsoids are set at the 50% probability
level.
In order to further characterize isolated Fe(I) complexes,
57Fe Mossbauer spectra of the iron(I) bis(phosphine)
complexes 3a−c and 12b,c were recorded at 80 K. Figure 8
shows the spectra for the dppe-containing series. The spectra of
the halide complexes 3a,b fit well to single iron species with
̈
The solid-state structure of 3c shows a distortion from
trigonal bipyramidal toward square-based pyramidal more
pronounced than that observed in complex 12a,27 while the
geometry about the iron in 3d is closer to trigonal bipyramidal.
This suggests that any subtle differences in electronically
preferred geometry are overridden by packing effects in the
crystal.
Mossbauer parameters of δ = 0.42 mm/s, ΔE = 0.51 mm/s
̈
Q
and δ = 0.42 mm/s, ΔEQ = 0.50 mm/s, respectively.
The analogous dpbz-containing complexes 12b,c (Figure S5,
Supporting Information) display similar isomer shifts (δ = 0.43
and 0.44 mm/s, respectively) and slightly larger quarupole
splittings (ΔEQ = 0.61 and 0.65 mm/s, respectively). The
asymmetry in the quadrupole split transitions observed in the
halide complexes with both dppe and dpbz ligation persists at 5
The spin Hamiltonian parameters used in the simulation of
the EPR spectrum (Table S1 in the Supporting Information) fit
well with the quantum chemical calculations based on a low-
34
1
spin (S = /2) Fe(I) center. Similarly, a density functional
theory (standard B3LYP functional, with -D2 dispersion
G
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Figure 9. EPR spectra (140 K) of (a) complex 12a (for comparison),
(b) a sample removed from the coupling of 8n with MgCl[PhBsBu3]
catalyzed by 1 in the presence of 10 mol % of ZnPh2, (c) reaction of 1
with 20 mol % of MgCl[PhBsBu3], (d) reaction of 1 with 20 mol % of
MgCl[9-BBN(iPr)4-tolyl], and (e) complex 12b (for comparison).
an EPR spectrum of the reaction mixture of complex 1 with 20
equiv of MgCl[PhBsBu3] in the absence of a zinc additive
(Figure 9, spectrum c) shows that 12b,d are both formed.
Similarly the equivalent reaction with MgCl[9-BBN(iPr)4-tolyl]
yields Fe(I) species, although in this case the chloride complex
12b predominates (Figure 9, spectrum d).
With regard to other group 13 nucleophiles, the spectrum36
of a sample removed from the coupling of 8n with MgCl[Al(4-
tolyl)4] catalyzed by 1 shows the presence of the halide
complex 12b, as do spectra recorded for the reactions of 1 with
excess MgCl[M(4-tolyl)4] (M = Al, In).36 In these cases there
was evidence to indicate the formation of some of the aryl
complex 12a as a minor component. The EPR spectrum of the
equivalent reaction mixture with MgCl[Ga(4-tolyl)4] showed
the solution to contain predominantly 12a;36 however, the
reaction was accompanied by significant precipitation and
therefore the spectrum may not accurately represent the
product distribution.
Figure 8. 57Fe Mossbauer spectra (80 K) of (A) complex 3a, (B)
complex 3b, and (C) complex 3c. For each spectrum the data (dots)
and best fits (solid lines) are shown.
̈
K (and at 250 K for complex 12b; see the Supporting
Information, Figures S5 and S6) and is likely to be due to
relaxation and/or texture effects.35 The spectrum of 3c is well-
fit to a single iron species with parameters of δ = 0.32 mm/s
and ΔEQ = 0.46 mm/s, where the decrease in isomer shift
observed upon 4-tolyl coordination is consistent with the
stronger electron-donating properties of this ligand in
comparison with the halide ligands.
In order to determine whether any of the isolated Fe(I)
complexes may be relevant in the coupling reactions of
tetraorganoborate nucleophiles, we recorded an EPR spectrum
of a sample removed from the catalytic reaction of 8n with
MgCl[PhBsBu3] catalyzed by 1 in the presence of substoichio-
It is apparent that dpbz and dppe complexes of iron readily
1
form low-spin (S = /2) five-coordinate complexes of the form
P4FeX in the presence of a range of nucleophilic substrates.
Indeed, the dppe analogues form even when the ratio of dppe
to iron is 1:1, implying rapid ligand redistribution.14c In
contrast, we recently showed that a bulky analogue of dpbz,
ligand 13, which has been exploited in a range of iron-catalyzed
cross-coupling reactions,14a,b does not give a P4FeX species.
Instead, the EPR spectroscopic data are consistent with the
1
formation of an S = /2 complex of the form FeX(13) with
inequivalent P-donor environments.14c
metric amounts of diphenylzinc. The spectrum (Figure 9,
1
spectrum b) shows two S =
/ complexes, 12b (minor
2
component) and a species assigned as [FePh(dpbz)2] (12d),
on the basis of the close similarity of its spectrum to that of
complex 12a (spectrum a).
A zinc additive is essential for optimum catalytic activity with
tetraorganoborate nucleophiles (see Table 2, entry 16), and
one possible role it may play is to provide an aryl zinc reagent
in situ that undergoes more effective transmetalation to the iron
center. Indeed, examining the reaction of the borate 10a with
0.2 equiv of ZnCl2 by 11B NMR spectroscopy36 showed a
diminution of the peak corresponding to the borate anion and
an increase in the amount of residual BsBu3, which is suggestive
of the concomitant formation of an arylzinc species. However,
We were interested to see whether a bulky analogue of dppe,
ligand 6, would behave in the same way as dppe and dpbz or
whether its bulk would prevent the formation of a five-
coordinate P4Fe(I) intermediate. Figure 10 shows the EPR
spectrum recorded after the reaction of FeCl2 with 6a and 20
equiv of MgCl[4-tolylBsBu3]. It is apparent that the solution
1
contains an S = /2 complex with a single ligand of 6 with
nonequivalent P-donor environments. The signals are very
H
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donors results from the iron centers adopting a higher
coordination number, most likely by a secondary interaction
with a π system on one of the ligands.38
CONCLUSIONS
■
We have demonstrated that a range of chelating diphosphine
complexes of iron(II) can be used as precatalysts in the Suzuki
coupling of tetraarylborates with benzyl halides. The judicious
choice of appropriate BR3 precursors allows the use of easily
prepared [ArBR3]− reagents, in place of tetraarylborates, in the
coupling with a range of electrophilic partners. Furthermore,
these coupling reactions can be extended to [MAr4]− or MAr3
nucleophiles (M = Al, Ga, In, Tl).
From a mechanistic perspective, we have presented evidence
that suggests that while lower average oxidation states can be
accessed upon reacting a representative precatalyst with [M(4-
tolyl)4]− (M = Al, In), only reduction to Fe(I) is fast enough to
be relevant in the catalytic cycle. EPR spectroscopic data show
that well-defined iron(I) complexes of the type P4FeX readily
form with smaller chelating diphosphine ligands under catalytic
conditions with [ArMR3]− (M = B, Al, In), in addition to our
previous observations that they can be produced in the
presence of arylzinc, aryl- and benzylmagnesium, and Li[PhB-
(pinacolato)(tBu)] reagents.27,12c,14c,21 With very bulky chelat-
ing diphosphines, EPR data suggest that P4FeX complexes do
not form; instead, low-spin Fe(I) species of the type P2FeX are
produced.
It should be noted that, while the data are consistent with the
formation of Fe(I) complexes under catalytic conditions, this
does not necessarily indicate that these species lie on the
catalytic cycle. In order to address this, we are currently
undertaking detailed kinetic experiments, the results of which
will be reported in due course.
Figure 10. EPR spectrum (140 K) of mixture obtained on reacting
FeCl2 with 2 equiv of 6 and 20 equiv of MgCl[(4-tolyl)BsBu3] in THF
at room temperature.
similar to those observed previously for FeX(13) and are
consistent with a low-spin Fe(I) complex of the form FeX(6)
(X = halide, 4-tolyl).
With regard to the nonequivalency of the P-donor
environments, it may seem reasonable to assume that these
iron(I) complexes of ligands 6 and 13 are three-coordinate T-
shaped species, analogous with d8 Pt(II) T-shaped complexes37
2
but with a singly occupied dz orbital. To probe this, we used
DFT to examine the ground-state structures of truncated
complexes using the model compounds 14a′−c′.
EXPERIMENTAL SECTION
■
General Procedures. All air-sensitive manipulations were carried
out using standard Schlenk-line and glovebox techniques. Solvents
were dried and purified using Anhydrous Engineering double-alumina
and alumina−copper catalyst dry columns. Commercial grade solvents
were used for chromatography and extraction. [FeCl2(dpbz)2],39
[FeCl2(dppe)],40 [FeCl2(dppp)],41 [FeCl2(dmpe)2],42 Ph-9-BBN and
iPr-9-BBN,43 and bromodiphenylthallium44 were synthesized accord-
ing to literature procedures. All other reagents were purchased from
commercial suppliers and used without further purification. X-band
EPR spectra of frozen solutions in THF (140 K) were recorded on a
Bruker EMX spectrometer operating at 100 kHz field modulation and
10 mW microwave power and equipped with a high-sensitivity Bruker
cavity (ER 4119HS). Spectral simulations were performed using the
Sim3245 and EasySpin46 software packages. All solid samples for 57Fe
The bromide-containing models 14a′,b′ display a pro-
3
nounced preference for high-spin (S = /2) ground states,
with calculated quartet−doublet gaps of 17.8 and 18.4 kcal/
mol, respectively. Similarly, a high-spin ground state is preferred
for 14c′ by 14.3 kcal/mol, although in this case the calculated
HS−LS gap is reduced to 11.3 kcal/mol on the inclusion of an
agostic interaction in the model (14c″). While the low-spin
models do show a pronounced tendency toward a T-shaped
geometry, the energetically more favorable high-spin systems
tend toward a Y-shaped geometry (Table 6).
The calculated preference for high-spin ground states for
1
models 14a′−c′ means that is highly unlikely that the S = /2
Mossbauer spectroscopy were run on nonenriched samples of the as-
̈
species observed in solution are three-coordinate. It is far more
probable that the observed nonequivalence of the phosphine
isolated complexes. All samples were prepared in an inert-atmosphere
glovebox equipped with a liquid nitrogen fill port to enable sample
freezing to 77 K within the glovebox. Each sample was loaded into a
Delrin Mossbauer sample cup for measurements and loaded under
̈
Table 6. Selected Bond Angles for Low-Spin and High-Spin
Models 14
liquid nitrogen. Low-temperature 57Fe Mossbauer measurements were
̈
performed using a SeeCo MS4Mossbauer spectrometer integrated
̈
with a Janis SVT-400T He/N2 cryostat for measurements at 5 and 80
Pa−Fe−X
Pb−Fe−X
Pa−Fe−Pb
K with a 0.07 T applied magnetic field. Isomer shifts were determined
LS-14a′
LS-14b′
LS-14c′
LS-14c″
HS-14a′
HS-14b′
HS-14c′
128.9
121.6
104.6
106.6
130.7
131.6
134.0
139.2
153.9
169.9
167.5
134.2
141.0
134.0
85.7
83.5
84.2
85.8
84.9
83.7
82.3
relative to α-Fe at 298 K. All Mossbauer spectra were fit using the
program WMoss (SeeCo).
̈
P r e p a r a t i o n o f 1 - ( D i p h e n y l p h o s p h i n o ) - 2 -
(diisopropylphosphino)benzene (4). (2-Bromophenyl)-
diphenylphosphine (0.3 mmol, 0.103 g) was dissolved in Et2O and
the solution stirred. n-Butyllithium (2.5 M in hexanes, 0.3 mmol, 0.120
mL) was added dropwise, and the reaction mixture was stirred for 30
min. Chlorodiisopropylphosphine (0.3 mmol, 0.046 g) was added, and
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the reaction mixture was stirred for a further 30 min. A saturated
aqueous solution of NH4Cl (5 mL) was added, and the aqueous layer
was washed with CH2Cl2 (3 × 5 mL). The combined organic fractions
were dried with MgSO4 and evaporated to dryness. The product was
(4H, m, PCH2), 0.22−0.18 (72H, m, SiCH3). 13C NMR (100 MHz,
CDCl3, 25 °C): δ 139.5 (d, J = 2.77 Hz), 138.5 (2C), 138.2 (d, J =
9.73 Hz, (2C)), 135.9, 24.79, −0.95. 31P NMR (160 MHz, CDCl3, 25
°C).: δ −12.53. HRMS (ESI) m/z: [M + H]+ calcd for C50H88P2Si8
975.4515; found 975.4588. Anal. Calcd for C50H88P2Si8: C, 61.54; H,
9.09. Found: C, 61.29; H, 8.91.
1
recrystallized from methanol (96%). H NMR (400 MHz, CDCl3, 25
°C): δ 7.2−7.1 (4H, m, ArH), 6.9−6.8 (10H, m, ArH), 1.9 (2H, hept,
J = 6.9 Hz, CH), 1.07 (3H, d, J = 7.1 Hz, CH3), 1.02 (d, J = 7.1 Hz,
CH3), 0.76 (3H, d, J = 6.9 Hz, CH3), 0.72 (3H, d, J = 6.9 Hz, CH3).
13C NMR (75 MHz, CDCl3, 25 °C): δ 146.3 (dd, J = 31.7 Hz, J = 9.8
Hz), 142.3 (dd, J = 30.0 Hz, J = 16.7 Hz), 138.0 (dd, J = 13.3 Hz, J =
6.3 Hz), 134.2 (d, J = 19.6 Hz), 133.8 (d, J = 8.1 Hz), 132.5, 129.0,
128.6, 128.3, 128.2, 24.7 (d, J = 13.9 Hz), 20.1 (d, J = 17.3 Hz), 19.5
(d, J = 10.3 Hz). 31P NMR (160 MHz, THF, 25 °C): δ −2.7 (d, J =
154 Hz), −10.0 (d, J = 154 Hz). MS (EI) m/z (%): 378.2 (13) [M]+,
335.1 (100), 301.1 (4), 292.1 (18), 183.0 (54). Anal. Calcd for
C24H28P2: C, 76.17; H, 7.46. Found: C, 76.26; H, 7.48.
Preparation of 1,3-[Bis(3,5-dimethylphenyl)phosphino]-
propane (7a). 1,3-Bis(dichlorophosphino)propane (1.0 mmol) was
dissolved in THF (5 mL) and the solution cooled to −78 °C. (3,5-
Dimethylphenyl)magnesium bromide (4.4 mmol, 1.5 M in THF) was
added, and the reaction mixture was stirred for 30 min before it was
warmed to room temperature. A degassed saturated solution of
aqueous NH4Cl was added, and the aqueous phase was extracted with
CH2Cl2. The combined organic fractions were dried with MgSO4,
filtered, and evaporated to dryness. The crude residue was triturated
with ethanol (2 × 3 mL) and the product obtained as a white
1
crystalline solid (48%). H NMR (400 MHz, CDCl3, 25 °C): δ 6.92
Preparation of 2-(Diphenylphosphino)-N,N-dimethylaniline
(5a). N,N-Dimethylaniline (120 mmol, 14.53 g) was dissolved in
hexane (40 mL). Over a period of 1 h, n-butyllithium (2.5 M in
hexanes, 100 mmol, 16.5 mL) was added. The solution was heated to
reflux and stirred for 5 h before being cooled to room temperature.
The solution was cooled to −78 °C, and chlorodiphenylphosphine
(100 mmol, 22.12 g) was added. The reaction mixture was stirred for 1
h before being warmed to reflux and stirred for a further 5 h. Once the
mixture was cooled, degassed water (30 mL) was added and the
mixture was stirred vigorously for 10 min. The organic layer was then
removed, and the aqueous layer was washed with Et2O (2 × 30 mL).
The combined organic fractions were dried over MgSO4, filtered, and
concentrated in vacuo. The product was recrystallized from CH2Cl2/
EtOH (42%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.37−7.20
(12H, m, ArH), 7.01 (1H, td, J = 7.7 Hz, J = 1.28 Hz, ArH), 6.81 (1H,
dd, J = 7.7 Hz, J = 1.28 Hz, ArH), 2.61 (6H, s, NCH3). 13C NMR (75
MHz, CDCl3, 25 °C): δ 158.1 (d, J = 20.0 Hz), 138.2 (d, J = 12.3 Hz),
134.5, (d, J = 8.4 Hz), 134.3, 133.7 (d, J = 20.0 Hz), 129.8, 128.3,
128.2, 124.4, 120.6 (d, J = 2.3 Hz), 45.5. 31P NMR (160 MHz, CDCl3,
25 °C): δ −13.87. MS (EI) m/z (%): 306.4 (100) [M + H]+, 228.3
(28), 214.3 (34), 185.2 (15). Anal. Calcd for C20H20NP: C, 78.67; H,
6.60; N, 4.59. Found: C, 78.36; H, 6.80; N, 4.50.
Preparation of 2-(Dicyclohexylphosphino)-N,N-dimethylani-
line (5b). 2-Bromo-N,N-dimethylaniline was dissolved in Et2O and
the solution cooled to −40 °C. Over a period of 1 h, n-butyllithium
(1.6 M in hexanes) was added and the resulting suspension was
warmed to room temperature. The solvent was removed by filter
cannula and the precipitate washed with cold hexanes and redissolved
in Et2O. Chlorodicyclohexylphosphine was added dropwise, and the
mixture was stirred for 48 h, following which all volatiles were
removed in vacuo. The residue was dissolved in CH2Cl2 and washed
with NaHCO3 and water, following which the organic layer was
collected, dried over MgSO4, and filtered and the volatiles were
removed in vacuo. The residue was dissolved in pentane and passed
through a plug of silica; removal of the solvent yielded the product as a
white solid (30%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.39−7.32
(1H, dt, J = 7.5, 2.0 Hz), 7.28 (1H, m), 7.13 (1H, m), 7.08−7.00 (1H,
td, J = 7.3, 1.3 Hz), 2.72 (6H, s), 1.93−1.50 (12H, m), 1.38−0.92
(10H, m). 13C NMR (400 MHz, CDCl3, 25 °C): δ 160.5, 133.4 (d, J =
3.2 Hz), 131.6, 129.4, 123.2, 119.8 (d, J = 3.4 Hz), 46.0 (d, J = 5.2 Hz),
33.9 (d, J = 14.4 Hz), 29.1 (d, J = 8.9 Hz), 27.5 (d, J = 11.7 Hz), 27.3
(d, J = 7.7 Hz), 26.6. 31P NMR (160 MHz, CDCl3, 25 °C): δ −12.68.
Preparation of 1,2-{Bis[3,5-bis(trimethylsilyl)phenyl]-
phosphino}ethane (6). 1,2-Bis(dichlorophosphino)ethane (1.0
mmol) was dissolved in THF (5 mL) and the solution cooled to
−78 °C. To this solution was added [3,5-bis(trimethylsilyl)phenyl]-
magnesium bromide (4.5 mmol, 0.42 M in THF) dropwise, and the
mixture was warmed to room temperature and stirred for 16 h. A
saturated aqueous solution of NH4Cl (5 mL) was added, and the
aqueous layer was washed with CH2Cl2 (3 × 5 mL). The combined
organic fractions were dried with MgSO4 and evaporated to dryness.
The crude residue was triturated with methanol (2 × 10 mL) and the
product obtained as a white solid (68%). 1H NMR (400 MHz, CDCl3,
25 °C): δ 7.59 (4H, app s, ArH), 7.57−7.53 (8H, m, ArH), 2.22−2.18
(4H, s, ArH), 6.90 (4H, s, ArH), 6.84 (4H, s, ArH), 2.18 (24H, s), 2.07
(4H, t, J = 8.07 Hz), 1.51 (2H, tt, J = 9.54 Hz, J = 2.93 Hz). 13C NMR
(100 MHz, CDCl3, 25 °C): δ 137.5, 137.3, 136.6, 136.5, 129.4, 129.2,
28.6, 21.7, 20.3. 31P NMR (160 MHz, CDCl3, 25 °C): δ −17.63.
HRMS (ESI) m/z: [M + H]+ calcd for C35H42P2 525.28; found
525.284.
Preparation of 1,3-{Bis[3,5-bis(trimethylsilyl)phenyl]-
phosphino}propane (7b). 1,3-Bis(dichlorophosphino)propane
(1.18 mmol, 0.290 g) was dissolved in THF and cooled to −78 °C.
[3,5-Bis(trimethylsilyl)phenyl]magnesium bromide (5.0 mmol, 0.18 M
in THF) was added dropwise, and the reaction mixture was stirred for
30 min before it was warmed to room temperature. A degassed
saturated solution of aqueous NH4Cl was added, and the aqueous
phase was extracted with CH2Cl2. The combined organic fractions
were dried with MgSO4, filtered, and evaporated to dryness. The
1
product was recrystallized from degassed ethanol (48%). H NMR
(400 MHz, CDCl3, 25 °C): δ 7.60 (4H, app q, J = 1.1 Hz, ArH), 7.53
(8H, app dd, J = 7.3 Hz, J = 1.1 Hz, ArH), 2.25 (4H, t, J = 7.7 Hz,
PCH2), 1.77 (m, CH2CH2CH2), 0.22 (72H, s, SiCH3). 13C NMR (100
MHz, CDCl3, 25 °C): δ 139.4 (d, J = 5.2 Hz), 138.3, 138.1 (d, J = 18.4
Hz), 136.3 (d, J = 14.5 Hz), 30.0 (t, J = 12.0 Hz), 23.3 (t, J = 16.4 Hz),
−1.1. 31P NMR (160 MHz, CDCl3, 25 °C): δ −17.53 (s). HRMS (CI)
m/z: [M + H]+ calcd for C51H91P2Si8 989.4754; found 989.4744. Anal.
Calcd for C51H91P2Si8: C, 61.88; H, 9.16; found: C, 61.06; H, 8.77.
General Procedure for Iron-Catalyzed Suzuki Coupling with
Na[BPh4] (GP1) (Table 1). The Fe salt or complex (0.025 mmol),
Na[BPh4] (0.214 g, 0.625 mmol), and ligand (0, 0.025, or 0.05 mmol
as appropriate) were dissolved in toluene (5 mL), and the mixture was
stirred at room temperature for 10 min. ZnPh2 (1.0 mL, 0.05 M, 0.05
mmol) was added, and the reaction mixture was stirred for a further 10
min. 8a (70 μL, 0.5 mmol) was then added and the reaction mixture
heated to 85 °C for 4 h. The mixture was cooled and quenched with
water (5 mL), and 1,3,5-trimethoxybenzene (0.084 g, 0.50 mmol) was
added as an internal standard. The organic fractions were extracted
into CH2Cl2 (3 × 10 mL), and the extracts were combined and filtered
through a plug of MgSO4. The solvent was removed in vacuo and the
1
residue redissolved in CDCl3 for H NMR analysis.
General Procedure for Iron-Catalyzed Suzuki Coupling with
MgCl[(sBu)3B(4-tolyl)] (10a) (GP2) (Table 2). Tri-sec-butylborane
(0.6 mmol, 1 M in THF) was placed in a Schlenk tube and cooled to
−78 °C. To this solution was added 4-tolylmagnesium chloride (0.6
mmol, 2 M in THF), and the mixture was warmed to room
temperature and stirred for a further 30 min. 1 (25 mg, 0.025 mmol)
and Zn(4-tolyl)2 (0.05 mmol, 0.05 M in THF) were added, resulting
in a red solution. Toluene (3 mL) and the appropriate alkyl halide (0.5
mmol) were then added, and the reaction mixture was heated to 85 °C
for 4 h. The reaction mixture was quenched and product distribution
determined as described in GP1. Alternatively, the product was
isolated using flash-column chromatography.
General Procedure for Iron-Catalyzed Suzuki Coupling with
Borates Generated from Ph-9-BBN (GP3) (Table 2). Ph-9-BBN
(0.105 mL, 0.5 mmol) was dissolved in THF and the solution cooled
to −78 °C. To this solution was added the appropriate organometallic
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reagent (0.5 mmol), and the mixture was warmed to room
temperature and stirred for a further 30 min. 1 (25 mg, 0.025
mmol) and ZnPh2 (0.05 mmol, 0.05 M in THF) were added, resulting
in a red solution. Toluene (3 mL) and 8a (0.5 mmol) were then
added, and the reaction mixture was heated to 85 °C for 4 h. The
reaction mixture was quenched and product distribution determined as
described in GP1.
General Procedure for Iron-Catalyzed Suzuki Coupling with
MgCl[9-BBN(iPr)(4-tolyl)] (10b) (GP4) (Table 2). iPr-9-BBN (0.119
mL, 0.6 mmol) was dissolved in THF and the solution cooled to −78
°C. To this solution was added 4-tolylmagnesium chloride (0.6 mmol,
2 M in THF), and the mixture was warmed to room temperature and
stirred for a further 30 min. 1 (25 mg, 0.025 mmol) and Zn(4-tolyl)2
(0.05 mmol, 0.05 M in THF) were added, resulting in a red solution.
Toluene (3 mL) and the appropriate alkyl halide (0.5 mmol) were
then added, and the reaction mixture was heated to 85 °C for 4 h. The
reaction mixture was quenched and product distribution determined as
described in GP1.
General Procedure for Iron-Catalyzed Coupling of Aryl
Aluminum Reagents with Benzyl Halides (GP5) (Table 4). AlCl3
(0.3 or 0.6 mmol as appropriate) was dissolved in THF (1 mL) and
cooled to 0 °C. To this solution was added 4-tolylmagnesium chloride
(3 or 4 equiv as appropriate), and the mixture was warmed to room
temperature and stirred for a further 30 min. 1 (25 mg, 0.025 mmol),
toluene (3 mL), and the appropriate benzyl halide (0.5 mmol) were
added, and the mixture was heated to 85 °C for 4 h. The reaction
mixture was quenched and product distribution determined as
described in GP1. Alternatively, the product was isolated using flash-
column chromatography.
added to the reaction mixture. Samples (0.1 mL) were taken and
quenched in 1/1 EtOAc/H2O at appropriate time points. For each
sample, the organic layer was removed and filtered and the solvent
removed in vacuo. The residue was redissolved in CDCl3 for 1H NMR
analysis.
Reduction Profiles for [FeCl2(dpbz)2] (1) with [In(4-tolyl)4]-
MgCl or [Al(4-tolyl)4]MgCl (Figure 3). The appropriate nucleophile
(1.0 mmol) was generated according to GP5 or GP7, toluene (5 mL)
was added, the mixture was heated to 85 °C, and a sample (0.1 mL)
was taken and quenched to determine the initial concentration of 4,4′-
bitolyl. In a separate Schlenk tube, 1 (50 mg, 0.05 mmol) and 1,3,5-
trimethoxybenzene (84 mg, 0.5 mmol) were suspended in toluene (to
give a total volume of 10 mL) and heated to 85 °C. The nucleophile
solution was added, and samples (0.1 mL) were taken and quenched
in 1/1 EtOAc/H2O at the appropriate time points. For each sample,
the organic layer was removed and filtered and the solvent removed in
1
vacuo. The residue was redissolved in CDCl3 for H NMR analysis.
Synthesis of [Fe(4-tolyl)(dppe)2] (3c). 4-Tolylmagnesium
chloride (10 mmol, 2 M in THF) was added to a deep red solution
of [FeCl(dppe)2] (200 mg, 225 μmol) in THF (10 mL). Dioxane (1
mL) was added to the reaction mixture, the colorless precipitates that
formed within 1 h were filtered off, and the solvent was removed from
the solution in vacuo. The crude product was dissolved in Et2O (5
mL), layered with hexanes (2 mL), and stored at −35 °C overnight.
The product was isolated by filtration as a deep red-brown amorphous
solid (51 mg, 24%) and characterized by EPR and Mossbauer
̈
spectroscopy and single-crystal X-ray analysis (see main text).
Magnetic moment: 1.78 μB (Evans method). Anal. Calcd for
C59H55FeP4: C, 75.08; H, 5.87. Found: C, 74.30; H, 5.91.
General Procedure for Iron-Catalyzed Coupling of Aryl
Gallium Reagents with 3-Methoxybenzyl Bromide (8a) (GP6)
(Table 4). GaCl3 (0.25 or 0.6 mmol, 0.5 M in pentane) was placed in
a Schlenk tube, and the solvent was removed in vacuo. The residue
was redissolved in Et2O (2 mL) and cooled to 0 °C, the appropriate
aryl Grignard reagent (4 equiv) was added, and the resulting
suspension was warmed to room temperature. 1 (25 mg, 0.025
mmol) and ZnAr2 (0.05 mmol where appropriate) were added,
resulting in a red solution. Toluene (3 mL) was added, the mixture
was heated to 85 °C, 8a (70 μL, 0.5 mmol) was added, and the
reaction mixture was stirred at the same temperature for 4 h. The
reaction mixture was quenched and product distribution determined as
described in GP1.
General Procedure for Iron-Catalyzed Coupling of Aryl
Indium Reagents with Alkyl Halides (GP7) (Table 4). InCl3 (0.25
or 0.5 mmol as appropriate) was dissolved in THF (2 mL) and cooled
to 0 °C. To this solution was added 4-tolylmagnesium chloride (3 or 4
equiv as appropriate), and the mixture was warmed to room
temperature and stirred for a further 30 min. 1 (25 mg, 0.025
mmol), Zn(4-tolyl)2 (0.05 mmol where appropriate), toluene (3 mL),
and the appropriate alkyl halide (0.5 mmol) were added, and the
mixture was heated to 85 °C for 4 h. The reaction mixture was
quenched and product distribution determined as described in GP1.
Alternatively, the product was isolated using flash-column chromatog-
raphy.
Iron-Catalyzed Coupling of Triphenylthallium with 3-
Methoxybenzyl Bromide (Table 4). Ph2TlBr (0.22 g, 0.5 mmol)
was dissolved in THF (2 mL) and cooled to −78 °C. To this solution
was added phenyllithium (0.27 mL, 1.8 M in THF), and the mixture
was warmed to room temperature and stirred for a further 30 min. 1
(25 mg, 0.025 mmol), ZnPh2 (0.05 mmol where appropriate), toluene
(3 mL), and 8a (70 μL, 0.5 mmol) were added, and the mixture was
heated to 85 °C for 4 h. The reaction mixture was quenched and
product distribution determined as described in GP1.
Reaction Profiles for Iron-Catalyzed Coupling of 3-Methox-
ybenzyl Bromide (8a) with [In(4-tolyl)4]MgCl or [Al(4-tolyl)4]-
MgCl (Figure 2). The appropriate nucleophile (1.0 mmol) was
generated according to GP5 or GP7. 1 (50 mg, 0.05 mmol), 1,3,5-
trimethoxybenzene (84 mg, 0.5 mmol), and toluene (5 mL) were then
added, and the mixture was heated to 85 °C. 8a (140 μL, 1.0 mmol)
was dissolved in toluene (sufficient to give a total reaction volume of
10 mL) in a separate Schlenk and heated to 85 °C, and this was then
Synthesis of [Fe(phenyl)(dppe)2] (3d). To a solution of
[FeCl2(dppe)] (1.90 mmol) in THF/dioxane (6/1, 20 mL) at −30
°C was slowly added phenylmagnesium bromide (1.0 M in THF, 4.18
mmol). After the mixture was stirred at −30 °C for 1 h, salts were
removed by filtration at −30 °C. The volume was reduced to ∼5 mL,
and 20 mL of hexane was added at −30 °C, leading to the isolation of
crystalline 3d (∼220 mg) as a purple-red solid which was characterized
by EPR spectroscopy and single-crystal X-ray analysis (see main text).
We were unable to obtain a satisfactory elemental analysis for 3d,
despite repeated attempts, presumably due to its sensitivity.
EPR Spectroscopic Study of Iron-Catalyzed Coupling of 3-
Methoxybenzyl Chloride and MgCl[(sBu)3BPh] (Figure 9b).
Phenylmagnesium chloride (2.0 mmol, 2 M in THF) was added to tri-
sec-butylborane (2.0 mmol, 1 M in THF) at −78 °C. The mixture was
warmed to room temperature, and ZnPh2 (0.2 mmol) and 1 (0.102 g,
0.1 mmol) were added followed by THF (1 mL), yielding a deep red
solution. 8n (0.291 mL, 2.0 mmol) was added, and an aliquot (120
μL) was removed after 2 min and analyzed by EPR spectroscopy,
showing the formation of two S = 1/2 species consistent with a mixture
of 12a (major) and 12b (minor).
In Situ Generation of Fe(I) by Reduction of [FeCl2(dpbz)2] (1)
with MgCl[(sBu)3BPh] (Figure 9c). Phenylmagnesium chloride (2.0
mmol, 2 M in THF) was added to tri-sec-butylborane (2.0 mmol, 1 M
in THF) at −78 °C. The mixture was warmed to room temperature,
and 1 (0.102 g, 0.1 mmol) was added followed by THF (1 mL),
yielding a deep red solution. An aliquot (120 μL) was removed and
1
analyzed by EPR spectroscopy, showing the formation of two S = /2
species consistent with a mixture of 12a (major) and 12b (minor).
In Situ Generation of [FeCl(dpbz)2] (12b) by Reduction of
[FeCl2(dpbz)2] (1) with MgCl[9-BBN(iPr)(4-tolyl)] (10b) (Figure
9d). 4-Tolylmagnesium chloride (2.0 mmol, 2 M in THF) was added
i
to a solution of Pr-9-BBN (2.0 mmol, 1 M in THF) at −78 °C. The
mixture was warmed to room temperature, and 1 (0.102 g, 0.1 mmol)
was added followed by THF (1 mL), yielding a deep red solution. An
aliquot (120 μL) was removed and analyzed by EPR spectroscopy,
showing the formation of 12b.
EPR Spectroscopic Study of Iron-Catalyzed Coupling of 3-
Methoxybenzyl Chloride (8n) and MgCl[Al(4-tolyl)4] or MgCl-
[In(4-tolyl)4] (Figure S3, Supporting Information). 4-Tolylmag-
nesium chloride (8.0 mmol, 2 M in THF) was added to a solution of
either InCl3 (2.0 mmol, 1 M in THF) or AlCl3 (2.0 mmol, 0.5 M in
K
dx.doi.org/10.1021/om500518r | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
THF) at 0 °C. The mixture was warmed to room temperature, and 1
(0.102 g, 0.1 mmol) was added, resulting in a deep red solution. 8n
(0.291 mL, 2.0 mmol) was added, and an aliquot (120 μL) was
removed after 2 min and analyzed by EPR spectroscopy.
In Situ Generation of [FeCl(dpbz)2] (12b) by Reduction of
[FeCl2(dpbz)2] (1) with MgCl[Al(4-tolyl)4] and MgCl[In(4-tolyl)4]
(Figure S3, Supporting Information). 4-Tolylmagnesium chloride
(8.0 mmol, 2 M in THF) was added to a solution of either InCl3 (2.0
mmol, 1 M in THF) or AlCl3 (2.0 mmol, 0.5 M in THF) at 0 °C. The
mixture was warmed to room temperature, and 1 (0.102 g, 0.1 mmol)
was added, resulting in a deep red solution. An aliquot (120 μL) was
removed and analyzed by EPR spectroscopy, showing the formation of
12b.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail for R.B.B.: r.bedford@bristol.ac.uk.
Dvorak
Sapountzis, I.; Korn, T. J.; Cahiez, G.; Knochel, P. Angew. Chem., Int.
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Notes
́
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This paper is dedicated to the memory of Michael Lappert, a
truly outstanding organometallic chemist. We thank the EPSRC
and Pfizer for funding under both the collaborative EPSRC
Programme for Synthetic Organic Chemistry with AZ-GSK-
Pfizer (E.C.N.) and for funding from Bristol Chemical
Synthesis Centre for Doctoral Training (P.M.C.) and the
EPSRC for student (N.J.G., J.N.) and postdoctoral (P.B.B.,
E.C.) support. This work was supported in part by start-up
funding from the University of Rochester (M.L.N.). The
authors thank Dr. William W. Brennessel for assistance in the
collection and analysis of X-ray crystallographic data (complex
3d). J.N.H. holds a Royal Society Wolfson Research Merit
Award.
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