Flow carbonylation of sterically hindered ortho-substituted iodoarenes

Article abstract of DOI:10.3762/bjoc.12.147

The flow synthesis of ortho-substituted carboxylic acids, using carbon monoxide gas, has been studied for a number of substrates. The optimised conditions make use of a simple catalyst system compromising of triphenylphosphine as the ligand and palladium acetate as the pre-catalyst. Carbon monoxide was introduced via a reverse "tube-in-tube" flow reactor at elevated pressures to give yields of carboxylated products that are much higher than those obtained under normal batch conditions.

Full text of DOI:10.3762/bjoc.12.147

Flow carbonylation of sterically hindered
ortho-substituted iodoarenes
Carl J. Mallia1, Gary C. Walter2 and Ian R. Baxendale*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1503–1511.
1Department of Chemistry, Durham University, South Road, Durham,
DH1 3LE, United Kingdom and 2Syngenta CP R&D Chemistry,
Jealott's Hill International Research Centre, Bracknell, Berkshire,
RG42 6EY, United Kingdom
doi:10.3762/bjoc.12.147
Received: 05 May 2016
Accepted: 27 June 2016
Published: 19 July 2016
Email:
Ian R. Baxendale* - i.r.baxendale@durham.ac.uk
This article is part of the Thematic Series "Automated chemical
synthesis".
* Corresponding author
Associate Editor: A. Kirschning
Keywords:
carbon monoxide; carbonylation of ortho-substituted substrates; flow
chemistry; gases in flow; “tube-in-tube”
© 2016 Mallia et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The flow synthesis of ortho-substituted carboxylic acids, using carbon monoxide gas, has been studied for a number of substrates.
The optimised conditions make use of a simple catalyst system compromising of triphenylphosphine as the ligand and palladium
acetate as the pre-catalyst. Carbon monoxide was introduced via a reverse “tube-in-tube” flow reactor at elevated pressures to give
yields of carboxylated products that are much higher than those obtained under normal batch conditions.
Introduction
Carbonylation reactions have received a great deal of attention starting material for several antibacterial drugs such as cipro-
both in batch as well as in flow (using plug/annular flow reac- floxacin (Cipro™), norfloxacin (Noroxin™) and pefloxacin
tors [1-5] or “tube-in-tube” reactors [6-10]) and generally (Peflacine™).
produce the desired products in good yields [11-14]. This is not
the case though for the carbonylation of ortho-substituted sub- The low catalyst turnover frequency (T.O.F.) and poor yields
strates which are much more challenging as highlighted by the associated with ortho-substituted transformations are attributed
limited literature precedence [15-17]. However, these products to the carbon monoxide coordination to the intermediate aryl
are of considerable industrial importance, especially the amide transition metal (i.e., Pd) complex which is inhibited by sterics
and ester derivatives, which are commonly found in agrochem- [15]. Following oxidative addition of the aryl halide, an associa-
ical active ingredients, for example tecloftalam, flutolanil, tive mechanism for the complexation of carbon monoxide on
fluopyram and diflufenican. Likewise, in pharmaceutical com- the d8 square planar intermediate would occur prior to the key
pounds such as 2,4,5-trifluorobenzoic acid, which serve as a migratory insertion step. In the complex, the aryl group would
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be oriented perpendicularly to the plane to minimise steric inter-
actions thus placing the ortho-substituent directly over an axial
site (Figure 1). The ortho-substituent therefore acts as a steric
buttress hindering the approach of the incoming carbon mon-
oxide thus slowing down the rate of the reaction. An X-ray
structure of trans-bromo(o-tolyl)bis(triphenylphosphine)palla-
dium(II) complex was reported by Cross et al. (Figure 2) [18].
The molecular structure of 1 comprises of a palladium atom
with near perfect square planar geometry with a slight out of
plane displacement of Br and C(1) where the Br–Pd–C(1) angle
is 170.9°. As a whole, the molecule has approximate Cs
symmetry with the PPh3 ligands almost eclipsing each other if
viewed along the P–Pd–P axis, with the tolyl group sandwiched
Figure 1: Steric interactions of the carbon monoxide coordination to
the aryl complex intermediate.
between the two phenyl groups (Figure 2, structure B). lower yields of the carbonylated product. In principle, increas-
Focusing on the tolyl group only, structure C (Figure 2) shows ing the carbon monoxide concentration (by increasing the car-
how the methyl of the tolyl group is placed straight over the bon monoxide pressure) together with an increase in tempera-
axial position of the palladium. Structure D (Figure 2) is a top ture, should promote the carbonylation. However, an increase in
view of the crystal structure illustrating how the methyl group carbon monoxide concentration can also decrease the amount of
sits directly over the axial position of the palladium which active Pd0 catalyst due to the π-acidic nature of carbon mon-
would introduce steric effects inhibiting the CO coordination on oxide as a ligand, thus slowing down the reaction. Additionally,
the intermediate aryl complex.
increasing the temperature will also increase the rate of side
product formation. Consequently, optimisation of the carbon
As the carbonylation step becomes slower, the competing monoxide concentration and temperature is critical to obtaining
dehalogenation pathway becomes dominant resulting in overall a good yield of carbonylated ortho-substituted products.
Figure 2: A) molecular structure of complex 1; B) ball and stick representation of X-ray structure; C) ball and stick representation of X-ray structure
showing the tolyl group only; D) topside view of X-ray structure [18].
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Results and Discussion
The application of flow chemistry [19,20] has been shown to be
beneficial for many reactions that involve gases [21-29]. The
efficient mixing along with high heat and mass transfer that are
achieved through the use of small dimensioned channels such as
those found in flow reactors, allow for the use of a wider range
of reaction conditions which are otherwise difficult or impos-
sible to achieve. The interfacial mixing area is also an impor-
tant characteristic when gases are involved as this is an essen-
tial factor determining the solubility of a gas in the liquid phase.
The interfacial area is generally very small when traditional
batch chemistry equipment is used such as round bottom flasks.
This also becomes proportionally smaller when larger volume
flasks are used as in scale up procedures making the mass
transfer even less efficient. In contrast, high interfacial areas can
be achieved in flow reactors especially microchannel reactors
(a = 3400–18000 m2 m−3) [30], which increases the mass
transfer and thus helps solubilise the gases in the liquid phase.
Figure 3: Reverse “tube-in-tube” reactor.
noted that the catalyst level could be reduced [34], but this
In our work a reverse “tube-in-tube” reactor [31-33] was used to amount allowed for an efficient catalytic process with short
deliver the carbon monoxide to the reaction (Figure 3), as this reaction times in the region of two hours, a good match for the
was shown to be more efficient than an alternative plug flow flow system assembly [8]. Five different phosphine ligands
system (Scheme 1) when evaluated on iodobenzene (2).
were subsequently tested, three of which were monodentate
with a variable cone angle (6–8; 145–256°) [35,36] and the
The “tube-in-tube” gas-liquid unit was attached to a commer- other two bidendate phosphine ligands namely 1,4-bis(di-
cial flow system; Vapourtec R2+ Series along with an R4 phenylphosphino)butane (DPPB, 9; βn = 98°) and Xantphos
heating unit. Having established the reactor design, we next (10; 104 and 133°) with differing bite angles (Figure 4) [37-39].
used 2-chloro-1-iodobenzene (4) as a model substrate for
screening and identification of a set of general reaction condi- Initially using 5 bar of carbon monoxide and a temperature of
tions (Scheme 2). Initially, a fixed 5 mol % of Pd(OAc)2 and 110 °C, the five ligands gave similar yields, with DPPB (9)
10 mol % of the phosphine ligand was investigated. It was giving marginally the highest and X-Phos (7) the lowest isolat-
Scheme 1: Comparison of plug flow reactor carbonylation (left) and “tube-in-tube” reactor carbonylation (right).
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Scheme 2: Schematic diagram of the flow process.
Figure 4: Phosphine ligands used for the ortho-carbonylation reaction.
ed yield. However, the highest selectivities for the desired prod- similar reactions in the literature [6,10]. However as anticipat-
uct were obtained with S-Phos (8) and triphenylphosphine (6) ed, an increase in carbon monoxide pressure did pertain to a
(Table 1, entries 2 and 5), with the difference between the raise in product yield to 62% (Table 1, entries 12 and 13). In ad-
conversion and the isolated yield mainly equating to the dehalo- dition the effect of gas contact time was evaluated by employ-
genated product namely, chlorobenzene.
ing two “tube-in-tube” reactors linked in series; albeit this
resulted in only a modest improvement in yield (Table 1, entry
Next changing the amount of triethylamine used from 1.1 equiv 15). A further increase in product yield was observed when a
to 1.6 equiv and 2.0 equiv, respectively, did not significantly larger excess of the triethylamine base (1.6 equiv) was used
change the isolated yield of 5. However, changing to the (Table 1, entry 16), but the isolated yield dropped with further
stronger base DBU (pKa in water at 25 °C = 13.5) [40] dramati- equivalents of triethylamine (2.0 equiv; Table 1, entry 17). This
cally reduced the isolated yield (Table 1, entry 8). A wider tem- indicated that the reaction was being inhibited by low pH which
perature range was also investigated (Table 1, entries 9–11). was generated at higher conversions when insufficient base was
This resulted in only a small increase in the yield on going from present to neutralise the carboxylic acid being formed. Interest-
100 °C to 120 °C and a marginal decrease when the tempera- ingly, the requirement for a higher excess of base during initial
ture was further increased to 130 °C. As there was no signifi- screening (Table 1, entries 6 and 7) had been masked due to the
cant difference between 110 °C and 120 °C (Table 1, entries 5 initial low conversions achieved.
and 10), the lower temperature was selected for the use in the
next set of experiments. Interestingly the addition of up to For comparison purposes, two batch carbonylation reactions
20 mol % of dimethylformamide (DMF) as an additive did not were performed. The first of these batch reactions (conducted in
improve the yield which had been suggested by evaluation of a conventional laboratory set-up) used the palladium triphenyl-
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Table 1: Optimisation for the carbonylation of ortho-substituted substrates in flow.
Entry
Ligand
Temperature (°C)
CO pressure (bar)
Conversion (%)
Isolated yield of 5 (%)
1
X-Phos
S-Phos
DPPB
Xantphos
PPh3
110
110
110
110
110
110
110
110
100
120
130
110
110
110
110
110
110
5
68
31
36
38
36
36
36
33
18
31
37
33
46
62
31
68
90
73
2
5
43
3
5
90
4
5
57
5
5
44
6a
PPh3
5
59
7b
PPh3
5
80
8c
PPh3
5
N/D
41
9
PPh3
5
10
11
12d
13d
14d,e
15f
16f,a
17f,b
PPh3
5
60
PPh3
5
N/D
67
PPh3
10
15
15
15
15
15
PPh3
74
PPh3
N/D
N/D
99
PPh3
PPh3
PPh3
99
a1.6 equiv of base. b2.0 equiv of base. c1.1 equiv of DBU used instead of NEt3. d10 mL reactor was not “tube-in-tube”. e20 mol % DMF added.
f2 × 15 mL “tube-in-tube” reactors used. N/D: not determined.
phosphine catalyst system under refluxing conditions with a when compared to the reactions conducted in flow, most
double-walled balloon to deliver the carbon monoxide probably arises from the fact that not enough carbon monoxide
(Scheme 3). This would constitute a normal set-up used by is being delivered to the reaction mixture. The dehalogenation
many laboratory chemists when reactions involving gases are pathway is then preferred yielding chlorobenzene as the main
attempted if no specialised equipment is available. Two parallel product.
reactions were preformed, one reaction was quenched after
2 hours and after purification yielded 5% of product 5, while the The second batch reaction set-up, conducted in the depart-
second reaction was quenched after 24 h yielding 9% of mental high pressure lab (HPL), was set up in a Parr autoclave
purified 5. The difference in the yields obtained in batch using carbon monoxide at 15 bar and 110 °C for 2 hours. After
Scheme 3: The batch carbonylation of 2-chloro-1-iodobenzene in conventional lab (top) and using a Parr autoclave in high pressure lab (bottom).
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purification, a yield of 87% for product 5 was obtained. This stituent to a bromo, fluoro or trifluoromethyl group. However, a
compares well with the flow protocol, however, the reaction slight decrease associated with the larger sizes of bromo and tri-
“processing” time is in reality much longer due to the long cool- fluoromethyl groups may be inferred (Scheme 4, 11, 13). A
ing and heating times (4 h 15 min “processing” time, see experi- more pronounced decrease in yield was obtained for substrates
mental section in Supporting Information File 1 for more 14 and 15 (Scheme 4, 63% and 60%, respectively) probably due
details). Also, the time required due to the extra precautionary to the larger size of these groups and as well as electronic
measures needed when high pressure laboratory equipment is effects (the more electron withdrawing trifluoromethyl group
used means that the turnaround time is much longer. This substrate 13 gave a 71% yield). For comparisons of the sizes of
makes the flow reactor more efficient in terms of processing the ortho-substituents used, A-values can be used as a guide
time. Additionally, the added safety and potential benefits (Cl: 0.43 kcal/mol, Br: 0.38 kcal/mol, F: 0.15 kcal/mol, OMe:
regarding scale up associated with the flow reactor makes this 0.60 kcal/mol, CF3: 2.10 kcal/mol and Me: 1.70 kcal/mol) [41].
even more favourable.
This indicates interplay between electronic and steric factors.
Having identified a set of reaction conditions for successful Using a pyridine as a heteroaromatic substrate gave a lower but
carbonylation, a number of additional substrates were assessed still acceptable yield of 16 compared to the phenyl equivalent
to determine the generality of the flow process. No significant (5). In general, substitution at the 4-position of the aryl gave
impact was seen on the overall yield by altering the ortho-sub- moderate to good yields (Scheme 4, 17–21) with weakly elec-
Scheme 4: Structures of ortho-substituted carboxylic acids prepared via a continuous flow hydroxy-carbonylation method.
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tron-withdrawing substituents or electron-donating groups proton-dehalogenation pathway becomes preferred, giving 1,3-
giving better yields (Scheme 4, compounds 17, 19–21) than the dimethoxybenzene as the main product, which was isolated in
more electron-withdrawing CF3 group (Scheme 4, compound 31% yield in the case of 29 and 3-chlorotoluene in the case of
18). In the case of 22 the attached aromatic ring introduces both 30 which was isolated in 52% yield (Scheme 4).
the ortho substituted sterics and the electronic effects from the
additional aromatic ring attached. For comparison 2-iodonaph- To demonstrate the potential scalability of the reaction condi-
thalene (31) was carboxylated under the same conditions to give tions, the synthesis of compound 20 was repeated at 16 mmol
2-naphthoic acid (32) showing that reducing the steric encum- scale, a factor of twelve times the original 1.3 mmol test scale
brance at the ortho position improves the yield by 10% for this (Scheme 6). The yield obtained for the larger scale was 85%
substrate (Scheme 5).
which is consistent with the original 89% obtained at the
1.30 mmol scale, indicating that the processes is robust and reli-
Moderate yields were obtained with 5-substited substrates ably delivering 1.19 g h−1 of 20 in 85% isolated yield.
(Scheme 4, compounds 23–30). Both electron-withdrawing
groups (Scheme 4, compounds 23–26) and electron-donating Conclusion
groups gave similar yields (Scheme 4, compounds 27 and 28) We have successfully demonstrated how flow chemistry can be
indicating that the inductive effects are not affecting the yields. used to enhance difficult transformations such as the palladium-
Comparing the yields obtained for 27 and 28 also indicates that catalysed hydroxy-carbonylation of ortho-substituted iodo-
the sterics at the 5-position are not affecting the yield with a arenes. The optimised conditions were also demonstrated to
large group at the 5-position of substrate 30 [42] (see X-ray work on a number of ortho-substituted substrates giving moder-
structure of substrate 33, Figure 5) actually leading to a better ate to good yields. Comparison of 22 with 32 also showed that
yield than obtained for product 27 which contains the smaller the steric encumbrance on the ortho position has an effect on
ethoxy group at the 5-position.
the yield even when other electronic effects are in place such as
those coming from the additional aromatic ring attached. A
The lowest yields of the array were obtained for compounds 29 scale-up of the reaction conditions was performed providing
and 30, demonstrating the importance of sterics and electronics comparable yields to those obtained from the initial smaller test
adjacent to the leaving group. In both cases, the carbon mon- scale. This method could thus be an efficient and scalable ap-
oxide insertion is assumed to be slow as both axial positions of proach to synthesising important intermediates containing
the aryl complex would be hindered, meaning the competing ortho-substituted carboxylic acids.
Scheme 5: Flow carbonylation of 2-iodonaphtalene.
Figure 5: X-ray structure of substrate 33.
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Scheme 6: Scale up synthesis of 2-chloro-4-fluorobenzoic acid (20).
Experimental
See Supporting Information File 1 for full experimental data.
Batsanov (Durham University, Department of Chemistry) for
solving the X-ray structure.
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