Synthesis of Polyfunctional Diorganomagnesium and Diorganozinc Reagents through In Situ Trapping Halogen–Lithium Exchange of Highly Functionalized (Hetero)aryl Halides in Continuous Flow

Article abstract of DOI:10.1002/anie.201706609

We report a halogen–lithium exchange performed in the presence of various metal salts (ZnCl₂, MgCl₂?LiCl) on a broad range of sensitive bromo- or iodo(hetero)arenes using BuLi or PhLi as the exchange reagent and a commercially available continuous-flow setup. The resulting diarylmagnesium or diarylzinc species were trapped with various electrophiles, resulting in the formation of polyfunctional (hetero)arenes in high yields. This method enables the functionalization of (hetero)arenes containing highly sensitive groups such as an isothiocyanate, nitro, azide, or ester. A straightforward scale-up was possible without further optimization.

Full text of DOI:10.1002/anie.201706609

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Accepted Article
Title: Preparation of Polyfunctional Diorgano-Magnesium and -Zinc
Reagents Using In Situ Trapping Halogen-Lithium Exchange of
Highly Functionalized (Hetero)aryl Halides in Continuous Flow
Authors: Marthe Ketels, Maximilian Andreas Ganiek, Niels Weidmann,
and Paul Knochel
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201706609
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10.1002/anie.201706609
Angewandte Chemie International Edition
COMMUNICATION
Preparation of Polyfunctional Diorgano-Magnesium and -Zinc
Reagents Using In Situ Trapping Halogen-Lithium Exchange of
Highly Functionalized (Hetero)aryl Halides in Continuous Flow
Marthe Ketels, Maximilian A. Ganiek, Niels Weidmann and Paul Knochel*
Abstract: We report a halogen-lithium exchange performed in the
presence of various metal salts (ZnCl2, MgCl2·LiCl) on a broad
range of sensitive bromo- or iodo-(hetero)arenes using BuLi or
knowledge for the first time aryl azides and that it can be
conducted using commercially available flow reactors.
PhLi as exchange reagent and
a commercially available
continuous flow setup. The resulting diarylmagnesium or
diarylzinc species were trapped with various electrophiles
resulting in the formation of polyfunctional (hetero)arenes in high
yields. This methodology enabled the functionalization of
(hetero)arenes containing highly sensitive groups such as an
isothiocyanate, nitro, azide or ester. A straightforward scale-up
was possible without further optimization.
Scheme 1. In situ trapping metalation and in situ trapping-exchange using
commercially available continuous flow setups.
Organolithiums are key organometallic intermediates in
organic synthesis.[1] The halogen-lithium exchange reaction is a
standard preparation of organolithium compounds[2]
and
First, we optimized the reaction conditions of the Br/Li-
exchange for 4-bromobenzonitrile (1a) using BuLi as exchange
reagent. Optimized flow conditions[11] without the addition of a
metal salt led after quenching with allyl bromide (2.5 equiv) and
CuCN·2LiCl[12] (10 mol%) to the allylated arene 4a in 17% GC-
yield (Scheme 2). This low yield may be due to the competitive
addition of the newly generated aryllithium or BuLi to the cyano
group. Addition of the well-soluble MgCl2·LiCl to the aryl bromide
1a and further optimization of the flow rate, reaction time and
temperature increased the GC-yield of 4a to 85%. Instead of
MgCl2·LiCl, also ZnCl2 was used as an in situ transmetalating
agent leading to 4a in 82% GC-yield.[13]
provides access after transmetalation to a broad variety of other
useful organometallic species.[3] The scope of halogen-lithium
exchange reactions is limited by the presence of sensitive
functional groups in these unsaturated substrates,[2,3] precluding
the presence of an ester, a nitro, an azide or an isothiocyanato
group.[4] These drawbacks were avoided to some extent by the
use of cryogenic temperatures[2a], special protecting groups[5] or
by fast consecutive transmetalations to less reactive
organometallics.[6] Continuous flow setups have emerged as a
powerful tool for solving synthetic problems.[6b,7] Thus, Yoshida
and others have utilized ultra-fast mixing and precise reaction
time control of custom-made flow setups for achieving the
generation of lithiated arenes bearing ester, isothiocyanate,
cyano or nitro groups.[8] Recently, we have shown that the scope
of metalations of arenes (Ar-H) with a strong base like TMPLi
(TMP = 2,2,6,6-tetramethylpiperidyl) is dramatically increased by
performing these metalations in the presence of metallic salts
(Met-Y).[9] The resulting organometallics (Ar-Met) were much
more stable than the initially generated lithium reagents and
could be broadly functionalized with a variety of electrophiles
(E+). The scope and reaction conditions of this in situ trapping
procedure are further improved by switching from a batch to a
continuous flow setup (Scheme 1a).[9b-c] Aware of the fast rate of
the halogen-lithium exchange,[10] we envisioned an analogous
exchange in situ trapping exchange procedure (Scheme 1b).
Herein, we wish to report a halogen-lithium exchange performed
in the presence of metallic salts for the convenient
functionalization of sensitive (hetero)arenes using continuous
flow technology (Scheme 1b). This in situ trapping halogen-
lithium exchange procedure has the advantage that it provides
excellent functional group tolerance including to the best of our
Scheme 2. Optimization of the in situ exchange-transmetalation sequence in
continuous flow for 4-bromobenzonitrile (1a).
The intermediate magnesium species 2a was used in
various quenching reactions such as iodolysis, addition to an
aldehyde and acylation resulting in functionalized benzonitriles
4b-d in 70-85% yield (Table 1, entries 1-3). The range of
substrates was extended to other bromobenzonitriles which
were converted to the corresponding diarylmagnesium species
(2b-c). After batch-quenching with ketones, allyl bromides or
acyl chlorides in the presence of CuCN·2LiCl the corresponding
products 4e-g were obtained in 68-74% yield (entries 4-6). It
was also possible to perform an I/Li-exchange on 2-iodo-
benzonitrile (1d) using similar conditions, providing the diarylzinc
species 2d. Allylation with 3-bromocyclohexene (3h) afforded
benzonitrile 4h in 80% yield (entry 7).
[*]
M. Ketels, M. A. Ganiek, N. Weidmann,
Prof. Dr. P. Knochel, Ludwig-Maximilians-Universität München,
Department Chemie
Butenandtstrasse 5-13, Haus F, 81377 München (Germany)
E-mail: paul.knochel@cup.uni-muenchen.de
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10.1002/anie.201706609
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COMMUNICATION
on a 10 mmol scale in 76% yield (entry 6) without further
optimization.[16]
Table 1. In situ exchange-transmetalation for sensitive aryl halides of type 1
leading via intermediate diorgano-zincs or -magnesiums of type
polyfunctional arenes of type 4.
2 to
To further demonstrate the broad applicability of in situ
trapping exchange reactions in flow, we investigated the
compatibility of these exchanges with aryl halides bearing
challenging functional groups such as an ester, a ketone, a nitro
and heterocumulene groups e.g. an azide or isothiocyanate.[2-6]
Notably, only halogen-lithium exchanges of o-nitroarenes[2,6] and
an alkenyl iodide containing an aliphatic azide[6] at -100 °C under
batch conditions are known, as well as several flow protocols for
ester-, ketone- and nitro-containing arenes applying ultrafast
micromixing and residence times down to 0.0015 s.[8] Again, we
found that in the absence of a metal salt 4-iodophenyl azide[17]
(5a) decomposes completely performing the reaction in flow.
However, screening of various in situ trapping exchange
conditions, e.g addition of soluble metal salts, flow rate and
temperature,[13] led to the desired allylated phenyl azide 7a in
72% isolated yield (Scheme 3).
metal species
T[°C]; flow rate [mL/min]; t[s]
entry
1
electrophile
product[a]
I2
2a: 0; 6; 2.5[b]
3b[c]
4b: 70%
2
2a[b]
3c[d]
4c: 83%
3
4
2a[b]
3d[e],[f]
4d: 85%
4e: 68%
2b: 0; 9; 1.7[b]
3e[g]
5
6
2c: 0; 9; 1.7[b]
3f[d]
4f: 74%
2c[b]
3g[e],[f]
4g: 74% (76%)[h]
Scheme 3. I/Li-exchange in presence of an azide group under various
reaction conditions a-c.
7
8
2d: 0; 6; 2.5[i]
2e: 0; 12; 25[b]
3h[g]
4h: 80%
4i: 68%
Scale-up of this reaction from 1 to 5 mmol provided the aryl
azide 7a in 60% yield. The analogous m-allyl azidobenzene (7b)
was obtained in 83% yield (Table 2, entry 1). Furthermore, nitro-,
ketone- and ester-groups were tested because of their pivotal
role in organic synthesis and well-known challenges in
organometallic chemistry due to competitive electron transfer
and nucleophilic addition reactions.[4-5] In order to access aryl
organometallics with such functional groups, the best lithiation
exchange reagent was found to be PhLi instead of BuLi.
3i[e],[f]
2f: 0; 6; 2.5[b]
3j: R = NO2
4j: 85%
[j]
9
10 2f[b]
3k: R = CO2Et[j] 4k: 70%
[a] Yield of analytically pure isolated product. [b] Metal species prepared from
the aryl bromide. [c] 2.0 equiv, 10 min, r.t.. [d] 1.1-1.5 equiv, 1-2 h, 0 °C. [e]
1.1 equiv CuCN·2LiCl was added. [f] 1.5 equiv, 1-2 h, 0 °C. [g] 2.5 equiv,
0.1 equiv CuCN·2LiCl, 30 min, 0 °C. [h] Reaction performed on 10 mmol
scale, 3 h, 0 °C. [i] Metal species prepared from the aryl iodide. [j] Negishi
cross-coupling was performed in batch, 1.5 equiv, 10 h, 25 °C after
transmetalation to ZnCl2.
Table 2. In situ exchange-transmetalation for highly sensitive aryl halides of
type 5 leading via intermediate diorgano-zincs or -magnesiums of type 6 to
polyfunctional arenes of type 7.
metal species
entry
electrophile
product[a]
T[°C]; flow rate [mL/min]; t[s]
6b: -40; 12; 1.25[b]
3a[c]
7b: 83%
Remarkably, these exchange reactions proceed at 0 °C in
contrast to the standard halogen-lithium exchanges in batch
which are performed at -78 °C.[2,6a] Also, electron-rich aryl
bromides (1e-f) were in situ transmetalated in the presence of
MgCl2·LiCl and quenched with acyl chloride 3i or subjected to a
Negishi cross-coupling[14] after batch-transmetalation with ZnCl2
using Organ’s catalyst PEPPSI-iPr.[15] The resulting products
4i-k were obtained in 68-85% yield (entries 8-10). While most
examples were performed on a 0.5 mmol scale, these in situ
trapping exchange reactions can be easily scaled up by simply
extending the runtime. Thus, benzophenone 4g was prepared
1
2
3
6c: -20; 12; 1.25[d]
6d: -20; 12; 0.10[d]
3l[c]
7c: 78%
7d: 73%
3a[c]
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10.1002/anie.201706609
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COMMUNICATION
The
preparation
of
polyfunctional
heterocyclic
organometallics is of key importance for the pharmaceutical and
agrochemical industry.[18,19] Thus, sulfur- and nitrogen-containing
heterocyclic halides were subjected to in situ trapping exchange
reactions. For instance, 3-bromothiophene (8a) was converted
to the reactive diheteroarylmagnesium species 9a. Further batch
addition to ketone 3h led to tertiary alcohol 10a in 77% yield
(Table 3, entry 1). To expand the range of substrates, different
pyridines and pyrimidines were subjected successfully to the
Br/Li-exchange. Thus, pyridine derivatives 8b-c underwent the in
situ trapping exchange (entries 2-3). Quenching of the bis-
pyridyl-zinc and -magnesium reagents 9b and 9c in batch led to
the tertiary alcohol 10b and allylated picoline 10c in 62-63%
yield (entries 2-3). Furthermore, 5-bromopyrimidine (8d) and the
fully substituted iodopyrimidine 8e were transmetalated in situ
using short reaction times (0.06-1.25 s) at -40-0 °C (entries 4-5).
By using PhLi, an ester was tolerated providing the acylated[12]
pyrimidine 10e in 68% yield (entry 5).
4
5
6e: -20; 20; 0.06[b]
3m[e]
7e: 60%
7f: 63%
6e[b]
3n[f]
6
7
6f: -60; 16; 0.08[b]
6g: -40; 20; 0.06[d]
3f[e]
3h[c]
3o[f]
7g: 73%
7h: 78%
7i: 70%
Table 3. In situ exchange-transmetalation for highly sensitive heteroaryl
halides of type 8 leading via intermediate diorgano-zincs or -magnesiums of
type 9 to polyfunctional heteroarenes of type 10.
8
9
6h: -40; 16; 0.94[b]
6i: -20; 12; 1.25[d]
metal species
T[°C]; flow rate [mL/min]; t[s]
entry
electrophile
product[a]
3p[c]
3a[c]
7j: 68%
7k: 65%
10 6j: -60; 18; 0.07[d]
11 6k: -60; 9; 0.15[d]
1
9a: 0; 6; 10[b]
3f[d]
10a: 77%
3a[c]
7l: 67%
2
3
9b: 0; 18; 0.83[e]
9c: 0; 6; 53[b]
3f[d]
10b: 62%
10c: 63%
12 6l: -20; 16; 0.94[d]
3q[f]
7m: 63%
3a[f]
[a] Yield of analytically pure isolated product. [b] Metal species prepared from
the aryl iodide. [c] 2.5 equiv, 0.1 equiv CuCN·2LiCl, 30 min, 0 °C. [d] Metal
species prepared from the aryl bromide. [e] 1.1-1.5 equiv, 1-2 h, 0 °C. [f]
1.5 equiv, 1.1 equiv CuCN·2LiCl, 1-2 h, 0 °C., 30 min, 0 °C.
4
9d: 0; 12; 1.25[b]
3h[f]
10d: 68%
Interestingly, also with PhLi, the exchange is faster than a
competitive transmetalation of PhLi therefore allowing an
efficient generation of diarylzincs and -magnesiums (6c-h). Thus,
bis-(nitroaryl)zincs 6c-f were generated from the corresponding
aryl halides 5c-f. Allylation, acylation and addition to indole
aldehyde 3m or ketone 3f in batch furnished the desired
functionalized nitro arenes 7c-g in 60-78% yield (entries 2-6).
Similarly, the aryl bromide 5g, containing a ketone was
functionalized by in situ trapping exchange reactions at -40 °C in
the presence of ZnCl2. Typical quenching conditions led to the
allylated product 7h from the diarylzinc 6g in 78% yield (entry 7).
Furthermore, ethyl 4-iodobenzoate (5h) led to 7i via the
diarylmagnesium 6h in 70% (entry 8). It was further possible to
perform an Br/Li-exchange on aryl bromides bearing an p-, m-,
and o-isothiocyanate moiety without subsequent additions to the
electrophilic isothiocyanate.[4d,e] After various copper mediated
allylations or acylations,[12] the desired products 7j-m were
obtained in 63-68% yield (entries 9-12).
5
9e: -40; 20; 0.06[e]
3r[c]
10e: 68%
[a] Yield of analytically pure isolated product. [b] Metal species prepared from
the aryl bromide. [c] 1.5 equiv, 1.1 equiv CuCN·2LiCl, 1-2 h, 0 °C. [d]
1.1 equiv, 1-2 h, 0 °C. [e] Metal species prepared from the aryl iodide. [f]
2.5 equiv, 0.1 equiv CuCN·2LiCl, 30 min, 0 °C.
In conclusion, we report a halogen-lithium exchange in the
presence of various metal salts (ZnCl2, MgCl2·LiCl) on a broad
range of sensitive bromo- or iodo-(hetero)arenes using BuLi or
PhLi as exchange reagent and
a commercially available
continuous flow setup. The resulting diarylmagnesium or
diarylzinc species were trapped with various electrophiles
resulting in the formation of polyfunctional (hetero)arenes in high
yields. This methodology enabled the functionalization of arenes
containing highly sensitive groups such as an isothiocyanate,
nitro, azide or ester since the reaction of the functional groups
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10.1002/anie.201706609
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COMMUNICATION
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Acknowledgements
We thank Dr. Benjamin Martin (Novartis Pharma AG) for fruitful
discussions, SFB 749 for financial and Vapourtec for technical
support. M. K. thanks the Foundation of German Business and
M. A. G. the German Academic Scholarship Foundation for
fellowships.
[17] For previous flow reactions with unstable aza-compounds, see: a) C. J.
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2nd Ed, Wiley, Weinheim, 2003.
Keywords: flow chemistry• lithiation• magnesiation• zincation• in
situ trapping
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[13] See Supporting Information for
a detailed survey of attempted
conditions and mixing dependencies as well as the order of exchange
and transmetalation.
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COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 2:
COMMUNICATION
Marthe Ketels, Maximilian A. Ganiek,
Niels Weidmann, Benjamin Martin, Paul
Knochel*
Page No. – Page No.
Preparation of Polyfunctional
Diorgano-Magnesium and -Zinc
Reagents Using In Situ Trapping
Halogen-Lithium Exchange of Highly
Functionalized (Hetero)aryl Halides in
Continuous Flow
Flow (ex)changes everything: A halogen-lithium exchange using BuLi or PhLi was
performed in the presence of various metal salts (ZnCl2, MgCl2·LiCl) on a broad
range of sensitive bromo- or iodo-hetero(arenes) using a commercially available
continuous flow setup. The resulting diarylmagnesium or diarylzinc species were
trapped with various electrophiles resulting in the formation of polyfunctional
(hetero)arenes in high yields and with excellent chemoselectivity.
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