The Halogen–Samarium Exchange Reaction: Synthetic Applications and Kinetics

Article abstract of DOI:10.1002/anie.201814373

Fast I/Sm and Br/Sm exchanges take place when various aromatic or heterocyclic iodides and bromides are treated with nBu₂SmCl?4 LiCl and nBu₃Sm?5 LiCl, respectively. The resulting organosamarium reagents were efficiently quenched with aldehydes, ketones, and imines. Also, they undergo acylations when treated with N,N-dimethylamides leading to ketones. The rate of the Br/Sm exchange for a typical aryl bromide was determined and found to be 8.5×10⁵ faster than the Br/Mg exchange, indicating that the rate of a metal-exchange is related to the ionic character of the carbon–metal bond and to the metal electronegativity.

Full text of DOI:10.1002/anie.201814373

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Accepted Article
Title: The Halogen-Samarium Exchange Reaction. Synthetic
Applications and Kinetics
Authors: Lucile Anthore-Dalion, Andreas D. Benischke, Baosheng Wei,
Guillaume Berionni, and Paul Knochel
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The Halogen-Samarium Exchange Reaction. Synthetic
Applications and Kinetics
Lucile Anthore-Dalion+,a, Andreas D. Benischke+,a, Baosheng Wei+,a, Guillaume Berionni*b, Paul
Knochel*,a
Fast I/Sm- and Br/Sm-exchanges take place when treating
various aromatic or heterocyclic iodides and bromides with
nBu2SmCl·4LiCl or nBu3Sm·5LiCl respectively. The resulting
organosamarium reagents were efficiently quenched with
aldehydes, ketones, and imines. Also, they undergo acylations
when treated with N,N-dimethylamides leading to ketones. The
rate of the Br/Sm-exchange on a typical aryl bromide was
determined and showed that it was 8.5·105 faster than the Br/Mg-
exchange, supporting that the rate of a metal-exchange is related
to the ionic character of the carbon-metal bond and to the metal
electronegativity.
Scheme 1. Relationship between the Pauling electronegativity12 of the metal
and the rate of the exchange reaction.
For proving this relation, we turned our attention to samarium
which has an electronegativity of 1.17 compared to 1.10 for
lanthanum and therefore we predicted that the halogen/samarium
exchange should be slower than the halogen/lithium, the
halogen/lanthanum and the halogen/cerium exchanges, and that
the functional group tolerance of the resulting arylsamarium(III)-
reagent should be better than those of the corresponding
lanthanum(III)-reagents. Moreover, organosamarium reagents
already proved their utility as nucleophiles, but only few
preparation methods of these organometallics have been
reported.13 A halogen/samarium-exchange will then provide a
new access to these reagents.
Herein, we report the preparation of halogen/samarium exchange
reagents and their use for performing efficient I/Sm- and Br/Sm-
exchanges. We describe also the reaction of these new aryl- and
heteroarylsamarium reagents with various electrophiles and
provide an approximate rate for the Br/Sm-exchange. Thus, we
have first prepared SmCl3·2LiCl starting from SmCl3·6H2O in
analogy to the preparation of LaCl3·2LiCl.14 Similar concentration
of SmCl3·2LiCl in THF (0.33M) was obtained. After extensive
screening, we identified three useful exchange reagents with the
tentative formulas nBu2SmCl·4LiCl (1), nBu2SmMe·5LiCl (2) and
nBu3Sm·5LiCl (3) obtained by the reaction of SmCl3·2LiCl with the
appropriate amounts of nBuLi and MeLi (Scheme 2).
Organometallics are key nucleophilic reagents for organic
synthesis,1 and their efficient preparation is an important synthetic
task. Besides the oxidative addition2 of a metal to an organic
halide, the halogen-metal exchange reaction3-5 is one of the most
important synthetic routes to aryl and heteroaryl organometallics.
The halogen/lithium-exchange3 is certainly the most used in
synthesis, however the functional group tolerance of aryllithiums
is rather moderate.4 As an alternative, the halogen/magnesium-
exchange5 is compatible with large number of functional groups,
nevertheless this exchange reaction is considerably slower than
the halogen/lithium exchange.6 Further the iodine/zinc exchange
proceeds at an even slower rate.7 Besides ate-metal reagents
such as manganates8 and cuprates9 undergo an exchange, but
the atom economy10 of these reactions is moderate.
Recently, we have reported a new halogen/lanthanum exchange
reaction and have found that this exchange is at least 106 faster
than the halogen/magnesium exchange. Also the I/La- or Br/La-
exchange displays a better functional group tolerance than the
halogen/lithium exchange.11 This led us to postulate that the rate
of the halogen/metal exchange depends on the ionic character of
the carbon-metal bond of the exchange reagent, and that the
functional group compatibility also depends on this ionic character
and therefore on the electronegativity12 of the metal (Scheme 1).
[a]
Dr. Ing. Lucile Anthore-Dalion,[+] Dr. A. D. Benischke,[+] Dr. B. Wei[+]
Prof. Dr. P. Knochel, Department Chemie, Ludwig-Maximilians-
Universität München Butenandtstrasse 5–13, Haus F, 81377
München (Germany)
E-mail: paul.knochel@cup.uni-muenchen.de
[b]
[+]
Dr. G. Berionni, Namur Institute of Structured Matter, University of
Namur, 61, rue de Bruxelles, B-5000 Namur (Belgium)
E.mail: guillaume.berionni@unamur.be
Scheme 2. Preparation of alkylsamarium reagents 1, 2 and 3.
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
First, nBu2SmCl·4LiCl (1) proved to undergo efficient exchange
reactions with aryl iodide of type 4.15 Aryl iodides 4a-d bearing
fluorinated or chlorinated substituents were converted into the
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corresponding diarylsamarium chlorides 5a-d within only 5 min at
−50 °C in THF. Subsequent trapping reactions with carbonyl
derivatives led to alcohols 6a-d in 65-87% yield. Similarly, 1-
bromo-4-iodobenzene (4e) underwent a selective I/Sm-exchange
with nBu2SmCl·4LiCl (1, 0.60 equiv) under those conditions
producing bis(4-bromophenyl)samarium(III) chloride (5e) in ca.
90% GC-yield, and its reaction with 2,4-dimethylpentan-3-one led
to alcohol 6e in 75% yield. Remarkably, no products resulting from
a competitive Br/Sm-exchange was detected. Aryl iodides bearing
electron-donating groups such as p-anisyl iodide (4f) also reacted
under those conditions, to give after trapping with a ketone the
tertiary alcohol 6f in 81% yield. More sensitive groups such as
nitriles or tert-butyl esters were also tolerated as exemplified by
the reaction of 3-iodobenzonitrile (4g), 4-iodobenzonitrile (4h) and
t-butyl 4-iodobenzoate (4i) which, after the I/Sm-exchange
followed by trapping reactions with carbonyl derivatives, led to
alcohols 6g-i in 62-98% yield. The result with the iodo-ester
derivative 4i is particularly instructive, since such substituents
were not tolerated in the case of the halogen-lanthanum
exchange showing that organosamariums are compatible with
more sensitive functional groups compared to the corresponding
organolanthanum derivatives.11 This observation is a strong
argument in favor of a dependence of the functional group
compatibility on the ionic character of the carbon-metal bond.
Finally, 2-iodothiophene (4j) and 3-iodopyridine (4k) and indole
(4l) reacted with nBu2SmCl·4LiCl (1, 0.60 equiv), furnishing
diheteroarylsamarium derivatives 5j-l which were further
transformed into alcohols 6j-l in 83-85% yield (Scheme 3).
Then, we turned our attention to the Br/Sm-exchange involving
the use of less expensive aryl bromides compared to aryl iodides.
Whereas nBu2SmCl·4LiCl (1) was a suitable exchange reagent
for I/Sm-exchange reactions, no Br/Sm-exchange with 4-
bromoanisole (7a) took place under those conditions even after
extended reaction time (Table 1, entry 1). In contrast, the mixed
species nBu2SmMe·5LiCl (2, 0.70 equiv), inspired from our
previous work on lanthanum,11 reacted at −30 °C, leading to full
conversion of 4-bromoanisole (7a) into the corresponding
diaryl(methyl)samarium reagent
8 within 1 h. Subsequent
reaction with ketone 9 provided the tertiary alcohol 10a in 69%
yield (Table 1, entry 2). Interestingly, we noticed that
nBu3Sm·5LiCl (3) exchanged all its three butyl residues, whereas
the related nBu3La·5LiCl was able to exchange only one butyl
residue.11a,c Indeed, the exchange of nBu3Sm·5LiCl (3, 0.40
equiv) with 7a led to full conversion into the corresponding
triarylsamarium derivative 11a within only 15 min at −30 °C. The
trapping reaction with ketone 9 then furnished alcohol 10a in 90%
yield (Table 1, entry 3).16 Interestingly, whereas the Br/La-
exchange on 7a was completed after less than 5 min at −50 °C ,11
the Br/Sm-exchange required 15 min at −30 °C , indicating a
somewhat slower rate than the corresponding Br/La-exchange.11
Table 1. Optimization of alkylsamarium(III) species for Br/Sm-exchanges.
Entry
Exchange
reagent
Conditions
Conversion
of 7a (%)
Yield (%)
1
2
3
nBu2SmCl·4LiCl
(1, 0.6 equiv)
>1 h
−50 °C
0
-
nBu2SmMe·5LiCl
(2, 0.70 equiv)
1 h
−30 °C
>95
>95
69
90
nBu3Sm·5LiCl
(3, 0.40 equiv)
15 min
−30 °C
Ar = 4-chlorophenyl
The exchange reagent nBu3Sm·5LiCl (3) was then used to
explore the scope of the Br/Sm-exchange. First, 1-bromo-2-
fluorobenzene (7b), underwent an exchange within 5 min at
−30 °C, furnishing triarylsamarium derivative 11b, and
subsequent trapping with a ketone provided the tertiary alcohol
10b in 85% yield. Interestingly,11a the same reaction with nBuLi
led only to degradation of aryl bromide 7b. Other fluorinated aryl
bromides 7c-d furnished the corresponding organosamarium
compounds 11c-d, and their reaction with a ketone or 3-
bromobenzaldehyde, provided alcohols 10c-d in 65-71% yield.
Similarly, the exchange on electron-rich aryl bromides such as 4-
bromoanisole (7a), 3,4,5-trimethoxybromobenzene (7e), 4-
bromothioanisole (7f), 4-bromo-1,2-(methylenedioxy)benzene
(7g) and 1-bromo-3-trimethylsilylbenzene (7h) with nBu3Sm·5LiCl
(3, 0.40 equiv) proceeded efficiently to form triarylsamarium
compounds 11a and 11e-h, although longer reaction times of up
to 30 min were required. After trapping with aldehydes or ketones,
alcohols 10e-j were obtained in 74-88% yield. Ellman tert-butyl
Scheme 3. I/Sm-exchanges with aryl iodides and subsequent trappings. (a)
Yields in our report are all calculated based on the amount of electrophile.
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sufinamide reacted with tris(1-naphthyl)samarium (11i), resulting
from the Br/Sm-exchange on 1-bromonaphthalene (7i), to furnish
the amine derivative 10k in 70% (dr=7:3). Finally, heteroaryl
bromides 7j-m underwent the Br/Sm-exchange providing
tri(heteroaryl)samarium derivatives 11j-m. These reacted with
either ketones or aldehydes, and alcohols 10l-o were obtained in
52-87% yields.
The difference of reactivity between nBu2SmMe·5LiCl (2) and
nBu3Sm·5LiCl (3) already observed with 4-bromoanisole (7a)
(Table 1) was confirmed in the case of substrates 7b,e-f. Their
reaction with 2 proceeded with 10-20% lower yields than with 3.
This difference is mainly explained by the formation of side-
products when nBu2SmMe·5LiCl (2) is used as exchange reagent,
namely the Ar-Me coupling product as well as an alcohol resulting
from the methyl attack onto the ketone (Scheme 4).
Scheme 5. I/Sm-exchange on 4-iodobenzonitrile (4h) with 2 at −30 °C
Finally, we wish to report the addition of triarylsamarium
derivatives of type 11 on N,N-dimethylamides of type 14 led
selectively to ketones of type 15, with only trace amounts of the
overaddition alcohol.11,17 This acylation was applied to various
aryl bromides and furnished, after Br/Sm-exchange followed by
addition to benzamides 14a-d, ketones 15a-d in 61-87% yields.
Interestingly, the reaction of tris(3-(trifluoromethoxy)phenyl)-
samarium (11f), obtained via an exchange reaction of aryl
bromide 7d and nBu3Sm·5LiCl (3), with 4-cyano-N,N-
dimethylbenzamide (14e) led selectively to ketone 15e. No
addition on the nitrile group was detected (Scheme 6).
Scheme 6. Acylation of Ar3Sm 13 with N,N-dimethylamides of type 14.
These results indicated clearly that the Br/Sm-exchange is slower
than the Br/La-exchange and confirm our intuition18 that the rate
of a halogen/metal-exchange is related to the ionic character of
the carbon-metal bond (and therefore related to the metal
electronegativity). To confirm this, we have quantitatively
determined the rate of the Br/Sm-exchange reaction. Thus,
absolute rate constant of Br/Sm-exchange reaction of 2,6-
dimethylbromobenzene 16 with nBu2SmMe·5LiCl 2 at 0 °C in THF
(Scheme 7) was calculated with the Eyring equation by
extrapolation of kinetic measurements performed between ‒40
and ‒60 °C (see the SI). The very small activation enthalpy H‡
of a few kJ.mol-1 indicated that the reaction is entropy-controlled.
Scheme 4. Br/Sm-exchanges with aryl iodides and subsequent trapping
reactions. (a) nBu2SmMe·5LiCl (0.70 equiv) was used. (b) the corresponding
Ellman tert-butyl sufinyl imine was used as trapping reagent.
Unfortunately, the Br/Sm-exchange was too slow compared to the
corresponding I/Sm-exchange, to tolerate either nitriles or esters.
Indeed, a faster I/Sm-exchange reaction on 4-iodobenzonitrile
(4h) at −30 °C with nBu2SmMe·5LiCl (2) was possible and led,
after trapping with ketone 12 to the tertiary alcohol 13 in 86% yield
(Scheme 5).
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Scheme 7. Comparison of the kinetics of Br/Sm and Br/Mg exchange reactions
in THF at 0 °C for 1 M solutions of reagents.
Quantitative comparison with the second-order rate constant of
Br/Mg exchange with the turbo-Grignard reagent iPrMgCl·LiCl
with 16 (Scheme 7) revealed that the Br/Sm exchange proceeds
8.5 × 105 faster.19 The Br/La exchange kinetics on 16 with the
reagent nBu2LaMe·5LiCl could not previously be measured even
in highly diluted solutions at -50°C because the exchange reaction
proceeded in only a few seconds. The t1/2 for Br/Sm-exchange on
16 is equal to 3 min under the same experimental conditions
(Figure S1 in SI), which led us to the conclusion that the Br/La
proceeds much faster (factor 10 or greater) than the Br/Sm
exchange for which only the lower limit is known.
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reaction on various aryl and heteroaryl iodides and bromides. The
resulting organosamarium reagents proved to be very useful
organometallic intermediates adding efficiently to aldehydes and
ketones and undergoing acylations with N,N-dimethylamides.
Compared to the corresponding aryllanthanum reagents prepared
by a related exchange reaction,11a-b a better functional group
tolerance was observed, an ester or an pyridyl ring being tolerated.
Furthermore, a kinetic study allowed us to determine the rate of
the Br/Sm-exchange, confirming that it is considerably faster than
the Br/Mg-exchange (8.5 × 105 faster) but somewhat slower than
the Br/La-exchange. This clearly supports that the metal
electronegativity is an important factor for predicting the rate of a
halogen/metal exchange.
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We thank the LMU Munich for financial support and the Humboldt
foundation for fellowship to L. A.-D. We also thank Albemarle
(Frankfurt) and BASF (Ludwigshafen) for the generous gift of
chemicals.
Keywords: halogen-samarium exchange • samarium • kinetics •
functionalized organometallics • lanthanides
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different. A summary of all samarium reagents prepared and tested are
included as Table S1 in the SI.
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202-204; Angew. Chem. 2008, 120, 208-210; b) L. Shi, Y. Chu, P.
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Entry for the Table of Contents (Please choose one layout)
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Lucile Anthore-Dalion+, Andreas D.
Benischke+, Baosheng Wei+, Guillaume
Berionni*, Paul Knochel*
Page No. – Page No.
The Halogen-Samarium Exchange
Reaction. Synthetic Applications and
Kinetics
Samarium stories. Aryl and heteroaryl iodides and bromides were converted to the
corresponding samarium(III)-reagents via halogen-samarium exchanges using nBu2SmCl
and nBu3Sm respectively. Remarkably, in contrast to the previously reported preparation
of aryllanthanum species, samarium reagents tolerates more functional groups (such as
an ester). This was explained by the higher electronegativity of samarium (compared to
lanthanum) which leads to an organosamarium species with a less ionic carbon-metal
bond. We found also that the Br/Sm-exchange is 8.5.105 faster than the Br/Mg-exchange.
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