Untangling the Complexity of Mixed Lithium/Magnesium Alkyl/Alkoxy Combinations Utilised in Bromine/Magnesium Exchange Reactions

Article abstract of DOI:10.1002/anie.202016422

While it is known that the addition of Group 1 alkoxides to s-block organometallics can have an activating effect on reactivity, the exact nature of this effect is not that well understood. Here we describe the activation of sBu₂Mg towards substituted bromoarenes by adding one equivalent of LiOR (R=2-ethylhexyl), where unusually both sBu groups can undergo efficient Br/Mg exchange. Depending on the substitution pattern on the bromoarene two different types of organometallic intermediates have been isolated, either a mixed aryl/alkoxide [{LiMg(2-FG-C₆H₄)₂(OR)}₂] (FG=OMe; NMe₂) or a homoaryl [(THF)₄Li₂Mg(4-FG-C₆H₄)₄] (FG=OMe, F). Detailed NMR spectroscopic studies have revealed that these exchange reactions and the formation of their intermediates are controlled by a new type of bimetallic Schlenk-type equilibrium between heteroleptic [LiMgsBu₂(OR)], alkyl rich [Li₂MgsBu₄] and Mg(OR)₂, with [Li₂MgsBu₄] being the active species performing the Br/Mg exchange process.

Full text of DOI:10.1002/anie.202016422

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Communications
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How to cite: Angew. Chem. Int. Ed. 2021, 60, 7626–7631
International Edition:
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doi.org/10.1002/anie.202016422
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Alkoxides Very Important Paper
Untangling the Complexity of Mixed Lithium/Magnesium Alkyl/
Alkoxy Combinations Utilised in Bromine/Magnesium Exchange
Reactions
Leonie J. Bole, Neil R. Judge, and Eva Hevia*
In memory of Victor Snieckus
Abstract: While it is known that the addition of Group
1 alkoxides to s-block organometallics can have an activating
effect on reactivity, the exact nature of this effect is not that well
understood. Here we describe the activation of sBu2Mg
towards substituted bromoarenes by adding one equivalent of
LiOR (R = 2-ethylhexyl), where unusually both sBu groups
can undergo efficient Br/Mg exchange. Depending on the
substitution pattern on the bromoarene two different types of
organometallic intermediates have been isolated, either
a mixed aryl/alkoxide [{LiMg(2-FG-C6H4)2(OR)}2] (FG =
OMe; NMe2) or a homoaryl [(THF)4Li2Mg(4-FG-C6H4)4]
(FG = OMe, F). Detailed NMR spectroscopic studies have
revealed that these exchange reactions and the formation of
their intermediates are controlled by a new type of bimetallic
Schlenk-type equilibrium between heteroleptic [LiMgsBu2-
(OR)], alkyl rich [Li2MgsBu4] and Mg(OR)2, with
[Li2MgsBu4] being the active species performing the Br/Mg
exchange process.
uted to the formation of mixed-metal aggregates.[5] Further-
more, Richey Jr has also found that aryl bromides and iodides
react with equimolar mixtures of dialkylmagnesium reagents
(R2Mg) and alkali-metal alkoxides to furnish the desired
products of Mg-halogen exchanges; whereas R2Mg reagents
on their own are inert towards these substrates.[6] While the
formation of alkali-metal magnesiates has been postulated in
some of these studies, tangible information on the constitu-
tion of these species is limited.
Expanding the synthetic potential of these bimetallic
combinations, recently we reported sBu2Mg·2LiOR (R = 2-
ethylhexyl) enables fast regioselective Mg/Br exchange of
dibromo(hetero)aromatics[7] as well as exchanges of less
activated aryl chlorides.[8]
Removing some of the mystery in this chemistry, here we
report our findings assessing the constitution of these
bimetallic alkyl/alkoxide combinations and their reactivity
towards bromoarenes, uncovering a complex equilibrium
between different bimetallic species where the final compo-
sition of the Br/Mg exchange intermediates appears influ-
enced by the substitution pattern in the aromatic substrate.
We started our studies by reassessing the reactivity of 2-
bromoanisole (1a) towards different combinations of Li and
Mg alkyl/alkoxide mixtures (Table 1).
T
ypified by the LIC-KOR superbase reagent, adding (or
more accurately, co-complexing) Group 1 alkoxides to s-
block organometallics can cause profound reactivity enhance-
ment of the latter, enabling unique chemical profiles.[1] These
type of reagents, have proved to be exceptionally powerful
bases for the deprotonative metallation of organic sub-
strates.[2] Interestingly, despite their numerous applications,
the constitution(s) and structure(s) of these bimetallic species
in solution have remained concealed for over five decades,
being a matter of intrigue and debate.[3] Shedding important
light in this matter, recent seminal work by Klett has revealed
that in the case of LiNp/KOtBu combinations (Np = neopen-
tyl), complicated aggregates are formed where the alkyl group
Using our originally reported combination of sBuLi and
Mg(OR)2 in a 2:1 ratio[8] led to the full magnesiation of two
equivalents of 1a after 40 minutes at room temperature
Table 1: Screening of Mg/Br-exchange capabilities of magnesium
reagents towards 2-bromoanisole (1a).
coordinates in a unique manner to both Li and K, leading to
[4]
À
hybrid Li/K C bond polarity. Within Group 1/Group 2
bimetallic combinations, early work by Screttas showed that
the addition of magnesium alkoxides to alkyl/aryl sodium (or
potassium) compounds induces the solubilisation of the
heavier alkali organyl in non-polar solvents, a benefit attrib-
Entry
Exchange reagent[a]
n
Yield [%][b]
1
2
3
4
5
6
2sBuLi + Mg(OR)2
2LiOR + sBu2Mg
LiOR + sBu2Mg
LiOR’ + sBu2Mg
sBu2Mg
2
2
1
1
99
99
99
20
5
0
[*] Dr. L. J. Bole, N. R. Judge, Prof. Dr. E. Hevia
Department fꢀr Chemie und Biochemie, Universitꢁt Bern
3012 Bern (Switzerland)
LiOR + sBu2Mg + 1.5Mg(OR)2
2[c]
0
[a] R=2-ethylhexyl and R’=CH2CH2N(Me)CH2CH2NMe2. [b] Yields have
been determined by GC analysis of reaction aliquots after an aqueous
quench using hexamethylbenzene as internal standard. Formation only
of anisole and unreacted 2-bromoanisole were observed. [c] n=2LiOR +
1.5Mg(OR)2. Optimized conditions highlighted in bold.
E-mail: eva.hevia@dcb.unibe.ch
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016422.
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(entry 1). The same conversion was observed when mixing
sBu2Mg with two equivalents of LiOR, hinting at the
formation of the same type of active mixed-metal organome-
tallic species, which contrasts with the almost total lack of
reactivity of sBu2Mg towards 1a (5% yield, entry 5). Inter-
estingly, 1a can also undergo quantitative Br/Mg exchange
when using a 1:1 mixture of sBu2Mg and LiOR (entry 3). This
observation is consistent with our previous work where
[{LiMg(2-anisyl)2(OR)}2] (2a) and LiOR are quantitatively
formed as products of the reaction of 1a with a 2:1 mixture of
sBuLi and Mg(OR)2 in toluene.[7] Exposing the key role of the
alkoxide employed to mediate this transformation, using the
relevant LiOR’ derived from 2-{[2-(dimethylamino)ethyl]me-
thylamino}ethanol (dmem(H)) only a modest 20% yield of
the magnesiation product was observed.[9] Studies using
different combinations of Li and Mg alkoxides also revealed
that when 1.5 equiv of Mg(OR)2 were added to the initial
LiOR/sBu2Mg combination the Br/Mg exchange totally shut-
down (entry 6, Table 1).
Figure 1. Molecular structures of 2b, 3a and 3b with displacement
Intrigued by these findings we next tested the reactivity of
equimolar amounts of sBu2Mg and LiOR with several
substituted bromoarenes (Scheme 1). 1H NMR monitoring
studies showed that the reaction with 2-bromoanisole led to
the quantitative formation of 2a. Similarly, when 2-bromo-
dimethylaniline was employed [{LiMg(2-NMe2-C6H4)2-
(OR)}2] (2b) was obtained which could be isolated in
a 46% yield as a solid.[10] X-ray crystallographic studies
established the bimetallic dimeric constitution of 2b, akin to
that reported for 2a (Figure 1).[7] Featuring a centrosymmetric
four-membered {MgOMgO} ring, in 2b each Mg binds to the
ipso C of two aryl groups, occupying the position previously
filled by a Br atom. In contrast, each Li coordinates to one
alkoxide bridge in addition to two NMe2 groups.
ellipsoids at 50% probability, all H atoms omitted, and with C atoms
in 2-ethylhexyl substituents and THF molecules drawn as wire frames
for clarity. Equivalent atoms of 2b generated by (1Àx, 1Ày, 1Àz)
symmetry operation.[20]
Adding further interest, when sBu2Mg and LiOR were
reacted with two equivalents of 4-bromo-anisole and 4-
bromo-fluorobenzene an entirely new type of mixed-metal
product was obtained (Scheme 1), namely the homoleptic
tetra(aryl)lithium magnesiates [(THF)4Li2MgAr4] (Ar= 4-
OMe-C6H4, 3a; Ar= 4-F-C6H4, 3b) in 74 and 60% isolated
yields, respectively.[11] In this case the Br/Mg exchange
reaction occurs with concomitant elimination of Mg(OR)2
1
(determined by H and 13C NMR).[10] The molecular struc-
tures of 3a and 3b were established by X-ray crystallography
(Figure 1). Diverging markedly from 2a and 2b, 3a and 3b
exhibit a monomagnesium constitution, with a 2:1 Li:Mg ratio
where all anionic groups are 4-substituted aryls resulting from
Br/Mg exchange. Both structures contain a C4-coordinated
Mg atom flanked by two THF-solvated Li cations. Unlike in
2a and 2b, here the lithium centres do not interact with the
donor substituents on the aromatic rings, which are now
located too remote, and instead they adopt a perpendicular
disposition to the aromatic rings, binding to their ipso C in
a similar manner as previously reported by Weiss for the
higher order sodium magnesiate [(PMDETA)2Na2MgPh4]
(PMDETA = N,N,N’,N’’,N’’-pentamethyldiethylenetria-
mine).[12]
1H NMR monitoring of the reactions of the LiOR/sBu2Mg
bimetallic combination with these different bromoarenes
showed that 2a,b and 3a,b are in each case the only
organometallic product of the reaction and that their
formation did not result from a disproportionation during
the crystallisation process. It should also be noted that these
species are the sole organometallic species present in solution
when using the original 2sBuLi/Mg(OR)2 combination,
offering further support to the in situ formation of the same
exchange reagent.[7]
Scheme 1. Reactions of sBu2Mg·LiOR (R=2-ethylhexyl) with 2-bromo
and 4-bromo-substituted arenes (258C, toluene, 30 min, using 2 equiv-
alents of the relevant bromoarene). For 3a and 3b amounts of THF
were introduced to facilitate their crystallisation.[10]
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Scheme 2. Comparative electrophilic interception studies of 2a–2b
and 3a,b prepared in situ via Br/Mg exchange with 0.6 equiv of the
2sBuLi·Mg(OR)2 combination. [a] ZnCl2 (1.3 equiv), E+ (0.8 equiv),
Pd(OAc)2 (4 mol%), SPhos (8 mol%). [b] E+ (1.2 equiv), 258C, 12 h.[10]
Figure 2. Proposed equilibrium of [LiMgsBu2(OR)] (6) with [Li2MgsBu4]
1
(7) and Mg(OR)2. Section of H NMR spectra (from 0.5 to À0.5 ppm)
in [D8]toluene of: a) LiOR + sBu2Mg; b) 2 sBuLi + sBu2Mg and c) LiOR
+ sBu2Mg + 1.5 Mg(OR)2. d) Section of 1H-EXSY NMR spectrum of
LiOR + sBu2Mg in [D8]toluene.
To ascertain whether the different composition of 2a,b
and 3a,b could have an influence on their further function-
alization, electrophilic interception studies were carried out
(Scheme 2).[10]
the same chemical shift as that found in the sBu2Mg/LiOR
Thus, transmetallation of 2a, 3a and 3b with ZnCl2
followed by Pd catalysed cross-coupling with 4-iodo-trifluor-
omethylbenzene or ethyl-3-iodobenzoate led to the formation
of asymmetric bis(arenes) 4a, 5a and 5b in good yields
ranging from 62 to 83%. In addition, 2b reacts with a Weinreb
amide to form acylation product 4b in a 56% yield. While
these quenching studies demonstrate the synthetic utility of
mixture (Figure 2b).
1
Furthermore, H-EXSY NMR experiments indicate that
lithium magnesiates 6 and 7 are in equilibrium with each other
(Figure 2d). An explanatory possible equilibrium is depicted
in Figure 2, with the combination of equimolar amounts of
sBu2Mg and LiOR forming mixed alkyl(alkoxide) co-com-
plex [LiMgsBu2(OR)] (6) which in turn is in equilibrium with
tetra(alkyl) [Li2MgsBu4] (7) and Mg(OR)2. Adding credence
to this interpretation it was found that adding increasing
amounts of Mg(OR)2 to a sBu2Mg/LiOR combination caused
the gradual decrease of 7 in solution, until it disappears
completely after 1.5 equiv of Mg(OR)2 in total. This suggests
that under these conditions the equilibrium lies towards the
formation of 6. While studies on homo(alkyl) alkali-metal
magnesiates have described that higher and lower order
species can be in equilibrium with each other,[14] the type of
equilibrium depicted in Figure 2 for mixed alkyl/alkoxide
species is, as far as we can ascertain, unknown in lithium
magnesiate chemistry. It bears a strong resemblance with the
well-established classical Schlenk equilibrium in Grignard
reagent chemistry where heteroleptic RMgX reagents are in
equilibrium with the homoleptic species, MgR2 and MgX2.
Related to these findings, OꢀHara has recently found that
a sodium dialkyl magnesiate supported by a biphenolate
ligand [Na2Mg(biphen)Bu2] is in equilibrium with all alkoxide
sodium magnesiate [Na2Mg(biphen)2] along with Na2MgBu4
although, in this case, all species involved within the
equilibrium have the same 2:1 alkali metal:magnesium
ratio.[13]
The high solubility of [LiMgsBu2(OR)] (6) and
[Li2MgsBu4] (7) in hydrocarbon solvents precluded their
crystallization. In the case of 6, using the alkoxide LiOR’
[R’ = CH2CH2N(CH3)CH2CH2N(CH3)2], which contains two
amide groups, in a co-complexation reaction with one
equivalent of sBu2Mg led to the isolation of dimeric
[LiMgsBu2(OR’)]2 (8) in a 35% crystalline yield (Figure 3).
X-ray crystallographic studies revealed a step ladder motif for
8, comprising outer Li-C rungs and inner Mg-O rungs.[15]
Along the ladder edge, internal NMe2 from the alkoxide
ligands and another sBu group coordinate to Li and Mg,
À
these mixed-metal complexes and their ability to undergo C
C bond forming processes, they also illustrate how on many
occasions the constitution of the active organometallic species
can remain concealed, limiting the understanding on how
such bimetallic reagents operate. This is particularly relevant
for these reactions, where using the same exchange reagent
and same reaction conditions, produces different types of
magnesium aryl species.
Puzzled by the contrasting compositions of 2a,b versus
3a,b, we next probed the constitution of the exchange reagent
sBu2Mg/LiOR in toluene solutions using a combination of
1
NMR experiments including H-DOSY NMR. These studies
revealed the presence of two distinct organometallic species
in solution containing sBu groups (Figure S1).[10] One species
whose M-CH fragment of the sBu groups resonates at d =
1
0.40 ppm in the H NMR spectrum (Figure 2a) belongs to
a compound which also contains alkoxide ligands (both sets of
resonances exhibit almost identical diffusion coefficients (D)
1
by H-DOSY NMR).[10] These data support the formation of
co-complexation product [LiMgsBu2(OR)] (6) (Figure 2a).
The second species displays an upfield shifted multiplet at d =
À0.26 ppm which is intermediate between those chemical
shifts found for the single metal reagents sBuLi (d =
À1.09 ppm) and sBu2Mg (d = 0.05 ppm). Moreover, 1H-
DOSY NMR experiments indicate that this signal belongs
to a species that contains only sBu groups,[10] which coupled
with the homoleptic constitution of the exchange products
3a–3b, suggest that this second species may be homoleptic
tetra(alkyl) [Li2MgsBu4] (7) (Figure 2a). Further support for
this interpretation was found when [Li2MgsBu4] (7) was
prepared independently via co-complexation of two equiv-
alents of sBuLi with sBu2Mg which gave a colourless oil with
1
its H NMR spectrum showing a multiplet at d = À0.26 ppm,
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Figure 3. Left: Synthesis of [LiMgsBu2(OR’)] (8); [R’=CH2CH2N-
(CH3)CH2CH2N(CH3)2] via co-complexation of LiOR’ with sBu2Mg and
Right: molecular structure of 8 with displacement ellipsoids at 30%
probability, all H atoms omitted, and with C atoms drawn as wire
frames (except for CH group of bridging sBu groups) for clarity.[20]
Figure 4. Synthesis of [(THF)2Li2Mg(2-OMe-C6H4)4] (9) via Br/Mg
exchange of four equivalents of 1a with [Li2MgsBu4] (7) and subse-
quent conversion in 2a via co-complexation with Mg(OR)2. Molecular
structure of 9 with displacement ellipsoids at 50% probability, all H
atoms omitted, and with C atoms in THF molecules drawn as wire
frames for clarity.[20]
respectively to complete the dimer. Alternatively, 8 can be
viewed as two dinuclear {LiOMgC} rings which combine
laterally through the Mg-O edges to form a tetranuclear
ladder.
While these findings shed light on the constitutions of
these mixed alkyl/alkoxide mixtures in solution, it was still not
clear why two completely different types of aryl lithium
magnesiates form during the Br/Mg exchange process with
substituted bromoarenes (Scheme 1). At first glance, it
appeared that 2a,b resulting from the reaction with 2-
bromo substituted substrates could be the products if the
exchange process was effected by [LiMgsBu2(OR)] (6). In
contrast, homoleptic 3a,b seemed more likely to be formed
via [Li2MgsBu4] (7). In order to establish if both systems, 6
and 7, could be active towards Mg/Br exchange, we first
carried out the reaction of 7 with four equivalents of 2-
bromoanisole (1a) in toluene to produce tetra(aryl) lithium
magnesiate [(THF)2Li2Mg(2-OMe-C6H4)4] (9) in a 90% iso-
lated yield (Figure 4) (GC analysis confirmed quantitative
conversion of 1a).[10] Exhibiting a contacted ion pair mag-
nesiate structure reminiscent to that of 3a, 9 displays
a distorted tetrahedral with Mg bonded to four ortho-
metallated molecules of anisole, while Li interacts with
OMe groups of two anisyl groups as well as a THF ligand.
To maximise these Li-OMe contacts the Li···Mg···Li vector in
9 is significantly more bent than in 3a [88.15(11) vs 159.05-
[Li2MgAr4] (I) intermediates that in the case of ortho-
substituted Ar groups can undergo co-complexation with
the concomitantly generated Mg(OR)2 furnishing 2a,b (as
depicted in Scheme 3). Alternatively the more kinetically
activated ortho-substituted bromoarenes[17] could react pref-
erentially with mixed(alkyl) alkoxide magnesiate 6 whereas,
for less reactive 4-substituted substrates, the exchange may
occur via tetra(alkyl) magnesiate 7. However, this second
option seems less likely as using isolated crystals of related
mixed(alkyl) alkoxide magnesiate 8 towards 1a led to
significantly lower conversions (20%, see Supporting Infor-
mation), a behaviour that we have also noticed in Zn/I
exchange reactions using [LiZnEt2(OR’)].[9] Especially
revealing was the fact that when an excess of 1.5 equiv of
Mg(OR)2 was added to the equimolar mixture LiOR and
sBu2Mg (pushing the bimetallic Schlenk equilibrium depicted
in Scheme 3 towards the exclusive formation of 6 in solution,
Figure 2c), the exchange process is completely suppressed
with no conversion seen after 40 minutes (entry 6, Table 1 and
Supporting Information).[18]
À
(8)8], while the lengths of the Mg C bonds in both complexes
are almost identical [average values, 2.223 and 2.2313 ꢁ for 9
and 3a, respectively]. Considering the alkyl-rich constitution
of higher order magnesiate 7,[16] coupled with its ability to
quantitatively promote Br/Mg exchange of 1a, the selective
formation of 2a when 1a was reacted with the LiOR/sBu2Mg
combination became even more puzzling. In an attempt to
mimic the reaction conditions when using this mixed alkyl/
alkoxide reagent, one molar equivalent of Mg(OR)2 (which is
the other product observed from disproportionation of 6 into
7) was introduced to a suspension of 9 in toluene which led to
the quantitative formation of 2a (Figure 4).
Adding a new layer of complexity, these results set at least
two different scenarios to explain the formation of mixed
alkyl/alkoxides 2a,b during the exchange reactions. On one
hand, in situ generated tetra(alkyl) magnesiate 7 could be
responsible for the Br/Mg exchange in all cases, furnishing
Scheme 3. Proposed organometallic species involved in lithium alkox-
ide mediated Mg/Br exchange of bromoarenes.
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Collectively our study confirms that addition of LiOR
Conflict of interest
kinetically activates the two alkyl groups on sBu2Mg to
effectively undergo Br/Mg exchange.[6,7] However this acti-
vation is not due to the formation of a more reactive alkyl/
alkoxide lithium magnesiate as initially thought, but better to
the presence of a bimetallic Schlenk equilibrium which
generates in situ a much more reactive species, namely
higher order tetra(alkyl) lithium magnesiate 7 (Scheme 3). It
is important to note that under the conditions of this study we
do not observe the disproportionation of 7 in solution to its
lower order derivative LiMgsBu3 and sBuLi (a process that
has been reported for related sodium tetra(alkyl) derivatives
Na2MgR4 (R = nBu, CH2SiMe3)), hinting that the choice of
alkali-metal adds to the complexity of this s-block hetero-
bimetallic systems.[19] Thus, the four alkyl groups on 7 are
active towards the exchange to form tetra(aryl) intermediates
[Li2MgAr4] (I) which have been trapped and structurally
authenticated for Ar= 4-OMe-C6H4, 3a; Ar= 4-F-C6H4, 3b.
However, for 2-substituted aryls (Ar= 2-OMe-C6H4, 2-NMe2-
C6H4), upon formation of I, co-complexation with Mg(OR)2
(which is also present in the reaction media as a consequence
of the Schlenk equilibrium referred above) can ensue,
The authors declare no conflict of interest.
Keywords: alkoxides · bromine/magnesium exchange · lithium ·
magnesium · mixed-aggregates
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furnishing
mixed-metal,
mixed-ligand
intermediate
[LiMgAr2(OR)] (II) (Scheme 3). Comparing the structures
of 2a,b vs. 3a,b, the preferred formation of II can be
attributed to the unique Lewis donor stabilisation achieved
by Li through simultaneous coordination to the donor ortho-
substituents on the aryl group and the oxygens of the alkoxide
ligand. This coordination environment is not available for
para-substituted arenes. Nevertheless, both intermediates I
and II undergo electrophilic interception as shown in
Scheme 3 affording in all cases the relevant functionalized
substituted arene 4a,b, 5a,b.
To conclude, by combining reactivity studies with detailed
NMR and structural studies, the mist surrounding the
complex constitutions of intermediates involved in lithium
alkoxide mediated Br/Mg exchange processes has begun to
clear.
[10] See Supporting Information for details.
[11] 1H NMR analysis of these reactions showed almost quantitative
conversions to these bimetallic species which are the only
organometallic species formed from the solution along with
Mg(OR)2.
[12] a) M. Geissler, J. Kopf, E. Weiss, Chem. Ber. 1989, 122, 1395;
b) E. Weiss, Angew. Chem. Int. Ed. Engl. 1993, 32, 1501; Angew.
Chem. 1993, 105, 1565.
The participation of several distinct lithium magnesiates,
exhibiting not only different ligand sets, but also different
Li:Mg ratios has been established, advancing understanding
of the modus operandi of these mixtures thus paving new
paths towards further synthetic applications.
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Acknowledgements
We thank Professor Paul Knochel and Alexandre Desaintjean
(Ludwig-Maximilians-Universitꢂt Mꢃnchen) for their helpful
discussions which inspired this work and for performing the
electrophilic interception studies which furnished 4a,b and
5a,b. We also thank the Swiss National Science Foundation
(200021_188573) and the University of Bern for the generous
financial support of this research.
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references therein; b) L. Davin, A. Hernꢅn-Gꢆmez, C.
McLaughlin, A. R. Kennedy, R. McLellan, E. Hevia, Dalton
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Chemie
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of charge by the joint Cambridge Crystallographic Data Centre
[18] After 24 h at room temperature 60% conversion was observed,
while after 48 h the reaction was complete.
and Fachinformationszentrum Karlsruhe Access Structures
service www.ccdc.cam.ac.uk/structures.
[19] T. X. Gentner, R. E. Mulvey, Angew. Chem. Int. Ed. 2021,
https://doi.org/10.1002/anie.202010963; Angew. Chem. 2021,
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[20] Deposition Number(s) 2049361 (2b), 2049362 (3a), 2049363 (3b),
2049364 (8) and 2049365 (9) contain(s) the supplementary
crystallographic data for this paper. These data are provided free
Manuscript received: December 10, 2020
Accepted manuscript online: January 6, 2021
Version of record online: February 26, 2021
Angew. Chem. Int. Ed. 2021, 60, 7626 –7631
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
7631