Nucleophilic Aromatic Substitution at Benzene with Powerful Strontium Hydride and Alkyl Complexes

Article abstract of DOI:10.1002/anie.201901548

Key to the isolation of the first alkyl strontium complex was the synthesis of a strontium hydride complex that is stable towards ligand exchange reactions. This goal was achieved by using the super bulky β-diketiminate ligand ^DIPePBDI (CH[C(Me)N-DIPeP]₂, DIPeP=2,6-diisopentylphenyl). Reaction of ^DIPePBDI-H with Sr[N(SiMe₃)₂]₂ gave (^DIPePBDI)SrN(SiMe₃)₂, which was converted with PhSiH₃ into [(^DIPePBDI)SrH]₂. Dissolved in C₆D₆, the strontium hydride complex is stable up to 70 °C. At 60 °C, H–D isotope exchange gave full conversion into [(^DIPePBDI)SrD]₂ and C₆D₅H. Since H–D exchange with D₂ is facile, the strontium hydride complex served as a catalyst for the deuteration of C₆H₆ by D₂. Reaction of [(^DIPePBDI)SrH]₂ with ethylene gave [(^DIPePBDI)SrEt]₂. The high reactivity of this alkyl strontium complex is demonstrated by facile ethylene polymerization and nucleophilic aromatic substitution with C₆D₆, giving alkylated aromatic products and [(^DIPePBDI)SrD]₂.

Full text of DOI:10.1002/anie.201901548

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Nucleophilic aromatic substitution at benzene with powerful
strontium hydride and alkyl complexes
Authors: Sjoerd Harder, Bastian Rösch, Thomas Xaver Gentner,
Holger Elsen, Christian A. Fischer, Jens Langer, and Michael
Wiesinger
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901548
Angew. Chem. 10.1002/ange.201901548
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http://dx.doi.org/10.1002/ange.201901548

10.1002/anie.201901548
Angewandte Chemie International Edition
Nucleophilic aromatic substitution at benzene
with powerful strontium hydride and alkyl complexes
Bastian Rösch, Thomas Xaver Gentner, Holger Elsen, Christian A. Fischer, Jens Langer, Michael
Wiesinger and Sjoerd Harder*
B. Rösch, T. X. Gentner, H. Elsen, C. A. Fischer, Dr. J. Langer, M. Wiesinger, Prof. Dr. S. Harder*
Chair of Inorganic and Organometallic Chemistry, Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058
Erlangen, Germany
Fax: (+49) 9131-8527387
E-mail: sjoerd.harder@fau.de
Supporting information for this article is available on the WWW under http//:www.angewandte.chemie.org or
from the author.
Keywords: Alkaline Earth Metal – Strontium – Alkylmetal – Nucelophilic Aromatic Substitution –DFT
calculations
Table of Contents
The first unstabilized alkylstrontium complex could be obtained using a very robust superbulky ß-
diketiminate ligand. The very high nucleophilicity of its Sr hydride and Sr ethyl complexes is
demonstrated by D-exchange reactions with C6D6 and D2. Catalytic deuteration of C6H6 with D2 is
presented.
1
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Metal alkyl complexes are typically extremely reactive and often burn upon contact with air. Despite
challenging syntheses and handling, they represent the earliest known examples of organometallic
compounds.[1,2] Schlenk’s alkyllithium[3] and Grignard’s alkylmagnesium reagents[4] are true
milestones in organometallic chemistry and essential to modern organic synthesis. Alkyl complexes
of the larger, more electropositive, heavier group 1 metals are even more reactive: the superbase
nBuK deprotonates THF already at -50 °C.[5,6] These bases and other smart mixtures with heavier
alkali metals[7] are routinely used in large scale specialty applications.[7] In contrast, heavier group 2
metal reagents remained elusive for at least a century.[8] Organocalcium chemistry is slowly
awakening but its recent developments mainly included syntheses of benzyl-,[9] aryl-,[10] allyl,[11] or
silylated methyl[12] compounds. The much more potent unsubstituted alkyl metal complexes were a
long sought after Holy Grail in alkaline earth metal chemistry and this quest has only very recently
been realized.[13-15]
Synthesis of highly reactive alkylcalcium complexes requires robust ligands and use of inert solvents
(THF should be avoided). Our endeavors to isolate an alkylcalcium reagent by reaction of a solvent-
free calcium hydride complex with the highly polarized C=C bond in a N-heterocyclic olefin gave
metallation of the ligand backbone (Scheme 1a).[16] Attempts to isolate a THF-free version of the
earlier reported ß-diketiminate calcium hydride complex [(DIPPBDI)CaH∙(THF)]2 (DIPPBDI = CH[C(Me)N-
DIPP]2, DIPP =2,6-diisopropylphenyl)[17] were frustrated by the Schlenk equilibrium, resulting in
(CaH2)∞ precipitation (Scheme 1b),[18,19] but using an excess of the hydride transfer reagent PhSiH3,
Hill and coworkers isolated [(DIPPBDI)CaH]2 (Scheme 1c).[13]
Scheme 1. (a) Reaction of an amidinate calcium hydride complex with a N-heterocyclic olefin led to backbone
deprotonation. (b) Attempted synthesis of [(DIPPBDI)CaH]2 using stoichiometric quantities of PhSiH3 led to ligand
exchange. (c) Using an excess of PhSiH3 gave the expected product which reacted with alkenes to give
alkylcalcium complexes.
2
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Although this effect of reagent stoichiometry is not fully understood, NMR investigations suggest
that equilibria involving hypervalent silicate species prevent destructive Schlenk equilibria (Figure
S23-24). Addition of alkenes to [(DIPPBDI)CaH]2 gave the first “true” alkylcalcium compounds (Scheme
1c).[13] Simultaneously, the Anwander group prepared Me2Ca by metal exchange between MeLi and
Ca[N(SiMe3)2]2.[14] The very high reactivity of these unstabilized alkylcalcium complexes was
demonstrated by alkyl/H exchange with the benzene solvent at 60 °C.[13] Without having explored the
full scope,[15,20] their unusual reactivity spurs the isolation of the increasingly more reactive alkyl
complexes of Sr and Ba. As alkylbarium complexes have so far only been discussed as fleeting
extremely reactive intermediates,[21] isolation of alkylstrontium complexes is a more realistic goal.
The main hurdle towards the synthesis of alkylstrontium complexes is the challenging isolation of a
simple, THF-free, strontium hydride complex LSrH with a robust spectator ligand L. It should be
soluble in non-polar solvents, stable towards ligand exchange reactions and decomposition but
reactive towards nonconjugated alkenes. Recently reported Sr hydride complexes[22-25] do not fulfill
these requirements. As amidinate ligands with extremely bulky 2,6-Ph2CH-phenyl substituents failed
to stabilize Sr hydride complexes[23,26] we investigated the Sr chemistry of recently introduced
DIPePBDI,[27] a very robust superbulky ß-diketiminate ligand that increases the solubility of polar
complexes in apolar solvents (Scheme 2).
Scheme 2. Synthesis and reactivity of strontium hydride and alkyl complexes.
Reaction of DIPePBDI-H with SrN’’2 (N’’ = N(SiMe3)2) in hexane at 70 °C reached full conversion to
(
DIPePBDI)SrN’’ after 14 days. Although sluggish, this direct ligand deprotonation is very convenient. In
contrast to (DIPPBDI)AeN’’ complexes (Ae = Sr, Ba),[28]
(
DIPePBDI)SrN’’ is extremely stable towards ligand
3
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10.1002/anie.201901548
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exchange: a toluene-d8 solution at 140 °C did even after 6 days not decompose. Complex
DIPePBDI)SrN’’ crystallizes as a monomer (Figure 1a) in which the N(SiMe3)2 ligand is asymmetrically
(
bound to Sr (Sr-N-Si1 = 130.5(1)°, Sr-N-Si2 = 106.1(1)°) on account of a strong Sr∙∙∙Me-Si agostic
interaction which is the shortest of its kind: Sr∙∙∙C = 3.057(2) Å. A somewhat longer agostic interaction
with the diisopentyl substituent (3.310(2) Å) complements the Sr coordination sphere,
demonstrating that the flexible arms of the DIPePBDI ligand are ideally suited to stabilize complexes
with larger metals by secondary interactions.
The hydride complex (DIPePBDI)SrH was obtained in 53% yield by warming a hexane solution of
(
DIPePBDI)SrN’’ and PhSiH3 from -80 °C to 20 °C. It crystallized as a dimer with bridging hydride ligands
that were freely refined (Figure 1b). The average Sr-(2)H distance of 2.35 Å is, considering a 0.03 Å
standard deviation, within the 2.26(5)-2.43(3) Å range for other dimeric Sr hydride complexes.[23,25]
The coordination sphere of Sr2 is supplemented by Sr∙∙∙C(isopentyl) agostic interactions, making all
bonds to Sr2 slightly longer. The ligand bulk in [(DIPePBDI)SrH]2 and [(DIPPBDI)CaH]2[13] was compared by
calculation of buried volume: the DIPePBDI ligand (Vbur = 61.5%) is substantially bulkier then the DIPPBDI
ligand (Vbur = 51.1%); Figure S59.
Figure 1. Crystal structures. (a) Complex
(
DIPePBDI)SrN’’. (b) Complex [(DIPePBDI)SrH]2. (c) Complex
[(DIPePBDI)SrEt]2. Selected distances are given in Å and angles in degrees.
The eight highly flexible isopentyl arms in [(DIPePBDI)SrH]2 make this complex very soluble in alkanes
1
or aromatic solvents. The H NMR spectrum in C6D6 shows for the hydride ligands a sharp singlet at
5.41 ppm fitting the 4.48-6.64 ppm range reported for Sr hydride complexes.[22-25] This C6D6 solution
of [(DIPePBDI)SrH]2 is remarkably stable and gave up to 70 °C no ligand exchange. Interestingly,
prolonged heating to 60 °C in C6D6 led overnight to complete H-D exchange with the deuterated
solvent giving [(DIPePBDI)SrD]2 and C6D5H (Scheme 2; somewhat slower H-D exchange is also observed
in toluene-d8). The Sr hydride complex gave fast H-D exchange with D2 (1.5 bar D2, C6D6, 20 °C: >99%
deuteration in 2 hours; Scheme 2). This scrambling reaction was found to be much faster than that
for a cationic Sr hydride cluster[24] and, more important, did not lead to decomposition. Therefore,
4
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[(DIPePBDI)SrH]2 could be used as a catalyst for the deuteration of C6H6 by D2 (TON = 27 in 5 days,
Figure S42-45). Although slow, Sr hydride catalyzed H-D isotope exchange in an unactivated aromatic
compound like benzene is unparalleled. H-D Exchange reactions are generally mediated by precious
metal catalysts,[29] following a Shilov-type oxidative addition of the C-H bond, or by strong Lewis
acids[30] in which case electrophilic substitution proceeds through a Wheland cyclohexadienyl cation.
We propose a mechanism in which benzene is activated for nucleophilic aromatic substitution by an
incipient -complex with the Sr metal, followed by formation of a cyclohexadienyl anion of the
Meisenheimer type (Scheme 2).
The very robust [(DIPePBDI)SrH]2 is the key to alkylstrontium complexes. In C6D6, it fully reacted with
1
ethylene at 20 °C and 1 bar within minutes (Scheme 2). The H NMR spectrum showed a variety of
alkylstrontium products characterized by multiplets in the -0.5/-1.5 ppm range. NMR and GC-MS
analysis indicated two competitive reactions. The Sr-Et intermediate reacts further with ethylene to
give Sr-Bu, Sr-hexyl and higher oligomers, a process reminiscent of Ziegler’s Et3Al/C2H4 “Aufbau
Reaktion”. Oligomerization competes with nucleophilic aromatic substitution at C6D6 giving
[(DIPePBDI)SrD]2 and a variety of RC6D5 products (R = Et, Bu, hexyl, etc.). The lower molecular weight
oligomers could be detected by GC-MS but also insoluble long-chain polyethylene has been isolated
(Figure S48). Room temperature ethylene (oligo-)polymerization at low pressure illustrates that
strontium hydrides may be promising s-block metal initiators for polymerization of unactivated
alkenes.[31] Previously reported [(DIPPBDI)CaEt]2 did not show ethylene polymerization but featured at
60 °C a similar nucleophilic aromatic substitution.[13]
The conversion of Sr-alkyl into Sr-D species by nucleophilic aromatic substitution with C6D6 was
circumvented by performing the synthesis in hexane, a solvent in which the isopentyl-decorated
species are highly soluble. Overnight reaction of [(DIPePBDI)SrH]2 in hexane with 1 bar ethylene
between -80 to -20 °C gave after crystallization [(DIPePBDI)SrEt]2 in 33% yield. Similar to
[(DIPPBDI)CaEt]2,[13] complex [(DIPePBDI)SrEt]2 crystallized as a centrosymmetric dimer with slightly
asymmetric bridging Et ligands (Figure 1c). It is stabilized by strong agostic Sr∙∙∙CH3 interactions of
3.034(2) Å, that are only 8.4% longer than the direct Sr-CH2 bond of 2.798(1) Å, and a secondary
Sr∙∙∙CH3(isopentyl) interaction of 3.240(4) Å. The freely refined H positions in the Et ligands show
essentially sp3-hybridized CH2 groups (the sum of the valence angles is 320.7°).
The Sr alkyl complex [(DIPePBDI)SrEt]2 is poorly soluble in alkanes but dissolves moderately in C6D6. In
agreement with the higher ionicity of the Sr-Et vs. Ca-Et bond, the Sr-CH2 1H NMR signal (-0.90 ppm)
is shifted highfield in respect to the Ca-CH2 signal in [(DIPPBDI)CaEt]2 (-0.79 ppm).[13] As the complex
instantaneously deprotonates THF, polar solvents should be avoided (Figure S58). The extreme
reactivity of [(DIPePBDI)SrEt]2 is demonstrated by fast reaction with H2 to give [(DIPePBDI)SrH]2 and
ethane (C6D6, 20 °C, 1.5 bar H2 , < 5 min); cf. reaction of [(DIPPBDI)CaEt]2 with H2 needed at least 48
hours).[20a] Its high reactivity is further demonstrated by very facile nucleophilic substitution at C6D6
which proceeds already at 20 °C giving full conversion to [(DIPePBDI)SrD]2 and EtC6D5 within 12 hours.
At 60 °C complete conversion was reached within five minutes (cf. conversion of [(DIPPBDI)CaEt]2 with
C6D6 at 60 °C needed 16 hours).[13] An intermediate in this unusually fast nucleophilic Et-D
substitution at C6D6 is the mixed Et-D species: (DIPePBDI)2Sr2(2-Et)2-D (Figure S50).
Since the DIPePBDI ligand inhibits ligand exchange in Sr complexes, catalytic insertion of ethylene in the
benzene C-H seemed feasible. Insertion of ethylene in the C-D bond of C6D6 with catalyst
[(DIPePBDI)SrEt]2, however, gave at room temperature apart from ethylbenzene a mixture of products.
5
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As ethylene oligomerization is competitive with alkyl-D exchange, Et(C2H4)nC6D5 (n = 0-10) and longer
alkanes (C2H4)nH2 (n = 5-18) were detected by GC-MS (Figure S56). In addition, double Et/D exchange
was verified by identification of C6D4(Et)2. A signal for the triple Et/D exchange product C6D3(Et)3 was
absent. This may be explained by the fact that the -electron density becomes higher with increasing
alkylation, making nucleophilic aromatic substitution progressively more difficult.
In order to provide further insight in this unusual nucleophilic substitution at benzene, an energy
profile following that previously reported for the reaction of [(DIPPBDI)Ca(n-hexyl)]2 with benzene[13]
was calculated (Scheme 3a). The calculated Sr-C distances in [(DIPePBDI)SrEt]2 are in excellent
agreement with those in the crystal structure. NPA Charges of +1.81 on Sr and -0.82 on the bridging
CH2 group confirm the strong polarity of the Sr-C bond. Formation of monomeric (M1) is
endothermic by +8.1 kcal/mol, a value considerably lower than the dissociation energy of 23.2
kcal/mol calculated for [(DIPPBDI)Ca(n-hexyl)]2.[13] The large Sr metal in (DIPPBDI)SrEt, which is formally
only three-coordinate, shows a strong Sr∙∙∙CH3 agostic interaction indicated by a Sr-C-C angle of 89.6°
that is considerably more acute than the Ca-C-C angle of 117.2° in (DIPPBDI)Ca(n-hexyl).[13] This agostic
interaction is lost by complexation of benzene (M2). The transition state for rear-side attack of Et at
the coordinated (activated) benzene ligand (M3*) is essentially barrier-free. The short C∙∙∙C and long
C∙∙∙H distance indicate that hydride transfer to Sr is at a very advanced state. Complex M4 releases
ethylbenzene to give (DIPPBDI)SrH (M5). The total energy gain of -9.2 kcal/mol reflects the lability of
the Sr ethyl educt M1 vs. the Sr hydride product M5. The formation of monomeric species M1 (+8.1
kcal/mol) is the most difficult step along the profile.
The problem with this monomeric model system is that rear-side Et attack can only proceed when
the Et group acts as a charge-separated nucleophile. Unfavorable Sr-Et cleavage may be facilitated by
solvent-stabilization but absence of polar solvents makes this route questionable. This is a common
dilemma in nucleophilic substitution reactions: the simple textbook picture of the SN2 transition state
+
(Nu-∙∙∙CH3 ∙∙∙X-) becomes much more complicated when a tightly bound metal cation is
considered.[32] How does the metal cation migrate from nucleophile to leaving group without paying
the high penalty for charge separation? Transition states in which Nu- and X- are bridged by a chain
of cations and anions were found to be high in energy and equally unsatisfying.[32] We do, however,
not rule out a mechanism in which the 4-membered Sr2Et2 ring in the dimer is opened, after which
Sr-benzene coordination and attack from the rear can occur. Using a simplified ß-diketiminate ligand
with Ph substituents (PhBDI), we calculated an energy profile for these much larger dimeric systems
(Scheme 3b).
6
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Scheme 3. Calculated energy profiles for benzene alkylation (B3PW91/def2tzvpp). Enthalpy differences (H)
are given in kcal/mol and selected distances in Å. The ß-diketiminate ligands are only partially shown. (a)
Energy profile for a monomeric model system (M) with a DIPePBDI ligand. (b) Energy profile for a dimeric model
system (D) with a PhBDI model system.
Simplifying the ligand has serious consequences for the cleavage energy of the dimer into monomers
which is strongly dependend on the bulk of the aryl substituent: DIPeP (+8.1 kcal/mol), DIPP (+21.2
kcal/mol) and Ph (+24.2 kcal/mol). Opening of a dimer, however, is much easier: insertion of benzene
in one of the four Sr-Et bonds in [(PhBDI)SrEt]2 costs only 11.5 kcal/mol and leads to loss of both
Sr∙∙∙CH3 agostic interactions and partial rupture of a second Sr-Et bond (D1). Intramolecular rear-side
attack (D2*) needs an activation energy of +26.4 kcal/mol. Formation of the Et∙∙∙C bond (2.231 Å)
results in transformation of benzene into a cyclohexadienyl anion; loss of aromaticity is indicated by
deviation from planarity. Product D3 contains an unstable Meisenheimer anion which transfers a
hydride anion to Sr through a low energy transition state D4*. The total reaction of [(PhBDI)SrEt]2 with
benzene to give ethylbenzene and the mixed ethyl-hydride dimer D6 is exothermic by -11.1 kcal/mol.
7
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Transition state D3* (+26.4 kcal/mol) is the highest point along the route. Although the rather high
activation energy is not conform the facile nucleophilic substitution at benzene, which proceeds at
room temperature, this dimeric model does not suffer from (DIPPBDI)Sr+/Et‾ charge separation, a step
that is currently not included in the monomeric model.
In summary, the group of highly reactive alkaline earth metal reagents has been extended with an
unstabilized alkyl complex of the heavy metal Sr. The key to isolation of [(DIPePBDI)SrEt]2 was the very
robust strontium hydride complex [(DIPePBDI)SrH]2 featuring the superbulky DIPePBDI ligand. This ligand
is tailor-made for larger metals, stabilizing complexes not only by its impressive bulk but also by
secondary agostic interactions with the isopentyl substituents. The potent nucleophilicity of
[(DIPePBDI)SrH]2 is demonstrated by an unprecedented H-D exchange with C6D6 and very fast H-D
exchange with D2. This reactivity is the basis for Sr hydride catalyzed deuteration of C6H6 by D2. The
even higher reactivity of [(DIPePBDI)SrEt]2 is exemplified by ethylene polymerization and facile Et-D
exchange with C6D6 proceeding already at room temperature. This first example of an unstabilized
alkylstrontium complex is currently the most polar and nucleophilic group 2 metal species. This
leaves us with the challenging goal to isolate even more reactive alkylbarium complexes.
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Eberhardt, US Pat. 3.321.479 (1967) and 3.567.703 (1971).
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Acknowledgements Steffen Brand is acknowledged for assistance in reactivity studies. S. H.
acknowledges the Deutsche Forschungsgemeinschaft for financial support (HA 3218/9-1).
Abstract: The key to isolation of the first alkylstrontium complex is the synthesis of a strontium
hydride complex that is stable towards ligand exchange reactions. This was achieved by using the
superbulky ß-diketiminate ligand DIPePBDI (CH[C(Me)N-DIPeP]2, DIPeP =2,6-diisopentylphenyl).
Reaction of DIPePBDI-H with Sr[N(SiMe3)2]2 gave (DIPePBDI)SrN(SiMe3)2 which was converted with
PhSiH3 to [(DIPePBDI)SrH]2. Dissolved in C6D6, the Sr hydride complex is stable is up to 70 °C. At 60 °C,
H-D isotope exchange gave full conversion to [(DIPePBDI)SrD]2 and C6D5H. Since H-D exchange with D2
is also facile, the strontium hydride complex is a catalyst for the deuteration of C6H6 by D2. Reaction
of [(DIPePBDI)SrH]2 with ethylene gave [(DIPePBDI)SrEt]2. The high reactivity of this first alkylstrontium
complex is demonstrated by facile ethylene polymerization and nucleophilic aromatic substitution
with C6D6 giving alkylated aromatic products and (DIPePBDI)SrD]2, both processes which proceed at
room temperature. DFT calculations illustrate that the latter Et-D exchange with C6D6 likely involves
the highly instable Meisenheimer anion C6D6Et‾ as an intermediate.
9
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