Facile access to tuneable Schwartz's reagents: Oxidative addition products from the reaction of amide N-H bonds with reduced zirconocene complexes

Article abstract of DOI:10.1002/anie.201305246

On the tracks of Schwartz's reagent: Two zirconocene hydrido amidate complexes are synthesized by formal oxidative addition of amide N-H bonds to reduced zirconocene fragments. Insertion reactions with alkenes show a different behavior than Schwartz's reagent by forming branched insertion products. The insertion product and the hydrido complex are characterized by X-ray analysis. Copyright

Full text of DOI:10.1002/anie.201305246

Angewandte
Chemie
DOI: 10.1002/anie.201305246
Zirconocene Hydrido Complexes
Facile Access to Tuneable Schwartzꢀs Reagents: Oxidative Addition
À
Products from the Reaction of Amide N H Bonds with Reduced
Zirconocene Complexes**
Martin Haehnel, Jacky C.-H. Yim, Laurel L. Schafer,* and Uwe Rosenthal*
Dedicated to Professor Christoph Marschner on the occasion of his 50th birthday
À
Since the development of Schwartzꢀs reagent and its use in
hydrozirconation reactions over the past decades,[1] the
chemistry of modified zirconocene hydride complexes con-
tinues to be of key interest to both inorganic and organic
chemists.[2] In contrast to the oligomeric structure of
Schwartzꢀs reagent, monomolecular terminal hydride com-
plexes [Cp’2Zr(H)E] (Cp’ = unsubstituted or substituted Cp
(Cp = h-C5H5), E = alkoxide,[3] amide,[4] alkyl,[5] aryl,[6] silyl,[7]
phosphanyl[8]) have been rarely reported and their syntheses
are typically limited to examples with special substituents on
the Cp rings and/or the E substituent. General approaches to
such complexes often exploit salt metathesis (starting from
[Cp’2Zr(H)Cl)][4,5d]) or protonolysis reactions (starting from
[Cp’2ZrH2)].[5c]) Herein we show that the formal oxidative
addition of organic amides to a masked reduced zirconocene
complex is an alternative and high yielding route to amidate-
ligated terminal hydrido zirconocene complexes.
as N H bonds, there are few reported examples with Group 4
metals,[9] and the only structurally characterized product
À
involves the oxidative addition of an N H bond of amino-
borane to yield zirconocene amidoborane complexes.[10]
À
While late-transition-metal complexes promote N H oxida-
tive addition,[11] characterized examples of the oxidative
addition of amides are restricted to Pt,[12] Ru,[13] and Ir.[11c,14]
À
To our knowledge, the oxidative addition of N H bonds of
amides at early transition metals has not yet been reported.
It has been shown recently that 4-membered hetero-
metallacycles can stabilize zirconocene hydrido complexes,[8]
although these complexes are limited to those with special
substituents on the cyclic unit. By comparison, amidates are
readily modified 4-membered heterometallacycles with tun-
able electronic and steric properties by simply changing the N
and carbonyl substituents.[15] Moreover, their coordination
modes are not limited to cyclic k2-binding modes, but can also
exist as k1-bound amidate ligands, resulting in a highly flexible
coordination sphere about the metal center. Furthermore, the
synthesis of such zirconocene hydrido amidate complexes is
easily accomplished by reaction of the established zircono-
cene source [Cp2Zr(py)(h2-Me3SiC2SiMe3)][16] with various
amides according to Scheme 2. This high-yielding route
À
Oxidative addition of E H bonds (E = OR, NR2, PR2,
CR3, SiR3, Scheme 1) to transiently generated ZrII species has
been invoked in reactivity investigations of a variety of
zirconocene complexes [Cp’2ZrX2].[5a,e,6,7a] The oxidative
À
À
addition of Si H and C H bonds to reduced zirconocene
derivatives has yielded monomolecular terminal hydride
complexes.[6a,b,7a] In the case of more polar E H bonds, such
À
À
Scheme 1. Oxidative addition of E H bonds to [Cp2Zr] fragments
(E=OR, NR2, PR2, CR3, SiR3).
[*] Dipl.-Chem. M. Haehnel, Prof. Dr. U. Rosenthal
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: uwe.rosenthal@catalysis.de
Scheme 2. Key steps in the formal oxidative addition of amides to
[Cp2ZrII].
B. Sc. J. C.-H. Yim, Prof. Dr. L. L. Schafer
Department of Chemistry, University of British Columbia
2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada)
E-mail: schafer@chem.ubc.ca
results in the preparation of mononuclear and tunable
alternatives to Schwartzꢀs reagent, that take advantage of
the stabilizing effect of heterometallacycles.[17] Herein we
show that the facile accessibility and variability of the amidate
ligands afford halogen-free, mononuclear versions of
Schwartzꢀs reagent that can afford branched insertion prod-
ucts selectively.
Two amides with different electronic and steric properties
were selected to evaluate the generality of this approach for
accessing oxidative-addition products. Amide 1a has alkyl
[**] We thank Philippa Payne for assistance with 600 MHz NMR
spectroscopy. We would like to thank the DFG for financial support.
J.C.H.Y. and L.L.S. thank NSERC for a postgraduate scholarship and
financial support. M.H. and U.R. thank DFG for funding. This
research was undertaken, in part, thanks to funding from the
Canada Research Chairs program.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305246.
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substituents with a small isopropyl group on the N atom, while
amide 1b has aryl substituents and is substantially more
sterically demanding with a N-2,6-diisopropylphenyl (Dipp)
group. The initial step in accessing the oxidative-addition
products is the substitution of the stabilizing pyridine ligand
with coordinated amide, as suggested by similar reactions
with N-alkylated lactams (cyclic amides).[18] The subsequent
formation of intermediates 2a and b would then take place by
a proton transfer from the Lewis acid activated amide to the
bis(trimethylsilyl)acetylene ligand to give amidate ligated s-
alkenyl complexes. Such metal vinyl species have been
Complexes 3a and 3b have been fully characterized, with
the solid-state molecular structure of 3a being provided as
a representative example (Figure 1). In complex 3a, the
zirconium center is surrounded by two Cp units, the chelating
amidate, and the hydrido ligand (H1). As known for
zirconium amidate complexes, the central metallacyclic unit
is almost planar (torsion angle 1.38).[15,23] The hydrogen atom
À
H1 is only slightly out of this plane. The Zr1 O1
À
(2.2595(8) ꢁ) and Zr1 N1 (2.2784(9) ꢁ) bond lengths are
typical for Zr amidate complexes,[15,23,24] and C1 O1
À
À
(1.2984(12) ꢁ) and C1 N1 (1.3096(13) ꢁ) bond lengths are
characterized as products of reactions of polar, protic E H
consistent with partial multiple bond character.[25] Spectro-
scopically, the resonance signals of the hydrido signals in the
1H NMR spectra appear at d = 5.01 (3a) and d = 5.72 ppm
(3b), respectively. Using 13C NMR spectroscopy the reso-
nance signals of C1 appear at d = 185.8 ppm (3a) and d =
176.9 ppm (3b), which are characteristic for the k2-metal-
lacyclic binding mode of the amidate ligand.[15,23,24]
À
bonds of lactams, amines, water, or acetylene with
[Cp2Zr(L)(h2-Me3SiC2SiMe3)] (L = pyridine, THF).[18–21]
In the case of 1a, this proton-transfer reaction could be
1
monitored by H NMR spectroscopy because the sterically
less-demanding N-(isopropyl)tert-butylamide (1a) allowed
for the spectroscopic characterization of 2a within the first
hour of the experiment at room temperature. Complex 2a
features the previously observed agostic interaction of the b-
hydrogen, as indicated by the downfield shift in the 1H NMR
spectrum (d = 8.08 ppm).[18] In the 13C NMR spectrum, char-
acteristic resonance signals of the s-alkenyl unit appear at d =
234.5 (Ca) and d = 113.8 ppm (Cb). This agostic interaction is
anticipated to be critical for the subsequent formation of the
terminal hydrido complex 3a. The characteristic resonance
signal of the amidate ligand in 2a appears at d = 168.1 ppm,
indicating a k1-binding mode. Additionally, complex 2a could
be characterized by mass spectrometry, in which a [M+H]
peak was observed at m/z 534.[22] However, isolation of pure
2a was not possible and the formal oxidative-addition product
3a was obtained exclusively.
Not surprisingly, the reaction of [Cp2Zr(py)(h2-Me3Si-
C2SiMe3)] with two equivalents of amide 1b results in the
formation of the bis(amidate) complex 4 (Scheme 3), analo-
Scheme 3. Formation of complex 4.
In contrast to previous results with lactams or amines,[19] it
was found that complexes 2 liberate bis(trimethylsilyl)acety-
lene by a hydride transfer to the Zr center, to give the desired
zirconocene hydrido amidate complexes 3 (Figure 1). Pre-
gous to that observed for reactions with amines.[19] Interest-
ingly, the evolution of H2 gas was detected by 1H NMR
spectroscopy (d = 4.47) along with free bis(trimethylsilyl)-
acetylene (d = 0.16). Notably, there was no evidence for
=
bis(trimethylsilyl)ethylene (Me3SiCH CHSiMe3) reduction
product. This result suggests that the formation of the
zirconocene hydrido complex 3b is an intermediate on the
path to the bis(amidate) complex. Indeed, the reaction of 3b
with one equivalent of 1b cleanly affords complex 4 together
with H2 gas.
The molecular structure of complex 4 is depicted in
Figure 2. The Zr center is surrounded by two Cp ligands and
two k1-O amidate bound ligands are coordinated to Zr in
a distorted tetrahedral coordination geometry. In the k1-O
À
amidate binding mode, the C O bond lengths (av. 1.336 ꢁ)
correspond more closely to a typical single bond while the
Figure 1. Crystal structure of 3a. Hydrogen atoms except for H1 are
omitted for clarity. The thermal ellipsoids set at 30% probability.
Selected bond lengths [ꢂ] and angles [8], C1-O1 1.2984(12), C1-N1
1.3096(13), Zr1-O1 2.2595(8), Zr1-N1 2.2784(9), Zr1-H1 1.768(17);
O1-C1-N1 113.51(9), O1-Zr1-N1 57.45(3).
À
À
C N bond lengths (av. 1.276 ꢁ) are closer to that of C N
double bonds.[18]
All the resonance signals in the room-temperature
1H NMR spectrum of complex 4 appear broad, most likely
a result of equilibria between k2-metallacycles and k1-O
bound amidate isomers in solution (Scheme 4). Low-temper-
ature NMR studies (À758C) confirmed such fluxionality and
resonance signals for the k1-O bound and k2-N,O bound
ligand could be observed.[26] This situation is most clearly seen
in the 13C NMR spectrum, in which the resonance signals for
the central carbon atom C1 of metallacyclic amidate ligand
sumably relief of steric strain upon bis(trimethylsilyl)acety-
lene elimination is the driving force for the reaction. This
notion is further supported by the observation that the
reaction proceeds only at elevated temperature (658C) with
1a, while the use of sterically demanding 1b results in product
3b at room temperature. With 1b the intermediate complex
2b could not be detected spectroscopically.
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an alkene is possible and the alternative branched product can
be prepared.[29] Efforts to access the branched product
selectively with traditional zirconocene type complexes have
been unsuccessful to date.[2b] Herein we show that by
modifying the ClÀ ligand of Schwartzꢀs reagent with an
amidate, variable reactivity can be targeted and most
importantly, rarely observed branched products can be
selectively obtained.
In contrast to Schwartzꢀs reagent, the reaction of excess 1-
octene with both complexes 3a and 3b at elevated temper-
atures results in the clean formation of 2-octene (Scheme 5).
Figure 2. Crystal structure of 4. Hydrogen atoms and the second
independent molecule of the unit cell are omitted for clarity. The
thermal ellipsoids set at 30% probability. Selected bond lengths [ꢂ]
and angles [8], corresponding values of the second molecule in the
asymmetric unit are given in square brackets: C1-O1 1.334(3)
[1.336(3)], C2-O2 1.333(3) [1.339(3)], C–N1 1.275(3) [1.277(3)], C2-N2
1.276(3) [1.275(3)], Zr1-O1 2.0073(16) [2.0191(15)], Zr1-O2 2.0096(15)
[2.0145(15)]; O1-C1-N1 124.4(2) [125.1(2)], O2-C2-N2 124.5(2)
[124.8(2)], O1-Zr1-O2 95.71(6) [97.69(6)]
Scheme 5. Selective isomerization of 1-octene to 2-octene by
2,1-insertion.
Such isomerization reactions using zirconium complexes are
known,[30] however, the proposed hydrido intermediates have
not been observed to date. The hydrido complexes 3a and 3b
act as catalysts for this reaction. In contrast to the reactivity of
Schwartzꢀs reagent, the initial key step is presumably the
formation of the branched insertion product as an intermedi-
ate, and subsequent b-hydride elimination to give 2-octene.
In an attempt to obtain the branched product selectively,
the reaction of 3a and 3b with styrene (having no allylic
1
hydrogen atoms) was monitored by H NMR spectroscopy.
Scheme 4. Possible isomers of complex 4 in the solution phase.
Schwartzꢀs reagent typically furnishes a mixture of branched
and linear products with this substrate.[28] At room temper-
ature using complex 3a an equilibrium between the branched
insertion product 5a and the starting terminal hydrido
complex 3a was observed (Scheme 6). Well resolved room-
(d = 172.1 ppm) differ significantly from the resonance of the
k1-O bound amidate ligand (d = 163.3 ppm). Also, in the
1H NMR spectrum two distinct ligand environments are
observed whereby the chemical shifts of the methine signals
of the isopropyl groups of one ligand appear as one septet at
d = 3.27 ppm (2H), while the other ligand has two independ-
ent septets at d = 3.63 (1H) and d = 3.54 ppm (1H) respec-
tively.
Hydrozirconation is an important reaction in synthetic
chemistry as it offers a direct route for the functionalization of
unactivated olefins.[1b,c] Hydrozirconation of terminal alkenes
gives the less sterically hindered primary zirconocene alkyl
complexes.[27] Such linear products are even observed when
internal alkenes are used as starting materials, because facile
b-hydride eliminations/reinsertions allow the Cp2Zr moiety to
“chain-walk” to the terminal position.[27] This regioselectivity
can be perturbed by the use of arylalkenes to give mixtures of
the branched and linear products.[28]
While Schwartzꢀs reagent typically provides excellent
regioselectivity for the linear product, it excludes the
possibility of accessing the branched hydrozirconation prod-
uct selectively. By using a half-sandwich Zr complex, Sita and
co-workers have shown that asymmetric hydrozirconation of
Scheme 6. Equilibrium between the two isomers of 5a and 3a/styrene.
temperature spectra showed that over 4 days a reaction
equilibrium can be reached: 42:58 ratio of free styrene:5a.
The 13C NMR spectrum is consistent with a k1-bound amidate
ligand for 5a’ (d = 166.4 ppm). However, at 708C this
equilibrium is shifted and only 3a and free styrene are
detected. Consequently, complex 5a could not be purified and
isolated for complete characterization.
In contrast, at room temperature styrene reacts with the
more sterically demanding 3b to give the branched insertion
product 5b (Scheme 7). In this case, by increasing the reaction
temperature to 708C, completion of the regioselective
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the amidate ligand into a modified Schwartzꢀs reagent favors
the formation of the branched insertion products, either as
isolable or proposed intermediates, with both aryl- and alkyl-
substituted alkenes. The mechanistic rationalization for the
preference for branched insertion products is being inves-
tigated and will be reported in due course. Notably, the
increased steric bulk of 3b specifically enhances the hemi-
lability of this ligand, thereby facilitating the high-yielding
preparation of the branched insertion product 5b exclusively.
Thus, the k1-bound amidate species, with its alkoxyimine
binding mode, is postulated as a key reactive intermediate.
In conclusion, we present a facile synthetic pathway to an
alternative and tunable Schwartzꢀs reagent by direct amide
Scheme 7. Insertion reaction of styrene with complex 3b to give 5b,
observed as an equilibrium.
reaction can be achieved within 4 h and polymerization of
styrene is not observed.
Similar to the 1H NMR spectrum of complex 4, the signals
of complex 5b appear broad at room temperature. This
broadening can be attributed to a similar equilibrium in
solution between the k1- and the k2-amidate isomers (5b’/
5b’’). Low-temperature NMR spectroscopic analysis (À358C)
revealed the existence of only the k1-O bound amidate
isomer. In the 13C NMR spectrum, the resonance of the
central carbon atom of the amidate ligand appears at d =
À
N H bond activation, to furnish the first examples of the
formal oxidative addition products of amides with reduced
early transition metals. In the case of 1a, we were able to
characterize the metal–vinyl intermediate by NMR spectros-
copy and the resulting mononuclear zirconocene hydrido
complex 3a in both the solution phase and by X-ray analysis.
Preliminary reactivity studies showed that 3a and 3b undergo
regioselective insertion reactions with 1-octene and styrene,
resulting in the preferential formation of the branched
insertion products. In the case of 5b, this product could be
structurally characterized by X-ray analysis. The variable
reactivity achieved between these easily prepared amidate
ligated terminal-hydride zirconocene complexes points
toward the tunable reactivity achievable with these systems.
Work in progress focuses on mechanistic investigations to
guide the development of new tunable Schwartzꢀs reagents
with easily installed amidate ligands for novel synthetic
applications in organic and organometallic chemistry.
1
156.1 ppm. In the H NMR spectrum, the methine signals of
the iPr groups of the ligand appear as independent septets at
d = 3.07 and d = 3.00 ppm. Most importantly, the diagnostic
doublet of the methyl group appears at d = 1.55 ppm (3J =
7.0 Hz) as a result of coupling with the methine proton of the
branched insertion product (d = 3.17 ppm). This low-temper-
ature solution-phase data matches the solid-state molecular
structure obtained by X-ray crystallography (Figure 3). The
Received: June 18, 2013
Published online: September 3, 2013
Keywords: amides · insertion · metallacycles ·
.
oxidative addition · zirconocenes
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