Synthesis, Structure, and Reactivity of a Terminal Magnesium Hydride Compound with a Carbatrane Motif, [TismPriBenz]MgH: A Multifunctional Catalyst for Hydrosilylation and Hydroboration

Article abstract of DOI:10.1021/jacs.7b06719

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl)]methyl ligand, [Tism^PriBenz], has been employed to form the magnesium carbatrane compound, [Tism^PriBenz]MgH, which possesses a terminal hydride ligand. Speci

Full text of DOI:10.1021/jacs.7b06719

Communication
pubs.acs.org/JACS
Synthesis, Structure, and Reactivity of a Terminal Magnesium
i
Hydride Compound with a Carbatrane Motif, [TismPr Benz]MgH: A
Multifunctional Catalyst for Hydrosilylation and Hydroboration
Michael Rauch, Serge Ruccolo, and Gerard Parkin*
Department of Chemistry, Columbia University, New York, New York 10027, United States
S
* Supporting Information
of which has been structurally characterized by X-ray diffraction.
ABSTRACT: The tris[(1-isopropylbenzimidazol-2-yl)-
i
For example, protolytic cleavage of the Mg-Me bond by H2S and
dimethylsilyl)]methyl ligand, [TismPr Benz], has been
PhNH2 affords uncommon terminal hydrosulfido15,16 and
employed to form the magnesium carbatrane compound,
i
anilido17 magnesium compounds, namely [TismPr Benz]MgSH
i
[TismPr Benz]MgH, which possesses a terminal hydride
i
i
(2) and [TismPr Benz]MgN(H)Ph (3). Metathesis of
ligand. Specifically, [TismPr Benz]MgH is obtained via the
i
i
reaction of [TismPr Benz]MgMe with PhSiH3. The reactivity
[TismPr Benz]MgMe with Me3SnX [X = F (4), Cl (5), Br (6), I
i
i
of [TismPr Benz]MgMe and [TismPr Benz]MgH allows access
i
(7)] provides the series of halide derivatives [TismPr Benz]MgX,
i
to a variety of other structurally characterized carbatrane
with [TismPr Benz]MgF being most noteworthy because terminal
i
derivatives, including [TismPr Benz]MgX [X = F, Cl, Br, I,
magnesium fluoride compounds are rare.14a
SH, N(H)Ph, CH(Me)Ph, O2CMe, S2CMe]. In addition,
i
[TismPr Benz]MgH is a catalyst for (i) hydrosilylation and
hydroboration of styrene to afford the Markovnikov
products, Ph(Me)C(H)SiH2Ph and Ph(Me)C(H)Bpin,
and (ii) hydroboration of carbodiimides and pyridine to
form N-boryl formamidines and N-boryl 1,4- and 1,2-
dihydropyridines, respectively.
i
Scheme 1. Reactivity of [TismPr Benz]MgMe
y comparison to other main group elements, terminal
Bhydride compounds of the alkaline earth Group 2 metals are
rare.1 For example, although we reported the first monomeric
t
terminal beryllium hydride, [TpBu ]BeH, in 1992,2,3 structurally
authenticated monomeric terminal magnesium hydride com-
pounds remained unknown until 2010, when Jones and Stasch
t
reported [NacnacBu ]Mg(DMAP)H.4−6 Moreover, structurally
characterized terminal calcium hydride compounds are currently
unknown.7 Despite their paucity, however, terminal hydride
compounds of these elements offer much potential, and are often
invoked as intermediates in a variety of catalytic cycles, including
hydrosilylation, hydroboration, hydroamination, and hydrogen-
ation.1,8,9 Such transformations involving Mg and Ca are of
particular interest because these are two of the most abundant
elements in the Earth’s crust. As such, efficient catalysis using Mg
and Ca offers potential societal benefits, especially considering
that, for example, industrial hydrosilylation reactions typically
employ precious metal catalysts.10 Therefore, we describe here
i
While each of the [TismPr Benz]MgX compounds adopts an
idealized trigonal bipyramidal geometry, a noteworthy difference
pertains to the location of the X substituent. Specifically, the X
i
substituents of [TismPr Benz]MgX [X = Me, F, Cl, Br, N(H)Ph]
the synthesis, structure, reactivity, and catalytic applications of
adopt an axial position that is trans to the atrane C-atom, whereas
i
i
the terminal magnesium hydride compound, [TismPr Benz]MgH,
the X substituents of [TismPr Benz]MgX [X = I, SH] adopt an
which features a carbatrane motif provided by the tris-
equatorial position. These two geometries are associated with
i
different coordination modes for the [TismPr Benz] ligand. Thus,
[(1‑isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand.
i
We recently described the use of the [TismPr Benz] ligand to
an axial X substituent is associated with a local C3 coordination
i
mode of the [TismPr Benz] ligand (with a trigonal arrangement of
afford the methyl magnesium carbatrane compound,
i
[TismPr Benz]MgMe (1).11,12 Here, we report that the reactivity
associated with the Mg-Me bond provides access to a variety of
Received: June 28, 2017
i
other [TismPr Benz]MgX derivatives (Schemes 1 and 2),13,14 each
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b06719
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Journal of the American Chemical Society
Communication
i
Scheme 2. Synthesis and Reactivity of [TismPr Benz]MgH
i
Figure 1. Molecular structures of [TismPr Benz]MgH and
i
[TismPr Benz]MgCH(Me)Ph.
provided by the observation that an EXSY experiment
demonstrates that the Mg-H and Si-H groups of
i
[TismPr Benz]MgH and PhSiH3 undergo degenerate exchange
N-atoms), whereas an equatorial X substituent is associated with
an idealized seesaw coordination mode (with a T-shaped
arrangement of N-atoms).
on the NMR time scale.27
PriBenz
The reactivity of [Tism
]MgH is not restricted to
metathesis reactions. In particular, the Mg-H bond of
In addition to metathesis reactions, the Mg-Me bond is also
subject to insertion reactions. Thus, CO2 and CS2 insert into the
Mg-Me bond to afford the acetate and dithioacetate derivatives,
PriBenz
[Tism
]MgH undergoes insertion of styrene to afford the
PriBenz
1-phenylethyl derivative [Tism
]MgCH(Me)Ph (11,
i
i
Scheme 2), which provides the first structurally authenticated
(Figure 1) example of the insertion of an olefin into a terminal
[TismPr Benz]Mg(κ2-O2CMe) (8) and [TismPr Benz]Mg(κ2-
S2CMe) (9), as illustrated in Scheme 1. The molecular structures
i
i
magnesium hydride bond.28,29 Of note, the alkyl ligand of
of [TismPr Benz]Mg(κ2-O2CMe) and [TismPr Benz]Mg(κ2-S2CMe)
were determined by X-ray diffraction, thereby revealing that the
PriBenz
[Tism
]MgCH(Me)Ph occupies an equatorial site of a
PriBenz
trigonal bipyramid, whereas that of [Tism
in an axial site.
]MgMe is located
acetate and dithioacetate ligands coordinate to the Mg center in a
i
κ2-manner, with the [TismPr Benz] ligand adopting a seesaw
coordination mode (Scheme 1). While magnesium acetate
compounds are well known,18,19 there are no structurally
characterized magnesium dithioacetate compounds listed in
Another interesting feature of the insertion of styrene is that it
affords selectively the branched 1-phenylethyl derivative,
PriBenz
[Tism
]MgCH(Me)Ph, rather than the less sterically
d e m a n d i n g l i n e a r 2 - p h e n y l e t h y l d e r i v a t i v e ,
the CSD.
PriBenz
[Tism
]MgCH2CH2Ph. Although the 2-phenylethyl deriv-
i
The formation of [TismPr Benz]Mg(κ2-O2CMe) is of some
ative is less sterically demanding, density functional theory
relevance to the mechanism of action of Rubisco, the most
abundant protein on Earth.20 Specifically, Rubisco is involved in
carbon fixation and employs Mg to mediate the formation of a
C-C bond between CO2 and the C2 position of D-ribulose 1,5-
bisphosphate to form a carboxylate derivative.20 The 2-carboxy-
D-arabinitol 1,5-bisphosphate transition state analogue has been
calculations demonstrate that the branched isomer,
i
[TismPr Benz]MgCH(Me)Ph, is 5.0 kcal mol−1 more stable than
i
the corresponding linear isomer, [TismPr Benz]MgCH2CH2Ph.
With respect to the observed selectivity, it is pertinent to note
that the insertion of an olefin into a M-H bond to afford a linear
isomer is often associated with the primary alkyl derivative being
generally more stable than secondary and tertiary derivatives.30,31
Electron-withdrawing substituents on the olefin, however, favor
formation of the branched isomer,32−34 which Jones attributed to
the insertion reaction being driven by the formation of the isomer
shown to possesses a magnesium−carboxylate interaction,21 and
i
so the formation of [TismPr Benz]Mg(κ2-O2CMe) from CO2
resembles an aspect of the mechanism of action of Rubsico,
which has otherwise received little attention from a synthetic
analogue perspective.22,23
The most significant transformation of [TismPr Benz]MgMe
with the stronger C-H bond.34 On this basis,
i
[TismPr Benz]MgCH(Me)Ph would be more thermodynamically
i
i
stable than [TismPr Benz]MgCH2CH2Ph because the former
possesses a strong methyl C-H bond whereas the latter possesses
involves the metathesis reaction with PhSiH3, which provides
access to the terminal magnesium hydride compound,
PriBenz
a weaker benzylic C-H bond.34
24
[Tism
]MgH (10, Scheme 2). The molecular structure
i
i
of [TismPr Benz]MgH was determined by X-ray diffraction (Figure
1), thereby demonstrating that the compound exists as a well-
defined monomeric species with a terminal hydride ligand in the
axial site of a trigonal bipyramidal coordination environment.
The Mg-H bond length is 1.85(3) Å, which is comparable to the
Analogous to [TismPr Benz]MgMe, the Mg-C bond of
i
[TismPr Benz]MgCH(Me)Ph also undergoes metathesis with
PhSiH3 to form the magnesium hydride complex,
i
[TismPr Benz]MgH, releasing Ph(Me)C(H)SiH2Ph and providing
i
a means by which [TismPr Benz]MgH may serve as a catalyst for
hydrosilylation of styrene (Scheme 3).35 The proposed
mechanism for the catalytic cycle is illustrated in Scheme 2,
and analysis of the reaction via 1H NMR spectroscopy indicates
sum of the covalent radii (1.72 Å).13,25 Spectroscopically, the
i
Mg-H moiety of [TismPr Benz]MgH is characterized by a singlet at
δ 6.78 in the 1H NMR spectrum.26
PriBenz
that the resting state of the system is the alkyl form,
i
Parallel to the reactivity observed for [Tism
]MgMe, the
[TismPr Benz]MgCH(Me)Ph.
hydride complex may likewise be employed to form
i
i
i
[TismP r B e n z ]MgSH, [TismP r B e n z ]MgN(H)Ph, and
Furthermore, since [TismPr Benz]MgH is obtained upon
i
i
[TismPr Benz]MgX (X = F, Cl, Br, I) via the respective metathesis
reactions with H2S, PhNH2, and Me3SnX (cf. Scheme 1). In
addition, a particularly noteworthy example of metathesis is
treatment of [TismP r B e n z ]MgMe with PhSiH3 ,
i
[TismPr Benz]MgMe may also serve as a precatalyst. Although
the activity is not high,36 the observation represents the first
B
DOI: 10.1021/jacs.7b06719
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Journal of the American Chemical Society
Communication
example of hydrosilylation of an olefin catalyzed by a magnesium
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b06719.
■
hydride compound.37 In addition, since styrene inserts into the
S
Mg-H bond to afford selectively the 1-phenylethyl isomer,
i
[TismPr Benz]MgCH(Me)Ph, the hydrosilylation proceeds in a
Markovnikov manner, in contrast to the anti-Markovnikov
selectivity that is commonly observed for transition metal
catalysts.38−40
Experimental details and computational data (PDF)
Crystallographic data (ZIP)
i
AUTHOR INFORMATION
Corresponding Author
*parkin@columbia.edu
Scheme 3. Catalytic Activity of [TismPr Benz]MgH
■
ORCID
Gerard Parkin: 0000-0003-1925-0547
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-1465095) for
support of this research. M.R. acknowledges the National Science
Foundation for a Graduate Research Fellowship under Grant No.
DGE-16-44869.
REFERENCES
i
i
■
[TismPr Benz]MgH and [TismPr Benz]MgMe likewise effect
catalytic hydroboration of styrene to form the Markovnikov
product, Ph(Me)C(H)Bpin. Although the activity is low,36 the
observation is noteworthy because alkylboronic esters are of
much utility in organic synthesis,41 and there are no other reports
(1) (a) Harder, S. Chem. Commun. 2012, 48, 11165. (b) Sarish, S. P.;
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(3) Tp = tris(pyrazolyl)hydroborate; Nacnac = β-diketiminate.
(4) Bonyhady, S. J.; Jones, C.; Nembenna, S.; Stasch, A.; Edwards, A. J.;
McIntyre, G. J. Chem. - Eur. J. 2010, 16, 938.
(5) In addition to [NacnacBu ]Mg(DMAP)H, only two other
structurally characterized terminal magnesium hydride compounds are
known: (a) Arrowsmith, M.; Maitland, B.; Kociok-Kohn, G.; Stasch, A.;
Jones, C.; Hill, M. S. Inorg. Chem. 2014, 53, 10543. (b) Schnitzler, S.;
Spaniol, T. P.; Maron, L.; Okuda, J. Chem. - Eur. J. 2015, 21, 11330. The
structure of a terminal hydride compound that is contaminated with the
hydroxide derivative has also been reported: (c) Lalrempuia, R.; Stasch,
A.; Jones, C. Chem. - Asian J. 2015, 10, 447.
(6) Bridging hydride compounds are also known. See, for example, refs
1, 4, and: Gallagher, D. J.; Henderson, K. W.; Kennedy, A. R.; O’Hara, C.
T.; Mulvey, R. E.; Rowlings, R. B. Chem. Commun. 2002, 376.
(7) Calcium compounds with bridging hydride ligands have,
nevertheless, been reported. See, for example: Spielmann, J.; Harder,
S. Chem. - Eur. J. 2007, 13, 8928.
(8) (a) Rochat, R.; Lopez, M. J.; Tsurugi, H.; Mashima, K.
ChemCatChem 2016, 8, 10. (b) Hill, M. S.; Liptrot, D. J.; Weetman,
C. Chem. Soc. Rev. 2016, 45, 972. (c) Arrowsmith, M. Enc. Inorg. Bioinorg.
Chem. 2015, 1−26. (d) Crimmin, M. R.; Hill, M. S. Top. Organomet.
Chem. 2013, 45, 191. (e) Revunova, K.; Nikonov, G. I. Dalton Trans.
2015, 44, 840.
of Mg-catalyzed hydroboration of olefins.42
i
In addition to hydroboration of styrene, [TismPr Benz]MgH is
t
also a catalyst for hydroboration of carbodiimides and pyridine
(Scheme 3). Hydroboration of carbodiimides has received
̈
relatively little attention,43 and so it is of note that
i
[TismPr Benz]MgH catalyzes the formation of N-boryl form-
amidines, RNC(H)N(R)Bpin, by addition of HBpin to
i
RNCNR (R = Cy, Pri). Significantly, [TismPr Benz]MgH is
an effective catalyst36 at room temperature, more active than the
only other reported catalyst for this transformation.43a Finally,
i
i
[TismPr Benz]MgH and [TismPr Benz]MgMe also effect catalytic
addition of HBpin to pyridine to afford N-boryldihydropyridine,
pinBNC5H6, as a mixture of 1,4- and 1,2-isomers, of which the
former isomer is obtained preferentially.44,45 The latter trans-
formation is of relevance because dearomatization is a useful
strategy in organic synthesis,46 and the dearomatization of
pyridine is of particular note because dihydropyridines (i) are a
component of pharmaceuticals and natural products and (ii)
serve as hydride sources in reductions.47
(9) (a) Dunne, J. F.; Neal, S. R.; Engelkemier, J.; Ellern, A.; Sadow, A.
D. J. Am. Chem. Soc. 2011, 133, 16782. (b) Schnitzler, S.; Spaniol, T. P.;
Okuda, J. Inorg. Chem. 2016, 55, 12997. (c) Spielmann, J.; Buch, F.;
Harder, S. Angew. Chem., Int. Ed. 2008, 47, 9434. (d) Buch, F.; Brettar, J.;
Harder, S. Angew. Chem., Int. Ed. 2006, 45, 2741.
In summary, the terminal hydride compound,
i
[TismPr Benz]MgH, has been synthesized via the reaction of
i
i
[TismPr Benz]MgMe with PhSiH3. [TismPr Benz]MgH exhibits
diverse reactivity with a variety of substrates to afford
i
[TismPr Benz]MgX [X = F, Cl, Br, I, SH, N(H)Ph, CH(Me)Ph].
(10) Meister, T. K.; Riener, K.; Gigler, P.; Stohrer, J.; Herrmann, W. A.;
Kuhn, F. E. ACS Catal. 2016, 6, 1274.
i
Of particular interest, [TismPr Benz]MgH undergoes insertion of
styrene into the Mg-H bond to afford the branched 1-
(11) Ruccolo, S.; Rauch, M.; Parkin, G. Chem. Sci. 2017, 8, 4465.
(12) For related atrane systems, see: Kuzu, I.; Krummenacher, I.;
Meyer, J.; Armbruster, F.; Breher, F. Dalton Trans. 2008, 5836.
(13) All reactions in Scheme 1 are performed at room temperature (rt).
Dashed Mg---C bonds (and open bonds in thermal ellipsoid plots)
indicate that there is a significant ionic component to the atrane
interaction, as observed in related carbatranes. In this regard, the Mg-C
bond lengths in these compounds range from 2.31 to 2.50 Å, longer than
the value of 2.17 Å predicted by the sum of covalent radii. Cordero, B.;
i
phenylethyl derivative, [TismPr Benz]MgCH(Me)Ph. The latter
i
compound reacts with PhSiH3 to afford [TismPr Benz]MgH and
Ph(Me)C(H)SiH2Ph, thereby providing a means by which
i
[TismPr Benz]MgH can serve as a catalyst for Markovnikov
i
hydrosilylation. In addition to hydrosilylation, [TismPr Benz]MgH
is also a catalyst for hydroboration of styrene, carbodiimides, and
pyridine.
C
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Journal of the American Chemical Society
Communication
Gom
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́
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(45) For example, whereas a 7:1 ratio of 1,4 to 1,2 isomers is obtained
at 60 °C (10% catalyst loading), an 11:1 ratio is obtained at 100 °C.
Mixtures of N-boryl-1,4- and 1,2-dihydropyridine isomers are observed
in other catalytic systems (refs 44b and 45a,b), and examples of selective
1,2-addition (refs 44b and 45c,d) and 1,4-addition (ref 9b) are known.
(a) Oshima, K.; Ohmura, T.; Suginome, M. J. Am. Chem. Soc. 2012, 134,
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(28) In this regard, although [Me3TACD·AlBui3]MgH does not react
with styrene, it does insert Ph2C=CH2 to afford linear and branched
isomers, neither of which has been structurally characterized. See ref 9b.
(29) It is also worth noting that hydromagnesiation of olefins using
“MgH2” or in situ generated “HMgX” species often employs transition
metal catalysis. See: Greenhalgh, M. D.; Thomas, S. P. Synlett 2013, 24,
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(35) Although there is negligible reduction of styrene to ethylbenzene
for 10% catalyst loading, higher concentrations of styrene, correspond-
ing to 1% catalyst loading, result in the formation of small amounts (ca.
15%) of ethylbenzene. For other examples of styrene hydrogenation
employing PhSiH3, see: (a) Baruah, J. B.; Osakada, K.; Yamamoto, T. J.
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i
(36) TOFs for [TismPr Benz]MgMe (10%) as a precatalyst:
PhCH=CH2/PhSiH3 (0.9 h−1 at 60 °C), PhCH=CH2/HBpin (0.3 h−1
at 60 °C), PriN=C=NPri/HBpin (8.5 h−1 at rt), CyN=C=NCy/HBpin
(6.5 h−1 at rt), and py/HBpin (0.9 h−1 for formation of the isomeric
mixture at 60 °C).
D
DOI: 10.1021/jacs.7b06719
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX