Barium-Mediated Cross-Dehydrocoupling of Hydrosilanes with Amines: A Theoretical and Experimental Approach

Article abstract of DOI:10.1002/anie.201502956

Alkaline-earth (most prominently barium) complexes of the type [Ae{N(SiMe₃)₂}₂(THF)_x] and [{N^N}Ae{N(SiMe₃)₂}(THF)_x] are very active and productive precatalysts (TON=396, TOF up

Full text of DOI:10.1002/anie.201502956

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502956
German Edition: DOI: 10.1002/ange.201502956
Synthetic Methods
Barium-Mediated Cross-Dehydrocoupling of Hydrosilanes with
Amines: A Theoretical and Experimental Approach**
ClØment Bellini, Jean-FranÅois Carpentier,* Sven Tobisch,* and Yann Sarazin*
Abstract: Alkaline-earth (most prominently barium) com-
plexes of the type [Ae{N(SiMe3)2}2·(THF)x] and [{N^N}Ae{N-
(SiMe3)2}·(THF)x] are very active and productive precatalysts
primary amines, ammonia, or hydrazine with phenyl-substi-
tuted hydrosilanes.[13a] Hill et al. have shown that [Ae{N-
(SiMe3)2}2] (Ae = Mg, Ca, Sr) operates under mild reaction
conditions, with the Ca precatalyst outperforming its Mg and
Sr congeners.[13b] As is often found in Ae-mediated cataly-
sis,[14] several subtleties linked to the size and charge density
of the Ae metal under consideration were revealed upon
investigating the mechanism. However, no precatalyst of the
larger Ba is known to date for the CDC of amines and
hydrosilanes. We report herein that highly active and selective
Ba complexes surpass their Ca and Sr analogues in the
(TON = 396, TOF up to 3600 hÀ1; Ca < Sr < Ba) for N H/H
Si cross-dehydrocoupling, with excellent chemoselectivity in
the reaction of (di)amines with (di)hydrosilanes. Experimental
and DFT investigations revealed that the reactions proceed by
nucleophilic attack of a metal amide at the incoming silane and
subsequent turnover-limiting hydrogen transfer to the metal
center.
À
À
À
À
S
ilazanides are valuable compounds used as ligands in
catalysis of N H/H Si CDC reactions. The main mechanistic
features are delineated by combining experimental data and
DFT computations.
coordination chemistry,[1] as bases,[2] O Si silylating agents,
or protecting groups in organic synthesis.[4] In addition, oligo-
and polysilazanes constitute excellent precursors of Si3N4
ceramics.[5] Historical protocols for the production of sila-
zanes involving metalation of amines or aminolysis of
chlorosilanes,[6] which generate stoichiometric amounts of
salts or HCl by-products, are being replaced by catalyzed
reactions. Although dependent on ready access to a broad
[3]
À
The bis(amido) compounds [Ae{N(SiMe3)2}2·(THF)x] (x =
0 for Ae = Mg, 1; x = 2 for Ae = Ca, 2; Sr, 3; Ba, 4) and their
derivatives bearing an iminoanilido ligand [{N^N}Ae{N-
(SiMe3)2}·(THF)x] (Ae = Ca, x = 1, 5; Sr, x = 2, 6; Ba, x = 2,
7; {N^N}À = [ArN(o-C6H4)C(H) = NAr]À with Ar= 2,6-
iPr2C6H3)[14d] all catalyze the CDC of pyrrolidine and
triphenylsilane at 258C, with a metal loading as low as 0.25
mol% (Table 1). The synthetically more demanding barium
alkyl complexes [Ba{CH(SiMe3)2}2·(THF)3] (8) and
[{N^N}Ba{CH(SiMe3)2}·(THF)2] (9)[14e] also proved very
effective.
À
À
range of hydrosilanes, the catalyzed N H/H Si cross-dehy-
drocoupling (CDC) of hydrosilanes and HNR1R2 amines (R1,
R2 = H, alkyl, aryl), with H2 being the sole by-product, is an
effective catalytic process. While this catalysis was at first
dominated by simple alkali hydrides[5a,d] or middle- and late-
transition-metal species,[4a,b,5b, 7] some breakthroughs have
now been achieved using borane organocatalysts[4d,8] or
complexes built on ions of oxophilic metals, for example,
Al3+,[9] Yb2+,[10] U4+,[11] Zn2+,[12] and Ae2+ (Ae = alkaline
earth).[10b, 13] However, high catalytic activity and selectivity,
for example, in the coupling of primary/secondary hydro-
silanes with primary amines or ammonia, remain challenging.
The Ae compounds known to catalyze the construction of
Key trends for the Ae-mediated CDC coupling of amines
and hydrosilanes, which must be used as purified, dry
substrates, emerge from Table 1 1) For both families of
Table 1: Alkaline-earth-catalyzed cross-dehydrocoupling of pyrrolidine
and triphenylsilane.[a]
À
N Si bonds are very promising. Notably, Sadowꢀs tris(oxazo-
linyl)borato (ToM) magnesium precatalyst couples either
Entry
Precatalyst
t [min]
Conv. [%][b]
TOF [hÀ1 [c]
]
1
2
3
4
5
6
7
8
9
1
2
3
4
4
5
6
7
7
8
8
9
15”60
15”60
15”60
5
2
0.5
19
26
2544
1584
11
[*] C. Bellini, Prof. Dr. J.-F. Carpentier, Dr. Y. Sarazin
Organometallics: Materials and Catalysis Centre
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS
UniversitØ de Rennes 1, Campus de Beaulieu
263 avenue du GØnØral Leclerc, 35042 Rennes Cedex (France)
E-mail: jean-francois.carpentier@univ-rennes1.fr
yann.sarazin@univ-rennes1.fr
72
99
53
99
42
74
32
94
75
99
54
15
15”60
15”60
15
2”60
5
20
512
188
3600
1584
864
Dr. S. Tobisch
10
11
12
University of St Andrews, School of Chemistry
Purdie Building, North Haugh, St Andrews KY16 9ST (UK)
E-mail: st40@st-andrews.ac.uk
15
15
[**] C.B. thanks the French Ministry for Higher Education and Research
for funding.
[a] Reactions at 298 K in C6D6 (0.5 mL), with [HN(CH2)4]0/[Ph3SiH]0/
[precatalyst]0 =400:400:1 and [Ae]0 =10.0 mm; reactions times not
1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502956.
optimized. [b] Conversion of Ph3SiH determined by H NMR spectros-
copy. [c] Turnover frequency.
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precatalysts, the catalytic activity increases with metal size,
electropositivity, and polarizability according to Mg ! Ca <
Sr< Ba; barium is the metal of choice for this catalysis.[15] The
most active precatalysts are 4 and 8, which fully convert the
substrates within 15 minutes and displayed a unique combi-
nation of productivity (TON = 396) and activity (TOF up to
3600 hÀ1). 2) The simple bis(amido) and bis(alkyl) barium
precatalysts 4 and 8, respectively, perform better than their
derivatives incorporating the bulky iminoanilide (7 and 9),
and hence the use of ancillary ligands is not mandatory. 3) The
barium alkyl precatalysts are more active than their amido
derivatives.[16] The superiority of barium alkyl precatalysts as
opposed to amido ones is attributed to the greater basicity of
the alkyl group, thus resulting in the formation of a larger
amount of barium pyrrolido species which are thought to be
the catalytically active species.[17] Along these line, the
stoichiometric reaction of pyrrolidine [HN(CH2)4] with 9
yields a barium pyrrolido compound upon release of CH2-
(SiMe3)2, whereas
7
reacts to give [{N^N}Ae{N-
(SiMe3)2}·{HN(CH2)4}2] upon displacement of THF, without
À
evidence for aminolysis of the Ba N(SiMe3)2 bond. More-
over, the NMR-scale reaction (C6D6, 258C) of two equiva-
lents of pyrrolidine with 8 releases CH2(SiMe3)2 quantita-
tively, whereas with 4 the evolution of free HN(SiMe3)2
cannot be detected.
The substrate scope was probed with the barium preca-
talysts 4 and 8, by employing, in particular, substrates with
multiple reactive sites. For example, HR2Si^^^SiR2H,
HxSiR4Àx (x ꢀ 2), and/or primary amines, for which chemo-
selectivity is a key aspect (Scheme 1).
The reactions were carried out in C6D6, but identical
results were obtained in C6D5Cl and mixtures of C6D6 and 1,2-
F2-C6H4. The choice of the precatalyst (4 versus 8) was
dictated by the selectivity that was sought and by the
reactivity of the substrates. Like Ph3SiH, mono- and diary-
lhydrosilanes react readily with pyrrolidine (10a–c). The
reaction of PhSiH3 with HN(CH2)4 can be selectively oriented
towards PhSi{N(CH2)4}3 (10b) or PhSiH{N(CH2)4}2 (10c). By
comparison, the reactions with trialkylhydrosilanes are slug-
gish (10d,e); the presence of aromatic ring(s) thus improves
reactions rates. This reactivity is also true for
HR2Si^^^SiR2H: 10 f, 10g, and even 10h [where BnNH2
replaced HN(CH2)4] can be obtained with high selectivities.
Scheme 1. Barium-catalyzed CDC of amines and silanes. Reactions in
C6D6 using precatalysts 4 or 8. For each product (10a–t), the
[amine]0/[silane]0/[precat]0 content, reaction temperature and time,
precatalyst, substrate conversion, and chemoselectivity between
mono-, di-, tri-, and tetrasilazane products are given. The conversion
and chemoselectivity were determined by NMR spectroscopy.
BnNH2 is
a reactive substrate, and the syntheses of
BnNHSiPh3 (10i) and Ph2Si(NHBn)2 (10j) are achieved
quantitatively and selectively, even with high substrate
loading for 10i. In contrast, bulky (tBuNH2, for 10k) or
poorly nucleophilic (anilines, for 10s) amines are less
reactive: even if they are selective towards the monocoupled
silazanes, the reactions necessitate careful control of more
forcing experimental conditions. HN(SiMe3)2 and Ph3CNH2
showed no or limited reactivity.[18] Diamines are valuable
substrates. Even at low precatalyst loading, the reactions of
piperazine or BnNHCH2CH2NHBn with Ph3SiH afford the
dicoupled products 10n and 10q, respectively. The catalyzed
reaction of Ph3SiH and N,N’-dimethylethylenediamine can be
directed towards the mono- or the dicoupled products 10l and
10m, respectively, by tuning of the experimental conditions.
Besides, depending solely on the initial ratio of substrates, the
reaction of H2NCH2CH2NH2 and Ph3SiH affords either the
mono- or the dicoupled products (10o and 10p, respectively).
Coupling occurs only once on each N atom, and further
dehydrocoupling, giving (Ph3Si)2NCH2CH2NH(SiPh3), is
never observed. The reaction of p-xylylenediamine and
Ph3SiH only generates the N,N’-disilazane 10r. Further
reaction of 10j (20 equiv) with Ph2SiH2 (40 equiv) affords
the four-membered cycle 10t. Overall, the combination of
activity, productivity, and chemoselectivity displayed by the
barium compounds 4 and 8 to produce the silazanes 10a–t,
many of which are accessed by CDC processes for the first
time, is unique.
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Kinetic analysis of the benchmark cou-
pling of HN(CH2)4 and Ph3SiH catalyzed
by 7, a slower barium precatalyst which
lends itself well to kinetic monitoring, was
performed in C6D6 at 298 K. The catalyzed
reaction proceeded with first-order kinet-
ics in [7] over a 15-fold concentration
range. Additional experiments using a 21-
fold excess of silane versus HN(CH2)4
indicated a partial zeroth order in [HN-
(CH2)4], and
a partial first-order in
[Ph3SiH] was deduced from the reverse
experiments. The second-order kinetic
rate law thus obeys Àd[Ph3SiH]/dt =
k[Ba]1 [Ph3SiH]1 [HN(CH2)4]0, with k =
4.801(13) ” 10À3 mÀ1 sÀ1. It matches that
found by Sadow and co-workers for their
magnesium complex[13a] but differs from
those reported by Hill et al. for amido
precatalysts.[13b] A primary kinetic isotope
effect (KIE) of kH/kD = 7.8(1) was mea-
sured for the coupling of Ph3SiD and
HN(CH2)4,[19] whereas none was found in
the catalyzed reaction of Ph3SiH and DN-
(CH2)4 [kH/kD = 1.1(1)]. These measure-
ments are consistent with the kinetic rate
Scheme 2. Proposed mechanism for cross-dehydrocoupling of amines and silanes mediated
by an iminoanilide barium silazanido precatalyst.
À
law and indicate Si H bond breaking to be
a key event in the rate-determining step.
The activation parameters for this reaction were derived from
Arrhenius and Eyring analyses:
representing the catalytically competent compound with the
aid of a reliable DFT methodology (dispersion-corrected
B97-D3 in conjunction with basis sets of triple-z quality and
a sound treatment of bulk solvent effects), which has been
demonstrated to reliably map the energy landscape of Ae-
mediated hydroamination.[14f,g] It enables us to exclude all but
one mechanistic pathway which involves nucleophilic attack
of the amide at the silane and subsequent hydrogen transfer
to the Ba center, and it accounts for all the key features
observed.
Scheme 2 depicts the mechanistic pathway that prevails
for CDC of amines with silanes by the iminoanilide barium
silazanido precatalyst 7 (ꢁ C2·(T)2). The initial conversion of
C2·(T)2 into the {N^N}barium pyrrolide compound, which is
predominantly present as the bis(amine) adduct C3·(A)2 and
shows no propensity toward dimer formation, is sufficiently
facile kinetically, as revealed by a recent computational study
of the hydroamination of styrene catalyzed by 7.[14f] As far as
catalytically competent silane adducted species (C3·S·(A)n)
are concerned, a single adducted pyrrolidine molecule
stabilizes C3·S greatly, whilst the presence of the rather
bulky Ph3SiH prevents the {N^N}Ba center from accommo-
dating a second amine molecule.
Ea = 16.2 Æ 2.3 kcalmolÀ1, DH° = 15.6 Æ 2.3 kcalmolÀ1, and
DS° = À13.3 Æ 0.7 calmolÀ1 KÀ1, that is, DG298° = 19.6 Æ
0.1 kcalmolÀ1 at 298 K. The value of DS° indicates an
associative mechanism, but it is small compared to those
given for Mg and Sr precatalysts,[13] and may reflect that the
large size of the Ba2+ ion (rionic = 1.38 Š) induces a less
constrained arrangement in the transition state.
Electronic effects were assessed by changing the sub-
stituent at the para position of one of the aromatic rings in the
hydrosilane. The data for the CDC of HN(CH2)4 with Ph2(p-
X-C6H4)SiH catalyzed by 7 under standard reaction condi-
tions, where X is either OMe, Me, H, F, or CF3, showed that
the reaction rate increased for electron-withdrawing p-
substituents, and varied according to X = OMe < Me < H <
F < CF3. Hammett analysis of the linear plot ln(kH/kX) =
sp(X)1 revealed a large, positive slope of 1 = 2.0 Æ 1.0,
evidence that electron-withdrawing groups at the para posi-
tion lower the activation barrier through stabilization of
a developing negative charge.
On the basis of these kinetic data, the generation of
silazane products (P) can be envisioned to proceed through
either stepwise or concerted pathways to furnish a barium
hydrido intermediate, which is transformed into the barium
pyrrolido compound thereafter (Scheme 2). In an attempt to
further inform our understanding, the CDC of pyrrolidine (A)
and Ph3SiH (S) by [{N^N}Ba{N(SiMe3)2}·(THF)2] (7, denoted
thereafter as C2·(T)2) was subjected to thorough computa-
tional examination. We studied various mechanistic pathways
conceivable for either a barium pyrrolide or barium silyl
The energetically prevalent mechanism (Scheme 3)
involves the initial nucleophilic attack of the barium amido
at the silane to furnish the transient silicate intermediate C4,
with subsequent hydrogen transfer to barium, thus affording
the {N^N}barium hydride C5 with the release of the silazane
product P. The most accessible pathway commencing from
C3·S·(A)n, featuring an only loosely associated silane mole-
cule, benefits from the participation of one associated
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of catalytically competent iminoanilide
barium amide and barium alkyl com-
pounds. The turnover-limiting hydrogen
transfer accounts for the substantial pri-
mary KIE and rate acceleration observed
for a silane having an electron-withdrawing
group at the para position. Its DFT-derived
barrier satisfactorily matches empirically
determined Eyring parameters. A similar
mechanism has been proposed for ToM
magnesium catalysts,[13a] but where nucleo-
philic attack is turnover limiting.[19] We are
now implementing this catalysis to obtain
polymeric materials, and to produce dis-
symmetric polysilazanes by stepwise CDC
of primary diamines and dihydrosilanes.
Scheme 3. Prevalent mechanistic pathway for cross-dehydrocoupling of pyrrolidine and
triphenylsilane by an iminoanilide barium pyrrolide catalyst complex.
pyrrolidine spectator molecule on both kinetic and thermo-
À
dynamic grounds. The initial N Si bond-forming nucleophilic
attack of the barium pyrrolide is seen to be kinetically
affordable (DG° = 17.4 kcalmolÀ1 relative to C3·(A)2). It is
linked to the energetically more demanding hydrogen trans-
fer to barium (DG° = 19.0 kcalmolÀ1 relative to C3·(A)2), via
a transient metastable silicate intermediate featuring a five-
coordinate silicon center. Scheme 3 reveals that P is likely
displaced readily at the bis(amine) adducted {N^N}barium
hydride (C5·(A)2) by an incoming amine. The stepwise
C3·(A)2!C3·S·A!C4·A!C5·P·A!C5·(A)2 formation of
the silazane product is virtually thermoneutral, with the
second slow hydrogen transfer to barium dictating the
kinetics of this reaction.
Keywords: amines · alkaline earth metals · barium ·
density functional calculations · reaction mechanisms
How to cite: Angew. Chem. Int. Ed. 2015, 54, 7679–7683
Angew. Chem. 2015, 127, 7789–7793
[1] M. Lappert, P. P. Power, A. Protchenko, A. Seeber, Metal Amide
Chemistry, Wiley, Chichester, 2009.
[2] L. F. Fieser, M. Fieser, Reagents in Organic Chemistry, Wiley,
New York, 1967.
[3] Y. Tanabe, T. Misaki, M. Kurihara, A. Iida, Y. Nishii, Chem.
Commun. 2002, 1628.
[4] a) D. W. Robbins, T. A. Boebel, J. F. Hartwig, J. Am. Chem. Soc.
2010, 132, 4068; b) S. Itagaki, K. Kamata, K. Yamaguchi, N.
Mizuno, Chem. Commun. 2012, 48, 9269; c) C. D. F. Kçnigs,
M. F. Müller, N. Aiguabella, H. F. T. Klare, M. Oestreich, Chem.
Commun. 2013, 49, 1506; d) L. Greb, S. Tamke, J. Paradies,
Chem. Commun. 2014, 50, 2318.
[5] a) D. Seyferth, G. H. Wiseman, J. Am. Ceram. Soc. 1984, 67,
C132; b) Y. D. Blum, K. B. Schwartz, R. M. Laine, J. Mater. Sci.
1989, 24, 1707; c) H. Q. Liu, J. F. Harrod, Organometallics 1992,
11, 822; d) N. R. Dando, A. J. Perrotta, C. Strohmann, R. M.
Stewart, D. Seyferth, Chem. Mater. 1993, 5, 1624; e) M. Birot, J.-
P. Pilot, J. Dunogu›s, Chem. Rev. 1995, 95, 1443.
[6] R. Fessenden, J. S. Fessenden, Chem. Rev. 1961, 61, 361.
[7] a) L. H. Sommer, J. D. Citron, J. Org. Chem. 1967, 32, 2470; b) Y.
Blum, R. M. Laine, Organometallics 1986, 5, 2081; c) C. Biran,
Y. D. Blum, R. Glaser, D. S. Tse, K. A. Youngdahl, R. M. Laine,
J. Mol. Catal. 1988, 48, 183; d) E. J. Matarasso-Tchiroukhine,
Chem. Soc. Chem. Commun. 1990, 681; e) W. D. Wang, R.
Eisenberg, Organometallics 1991, 10, 2222; f) C. K. Toh, H. T.
Poh, C. S. Lim, W. Y. Fan, J. Organomet. Chem. 2012, 717, 9.
[8] A. Kawachi, M. Zaima, A. Tani, Y. Yamamoto, Chem. Lett. 2007,
36, 362.
All our attempts to localize a low-energy TS structure
À
À
describing Ba N/Si H s-bond-breaking metathesis, thus
describing a concerted process analogue (see Scheme 2),
have been unsuccessful. It can thus confidently be concluded
that such a process would require a much higher, insurmount-
able barrier to overcome and is thus unlikely to have any
relevance for catalyst turnover.
The ensuing protonolysis of the pyrrolidine by the
{N^N}barium hydride converts C5 back into the catalytically
competent {N^N}Ba pyrrolide complex for another catalyst
turnover through the release of one equivalent of H2. Similar
to what has been found for the stepwise silazane formation,
another adducted amine spectator molecule facilitates the
process. The pyrrolidine C5·(A)2 (+ A)!C3·(A)2 + H2
protonolysis is kinetically easy (DG° = 11.2 kcalmolÀ1 rela-
tive to C5·(A)2; Scheme 3) and almost thermoneutral. Hence,
the {N^N}barium hydride C5·(A)2 and {N^N}barium pyrro-
lide C3·(A)2 compounds can readily interconvert, thus
participating in a mobile equilibrium.
[9] L. K. Allen, R. Garcia-Rodriquez, D. S Wright, Dalton Trans.
2015, DOI: 10.1039/C5DT00662G.
In conclusion, barium precatalysts exhibit unique activ-
ities and selectivities towards a broad range of substrates in
the cross-dehydrocoupling of amines and hydrosilanes. Our
collective experimental and computational approach provide
compelling evidence in support of the prevailing mechanism
[10] a) K. Takaki, T. Kamata, Y. Miura, T. Shishido, K. Takehira, J.
Org. Chem. 1999, 64, 3891; b) F. Buch, S. Harder, Organo-
metallics 2007, 26, 5132; c) W. Xie, H. Hu, C. Cui, Angew. Chem.
Int. Ed. 2012, 51, 11141; Angew. Chem. 2012, 124, 11303.
[11] J. X. Wang, A. K. Dash, J.-C. Berthet, M. Ephritikhine, M. S.
Eisen, J. Organomet. Chem. 2000, 49, 610.
[12] T. Tsuchimoto, Y. Iketani, M. Sekine, Chem. Eur. J. 2012, 18,
9500.
[13] a) J. F. Dunne, S. R. Neal, J. Engelkemier, A. Ellern, A. D.
Sadow, J. Am. Chem. Soc. 2011, 133, 16782; b) M. S. Hill, D. J.
À
being a stepwise pathway comprising N Si bond-forming
nucleophilic attack of the barium pyrrolide at the silane and
subsequent turnover-limiting hydrogen transfer to the Ba
center. This step is linked to kinetically easy amine proto-
nolysis for CDC of amines and organosilanes in the presence
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Chemie
Liptrot, D. J. MacDougall, M. F. Mahon, T. P. Robinson, Chem.
Sci. 2013, 4, 4212.
[16] When, other things being equal, Ph3SiH was replaced at 333 K by
the less reactive Et3SiH, 8 and 4 gave respectively 93 and 5%
conversion.
[14] a) A. G. M. Barrett, C. Brinkmann, M. R. Crimmin, M. S. Hill, P.
Hunt, P. A. Procopiou, J. Am. Chem. Soc. 2009, 131, 12906; b) M.
Arrowsmith, M. R. Crimmin, A. G. M. Barrett, M. S. Hill, G.
Kociok-Kçhn, P. A. Procopiou, Organometallics 2011, 30, 1493;
c) C. Brinkmann, A. G. M. Barrett, M. S. Hill, P. A. Procopiou, J.
Am. Chem. Soc. 2012, 134, 2193; d) B. Liu, T. Roisnel, J.-F.
Carpentier, Y. Sarazin, Angew. Chem. Int. Ed. 2012, 51, 4943;
Angew. Chem. 2012, 124, 5027; e) B. Liu, T. Roisnel, J.-F.
Carpentier, Y. Sarazin, Chem. Eur. J. 2013, 19, 13445; f) S.
Tobisch, Chem. Eur. J. 2014, 20, 8988; g) S. Tobisch, Chem. Eur. J.
2015, 21, 6765.
[17] [Ba(SiPh3)2·(THF)3], obtained in THF from BaI2 and KSiPh3,
does not catalyze the reaction of Ph3SiH and pyrrolidine.
[18] The reactions of HN(SiMe3)2 (20 equiv vs. 4) or Ph3CNH2
(20 equiv vs. 8) with Ph3SiH (20 equiv) at 333 K gave respec-
tively 0 and 24% conversion. The absence of reactivity of
HN(SiMe3)2 with Ph3SiH was reported for [Ae{N(SiMe3)2}2];
Ref. [13b].
[19] No significant KIE was found for the reaction between
{ToM}MgNHtBu and PhMeSiD2 at 273 K: kH/kD = 1.0(2);
Ref. [13a].
[15] This finding contrasts with Hillꢀs report on [Ae{N(SiMe3)2}2]
CDC precatalysts, for which activity increased with Ae = Mg <
Sr< Ca; Ref. [13b]. Under our experimental conditions
([Ph3SiH]0/[HN(CH2)4]0/[Ae]0 = 400:400:1, 298 K, C6D6), we
found that [Ae{N(SiMe3)2}2] (Ae = Ca–Ba) and their THF
adducts 2–4 behaved almost identically.
Received: March 31, 2015
Published online: May 26, 2015
Angew. Chem. Int. Ed. 2015, 54, 7679 –7683
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7683