Sequential Barium-Catalysed N?H/H?Si Dehydrogenative Cross-Couplings: Cyclodisilazanes versus Linear Oligosilazanes

Article abstract of DOI:10.1002/chem.201603191

Starting from Ph₃SiH, the barium precatalyst Ba[CH(SiMe₃)₂]₂?(THF)₃was used to produce the disilazane Ph₃SiN(Bn)SiPh₂NHBn (4) by sequential N?H/H?Si dehydrogenative couplings with BnNH₂and Ph₂SiH₂. Substrate scope was extended to other amines and hydrosilanes. This smooth protocol gives quantitative yields and full chemoselectivity. Compound 4 and the intermediates Ph₃SiNHBn and Ph₃SiN(Bn)SiHPh₂were structurally characterised. Further attempts at chain extension by dehydrocoupling of Ph₂SiH₂with 4 instead resulted in cyclisation of this compound, forming the cyclodisilazane c-(Ph₂Si-NBn)₂(5) which was crystallographically authenticated. The ring-closure mechanism leading to 5 upon release of C₆H₆was determined by complementary experimental and theoretical (DFT) investigations. Ba[CH(SiMe₃)₂]₂?(THF)₃and 4 react to afford the reactive Ba{N(Bn)SiPh₂N(Bn)SiPh₃}₂, which was characterised in situ by NMR spectroscopy. Next, in a stepwise process, intramolecular nucleophilic attack of the metal-bound amide on the terminal silicon atom generates a five-coordinate silicate. It is followed by turnover-limiting β-C₆H₅transfer to barium; this releases 5 and forms a transient [Ba]?Ph species, which undergoes aminolysis to regenerate [Ba]?N(Bn)SiPh₂N(Bn)SiPh₃. DFT computations reveal that the irreversible production of 5 through such a stepwise ring-closure mechanism is much more kinetically facile (ΔG^≠=26.2 kcal mol^?1) than an alternative σ-metathesis pathway (ΔG^≠=48.2 kcal mol^?1).

Full text of DOI:10.1002/chem.201603191

DOI: 10.1002/chem.201603191
Full Paper
&
Cross-Coupling
Sequential Barium-Catalysed NꢀH/HꢀSi Dehydrogenative Cross-
Couplings: Cyclodisilazanes versus Linear Oligosilazanes
Clꢀment Bellini,[a] Thierry Roisnel,[b] Jean-FranÅois Carpentier,*[a] Sven Tobisch,*[c] and
Yann Sarazin*[a]
Abstract: Starting from Ph3SiH, the barium precatalyst
Ba[CH(SiMe3)2]2·(THF)3 was used to produce the disilazane
Ph3SiN(Bn)SiPh2NHBn (4) by sequential NꢀH/HꢀSi dehydro-
genative couplings with BnNH2 and Ph2SiH2. Substrate scope
was extended to other amines and hydrosilanes. This
smooth protocol gives quantitative yields and full chemose-
lectivity. Compound 4 and the intermediates Ph3SiNHBn and
Ph3SiN(Bn)SiHPh2 were structurally characterised. Further at-
tempts at chain extension by dehydrocoupling of Ph2SiH2
with 4 instead resulted in cyclisation of this compound,
forming the cyclodisilazane c-(Ph2Si-NBn)2 (5) which was
crystallographically authenticated. The ring-closure mecha-
nism leading to 5 upon release of C6H6 was determined by
complementary experimental and theoretical (DFT) investi-
gations. Ba[CH(SiMe3)2]2·(THF)3 and 4 react to afford the reac-
tive Ba{N(Bn)SiPh2N(Bn)SiPh3}2, which was characterised in
situ by NMR spectroscopy. Next, in a stepwise process, intra-
molecular nucleophilic attack of the metal-bound amide on
the terminal silicon atom generates a five-coordinate silicate.
It is followed by turnover-limiting b-C6H5 transfer to barium;
this releases 5 and forms a transient [Ba]ꢀPh species, which
undergoes aminolysis to regenerate [Ba]ꢀN(Bn)SiPh2N-
(Bn)SiPh3. DFT computations reveal that the irreversible pro-
duction of 5 through such a stepwise ring-closure mecha-
nism is much more kinetically facile (DG° =26.2 kcalmolꢀ1)
than an alternative s-metathesis pathway (DG° =48.2 kcal
molꢀ1).
Introduction
oligo- and polysilazanes involve the dehydrocoupling of
amines and hydrosilanes, which can be catalysed by a variety
Silazanes are common and versatile compounds containing Nꢀ of complexes of late transition metals,[7] titanium,[8] or the
Si bonds which can be used in coordination chemistry,[1] as
bases,[2] silylating agents[3] or protecting groups for amines, in-
doles and anilines in organic synthesis.[4] Oligo- and polysila-
zanes are commonly prepared by alkali-mediated ring-opening
polymerisation of cyclosilazanes following the Seyferth-Wise-
man procedure.[5] Although readily implemented, this oligo/
polymerisation method necessitates the synthesis of cyclosila-
zanes that are not easily characterised.[5,6] Alternative routes to
older polycondensation by ammonolysis and aminolysis reac-
tions.[6a,9] Polycarbosilazanes are related preceramics containing
-(SiꢀCꢀN)n- backbones; amorphous SiCN ceramics are useful
for their corrosion resistance, high-temperature stability and
long-term durability for applications as structural materials.[10]
Polycarbosilazanes are commonly synthesised by aminolysis of
dichlorosilanes with 1,2-ethylenediamine[11] or other diam-
ines[12] upon release of ammonium chlorides as by-products.
Original dendrimers were obtained in an iterative two-step
strategy employing first Karstedt’s catalyst for the hydrosilyla-
tion of tris(vinyldimethylsilyl)amine with chlorodimethylsilane,
followed by treatment with alkali bis(vinyldimethylsilyl)am-
ide.[13] Nevertheless, until recently, the most atom-efficient syn-
thetic protocol relied on the cross-dehydrocoupling (CDC) of
1,4-bis(dimethylsilyl)benzene and ammonia catalysed by
Pd2(dba)3, an attractive process even though the reaction re-
quires 72 h to convert only a portion of 200 equiv of the co-
monomers.[14]
[a] C. Bellini, Prof. Dr. J.-F. Carpentier, Dr. Y. Sarazin
Organometallics, Materials and Catalysis Department
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS
Universitꢀ de Rennes 1, Campus de Beaulieu, 35042 Rennes (France)
E-mail: jean-francois.carpentier@univ-rennes1.fr
yann.sarazin@univ-rennes1.fr
[b] Dr. T. Roisnel
Centre de Diffractomꢀtrie des Rayons X
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS
Universitꢀ de Rennes 1, 35042, Rennes (France)
[c] Dr. S. Tobisch
We have been keen on implementing alkaline earth com-
plexes (Ae=Ca, Sr, Ba) in catalysed NꢀH/HꢀSi dehydrocoupling
reactions, a clean route to the production of silazanes[15] and
polycarbosilazanes.[16] We have shown that barium precatalysts
display a unique combination of high activity and chemoselec-
tivity in the couplings of a variety of amines, (di)hydrosilanes,
a,w-diamines and a,w-di(hydrosilanes), which has enabled
School of Chemistry, University of St Andrews
Purdie Building, North Haugh, St Andrews KY16 9ST (UK)
E-mail: st40@st-andrews.ac.uk
Supporting information, including general procedures, synthetic protocols,
complete characterisation for all new compounds, crystallographic data, ki-
netic measurements and DFT calculations, as well as the ORCID number(s)
for the author(s) of this article are available under http://dx.doi.org/
10.1002/chem.201603191.
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access to a range of silazanes and linear disilazanes. Other cat-
alytic systems exist for these types of reactions. Some are built
on middle and late transition metals, for example, Al2O3-sup-
ported palladium species and graphene-supported palladium
nanoparticles,[17] Ru3(CO)12 and Rh6(CO)16 clusters,[7a–b,18] as well
as discrete ruthenium,[4b,e] rhodium,[4c,7c] chromium[19] or copper
complexes.[20] Precatalysts based on oxophilic metals such as ti-
tanium,[8,21] aluminium,[22] yttrium,[23] ytterbium(II),[24] urani-
um(IV),[25] zinc,[26] alkali metals[27] and alkaline earths,[15,28] for
which s-bond metathesis or Si-to-metal b-hydride transfer are
key mechanistic features, have proved very effective of late.
We report here the utilisation of Ba[CH(SiMe3)2]2·(THF)3,
a most efficient barium-based precatalyst for NꢀH/HꢀSi dehy-
drocouplings,[15] for the preparation of original compounds
containing several silazanyl linkers. They are obtained by a con-
trolled extension process upon sequential coupling of a variety
dihydrosilazanes and diamines. It is shown how the same pre-
catalyst can also readily lead to the ring-closing formation of
4-membered cyclodisilazanes. The results of DFT computations,
aimed at probing the rival mechanistic pathways leading to
cyclisation, are presented.
triphenylsilane and benzylamine ([Ph3SiH]0/[BnNH2]0/[A]0 =
400:400:1, [BnNH2]0 =4.0m in benzene, Bn=benzyl) at 258C
within 2 h to afford the known monocoupled product
Ph3SiNHBn (1) with complete chemoselectivity. Single crystals
of 1, the starting material for the subsequent work described
here, were obtained and its molecular solid-state structure was
established. Under these experimental conditions, the forma-
tion of the di-coupled product, (Ph3Si)2NBn (2), or that of cyclic
disilazanes were not detected. Compound 2 could only be ob-
tained in poor yields by coupling of Ph3SiH with pre-isolated
1 (12 h, 258C, yield: 15%; 608C, yield: 22%); high precatalyst
loading was required, typically [Ph3SiH]0/[1]0/[A]0 =20:20:1,
with [1]0 =0.20m in benzene (Scheme 1).[29] The difficulties en-
countered in obtaining 2 are reminiscent of the little propensi-
ty shown by R3SiNHR’ silazanes (R, R’=aryl, alkyl) to engage in
Ae-mediated cross-dehydrocoupling reactions with bulky hy-
drosilanes such as Ph3SiH, and can probably be ascribed to ex-
cessive steric congestion.[15]
With a less cumbersome and more reactive hydrosilane, the
coupling of high loadings of 1 with diphenylsilane (Ph2SiH2)
catalysed by A ([Ph2SiH2]0/[1]0/[A]0 =400:400:1, [1]0 =4.00m in
benzene) was quantitative within 2 h at 258C and afforded the
asymmetric disilazane Ph3SiN(Bn)SiHPh2 (3) with complete se-
lectivity (Scheme 2). This compound was isolated and dehydro-
coupled with BnNH2 ([3]0/[BnNH2]0/[A]0 =100:100:1, [BnNH2]0 =
1.0m in benzene) at 608C within 2 h, to give the linear product
Ph3SiN(Bn)SiPh2NHBn (4) quantitatively. See the Supporting In-
formation for complete product characterisation.
Results and Discussion
Initial reactivity studies
The barium precatalyst Ba[CH(SiMe3)2]2·(THF)3 (A), which has re-
cently proved very effective in NꢀH/HꢀSi CDC reactions,[15,16]
was selected for the present study. It catalyses the coupling of
Scheme 1. Synthesis of Ph3SiNHCH2C6H5 (1) catalysed by Ba[CH(SiMe3)2]2·(THF)3 (A).[29]
Scheme 2. Stepwise syntheses of silazanes and (cyclo-)disilazanes catalysed by Ba[CH(SiMe3)2]2·(THF)3 (A). All reactions were carried out in benzene.[29]
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We tried to extend this stepwise chain growth to the pro-
duction of longer molecules with the ambition to obtain oligo-
and, ultimately, polycarbosilazanes. However, the attempted
coupling of 4 with Ph2SiH2 (0.2m for each substrate) did not
yield the expected Ph3SiN(Bn)SiPh2N(Bn)SiPh2H, but instead re-
turned quantitatively the cyclodisilazane c-(Ph2SiꢀNBn)2 (5) and
unreacted Ph2SiH2. The catalytic (5.0 mol% of A) formation of 5
upon cyclisation of 4 with concomitant release of benzene
(vide infra) also occurs at higher substrate concentrations
(2.0m), or even in the absence of Ph2SiH2 under otherwise
identical experimental conditions (608C, 2 h, [4]0/[A]0 =20:1,
[4]0 =0.2m).[30]
Overall, the multistep sequential Ba-promoted process form-
ing 4 from triphenylsilane is unusual, as well as being highly
effective and chemoselective. Like 1, silazanes 3–5 were all iso-
lated as colourless solids, and crystals were grown from con-
centrated pentane solutions. Their identities were established
on the basis of the NMR spectroscopic (1H, 13C{1H}, 29Si{1H}),
mass spectrometric and crystallographic data of the purified
products. For these and the following related compounds, ele-
mental analyses were often thwarted because of high silicon
contents, but they were overall consistent with the proposed
formulations.[31,32]
(MeHN^^^NHMe) incorporating two secondary amines with
N,N’-dimethyl substituents was also tested in the coupling with
3, but no conversion to Ph3SiN(Bn)SiPh2N(Me)CH2C6H4CH2NHMe
was detected. Similar observations were made previously in
the couplings with Ph3SiH upon going from BnNH2, which is
highly reactive, to the secondary amine Bn2NH, which showed
almost no reactivity.[15b] CDC reactions are thus greatly sensitive
to steric effects.
Starting materials other than 1 and 3 are also amenable to
couplings catalysed by A (Scheme 4). It has been shown that
the coupling of 2 equiv of Ph3SiH with 1,4-phenylenedimethan-
amine selectively affords Ph3SiNHCH2C6H4CH2NHSiPh3 (9).[15b]
This precursor was further subjected to coupling with 2 equiv
of Ph2SiH2. The reaction was quantitative (258C, 2 h, [Ph2SiH2]0/
[9]0/[A]0 =200:100:1) and produced the bis(disilazane)
Ph2HSi(Ph3Si)NCH2C6H4CH2N(Ph3Si)-SiHPh2 (10), which contains
two hydrosilanyl functional groups in the a and w positions.
The subsequent coupling of the difunctional 10 with 1 equiv
of H2N^^^NH2 at 258C for 1 h ([10]0/[H2N^^^NH2]0/[A]0 =
20:20:1) yielded an insoluble white solid, probably a cross-
linked polycarbosilazane resulting from dehydropolymerisa-
1
tion; the H and 29Si NMR data of this material could not inter-
preted. The reaction of 10 with 5 or 10 equiv of H2N^^^NH2
versus A ([10]0/[H2N^^^NH2]0/[A]0 =20:X:1) gave intractable
mixtures. HSiPh2N(Me)CH2C6H4CH2NHMe (11) is a starting mate-
rial for the grafting of either a dihydrosilane or an amine, on
the -NHMe or the -NMeSiPh2H tail-ends, respectively, or as a bi-
functional AB monomer for dehydropolymerisation reactions
to obtain high molecular polycarbosilazanes.[16] It can be ob-
tained from the quantitative and chemoselective coupling of
Reaction scope
Starting from 3, the method employed to obtain 4 by coupling
with BnNH2 was extended to other amines (Scheme 3).
Ph3SiN(Bn)SiPh2N(CH2)4 (6) was obtained in a 42% unoptimised
yield upon dehydrocoupling of 3 and pyrrolidine under mild
conditions. The coupling of 3 with diamines or with primary
amines is more pertinent. Instead of BnNH2 leading to the for-
mation of 4, the CDC of 3 with MesCH2NH2 (Mes=mesityl)
yielded Ph3SiN(Bn)SiPh2NHMes (7) quantitatively. The coupling
of 3 with 1,4-phenylenedimethanamine (H2N^^^NH2) afforded
Ph3SiN(Bn)SiPh2NHCH2C6H4CH2NH2 (8). Cyclisation and forma-
MeHN^^^NHMe with
(Scheme 4).
a
single equivalent of Ph2SiH2
Substituents can be introduced onto the amines, as in 7 (see
Scheme 3), or alternatively in the hydrosilanes. For instance, (p-
CF3C6H4)Ph2SiNHBn (12) was prepared upon coupling of (p-
CF3C6H4)Ph2SiH and BnNH2 (Scheme 4). The coupling of 12
tion of a [Si2N2] cyclic moiety giving a putative [Si2N2]ꢀ with Ph2SiH2 ([Ph2SiH2]0/[12]0/[A]0 =20:20:1) smoothly gave (p-
CH2C6H4CH2NH2 species could perhaps occur in 8. To prevent
such scenario, 1,1’-(1,4-phenylene)bis(N-methylmethanamine)
CF3C6H4)Ph2SiN(Bn)SiHPh2 (13), which could be further reacted
with BnNH2 to afford (p-CF3C6H4)Ph2SiN(Bn)SiPh2NHBn (14). In
Scheme 3. Synthesis of linear disilazanes catalysed by Ba[CH(SiMe3)2]2·(THF)3 (A) starting from Ph3SiN(Bn)SiHPh2 (3). Experimental conditions: Precatalyst A
(5.0 mmol), [3]0/[amine]0/[A]0 =20:20:1, [amine]0 =0.2m in benzene, 608C, 2 h.[29]
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Scheme 4. Sequential grafting of various amines and hydrosilanes in CDC reactions catalysed by Ba[CH(SiMe3)2]2·(THF)3 (A).[29] All reactions performed in ben-
zene.
the way seen for the cyclisation from 4 to 5, upon exposure to
precatalyst A at 608C for 12 h, the linear 14 led to the forma-
tion of 5 by selective elimination of 1 equiv of C6H5CF3, as es-
tablished by 1H and 19F NMR spectroscopy. The mechanism
proposed for the cyclisation, with formation of a transient pen-
tavalent silicate and transfer of a partially negatively charged
aromatic moiety to barium stabilised by para electron-with-
drawing groups, accounts for the selectivity towards the elimi-
nation of p-CF3C6H5 in the production of 5 from 14 (vide infra).
This series of reactions also works for p-methyl-substituted ar-
ylhydrosilanes. However, they are more sluggish reagents, for
example requiring heating for 12 h at 608C to afford (p-
CH3C6H4)Ph2SiN(Bn)SiPh2NHBn. Further cyclisation of this com-
pound does not even reach 10% conversion (upon selective
elimination of benzene, while the release of toluene is not de-
tected) after heating for 12 h at 608C. In a rough empirical
evaluation, the reaction rate therefore seems to increase when
electronic density on the silicon atom decreases (Me<H<CF3),
and the proposed mechanism detailed in the following sec-
tions is consistent with this observation.
The formation of the cyclic product 5 upon elimination of
an aromatic molecule, benzene in the case of 4 and trifluoro-
methylbenzene for 14, is not the sole method for the prepara-
tion of cyclosilazanes. The coupling of BnNHCH2CH2NHBn with
Ph2SiH2 catalysed by A or even Ba[N(SiMe3)2]2·(THF)2 takes
place readily to give the corresponding 5-membered cycle 15,
c-[Ph2SiN(Bn)CH2CH2N(Bn)] (Scheme 5). Furthermore, com-
pound 5 itself had previously been obtained by barium-medi-
ated H2-releasing dehydrocoupling of Ph2SiH2 with
BnNHSiPh2NHBn.[15a]
The peculiarity of the formation of 5 from 4 or 14 lies in the
fact that it is formed not by cross-dehydrocoupling of an
amine and hydrosilane with hydrogen evolution, but upon
ring-forming, barium-catalysed SiꢀC bond cleavage with con-
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after isolation and re-dissolution in benzene in the presence of
A (0.05 equiv, 608C, 2 h; see Scheme 2), cleanly and quantita-
tively yielded 5 in a catalysed process, suggests an intriguing
reactivity pattern. We are unable at this time to propose a con-
clusive explanation for this phenomenon. One conceivable hy-
pothesis is that during the formation of 4 by coupling of
BnNH2 and 3, the catalyst resting state is under the form of
a putative “[Ba(NHBn)2(S)x]n” species, in which S is THF or
BnNH2.[41] This species may be insufficiently basic to deproto-
nate the newly-formed 4 present in the reaction mixture. In
contrast, reaction of the pre-isolated, purified 4 with the highly
basic barium bis(alkyl) A must produce a species akin to
“Ba[N(Bn)SiPh2N(Bn)SiPh3]2·(THF)n” (DFT computations favours
n=0, vide infra) by aminolysis and irreversible release of
CH2(SiMe3)2. The fact that the relatively less basic Ba[N(Si-
Me3)2]2·(THF)2 fails entirely to react with 4 (only unreacted ma-
terials are returned) is congruent with this working hypothesis.
In the absence of any reactive substrate other than 4, if this
new compound “Ba[N(Bn)SiPh2N(Bn)SiPh3]2·(THF)n” evolves, it
can only be towards the formation of the cyclodisilazane 5. If
so, this may for instance occur by nucleophilic attack of the
Namide atom onto the terminal Si atom creating a hypervalent
silicate followed by b-C6H5 transfer to Ba, or perhaps by a less
likely s-metathetical pathway. Such processes should give rise
to a transient [Ba]ꢀPh species, which is basic enough to de-
protonate more incoming 4 and, in doing so, regenerate
Scheme 5. Formation of cyclo(di)silazanes by barium-mediated CDC reac-
tions.[29]
comitant elimination of one stoichiometric equivalent of aro-
matic by-product. Although not common, the rupture of SiꢀC
bonds has already been documented under other circumstan-
ces. The cleavage of SiꢀC bonds under acidic conditions has
long been known.[33] More recently, several cases of metal-pro-
moted SiꢀC bond ruptures have been disclosed for both Csp2
and Csp3 carbon atoms. For instance, the lutetium hydride
[(C5Me5)2Lu(m-H)]2 was reported to cleave the SiꢀCsp2 bond in
PhSiH3 to generate benzene and cross-linked polysilanes
(SiHx)y.[34] Nickel and palladium silyl pincer complexes can un-
dergo structural rearrangements implicating reversible SiꢀCsp3
and SiꢀCsp2 bond activation.[35] The rhodium-catalysed coupling
of 2-trimethylsilylphenylboronic acid with internal alkynes
“Ba[N(Bn)SiPh2N(Bn)SiPh3]·(THF)n”
(Scheme 6).
and
release
benzene
produces
2,3-disubstituted
benzosilole
derivatives.[36]
Rh(H)(CO)(PPh3)3 triggers SiꢀCR bond cleavage in [o-
(Ph2P)C6H4]2SiMeR (R=Ph, Me, Et); it was shown that the facili-
ty of bond activation increased with temperature and accord-
ing to SiꢀEt<SiꢀMe<SiꢀPh.[37] Perhaps more relevant to the
present work, the stoichiometric conversion of SiꢀCaryl s-bonds
into Siꢀheteroatom ones in the reactions of h3-a-silabenzyl
molybdenum and tungsten complexes with 2-substituted pyri-
dines has been reported very recently.[38] The cleavage of SiꢀC
bonds is also promoted by supercritical water[39] or by mont-
morillonite.[40]
Regarding the mechanism of the cyclisation reaction
The NMR-scale reaction of A with 2 equiv of 4 in [D6]benzene
1
(2 h, 258C) was examined by H NMR spectroscopy (Figure 1).
It generated preponderantly a new barium compound, the
1H NMR data of which showed in particular two sharp singlets
integrating for two hydrogens each at d1H 4.59 and 4.13 ppm
and corresponding to two types of C6H5CH2 methylene hydro-
gens. This data agreed with the formulation Ba[N(Bn)-
SiPh2N(Bn)SiPh3]2 (Figure 1). Quantitative release of CH2(SiMe3)2
was also detected, together with the presence of 5 (about
The fact that 4 could be synthesised from 3 under catalytic
conditions and without contamination by 5, and that pure 4,
Scheme 6. Possible stepwise mechanism involving a transient [Ba]ꢀPh species for the quantitative synthesis of c-(Ph2SiNBn)2 (5) starting from
Ph3SiN(Bn)SiPh2NHBn (4) and catalysed by Ba[CH(SiMe3)2]2·(THF)3 (A).
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Figure 1. 1H NMR spectrum ([D6]benzene, 258C, 400.1 MHz) of the NMR-scale reaction of Ba[CH(SiMe3)2]2·(THF)3 (A) with 2 equiv of Ph3SiN(Bn)SiPh2NHBn (4),
&
showing the quantitative release of CH2(SiMe3)2 and formation of a minor amount of c-(Ph2SiNBn)2 (5). *=unreacted 4. =residual Et2O.
40%, diagnostic singlet at d1H 4.08 ppm) and minute amounts
of unreacted 4. The preparation of an authentic sample of this
complex by reacting A with 2.0 equiv of 4 showed that the
0.1 calmolꢀ1 Kꢀ1 (DG° =23.4ꢁ0.1 kcalmolꢀ1 at 298 K) were ex-
tracted by Eyring analysis of kinetic data recorded in the tem-
perature range 328–353 K (five data points). The cyclisation
4!5 upon exclusive elimination of C6H5CF3 from 14 proceeds
with kobs =5.16ꢁ0.1ꢂ10ꢀ5 sꢀ1. Enhancement of the cyclisation
rate upon addition of an electron-withdrawing group in the
para position of one of the aryl rings on the silicon atom that
undergoes ring-closure, is consistent with the existence of
a negative charge developing on this Si atom in the transition
state. Finally, the formation of (C6D5)3SiN(Bn)SiPh2NHBn (or its
(C6D5)x(Ph)3ꢀxSiN(Bn)Si(C6D5)x’(Ph)2ꢀx’NHBn precursors) is not de-
tected when the benzene-releasing transformation 4!5 is car-
ried out in [D6]benzene over several hours, revealing that this
ring-closing process is irreversible. Considered collectively,
these observations are compatible with a mechanism such as
that described in Scheme 6.
1
THF molecules did not remain metal-bound. The H NMR data
of the product obtained after work-up was consistent with
that given in Figure 1, except for the fact that no resonance
for THF was detectable. These data confirm that Ba[N(Bn)-
SiPh2N(Bn)SiPh3]2 can be made, but also that it is unstable at
room temperature. It spontaneously evolves in solution to gen-
erate 5 in increasing amounts over the course of 6 h, and also
partly decomposes to release 4 together with unidentified
barium species.
The formation of 5 upon addition of 20 equiv of 4 to A in
[D8]toluene at 608C, that is, under catalytic conditions, was
monitored spectroscopically over the course of several hours.
Gradual but eventual quantitative formation of 5, together
with the release of benzene could be visualised (sharp singlet
Importantly,
no
ring-closure
takes
place
when
resonances at d1H 7.13 ppm and d 128.53 ppm) concomitant-
Me3SiN(Bn)SiPh2NHBn (18), with alkyl instead of aryl substitu-
ents on the tail-end silicon atom, is heated in [D6]benzene for
12 h at 608C in the presence of 5.0 mol% of A. The compound
remains unreacted throughout the course of the experiment,
with no evolution detected spectroscopically. The release of
benzene in the cyclisation process from 4 to 5 (instead of an
alkane from 18) seems to be a key driving force, or at least
one can safely conclude that for this process, SiꢀCsp2 bonds are
more likely to undergo activation than SiꢀCsp3 bonds. Com-
pound 18 had first been isolated after the two-step sequential
procedure (Scheme 7) involving the coupling of the known
13C
ly with the consumption of 4. The kinetic rate law exhibits par-
tial first order in both catalyst and substrate concentrations.
The value of the observed rate constant, kobs,H =1.98ꢁ0.1
ꢂ10ꢀ5 sꢀ1, was determined from the semi-logarithmic plot of
conversion versus reaction time. No primary kinetic isotope
effect (KIE) was seen (the cyclisation of Ph3SiN(Bn)SiPh2NDBn
also proceeded with kobs,D =2.00ꢁ0.1ꢂ10ꢀ5 sꢀ1, kobs,H/kobs,D
=
0.99ꢁ0.1), indicating that the rupture of the NꢀH bond is not
a key component of the turnover-limiting step. The activation
parameters DH° =20.6ꢁ0.2 kcalmolꢀ1 and DS° =ꢀ9.5ꢁ
Scheme 7. Catalytic synthesis of Me3SiN(Bn)SiPh2NHBn (18).
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Me3SiNHBn (16) with Ph2SiH2 to afford Me3SiN(Bn)SiHPh2 (17),
followed by coupling with BnNH2.[42]
one THF for an amine molecule and evolves through a meta-
thesis-type transition-state (TS) structure to deliver intermedi-
ate CI2·(T)2·RSi featuring a barium-bound H2C(SiMe3)2 moiety.
Its rapid displacement by another amine molecule prepares for
protonolytic cleavage of the second BaꢀC alkyl bond through
a similar metathesis-type TS structure that decays thereafter
into C2 with the rapid release of H2C(SiMe3)2 and THF. The Baꢀ
C alkyl bond protonolysis has an affordable barrier of 16.7 kcal
molꢀ1 (for aminolysis of the first BaꢀC bond, see Figure S18) to
overcome and is driven by a huge thermodynamic force that
amounts to 35.3 kcalmolꢀ1. Thus, one can safely conclude that
barium bis-alkyl C1·(T)3 is likely converted irreversibly in a quan-
titative fashion by 4Me into the competent barium bis-silazanyl
amido compound C2.
It has been shown above that 4 exhibits a distinct reactivity
pattern under varying experimental conditions, on the one
hand, when generated under conditions of CDC catalysis
(hence in the presence of benzylamine), or alternatively in the
absence of any amine substrate other than 4. Thus, we
deemed it instructive to study the aminolysis of C1·(T)3 by
NH2Bn and the conversion of the thus formed barium bis-ben-
zylamido C3 into C2 as well. The sequential protonolytic dis-
placement of a hydrocarbyl with an amido group by either 4Me
or NH2Bn, share structural features and similar energy profiles.
For the less encumbered primary NH2Bn, the most accessible
pathway (see Figure S22, but also Figures S21, S23, S24 in the
Supporting Information) proceeds after the initial displacement
of all three THF molecules by amine with a moderate overall
barrier of 7.5 kcalmolꢀ1 to engage thereafter in the cleavage of
the remaining BaꢀC bond. This affords the NH2Bn adduct
C3·(Am)3 of the barium bis-benzylamido compound. Overall,
the conversion of starting material C1·(T)3 in the presence of
The benzene-releasing cyclisation from 4 to 5 is not specific
to barium precatalysts; it is also catalysed by, for instance,
Ca[CH(SiMe3)2]2·(THF)2 (B), the calcium analogue of A. In a con-
trol experiment performed under otherwise identical experi-
mental conditions ([4]0/[precatalyst]0 =20:1, 608C in benzene,
[precatalyst]0 =10 mm), B catalyses the reaction at a rate very
comparable to that of A, kobs =9.16ꢁ0.3ꢂ10ꢀ6 sꢀ1 and 1.98ꢁ
0.1ꢂ10ꢀ5 sꢀ1, respectively. Eyring analyses in the temperature
range 328–353 K (five data points) allowed for the calculation
of the activation parameters for B, DH° =16.8ꢁ0.1 kcalmolꢀ1
and DS° =ꢀ22.3ꢁ0.1 calmolꢀ1 Kꢀ1, with DG° =23.4ꢁ0.1 kcal
molꢀ1 at 298 K. This final value replicates that for A, but the re-
spective enthalpic and entropic contributions are very differ-
ent. Due to its much smaller size compared to barium (effec-
tive rionic for C.N.=6: Ca2+, 1.00 ꢃ; Ba2+, 1.38 ꢃ), calcium has
a greater entropic factor that becomes unfavourable as the
temperature of the reaction is increased. The energy of activa-
tion determined by Arrhenius analysis is actually lower for the
calcium precatalyst B (Ea =17.5ꢁ0.1 kcalmolꢀ1; R2 =0.9938)
than for the barium A (Ea =21.3ꢁ0.2 kcalmolꢀ1; R2 =0.9928).
Computational investigations
A reliable state-of-the-art density functional theory (DFT)
method has been employed to complement the mechanistic
studies reported above with an aim to further enhance our un-
derstanding of mechanistic intricacies behind the generation
of cyclodisilazanes. We have examined various mechanistic
pathways conceivable for the conversion of disilazane 4 into
the 4-membered cyclodisilazane 5 by the barium precatalyst A
(denoted C1·(T)3 hereafter). For the disilazane, the two silicon-
bound phenyl spectator groups have been replaced by methyl
ones (4Me) for the sole purpose of expediting computations,
but no further simplifications of any kind have been imposed
for any of the key species involved. The computational meth-
odology employed (dispersion-corrected B97-D3 in conjunc-
tion with triple-z basis sets and a sound treatment of bulk sol-
vent effects; see the computational methodology in the Sup-
porting Information) adequately simulated the authentic reac-
tion conditions and has been demonstrated before to reliably
map the free-energy landscape of alkaline earth-mediated hy-
droelementation reactions.[15,43] This has allowed mechanistic
conclusions with substantial predictive value to be drawn.
We started with an examination of the BaꢀC alkyl bond ami-
nolysis at starting material C1·(T)3 (ꢂA) by 4Me, thereby trans-
forming C1·(T)3 into the related [Ba{N(Bn)SiMe2N(Bn)SiMe2Ph}2]
barium bis-amido compound C2, which is likely competent to
trigger the 4Me!5Me conversion. The activation of C1·(T)3 en-
tails that amine binds initially at barium. Several pathways,
which are distinguished by the number of THF molecules to
remain bound at barium, have been examined. The most ac-
cessible case (Figure S18) together with the complete account
of all the studied pathways (Figure S17–S20) can be found in
the Supporting Information. The energetically prevalent path-
way for BaꢀC alkyl bond aminolysis sees the first exchange of
4
Me or NH2Bn into the respective barium bis-amido compounds
is found affordable kinetically and strongly downhill, driven by
a thermodynamic force of comparable substantial amount.
For the experimental setup of CDC catalysis to be applied,
the barium bis-benzylamido, which is predominantly present
as amine adduct C3·(Am)3, is likely representing the catalyst
resting state.[15] Hence, under such conditions, C3 needs first to
be converted into C2 to proceed along the various conceivable
avenues that lead to the formation of the cyclodisilazane. In-
terestingly, the sequential exchange of benzylamido by silazan-
yl amido appears to be equally kinetically viable, featuring an
overall activation energy of 11.3 kcalmolꢀ1. This barrier is
linked to aminolysis of the first BaꢀN amido bond, along the
energetically prevalent pathway (see Figure S26 but also Figur-
es S25, S27, S28 in the Supporting Information) that proceeds
from the bis-silazane adduct C3·(4Me)2. However, the thermody-
namic force associated with the overall transformation of C3
into C2 is found strikingly different to the findings for starting
material C1·(T)3. The huge negative reaction free energy pre-
dicted there is indicative of an irreversible and essentially
quantitative conversion of C1·(T)3 into the respective barium
bis-amido compounds. This is in sharp contrast to the process
here, which does not benefit from a massive thermodynamic
driving force, but is rather thermoneutral with C3·(Am)3QC2
likely in a mobile equilibrium that does not favour C2 over C3.
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The benzylamido ligand is thus predicted to effect deprotona-
tion of 4Me substantially less so than does the more basic hy-
drocarbyl ligand. The ineffectiveness of the protonolytic ex-
change of benzylamido by silazanyl amido at C3·(Am)3 due to
unfavourable thermodynamics limits the population of C2
under conditions optimal for CDC catalysis. Given that C2
cannot be expected to be present in appreciable amounts
under such conditions, it explains why the formation of cyclo-
disilazane 5 could not be observed during catalytic experi-
ments.
the TS structure decays into a metastable nucleophilic silicate
intermediate C4 with a five-coordinate silicon centre carrying
a partial negative charge. The cyclisation has an activation bar-
rier of 18.7 kcalmolꢀ1 to overcome along the most accessible
pathway (Figure 2), which does not see the participation of ad-
ditional metal-bound amine molecules (see Figure S30), to
afford the metastable high-energy barium silicate C4. It charac-
terises CꢀN bond-forming ring closure to be kinetically afforda-
ble and reversible, with C2QC4 in a mobile equilibrium that
strongly favours C2. A metal-bound amine molecule appears
to counterbalance to some extent the weakening of the BaꢀN
linkage in the TS structure and also the silicate intermediate.
However, additional 4Me is found to not stabilise the key spe-
cies involved (see Figure S30).
The barium bis-silazanyl amido compound C2 is predicted
to be predominantly present without amine or THF molecules
directly bound to barium. It features a metal centre that is ca-
pable of accommodating THF molecules at moderate costs,
whereas the propensity of 4Me to bind is substantially less pro-
nounced.
As already indicated by its profoundly elongated SiꢀPh link-
age (2.09 ꢃ) together with a suitably towards barium oriented
phenyl group, silicate C4 shows a distinct aptitude to undergo
b-C6H5 abstraction from the five-coordinate terminal silicon
onto barium. It evolves through a TS structure with distances
of approximately 2.83 and 2.75 ꢃ for vanishing SiꢀC and
emerging BaꢀC phenyl bonds (see Figure S31), respectively,
and generates the cyclodisilazane 5Me bound to barium silazan-
yl amido phenyl C5.
Focusing at first on the intrinsic reactivity, an additionally as-
sociated 4Me molecule is seen facilitating the process on both
kinetic and thermodynamic grounds (see Figures 3 and S32 in
the Supporting Information). Phenyl transfer along the favoura-
ble C4·4Me!C5·5Me·4Me pathway features a rather modest in-
trinsic activation barrier (DG°int =9.0 kcalmolꢀ1, relative to
C4·4Me,) and is virtually thermoneutral (DGint =0.9 kcalmolꢀ1,
relative to C4·4Me). Hence, as far as intrinsic reactivity is con-
cerned, phenyl transfer is found to proceed more rapidly than
preceding cyclisation, with C4·4Me and C5·5Me·4Me expected to
participate in a mobile equilibrium, but without a distinct ther-
Turning now to the plausible mechanistic proposal outlined
in Scheme 6, it comprises NꢀSi bond-forming cyclisation at C2
with b-C6H5 abstraction from the thus formed barium silicate
C4. These processes could proceed through stepwise or meta-
thesis-like pathways, to furnish a barium amido phenyl com-
pound C5 to which cyclodisilazane 5Me may be bound. Proto-
nolytic cleavage of the BaꢀC phenyl bond at C5 by another
molecule of 4Me would regenerate C2 with release of 1 equiv
of benzene.
Commencing from C2, the initial NꢀSi bond-forming cyclisa-
tion evolves through a TS structure that describes ring closure
by nucleophilic attack of the metal-bound amido nitrogen at
the terminal silicon within the vicinity of the barium centre at
distances of around 2.41 and 2.67 ꢃ (see Figure S29 in the Sup-
porting Information) for the emerging NꢀSi and dative BaꢀN
bonds, respectively. The terminal phenyl group, featuring an al-
ready elongated SiꢀC linkage (1.94 ꢃ) is suitably aligned to-
wards the barium centre. Following the reaction path further,
Figure 2. NꢀSi bond-forming cyclisation at barium bis-silazanyl amido C2. Energies are given in kcalmolꢀ1 relative to {C2+nꢂ4Me}.
Figure 3. b-C6H5 transfer onto the metal centre at barium silicate C4. Energies are given in kcalmolꢀ1 relative to {C2+nꢂ4Me}.
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modynamic preference for either side. However, the unfavoura-
ble thermodynamic profile of the C2QC4 ring closure (togeth-
er with 4Me complexation at C4) markedly favours C2 over
C4·4Me, which renders the second phenyl transfer of the step-
wise C2QC4(+4Me)QC4·4MeQC5·5Me·4Me more kinetically de-
manding.
of disilazane 4 observed under varying experimental setups
and defines the most accessible among the various examined
pathways for its conversion into cyclodisilazane 5. It involves:
(1) kinetically affordable and reversible NꢀSi bond-forming cyc-
lisation through nucleophilic attack of the barium silazanyl
amido at the terminal silicon, (2) reversible b-C6H5 transfer onto
the barium centre at the thus generated barium silicate and
(3) rapid and strongly downhill BaꢀC phenyl-bond aminolysis
to regenerate the barium bis-silazanyl amido with concomitant
release of benzene. The TS structure for b-C6H5 transfer repre-
sents the species of highest energy traversed along the identi-
fied minimum-energy pathway. The DFT-derived effective barri-
er of 26.2 kcalmolꢀ1 for conversion of 4Me into 5Me mediated
by barium precatalyst A compares favourably with experimen-
tally determined Eyring parameters for the 4!5 transforma-
tion. The operative mechanism for the novel Ae-mediated con-
version of disilazane 4 into cyclodisilazane 5 established here,
through complementary kinetic analysis and DFT investiga-
tions, is consistent with all relevant process features. This in-
cludes the rate dependency observed upon varying concentra-
tions of precatalyst and/or substrate,[44] the absence of a pri-
mary KIE to be detectable and the enhancement of the rate
observed for disilazanes with a para-phenyl-substituted termi-
nal silicon centre featuring electron-withdrawing groups.
As already mentioned above, BaꢀN/SiꢀC s-bond breaking
metathesis commencing from C2 describes the concerted ana-
logue to the thus far studied stepwise generation of C5 (see
the Supporting Information for more details). The protonolytic
cleavage of the BaꢀC phenyl bond at C5 by another molecule
of 4Me regenerates barium bis-silazanyl amido C2, which is ac-
companied by the release of benzene. The process commenc-
ing from the mono-amine adduct C5·5Me·4Me evolves through
a metathesis-type TS structure (see Figure S33 in the Support-
ing Information). This TS decays through facile liberation of
benzene initially into a 5Me adduct of C2, from which 5Me is
readily released. The protonolytic expulsion of the phenyl
ligand is seen to be astonishingly kinetically facile, featuring an
intrinsic barrier of 3.7 kcalmolꢀ1 (relative to C5·5Me·4Me), and is
strongly exergonic (Figure 4). The subsequent rapid release of
the cyclodisilazane from C2·5Me drives the overall aminolysis
step even further downhill. Furthermore, C5·5Me·4Me is likely
showing a higher propensity towards undergoing forward pro-
tonolytic BaꢀC phenyl bond cleavage rather than reverse SiꢀC
bond-forming phenyl transfer. In light of the predicted kinetic
Conclusions
gap of 4.4 kcalmolꢀ1 (DDG° between C5·5Me·4Me!C2·5Me
+
PhH and C5·5Me·4MeQC4·4Me) in favour of the forward protonol-
ysis, C5·5Me·4Me is likely a short-lived intermediate that converts
into C2·5Me almost immediately after it is generated. Hence,
TS[C5·5Me·4MeꢀC2·5Me] of the favourable protonolysis pathway
shown in Figure 4 does not define the highest point to be
crossed along the minimum-energy pathway for generation of
the cyclodisilazane. Instead, TS[C4·4MeꢀC5·5Me·4Me] for b-C5H6
transfer onto barium (Figure 3) represents the species of high-
est free energy.
The barium alkyl precatalyst Ba[CH(SiMe3)2]2·(THF)3 enables the
preparation of unusual silazanes of various sizes and composi-
tions upon iterative dehydrocouplings of primary amine and
secondary dihydrosilanes. Until now, molecules with SiꢀNꢀSiꢀ
N sequences of up to four backbone heteroatoms have been
obtained in quantitative yields and complete chemoselectivity
under mild conditions. We are hoping to extend this method-
ology of sequential dehydrocouplings to higher degrees, that
is, for the syntheses of oligo- and polycarbosilazanes of in-
creasing sizes and multiple compositions. For now, this ambi-
tion has been hampered by an unforeseen phenomenon of
cyclisation leading to the formation of thermodynamically
stable four-membered Si2N2 cyclodisilazanes such as c-(Ph2Siꢀ
NBn)2 (5). The main lines of the mechanism leading to the ben-
zene-releasing cyclisation of Ph3SiN(Bn)SiPh2NHBn (4) have
been clearly delineated. The identified operative stepwise pro-
cess involves the initial facile formation of a [Ba{N(Bn)-
SiPh2N(Bn)SiPh3}2] species, in which the intramolecular nucleo-
Over the course of exploring the existence of alternative
mechanistic avenues, we managed to successfully locate a mul-
ticentre TS structure that is linked to barium silicate C4·4Me de-
scribing b-C6H5 transfer with concomitant 4Me amine-proton
delivery onto phenyl. This all occurs outside of the immediate
vicinity of the metal centre (see the Supporting Information for
more details).
Overall, the reported computational examination of several
mechanistic avenues rationalises the distinct reactivity pattern
Figure 4. BaꢀC phenyl-bond protonolysis at barium phenyl amido C5. Energies are given in kcalmolꢀ1 relative to {C2+nꢂ4Me}.
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Experimental procedures are given in the Supporting Information.
CCDC 1435967 (1), 1435968 (3), 1435969 (4) and 1435970 (5) con-
tain the supplementary crystallographic data for this paper. These
data are provided free of charge by The Cambridge Crystallograph-
ic Data Centre.
Acknowledgements
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Keywords: barium
coupling · sequential chain extension · silazanes
·
cyclisation
·
HꢀSi coupling
·
NꢀH
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NꢀH/HꢀSi CDC reactions, barium silyl species do not intervene in this
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[42] The otherwise commercially available Me3SiNHBn (16; CAS number
14856-79-2) was prepared by metathesis between Me3SiCl and BnNHLi,
because the CDC of amines with hydrosilanes bearing aliphatic alkyl
groups is rather sluggish, see ref. [15].
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[29] The conversions here and in the following sections are given for opti-
mised reaction times and temperatures for reactions performed in NMR
tubes. For large-scale syntheses followed to ensure complete conver-
sion of the substrates, other conditions were used, as given in the ex-
perimental protocols. See the Supporting Information for details.
[30] It has been shown before that 5 could be prepared in a more princi-
pled fashion by dehydrocoupling of BnNHSiPh2NHBn with Ph2SiH2 cata-
lysed by Ba[N(SiMe3)2]2·(THF)2, see ref. [15a]. As a test, this same reac-
tion catalysed by A gave full conversion to 5 under mild conditions
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presence of barium bis-alkyl precatalyst A, which sees the initial rapid
and quantitative conversion of the barium bis-alkyl into the barium bis-
silazanyl amido compound, competent of triggering 4!5 transforma-
tion to involve substrate-assisted b-C6H5 transfer onto barium as the
most kinetically demanding step, conforms with the observed rate law,
describing a first-order dependence on the concentration of both cata-
lyst and substrate.
([Ph2SiH2]0/[BnNHSiPh2NHBn]0/[A]0 =100:100:1,
1.0m in benzene, 258C, 2 h).
[BnNHSiPh2NHBn]0 =
[31] Because of the high contents in silicon resulting in the formation of
non-pyrolysable silicon carbides, the results for the analysis of carbon
contents were lower than expected in a number of occasions, see the
Supporting Information.
[32] The 29Si NMR spectra for 3 and 4 are characterised by two singlet reso-
nances of equal intensities, at d29 Si =ꢀ9.87 and ꢀ13.18 ppm for 3, and
at ꢀ11.19 and ꢀ15.97 ppm for 4. A single resonance at ꢀ11.50 ppm is
found in the 29Si NMR spectrum of 5. These values fall within the typical
range of 29Si chemical shifts expected for aryl-substituted silazanes.
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Received: July 4, 2016
Published online on && &&, 0000
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FULL PAPER
&
Cross-Coupling
C. Bellini, T. Roisnel, J.-F. Carpentier,*
S. Tobisch,* Y. Sarazin*
&& – &&
Sequential Barium-Catalysed NꢀH/Hꢀ
Si Dehydrogenative Cross-Couplings:
Cyclodisilazanes versus Linear
Oligosilazanes
Power to barium: The barium complex
Ba[CH(SiMe3)2]2·(THF)3 catalyses the con-
trolled, highly active and chemoselec-
tive formation of linear or cyclic “SiꢀNꢀ
SiꢀN” disilazanes upon sequential cross-
dehydrocoupling of simple amines and
hydrosilanes. The mechanistic pathways
have been established by combination
of experimental and computational
data.
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Chem. Eur. J. 2016, 22, 1 – 12
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ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!