N-Methyl-Benzothiazolium Salts as Carbon Lewis Acids for Si?H σ-Bond Activation and Catalytic (De)hydrosilylation

Article abstract of DOI:10.1002/chem.201604613

N?Me-Benzothiazolium salts are introduced as a new family of Lewis acids able to activate Si?H σ bonds. These carbon-centred Lewis acids were demonstrated to have comparable Lewis acidity towards hydride as found for the triarylboranes widely used in Si?H σ-bond activation. However, they display low Lewis acidity towards hard Lewis bases such as Et₃PO and H₂O in contrast to triarylboranes. The N?Me-benzothiazolium salts are effective catalysts for a range of hydrosilylation and dehydrosilylation reactions. Judicious selection of the C2 aryl substituent in these cations enables tuning of the steric and electronic environment around the electrophilic centre to generate more active catalysts. Finally, related benzoxazolium and benzimidazolium salts were found also to be active for Si?H bond activation and as catalysts for the hydrosilylation of imines.

Full text of DOI:10.1002/chem.201604613

DOI: 10.1002/chem.201604613
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Frustrated Lewis Pairs |Hot Paper|
N-Methyl-Benzothiazolium Salts as Carbon Lewis Acids for SiÀH
s-Bond Activation and Catalytic (De)hydrosilylation
Valerio Fasano, James E. Radcliffe, Liam D. Curless, and Michael J. Ingleson*[a]
Abstract: NÀMe-Benzothiazolium salts are introduced as
a new family of Lewis acids able to activate SiÀH s bonds.
These carbon-centred Lewis acids were demonstrated to
have comparable Lewis acidity towards hydride as found for
the triarylboranes widely used in SiÀH s-bond activation.
However, they display low Lewis acidity towards hard Lewis
bases such as Et3PO and H2O in contrast to triarylboranes.
The NÀMe-benzothiazolium salts are effective catalysts for
a range of hydrosilylation and dehydrosilylation reactions.
Judicious selection of the C2 aryl substituent in these cations
enables tuning of the steric and electronic environment
around the electrophilic centre to generate more active cata-
lysts. Finally, related benzoxazolium and benzimidazolium
salts were found also to be active for SiÀH bond activation
and as catalysts for the hydrosilylation of imines.
Introduction
The use of hydridophilic main-group Lewis acids, particularly
B(C6F5)3, in SiÀH bond activation,[1] and more broadly in “frus-
trated Lewis pair” (FLP) chemistry,[2] has generated significant
recent breakthroughs.[3] This includes their use as versatile cat-
alysts for dehydrosilylation and hydrosilylation reactions.[4] Gen-
erally, the Lewis acids employed are tri(fluoroaryl)boranes
which have sufficient Lewis acidity towards hydride to hetero-
lytically cleave SiÀH bonds (in combination with an appropri-
ate Lewis base) and generate borohydrides that are able sub-
sequently to reduce electrophilic substrates. Mechanistic stud-
ies revealed that SiÀH heterolysis is an SN2 type process pro-
ceeding via a “partially” activated SiÀH bond, which can be
viewed as a “Si-H-B” 3c–2e interaction,[5,6] with one example re-
cently crystallographically characterised.[7] In the absence of an
appropriate nucleophile no silylium ions are formed from com-
bining B(C6F5)3 and R3SiH, although silane H/D scrambling still
Scheme 1. Silane activation with B(C6F5)3.
which carbon is the locus of electrophilic character have signif-
icant potential in this area as the higher electronegativity of
carbon (relative to boron) results in a reduction in “hard” Lewis
acidity.[12] Trityl salts are amongst the most widely utilised
carbon Lewis acids including in catalytic applications. However,
these catalytic transformations generally proceed by activation
of the substrate by coordination to the electrophilic carbon
centre in trityl and not by an FLP-type mechanism.[12,13] The
use of trityl salts and other carbon Lewis acids in FLP chemis-
try, including the activation of H2, has significantly less prece-
dent, although a limited number of examples have been re-
cently reported.[14]
proceeds via
Scheme 1).[7]
a
four-membered transition state (inset
In more recent studies, weaker boron Lewis acids, such as
BPh3, also have been shown also to be effective in SiÀH bond
activation.[8] Nevertheless, the high oxophilicity of boron Lewis
acids requires rigorously dried conditions (or an excess of hy-
dride)[9] and leads to substrate scope limitations.[10] Conse-
quently, the development of new Lewis acids that have low ox-
ophilicity but retain sufficient Lewis acidity towards hydride to
activate EÀH (E=H or R3Si) bonds is desirable.[11] Lewis acids in
Carbocations including trityl, are well documented to irrever-
sibly cleave SiÀH bonds to form silylium cations and Ph3CH.[15]
In contrast, carbon Lewis acids that “partially” activate SiÀH
bonds (e.g., exhibit analogous reactivity to B(C6F5)3) are ex-
tremely rare to the best of our knowledge. One recent exam-
ple from our group are N-methyl-acridinium salts which acti-
vate SiÀH bonds (Scheme 2) as indicated by SiÀH/SiÀD scram-
bling experiments but no silylium cations are observed in solu-
tion.[16] However, the Lewis acidity of the NÀMe-acridinium
[17]
cation (1+) towards hydride is greater than that of B(C6F5)3.
[a] V. Fasano, Dr. J. E. Radcliffe, Dr. L. D. Curless, Dr. M. J. Ingleson
School of Chemistry, University of Manchester
Manchester, M13 9PL (UK)
This makes the conjugate organic hydride, N-Me-acridane (1-
H), formed for example on Si-H heterolysis by 1+ and an ap-
propriate nucleophile, a poor reductant. This fact combined
with the propensity of NÀMe-acridinium salts to initiate photo-
activated radical reactivity[18] led us to search for other carbon
E-mail: Michael.ingleson@manchester.ac.uk
Supporting information for this article and ORCID(s) from the authors can
be found under:
http://dx.doi.org/10.1002/chem.201604613.
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Scheme 2. EÀH bond activation using [N-Me-acridinium]+.
Scheme 4. Relative (to BEt3) HIA values of [2]+ and [3]+.
Lewis acids able to “partially” activate SiÀH bonds but that are
weaker Lewis acids towards hydride than 1+.
Due to its suitable calculated HIA value N-methyl-2-phenyl-
benzothiazolium iodide was synthesized and metathesized
with AgOTf, NaBPh4 and NaBArCl (BArCl=[B(3,5-Cl2-C6H3)4]À) to
afford [2][Anion] (Anion=OTf, BPh4, BArCl, respectively) in
moderate to good yields in each case. Single crystals of the tri-
flate salt were obtained from MeCN/ ortho-dichlorobenzene (o-
DCB) which revealed a non-planar cation with an angle of
47.88 between the plane of the phenyl ring and that of the
thiazole ring, with the NÀMe group preventing a co-planar ar-
rangement (Figure 1). The closest cation/anion contact involv-
ing the electrophilic C2 position is long and involves a triflate
oxygen located at 3.324 ꢁ, a distance that is significantly great-
er than the combined covalent radii of carbon and oxygen.[21]
Previous work has shown that C2 substituted benzothiazo-
lines are highly effective organic hydrides for the reduction of
imines catalyzed by phosphoric acids (Scheme 3, top).[19] Based
on this precedence, we targeted the oxidised form, the benzo-
thiazole, as a potential carbon Lewis acid after methylation at
nitrogen to increase the electrophilicity at the C2 position. N-
Me-2-R-benzothiazolium cations are attractive Lewis acids as
they are simple to make using established routes and can be
readily fine-tuned e.g., by altering the C2 substituent.[19] Fur-
thermore, they represent a hitherto underexplored class of
Lewis acid in FLP-type catalysis, namely a Lewis acid based on
an iminium cation, a moiety which is generally considered as
a substrate for reduction and not as a catalyst. Herein we dem-
onstrate that N-Me-2-aryl-benzothiazolium salts are able to ac-
tivate SiÀH bonds and are effective as catalysts for dehydrosily-
lation and hydrosilylation reactions.
Figure 1. Synthesis and X-ray structure of [2][Anion]. Red=oxygen, yel-
low=sulfur, grey=carbon, blue=nitrogen, light green=fluorine.
The low oxophilicity of [2]+ indicated by this structure was
confirmed by the lack of any evidence for binding of H2O to
[2][BArCl]; furthermore, no OÀH heterolytic cleavage was ob-
served on addition of 2,6-lutidine and H2O to a DCM solution
of [2][BArCl] (in contrast to what is observed with B(C6F5)3 and
other Lewis acidic boranes).[22] The combination of one equiva-
lent of Et3PO and three equivalents of [2][BArCl] only led to
a small downfield shift in the 31P{1H} resonance (Dd=4.4 ppm)
confirming the weak Lewis acidity of [2]+ towards hard Lewis
bases (for comparison B(C6F5)3 gives a Dd of 33.7 ppm).[23]
Seeking to experimentally assess the relative Lewis acidity of
[2]+ towards hydride, compound 2-H and B(C6F5)3 were com-
bined in DCM. This led to rapid (<5 minutes) hydride transfer
and formation of [2][HB(C6F5)3], thus B(C6F5)3 is a stronger
Lewis acid towards hydride than [2]+. In contrast, combining
[2-H] and BPh3 led to no hydride transfer, suggesting the Lewis
acidity of [2]+ towards hydride lies between that of BPh3 and
B(C6F5)3. Both these boranes can activate H2 and SiÀH
bonds,[1,8b,24] thus analogous bond activations using [2][Anion]
should be thermodynamically viable. The activation of HÀH or
SiÀH is only the initial step in a putative catalytic cycle, with
subsequent hydride transfer to the substrate also required.
Recent studies have highlighted the importance of correctly
balancing the electrophilicity of the substrate (e.g., an iminium
cation) and the reducing power of the organic hydride to ach-
ieve successful transfer hydrogenation.[25] Thus, we investigated
Scheme 3. Top, Brønsted acid catalyzed transfer hydrogenation of imines
using stoichiometric benzothiazoline. Bottom, benzothiazolium cation cata-
lyzed hydrosilylation and dehydrosilylation using stoichiometric silane.
Results and Discussion
Initially, the hydride ion affinity (HIA)20 of the N-methyl-2-phe-
nylbenzothiazolium cation (2+) relative to BEt3 was computa-
tionally determined and found to be À45 kcalmolÀ1
(Scheme 4). This is comparable to that calculated previously for
B(C6F5)3 (À41, kcalmolÀ1 M06-2X/6–311G(d,p) with dichlorome-
thane (DCM) solvation (Polarizable Continuum model, PCM).
Furthermore, this HIA value is less than that found for 1+
(À53 kcalmolÀ1)[15] suggesting [2]+ is a more appropriate Lewis
acid for use in catalytic imine reductions as its conjugate or-
ganic hydride will be more reducing than 1-H. The C2-penta-
fluorophenyl analogue, [3]+ also was calculated and found to
,
have an HIA of À51 kcalmolÀ1 indicating it is less suitable for
use in catalytic reductions (as its conjugate hydride, 3-H, will
be a poorer reductant).
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the diphenylphosphoric acid initiated reduction of N-benzyli-
dene–aniline, employing [2-H] as the hydride source
(Scheme 5). Using equimolar ratios this produced N-benzylani-
line in around 50% conversion, with conversion limited due to
the amine product deprotonating the iminium cation (full
imine reduction can be achieved by using >2 equivalent of
the phosphoric acid). These reactions confirm that 2-H is
a more accessible source of hydride for reductions than 1-H.
bulk provided by a tBu substituent to weaken cation–anion in-
teractions. However, combinations of [2][BArCl] or [4][BArCl]
and these amines resulted in no H2 activation (1008C in o-DCB
for 16 h, ca. 4 atm. of H2). FLPs containing [4]+ and stronger
Lewis bases, such as PtBu3 and Verkade’s base also showed no
propensity to activate H2 (ca. 4 atm. at 608C or 1008C). In
these cases slow demethylation of the benzothiazolium salt
proceeded to form, for example, [Me-PtBu3]+ (Scheme 6).
Scheme 5. Phosphoric acid initiated N-benzylidine-aniline reduction with
2-H.
Scheme 6. Activation of EÀH bonds using benzothiazolium salts. LB for ex-
ample=PtBu3.
EÀH bond activation studies
Catalytic studies
Compounds [2][BPh4] and [2][BArCl] (5 mol %) both induced
H/D scrambling between Et3SiD and PhMe2SiH, albeit only at
raised temperatures (808C in MeCN for the former and at 608C
in DCM for the latter in sealed tubes). In contrast, no scram-
bling was observed between Ph3SiH and Et3SiD using [2]
[BArCl] (at 608C in DCM for 24 h) presumably due to the great-
er steric bulk of Ph3SiH. This is consistent with the less bulky
silane Ph2MeSiH, that has a similar kinetic nucleophilicity to
Ph3SiH (N parameter values of 2.72 and 2.65, respectively),[26]
undergoing H/D exchange with Et3SiD in the presence of [2]
[BArCl]. Triarylboranes are also competent at activating SiÀH
bonds,1 therefore control reactions were performed to pre-
clude the possibility that a Lewis acidic borane, potentially
formed in situ by anion decomposition by trace protic impuri-
ties,[27] is leading to SiÀH activation. Notably, utilising the iden-
tical batch of NaBPh4 used to prepare [2][BPh4] no H/D scram-
bling between Et3SiD and PhMe2SiH was observed under iden-
tical conditions (at 5 mol % NaBPh4 loading for 20 h at 808C in
MeCN), thus silane H/D scrambling is being mediated by the
carbon Lewis acid [2]+. This is further confirmed by [2][OTf]
also resulting in H/D scrambling between PhMe2SiH and Et3SiD
(at 808C in MeCN) albeit more slowly than observed with both
borate anions. The requirement for raised temperatures for H/
D scrambling with [2]+ may in part be attributed to strong
anion cation interactions which need to be overcome before
silane activation occurs. Indeed the diffusion coefficients (by
diffusion ordered spectroscopy (DOSY) experiments) are identi-
cal for the cationic and anionic components of [2][BArCl] in
DCM and in acetone, consistent with the existence of intimate
ion pairs in these solvents.[28]
Three different benzothiazolium cations containing phenyl
([2]+), p-tBu-phenyl ([4]+) and 1-naphthyl ([5]+) C2-substitu-
ents were evaluated as catalysts in dehydrosilylation and hy-
drosilylation reactions. Cations [4]+ and [5]+ were selected to
alter steric bulk distal (in the case of [4]+) and proximal ([5]+)
to the C2 electrophilic centre relative to that in [2]+. It should
be noted that the incorporation of a para-tBu substituent will
also affect the electrophilicity at the C2 position, an effect con-
firmed by HIA calculations which revealed that [4]+ is 1 kcal
molÀ1 less Lewis acidic towards hydride relative to [2]+. The in-
itial reaction studied was the dehydrosilylation of benzyl alco-
hol using PhMe2SiH.[30] Although the parent iodide salts (e.g.
[2][I]) resulted in no dehydrosilylation using the [OTf] and
[BPh4] salts dehydrosilylation occurred in DCM but was ex-
tremely slow (Table 1, entries 1–3). Dehydrosilylation could be
accelerated to some extent for the [BPh4] salts by using a more
polar solvent (e.g., MeCN, Table 1, entries 4,5). The [BArCl] salts
were significantly more active catalysts and thus only salts con-
Table 1. Dehydrosilylation of benzyl alcohol with benzothiazolium salts.
Entry
Catalyst
Solvent
Time [h]
Temp [8C]
Yield [%][a]
1
2
3
4
5
6
7
8
9
10
[2][BPh4]
[5][BPh4]
[5][OTf]
[2][BPh4]
[5][BPh4]
[2][BArCl]
[2][BArCl]
[5][BArCl]
[4][BArCl]
[4][BArCl]
DCM
DCM
DCM
MeCN
MeCN
DCM
MTBE
DCM
DCM
MeCN
40
40
45
24
24
1
1
1
1
24
60
60
60
80
80
20
20
20
20
20
8
13
12
76
72
94
94
86
84
60
FLPs were generated by combining [2][BArCl] with equimo-
lar 2,6-lutidine, 1,8-bis(dimethylamino)naphthalene and 4-
DMAP. The absence of any observable Lewis adduct between
[2]+ and 4-DMAP is notable and in contrast to the reactivity of
both [1]+ and B(C6F5)3 towards 4-DMAP,[16,29] further indicating
the low Lewis acidity of [2]+ towards hard Lewis bases. A
modified benzothiazolium salt containing a para-tBu-phenyl
C2-substituent, [4][BArCl], was also explored in FLPs with
these three amine Lewis bases in an attempt to use the steric
[a] Conversion by NMR spectroscopy versus mesitylene as an internal
standard.
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taining this anion were studied hereon. The three benzothiazo-
lium [BArCl] salts showed minimal differences as catalysts for
benzyl alcohol (BnOH) dehydrosilylation (entries 6–9) which
proceeded rapidly at 208C in DCM in each case.
Table 2. Hydrosilylation of benzaldehyde and acetophenone.
A number of control reactions were performed to ensure
that protonolysis of the borate anion is not leading to an
active borane catalyst under these conditions. Anion decompo-
sition has been previously reported on heating NaBPh4 in wet
solvents which led to the formation BPh3.[27,31] Using 5 mol%
Na[BPh4] (at 808C for 24 h in MeCN) or 5 mol% Na[BArCl] (20 h
at 208C or 5 h at 808C in MeCN) no dehydrosilylation of BnOH
was observed. In contrast, under identical conditions both [2]
[BPh4] and [4][BArCl] lead to significant dehydrosilylation (en-
tries 4 and 10). Moreover, BPh3 was confirmed to be an active
catalyst for the dehydrosilylation of BnOH in MeCN under
these conditions (40% in 20 h at 808C), consistent with
Okuda’s original observation of EtOH dehydrosilylation cata-
lyzed by BPh3.[8b] Thus the complete absence of BnOH dehy-
drosilylation using Na[B(Aryl)4] in MeCN indicates that B(Aryl)3
(or any other borane able to initiate dehydrosilylation) is not
being formed by anion decomposition. These results combined
confirm that under these conditions catalytic BnOH dehydrosi-
lylation is initiated by the benzothiazolium salts and not by
Lewis acidic boranes derived from anion decomposition.[32]
[5][BArCl] also catalyzes the dehydrosilylation of BnOH in
DCM with Ph3SiH with 57% conversion at 608C after 24 h. This
Entry
Catalyst
R
Time [min]
Yield [%][a]
1
2
3
4
5
6
[2][BArCl]
[5][BArCl]
[4][BArCl]
[2][BArCl]
[5][BArCl]
[4][BArCl]
H
H
H
Me
Me
Me
60
60
60
15
15
15
92
10
90
>99
40
>99
[a] Conversion by NMR spectroscopy versus mesitylene as an internal
standard.
substrate, benzaldehyde, preferentially in the presence of
5 mol% [4][BArCl]. Moreover, attempts to selectively hydrosily-
late ethyl benzoate using 1.2 equivalents of PhMe2SiH in the
presence of [4][BArCl] led to mixtures of products consistent
with the silyl acetal product undergoing further reduction
competitively to ester hydrosilylation, again analogous to that
observed using B(C6F5)3.[5] Finally, a control reaction using
5 mol% NaBArCl in the hydrosilylation of acetophenone result-
ed in no reaction (after 60 minutes at 208C in DCM) indicating
that catalytic activity is again due to the benzothiazolium salt.
Benzothiazolium[BArCl] salts were also effective catalysts for
the hydrosilylation of imines, albeit at raised temperatures.
Whilst all three benzothiazolium catalysts reduced N-benzyli-
dene aniline at 1008C in o-DCB (Table 3) considerable dispari-
ties in catalyst activity were observed in the hydrosilylation of
the less electrophilic imine N-benzylidene-tert-butylamine. Both
[2][BArCl] and [5][BArCl] were inactive for the hydrosilylation
of N-benzylidene-tert-butylamine with PhMe2SiH at 608C,
whilst catalysis did proceed at 1008C it was extremely slow
with both salts. In contrast, [4][BArCl] was able to catalyse the
hydrosilylation of both of these imines, including N-benzyli-
[30]
is slower than with B(C6F5)3 using the same silane/alcohol
which proceeds at room temperature. This indicates a signifi-
cantly greater kinetic barrier using the benzothiazolium salt as
catalyst. Phenol also underwent dehydrosilylation catalyzed by
[4][BArCl] (76% conversion after 2 h at 208C in DCM) preclud-
ing an alcohol dehydrogenation/ carbonyl hydrosilylation
mechanism. Benzothiazolium salts also dehydrosilylate water
to form the respective siloxane, thus alcohol dehydrosilylation
proceeds using non-purified solvents (using excess silane) and
also in more environmentally friendly solvents such as methyl
tert-butyl ether (MTBE).
Another established B(C6F5)3 catalyzed reaction is the hydro-
silylation of carbonyls reported by Piers and co-workers.[5]
Using the three benzothiazolium[BArCl] salts for carbonyl hy-
drosilylation led to differences in the rate of benzaldehyde
(Table 2 entries 1–3) and acetophenone (Table 2 entries 4–6)
hydrosilylation. With both substrates the C2-(1-naphthyl) sub-
stituted catalyst [5][BArCl] results in the slowest rate of hydro-
silylation suggesting that either greater steric hindrance
around the C2 centre or stronger cation–anion interactions are
retarding the rate of hydrosilylation using this catalyst. HIA cal-
culations on [5]+ revealed it has an effectively identical Lewis
acidity towards hydride as found for [2]+ (À45.5 and
À45.4 kcalmolÀ1, respectively) precluding the reactivity dispari-
ty originating from different degrees of Lewis acidity to hy-
dride (which in turn would lead to differing degrees of silane
activation and differing reducing powers of 5-H and 2-H).
Other features of carbonyl hydrosilylation using benzothia-
zolium salts are comparable to that reported for B(C6F5)3. For
example, an equimolar mixture of acetophenone/benzalde-
hyde/PhMe2SiH led to hydrosilylation of the more nucleophilic
Table 3. Imine hydrosilylation using [n][BArCl] as catalyst.
Catalyst
HIA [kcalmolÀ1
]
Yield [%][a]
Reaction A
Reaction B
Reaction C
[2][BArCl]
[4][BArCl]
[5][BArCl]
À45.4
À44.4
À45.5
76
60
54
0
97
0
24
76
20
[a] Conversion by NMR spectroscopy versus mesitylene as an internal
standard.
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dene-tert-butylamine at 608C in DCM. The disparities in activity
between the benzothiazolium salts are attributed to the lower
HIA of [4]+ relative to [2]+ and [5]+. This results in the conju-
gate hydride of [4]+, 4-H, being a stronger reducing agent
thus more effective at reducing the silylated iminium cation
derived from N-benzylidene-tert-butylamine. The effectiveness
of benzothiazolium salts in catalytic imine hydrosilylation is
therefore dependent on the difference in electrophilicity be-
tween the silylated iminium cation and the benzothiazolium
cation.
Table 4. Imine hydrosilylation using [4][BArCl] as catalyst.
Entry
R
R1
R2
Time [h]
Yield [%][a]
1
2
3
4
5
6
tBu
tBu
Ph
Ph
CH2Ph
Me
Me
Me
Me
Ph
Me
Me
Me
H
Me
Me
Me
Me
17
17
3
44
72
48
76
98
60
73
43[b]
22[b]
A decrease in the HIA of the benzothiazolium cation (relative
to [4]+) was explored targeting more rapid iminium cation re-
duction by increasing the reducing power of the conjugate hy-
dride.[26] Thus [6][BArCl] (Scheme 7, left), containing a MeO
[a] % Conversion by NMR spectroscopy versus mesitylene as an internal
standard. [b] Imine consumption.
bonyls and imines it does not catalyse the hydrosilylation of al-
kynes (e.g., phenylacetylene or 1-phenyl-1-propyne) with
PhMe2SiH (even at 1008C), consistent with the lower Lewis
acidity of the [4]+ towards hydride (relative to B(C6F5)3) leading
to a lower degree of silane activation and thus a weaker silicon
electrophile.[8a]
Scheme 7. Imine hydrosilylation using [6][BArCl].
To compare the catalytic activity of [4][BArCl] in another re-
action involving a nucleophile that forms a Lewis adduct with
B(C6F5)3 the reduction of a phosphine oxide was investigated.
Oestreich, Stephan and co-workers have recently utilized
B(C6F5)3 to reduce phosphine oxides with silanes.[33] Replacing
B(C6F5)3 with [4][BArCl] under identical conditions led to the
quantitative reduction of Ph3PO to Ph3P (Scheme 8), although
at a slower rate relative to that catalyzed by B(C6F5)3. This is de-
spite the absence of any observable Lewis adduct on combin-
ing equimolar [4]+ and Ph3PO. A control reaction using
5 mol% NaBArCl resulted in no significant phosphine oxide re-
duction under identical conditions again indicating catalytic
activity is initiated by [4]+.
group in the para position of the C2-phenyl substituent was
synthesized using standard procedures. HIA calculations on
[6]+ confirmed a reduced HIA value, with [6]+ being 0.8 kcal
molÀ1 less Lewis acidic towards hydride than [4]+, indicating
the conjugate hydride 6-H should be more reducing than 4-H.
Utilizing 5 mol% of [6][BArCl] the hydrosilylation of N-benzyli-
dene-tert-butylamine led to more rapid hydrosilylation relative
to [4][BArCl], with around 50% conversion after 3 h at 1008C.
However, heating this reaction for longer did not lead to any
further imine hydrosilylation. This is attributed to a catalyst de-
activation process as the use of 10 mol% of [6][BArCl] led to
86% hydrosilylation of N-benzylidene-tert-butylamine in
2 hours (at 1008C in o-DCB).
With [4][BArCl] identified as the more robust catalyst (rela-
tive to [6][BArCl]) a brief substrate scope exploration was per-
formed. This revealed that using the less bulky silane PhMeSiH2
in place of PhMe2SiH led to more rapid imine hydrosilylation
(Table 4, entries 1 and 2), whilst the bulkier silane Ph2MeSiH
significantly retarded the rate of hydrosilylation (entries 3,4).
The hydrosilylation of the less hindered imines N-benzylidene-
benzylamine and N-benzylidene-methylamine were both ex-
tremely slow using [4][BArCl] (entry 5 and 6). The 1H NMR
spectra for [4][BArCl] revealed no significant changes to the
resonances for [4][BArCl] before and after addition of N-ben-
zylidene-methylamine, even with 20 equivalents of N-benzyli-
dene-methylamine, precluding any appreciable Lewis adduct
formation. In contrast, mixtures of B(C6F5)3 and this imine form
a strong Lewis adduct.[10] However, when [4][BArCl] was com-
Scheme 8. Ph3PO reduction using [4][BArCl].
Benzoxazolium and benzimidazolium cations as Lewis acids
With an understanding of the catalytic ability of N-Me-benzo-
thiazolium[BArCl] salts in hand the related cations, N-Me-2-
phenyl-benzoxazolium, [7]+, and N,N-Me2-2-Ph-benzimidazoli-
um, [8]+ were investigated. From previous calorimetry studies
N-Me-2-Ph-benzoxazoline, 7-H, is reported to have a significant-
ly lower, and N,N-Me2-2-Ph-benzimidazoline, 8-H, a significantly
higher hydride donating ability relative to 2-H.[34] The latter
was consistent with HIA calculations (Scheme 9), however, [8]+
and [2]+ were calculated to have HIA values within 2 kcal
molÀ1 of each other. In fact, the only major significant calculat-
ed difference between [2]+ and [7]+ is related to the charge
distribution, with a more polarized s-bonding framework in
1
bined with 1 or 5 equivalents of this imine the NÀMe H NMR
resonance was significantly broadened suggesting a non-cova-
lent interaction (e.g., H-bonding or p-stacking) between the
imine and [4]+ which maybe impacting its rate of hydrosilyla-
tion. Although [4][BArCl] catalyses the hydrosilylation of car-
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[7]+ leading to a greater positive NBO charge localized at C2
in [7]+ relative to [2]+ (+0.56 and +0.20, respectively).
[BArCl]. The small differences in relative conversions are attrib-
uted to different degrees of silane activation (lower with the
less Lewis acidic [8]+) and reducing powers of the conjugate
hydride (higher in [8-H]).
Salts [7][I] and [8][I] were obtained by methylating the neu-
tral precursors with MeI. Subsequent metathesis with NaBArCl
provided [7][BArCl] and [8][BArCl]. Confirmation of the greater
hydride donating ability of 8-H relative to 2-H and 7-H was
confirmed by the combination of 8-H with [2][BArCl] (or [7]
[BArCl]). This led to complete consumption of 8-H and the for-
mation of [8][BArCl] and 2-H (or 7-H), after heating to 608C
for 2 h in DCM (Scheme 9). Despite the increased magnitude
Conclusion
N-Me-C2-Aryl-benzothiazolium cations represent a new family
of readily tuned Lewis acids that show activity in frustrated
Lewis pair (FLP) chemistry. They are based on cationic iminium
moieties containing an electrophilic carbon centre that has
a Lewis acidity towards hydride comparable to the triarylbor-
anes widely used in FLP reactivity. However, in contrast to the
triarylboranes these cations show little propensity to bind hard
Lewis bases such as, H2O, Et3PO and 4-DMAP. A range of ben-
zothiazolium salts “partially” activate the SiÀH bond of silanes,
as indicated by H/D scrambling. Furthermore, they are effective
catalysts[35] in a range of established FLP-type (de)hydrosilyla-
tion reactions with rational tuning of the C2-aryl substituent
enhancing catalytic activity. The ability of a cationic iminium
moiety to initiate catalytic (de)hydrosilylation reactions by acti-
vation of silane s-bonds is notable as these moieties are
viewed generally as substrates for reduction in FLP chemistry
and not as catalysts themselves.
Scheme 9. Calculated (relative to BEt3) hydride ion affinity values of [7]+/
[8]+.
of positive charge at C2 in [7][BArCl] there is no evidence for
binding of H2O or one equivalent of Et3PO (by NMR spectros-
copy) indicating [7]+ is a weak Lewis acid toward hard Lewis
bases. Consistent with this the combination of [7][BArCl] with
2,6-lutidine, 4-DMAP and PtBu3 resulted in FLP formation with
no evidence for any coordination to [7]+. However, no H2 acti-
vation was observed for any of these FLP combinations. In
contrast, both [7][BArCl] and [8][BArCl] were effective for the
activation of silanes, with H/D exchange observed between
Et3SiD and PhMe2SiH. To assess the catalytic activity of [7]
[BArCl] and [8][BArCl] relative to [2][BArCl] (the latter selected
as it has an identical C2-substitutent) the hydrosilylation of N-
benzylidene-tert-butylamine was explored (Table 5). This was
selected as it is a reaction that was found to be sensitive to
variation in the C2-substituents of the benzothiazolium salts.
Both [7][BArCl] and [8][BArCl] were active for the hydrosilyla-
tion of imines confirming that they are also effective Lewis
acid catalysts, with conversions similar to that observed for [2]
Experimental Section
Supporting Information for this article includes experimental de-
tails, spectra, computational and crystallographic data.
CCDC 1501133 contains the supplementary crystallographic data
for this paper. These data are provided free of charge by The Cam-
bridge Crystallographic Data Centre.
Acknowledgements
This work was made possible by financial support from the
EPSRC (EP/K03099X/1 and EP/M023346/1), and the Royal Soci-
ety (for the award of a University Research Fellowship). Dr Alex
Pulis is thanked for useful discussions. Additional research data
supporting this publication are available as supplementary in-
formation accompanying this publication.
Keywords: benzothiazolium salts · frustrated Lewis pairs ·
hydrosilylation · Lewis acids · organocatalysis
Table 5. Imine hydrosilylation using [n][BArCl] as catalyst.
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E (Catalyst)
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2
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S ([2][BArCl])
O ([7][BArCl])
NMe ([8][BArCl])
24
25
38
[a] conversion by NMR spectroscopy versus mesitylene as an internal
standard.
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Manuscript received: September 29, 2016
Accepted Article published: October 25, 2016
Final Article published: && &&, 0000
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FULL PAPER
Soft skills: N-methyl-benzothiazolium
cations are introduced as effective cata-
lysts for SiÀH s-bond activation, with
particular focus on (de)hydrosilylation
reactions. Substitution of the C2 aryl
group or replacement of the sulphur
with other heteroatoms has been inves-
tigated, both computationally and ex-
perimentally.
&
Frustrated Lewis Pairs
V. Fasano, J. E. Radcliffe, L. D. Curless,
M. J. Ingleson*
&& – &&
N-Methyl-Benzothiazolium Salts as
Carbon Lewis Acids for SiÀH s-Bond
Activation and Catalytic
(De)hydrosilylation
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ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!