Silver-catalyzed silicon-hydrogen bond functionalization by carbene insertion

Article abstract of DOI:10.1039/c2dt31460f

The catalytic functionalization of silicon-hydrogen bonds by means of the insertion of carbene units:CHCO₂Et from ethyl diazoacetate (EDA) has been achieved using a silver-based catalyst, constituting the first example of this metal to promote this transformation. Competition experiments have revealed that the relative reactivity of substituted silanes depends on the bond dissociation energy of the Si-H bond (tertiary > secondary > primary for ethyl substituted). In the presence of bulky substituents such order reverts to secondary > primary ≈ tertiary (for phenyl substituted). Screening with other diazo compounds has shown that N₂C(Ph)CO₂Et displays similar reactivity to that of EDA, whereas other N₂C(R)CO ₂Et (R = Me, CO₂Et) gave lower conversions. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt31460f

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Silver-catalyzed silicon–hydrogen bond
functionalization by carbene insertion†‡
Cite this: Dalton Trans., 2013, 42, 1191
M. José Iglesias,a M. Carmen Nicasio,b Ana Caballero*a and Pedro J. Pérez*a
The catalytic functionalization of silicon–hydrogen bonds by means of the insertion of carbene units :
CHCO2Et from ethyl diazoacetate (EDA) has been achieved using a silver-based catalyst, constituting the
first example of this metal to promote this transformation. Competition experiments have revealed that
the relative reactivity of substituted silanes depends on the bond dissociation energy of the Si–H bond
(tertiary > secondary > primary for ethyl substituted). In the presence of bulky substituents such order
reverts to secondary > primary ≈ tertiary (for phenyl substituted). Screening with other diazo compounds
has shown that N2C(Ph)CO2Et displays similar reactivity to that of EDA, whereas other N2C(R)CO2Et
(R = Me, CO2Et) gave lower conversions.
Received 4th July 2012,
Accepted 17th October 2012
DOI: 10.1039/c2dt31460f
www.rsc.org/dalton
co-workers introduced Rh2(OAc)4 as an effective catalyst for the
Introduction
reaction of ethyl diazoacetate with silanes,4 which could be con-
The conversion of carbon–hydrogen bonds into valuable pro- sidered as the seminal work of this transformation. After this
ducts currently constitutes an area of continuous research.1 discovery, a series of contributions based on this metal
One of the emerging strategies in the last few decades consists appeared in the literature,5,6 including the asymmetric version.6
of the metal-catalyzed transfer of carbene groups from diazo In addition to rhodium catalysts, only a few examples involving
compounds (Scheme 1, route a).2 The same route has been copper,7 iron8 or iridium9 have been reported to date to induce
scarcely applied to the silicon–hydrogen bond, in spite of this silane functionalization. Given the interest in the chiral
the interest in silicon-containing reagents (Scheme 1, route b). version,6,7,9 it seems appropriate to expand the yet reduced
First examples were independently described by Janssen3a number of potential metals. On the basis of our previous work
and Brook3b with copper, at low conversions. Later, Doyle and on the related C–H functionalization by carbene insertion with
group 11 metal-based catalysts,10 we decided to test the catalytic
capabilities of copper and silver complexes bearing the hydro-
tris(3,4,5-tribromopyrazolyl)borate ligand (TpBr3),11 which have
already been employed in the aforementioned C–H bond func-
tionalization reaction.12 As a result of this study, we found that
both complexes induce the silane functionalization to a signifi-
cant degree, in the first example of a silver-mediated Si–H deri-
vatization by carbene insertion from diazo compounds.
Scheme 1 The metal-catalyzed functionalization of C–H and Si–H bonds by
carbene insertion from diazo compounds.
aLaboratorio de Catálisis Homogénea, Departamento de Química y Ciencia de los
Materiales, Unidad Asociada al CSIC, Centro de Investigación en Química Sostenible
(CIQSO), Universidad de Huelva, Campus de El Carmen, 21007-Huelva, Spain.
E-mail: perez@dqcm.uhu.es, ana.caballero@dqcm.uhu.es; Fax: +34 959 219942;
Tel: +34 959 219956
Results and discussion
bDepartamento de Química Inorgánica, Universidad de Sevilla, Aptdo 1203, 41071
Catalytic activities of the complexes TpBr3Cu(NCMe) (1) and
Sevilla, Spain. E-mail: mnicasio@us.es; Fax: +34 954 233765; Tel: +34 954 557160
†Dedicated to Professor David Cole-Hamilton on the occasion of his retirement
[TpBr3Ag]2 (2) in the reaction of silanes with ethyl diazoacetate
and for his outstanding contribution to transition metal catalysis.
Given the well-known catalytic capabilities of the copper and
‡Electronic supplementary information (ESI) available: NMR data for the func-
silver complexes TpBr3Cu(NCMe)12a (1) and [TpBr3Ag]2
(2)
12b
tionalized silanes. See DOI: 10.1039/c2dt31460f
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Scheme 3 Alkyl- and aryl-silanes functionalization carried out in this work.
Scheme 2 Triethylsilane functionalization by carbene insertion from ethyl di-
azoacetate using complexes 1 or 2 as the catalyst precursor. aDiazo compound
added in one portion. bSlow addition of diazo compound for 12 h.
Table 1 Catalytic functionalization of silanes with complexes 1 or 2 and ethyl
diazoacetate (EDA)a
Catalyst/diazo additionb
toward the decomposition of ethyl diazoacetate (N2CHCO2Et,
EDA) and the subsequent transfer of the :CHCO2Et group to
alkanes, we decided to expand their scope for silanes. As a
probe substrate we have chosen Et3SiH in order to compare
the relative reactivity of C–H and Si–H bonds toward this trans-
formation. As shown in Scheme 2, when this silane was
reacted with EDA in the presence of catalytic amounts of the
copper complex 1, the corresponding functionalized silane 3
was obtained in 68% yield, referred to the initial EDA. The
reaction was performed using a 1 : 50 : 630 ratio of [catalyst] :
[EDA] : [silane], adding the diazo compound in one portion at
the beginning of the reaction (overall reaction time 12 h). That
yield was enhanced up to 76% when adding the diazo com-
1/one
1/slow
2/one
2/slow
Entry Silane
portionc
additiond
portionc
additiond
1
2
3
4
5
6
7
8
9
Et3SiH
68
77
39
81
49
44
76
79
42
78
13
94
37
15
12
11
7
72
84
Et2SiH2
Ph3SiH
Ph2SiH2
PhSiH3
71
43
n.p.e
80
n.p.e
78
nPr3SiH
82
92
80
70
12
10
72
40
t
Me2 BuSiH 62
Me2PhSiH 74
MePh2SiH 75
a [catalyst] : [EDA] : [silane] = 1 : 50 : 630, 0.01 mmol of catalyst, 12 h of
reaction time. b Percentage of the functionalization product, based on
pound with the aid of a syringe pump for 12 h. In addition to initial EDA. The remaining diazo compound was converted into
diethyl fumarate and maleate. c EDA was added in one portion. d EDA
3, diethyl fumarate and maleate accounted for all initial EDA.
was added for 12 h from a syringe pump. See the Experimental section
for details. e Not performed.
The silver-based catalyst 2 yielded 3 in 42% yield when
adding EDA in one portion, with some other minor byproducts
derived from the insertion of the carbene group into the ethyl
C–H bonds being observed. However, when the reaction was
performed using a slow addition device, only 3 was obtained tri-phenyl substituted silanes were also employed as the sub-
in 72% yield, in addition to diethyl fumarate and maleate. The strate. No reaction onto the aromatic ring (either addition to the
reaction was therefore quite selective toward the Si–H bond, in double bond or insertion into the C–H bonds) was observed,
spite of a larger number of C–H bonds being available in the only the modification of the Si–H and the corresponding for-
substrate. This is in agreement with the difference between the mation of compounds 5–7 were detected. Yields were moderate
bond dissociation energies of silanes (ca. 90 kcal mol−1 13
and to high both with copper and silver, with a very high value in
)
non-activated C–H bonds (ca. 94–97 kcal mol−1).14 It is also the case of Ph2SiH2 being obtained with 2 as the catalyst and
worth mentioning that the tetrahedral environment forced by adding the EDA in one portion. A series of silanes with two
the Tpx ligand seems to be crucial in the reaction outcome. different substituents (entries 6–9) have also being studied, with
The complexes bearing an N-heterocyclic carbene ligand such a reactivity pattern similar to that already commented on.
as IPr, either neutral or cationic, did not induce this trans- Copper gave moderate to high yields whereas silver induced
formation. The naked-silver precursor AgOTf did not catalyze variable yields. Thus MePh2SiH was functionalized at 40% with
this transformation either (Scheme 2).
2 as the catalyst (entry 9), similarly to the value obtained with
After those preliminary results, we carried out a full set of Ph3SiH as the substrate (40%, entry 3). Analogously, Me2PhSiH
experiments with a series of silanes bearing either alkyl or aryl and Et3SiH provided yields around 70%. These data support the
substituents, and with primary, secondary and tertiary Si–H proposal of a delicate balance between steric and electronic
bonds (Scheme 3). The results are shown in Table 1. Diethyl effects of the substituents. Finally, trialkyl-substituted silanes
silane was functionalized in a similar manner to triethylsilane, with bulky groups gave low conversions.
the silver-based catalyst being similar or slightly more active
These results constitute, to the best of our knowledge, the
than the copper catalyst. A series of three mono-, di- and first example of a silver-based catalyst capable of inducing the
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catalytic functionalization of silanes by carbene insertion. Given
the aforementioned reduced number of metals yet4–9 reported
for this transformation (rhodium, iridium, copper), the incor-
poration of another metal could be interesting per se. It has to
be pointed out that the catalytic activity of this catalyst is not as
high as those of the above metals. However, we believe that the
use of silver integrates some value added to the field. First, it
does not oxidize to the +2 oxidation state as copper does, a well-
known decomposition route for most of the Cu-based catalysts.
Second, there are several silver-based chiral complexes and salts
that could be employed to develop the chiral version of this
reaction, which could make them suitable as potential catalysts
and, if positive, would challenge chiral rhodium6 and iridium-
based9 catalysts from an economical point of view.
Scheme 4 Competition experiments with different silanes.
Relative reactivity of silane bonds toward carbene insertion
It is a well-known feature in the related carbon–hydrogen bond
functionalization reaction with diazo compounds that the rela-
tive reactivity follows the trend tertiary C–H > secondary C–H >
primary C–H, i.e., it is coincident with the bond dissociation
energy (BDE).12 This is in good agreement with a transition
state in which the C–H bond interacts with the metallocarbene
carbon atom, with the concerted breaking of such C–H bonds
and the concomitant formation of the new C–C and C–H
bonds.15 In order to ascertain if such behavior can be extra-
polated to the Si–H bonds, we have run a competition experiment
with the three silanes bearing the phenyl rings, using a
2 : 1 : 0.67 molar mixture of Ph3SiH : Ph2SiH2 : PhSiH3, respect-
ively, which ensures the same number of potential Si–H reac-
tion sites for each reagent. At the end of the reaction, a
mixture of compounds 5–7 was obtained (Scheme 4), the reso-
nance of the CH2 group of the Si–CH2–CO2Et moiety in the
1H NMR spectrum of the reaction mixture providing the exact
ratio of products obtained (Fig. 1). The 5 : 6 : 7 ratio was quite
similar for both the copper and silver catalysts: 1.0 : 1.4 : 1.2 for
the former and 1.0 : 1.2 : 1.1 for the latter. In both cases, the
secondary > primary ≈ tertiary trend seems to be distinct from
Si–H bond dissociation energies.13,14,16 The incorporation of a
phenyl ring attached to the C–H moiety induces, by conju-
gation, a decrease in the BDE by 17 kcal mol−1 (89 kcal mol−1
in toluene vs. 105 kcal mol−1 in methane);14 on the other
hand, such substitution barely originates
a substantial
decrease of the BDE of a Si–H bond attached to a Ph ring: the
91.8 kcal mol−1 of a Si–H attributed to SiH4 dropped to 91.3 in
PhSiH3 and 88.7 in Ph3SiH.13 Thus, the narrow interval of less
than 3 kcal mol−1 and the considerable difference in bulkiness
within the three silanes employed could explain that steric
effects influence to a larger extent the Si–H functionalization
reaction. To verify our hypothesis, we have run a second compe-
tition experiment using Et3SiH and Et2SiH2, and complex 1 as
the catalyst, from which a 1.8 : 1 relative reactivity for tertiary :
secondary Si–H bonds has been obtained. It seems clear that
the decrease of the steric pressure around the functionalization
site shows that the reaction is electronically-driven.
Functionalization with other diazo compounds
that described for the related C–H functionalization reaction. Other diazo compounds have also been tested with both cata-
However, there is a significant difference between C–H and lysts in order to check the effect of the substituents at the
Fig. 1 Selected region of the 1H NMR spectrum corresponding to competition experiments with different silanes. Left: Experiment carried out with Ph3SiH, Ph2SiH2
and PhSiH3 leading to a mixture of 5–7. Right: Experiment carried out with Et3SiH, Et2SiH2 affording 3 and 4 (see Scheme 4 for numbering).
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were purchased from Aldrich and used without further purifi-
cation. GC data were collected with a Varian 3900 instrument.
NMR spectra were recorded on a Varian Mercury 400 MHz. The
complexes TpBr3Cu(NCMe) (1) and [TpBr3Ag]2 (2) were prepared
according to the literature.12a,b
Table 2 Catalytic functionalization of triethylsilane with several diazo
compoundsa
General procedure for the silane functionalization reaction
Complexes 1 or 2 (0.01 mmol) were dissolved in a mixture of
dichloromethane (3 mL) and the corresponding silane
(6.3 mmol). Ethyl diazoacetate (52.6 μL, 0.5 mmol) was added
in one portion or dissolved in dichloromethane (3 mL) and
added for 12 h with the aid of a syringe pump, at room temp-
erature. No ethyl diazoacetate was detected by GC after 12 h in
both cases. The solvent was removed under reduced pressure
and the residue was dissolved in CDCl3 to be analyzed by
1H NMR spectroscopy. 2-Iodoanisole as an internal standard
was added (39.1 μl, 0.3 mmol), the conversion and yields being
Entry
R1
1
2
1
2
3
4
H
Me
Ph
72
15
70
36
76
0
33
14
CO2Et
a See the Experimental section for details. Although previously
reported, the NMR data of the products are given in the ESI‡ for the
sake of completeness.
1
estimated by H NMR. Although previously reported, the NMR
diazo functionality. Thus, in addition to EDA, three more
diazo compounds bearing common CO2Et substituents have
been studied, varying the other group among Me, Ph and
CO2Et [eqn (1)], and with triethylsilane as the substrate. As
shown in Table 2, the group of diazo reagents induced quite
distinct yields. This is related to the different electronic behav-
ior of the substituents.17 It seems that for this transformation,
the preferred diazo compounds are those bearing H or Ph
groups, that is, an acceptor or a donor–acceptor diazo source,
respectively. The acceptor–acceptor diazomalonate is less reac-
tive due to the stability of the diazo compound.2,17 Interest-
ingly, the donor–acceptor CH3C(N2)CO2Et does not work in a
similar manner to that containing a Ph group. The role of the
phenyl substituent in these transformations has been exten-
sively studied by Davies and co-workers in a number of
transformations.17
data of the products are given in the ESI‡ for the sake of
completeness.
Competition experiments with Ph3SiH, Ph2SiH2 and PhSiH3
0.01 mmol of complexes 1 or 2 were dissolved in CH2Cl2
(3 mL) and the solution was charged with 2 mmol of Ph3SiH,
1 mmol of Ph2SiH2 and 0.67 mmol of PhSiH3. Ethyl diazoace-
tate (52.6 μl, 0.5 mmol) was added, in one portion, to the
above solution. The ratio of products was determined by 1H
NMR spectroscopy (see Fig. 1) as 1.0 : 1.4 : 1.2 with 1 and
1.0 : 1.2 : 1.1 with 2.
Competition experiment with Et3SiH and Et2SiH2
Following
a similar procedure to that described above,
0.01 mmol of complex 1 was dissolved in CH2Cl2 and the sol-
ution was charged with 2 mmol of Et3SiH and 1 mmol of
Et2SiH2. A solution of ethyl diazoacetate (52.6 μl, 0.5 mmol) in
dichloromethane (5 mL) was slowly added for 12 h with the
aid of a syringe pump into the above solution. The ratio of pro-
ducts 3 : 4 was determined by 1H NMR spectroscopy (see
Fig. 1) as 1.8 : 1.
Conclusions
We have found that the complexes TpBr3Cu(NCMe)12a (1) and
[TpBr3Ag]2
(2) catalyze the transfer of carbene units from
12b
diazo compounds, to Si–H bonds, with moderate to high
yields, the silver case being the first example based on this
metal for such transformation. The relative reactivity seems to
be strongly influenced by steric factors, although in their
absence the observed trend seems to follow the bond dis-
sociation energy order. The discovery of silver as a metal to
promote this transformation could be of interest to increase
the knowledge of the chiral version of this reaction.
Acknowledgements
We thank the MINECO (CTQ2011-28942-CO2-01) and the Junta
de Andalucía (Proyecto P07-FQM-02745). MJI thanks the Junta
de Andalucía for a research fellowship.
Notes and references
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Functionalization of C–H Bonds, ed. K. I. Goldberg and
A. S. Goldman, (ACS Symp. Ser. 885, Am. Chem. Soc.,
Washington DC, 2004).
Experimental
General
All preparations and manipulations were carried out using
Schlenk techniques or a glovebox. The solvents were dried
using a Solvent Purification System (MBraun). All the reagents
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