Metal-catalyzed reduction of HCONR'2, R' = Me (DMF), et (DEF), by silanes to produce R'2NMe and disiloxanes: A mechanism unraveled

Article abstract of DOI:10.1021/ja2101246

We demonstrate that using Mo(CO)₆, Mo(CO)₅NMe ₃, and (η⁵-C₅H₅)Mn(CO) ₃ as catalysts for the silane, R₃SiH, reduction of N,N-dimethylformamide (DMF), and N,N-diethylformamide (DEF), we can observe, intercept, and isolate, the important siloxymethylamine intermediates, R ₃SiOCH₂NR'₂, R' = Me, Et, for the first time. In the presence of excess DMF such intermediates thermally react with a variety of silanes to form the corresponding disiloxanes in the absence of a metal catalyst. We also show that the germanium hydrides, Et₃GeH and Bu₃GeH, also reduce DMF to form trimethylamine and the corresponding digermoxane but observe no intermediates R₃GeOCH₂NMe ₂. Bu₃SnH reduces DMF, but along with the low yields of Bu₃SnOSnBu₃ (but no Bu₃SnOCH ₂NMe₂) significant side products are obtained including (Bu₃Sn)₂ and Bu₄Sn. In the absence of DMF the siloxymethylamines can undergo metal-catalyzed reactions with silanes, germanes and stannanes to form disiloxanes, and R₃SiOER₃ E = Ge, Sn, respectively. To date, the most efficient catalyst for this latter process is (η⁵-C₅H₅)Mo(CO)₃CH ₃ via a photochemical reaction.

Full text of DOI:10.1021/ja2101246

Communication
pubs.acs.org/JACS
Metal-Catalyzed Reduction of HCONR′2, R′ = Me (DMF), Et (DEF), by
Silanes to Produce R′2NMe and Disiloxanes: A Mechanism Unraveled
Renzo Arias-Ugarte, Hemant K. Sharma, Andrew L.C. Morris, and Keith H. Pannell*
Department of Chemistry, The University of Texas at El Paso, El Paso, Texas 79968-0513, United States
S
* Supporting Information
reported to back up a general mechanism, even when
postulated.5
ABSTRACT: We demonstrate that using Mo(CO)6,
Mo(CO)5NMe3, and (η5-C5H5)Mn(CO)3 as catalysts for
the silane, R3SiH, reduction of N,N-dimethylformamide
(DMF), and N,N-diethylformamide (DEF), we can
observe, intercept, and isolate, the important siloxymethyl-
amine intermediates, R3SiOCH2NR′2, R′ = Me, Et, for the
first time. In the presence of excess DMF such
intermediates thermally react with a variety of silanes to
form the corresponding disiloxanes in the absence of a
metal catalyst. We also show that the germanium hydrides,
Et3GeH and Bu3GeH, also reduce DMF to form
trimethylamine and the corresponding digermoxane but
observe no intermediates R3GeOCH2NMe2. Bu3SnH
reduces DMF, but along with the low yields of
Bu3SnOSnBu3 (but no Bu3SnOCH2NMe2) significant
side products are obtained including (Bu3Sn)2 and
Bu4Sn. In the absence of DMF the siloxymethylamines
can undergo metal-catalyzed reactions with silanes,
germanes and stannanes to form disiloxanes, and
R3SiOER3 E = Ge, Sn, respectively. To date, the most
efficient catalyst for this latter process is (η5-C5H5)Mo-
(CO)3CH3 via a photochemical reaction.
We have reported that (η5-C5H5)Fe(CO)2CH3, (1a) is also
efficient in the photolytic reduction of N,N-dimethylformamide
(DMF) by silanes leading to trimethylamine and disiloxane
formation,9a an observation that was later corroborated by
others.9b Continuation of this research led us to use the
molybdenum analogs (η5-C5H5)Mo(CO)2(L)CH3, L = CO
(2a), PPh3 (2b), and we observe that 2a is more efficient
photochemically than 1a, Table 1, entries 6 vs 4 and 16 vs 14.
Use of the phosphine-substituted complex 2b to generate the
needed 16-electron species thermally, thus removing the need
for photochemical equipment, also led to reduction of DMF to
Me3N, Table 1, entries 7 and 17. Using 2a/2b as catalysts the
transformation of germanes to digermoxanes was also
performed for the first time, eq 2, Table 1. Whereas only
2R GeH + HC(O)NMe
3
2
2a(2b)
⎯⎯⎯⎯⎯⎯→⎯ R Ge − O − GeR + Me N
3
3
3
n
R = Et , Bu
(2)
3
trace amounts of the digermoxane can be observed from the
reactions between R3GeH (R = Et, Bu) and DMF catalyzed by
either 1a or 1b, Table 1, entry 22, 27, such reactions are
superior when using 2 as catalyst, over long reaction times,
Table 1, entries 23, 24, 28 and 29.
We initially suggested that the first step in the silane
reduction process was the hydrosilylation of the amides to form
siloxymethylamines (R3SiOCH2NMe2) subsequent to DMF
activation via coordination with the metal complex.9a We
proposed this intermediate could either react directly with
excess R3SiH or via a more complex route involving the key
intermediacy of (η5-C5H5)Fe(CO)2CH2NMe2 to form the
observed disiloxane products. We could not observe either of
these materials when using the Fe catalysts 1a or 1b.
The literature contains reports of DMF acting as a metal
ligand, and the work of Dobson and Sheline clearly detailed the
formation of Mo(CO)5(DMF) from the reaction of Mo(CO)6
and DMF.10 We decided to investigate the use of Mo(CO)6
(3a) as a catalyst for the reaction of tertiary silanes with DMF.
In all cases it is an excellent catalyst, Table 1, entries 1, 8, 18,
21, 25, 30, and 34. Thermal reactions were performed at 90 °C
he transition metal-catalyzed reduction of amides by
T
hydrosilanes is an efficient method for amine synthesis
along with the appropriate disiloxane,1−8 eq 1.
2R SiH + R′C(O)NR″
3
2
Catalyst
⎯⎯⎯⎯⎯⎯⎯→⎯ R Si − O − SiR + R′CH NR″
2
(1)
3
3
2
In 1985, the Voronkov group first reported that the reaction
of various silanes R2R′SiH (R2R′ = Cl2Me, Cl2Et; Et2Me and
Et3) with DMF, in the presence of the metal complexes
(NO)2PtCl6 or [Me2NH2]+[Rh(CO)2Cl2]−, led to the
formation of the corresponding disiloxanes.1 More recently
both Fe2(CO)9 and Fe3(CO)12 have been used as catalysts for
the efficient reduction of N,N-dimethylbenzamide with the
formation of benzyldimethylamine2 and zinc acetate was shown
to catalyze the reduction of aromatic heterocyclic amides.3 In
between these reports a number of groups have reported
variations on the theme using different metal catalysts; “green”
4a
Pt resins;4 Ru3(CO)12 and related variants;5 a range of other
metal carbonyl complexes,6 and the main group metal salt
InBr3.7 A variety of “plausible” mechanisms have been
suggested but to date, no process involving possible organo-
silicon intermediates and metal catalytic transients has been
Received: October 27, 2011
Published: December 19, 2011
© 2011 American Chemical Society
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Journal of the American Chemical Society
Communication
Table 1. Synthesis of R3EOER3 (E = Si, Ge, Sn) using Fe, Cr,
Mo, W and Mn Catalysts with excess of DMF
ab
,
entry
R3
Et3
E
cat.
hv/Δ
time (h) yield /%
1
Si
3a
3b
5
hv/Δ
hv/Δ
hv
1/3
1/3
28
(100)
2
Et3
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
(100)
3
Et3
(90)
4
PhMe2
PhMe2
PhMe2
PhMe2
PhMe2
PhMe2
PhMe2
PhMe2
PhMe2
PhMe2
Ph2Me
Ph2Me
Ph2Me
Ph2Me
Ph2Me
Ph3
1a
1b
2a
2b
3a
3b
5
hv
14
(100)
5
Δ
hv
48
TR
6
3
65(100)
75(100)
80(100)
80(100)
7
Δ
6
8
hv/Δ
hv/Δ
hv/Δ
hv/Δ
hv/Δ
hv/Δ
hv
2/6
2/8
18/24
12/24
4/1
7/7
18 h
2d
9
c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
(80)
d
c
5
(85)
6
93(100)
81(100)
(100)
TR
7
1a
1b
2a
2b
3a
hv
hv
3 h
2d
60(100)
65(100)
65(100)
NR
Δ
Δ
hv/Δ
Δ
Δ
12 h
48
Figure 1. NMR monitoring of the reaction of PhMe2SiH and DMF
catalyzed by Mo(CO)6, 3a. (Left) 29Si NMR showing the
disappearance of PhMe2SiH, −17.3 ppm (*) and transient formation
of 4a (5.4 ppm) en route to (PhMe2Si)2O, −0.6 ppm (◊): (Right)
13C spectra illustrating transient formation of PhMe2SiOCH2NMe2
(4a).
1a,1b,2a
2b
Ph3
24
TR
c
Ph3
3a
48
50(100)
TR
Et3
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Sn
1a,1b
2a
hv/Δ
hv
120
24
Et3
(30)
Et3
2b
Δ
Δ
Δ
hv/Δ
hv
120
72
55(80)
70(100)
(100)
TR
3a
Et3
3a
R3SiH + HC(O)NMe2 → R3Si − OCH2NMe2
Et3
3b
48
Bu3
1a,1b
2a
168
24
R3 = Ph3, Ph2Me, PhMe2, Et3
(3)
Bu3
(30)
Bu3
2b
Δ
Δ
hv
168
120
120
168
168
168
12
45(55)
70(100)
TR
Full experimental details for the isolation of 4a are provided
in the Supporting Information, and we also isolated and
characterized Ph2MeSiOCH2NMe2 (4b), Ph3SiOCH2NMe2
(4c) and Et3SiOCH2NMe2 (4d) (Table 2). The 13C NMR
Bu3
3a
Bu3
3a, 3b
1a,1b
2a,2b
3a
Ph3
hv/Δ
hv/Δ
Δ
NR
Ph3
TR
c
Ph3
60(100)
TR
Table 2. Synthesis of R3SiOCH2NMe2 using 3a
Bu3
1a,2a
1b
hv
NMR data for R3SiOCH2NMe2
Bu3
Sn
Δ
48
5(15)
a
b
time/
h
%
13C
Isolated yields. Yields based upon 1H NMR in parentheses.
Reaction performed at 120 °C. DEF. TR: traces; NR: no reaction.
ab
,
29Si
(CH2) (Me)2N
MeSi
c
d
R3
yield (NMR)
CpFe(CO)2Me (1a); CpFe(CO)2(Ph3P)Me (1b); CpMo(CO)3Me
(2a); CpMo(CO)2(Ph3P)Me (2b); Mo(CO)6 (3a); Mo(CO)5NMe3
(3b); CpMn(CO)3 (5); Cr(CO)6 (6); W(CO)6 (7).
Me3
13.8
15.3
5.4
81.5
82.3
82.4
82.5
82.9
40.4
41.1
41.1
40.8
40.9
−0.86
Et3
2
2
5
5
90(100)
25(60)
10(25)
(10)
PhMe2
Ph2Me
Ph3
−1.42
−3.04
−5.1
−15.2
1
and photochemical reactions at ambient temperature. Monitor-
ing the thermal reaction between PhMe2SiH and DMF ([SiH]/
[DMF] = 1:2) catalyzed by 3a in C6D6 using 29Si NMR, clearly
illustrated the efficient transformation of the silane to the
corresponding disiloxane (PhMe2Si)2O; however, it is clear that
a transient silicon-containing compound is formed with a 29Si
NMR resonance at 5.4 ppm, Figure 1. Monitoring of the same
reaction by 13C NMR also indicated the formation of a
transient species with three 13C NMR resonances at 82.4
(CH2), 41.1 (Me2N) and −1.4 (Me2Si) ppm, Figure 1.
Isolation of this transient by stopping the overall reaction prior
to completion permitted us to identify it as
PhMe2SiOCH2NMe2 (4a) and, for the first time, observe the
initial hydrosilylation product of DMF, illustrating it is an
intermediate in the overall DMF reduction process, eq 3.
a
b
Isolated yield. Yields based upon H NMR in parentheses.
data for 4a-d are almost equivalent to that of
Me3SiOCH2NMe2 (4f) the only previously reported member
of this family.11
We examined the thermal reactivity of 4 in the presence of
various silanes and observed no apparent chemical reaction
when the reaction was performed in deuterobenzene, the
solvent of choice for monitoring the reaction progress using
NMR spectroscopy. However, upon addition of excess DMF to
these same C6D6 solutions, under the thermal conditions
similar to those used for the chemistry observed in Figure 1, but
without the metal catalyst, the appropriate disiloxane and Me3N
were formed in high yield, eq 4, see Supporting Information.
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Journal of the American Chemical Society
Communication
HSiR3 coordination may be sufficient for initial reduction with
the amide CO group. Similarly the electron-deficiency at the
amide C atom induced by co-ordination to the metal center
may be sufficient for the interaction with the silane. However,
we have included the dual coordination step in the mechanism
since although we have not observed any Mo-tetracarbonyl
transients they have been observed in related chemistry.14d
Interestingly, the use of 2a as catalyst for photochemical
reactions of 4f with the group 14 hydrides in C6D6 solutions,
but in the absence of DMF, also led to the formation of the
appropriate disiloxane, siloxygermane15a or siloxystannane,15b
eq 5, Table 3.
R SiH + R Si − O − CH NMe
2
3
3
2
DMF
⎯⎯⎯⎯⎯→ R Si − O − SiR + NMe
3
3
3
R = Ph , Ph Me, PhMe , Et
3
(4)
3
3
2
2
This new reaction is reminiscent of similar reactions of
silanes with hydroxylic reagents and related systems where
activation of the Si−H bond via DMF coordination to form 5-
and 6-coordinated transients has been proposed and in some
cases observed.12 The Mironov group has reported that 4f
readily reacts with chlorosilanes to form disiloxanes and
ClCH2NMe2.13
From these results we now propose the thermal reduction of
amides to amines occurs via the cycle noted in Scheme 1 in the
R3EH + Me3Si − O − CH2NMe2
2a
⎯⎯⎯⎯→⎯ R3E − O − SiMe3 + NMe3
C D
6
6
Scheme 1. Proposed Catalytic Cycle for the Formation of
Me3N and R3SiOSiR3 from the Reaction between R3SiH and
Excess DMF Catalyzed by 5 mol % Mo(CO)6 (3a)
E = Si, R3 = Ph3, Ph2Me, PhMe2;
n
n
E = Ge: R3 = Et3, Bu3; E = Sn, R3 = Bu3
(5)
Table 3. Photochemical Synthesis of Me3SiOER3 Catalyzed
by 2a, eq 5
NMR data for R3EOSiMe3
a
b
R3
Ph3
E
time/h yield /% (NMR)
29Si
119Sn
Si
6
4
83(100)
60(100)
70(100)
40(80)
(100)
11.1
10.2
9.2
−20.0
−11.0
−1.6
Ph2Me
PhMe2
Ph3
Si
Si
4
Ge
Ge
Sn
12
18
2
8.3
Et3
6.8
Bu3
80(100)
5.6
76.4
a
b
1
Isolated yield. Yield based upon H NMR in parentheses.
The same reactions occur using 3a as the catalyst both
thermally and photochemically; however, to date, yields are
lower and reaction times are longer compared to those when
using 2a. Clearly two distinct mechanisms are occurring, DMF-
catalyzed and/or metal-catalyzed for the transformation of
R3SiOCH2NMe2 to disiloxanes in the presence of R3SiH. The
unsymmetrical disiloxanes, siloxygermanes and siloxystannanes,
R3Si = Me3Si, were also synthesized independently from the
direct thermal reactions of 4f with R3ECl following the
Mironov procedure.13
Regarding the metal-catalyzed reaction between 4f and R3EH
in an “inert” solvent, we have attempted to form molybdenum
complexes of 4f from reactions with either 2a or 3a but these
attempts have, to date, proven unsuccessful and led to
unrelated chemistry. We suggest that the metal-catalyzed
reactions outlined in eq 5 involve the metal-silane transients
and our studies are continuing on this aspect of the chemistry.
By careful examination of the 13C NMR data of the reactions
between R3SiH and DMF catalyzed by 3a, we can observe trace
amounts of the (CO)5MoNMe3, 3b. We have independently
synthesized 3b16 and in a separate experiment shown that it
acts as a catalyst for the overall transformation of R3SiH to
R3SiOSiR3 in the presence of DMF, Table 1, entries 9 and 26.
Other metal systems that have been reported to activate
R3SiH, and where DMF coordination can play a role, include
(η5-C5H5)Mn(CO)3, 5, Cr(CO)6, 6, and W(CO)6, 7.17,18 Use
of these materials as catalysts for the silane reduction of DMF
case of using 3a as a catalyst. The initial hydrosilylation can
proceed either via initial amide coordination to molybdenum as
has been reported,10 followed by η2-silane coordination after
further loss of CO, or vice versa, initial coordination of the η2-
SiH, followed by the amide. Molybdenum η2-silane σ-
1
complexes, (CO)5Mo(η2-HSiR3) have been detected by H
NMR spectroscopy in the photochemical reaction of group 6
metal carbonyls and the R3SiH.14 Migration of R3Si group to
oxygen can lead to the reductive elimination of the siloxy-
methyl amines R3SiOCH2NMe2, 4. It is significant that direct
hydrosilylation of acetone by W(CO)5(η2-HSiR3) to yield
R3SiOCHMe2 has also been observed previously.14d,e Whether
both reagents need to be coordinated to the metal center is an
open question since the activation of the hydrogen via the η2-
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Journal of the American Chemical Society
Communication
different route. Kozyukov, V. P.; Kozyukov, V. P.; Mironov, V. F. Zh.
Obshch. Khim. 1983, 53, 119−126, . Full experimental and
characterization data are in the Supporting Information.
and DEF chemistry was also successful, Table 1, entries 10−13.
The intermediacy of organosilicon compounds 4 was also
observed for each of these new catalysts.
(12) (a) Chrusc
(b) Weinmann, M.; Walter, G.; Huttner, G.; Heinrich, J. J.
́
iel, J. J. Can. J. Chem. 2005, 83, 508−516.
In the case of the reduction of DMF by germanes and
stannanes we have, to date, been unable to observe any
R3EOCH2NMe2 (E = Ge, Sn) intermediates when using any of
the catalysts noted above. It is possible that these group 14-
substituted amines react more rapidly under the reaction
conditions than their silicon analogs. Alternatively another
mechanism may be operative for Ge and Sn hydride reductions.
In the case of the Sn chemistry the range of other products
suggests a considerable radical participation as noted in other
stannane reduction reactions.19 Also, for the amide reductions
catalyzed by 1a, 1b, 2a and 2b where no R3SiOCH2NMe2
intermediates are observed, the overall mechanism may be
different to that proposed for the use of 3a.
Organomet. Chem. 1998, 561, 131−141. (c) Marciniec, B.; Gulin
H. J. Organomet. Chem. 1978, 146, 1−5.
́
ska,
(13) Kozyukov, V. P.; Mironov, V. F Zh. Obshch. Khim 1983, 53,
159−156.
(14) (a) Mathews, S. L.; Pons, V.; Heinekey, D. M. Inorg. Chem.
2006, 45, 6453−6459. (b) Stosur, M.; Kochel, A.; Keller, A.;
Szyman
(c) Adrjan, B.; Szyman
2163−2170. (d) Gadek, A.; Szyman
́
ska-Buzar, T. Organometallics 2006, 25, 3791−3794.
́
́
1441−1448. (e) We have photolyzed Mo(CO)6 in the presence of
Et3SiH and observed the formation of (CO)5Mo(η2-H-SiEt3), 1H
NMR resonance at −8.4 ppm for Mo-H. Addition of DMF to this
solution at room temperature results in the formation of
Et3SiOCH2NMe2. Experimental details and spectroscopic monitoring
are available in the Supporting Information.
(15) (a) R3SiOSnBu3: R3 = Ph3, Ph2Me, PhMe2: Takamizawa, M.;
Yamamoto, Y.; Takano, K. Jpn. Kokai Tokkyo Koho 1978, JP 53097045
A 19780824. (b) R3SiOGeEt3: R3 = Ph3, Ph2Me, PhMe2, Et3:
Hreczycho, G.; Frackowiak, D.; Pawluc, P.; Marciniec, B. Tetrahedron
Lett. 2011, 52, 74−6.
(16) Strohmeier, W.; Guttenberger, J. F.; Blumenthal, H.; Albert, G.
Chem. Ber. 1966, 99, 3419−3424.
(17) (a) Palmer, B. J.; Hill, R. H. Can. J. Chem. 1996, 74, 1959−1967.
(b) Schubert, U.; Grubert, S. Monats. Chem. 1998, 129, 437−443.
(18) Sheng, T.; Dechert, S.; Hyla-Kryspin, I.; Winter, R. F.; Meyer, F.
Inorg. Chem. 2005, 44, 3863−3874.
(19) (a) Dakternieks, D.; Perchyonok, V. T.; Schiesser, C. H.
Tetrahedron: Asymmetry 2003, 14, 3057−3068. (b) Studer, A.; Amrein,
S. Synthesis 2002, 835−849.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and characterization of
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
kpannell@utep.edu
■
ACKNOWLEDGMENTS
■
This work was supported by the Welch Foundation, Houston,
TX (Grant # AH-0546) and NIH-MARC program. We thank
Mr. Eduardo Cervantes for experimental help with reactions
involving catalysts 6 and 7.
REFERENCES
■
(1) Kopylova, L. I.; Ivanova, N. D.; Voronkov, M. G. Zhur. Obshch.
Khim. 1985, 55, 1649−51.
(2) Zhou, S.; Junge, K.; Adis, D.; Das, S.; Beller, M. Angew. Chem., Int.
Ed. 2009, 48, 9507−9510.
(3) (a) Das, S.; Addis, D.; Zhou, S.; Junge, K.; Beller, M. J. Am. Chem.
Soc. 2010, 132, 1770−1771. (b) Das, S.; Addis, D.; Junge, K.; Beller,
M. Chem.Eur. J. 2011, 17, 12186−12192.
(4) (a) Motoyama, Y.; Mitsui, K.; Ishida, T.; Nagashima, H. J. Am.
Chem. Soc. 2005, 127, 13150−13151. (b) Hanada, S.; Motoyama, Y.;
Nagashima, H. Terahedront. Lett. 2006, 47, 6173−6177. (c) Miles, D.;
Ward, J.; Foucher, D. A. Macromolecules 2009, 42, 9199−9203.
(5) Matsubara, K.; Iura, T.; Maki, T.; Nagashima, H. J. Org. Chem.
2002, 67, 4985−4988.
(6) (a) Sunada, Y.; Kawakami, H.; Imaoka, T.; Motoyama, Y.;
Nagashima, H. Angew. Chem., Int. Ed. 2009, 48, 9511−9514.
(b) Motoyama, Y.; Aoki, M.; Takaoka, N.; Aoto, R. Chem. Commun.
2009, 1574−1576.
(7) (a) Sakai, N.; Fujii, K.; Konokahara, T. Tetrahedron Lett. 2008,
49, 6873−75.
(8) (a) Barbe, G.; Charette, A. B. J. Am. Chem. Soc. 2008, 130, 18−
19. (b) Pelletier, G.; Bechara, W. S.; Charette, A. B. J. Am. Chem. Soc.
2010, 132, 12817−12819.
(9) (a) Sharma, H. K.; Pannell, K. H. Angew. Chem., Int. Ed. 2009, 48,
7052−7054. (b) Itazaki, M.; Ueda, K.; Nakazawa, H. Angew. Chem., Int.
Ed. 2009, 48, 6938.
(10) Stolz, I.; Dobson, G. R.; Sheline, R. K. Inorg. Chem. 1963, 2,
322−6.
(11) We have synthesized the new materials R3SiOCH2NMe2, in
good yields for R3 = Ph3, Ph2Me, PhMe2, Et3 and the procedure can be
readily extended to a wide variety of silyl groups. Previously only the
Me3Si compound had been reported by the Mironov group by a
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