Regio- and stereoselective hydrosilylation of unsymmetrical alkynes catalyzed by a well-defined, low-valent cobalt catalyst

Article abstract of DOI:10.1021/acs.orglett.6b01987

Herein, the use of a well-defined low-valent cobalt(I) catalyst [HCo(PMe₃)₄] capable of performing the highly regio- and stereoselective hydrosilylation of internal alkynes is reported. The reaction can be applied to a variety of hyd

Full text of DOI:10.1021/acs.orglett.6b01987

Letter
pubs.acs.org/OrgLett
Regio- and Stereoselective Hydrosilylation of Unsymmetrical Alkynes
Catalyzed by a Well-Defined, Low-Valent Cobalt Catalyst
Alejandro Rivera-Hernandez,‡ Brendan J. Fallon,‡ Sandrine Ventre, Cedric Simon,
́
́
Marie-Helene Tremblay, Geoffrey Gontard, Etienne Derat, Muriel Amatore, Corinne Aubert,
and Marc Petit*
́
̀
UPMC UNIV Paris 06, Institut Parisien de Chimie Molec
France
́
ulaire, UMR CNRS 8232, Case 229, 4 Place Jussieu, Paris 75252 Cedex 05,
S
* Supporting Information
ABSTRACT: Herein, the use of a well-defined low-valent
cobalt(I) catalyst [HCo(PMe3)4] capable of performing the
highly regio- and stereoselective hydrosilylation of internal
alkynes is reported. The reaction can be applied to a variety of
hydrosilanes, symmetrical and unsymmetrical alkynes, giving in
many cases a single hydrosilylation isomer. Experimental and
theoretical studies suggest the key step to be a hydro-
cobaltation and that the reaction proceeds through a classical
Chalk−Harrod mechanism.
Scheme 1. Cobalt-Catalyzed Hydrosilylation of Alkynes
ransition metal-catalyzed hydrosilylation of alkynes
Trepresents one of the most straightforward methods to
obtain synthetically valuable vinylsilanes.1 However, a pertinent
issue impeding the widespread use of this method is the
difficultly in achieving regio- and stereocontrol, both of which
are still currently challenging.2 There have been several reports
over the past decade on the transition-metal-catalyzed hydro-
silylation of alkynes largely based on the use of noble metals
(Y,3 Ru,4 Rh,5 Pt).6 In contrast, reports on the use of earth
abundant metals (Fe,7 Ni,8 Cu)9 have been sparse, and
considering the need to develop new efficient, economic, and
environmentally benign catalysts this remains an appealing area
of study. Owing to its high abundancy, low cost, and rich redox
chemistry, cobalt offers an attractive alternative to other
transition-metal-based catalysts. While several groups using
low-valent cobalt catalysis recently explored the hydrosilylation
of alkenes, reports on alkynes remain rare in comparison.10 The
group of Konno demonstrated the catalytic activity of
Co2(CO)8 toward the highly regio- and stereoselective
hydrosilylation of fluoroalkylated alkynes with HSiEt3 (Scheme
1a).11a Isobe initially reported the hydrosilylation of unsym-
metrical alkynes which suffered from poor regioselectivity.11b
Later, he described the syn-α-hydrosilylations with internal
H2SiPh2; switching to a tertiary hydrosilane led to a complete
shutdown of the catalytic activity.11g
Our group has recently been interested in the application of
well-defined low-valent cobalt complexes, in particular, HCo-
(PMe3)4 and Co(PMe3)4, toward C−H bond activation and [2
+ 2 + 2] enediyne cycloadditions.12,13 Herein, we report the
highly regio- and stereoselective hydrosilylation of internal
alkynes. The present study is notable for a number of features:
(i) high regio- and stereoselectivities have been obtained for a
large variety of unsymmetrical alkynes, (ii) the reaction is
compatible with tertiary silanes and alkoxysilanes, and (iii) the
one-step synthesis of catalyst is inexpensive and easy on a large
phenylthioalkynes (Scheme 1b).11c−e Butenschon described the
̈
hydrosilylation of a narrow range of internal alkynes using both
HSi(OEt)3 and HSiEt3 albeit with poor control of the
regioselectivity (Scheme 1c).11f Recently, Deng and co-workers
reported the use of low-coordinated cobalt(I) complexes
stabilized by a bulky N-heterocyclic carbene ligand (Scheme
1d). Such complexes have proven to be highly efficient for the
regio- and stereoselective hydrosilylation of a large variety of
alkynes. However, the reaction was only compatible with
Received: July 8, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01987
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
scale. To begin this study, we first examined the capacity of
complex HCo(PMe3)4 to perform the hydrosilylation of
diphenylacetylene 1a with various hydrosilanes (Table 1).
Table 2. Hydrosilylation of Unsymmetrical Alkynes
Table 1. Hydrosilylation of Diphenylacetylene with Various
a
Hydrosilanes
a
b
Diphenylacetylene (0.5 mmol), silane (0.55 mmol). Certain
reactions could also be performed at lower temperatures but generally
required longer reaction times and led to slightly lower yields (see the
c
d
Supporting Information). Isolated yields. Yields obtained using
Co(PMe3)4 as catalyst.
Pleasingly, we encountered a reaction amenable with an ample
variety of aryl/alkyl tertiary silanes and alkoxysilanes (Table 1,
entries 1−7), achieving good yields up to 94%. Notably, these
reaction conditions successfully allow for the hydrosilylation of
a highly functionalized and sterically demanding silane 2g
(Table 1, entry 7). In general the products of syn-hydro-
silylation were observed as major isomers, with high (E/Z)
ratios. It is worth highlighting the unique ability of this complex
to carry out the hydrosilylation of alkynes with a wide variety of
hydrosilanes which has not been previously described under
cobalt catalysis. As shown in our previous work, terminal
alkynes only afford dimerization compounds under these
conditions.13a
a
b
c
Alkyne (0.5 mmol), HSiR3 (0.55 mmol). Isolated yields. For
detailed information regarding the (E/Z) and (α/β) ratios of the
obtained products (see the Supporting Information). Reaction was
performed at 60 °C for 24 h. Reaction was performed at 100 °C for
24 h. Regiochemistry of the major compound was not determined.
d
e
f
We then turned out attention to the more challenging
hydrosilylation of internal unsymmetrical alkynes (Table 2).
Such alkynes were substituted by functional groups with
different steric hindrances, described as RS (smaller group) and
RL (larger group). The complex HCo(PMe3)4 along with the
silanes HSi(OEt)3 and HSiPh3 were chosen due to positive
preliminary results regarding the high yields and syn selectivity
observed. Using less reactive alkynes (compared to dipheny-
lacetylene), new optimized reactions conditions were estab-
lished, i.e., toluene heated at 160 °C for 2 h (see the SI).
Starting the initial scope with the silane 2a (HSiPh3), the
product of hydrosilylation of 1-phenyl-1-hexyne was isolated in
92% yield (Table 2, entry 1) in favor of the syn adduct 3ba
(product of type 3 with the H atom positioned close to the
larger substituent). Variation of the electronics on the phenyl
moiety has no effect on the yield, but a better regioselectivity
was observed in the presence of an electron-withdrawing group
(Table 2, entries 2 and 3). Next, we examined the reactivity of
TMS-protected alkynes 1e−k. Previous reports on the
hydrosilylation of these kind of alkynes indicated that both
(E)-α,β5d,6 or (E)-α,α-disilylalkenes11g products could be
selectively obtained. Under our reaction conditions, and using
HSiPh3, the hydrosilylation of TMS-protected arylacetylenes
1e−i (Table 2, entries 4−8) led exclusively to the
corresponding disilylalkenes in high yields and complete
regioselectivity toward the adducts of type 3, i.e., (E)-α,β-
disilylalkenes.
The catalytic system also allowed the use of heteroatoms
with TMS-protected pyridylalkynes 1j−k, isolating only the
products of type 3 in moderate to good yields (Table 2, entries
9 and 10). It is worth noting that the regioselectivity oberved in
our reaction with TMS-containing alkynes is opposite to the
one previously reported by Deng11g (see below for more
detailed discussion). Adding two methyl groups in the ortho-
position on a phenyl ring generates a steric bulk on one side of
the alkyne, which directs the addition of the H atom to the
sterically hindered carbon to give product 3la (Table 2, entry
11). 1-Phenylpropyne 1m followed the same trend of steric
dependence of the alkyne substituents in the formation of the
final product 3ma. The low yield is presumably due to a
stability issue of the alkyne (Table 2, entry 12). On the other
hand, the reaction of nonsterically differentiated unsymmetrical
alkynes but bearing electronically distinct substituents (1n, 1o,
and 1p) gave 50/50 mixtures of vinylsilanes of type 3 and 3′
(Table 2, entries 13−15). These latter results imply that the
regio- and stereocontrol of the reaction is predominantly
governed by steric factors of the starting substrates, while
electronic factors have no significant effect on the outcome of
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the reaction. Finally, the use of ester-substituted substrates 1q
and 1r in the hydrosilylation with HSiPh3 yields solely the
product of type 3′, presumably due to the chelating effect of the
ester which appears to overcome the steric control (Table 2,
entries 16 and 17).12,14 We obtained analogous results when we
addressed the hydrosilylation of unsymmetrical internal alkynes
using the silane HSi(OEt)3 2b, which again gave almost
exclusively the syn-adducts of type 3 (Table 2, entries 18−27).
This time, products 3bb and 3cb showed complete
regioselectivity, presumably due to the larger steric bulk
provided by the triethoxy moieties in the silane (Table 2,
entries 18 and 19). Further exploration into the scope of the
current reaction showed identical results by employing TMS-
protected arylacetylenes, with complete selectivity toward the
corresponding (E)-α,β-disilylalkenes of type 3 and regardless of
the aromatic group linked to the alkyne. Again, alkynes 1l and
1n gave similar regioselectivities as previously observed with
triphenylsilane (Table 2, entries 26 and 27). Extension of this
methodology to an intramolecular version was also possible,
which provides an alternative route to benzosilole derivatives
via a hydrosilylation/double migration sequence (Scheme 2).15
smallest activation barrier, shown in Figure S1 (see the SI). The
main question addressed is whether the alkyne insertion
initially occurs into the Co−H bond (hydrocobaltation) or into
the Co−Si bond (silacobaltation). According to the results, the
barrier for the hydro-obaltation process is 3.18 kcal/mol, while
the silacobaltation barrier is 19.42 kcal/mol. Thus, our cobalt
catalyst would follow the classical Chalk−Harrod mechanism.18
Another important question addressed by the DFT calculations
was that, despite the use of a hydridocobalt catalyst with a
silane, no H2 evolution was experimentally observed. This
could be explained since starting from the cobalt complex 6
bearing two hydrogen atoms the reductive elimination of H2 is
an endothermic process with a transition state residing above
by 3.36 kcal/mol, while the reverse barrier (i.e., H2 addition on
the cobalt center) is very small (0.47 kcal/mol). Thus, even if
H2 is produced during the catalytic process, it will quickly
rebound and split onto the cobalt center to regenerate the
active species (Figure S2, SI).
Based on the experimental and theoretical information (see
also Tables S2 and S3 in the SI), we propose the following
catalytic cycle (Figure 1). Initial oxidative addition of the silane
Scheme 2. Intramolecular Hydrosilylation
Focusing our attention now on the reaction mechanism, we
studied the stoichiometric reaction of hydridocobalt complex
HCo(PMe3)4 and triphenylsilane (Scheme 3a). Having
Scheme 3. Reaction of HCo(PMe3)4 with HSiPh3
Figure 1. Proposed mechanism.
to the cobalt center, followed by coordination of the alkyne,
gives the dihydridocobalt(III) intermediate III, which under-
goes alkyne insertion into a Co−H bond to furnish the vinyl
organometallic IV. At this point, the H atom is directed toward
the more bulky substituent in order to place the cobalt moiety
in the less hindered plane of the molecule. Direct reductive
elimination releases the vinylsilane V as the major product with
the observed regioselectivity and regenerates the catalytically
active species I. To a much lesser extent, an opposite migratory
insertion would yield the adduct isomer VI, which eventually
results in the unfavored product VII.
A model to compare the regioselectivity between our results
and Deng’s11g is shown in Scheme 4. In the model proposed by
Deng, the sila-cobaltation step is controlled by the very bulky
ligand IAd (1,3-diadamantylimidazol-2-ylidene), and this
determines the regioselectivity of the reaction. For unsym-
metrical internal alkynes, this control based on a bulky ligand is
efficient when the steric differentiation between the two
substituents is significant. Otherwise, it leads to low selectivity
as shown by Deng using 1-phenylpropyne which yielded a 50/
50 mixture of regioisomers. In our system, we proposed that a
hydrocobaltation step controls the selectivity. In this process
successfully isolated the dihydrocobalt(III) complex 6, we
began to explore its catalytic activity.16,17 To this end, the
reaction of diphenylacetlyene and triphenylsilane in the
presence of 5 mol % of 6 afforded the desired hydrosilylation
product 3aa in good yields and the same selectivity as that
observed with HCo(PMe3)4 (Scheme 3b). This result suggests
a potential participation of the dihydrodocobalt(III) complex to
trigger the catalytic process. Moreover, the stoichiometric
reaction of diphenylacetylene with HCo(PMe3)4 shows only a
ligand exchange and no evidence of hydrocobaltation (Scheme
3c).
To gain deeper insight into the mechanistic details, DFT
(density functional theory) calculations were performed on a
system based on complex 6 in conjunction with 1-phenyl-
propyne and HSi(OMe)3. Following our previous studies,13
parameters were set as functional B3LYP and basis set SVP,
while all the DFT calculations were performed with
Gaussian09. We present the conformation, which led to the
C
DOI: 10.1021/acs.orglett.6b01987
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b01987.
Tetrahedron Lett. 2005, 46, 381. (f) Yong, L.; Kirleis, K.; Butenschon,
̈
Experimental procedures and analytical data (PDF)
Crystallographic data for 3ha, 3ia, and 3la (CIF)
H. Adv. Synth. Catal. 2006, 348, 833. (g) Mo, Z.; Xiao, J.; Gao, Y.;
Deng, L. J. Am. Chem. Soc. 2014, 136, 17414.
(12) Ventre, S.; Simon, C.; Rekhroukh, F.; Malacria, M.; Amatore,
M.; Aubert, C.; Petit, M. Chem. - Eur. J. 2013, 19, 5830.
(13) (a) Ventre, S.; Derat, E.; Amatore, M.; Aubert, C.; Petit, M. Adv.
Synth. Catal. 2013, 355, 2584. (b) Fallon, B. J.; Derat, E.; Amatore, M.;
Aubert, C.; Chemla, F.; Ferreira, F.; Perez-Luna, A.; Petit, M. J. Am.
Chem. Soc. 2015, 137, 2448. (c) Fallon, B. J.; Garsi, J.-B.; Derat, E.;
Amatore, M.; Aubert; Petit, M. ACS Catal. 2015, 5, 7493. (d) Fallon,
B. J.; Derat, E.; Amatore, M.; Aubert, C.; Chemla, F.; Ferreira, F.;
Perez-Luna, A.; Petit, M. Org. Lett. 2016, 18, 2292.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: marc.petit@upmc.fr.
Author Contributions
‡A.R-H. and B.J.F. contributed equally.
Notes
(14) The strong electron-withdrawing effect of the ester may also
explain the observed selectivity.
The authors declare no competing financial interest.
(15) Onoe, M.; Baba, K.; Kim, Y.; Kita, Y.; Tobisu, M.; Chatani, N. J.
Am. Chem. Soc. 2012, 134, 19477.
ACKNOWLEDGMENTS
■
This work was supported by CNRS, MRES, UPMC, and ANR
(ANR-12-BS07-0031- 01COCACOLIGHT), which we grate-
fully acknowledge. We also thank CONACYT for the
postdoctoral fellowship granted to A.R.-H. (Registry No.
252113).
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