Platinum-catalyzed one-pot alkenylation of aldehydes using alkynes and triethylsilane: Dual catalysis by platinum(II) chloride

Article abstract of DOI:10.1021/ol4026952

The PtCl₂-catalyzed hydrosilylation of terminal alkynes with triethylsilane and subsequent alkenylation of aldehydes with the resultant (E)-alkenylsilanes in a one-pot manner are described. By adding p-benzoquinone and LiI, the latter alkenylation step proceeds smoothly to give allyl silyl ethers in moderate to high yields. This one-pot alkenylation is tolerant to a reasonable range of functional groups. PtCl₂ plays a dual role as hydrosilylation and alkenylation catalysts.

Full text of DOI:10.1021/ol4026952

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Platinum-Catalyzed One-Pot Alkenylation
of Aldehydes Using Alkynes and
Triethylsilane: Dual Catalysis by
Platinum(II) Chloride
Hidenori Kinoshita, Ryousuke Uemura, Daiki Fukuda, and Katsukiyo Miura*
Department of Applied Chemistry, Graduate School of Science and Engineering,
Saitama University, Sakura-ku, Saitama 338-8570, Japan
kmiura@apc.saitama-u.ac.jp
Received September 18, 2013
ABSTRACT
The PtCl2-catalyzed hydrosilylation of terminal alkynes with triethylsilane and subsequent alkenylation of aldehydes with the resultant
(E)-alkenylsilanes in a one-pot manner are described. By adding p-benzoquinone and LiI, the latter alkenylation step proceeds smoothly to
give allyl silyl ethers in moderate to high yields. This one-pot alkenylation is tolerant to a reasonable range of functional groups. PtCl2 plays a dual
role as hydrosilylation and alkenylation catalysts.
Since allylic alcohols and their protected forms are very
useful synthetic intermediates, various methods have been
developed so far for efficient synthesis of this important
class of compounds.1 The transition-metal-mediated re-
ductive coupling between carbonyls and alkynes, a kind of
carbonyl alkenylation, provides a straightforward method
for this end.2ꢀ9 Particularly, the Ni-catalyzed reaction
using Et3B or a hydrosilane as a reducing agent has a wide
scope of available alkynes.2ꢀ4 In this context, intermole-
cular regiocontrol of the Ni-catalyzed alkenylation with
unsymmetrical alkynes is an important issue and has been
extensively studied by Montgomery’s and Jamison’s
groups for the past decade. In general, the CꢀC bond
formation with unsymmetrical alkynes occurs selectively
at the sterically less hindered sp-carbon. Introduction of an
alkene moiety3aꢀc or a TMS group2c,d,3f into alkynes or a
proper choice of NHC ligands2a,b can reverse the sense of
regioselection. However, completeregiocontroltoward the
CꢀC bond formation at the internal sp-carbon of terminal
alkynes remains as a subject of further study.10 We herein
report a Pt-catalyzed one-pot alkenylation of aldehydes
(1) Hodgson, D. M.; Humphreys, P. G. In Science of Synthesis;
Clayden, J., Ed.; Thieme: Stuttgart, 2008; Vol. 36, p 583.
(2) (a) Malik, H. A.; Sormunen, G. J.; Montgomery, J. J. Am. Chem.
Soc. 2010, 132, 6304. (b) Malik, H. A.; Chaulagain, M. R.; Montgomery,
J. Org. Lett. 2009, 11, 5734. (c) Sa-ei, K.; Montgomery, J. Tetrahedron
2009, 65, 6707. (d) Chaulagain, M. R.; Sormunen, G. J.; Montgomery, J.
J. Am. Chem. Soc. 2007, 129, 9568. (e) Sa-ei, K.; Montgomery, J. Org.
Lett. 2006, 8, 4441. (f) Mahandru, G. M.; Liu, G.; Montgomery, J.
J. Am. Chem. Soc. 2004, 126, 3698. (g) Oblinger, E.; Montgomery, J.
J. Am. Chem. Soc. 1997, 119, 9065.
(3) (a) Moslin, R. M.; Jamison, T. F. Org. Lett. 2006, 8, 455. (b)
Luanphaisarnnont, T.; Ndubaku, C. O.; Jamison, T. F. Org. Lett. 2005,
7, 2937. (c) Miller, K. M.; Jamison, T. F. J. Am. Chem. Soc. 2004, 126,
15342. (d) Miller, K. M.; Luanphaisarnnont, T.; Molinaro, C.; Jamison,
T. F. J. Am. Chem. Soc. 2004, 126, 4130. (e) Miller, K. M.; Huang, W.-S.;
Jamison, T. F. J. Am. Chem. Soc. 2003, 125, 3442. (f) Huang, W.-S.;
Chan, J.; Jamison, T. F. Org. Lett. 2000, 2, 4221.
(7) Oppolzer, W.; Radinov, R. N.; El-Sayed, E. J. Org. Chem. 2001,
66, 4766 and references therein.
(4) Ogoshi, S.; Arai, T.; Kurosawa, H. Chem. Commun. 2008, 1347.
(5) (a) Skucas, E.; Ngai, M.-Y.; Komanduri, V.; Krische, M. J. Acc.
Chem. Res. 2007, 40, 1394. (b) Cho, C.-W.; Krische, M. J. Org. Lett.
2006, 8, 891. (c) Komanduri, V.; Krische, M. J. J. Am. Chem. Soc. 2006,
128, 16448. (d) Jang, H.-Y.; Huddleston, R. R.; Krische, M. J. J. Am.
Chem. Soc. 2004, 126, 4664. (e) Huddleston, R. R.; Jang, H.-Y.; Krische,
M. J. J. Am. Chem. Soc. 2003, 125, 11488.
(8) Wipf, P.; Nunes, R. L. Tetrahedron 2004, 60, 1269 and references
therein.
(9) (a) Takai, K.; Sakamoto, S.; Isshiki, T.; Kokumai, T. Tetrahedron
2006, 62, 7534. (b) Takai, K.; Sakamoto, S.; Isshiki, T. Org. Lett. 2003, 5,
653. (c) Inanaga, J.; Katsuki, J.; Ujikawa, O.; Yamaguchi, M. Tetra-
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Lett. 1990, 31, 4765.
(6) (a) Leung, J. C.; Patman, R. L.; Sam, B.; Krische, M. J. Chem.;
Eur. J. 2011, 17, 12437. (b) Patman, R. L.; Chaulagain, M. R.; Williams,
V. M.; Krische, M. J. J. Am. Chem. Soc. 2009, 131, 2066.
(10) Takai and co-workers have reported that the Cr(II)-mediated
reductive coupling between aldehydes and terminal alkynes proceeds at
the internal sp-carbon with high regioselectivity. See refs 9a and 9b.
r
10.1021/ol4026952
XXXX American Chemical Society

with terminal alkynes and triethylsilane (Et3SiH), which
solves a part of the problem of regiocontrol in carbonyl
alkenylation.
pure sample of 2a was carried out under the conditions
shown in Scheme 1. As a result, the reaction carried out for
18 h gave the corresponding alkenylation product 4aa in
75% yield. This result ensured sufficient reactivity of 2a.
With these encouraging results, we next investigated the
one-pot alkenylation of 3a using the reaction mixture
obtained from 1a and Et3SiH under catalysis by 5 mol %
PtCl2. After the hydrosilylation of 1a (1 mmol) with
Et3SiH (1 mmol) in DCE was completed, LiI (0.1 mmol)
and 3a (0.5 mmol) were added to the reaction vessel.15,16
The mixture was heated to 70 °C and stirred for 24 h. This
one-pot operation afforded the desired product 4aa but
with low reproducibility (Table 1, entry 1). As described in
our previous report, the present alkenylation is hardly
catalyzed by Pt(0) and Pt(0) complexes.11 Therefore, it
seemed that the deactivation of the active Pt(II) species by
the remaining Et3SiH, that is, the generation of an inactive
Pt(0) species, caused the low reproducibility. To avoid the
reductive deactivation, excess amounts of 1a were used
for fast, complete conversion of Et3SiH. The reaction with
1.5 mmol of 1a gave 4aa in moderate yield with good
reproducibility (entry 2). Use of a small excess (1.2 mmol)
of 1a led to a successful result (entry 3). In entry 2, the
remaining 1a may retard the alkenylation step by compe-
titive coordination to the active Pt(II) species. On the other
hand, an excess of Et3SiH completely inhibited the alke-
nylation (entry 4). In this case, the formation of platinum
black was observed during the hydrosilylation step. These
results obtained by variation of the reactant ratio support
the deactivation of the Pt(II) catalyst by Et3SiH.
Introduction of oxidizing agents (0.1 mmol) was also
examined for more efficient alkenylation by conversion of
the inactive Pt(0) species into the active Pt(II) species.
When the crude 2a obtained from an equimolar mixture
of 1a and Et3SiH was used, N-methylmorpholine N-oxide
(NMO), CuCl2, and K2S2O8 were not effective in promot-
ing the alkenylation (entries 5ꢀ7). To our delight, the
reaction using p-benzoquinone (BQ) proceeded efficiently
with good reproducibility (entries 8 and 9). It was com-
pleted in 12 h, and the reaction for 12 h achieved a better
yield of 4aa than that for 24 h due tothe suppression of side
reactions. Alkenylsilane 2a0 was recovered from the reac-
tion mixture, which indicates that the regioisomer does not
participate in the alkenylation step.
Recently we have reported the Pt(II)-catalyzed alkenyla-
tion of aldehydes with alkenylsilanes (Scheme 1).11 Reac-
tions of alkenylsilanes with carbon and heteroatom
electrophiles take place usually at the position R to silicon,12
while the alkenylation developed by us proceeds only at
the β-position. Additionally, alkenylsilanes can be readily
prepared by the Pt-catalyzed hydrosilylation of terminal
alkynes with hydrosilanes.13 Our interest was therefore
focused on a one-pot hydrosilylationꢀalkenylation reac-
tion utilizing dual catalysis by a single platinum catalyst
to develop a more convenient method for aldehyde
alkenylation.
Scheme 1. Pt(II)-Catalyzed Alkenylation of Aldehydes with
Alkenylsilanes
We initially examined the PtCl2-catalyzed reaction of
oct-1-yne (1a, 1 mmol) with Et3SiH (1 mmol) in 1,2-
dichloroethane (DCE, 1.5 mL) (Scheme 2). As expected,
the desired hydrosilylation proceeded smoothly at room
temperature in the presence of 5 mol % PtCl2. Under these
conditions, 1a was completely consumed in 3 h, and a
mixture of (E)-alkenylsilane 2a and its regioisomer 2a0
(2a:2a0 = 89:11) was obtained in 88% yield.14
Scheme 2. Hydrosilylation of Oct-1-yne with Et3SiH
Our previous work on the Pt(II)-catalyzed alkenylation
disclosed that an (E)-alkenylsilane showed lower reactiv-
ity than its (Z)-isomer.11 To test the reactivity of (E)-
alkenylsilane 2a, the reaction of benzaldehyde (3a) with a
Withthe optimized reactionconditionsin hand (Table 1,
entry 9),17 our attention was turned to defining the
scope and functional group compatibility of this one-pot
(11) Miura, K.; Inoue, G.; Sasagawa, H.; Kinoshita, H.; Ichikawa, J.;
Hosomi, A. Org. Lett. 2009, 11, 5066.
(15) LiI is quite effective in promoting the Pt(II)-catalyzed alkenyla-
tion as described in ref 11. It likely serves to enhance the π-Lewis acidity
of the postulated active catalyst, Li[PtX3] (X = Cl, I), by introduction of
the iodide ion.
(16) It is necessary to add LiI after the hydrosilylation step. The
combined use of PtCl2 and LiI induced the dehydrogenative coupling
between 1a and Et3SiH to give 1-triethylsilyloct-1-yne mainly. Voronkov,
M. G.; Pukharevich, V. B.; Ushakova, N. I.; Tsykhanskaya, I. I.;
Albanov, A. I.; Vitkovskii, V. Y. J. Gen. Chem. USSR (Engl, Transl.)
1985, 55, 80.
(12) (a) Hosomi, A.; Miura, K. In Comprehensive Organometallic
Chemistry III; Crabtree, R. H., Mingos, M. P., Eds.; Elsevier: Amsterdam,
2007; Vol. 9, p 297. (b) Brook, M. A. Silicon in Organic, Organometallic,
and Polymer Chemistry; Wiley: New York, 2000. (c) Bassindale, A. R.;
Taylor, P. G. In The Chemistry of Organic Silicon Compounds, Part 2;
Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1989; p 893.
(13) Marciniec, B. In Hydrosilylation, A Comprehensive Review on
Recent Advances; Marciniec, B., Ed.; Spinger: Dordrecht, Netherlands,
2009; p 53.
(14) The H2PtCl6-catalyzed reaction of 1-hexyne with Et3SiH at
20 °C shows a similar regioselectivity (E:regio = 82:18). Pukhnarevich,
V. B.; Kopylova, L. I.; Trofimov, B. A.; Voronkov, M. G. J. Gen. Chem.
USSR (Engl, Transl.) 1975, 45, 2600.
(17) Other hydrosilanes, BuMe2SiH and PhMe2SiH, were also used
for the reaction of 3a with 1a under the optimized conditions. However,
the alkenylation resulted in low yields of the corresponding silyl ethers
(35% with BuMe2SiH, 39% with PhMe2SiH).
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Optimization for One-Pot Reaction among Oct-1-yne,
Et3SiH, and Benzaldehydea
Table 2. One-Pot Alkenylation of Aldehydes with Alkynes and
Et3SiHa
1a
Et3SiH
(mmol)
isolated
yield
product (%)b
entry
(mmol)
oxidant
yield (%)
entry
R1 in 1
R in 3
1
2
3
4
5
6
7
8
9c
1
1
none
none
none
none
NMO
CuCl2
K2S2O8
BQ
35ꢀ65
52
1
n-C6H13 (1a)
n-C6H13
4-ClC6H4 (3b)
4-BrC6H4 (3c)
4-O2NC6H4 (3d)
4-MeO2CC6H4 (3e)
4-MeC6H4 (3f)
4-AcOC6H4 (3g)
1-naphthyl (3h)
c-C6H11 (3i)
Ph(CH2)2 (3j)
Ph (3a)
4ab
4ac
4ad
4ae
4af
4ag
4ah
4ai
4aj
92
2
85
1.5
1.2
1
1
1
73
3c
4
n-C6H13
80
n-C6H13
73
1.5
1
0
1
19
5
n-C6H13
64
1
1
trace
40
79b
85
6
n-C6H13
37
1
1
7
n-C6H13
73
1
1
8
n-C6H13
78
1
1
BQ
9
n-C6H13
63
10
11
12
13e
14c,e
15c
16c,f
c-C6H11 (1b)
Cl(CH2)3 (1c)
Cl(CH2)3
Ph (1d)
4ba
4ca
4ci
77
a All reactions were carried out with 1a (1ꢀ1.5 mmol), Et3SiH
(1ꢀ1.5 mmol), PtCl2 (0.05 mmol), LiI (0.1 mmol), an oxidant
(0.1 mmol), and 3a (0.5 mmol) in DCE (1.5 mL). b The desilylated
alcohol was detected. c The alkenylation time is 12 h.
Ph
90
c-C6H11
83
Ph
4da
4dj
4ea
4fa
4fi
86
Ph
Ph(CH2)2
73d
75d
83d
51d
51d
BnO(CH2)3 (1e)
Ph
PhCO2(CH2)4 (1f) Ph
transformation (Table 2). Aromatic aldehydes bearing an
electron-withdrawing group efficiently underwent the Pt-
catalyzed alkenylation with 1a (entries 1ꢀ4). Introduction
of an electron-donating group into the aromatic ring
decreased the yield of 4 (entries 5 and 6). 1-Naphthalde-
hyde (3h) gave the corresponding allyl silyl ether 4ah in
73% yield (entry 7). These results roughly agree with our
previous results using (Z)-1-(trimethylsilyl)dodec-1-ene
(5), an isolated alkenylsilane.11 The one-pot alkenylation
of aliphatic aldehydes 3i and 3j proceeded smoothly under
catalysis by PtCl2ꢀLiI (entries 8 and 9). In the previous
work, the PtCl2ꢀLiI system was not effective in the
alkenylation of 3i and 3j with 5 because of competitive
R-deprotonation leading to the silyl enolate of 3i or aldol
products from 3j. The reasonable results may come from
the use of a triethylsilyl group although it is not clear at
present.
Other terminal alkynes 1bꢀg also served in the alkenyla-
tion. Under the standard conditions, the reactions of
cyclohexylacetylene (1b) and 5-chloropent-1-yne (1c) af-
forded alkenylation products 4 in good to high yields
(entries 10ꢀ12). The sp3-CꢀCl bond did not suffer from
any possible side reactions such as reduction and elimina-
tion. The alkenylsilane derived from phenylacetylene (1d)
showed low reactivity and required severe conditions
(entries 13 and 14). Pent-4-yn-1-ol could not be used for
the alkenylation while the protected form 1e served the
purpose (entry 15). Ester and imide functionalities are also
compatible with the Pt(II)-catalyzed alkenylation (entries
16ꢀ18). However, these alkynes 1f and 1g are inferior to 1a
in reactivity. In the reaction of 1f with 3i, MnI2 was more
effective than LiI in suppressing the formation of the silyl
enolate of 3i (entry 17). Oct-4-yne, an internal alkyne, was
also subjected to the successive reaction with Et3SiH and 3a.
17f,g,h PhCO2(CH2)4
18
PI(CH2)4 (1g)i
c-C6H11
Ph
4ga
a All reactions were carried out with 1 (1 mmol), Et3SiH (1 mmol),
PtCl2 (0.05 mmol), LiI (0.1 mmol), BQ (0.1 mmol), and 3 (0.5 mmol) in
DCE (1.5 mL). b Isolated yield based on 3. c The alkenylation time was 24 h.
d For ease of isolation, the product was converted into the corresponding
allylic alcohol by desilylation with TBAF. e The alkenylation step was
carried out at 100 °C. f The hydrosilylation time was 5 h. g MnI2 (0.05 mmol)
was used instead of LiI. h The alkenylation time is 15 h. i PI stands for
phthalimide.
Although the hydrosilylation with Et3SiH proceeded, no
alkenylation of 3a was observed.
To ascertain the role of BQ as a catalyst activator, the
reaction of 3a with 2a in the presence of Pt(0)-supporting
CaCO3 and LiI was investigated. Without any other
additives, it affored 4aa in low yield (Table 3, entry 1).
Use of LiCl as an additive led to a similar result (entry 2).
In contrast, adding BQ was effective in promoting the
alkenylation (enrty 3). The combined use of BQ and LiCl
achieved an efficient conversion of 3a into 4aa (entry 4).18
(18) As described in the previous report, the presence of a chloride ion
is important for enhancing the activity of the Pt catalyst although the
reason is not clear. See ref 11.
(19) Successive (Domino) reactions promoted by a Lewis acidic
ꢀ
ꢀ
Pt(II) salt or complex: (a) Barluenga, J.; Fernandez, A.; Dieguez, A.;
~ ꢀ
´
Rodrıguez, F.; Fananas, F. J. Chem.;Eur. J. 2009, 15, 11660. (b)
Fananas, F. J.; Fernandez, A.; C-evic, D.; Rodrı
2009, 74, 932. (c) Barluenga, J.; Mendoza, A.; Rodrı
F. J. Chem.;Eur. J. 2008, 14, 10892. (d) Miura, K.; Horiike, M.; Inoue,
G.; Ichikawa, J.; Hosomi, A. Chem. Lett. 2008, 37, 270. (e) Pujanauski,
B. G.; Prasad, B. A. B.;Sarpong, R. J. Am. Chem. Soc. 2006, 128, 6786. (f)
Miura, K.; Itaya, R.; Horiike, M.; Izumi, H.; Hosomi, A. Synlett 2005,
~
ꢀ
ꢀ
´
guez, F. J. Org. Chem.
~ ꢀ
guez, F.; Fananas,
´
ꢀ
3148. (g) Cadran, N.; Cariou, K.; Herve, G.; Aubert, C.; Fensterbank, L.;
Malacria, M.; Marco-Contelles, J. J. Am. Chem. Soc. 2004, 126, 3408. (h)
Nishibayashi, Y.; Yoshikawa, M.; Inada, Y.; Milton, M. D.; Hidai, M.;
Uemura, S. Angew. Chem., Int. Ed. 2003, 42, 2681.
Org. Lett., Vol. XX, No. XX, XXXX
C

coupling between aldehydes and alkynes. This one-pot
Pt(II)-catalyzed reaction is valuable and intriguing not
only from the synthetic point of view but also from the
viewpoint of catalytic action. Several successive reactions
involving two different catalytic cycles driven by a single
Pt catalyst have been developed so far.19,20 In most cases,
however, the Pt catalyst functions as a π-Lewis acid or
Lewis acid for electrophilic activation of CꢀC multiple
bonds and Cꢀheteroatom bonds. The present study on
dual catalysis by PtCl2 provides a rare example of Pt-
catalyzed successive reaction.21 We are now studying the
alkenylation of other carbon electrophiles by this one-pot
method.
Table 3. Pt(0)-Catalyzed Alkenylationa
isolated
entry
additive
yield (%)
1
2
3
4
none
12
16
50
73
LiCl
BQ
BQ þ LiCl
Acknowledgment. This work was partly supported by a
Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
a All reactions were carried out with 2a (1 mmol), 3a (0.5 mmol), 5%
Pt(0)/CaCO3 (0.05 mmol of Pt), LiI (0.1 mmol), and an additive(s)
(0.1 mmol) in DCE (1.5 mL).
Supporting Information Available. Experimental details
and characterization data (1H NMR, 13C NMR, IR, MS).
This material is available free of charge via the Internet at
http://pubs.acs.org.
Although we do not have any direct evidence of the
formation of a Pt(II) species by oxidation with BQ, these
results suggest that the inactive Pt(0) species is changed
into an active Pt(II) species by the action of BQ.
In conclusion, we have developed a new one-pot method
for the alkenylation of aldehydes with terminal alkynes
and a hydrosilane. The Pt(II)-catalyzed alkenylation takes
place only at the internal sp-carbon to give single regioi-
somers of allyl silyl ethers and shows reasonable compat-
ibility with polarfunctionalities including CꢀCl and CꢀBr
bonds. It is complementary to the Ni-catalyzed reductive
(20) A successive hydroborationꢀcross-coupling reaction by
a
PtꢀNHC complex: Lillo, V.; Mata, J. A.; Segarra, A. M.; Peris, E.;
Fernandez, E. Chem. Commun. 2007, 2184.
(21) One-pot hydrosilylationꢀcross-coupling reactions using two
transition metal catalysts: (a) Thiot, C.; Schumtz, M.; Wagner, A.;
Mioskowski, C. Chem.;Eur. J. 2007, 13, 8971. (b) Denmark, S. E.;
Wang, Z. Org. Synth. 2005, 81, 54. (c) Tamao, K.; Kobayashi, K.; Ito, Y.
Tetrahedron Lett. 1989, 30, 6051.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX