Re(I)-Catalyzed Hydropropargylation of Silyl Enol Ethers Utilizing Dynamic Interconversion of Vinylidene-Alkenylmetal Intermediates via 1,5-Hydride Transfer

Article abstract of DOI:10.1021/jacs.8b02903

Re(I)-catalyzed hydropropargylation reaction of silyl enol ethers was realized utilizing dynamic interconversion of vinylidene-alkenylmetal intermediates, where alkenylmetals underwent 1,5-hydride transfer of the α-hydrogen to generate vinylidene intermediates. Furthermore, this process was found to be in an equilibrium.

Full text of DOI:10.1021/jacs.8b02903

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Communication
Re(I)-Catalyzed Hydropropargylation of Silyl Enol Ethers
Utilizing Dynamic Interconversion of Vinylidene–
Alkenylmetal Intermediates via 1,5-Hydride Transfer
Nobuharu Iwasawa, Shoya Watanabe, Akane Ario, and Hideyuki Sogo
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02903 • Publication Date (Web): 12 Jun 2018
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Re(I)-Catalyzed Hydropropargylation of Silyl Enol Ethers Utilizing
Dynamic Interconversion of Vinylidene–Alkenylmetal Intermediates
via 1,5-Hydride Transfer
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Nobuharu Iwasawa,* Shoya Watanabe, Akane Ario, and Hideyuki Sogo
Department of Chemistry, School of Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551,
Japan
Supporting Information Placeholder
yields (Scheme 1a).9–11 In this reaction, α,β-unsaturated
ABSTRACT: Re(I)-catalyzed hydropropargylation re-
carbene complex intermediates reacted with si-
loxydienes at the carbene moiety to give cyclopropane
intermediates, which further underwent divinylcyclo-
propane rearrangement to give the products. We then
thought of the possibility of utilizing the α,β-unsaturated
carbene complex intermediates as a Michael accep-
tor.12,13 For this purpose, we expected that use of silyl
enol ethers with a substituent on the β-carbon would
disfavor the reaction at the carbene moiety due to the
steric repulsion and 1,4-addition of the silyl enol ethers
would give addition intermediate, which we initially
expected to give allylation products by hydrolysis.12 In
this paper, we describe exploitation of this approach,
which has resulted in a discovery of entirely unexpected
interconversion of vinylidene–alkenylmetal intermedi-
ates to realize a novel hydropropargylation reaction of
silyl enol ethers.
action of silyl enol ethers was realized utilizing dynamic
interconversion of vinylidene–alkenylmetal intermedi-
ates, where alkenylmetals underwent 1,5-hydride trans-
fer of the α-hydrogen to generate vinylidene intermedi-
ates. Furthermore, this process was found to be in an
equilibrium.
Vinylidene complexes have attracted a lot of attention
as a useful reactive species in transition metal-catalyzed
reactions.1 In these reactions, vinylidene complexes are
usually generated from terminal alkynes and their cen-
tral carbons show high electrophilicity, which has been
utilized for various synthetically useful catalytic trans-
formations such as anti-Markovnikov additions,2 cycloi-
somerization reactions,3 etc.4 In most of these reactions,
addition of nucleophilic reagents to the central vinyli-
dene carbon generates alkenylmetallic species, which
further undergo transformations such as protonation with
regeneration of the catalytic species. On the contrary, its
reverse reaction, that is, generation of vinylidene com-
plexes from alkenylmetallic species is rather limited
even in the stoichiometric reactions, 5–7 and elimination
of α-hydrogen of alkenylmetallic species as hydride for
the generation of vinylidene intermediates has not been
utilized for catalytic reactions, except for the α-
hydrogen elimination proposed for generation of vinyli-
dene complexes from terminal alkynes.8 In this paper,
we report Re(I)-catalyzed stereoselective hydropropar-
gylation of silyl enol ethers by utilizing dynamic inter-
conversion of vinylidene–alkenylmetal intermediates
through reversible 1,5-hydride transfer of α-hydrogen of
alkenylrheniums.
Scheme 1. Previous work/This work
β,β-Dimethyl-substituted silyl enol ether 2a was cho-
sen as a nucleophile and the reaction with primary pro-
pargyl ether 1 was examined.14 The reaction was carried
out by heating a 1.25 : 1 mixture of 1 and 2a using 3.1
mol% amount of ReI(CO)5 in dioxane at 100 °C for 10
hours. The reaction was found to proceed cleanly, and
We previously reported Re(I)-catalyzed generation of
α,β-unsaturated carbene complex intermediates from
easily available propargyl ethers and their reactions with
siloxydienes to give cycloheptadiene derivatives in high
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surprisingly, instead of the expected allylation product,
hydropropargylation product 3a was obtained in 85%
yield. (eq. 1)
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containing the deuterium atom at α-position of the silyl
ether (95%D) was obtained in good yield. In a similar
manner, terminally deuterated propargyl ether 1D’
(>98%D) gave terminally deuterated product 3aD’ with
93% deuterium incorporation (for details, see SI).20 The-
se results supported the proposed mechanism (Scheme
2). Thus hydride transfers associated with dynamic vi-
nylidene-alkenylmetal interconversion enabled this
unique transformation.
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8
Based on the structure of the product, the reaction was
thought to proceed as follows (Scheme 2); in a similar
manner to the previously reported reaction,9 the α,β-
unsaturated carbene complex intermediate B was gener-
ated via 1,5-hydride transfer to the vinylidene intermedi-
ate A followed by elimination of benzophenone with
electron-donation from the metal. Then silyl enol ether 2
added in a 1,4-addtion manner to give alkenylrhenium(I)
intermediate C having a silyloxonium moiety. At this
point, instead of protonation of the alkenylrhenium(I)
intermediate C, a novel 1,5-hydride transfer of the α-
hydrogen of the alkenylrhenium moiety to the silyloxo-
nium moiety proceeded to give a vinylidene complex D
containing a silyl ether moiety. It should be noted that
hydride transfer from sp2-carbon to generate sp-carbon
center is a quite rare process.15,16 The generated vinyli-
dene complex released the terminal alkyne product 3 as
a reverse reaction of vinylidene complex formation with
regeneration of the Re(I) catalyst.17 The overall trans-
formation is a hydropropargylation of a silyl enol ether,
a rare type of alkene functionalization reaction.18,19 In
this reaction, the vinylidene complex works as a hydride
acceptor first to generate α,β-unsaturated carbene com-
plex intermediate B, and then, after addition of the silyl
enol ether, the produced alkenylrhenium(I) C worked as
a hydride donor to generate another vinylidene complex
D. To support this mechanism, a deuterium-containing
propargyl ether 1D containing a deuterium atom at the
9
We then examined the generality of the present reac-
tion. The reaction of aromatic silyl enol ethers 3b pro-
ceeded smoothly to give the corresponding hydropro-
pargylation product in high yield. The reaction of silyl
enol ether 3c derived from isobutyraldehyde also pro-
ceeded without problem. As the second hydride transfer
step would occur intramolecularly, the reaction of cyclic
silyl enol ethers was examined next expecting high ste-
reoselectivity. The reaction of 1-siloxycyclopentene
gave the expected trans-2-propargyl-1-cyclopentanol
derivative 3d in good yield as a single stereoisomer ac-
companied by a small amount of a cyclopropanation
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Table 1. Generality of the silyl enol ether
Scheme 2. Deuterium experiment and proposed reaction
mechanism
diphenylmethyl ether moiety (96%D) was subjected to
this reaction, and the hydro-propargylation product 3aD
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product 12d. In the case of 1-siloxycyclohexene deriv-
mixture of two isomers in a ratio of 88 : 12 (entry 1).
The structure of the major isomer was determined by
NMR analysis. Tertiary propargyl ether 5b was also em-
ployable to give the cis-decalin 6b as a single isomer
(entry 2). The reaction of regioisomeric 1-
siloxycyclohexene substrate 5c also proceeded without
problem to give a bicyclo[3.3.1]octane derivative 6c in
82% yield as a single isomer (entry 3). It is also noted
that acyclic silyl enol ether 5d gave the monocyclic
compound 6d with high diastereoselectivity (entry 4).
The structure of 6b, 6c and 6d(major) was determined
by single crystal X-ray analysis. Thus this protocol af-
fords a useful method for the stereoselective synthesis of
polycyclic compounds.
1
ative, trans-2-propargyl-1-cyclohexanol derivative 3e
was obtained in good yield, but in this case a small
amount of cis-isomer was also obtained. 2-Methyl-1-
siloxycyclohexene derivative gave the desired product 3f
again as a single isomer. It is likely that 2-methyl group
inhibited the 1,5-hydride transfer from the opposite face.
Siloxy-indene or dihydronaphthalene derivatives also
gave the desired trans-2-propargyl-1-cycloalkanol de-
rivatives 3h-k as a single isomer. Several functional
groups such as cyclic acetal and imide were compatible
under the reaction conditions to give the desired hydro-
propargylation products 3l,m in good yield. Thus, the
reaction showed wide generality concerning the silyl
enol ether. On the contrary, the reaction of secondary or
tertiary propargyl ethers was rather sluggish, and in one
instance, hydropropargylation product 3p was obtained
in about 45% yield by the reaction of BOM ether of 3-
butyn-2-ol with a silyl enol ether derived from 2-
norbornanone.
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Table 2. Generality of the intramolecular reaction
During the examination of the generality of the reac-
tion, another dynamic nature of the reaction was discov-
ered in the product-forming step. As shown in Table 1,
the cycloheptene derivative gave two isomeric products
3g in 67 : 33 ratio, and the major trans product was iso-
lated as a diastereomerically pure compound. Interest-
ingly, on treatment of this trans product 3gmajor with a
catalytic amount of ReI(CO)5 at 100 °C in dioxane,
isomerization reaction was found to proceed to give
again a 67 : 33 mixture of two diastereomeric products.
(eq 2) This result suggests the presence of the reversible
pathway for the hydride transfer of the alkenylrhenium
intermediates, that is, the vinylidene complex E is gen-
erated from the product and the hydrogen at the carbon
connected to the siloxy group undergoes 1,5-hydride
transfer to the vinylidene carbon to give alkenylrhenium
species F.21 Thus, this reaction is comprised of dynamic
interconversion of vinylidene-alkenylmetal intermedi-
ates all through the reaction sequence.
In summary, we have developed the hydro-
propargylation reaction of silyl enol ethers utilizing the
dynamic interconversion of vinylidene–alkenylmetal
intermediates via reversible 1,5-hydride transfer process.
The siloxycycloalkene gave the trans-2-propargyl-1-
siloxy-cycloalkane derivatives with high stereoselectivi-
ty. The reaction was further applied to intramolecular
reactions to give useful polycyclic compounds stereose-
lectively.
This hydropropargylation reaction was further applied
to the intramolecular reaction to realize a facile con-
struction of polycyclic compounds with high diastere-
oselectivity (Table 2). A siloxy-cyclohexene derivative
containing a propargyl ether moiety 5a gave the cis-
bicyclo[4.4.0]decane derivative 6a in 80% yield as a
ASSOCIATED CONTENT
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Page 4 of 6
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AUTHOR INFORMATION
Corresponding Author
niwasawa@chem.titech.ac.jp
ORCID
8
9
(7) 1,3-H migration of alkenylmetallic species to the nitrogen of
azavinylidene ligand of Os complex was reported. Castarlenas, R.;
Esteruelas, M. A.; Oñate, E. Organometallics 2000, 19, 5454–5463.
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(b) Arndt, M.; Salih, K. S. M.; Fromm, A.; Goossen, L. J.; Menges,
F.; Niedner-Schatteburg, G. J. Am. Chem. Soc. 2011, 133, 7428–7449.
(c) Maity, B.; Goossen, L. J.; Koley, D. Chem. Sci. 2015, 6, 2532–
2552. (d) Maity, B.; Koley, D. ChemCatChem 2018, 10, 566–580.
(9) Sogo, H.; Iwasawa, N. Angew. Chem. Int. Ed. 2016, 55, 10057–
10060.
(10) For reviews on rhenium-catalyzed reactions, see: (a) Kunino-
bu, Y.; Takai, K. Chem. Rev. 2011, 111, 1938–1953. (b) Mao, G.;
Huang, Q.; Wang, C. Eur. J. Org. Chem. 2017, 2017, 3549–3564.
(11) For examples of catalytic reactions utilizing rhenium vinyli-
dene complexes, see: (a) Fukumoto, Y.; Daijo, M.; Chatani, N. J. Am.
Chem. Soc. 2012, 134, 8762–8765. (b) Xia, D.; Wang, Y.; Du, Z.;
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Nobuharu Iwasawa: 0000-0001-6323-6588
Note
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
This research was supported by a Grant-in-Aid for Scien-
tific Research on Innovative Areas "Precise Formation of a
Catalyst Having a Specified Field" (No. 15H05800) from
the Ministry of Education, Culture, Sports, Science, and
Technology of Japan. H. S. thanks JSPS for a fellowship.
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product was observed by mass analysis (for details, see SI).
(21) No isomerization occurred when 3g-Me, in which terminal al-
kyne of 3gmajor was methylated, was subjected to the reaction condi-
tions (for details, see SI).
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alkenes has several precedents. For examples, see: (a) Haruta, J.;
Nishi, K.; Matsuda, S.; Akai, S.; Tamura, Y.; Kita, Y. J. Org. Chem.
1990, 55, 4853–4859. (b) Shibata, I.; Kano, T.; Kanazawa, N.; Fuku-
oka, S.; Baba, A. Angew. Chem. Int. Ed. 2002, 41, 1389–1392. (c) Lee,
K.; Kim, H.; Miura, T.; Kiyota, K.; Kusama, H.; Kim, S.; Iwasawa,
N.; Lee, P. H. J. Am. Chem. Soc. 2003, 125, 9682–9688. (d) Meng, F.;
Li, X.; Torker, S.; Shi, Y.; Shen, X.; Hoveyda, A. H. Nature 2016,
537, 387–393.
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(19) For reviews of transition metal-catalyzed propargylic substitu-
tion reactions, see: (a) Miyake, Y.; Uemura, S.; Nishibayashi, Y.
ChemCatChem. 2009, 1, 342–356. (b) Ding, C.-H.; Hou, X.-L. Chem.
Rev. 2011, 111, 1914–1937. (c) Debleds, D.; Gayon, E.; Vrancken,
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TOC Graphic
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OTIPS
R3
R1 R2
OTIPS
R3
R2
cat. ReI(CO)5
OCHPh2
H
R1
+
1,4-dioxane
100 °C
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[Re]
OTIPS
[Re]
H
1,5-H transfer
OTIPS
R3
H
R3
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R1 R2
R1 R2
Vinylidene-Alkenylmetal Dynamic Interconversion
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