Rhodium-catalyzed hydroalkynylation of internal alkynes with silylacetylenes: An alkynylrhodium(I) intermediate generated from the hydroxorhodium(I) complex [Rh(OH)(binap)]2

Article abstract of DOI:10.1002/adsc.200700334

A highly selective hydroalkynylation of internal alkynes with silylacetylenes giving 1,3-enynes was realized by use of a hydroxorhodium catalyst. As a key intermediate in the catalytic cycle, an alkynylrhodium(I) complex was isolated and investigated for its structure and reactivity.

Full text of DOI:10.1002/adsc.200700334

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DOI: 10.1002/adsc.200700334
Rhodium-Catalyzed Hydroalkynylation of Internal Alkynes with
Silylacetylenes: An Alkynylrhodium(I) Intermediate Generated
from the Hydroxorhodium(I) Complex [Rh(OH)(binap)]2
A
Takahiro Nishimura,a,* Xun-Xiang Guo,a Kohei Ohnishi,a and Tamio Hayashia,*
a
Department of Chemistry, Graduate Schoolof Science, Kyoto University, Kyoto 606-8502, Japan
Fax : (+81)-75-753-3988; e-mail: tnishi@kuchem.kyoto-u.ac.jp or thayashi@kuchem.kyoto-u.ac.jp
Received: July 9, 2007; Published online: November 27, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A highly selective hydroalkynylation of
internal alkynes with silylacetylenes giving 1,3-
enynes was realized by use of a hydroxorhodium
catalyst. As a key intermediate in the catalytic
cycle, an alkynylrhodium(I) complex was isolated
and investigated for its structure and reactivity.
Keywords: alkynes; alkynylation; dimerization; rho-
dium
dicates that the hydroalkynylation took place in a syn
fashion.[8]
In previous studies on the rhodium-catalyzed di-
merization of terminal alkynes, alkynylrhodium(III)
G
hydride or vinylidene-rhodium species have been pro-
posed as key intermediates.[2a,b,9] In our studies, we
succeeded in isolating and characterizing an alkynylr-
As a straightforward and economicalmethod for pre-
paring conjugate enynes, which are important building hodium(I) complex, which gave us significant insights
blocks in organic synthesis,[1] the catalytic dimeriza- into the mechanism of the present hydroalkynylation.
tion of alkynes has attracted considerable attention. Thus, treatment of [Rh(OH)((R)-binap)]2 (5) with si-
Although several protocols have been developed for lylacetylene 2m and triphenylphosphine in toluene at
[2,3]
the homo-dimerization of terminalaklynes,
there 808C for 1 h brought about the selective formation of
are only a few examples of selective cross-dimeriza- alkynylrhodium(I) complex 6 coordinated with binap
[4,5]
tion between terminalaklynes and internalones,
mainly because the terminal alkynes are highly reac-
tive towards homo-dimerization or oligomerization.
Here we describe that a hydroxorhodium(I) complex
efficiently catalyzes the addition of silylacetylenes to
internalakl ynes giving conjugate enynes with high se-
lectivity and that its catalytic cycle involves an alky-
nylrhodium(I) complex as a key intermediate.
and triphenylphosphine, which was isolated in 86%
A hydroxorhodium(I) complex coordinated with
binap was found to be highly effective in catalyzing
the addition of silylacetylenes to internal alkynes to yield [Eq. (2)].[10,11] The 31P NMR spectrum of the
give high yields of the cross-dimerization products complex 6 in C6D6 showed three ddd peaks (32.6,
with high selectivity. Thus, the reaction of 1-phenyl-1- 35.5, and 37.8 ppm) [Figure 1, (i)], which are charac-
propyne (1a) with (triphenylsilyl)ethyne (2m) (1a/ teristic for square planar rhodium complexes coordi-
2m=1/1.5) in the presence of a catalytic amount of nated with three non-equivalent phosphorus atoms.[6]
[6]
[Rh(OH)((R)-binap)]2 (5) (5 mol% Rh) in 1,4-diox-
The complex 6 was also formed in the reaction of hy-
ane at 408C for 1 h gave a 90% yield of (E)-1-triphe- droxorhodium 5 with propargylic alcohol 7 in place of
nylsilyl-3-methyl-4-phenylbut-1-yn-3-ene (3am) to- 2m, which should proceed via b-alkynyl elimination
gether with a minor amount (4%) of its regioisomer on an alkoxorhodium intermediate.[12] The alkynyl
(E)-4am [Eq. (1)].[7] The formation of (E)-isomers in- complex 6 was fully characterized by introduction of
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Takahiro Nishimura et al.
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Figure 1. 31P and 13C NMR spectra (at 202 MHz for 31P and 125 MHz for 13C in C6D6 at room temperature) of alkynylrhodi-
um complexes. (i) 31P NMR spectrum of complex 6. (ii) 31P NMR spectrum of 6-13C(a). (iii) 13C NMR spectra of 6-13C(a)
[left, C(a)] and 6-13C(b) [right, C(b)].
the silylethynyl group incorporated with 13C at either of acetic acid (1 equiv.) was found to proceed at 808C
the a or b position.[13] The 13C-labeled (90% 13C) rho- to give a 78% yield of the hydroalkynylation product
dium complexes, 6-13C(a) and 6-13C(b), were obtained [3am/4am=90/10, Eq. (4)]. The high reaction temper-
in high yields by the elimination reaction with specifi-
cally 13C-labeled propargylic alcohols 7 [Eq. (3)]. The
ature compared with that (408C) for the catalytic re-
action is probably because the strongly coordinating
triphenylphosphine ligand occupies a coordination
site required for the activation of the alkyne prior to
its insertion into the alkynyl-rhodium bond. Unfortu-
nately, attempts to isolate a but-3-yn-1-enylrhodium
intermediate before protonolysis were not successful,
probably due to its instability under the reaction con-
31P NMR spectrum of the complex 6-13C(a) in C6D6 ditions. On monitoring the reaction of 1a with 2m cat-
consisted of three dddds, where each peak has an ad- alyzed by complex 6 (5 mol%) at 80 8C, which gave a
ditionalcoupilng with 13C(a), the coupling constants high yield of the hydroalkynylation products in 3 h
(2JC,P) being 21, 22, and 92 Hz for the peaks at 32.6, [Eq. (5)], 31P NMR spectrometry of the reaction mix-
35.5, and 37.8 ppm, respectively [Figure 1, (ii)]. The
13C NMR signalof the compelx 6-13C(a) appeared at
155.0 ppm as dddd where the coupling constant be-
tween rhodium and 13C (1JRh,C(a)) is 43 Hz [Figure 1,
(iii)]. In the 13C NMR spectrum of 6-13C(b) (dddd,
117.7 ppm), all four coupling constants including
2JRh,C(b) =10 Hz are much smaller than those observed
for 6-13C(a). These 31P and 13C NMR spectra clearly ture showed that the alkynylrhodium complex 6 is a
indicate the direct connectivity between a rhodium dominant species while the catalytic hydroalkynyla-
center and the silylethynyl group.
tion is proceeding.
A stoichiometric reaction of the alkynylrhodium
On the basis of the results observed above which
complex 6 with alkyne 1a (6/1a=1/1) in the presence demonstrated the intermediacy of the alkynylrhod-
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Rhodium-Catalyzed Hydroalkynylation of Internal Alkynes with Silylacetylenes
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Table 1. Rhodium-catalyzed addition of silylacetylenes.[a]
ium(I) species in the present hydroalkynylation, a cat-
alytic cycle is proposed as illustrated in Scheme 1. A
hydroxorhodium complex A which is in equilibrium
Entry Alkyne 1 Alkyne 2 Time [h] Yield [%][b] 3/4[c]
1
2
3
1b
1c
1d
1d
1e
1f
1e
1e
2m
2m
2m
2m
2m
2m
2n
1
2
2
3
4
12
4
4
90
95
92
87
93
80
92
96
97/3
95/5
98/2
98/2
94/6
93/7
98/2
98/2
4[d]
5[e]
6[e,f]
7[e]
8[e]
Scheme 1. Proposed catalytic cycle.
with dimeric hydroxorhodium 5,[14] undergoes a reac-
tion with the terminal alkyne to form an alkynylrho-
dium(I) B and water. Insertion of the internalakl yne
into the carbon-rhodium bond in B forms an alkenylr-
hodium species C. s-Bond metathesis between alke-
nylrhodium C and the terminal alkyne or hydrolysis
followed by alkynylation by way of A gives the enyne
product to regenerate the alkynylrhodium intermedi-
ate B.[5] The observation of the alkynylrhodium com-
plex 6 as a dominant rhodium species during the cata-
lytic reaction may indicate that alkynylrhodium B is a
resting stage in the catalytic cycle.
2o
[a]
Reaction conditions: [Rh(OH)((R)-binap)]2 (5) (5 mol%
of Rh), internalakl yne 1 (0.20 mmol), terminal alkyne 2
(0.30 mmol), 1,4-dioxane (0.4 mL) at 408C.
Isolated yields of 3 and 4.
[b]
[c]
[d]
1
Determined by H NMR.
The reaction was conducted in 2.0 mmolscael with
[Rh(OH)((R)-binap)]2 (5) (2 mol% of Rh) at 80 8C.
Performed at 608C.
[e]
[f]
Alkyne 2m (0.4 mmol) was used.
As summarized in Table 1, [Rh(OH)((R)-binap)]2
(5) efficiently catalyzed the addition of (triphenyl-
silyl)ethyne (2m) to severaltypes of phen-yl
A
the corresponding enynes 3 with high regioselectivity
(entries 1–3 and 5). The reaction of 2d on a 2-mmol
scale with a reduced amount of the rhodium catalyst
(2 mol%) successfully proceeded to give enyne 3dm
in 87% yield (entry 4). The addition to alkenylacety-
lene 1f also proceeded with high regioselectivity rhodium(I) complex was isolated and investigated for
(entry 6). As a terminal acetylene, (triethylsilyl)- (2n) its structure and reactivity.
and (triisopropylsilyl)ethyne (2o) can be used as
well,[15] the addition to propargylic alcohol 1e giving
high yields of the corresponding enynes 3 (entries 7
and 8). For the addition to dialkylacetylenes, 4-octyne
Experimental Section
(1g) and 1,4-dimethoxy-2-butyne (1h), a rhodium
complex coordinated with 1,6-bis(diphenylphosphi-
Typical Procedure
To
a
solution of [Rh(OH)((R)-binap)]2 (5) (7.4 mg,
no)hexane (dpph), generated in situ from [Rh(OH)-
(cod)]2 and dpph, was more effective than the binap
0.010 mmolof Rh) in 1,4-dioxane (0.40 mL) in a screw-cap
test tube was added (triphenylsilyl)acetylene (85.3 mg,
0.30 mmol) and 1-phenyl-1-propyne (23.2 mg, 0.20 mmol)
successively, and the tube was capped tightly. Then, the mix-
ture was allowed to stir at 408C (bath temperature) for 1 h.
The reaction mixture was passed through a short silica gel
column eluting with Et2O. Evaporation of the solvent fol-
lowed by preparative thin-layer chromatography on silica
ACHTREUNG
complex 5 to give enynes 3gm and 3hm in higher
yields [Eq. (6)].[16] The addition of 2m to 2-butynoate
1i was also catalyzed by the dpph-rhodium catalyst.
In summary, highly selective hydroalkynylation of
internal alkynes with silylacetylenes giving 1,3-enynes
was realized by use of a hydroxorhodium catalyst. As
a key intermediate in the catalytic cycle, an alkynyl- gel(eul ent: n-hexane/ethylacetate =10/1) gave a mixture of
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Takahiro Nishimura et al.
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3am and 4am (3am/4am=96/4) as a colorless oil; yield:
75.3 mg (0.19 mmol, 94%).
c) T. Hirabayashi, S. Sakaguchi, Y. Ishii, Adv. Synth.
Catal. 2005, 347, 872; d) U. Lꢄcking, A. Pfaltz, Synlett
2000, 1261.
[5] For an example of the addition of terminal alkynes to
internalaklynes, see: C. S. Yi, N. Liu, Organometallics
1998, 17, 3158. Very recently, Miura reported rhodium-
catalyzed cross dimerization of silylacetylenes and in-
ternalaklynes: T. Katagiri, H. Tsurugi, A. Funayama,
T. Satoh, and M. Miura, Chem. Lett. 2007, 36, 830. Al-
though the present reaction in this manuscript is similar
to that reported by Miura, we have studied mechanistic
aspects of the reaction including characterization of the
intermediates in the catalytic cycle as well as its syn-
thetic aspects.
Acknowledgements
This work was supported in part by a Grant-in-Aid for Scien-
tific Research on Priority Areas “Advanced Molecular Trans-
formations of Carbon Resources” from the Ministry of Edu-
cation, Culture, Sports, Science and Technology, Japan
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