Rhodium-catalyzed asymmetric hydroalkoxylation and hydrosulfenylation of diphenylphosphinylallenes

Article abstract of DOI:10.1039/b900976k

The rhodium-catalyzed intermolecular asymmetric hydroalkoxylation and hydrosulfenylation of diphenylphosphinylallenes gave chiral allylic phosphine oxides substituted with vinyl ether and thioether moieties in high yields with high enantioselectivities.

Full text of DOI:10.1039/b900976k

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Chemical Communications
Title: Rhodium-catalyzed asymmetric hydroalkoxylation and
hydrosulfenylation of diphenylphosphinylallenes
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Number 24 | 28 June 2009 | Pages 3477–3640
Rhodium-catalyzed intermolecular asymmetric hydroalkoxylation
and hydrosulfenylation of diphenylphosphinylallenes gave chiral
allylic phosphine oxides substituted with vinyl ether and thioether
moieties in high yields with high enantioselectivity.
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www.rsc.org/chemcomm | ChemComm
Rhodium-catalyzed asymmetric hydroalkoxylation and hydrosulfenylation
of diphenylphosphinylalleneswz
Takahiro Kawamoto, Sho Hirabayashi, Xun-Xiang Guo,
Takahiro Nishimura* and Tamio Hayashi*
Received (in Cambridge, UK) 16th January 2009, Accepted 17th February 2009
First published as an Advance Article on the web 13th March 2009
DOI: 10.1039/b900976k
The rhodium-catalyzed intermolecular asymmetric hydro-
alkoxylation and hydrosulfenylation of diphenylphosphinylallenes
gave chiral allylic phosphine oxides substituted with vinyl ether
and thioether moieties in high yields with high enantioselectivities.
The catalytic addition of a hydrogen–oxygen bond to an
unsaturated carbon–carbon bond, hydroalkoxylation, is a
Scheme
1 Asymmetric hydroarylation and hydroalkynylation
simple and atom-economical route to construct carbon–oxygen
bonds, and its asymmetric variant is of great value to provide
useful chiral compounds. As one of the asymmetric hydro-
alkoxylations, oxa-Michael-type addition to electron-deficient
olefins has recently been developed under organic and Lewis
acid catalysis.1 On the other hand, despite the recent progress
of hydroalkoxylation based on transition metal catalysts,2,3
there are only a few examples of asymmetric hydroalkoxylation.
For example, the gold-catalyzed asymmetric intramolecular
hydroalkoxylation of allenes for the synthesis of chiral
oxygen heterocycles was reported independently by Zhang
and Widenhoefer,4 and Toste et al.5 The palladium-catalyzed
asymmetric intramolecular hydroalkoxylation of alkynes
reported by Yamamoto et al. was proposed to proceed via
in situ-generated allenes.6 Meanwhile, there have been no
reports on the intermolecular asymmetric hydroalkoxylation
of allenes, to the best of our knowledge. Recently, we reported
the rhodium-catalyzed asymmetric hydroarylation7 and hydro-
alkynylation8 of diphenylphosphinylallenes,9 where a p-allyl-
rhodium species A is formed as a key intermediate, and the
following regioselective protonation determines the enantio-
selectivity (Scheme 1). In this context, we focused on oxygen
nucleophiles for the asymmetric addition to allenes, giving
chiral vinyl ethers. Here, we report the rhodium-catalyzed
asymmetric addition of phenols to diphenylphosphinylallenes.
The results of asymmetric hydrosulfenylation are also reported.
Our initial studies were focused on the asymmetric addition
of p-methoxyphenol (2m) to 3-(diphenylphosphinyl)-1,2-butadiene
(1a) to give chiral vinyl ether 3am in the presence of a
catalyzed by rhodium complexes.
hydroxorhodium complex coordinated with a chiral bis-
phosphine ligand (Table 1). Treatment of allene 1a with
10
phenol 2m (1 equiv.) in the presence of [Rh(OH)((R)-binap)]2
(5 mol% of Rh) in toluene at 80 1C for 12 h gave 3am in 71%
yield with 30% ee (entry 1). The ratio of allene 1a to phenol
2m had a significant influence on the enantioselectivity
(entries 1–3); thus, the use of excess allene 1a (2 equiv.) gave
55% ee of 3am (entry 3). Other axially chiral biarylbisphosphines
related to binap were also examined, where the reaction was
catalyzed with 5 mol% rhodium catalyst generated in situ
11
by mixing [Rh(OH)(cod)]2 with the ligands. The use of
(R)-segphos12 gave 3am in 32% yield with 64% ee (entry 4).
Sterically bulky bisphosphine ligand (R)-DTBM-segphos12
Table 1 The asymmetric addition of p-methoxyphenol (2m) to
diphenylphosphinylallene 1aa
1/mmol,
Entry 2/mmol
NMR
yield (%)b ee (%)c
Ligand
Solvent
1d
2d
3d
4
0.2, 0.2
0.2, 0.4
0.4, 0.2
0.4, 0.2
0.4, 0.2
(R)-binap
(R)-binap
(R)-binap
(R)-segphos Toluene 32
(R)-DTBM- Toluene 69
segphos
Toluene 71
Toluene 85
Toluene 72
30
23
55
64
80
Department of Chemistry, Graduate School of Science,
Kyoto University, Sakyo, Kyoto 606-8502, Japan.
E-mail: tnishi@kuchem.kyoto-u.ac.jp, thayashi@kuchem.kyoto-u.ac.jp;
Fax: +81 75 753 3983; Tel: +81 75 753 3988
w Electronic supplementary information (ESI) available: Experimental
procedures and compound characterization data. CCDC 713591. For
ESI and crystallographic data in CIF or other electronic format, see
DOI: 10.1039/b900976k
z This article is part of a ChemComm ‘Catalysis in Organic Synthesis’
web-theme issue showcasing high quality research in organic chem-
istry. Please see our website (http://www.rsc.org/chemcomm/organic
webtheme2009) to access the other papers in this issue
5
6
0.4, 0.2
(R)-DTBM- t-BuOH 99
segphos
82
a
Reaction conditions: [Rh(OH)(cod)]2 (5 mol% of Rh), ligand
b
(6 mol%), solvent (0.4 mL), 80 1C, 12 h. Determined with CH3NO2
as an internal standard. Determined by HPLC analysis with a chiral
d
stationary phase column: Chiralcel OJ–H. [Rh(OH)((R)-binap)]2
(5 mol% of Rh) was used.
c
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This journal is The Royal Society of Chemistry 2009
3528 | Chem. Commun., 2009, 3528–3530

Table 2 Asymmetric hydroalkoxylation of allene 1aa
Scheme 3 The asymmetric hydrosulfenylation of allene 1a.
corresponding vinyl thioethers, 6 and 7, in 81 and 75% ee,
respectively.
Entry
2
Ar
3
Yield (%)b
ee (%)c
NMR studies on the catalytic reaction using hydroxorhodium
complex [Rh(OH)((R)-binap)]2 (8) provided us with significant
information on the catalytic cycle (Scheme 4). Treatment of 8
with p-cresol (2o) (10 equiv. to Rh) in C6D6 at 30 1C for 1 h
brought about the selective formation of a new rhodium
complex. The 31P NMR spectrum of the reaction mixture
showed a doublet peak at 45.2 ppm (JRh–P = 207 Hz),
which was assigned to be p-phenoxorhodium complex 9
by the similarity of its 31P NMR spectrum to that of
[Rh(PPh3)2(p-C6H5O)]ꢁ2(C6H5OH).14 The 1H NMR spectrum
of 9, which showed four non-equivalent aromatic protons
(3.40 (dd, J = 7.3, 1.9 Hz, 1H), 4.02 (dd, J = 6.8, 2.1 Hz, 1H),
5.56 (dd, J = 6.8, 1.4 Hz, 1H), 6.34 (dd, J = 7.3, 1.9 Hz, 1H) ppm)
on the coordinated benzene ring, also supported the structure
of complex 9. To the solution of rhodium complex 9 (Fig. 1b)
generated from [Rh(OH)((R)-binap)]2 (8) (Fig. 1a) and
10 equiv. of phenol 2o, was added 5 equiv. of allene 1a
(Fig. 1c), and the reaction was monitored by 31P NMR.
Within 1.5 h, the formation of a new rhodium complex was
observed (Fig. 1d), which was assigned to be p-allylrhodium
complex 1015 by 31P NMR, which consisted of a doublet at
1
2
3
2m
2n
2o
2p
2q
2r
2s
2t
2u
2v
2w
4-MeOC6H4
3,4-(OCH2O)C6H3
4-MeC6H4
2-MeC6H4
5-(i-Pr)-2-MeC6H3
C6H5
4-FC6H4
4-ClC6H4
3-ClC6H4
2-naphthyl
1-naphthyl
3am
3an
3ao
3ap
3aq
3ar
3as
3at
96
93
96
91
86
99
99
92
99
99
99
84
80
80
87
85
85
84
4d
5d
6
7
8
9
10
11
93(R)e
94
91
3au
3av
3aw
97
a
Reaction conditions: 1a (0.4 mmol), 2 (0.2 mmol), [Rh(OH)(cod)]2
(5 mol% of Rh), (R)-DTBM-segphos (6 mol%), t-BuOH (0.4 mL),
b
c
80 1C, 24 h. Isolated yields. Determined by chiral HPLC analysis.
For 48 h. Determined by X-ray analysis.
d
e
displayed a high enantioselectivity (80% ee), although the
yield was modest (69% yield; entry 5). The reaction in
tert-butyl alcohol as a solvent gave a quantitative yield of
3am with 82% ee (entry 6).
Table 2 summarizes the results obtained for the reactions of
phenols 2 with diphenylphosphinylallene 1a, which were
carried out in the presence of rhodium/(R)-DTBM-segphos
as a catalyst (5 mol% of Rh). Phenols 2m–2u bearing either
electron-donating or -withdrawing groups were introduced
onto allene 1a with high enantioselectivities, to give the
corresponding vinyl ethers, 3am–3au, in 80–94% ee
(entries 1–9). The ortho-substituent (o-Me; entries 4 and 5)
and the electron-withdrawing group (Cl; entries 8 and 9) on
the benzene ring increased the enantioselectivity. High
enantioselectivities were also observed for the addition of
naphthols 2v and 2w, giving the corresponding vinyl ethers,
3av and 3aw, in 91 and 97% ee, respectively (entries 10 and 11).
The absolute configuration of 3at was determined to be
R by X-ray crystallography (ESIw).
35.7 (JPꢂP = 14 Hz), a ddd at 39.8 (JRhꢂP = 193 Hz, JPꢂP
=
39, 14 Hz) and a dd at 43.5 (JRhꢂP = 193 Hz, JPꢂP = 39 Hz)
ppm.7,8 During the catalytic reaction of 1a and 2o to give 3ao,
both rhodium complexes 9 and 10 were observed, with the
ratio of 9 to 10 being changed depending on the conversion
(Fig. 1d–g). Allene 1a was completely converted to 3ao after
80 h at 30 1C, where complexes 8 and 9 were the observable
rhodium complexes (Fig. 1h). These results support the cata-
lytic cycle shown in Scheme 4,16 where protonolysis8,17 of the
p-allylrhodium with phenol 2o gives hydroalkoxylation
product 3ao, regenerating phenoxorhodium 9. The observa-
tion of both key intermediates 9 and 10 at the same time
throughout the catalytic reaction indicates a comparable
reaction rate for the two key steps, p-allylrhodium formation
and its protonation, in the catalytic cycle. Provided that p-allyl
The hydroalkoxylation of allenes substituted with Et (1b),
Bu (1c) and Ph (1d) groups also took place to give the
corresponding vinyl ethers, the enantioselectivity being lower
with more bulky substituents (Scheme 2).
This catalytic system was successfully applied to the hydro-
sulfenylation of allene 1a (Scheme 3).13 Thus, the addition
of benzenethiol (4) and naphthalene-2-thiol (5) gave the
Scheme 2 The asymmetric addition of 2t to diphenylphosphinylallenes 1.
Scheme 4 Observation of the intermediates in the catalytic cycle.
Chem. Commun., 2009, 3528–3530 | 3529
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This journal is The Royal Society of Chemistry 2009

Fig. 1 Time course of the reaction monitored by 31P NMR in C6D6 at 30 1C.
intermediate 10 has a similar structure to that decided for Au: A. Hoffmann-Roder and N. Krause, Org. Lett., 2001, 3, 2537;
¨
N. Krause, A. Hoffmann-Roder and J. Canisius, Synthesis, 2002,
the hydroarylation of 1a,7 the absolute configuration R of
the addition product 3at implies that the p-allyl undergoes
protonation from the side opposite to rhodium.
¨
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This work was supported by a Grant-in-Aid for Scientific
Research from the MEXT, Japan. We thank Takasago
International Corporation for the gift of (R)-DTBM-segphos.
T. K. thanks a research fellowship of the Japan Society for the
Promotion of Science for Young Scientists.
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This journal is The Royal Society of Chemistry 2009
3530 | Chem. Commun., 2009, 3528–3530