Rhodium(I)-catalyzed 1,4-silicon shift of unactivated silanes from aryl to alkyl: Enantioselective synthesis of indanol derivatives

Article abstract of DOI:10.1002/anie.201005399

Let's swap! Highly reactive alkyl rhodium species undergo facile C-Si bond activation resulting in an overall 1,4-Si/Rh positional switch. This reactivity is utilized to access densely functionalized indanol derivatives in a highly enantioselective manner

Full text of DOI:10.1002/anie.201005399

Angewandte
Chemie
DOI: 10.1002/anie.201005399
Asymmetric Catalysis
Rhodium(I)-Catalyzed 1,4-Silicon Shift of Unactivated Silanes from
Aryl to Alkyl: Enantioselective Synthesis of Indanol Derivatives**
Tobias Seiser and Nicolai Cramer*
In the search for atom-economic and sustainable transforma-
dominated by activated silyl groups such as halo- and
hydrosilanes or strained silacyclobutanes. Besides reactions
proceeding by discrete penta- or hexa-coordinated silicate
species prior to bond cleavage, tetraorganosilanes are much
less frequently used.[7] Chatani and co-workers recently
ꢀ
tions, the selective functionalization of unactivated C H
bonds is a major topic of organometallic chemistry.[1]
A
specific class of such activations are 1,4-hydrogen–metal shifts
which are mostly observed with organorhodium and -palla-
dium species.[2] One major driving force for these shifts is a
stabilization of the system by the formation of a stronger
reported an elegant work on rhodium(I)-catalyzed direct
[8]
ꢀ
insertion into the Me Si bond of aryltriorganosilanes. This
carbon–metal bond. Therefore, mostly migrations from Calkyl
-
report corroborates the potential and the need to further
develop methods allowing to capitalize on the ease of access,
robustness, and versatility of tetraorganosilanes.
3
2
2
[3]
ꢀ
ꢀ
ꢀ
(sp ) M or Cvinyl(sp ) M to Caryl(sp ) H are observed. This
reactivity trend can be exploited to relay organometallic
species. While such tactic often leads to versatile aryl metal
species, the originating position of the metal remains unfunc-
tionalized. From a practical point of view, methods that would
allow for a positional switch of two reactive functional groups
are desirable. Therefore we investigated reactions involving
the transfer of a synthetically versatile silyl group instead of a
hydrogen atom, thus representing overall a silicon/metal swap
[Eq. (1)].[4] To the best of our knowledge such a concept has
not yet been explored.
Herein, we report a 1,4-Rh/Si shift and a second rhodium-
ꢀ
promoted C Si bond cleavage of the arising products. The
envisioned process is initiated by b-carbon elimination from
tert-cyclobutanol 1 (Scheme 1). The thereof generated highly
b-Carbon cleavages from strained tert-cyclobutanols pro-
vide a suitable source to obtain the requisite alkyl organo-
metallic species. This method has been proven to be a
convenient and robust way to generate highly reactive
alkylmetal species in
a well defined and reagent-free
manner.[5] Recently, we and Murakami demonstrated the
capability of such intermediates to undergo 1,4-rhodium shifts
Scheme 1. Proposed pathway of the targeted rhodium/silicon swap.
leading to indanols.[5k,l] The metal-catalyzed activation of C
ꢀ
Si groups has been well established for Hiyama–Denmark
cross-coupling reactions.[6] These reactions are by and large
reactive alkylrhodium species 2 could undergo Caryl(sp ) H
2
ꢀ
activation giving indanol 3.[5k,l] However, in the presence of a
suitable silyl group, we hypothesized that an oxidative
addition leading to presumed rhodium(III) complex 4 might
become the preferred pathway.[9] In turn, this complex could
reductively eliminate to give the envisioned intermediate 5.
[*] T. Seiser, Dr. N. Cramer
Laboratorium fꢀr Organische Chemie, ETH Zꢀrich
Wolfgang-Pauli-Strasse 10, HCI H 304, 8093 Zꢀrich (Switzerland)
Fax: (+41)44-632-1328
E-mail: nicolai.cramer@org.chem.ethz.ch
Homepage: http://www.cramer.ethz.ch
ꢀ
ꢀ
We anticipated that the higher stability of Caryl Rh vs. Calkyl
Rh bonds would drive the reaction in the desired direction.[10]
Species 5, in turn, could add across the carbonyl group, and
subsequent protonation of the rhodium alkoxide formed
would deliver indanol 6 and close the catalytic cycle.
[**] We thank the Swiss National Foundation for funding (21-
119750.01), Solvias AG for Taniaphos, Prof. Dr. A. Alexakis for
Simplephos and Prof. Dr. E. M. Carreira for generous support. The
Fonds der Chemischen Industrie is acknowledged for a Liebig
Fellowship (N.C.) and a Kekulꢁ Fellowship (T.S.).
During our investigations we detected compounds 3 and 6,
but observed additionally silacycles 7 in significant amounts
(Scheme 2). The origin of compounds 7 might be rationalized
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005399.
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10163

Communications
(entry 2). Difluorphos (L1; Scheme 3) showed the highest
selectivity for the formation of trans-6a and the use of
ꢀ
mesitylene as solvent mitigated concurrent C H activation
(entry 3).[14,15] Contrasting these findings, taniaphos (L2) and
simplephos (L3)[16] displayed both a high selectivity for the
ꢀ
Scheme 2. Second rhodium-catalyzed C Si bond activation.
Table 1: Optimization studies for the individual products.[a]
Scheme 3. Employed ligands.
Entry L*[b] Solvent
Additive T [8C] trans-6/cis-6/7/3[c] ee [%]
isomer cis-6a, although it was formed virtually racemic
(entries 4 and 5). However, even in the presence of cesium
carbonate at high reaction temperatures, no tricyclic product
7 was detected. Led by the high cis selectivity of [{Rh-
(cod)OH}2] in the absence of phosphine ligands, we evaluated
chiral dienes for the synthesis of 7. Dolefin (L4), developed by
Carreira et al.,[17] induced high enantioselectivity, albeit it
displayed the inverse diastereoselectivity, favoring trans-6a
over cis-6a (entry 6). On the other hand, diene L5, reported
by Hayashi et al.,[18] showed the desired cis selectivity, but the
1
–
–
dioxane
toluene
mesitylene
dioxane
mesitylene
dioxane
H2O
Cs2CO3 130
–
H2O
–
110
14:72:14:0
8:4:88:0
72:11:0:17
12:86:2:0
7:93:0:0
–
–
2[d]
3
L1
L2
L3
L4
L5
100
110
100
110
100
97[e]
4
<5
<5
96[h]
18
5[f]
6[g]
7[g]
H2O
–
78:22:0:0
29:71:0:0
mesitylene
[a] Conditions: 0.05 mmol 1a, 0.20m in solvent, 2.5 mol% [{Rh(OH)-
(cod)}2], 6 mol% L*, 1.5 equiv additive, 12 h. [b] See Scheme 3. [c] Ratio
determined by 1H NMR spectroscopy. [d] 0.50m. [e] (1S,3S)-6a.
[f] 12 mol% L3. [g] 2.5 mol% [{Rh(OAc)(C2H4)2}2]. [h] (1R,3R)-6a.
ꢀ
enantioselectivity and its potential for the second C Si
activation are low (entry 7).
from the rhodium tert-alkoxide derived form cis-6.[11] Paral-
leling a recent report of Hiyama and co-workers who used 4-
hydroxy-functionalized silanes as reagents for palladium-
catalyzed methylation and alkyla-
Under the optimized conditions of entry 3, we explored
the scope of the reaction with special focus on different silyl
groups (Table 2). The substituents R and R’ can be varied
tion of aryl halides,[12] the forma-
[a]
ꢀ
Table 2: Scope and limitation of the C Si activation.
tion of a methyl rhodium species
and the concomitant release of 7 is
plausible. Liberation of methane
by protonation would regenerate
the active RhI catalyst.
For the silyl group transfer, we
initially investigated conditions
controlling the C Si vs. C H acti-
vation ratio and the diastereo- as
well as enantioselectivity of the
process.[13] In addition, we tried to
Entry
R
R’
Si
trans-6
d.r.[b]
yield [%][c]
ee [%]
1
2
3
4
Et
tBu
p-ClC6H4
p-ClC6H4
Me
Me
Me
SiMe3
SiMe3
SiMe3
SiMe3
6a
6b
6c
6d
6:1
20:1
10:1
8:1
82
82
80
93
97
96
96
99
ꢀ
ꢀ
CH2OBn
ꢀ
5[d]
Me
SiMe3
–
70
85
promote selectively the C Si bond
activation leading to tricycle 7.
When [{Rh(cod)OH}2] was used
as rhodium source in the absence
of phosphine ligands, cis-6 was
obtained as the major product
(Table 1, entry 1). Increasing the
concentration to 0.5m M and the
reaction temperature to 1308C, as
well as adding cesium carbonate, 7
becomes the dominant product
6
7
8
9
10
11
12
Et
Et
Et
Et
Me
Me
Ph
Me
Me
Me
Me
SiMe2Bn
SiMe2Ph
SiMe2Ph
SiMe2(HC CH2)
SiMe2(HC CH2)
6 f
6g
6h
6i
6j
6k
6l
9:1
9:1
9:1
61
72
72
90
75
75
22
97
97
82
96
99
98
86
=
=
8:1
=
HC CH2
Et
Et
19:1
13:1
1.2:1
=
SiMe2(H2CCH CH2)
SiEt3
[a] Conditions: 0.10 mmol 1, 0.20m in mesitylene, 2.5 mol% [{Rh(OH)(cod)}2], 6.0 mol% L1, 1008C,
12 h. [b] Ratio determined by H NMR spectroscopy. [c] Yield of isolated product 6.[19] [d] With cis-1e.
1
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Angewandte
Chemie
without affecting the outcome of the reaction, providing 6a–
6d in good yields and with high enantioselectivities
(entries 1–4). Notably, with a 2-thienyl group, indanone 8e
is formed instead of 6e (entry 5).[5p] Silyl groups possessing
(cod and L4) cleanly provided the silyl-transfer product trans-
6m, whereas the use of phosphine ligand L1 led to a mixture
of the isomers of 3m and 9m (Scheme 4).
[a]
ꢀ
ꢀ
Table 3: C Si vs. C H bond activation.
Entry
L*[b]
3l/6l[c]
Entry
L*
3 l/6l[c]
1
L1
L6
L7
–
78:22[d]
98:2
<5:95
<5:95
5
6
7
8
L4
L5
L8
L9
<5:95[g]
<5:95
<5:95
2[e]
3
ꢀ
Scheme 4. L4 enables selective C Si activation of 1m.
4[f]
–
[h]
[a] Conditions: 0.05 mmol 1l, 0.20m in mesitylene, 12 h for full
conversion. [b] See Scheme 3. [c] Ratio determined by 1H NMR spec-
troscopy. [d] 72% 3k, 86% ee. [e] 12 mol% L6. [f] [{Rh(OH)(cod)}2].
[g] 89% 6l, d.r. =2:1 (trans-6l), 90% ee. [h] No reaction.
ꢀ
We next examined the second C Si bond activation under
phosphine-free conditions leading to tricyclic products 7
(Table 4). For a trimethylsilyl group, the reaction yielded,
independent of the substitution of the 3-position of the
cyclobutanol, the silacycle (7a and 7n, entries 1 and 2).
ꢀ
ꢀ
another potentially reactive insertion site, like SiMe2Bn,
Furthermore, a Si Me bond was selectively cleaved over a Si
Bu bond (entry 3). However, we observed no diastereoselec-
=
=
SiMe2Ph, SiMe2(HC CH2), and SiMe2(H2CCH CH2), are
cleaved exclusively at the reported
positions (entries 6–11). This selec-
tivity can be rationalized by a more
[a]
ꢀ
Table 4: Scope for the double C Si activation process leading to 7.
favorable five-membered rhodacy-
cle 4 compared to a hypothetical
six-membered rhodacycle that
would arise from the alternative
insertion positions. The triethylsilyl
group proved to be more reluctant
Entry
1
1
Si
7
yield [%][c]
80
ee [%][d]
ꢀ
and the C Si cleavage proceeded
only in modest yields, whereas the
ꢀ
trans-1a
SiMe3
7a
7n
7o
7p
–
concurrent C H activation pathway
became dominant (entry 12).
This outcome with bulkier silyl
groups prompted us to re-evaluate
2
3
4
trans-1n
trans-1o
trans-1p
SiMe3
71
–
–
–
ꢀ
the decisive variables governing C
[20]
ꢀ
Si vs. C H activation (Table 3).
Whereas phosphorus containing
ligands displayed a high preference
SiMe2Bu
SiMe2iPr
72 (d.r. 1:1)
28 (d.r. 2:1)
ꢀ
for the C H pathway forming 3l
(entries 1 and 2), diene ligands
ꢀ
promoted virtually exclusively C
Si activation, though with varying
enantio- and diastereoselectivity of
the product 6l (entries 4–7). L4
worked best and provided ent-6l in
90% ee (entry 5). Notably, P-olefin
ligand L7 (entry 3) gave selectively
6l and bis-sulfoxide L9 did not
catalyze the reaction (entry 8).
5[b]
cis-1a
cis-1q
SiMe3
SiMe3
(S,R)-7a
(S,S)-7q
57
35
96
82
6[b]
The influence of the ligand is
even more striking with substrate
[a] Reaction conditions: 0.10 mmol 1, 0.3m in toluene, 5 mol% [{Rh(OH)(cod)}2], 3.0 equiv Cs2CO3,
1308C, 8 h. [b] 0.2m in mesitylene, 2.5 mol% [{Rh(OH)(cod)}2], 6.0mol% L1, , 1008C, 5 h, then
3.0 equiv Cs2CO3, 5mol% [{Rh(OH)(cod)}2], 1308C, 8 h. [c] Of isolated product. [d] Determined on an
ꢀ
1m possessing three potential C H
insertion positions. Diene ligands aliquot of cis-6.
Angew. Chem. Int. Ed. 2010, 49, 10163 –10167
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www.angewandte.org 10165

Communications
gel (pentane/EtOAc 15:1, Rf = 0.19) giving 21.6 mg (82%, d.r. = 6:1,
97 % ee) of indanol (1S,3S)-6a as a colorless oil.
tion of the two methyl groups of intermediate cis-6o, thus 7o
was formed as a 1:1 mixture of diastereomers at the silicon
stereogenic center. A moderate diastereoselectivity of 2:1
could be obtained by replacing the butyl group by the bulkier
isopropyl group (7p, entry 4). Additionally, optically active
silacycles were accessible by a sequential procedure in
moderate yields: First, enantioenriched indanols cis-6a and
cis-6q were prepared selectively with L1 from cyclobutanols
cis-1a and cis-1q, respectively.[21] After complete conversion,
Received: August 29, 2010
Revised: October 25, 2010
Published online: December 1, 2010
ꢀ
Keywords: asymmetric catalysis · C C activation ·
.
ꢀ
C Si activation · cyclobutanes · rhodium
ꢀ
5 mol% of [{Rh(OH)(cod)}2] were added and the Si Me
bond cleavage was induced at 1308C giving (R,S)-7a and
(S,S)-7o in 96% and 82% ee, respectively.
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Fleming oxidation into the corresponding diol 10 in 80%
yield (Scheme 5). Other functionalized silanes are as well
ꢀ
J. A. Ellman, Chem. Rev. 2010, 110, 624 – 655; g) C H Activation
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b) D. A. Rooke, E. M. Ferreira, J. Am. Chem. Soc. 2010, 132,
11926 – 11928.
Scheme 5. Transformations of indanols 6. TBAF: tetrabutylammonium
fluoride, DMF: dimethylformamide.
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suited for further reactions. For instance, treating bis vinyl-
substituted indanol 6i with 10 mol% of toluenesulfonic acid
(TsOH) resulted in a smooth dehydration to diene 11 which
underwent upon heating to 808C a Diels–Alder reaction to
give tetracycle 12 in 71% yield as a single diastereomer.
In summary, we reported a rhodium(I)-promoted activa-
tion and 1,4-positional swap of unactivated tetraorganosi-
ꢀ
lanes. As an extension to this process, a second Si C bond
activation is initiated by rhodium tert-alkoxides. Both reac-
tivities underline the potential of rhodium(I) species to
undergo unexplored reactivity leading to reorganizations of
the molecular framework. Ongoing research focuses on a
further understanding of the decisive factors of the individual
activation modes and their exploitation for synthetic applica-
tions.
Experimental Section
Cyclobutanol 1a (26.2 mg, 0.10 mmol), [{Rh(cod)(OH)}2] (1.14 mg,
2.50 mmol), and (R)-difluorphos (L1) (4.10 mg, 6.00 mmol) were
weighed into an oven-dried vial equipped with a magnetic stir bar,
sealed with a rubber septum, and flushed with nitrogen. After the
addition of 0.5 mL of mesitylene, the reaction mixture was degassed
by three freeze–pump–thaw cycles, stirred for 5 min at 238C, and
subsequently immersed into a preheated oil bath (1008C) for 5 h.
After thin layer chromatography showed the complete conversion,
the reaction mixture was cooled to 238C and directly purified on silica
[7] For a review, see: a) G. R. Jones, Y. Landais, Tetrahedron 1996,
52, 7599 – 7662; for examples in transition-metal catalysis
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Angew. Chem. Int. Ed. 2010, 49, 10163 –10167

Angewandte
Chemie
exploiting intramolecular alkoxide activation of tetraorganosi-
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Hiyama, J. Am. Chem. Soc. 2005, 127, 6952 – 6953; c) Y. Nakao,
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Shintani, T. Hayashi, J. Am. Chem. Soc. 2007, 129, 9137 – 9143;
d) J. Chen, M. Tanaka, M. Takeda, A. K. Sahoo, A. Yada, Y.
Nakao, T. Hiyama, Bull. Chem. Soc. Jpn. 2010, 83, 554 – 569.
[8] M. Tobisu, M. Onoe, Y. Kita, N. Chatani, J. Am. Chem. Soc. 2009,
131, 7506 – 7507.
[9] The generated carbonyl group might assist the metal insertion by
coordination to the silicon atom. For a related effect, see: W.
Rauf, J. M. Brown, Angew. Chem. 2008, 120, 4296 – 4298; Angew.
Chem. Int. Ed. 2008, 47, 4228 – 4230.
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2010, 122, 4549 – 4552; Angew. Chem. Int. Ed. 2010, 49, 4447 –
4450.
[13] For a comprehensive optimization table see the Supporting
Information.
[14] The absolute configurations of compounds 6 and 7 were assigned
in analogy to the reported cleavage site; see references [5h,k,m].
ꢀ
ꢀ
[15] For an example of a competition between C H and C Si bond
activation, see: W. Rauf, A. L. Thompson, J. M. Brown, Chem.
Commun. 2009, 3874 – 3876.
[16] L. Palais, I. S. Mikhel, C. Bournaud, L. Micouin, C. A. Falciola,
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Commun. 2009, 5713 – 5715.
[19] The remaining product mass can mostly be attributed to the
corresponding products 3; see the Supporting Information for
the individual amounts.
[20] For a comprehensive Table 3, see the Supporting Information.
[21] Cis refers to the orientation of the silyl-substituted aryl group
relative to the hydroxy group in 1.
[22] J. H. Smitrovich, K. A. Woerpel, J. Org. Chem. 1996, 61, 6044 –
6046.
[10] The selectivity of the insertion site is opposite to that recently
reported by Chatani and co-workers who observed for a related
ꢀ
vinyl rhodium species an exclusive insertion into the Si CH3
bond. See reference [8].
[11] For similar reactions promoted by main-group metal alkoxides,
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