General and functional group-tolerable approach to allenylsilanes by rhodium-catalyzed coupling between propargylic carbonates and a silylboronate

Article abstract of DOI:10.1021/ol902339a

[Chemical Equation Presented] The Rh-catalyzed coupling reaction between propargylic carbonates and a silylboronate afforded allenylsilanes with different substitution patterns In high yields. The reaction tolerates a variety of functional groups in propa

Full text of DOI:10.1021/ol902339a

ORGANIC
LETTERS
2009
Vol. 11, No. 24
5618-5620
General and Functional Group-Tolerable
Approach to Allenylsilanes by
Rhodium-Catalyzed Coupling between
Propargylic Carbonates and a
Silylboronate
Hirohisa Ohmiya, Hideto Ito, and Masaya Sawamura*
Department of Chemistry, Faculty of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan
sawamura@sci.hokudai.ac.jp
Received October 9, 2009
ABSTRACT
The Rh-catalyzed coupling reaction between propargylic carbonates and a silylboronate afforded allenylsilanes with different substitution
patterns in high yields. The reaction tolerates a variety of functional groups in propargylic carbonates. The reaction of an optically active
propargylic carbonate proceeded with excellent chirality transfer with 1,3-anti stereochemistry to give an axially chiral allenylsilane.
Allenylsilanes are useful organometallic reagents in organic
synthesis because of their applicability toward various
transformations.1 In particular, the addition to carbonyl
compounds offers a general approach to homopropargylic
alcohols. Among a number of routes to allenylsilanes, the
SN2′ displacement strategies, the substitution of γ-silyl-
substituted propargylic alcohol derivatives with organocopper
reagents,2 or silylation of propargylic alcohol derivatives with
silylcuprate reagents3 are most straightforward. However,
these methods often encounter problems of functional group
compatibility because the organocopper reagents must be
prepared from basic organometallic reagents such as Grignard
or organolithium reagents, and the silylcuprate reagents are
also strongly basic.
Herein, we report rhodium-catalyzed coupling between
propargylic carbonates and a silylboronate as a general
synthetic approach toward the preparation of allenylsi-
lanes.4-6 This reaction tolerated various functional groups
in propargylic carbonates and afforded functionalized alle-
(1) For reviews on allenylmetals, see: (a) Marshall, J. A.; Gung, B. W.;
Grachan, M. L. Modern Allene Chemistry; Krause, N., Hashmi, A. S. K.,
Eds.; Wiley-VCH: Weinheim, 2004; Vol. 2, pp 493-592. (b) Krause, N.;
Hoffmann-Roder, A. Tetrahedron 2004, 60, 11671–11694. (c) Brummond,
K. M.; DeForrest, J. E. Synthesis 2007, 795–818. (d) Marshall, J. A. J.
Org. Chem. 2007, 72, 8153–8166. (e) Masse, C. E.; Panek, J. S. Chem.
ReV. 1995, 95, 1293–1316.
Scheme 1
.
Rh(I)-Catalyzed Coupling between Propargylic
Carbonate 1a and Silylboronate 2
(2) For SN2′ substitutions of γ-silyl-substituted propargylic alcohol
derivatives with organocopper reagents, see: (a) Danheiser, R. L.; Stoner,
E. J.; Koyama, H.; Yamashita, D. S.; Klade, C. A. J. Am. Chem. Soc. 1989,
111, 4407–4413. (b) Westmijze, H.; Vermeer, P. Synthesis 1979, 390–392.
(3) For silylations of propargylic alcohol derivatives with silylcuprate
reagents, see: (a) Fleming, I.; Terret, N. K. J. Organomet. Chem. 1984,
264, 99–118. (b) Fleming, I.; Terret, N. K. Tetrahedron Lett. 1983, 24,
4153–4156. (c) Fleming, I.; Waterson, D. J. Chem Soc., Perkin Trans. 1
1984, 1809–1813. (d) Marshall, J. A.; Maxson, K. J. Org. Chem. 2000, 65,
630–633.
10.1021/ol902339a  2009 American Chemical Society
Published on Web 11/12/2009

nylsilanes that are difficult to prepare by other methods. The
reaction of an optically active propargylic carbonate took
place with excellent point-to-axial chirality transfer with anti
stereochemistry to give an axially chiral allenylsilane.
The reaction of propargylic carbonate 1a with
PhMe2SiB(pin)7 (2) (1.5 equiv) in the presence of
[Rh(cod)2][BF4] (5 mol %) and Et3N (2.5 equiv) in acetone
at 50 °C (18 h) gave allenylsilane 3a in 81% yield (Scheme
1). The allenylsilane 3a is a formal SN2′ product, assuming
silyl anion PhMe2Si- as a nucleophile. No SN2 product was
observed. Notably, the rhodium-catalyzed coupling could be
performed even under air without affecting the product yield.
Several observations regarding the optimum reaction condi-
tions are to be noted. Although the catalytic reaction proceeded
without Et3N, the yield was significantly reduced (31% yield).
A neutral Rh complex [RhCl(cod)]2 was less effective (17%
yield) than the cationic [Rh(cod)2][BF4]. The use of other
silylboron reagents7 such as FMe2SiB(pin), (i-PrO)Me2SiB(pin),
and (Et2N)Me2SiB(pin) instead of 2 resulted in no reaction under
otherwise identical conditions. DMF was as effective as acetone
as a solvent (76% yield). DME, THF, toluene, hexane, and 1,4-
dioxane were also useful, while the rate of conversion of 1a
was slightly decreased (62%, 58%, 56%, 51%, and 49%,
respectively). CH3CN and 1,2-dichloroethane were even less
effective (26% and 10%).
Table 1. Synthesis of Various Allenylsilanesa
The rhodium-catalyzed reaction could be applied to the
synthesis of the allenylsilanes with different substitution
patterns (Table 1). The secondary propargylic carbonate 1b
bearing bulky cyclohexyl groups at both the R- and γ-posi-
tions underwent the coupling efficiently (entry 1). The
secondary benzyl alcohol derivative 1c with a phenyl group
at the R-position was also silylated efficiently, giving the
conjugated allenylsilane 3c (entry 2). The tertiary propargylic
carbonates 1d and 1e were converted into the corresponding
fully substituted allenylsilanes 3d and 3e, respectively (entries
3 and 4). Furthermore, the tertiary propargylic carbonate 1f
bearing a bulky cyclohexyl group at the γ-position was also
efficiently coupled with 2, giving sterically more congested
allenylsilane 3f (entry 5). On the other hand, the reaction of
the primary propargylic carbonate 1g with no R-substituent
resulted in a low conversion and a poor yield (17%) (entry
6). The reaction of terminal alkyne 1h resulted in the complex
mixtures (entry 7).
a Conditions: [Rh(cod)2][BF4] (5 mol %), Et3N (0.5 mmol),
PhMe2SiB(pin) (2) (0.3 mmol), 1 (0.2 mmol), acetone (1.0 mL), 50 °C.
b Isolated yield. c [Rh(cod)2][BF4] (10 mol %), Et3N (0.5 mmol),
PhMe2SiB(pin) (2) (0.3 mmol), 1 (0.2 mmol), acetone (1.0 mL), 50 °C.
The rhodium-catalyzed reaction was capable of affording
a variety of functionalized allenylsilanes (Table 1). Functional
groups such as carbamates and esters were tolerated in the
propargylic carbonates (entries 8 and 9). The propargylic
carbonate (1k) having a benzyloxy group at the opposite
propargylic position underwent the reaction smoothly (entry
10). The substrate with the triisopropylsiloxy group 1l was
less reactive but led to completion with 10 mol % catalyst
loading (entry 11). The free hydroxyl group in substrate 1m
did not inhibit the reaction, giving the corresponding hydroxy
allenylsilane 3m in 93% yield (entry 12). The neutral nature
of the silylboronate reagent and the rhodium complex seems
to contribute to the remarkable functional group tolerance
of this catalytic reaction.
(4) For other approaches to allenylsilanes, see: (a) Kobayashi, S.; Nishio,
K. J. Am. Chem. Soc. 1995, 117, 6392–6393. (b) Han, J. W.; Tokunaga,
N.; Hayashi, T. J. Am. Chem. Soc. 2001, 123, 12915–12916. (c) Suginome,
M.; Matsumoto, A.; Ito, Y. J. Org. Chem. 1996, 61, 4884–4885. (d) Brawn,
R. A.; Panek, J. S. Org. Lett. 2007, 9, 2689–2692. See also ref.1a
(5) For the synthesis of allenylboronates by the Cu-catalyzed substitution
of propargylic carbonates with bis(pinacolato)diboron, see: Ito, H.; Sasaki,
Y.; Sawamura, M. J. Am. Chem. Soc. 2008, 130, 15774–15775.
(6) For the synthesis of allenes by Cu-catalyzed reduction of propargylic
carbonates with hydrosilanes, see: Zhong, C.; Sasaki, Y.; Ito, H.; Sawamura,
M. Chem. Commun. 2009, 5850–5852.
(7) For the preparation and reactivities of silylboronate reagents, see:
(a) Suginome, M.; Matsuda, T.; Ito, Y. Organometallics 2000, 19, 4647–
4649. (b) Ohmura, T.; Masuda, K.; Furukawa, H.; Suginome, M. Organo-
metallics 2007, 26, 1291–1294. (c) Ohmura, T.; Suginome, M. Bull. Chem.
Soc. Jpn. 2009, 82, 29–49.
The reaction of an optically active propargylic carbonate
(S)-1n (97% ee, 0.2 mmol) and 2 (0.3 mmol) was carried
Org. Lett., Vol. 11, No. 24, 2009
5619

out in the presence of [Rh(cod)2][BF4] (10 mol %) and Et3N
(0.5 mmol) in DMF/CH3CN (100:1, 1.0 mL)8 at 25 °C
(Scheme 2). The allenylsilane 3n was isolated in 77% yield.
The subsequent TiCl4-mediated addition of 3n to isobutyral-
dehyde afforded homopropargylic alcohol (3S,4S)-4n (95%
ee) with only slightly decreased enantiomeric purity.4c The
diastereomer ratio (syn/anti, dr) with respect to the consecu-
tive chiral centers was >20:1. The excellent syn-selectivity
suggests that the addition of the allenylsilane 3n to the
aldehyde proceeded through an acyclic antiperiplanar transi-
tion state.1e,9 On the basis of the established 1,3-anti
stereochemical pathway on the Lewis acid-mediated addition
of allenylsilanes to aldehydes,1e the absolute configuration
of the axially chiral allenyl silane 3n was deduced to R.
Therefore, it was revealed that the point-to-axial chirality
transfer in the Rh-catalyzed coupling proceeded with an anti-
stereochemistry.
rhodium species across the C-C triple bond of 1 to form an
alkenylrhodium intermediate (A) and that the addition
reaction be followed by anti-ꢀ-elimination to give allenyl-
silane 3 (Figure 1).11,12
Figure 1. Proposed anti-ꢀ-elimination from an alkenylrhodium
intermediate.
In summary, the Rh-catalyzed coupling reaction between
propargylic carbonates and a silylboronate is a general and
functional group tolerable approach to allenylsilanes. The
excellent point-to-axial chirality transfer with anti-stereo-
chemistry was demonstrated in the preparation of an axially
chiral allenylsilane from an optically active propargylic
carbonate.
Scheme 2
.
Synthesis and Lewis Acid Addition of an Axially
Chiral Allenylsilane (R)-3n
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research (B) (No. 21350020) and for
Young Scientists (B) (No. 21750086), JSPS. H.I. thanks JSPS
for a fellowship.
Supporting Information Available: Experimental pro-
cedures and NMR spectra for new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL902339A
(10) For the Rh-catalyzed conjugate addition to R,ꢀ-unsaturated carbonyl
compounds with PhMe2SiB(pin), see: (a) Walter, C.; Auer, G.; Oestreich,
M. Angew. Chem., Int. Ed. 2006, 45, 5675–5677. (b) Walter, C.; Oestreich,
M. Angew. Chem., Int. Ed. 2008, 47, 3818–3820. (c) Walter, C.; Fro¨hlich,
While it is not clear how silylboronate 2 reacts with a Rh
complex,10 the stereochemical outcome of the present
rhodium-catalyzed reaction suggests that the reaction path-
way may involve regioselective cis-1,2-addition of a silyl-
R.; Oestreich, M. Tetrahedron 2009, 65, 5513–5520
.
(11) For the Rh-catalyzed substitutions of propargylic derivatives with
arylboronic acid, see: (a) Murakami, M.; Igawa, H. HelV. Chim. Acta 2002,
85, 4182–4188. (b) Miura, T.; Shimada, M.; Ku, S.-Y.; Tamai, T.;
(8) The Rh-catalyzed reactions with DMF and CH3CN, instead of DMF/
CH3CN (100:1), afforded 3n with 93% ee (89% yield) and 97% ee (43%
yield), respectively. These results suggest that the coordination of CH3CN
may have played a beneficial role in terms of the efficiency of the chirality
transfer.
(9) (a) Danheiser, R. L.; Carini, D. J. J. Org. Chem. 1980, 45, 3925–
3927. (b) Danheiser, R. L.; Carini, D. J.; Kwasigroch, C. A. J. Org. Chem.
1986, 51, 3870–3878. See also ref.3d
Murakami, M. Angew. Chem., Int. Ed. 2007, 46, 7101–7103
.
(12) Similar mechanisms have been proposed for the Cu(I)-catalyzed
reactions of allylic carbonates with bis(pinacolato)diboron. See: (a) Ito, H.;
Kawakami, C.; Sawamura, M. J. Am. Chem. Soc. 2005, 127, 16034–16035.
(b) Ito, H.; Ito, S.; Sasaki, Y.; Matsuura, K.; Sawamura, M. J. Am. Chem.
Soc. 2007, 129, 14856–14857. (c) Ito, H.; Kosaka, Y.; Nonoyama, K.;
Sasaki, Y.; Sawamura, M. Angew. Chem., Int. Ed. 2008, 47, 7424–7427
.
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