Highly Regio- and Stereoselective Isomerization of Sylyl Enol Ethers Catalyzed by LBA. A Remarkable Enantiomer Discrimination of Chiral LBA

Full text of DOI:10.1021/jo9812936

6
444
J . Org. Chem. 1998, 63, 6444-6445
High ly Regio- a n d Ster eoselective
Sch em e 1
Isom er iza tion of Silyl En ol Eth er s Ca ta lyzed
by LBA. A Rem a r k a ble En a n tiom er
Discr im in a tion of Ch ir a l LBA
†
‡
Kazuaki Ishihara, Hiroko Nakamura,
‡
,‡
Shingo Nakamura, and Hisashi Yamamoto*
Research Center for Advanced Waste and Emission
Management (ResCWE), and Graduate School of Engineering,
Nagoya University, CREST, J ST, Furo-cho,
Chikusa, Nagoya 464-8603, J apan
Received J uly 6, 1998
The thermodynamic equilibration of trimethylsilyl enol
ethers catalyzed by a Brønsted acid such as p-toluenesulfonic
acid was first reported by Stork and Hudrlik in 1968.1
Unfortunately, this type of equilibration was not established
as a synthetically useful procedure for a long time, since the
use of Brønsted acid was seriously complicated by the
concurrent formation of substantial amounts of higher-
molecular-weight materials and ketones.1b,c In this paper,
we describe the highly regio- and stereoselective isomeriza-
tion of a “kinetic” silyl enol ether to a “thermodynamic” silyl
enol ether catalyzed by Lewis acid-assisted Brønsted acids
Ta ble 1. Isom er iza tion of TBDMS En ol Eth er sa
a
recovered silyl enol ether
ArOH-SnCl4
yield (%) 6a :7a yield (%) 6b:7b
LBAs).2
,3
5
4
(
90
84
2:98
5:95
(
89
94
82
1:99
2:98
87:13
In general, both protodesilylation and isomerization are
R)-2c
able to occur in the reaction of silyl enol ethers with a
Brønsted acid (Scheme 1). The greater stability of the
oxygen-silicon bond in silyl enol ethers and the milder
nucleophilicity of the conjugate base favors the latter
process. In the previously reported enantioselective proto-
nation of “thermodynamic” trimethylsilyl enol ethers with
an optically active binaphthol (BINOL)-tin tetrachloride
3
guaiacol-SnCl4
,6-dimethylphenol-SnCl4
71
94:6
2
91
92:8
a
Unless otherwise noted, the isomerization was carried out
b
using LBA (10 mol % for 6a , 5 mol % for 6b). Initial ratio: 6a :
7
a ) 92:8; 6b:7b ) 98:2. c 5 mol % of (R)-2 was used.
complex 1 in toluene, BINOL was rapidly transformed to
ethers. This theory was confirmed by a preliminary experi-
ment using TBDMS enol ethers 6a and 6b derived from
-substituted cyclohexanone (Table 1). As expected, 6a was
isomerized to 7a in the presence of catalytic amounts of (R)-2
or 4. LBA 5 was a more effective catalyst than 4 for the
isomerization of 6b. However, the use of the coordinated
complex of tin tetrachloride with 2,2′-dihydroxy-1,1′-biphenyl
8) or other monoaryl alcohols predominantly afforded the
corresponding ketones via protodesilylation, and the recov-
ered silyl enol ethers were only slightly isomerized.
the corresponding monosilyl ether.2
a,b
On the other hand,
in the reaction with a monomethyl ether of BINOL (BINOL-
2
Me)-tin tetrachloride complex 2 under similar conditions,
chlorotrimethylsilane was generated in place of the silyl
ether of BINOL-Me.2
c,4
These results reveal that the nu-
cleophilicity of the conjugate base (aryloxy anion) of LBA 2
to the silicon atom is milder than that of LBA 1, probably
due to some steric reason. Furthermore, the silyl transfer
was much slower in the case of bulky tert-butyldimethylsilyl
(
(TBDMS) enol ether.
To explore the generality and scope of LBA 5-catalyzed
isomerization, the reaction was examined with various
structurally diverse ketones (Table 2). In most cases, the
reaction proceeded cleanly, and the desirable “thermody-
namic” regioisomer was obtained in high yield. Interest-
ingly, in the case of silyl enol ethers derived from menthone,
the isomerization of cis isomer 10 occurred more rapidly than
that of trans isomer 9 (entries 3 vs 4). In addition, in silyl
enol ethers derived from 1-decalone, the isomerization of cis
isomer 12 was extremely fast, while no isomerization of
trans isomer 11 occurred (entries 5-7). Catalyst 5 was
useful for reacting not only cycloalkanones but also acyclic
ketones. The isomerization of acyclic silyl enol ethers
stereoselectively gave Z isomer (entries 9-12). Notably, the
We envisioned that chiral LBA 2 and its achiral analogues
would facilitate isomerization rather than protodesilylation
for hydrolytically more stable “kinetic” trialkylsilyl enol
†
ResCWE.
Graduate School of Engineering.
‡
(
1) (a) Stork, G.; Hudrlik, P. F. J . Am. Chem. Soc. 1968, 90, 4462. (b)
House, H. O.; Czuba, L. J .; Gall, M.; Olmstead, H. D. J . Org. Chem. 1969,
4, 2324. (c) House, H. O.; Fischer, W. F., J r.; Gall, M.; McLaughlin, T. E.
J . Org. Chem. 1971, 36, 3429. (d) Poirier, J .-M. Org. Proc. Int. 1998, 20,
17. (e) Deyine, A.; Dujardin, G.; Mammeri, M.; Poirier, J .-M. Synth.
Commun. 1998, 28, 1817.
2) For enantioselective protonation using LBA, see: (a) Ishihara, K.;
3
3
(
Kaneeda, M.; Yamamoto, H. J . Am. Chem. Soc. 1994, 116, 11179. (b)
Ishihara, K.; Nakamura, S.; Yamamoto, H. Croat. Chem. Acta 1996, 69,
13. (c) Ishihara, K.; Nakamura, S.; Kaneeda, M.; Yamamoto, H. J . Am.
Chem. Soc. 1996, 118, 12854. (d) Yanagisawa, A.; Ishihara, K.; Yamamoto,
H. Synlett 1997, 411. (e) Ishihara, K.; Ishida, Y.; Nakamura, S.; Yamamoto,
H. Synlett 1997, 758. (f) Taniguchi, T.; Ogasawara, K. Tetrahedron Lett.
5
5
use of (R)-LBA 2 increased the Z selectivity (entry 9).
This catalytic LBA system was applied to the enantiomer-
selective isomerization of racemic 6a . Isomerization of 6a
with (R)-2 gave 42% of (R)-6a in 97% ee together with 53%
of 7a . This absolute stereopreference is consistent with that
in the enantioselective protonation of 7a with (R)-1 to afford
1
997, 38, 6429.
3) For recent reports of the generation of more-substituted enolates,
(
see: (a) Saito, S.; Ito, M.; Yamamoto, H. J . Am. Chem. Soc. 1997, 119, 611
and references therein. (b) Mahrwald, R.; G u¨ ndogan, B. J . Am. Chem. Soc.
1
998, 120, 413 and references therein.
4) The enantioselective protonation of prochiral silyl enol ethers has been
(
realized using a catalytic amount of (R)-BINOL-Me in place of (R)-BINOL
in the presence of a stoichiometric amount of 2,6-dimethylphenol as an
achiral proton source in toluene.2c
(5) The Z-selectivity of the isomerization was not affected by the steric
demands of the alkyl moiety in monoalkyl ethers of BINOL and 8.
S0022-3263(98)01293-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/29/1998

Communications
Ta ble 2. Isom er iza tion of TBDMS En ol Eth er sa
J . Org. Chem., Vol. 63, No. 19, 1998 6445
mechanism, while the latter takes place via an anti SE′
mechanism. cis-Silyl enol ether 12, as well as 10, gave only
the syn isomer due to an anti SE′ pathway.8
Sch em e 2
The anti SE′ pathway for the isomerization of 10 and 12
can be explained by the product developing control via the
product-like transition state assemblies A and B, respec-
tively, if the rate-determining step is near the C-H bond-
breaking (deprotonation) step rather than the protonation
step to the substrates. This postulate is consistent with the
experimental finding that 2H was located at a pseudoaxial
position of the isomerized silyl enol ether. On the other
hand, the syn SE′ pathway for the isomerization of 9 can be
understood in terms of the stepwise mechanism via the
favored intermediate D. In the latter case, the anti SE′
pathway is prevented because the free energy of the product-
like transition-state assembly C for 9 becomes relatively
higher than that of A by pseudoaxial disposition of methyl
group. The predominance of protodesilylation for trans
isomer 11 can be attributed to the great conformational
stability in the analogous intermediate.9
a
Unless otherwise noted, the isomerization was carried out
using LBA 5 (5 mol %). b Stirred for 5 h. Stirred for 2 min using
R)-2 (1 mol %) in place of 5. Stirred for 3 h. (R)-2 was used in
place of 5.
c
d
e
(
(
S)-2-phenylcyclohexanone via the transition-state assembly
previously reported.
from racemic 6a to (S)-2-phenylcyclohexanone was attained
by the combination of the isomerization of 6a catalyzed by
R)-2 and the subsequent enantioselective protonation of 7a
2
a,d
Furthermore, a one-pot procedure
(
catalyzed by (R)-2 in the presence of 2,6-dimethylphenol, tin
tetrachloride, and chlorotrimethylsilane.
2
c
To clarify the stereochemical course of the protonation
step to the vinyl carbon, silyl enol ethers 9, 10, and 12 were
isomerized in the presence of 1.5 equiv of 2-hydroxy-2′-
methyloxy-1,1′-biphenyl-d (13-d) and 0.5 or 1.5 equiv of tin
Su p p or tin g In for m a tion Ava ila ble: Experimental proce-
dures and analytical data for all compounds (9 pages).
6
2
tetrachloride (Scheme 2). The H-location in the products
1
was determined by H NMR analysis of the corresponding
ketones following protodesilylation with LBA 3.7 Surpris-
ingly, in the case of silyl enol ethers derived from menthone,
only the identical syn isomer was obtained from the trans-
and cis-silyl enol ethers 9 and 10 in good deuterated yield,
respectively. These results show that the stereochemical
course of the isomerization is dependent on the structure of
substrates: the former reaction takes place via a syn SE′
J O9812936
(8) The low deuterated yield observed in the isomerization of 12, which
is an extremely highly reactive silyl enol ether (see, Table 2, entry 6), may
be due to the catalysis of LBA during the addition of tin tetrachloride.6
(9) For mechanism of protodesilylation of allylsilanes, see: (a) Fleming,
I.; Marchi, D.; Patel, S. K. J . Chem. Soc., Perkin Trans. 1 1981, 2518. (b)
Hayashi, T.; Ito, H.; Kumada, M. Tetrahedron Lett. 1982, 23, 4605. (c)
Wilson, S. R.; Price, M. F. Tetrahedron Lett. 1983, 24, 569. (d) Fleming, I.;
Terrett, N. K. Tetrahedron Lett. 1983, 24, 4153. (e) Fleming, I.; Higgins, D.
J . Chem. Soc., Perkin Trans. 1 1989, 206.
(6) Tin tetrachloride was added in a mixed solution of silyl enol ethers
and 13-d in toluene at -78 °C.
1
(
7) For H NMR (500 MHz) analysis, see the Supporting Information.