C2-symmetric Cu(II) complexes as chiral Lewis acids. Catalytic enantioselective Michael addition of silylketene acetals to alkylidene malonates [21]

Full text of DOI:10.1021/ja983864h

1994
J. Am. Chem. Soc. 1999, 121, 1994-1995
Scheme 1
C2-Symmetric Cu(II) Complexes as Chiral Lewis
Acids. Catalytic Enantioselective Michael Addition of
Silylketene Acetals to Alkylidene Malonates
David A. Evans,* Tomislav Rovis, Marisa C. Kozlowski, and
Jason S. Tedrow
Department of Chemistry and Chemical Biology
HarVard UniVersity, Cambridge, Massachusetts 02138
ReceiVed NoVember 9, 1998
The conjugate addition of silylketene acetals and enolsilanes
to R,â-unsaturated carbonyl derivatives, the Mukaiyama-Michael
reaction, has been shown to be a mild, versatile method for
carbon-carbon bond formation.1 While the development of
catalytic asymmetric variants of this process would provide access
to chiral enantioenriched 1,5-dicarbonyl synthons, success in such
endeavors has proven elusive.2 C2-symmetric bisoxazoline copper-
(II) complexes have been shown to be highly effective as chiral
Lewis acids in such transformations as the Diels-Alder,3 hetero
Diels-Alder,4 the Mukaiyama aldol,5 and carbonyl ene reactions.6
The unifying structural motif in these studies has been the
presence of functional groups in the substrate capable of bidentate
chelation to the metal center. We had two goals in initiating this
study: to extend the range of substrates meeting this criterion
and to develop a catalytic asymmetric Mukaiyama-Michael
reaction of silylketene acetals to alkylidene malonates (Scheme
1, eq 1).7 The alkylmalonate products of these reactions may be
induced to undergo decarboxylation to afford differentiated chiral
glutarate esters, a valuable set of chiral synthons.8
expected adduct 4a in 86% ee. However, attempts to render the
process catalytic were unsuccessful, presumably as a consequence
of the stability of the product Cu(II)-malonyl enolate complex.11
After a number of strategies for promoting catalyst turnover were
investigated, it was discovered that addition of 2 equiv of hexa-
fluoro-2-propanol (HFIP) to the reaction mixture increased
conversion from 10 to 40% (Table 1, entry 1).12 Although catalyst
turnover was achieved, conversion suffered due to competitive
HFIP-induced hydrolysis of the silylketene acetal mediated by
the copper catalyst. This side reaction could be minimized by
the slow addition of the nucleophile (entry 2); however, this failed
to provide a general solution.13 Taking advantage of the limited
solubility of HFIP at low temperatures, we found that an increase
in the concentration of alkylidene malonate from 0.2 to 0.5 M
resulted in an increase in conversion from 40 to 87% without
recourse to a slow addition procedure (Table 1, entry 3). When
the dielectric constant of the medium was lowered using a
dichloromethane/toluene mixture complete conversion at 0.2 M
concentration (entries 4 and 5) was observed. The use of toluene
without a cosolvent led to slightly longer reaction times with no
discernible benefits (entry 6).14
A variety of â-substituted alkylidene malonates were examined
in this reaction using the optimized conditions (Table 2). Aromatic
substituents such as phenyl, 2-furyl, â-naphthyl and 3-indolyl
provide high selectivities in the reaction, (entries 1-4). Ortho
substitution on the aromatic ring is well-tolerated, with the ortho-
anisyl substituent affording the addition product 4e in 99% ee,
(entry 5). Sterically demanding alkyl substituents are also well-
tolerated. Cyclohexyl, isopropyl, and even tert-butyl alkylidene
malonates all provide the addition products in enantioselectivities
of 95, 93, and 90% respectively, (entries 6, 9, 10). Interestingly,
the methyl-substituted derivative 2i provides the addition adduct
4i in only 43% ee with opposite facial selectivity, (entry 11). The
reasons for this turnover in selectivity are not clear at this time.
From a practical standpoint, it is important to note that the reaction
can be conducted using lower catalyst loadings (5 mol %), (Table
2, entry 7). Under these conditions, the cyclohexyl alkylidene
malonate 2f provides a 99% yield of the addition product 4f in
95% ee on a gram scale (Table 2, entry 8).15 The absolute
stereochemistry of the adducts was established by X-ray crystal-
lography for 4a, and by chemical correlation for 4e, 4f, 4g, and
4i. The other products were assigned by analogy.
Initial experiments revealed that the [Cu((S,S)-t-Bu-box)]-
(SbF6)2 complex 19 (100 mol %) efficiently mediates the enan-
tioselective addition of silylketene acetal 310 to the phenyl-
substituted alkylidene malonate 2a (CH2Cl2, -78 °C) to give the
(1) Oare, D. A.; Heathcock, C. H. Top. Stereochem. 1991, 20, 87. For a
seminal reference, see: Narasaka, K.; Soai, K.; Mukaiyama, T. Chem. Lett.
1974, 1223-1224.
(2) Kobayashi, S.; Suda, S.; Yamada, M.; Mukaiyama, T. Chem. Lett. 1994,
97-100. For related investigations involving chiral tin enolates, see: (a) Yura,
T.; Iwasawa, N.; Mukaiyama, T. Chem. Lett. 1988, 1021-1024. (b) Yura, T.;
Iwasawa, N.; Narasaka, K.; Mukaiyama, T. Chem. Lett., 1988, 1025-1026.
(3) (a) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem. Soc. 1993,
115, 6460-6461. (b) Evans, D. A.; Murry, J. A.; von Matt, P.; Norcross, R.
D.; Miller, S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 798-800. (c) Evans,
D. A.; Kozlowski, M. C.; Tedrow, J. S. Tetrahedron Lett. 1996, 37, 7481-
7484. (d) Evans, D. A.; Barnes, D. M. Tetrahedron Lett. 1997, 38, 57-58.
(e) Evans, D. A.; Johnson, J. S. J. Org. Chem. 1997, 62, 786-787. (f) Evans,
D. A.; Shaughnessy, E. A.; Barnes, D. M. Tetrahedron Lett. 1997, 38, 3193-
3194. (g) Davies. I. W.; Senanayake, C. H.; Larsen, R. D.; Verhoeven, T. R.;
Reider, P. J. Tetrahedron Lett. 1996, 37, 1725-1729. (h) Ghosh, A. K.;
Mathivanan, P.; Cappiello, J. Tetrahedron Lett. 1996, 37, 3815-3819.
(4) (a) Evans, D. A.; Johnson, J. S. J. Am. Chem. Soc. 1998, 120, 4895-
4896. (b) Johannsen, M.; Jørgensen, K. A. J. Org. Chem. 1995, 60, 5757-
5758. (c) Johannsen, M.; Jørgensen, K. A. Tetrahedron 1996, 52, 7321-
7328. (d) Ghosh, A. K.; Mathivanen, P.; Cappiello, J.; Krishnan, K.
Tetrahedron: Asymmetry 1996, 8, 2165-2169. (e) Thorhauge, J.; Johannsen,
M.; Jørgensen, K. A. Angew. Chem., Int. Ed. 1998, 37, 2404-2406.
(5) (a) Evans, D. A.; Murry, J. A. J. Am. Chem. Soc. 1996, 118, 5814-
5815. (b) Evans, D. A.; Kozlowski, M. C.; Burgey, C. S.; MacMillan, D. W.
C. J. Am. Chem. Soc. 1997, 119, 7893-7894.
(9) For a detailed procedure for the preparation of ((S,S)-t-BuBox)Cu(SbF6)2
complex 1 see: Evans, D. A.; Peterson, G. S.; Johnson, J. S.; Barnes, D. M.;
Campos, K. R.; Woerpel, K. A. J. Org. Chem. 1998, 63, 4541-4544.
(10) For an analogy to the preparation of 3 see: Kuwajima, I.; Kato, M.;
Sato, T. J. Chem. Soc., Chem. Commun. 1978, 478-479.
(11) Trimethylsilyl trifluoromethanesulfonate (TMSOTf) was ineffective
as an additive. For its effectiveness in promoting turnover in the Mukaiyama
aldol reaction, see ref 5b.
(12) The impact of HFIP on asymmetric Michael reactions is prece-
dented: Kitajima, H.; Katsuki, T. Synlett 1997, 568.
(13) Variation of the â-substituent on the alkylidene malonate necessitated
reoptimization of addition times.
(14) The use of the cosolvent is preferred for reasons of operational
simplicity. The catalyst cannot be formed in toluene, and although the
dichloromethane can be removed once the catalyst is formed, we have found
the mixed solvent system to be superior in certain cases likely owing to
solubility issues. Full details of this work will be published in due course.
(15) Ligand recovery was 63%: see Supporting Information for details.
(6) Evans, D. A.; Burgey, C. S.; Paras, N. A.; Vojkovsky, T.; Tregay, S.
W. J. Am. Chem. Soc. 1998, 120, 5824-5825.
(7) Mukaiyama-Michael additions to the related 2-carbomethoxycyclo-
pentenone have been reported using stoichiometric amounts of Cu(II) box
complexes achieving ee’s up to 66%: Bernardi, A.; Colombo, G.; Scolastico,
C. Tetrahedron Lett. 1996, 37, 8921-8924. The authors report a single
example of a catalytic reaction, where 20 mol % catalyst gave 65% yield of
63% ee material. Ti-TADDOL was reported to be less selective as a
stoichiometric promoter with the same system: Bernardi, A.; Karamfilova,
K.; Boschin, G.; Scolastico, C. Tetrahedron Lett. 1995, 36, 1363-1364.
(8) (a) ApSimon, J. W.; Blackwell, B. A.; Edwards, O. E.; Fruchier, A.
Tetrahedron Lett. 1994, 35, 7703-7706. (b) Poch, G. D.; Powell, R. G.
Tetrahedron Lett. 1994, 35, 7707-7710. (c) Boyle, C. D.; Kishi, Y.
Tetrahedron Lett. 1995, 36, 4579-4582. (d) Singh, S. B.; Liesch, J. M.;
Lingham, R. B.; Silverman, K. C.; Sigmund, J. M.; Goetz, M. A. J. Org.
Chem. 1995, 60, 7896-7901. (e) Iwasawa, Y.; Nonoshita, K.; Tomimoto, K.
Tetrahedron Lett. 1995, 36, 7459-7462.
10.1021/ja983864h CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/19/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 9, 1999 1995
Table 1. Effect of Concentration and Solvent on Conversion in the
Michael Reaction between 2 and 3 (eq 2)a
entry concn (M)
solvent
conversion (%)b ee (%)c
1
2
3
4
5
6
0.2
0.2
0.5
0.2
0.2
0.2
CH2Cl2
40
>98d
87
90
91
91
93
93
92
CH2Cl2
CH2Cl2
CH2Cl2/PhMe(1:1)
CH2Cl2/PhMe(1:3)
PhMe
>98e
>98
>98
a Reaction conducted with 2.2 equiv of 3 and 2 equiv of hexafluoro-
2-propanol (HFIP) relative to 2. b Determined by 1H NMR spectroscopy.
c Determined by chiral HPLC (see Supporting Information). d Slow ad-
dition of 3 to the reaction over a period of 8 h. e Isolated yield is 88%.
Figure 1. X-ray structure of [Cu(t-Bubox)‚2a](SbF6)2 (5). Selected bond
lengths and angles: Cu-N1, 1.926 Å; Cu-N2, 1.955 Å; Cu-O1, 1.964
Å; Cu-O2, 1.950 Å; C1-N1-Cu-O1, 16.6°; C2-N2-Cu-O2, 7.0°.
Table 2. Scope of the Catalyzed Michael Reaction between 2 and
3 (eq 3)a
(SbF6)2.10 The sense of asymmetric induction is consistent with
attack of the silylketene acetal nucleophile from the less hindered
si-face of the complexed alkylidene malonate, the stereochemical
outcome predicted from the crystal structure (Figure 1). In cases
where the â-substituent (R) on the Michael acceptor is sterically
demanding, we speculate that the trajectory of the nucleophilic
addition could also be important in achieving asymmetric induc-
tion. As the nucleophile is forced to approach the Michael acceptor
on a nonvertical trajectory to avoid nonbonding interactions with
substituent (R), the ligand is able to provide π-facial discrimina-
tion since the nucleophile is now forced to interact with the two
ligand substituents unequally.18
entry substrate
R
time (h) yield (%)b ee (%)c
1
2
2a
2b
2c
2d
2e
2f
Phenyl
2-Furyl
3
5
91
88
90
99d
92
95
96e
99f
93
89
91
93
94
93
3
2-Naphthyl
3-Ts-Indolyl
2-MeOPh
Cyclohexyl
Cyclohexyl
Cyclohexyl
i-Pr
10
48
12
5
12
20
6
8
5
4
86
5
99
6
95
7
2f
93
8
2f
95
The malonate adducts readily undergo Krapcho decarboxyla-
tion19 (NaCl, wet DMSO, 130 °C, 24 h) to afford the differentiated
glutarate esters 8 in high yield, formally the conjugate adducts
9
2g
2h
2i
93
10
11
t-Bu
90
Methyl
-43
a Reaction conducted with 2.2 equiv of 3 and 2 equiv of hexafluoro-
2-propanol (HFIP) relative to 2 in PhMe/CH2Cl2 (3:1). b Isolated yield.
c Determined by chiral HPLC (see Supporting Information). d 20 mol
% catalyst used and 2.5 equiv of 3 added. e Five mol % catalyst used.
f Five mol % catalyst used on a 5-mmol scale.
At the outset of this investigation, it was not clear that the
[Cu((S,S)-t-Bu-box)](2+) complex 1 would provide an effective
facial bias upon binding the alkylidene malonate substrate since
the prochiral carbon in the undergoing reaction lies along the C2
axis of the chiral ligand. This is to be contrasted with the analo-
gous complexes of unsaturated imides 6, pyruvates, 7 (R ) Me),
of ester enolates and substituted acrylate esters (eq 4). As a
demonstration of the orthogonality of the ester groups, oxidative
hydrolysis (Br2 or NBS in THF/H2O)20 of the thioester yields the
corresponding free acid without interference from the methyl ester.
While space constraints do not permit a more detailed discus-
sion of the role of alcohol addends in rendering the preceding
process catalytic, it is noteworthy that this variant of the
Mukaiyama-Michael reaction has now been generalized to two
other processes that will be reported in due course.
Acknowledgment. Support has been provided by the NSF, Merck,
Pfizer, and NSC Technologies. T.R. gratefully acknowledges an NSERC
(Canada) Postdoctoral Fellowship.
Supporting Information Available: Experimental procedures, spec-
tral data for all compounds, crystallographic data, and stereochemical
proofs (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
and glyoxylate esters 7 (R ) H), that exhibit high levels of asym-
metric induction in the Diels-Alder,3a,3b aldol,5 and ene5 reactions.
Direct evidence for the structure of the catalyst-alkylidene
malonate complex 5 was obtained by single-crystal X-ray
diffraction (Figure 1).16 Interestingly, there is significant distortion
in the alkylidene malonate-catalyst complex 5. The chelated
substrate forms a boat conformation with the copper atom at the
apex.17 This distortion of the bis(oxazoline) ligand out of the plane
approximately defined by the coordinated ester carbonyls is
probably essential for good levels of asymmetric induction. This
represents the first crystallographic characterization of a substrate-
bisoxazoline copper(II) complex and confirms our hypothesis3-6
that the substrate binds to copper in a distorted square planar
geometry in analogy to that observed in the X-ray structure of
the analogous dihydrate complex [Cu((S,S)-t-Bu-box)(H2O)2]-
JA983864H
(16) X-ray data for Cu(t-Bubox)‚2a(SbF6)2: blue-green crystal, fw 1049.69,
crystal system trigonal, space group P32, a ) 12.9635(4) Å, b ) 12.9635(4)
Å, c ) 20.4970(8) Å, R ) 90°, â ) 90°, γ ) 120°, V ) 2983.1(2) Å3, Z )
3, R1 ) 0.0397, wR2 ) 0.0980, data/parameters ) 7940/469, GOF ) 1.019.
(17) For an X-ray structure of an alkylidene malonate bound to TiCl4, see:
Kakkonen, H. J.; Pursuiainen, J.; Pakkanen, T. A.; Ahlgre´n, M.; Iiskola, E. J.
Organomet. Chem. 1993, 453, 175-184.
(18) In a related case the impact of nonbonding interactions imposed by
carbonyl substituents in diastereoselective carbonyl addition has previously
been noted: (a) Heathcock, C. H.; Flippin, L. A. J. Am. Chem. Soc. 1983,
105, 1667-1668. (b) Heathcock, C. H. Aldrichimica Acta 1990, 23, 99-111.
(19) Krapcho, A. P. Synthesis 1982, 805-822.
(20) (a) Minato, H.; Kodama, H.; Miura, T.; Kobayashi, M. Chem. Lett.
1977, 413. (b) Minato, H.; Takeda, K.; Miura, T.; Kobayashi, M. Chem. Lett.
1977, 1095.