Regulation of equilibria in the catalytic asymmetric allylic transfer reaction: Unusual 1,2-carbonyl addition of 3-trimethylsilyl-2-propenylstannane

Article abstract of DOI:10.1039/b304356h

Catalytic enantioselective addition of 3-trimethylsilyl-2-propenylstannane to aldehydes promoted by BINOL-Ti(IV) catalyst along with synergistic reagent provides unusual 1,2-adducts with high levels of enantioselectivity.

Full text of DOI:10.1039/b304356h

Regulation of equilibria in the catalytic asymmetric allylic transfer
reaction: unusual 1,2-carbonyl addition of
3-trimethylsilyl-2-propenylstannane†
Chan-Mo Yu,* Ji-Min Kim, Mi-Sook Shin and Mee-Ok Yoon
Department of Chemistry and Laboratory for Metal-Catalyzed Reactions, Sungkyunkwan University, Suwon
440-746, Korea. E-mail: cmyu@chem.skku.ac.kr; Fax: +82 31 290 7075; Tel: +82 31 290 7067
Received (in Cambridge, UK) 23rd April 2003, Accepted 29th May 2003
First published as an Advance Article on the web 18th June 2003
Catalytic enantioselective addition of 3-trimethylsilyl-2-pro-
penylstannane to aldehydes promoted by BINOL–Ti(IV
catalyst along with synergistic reagent provides unusual
1,2-adducts with high levels of enantioselectivity.
The first study for preliminary experiments focused on the
feasibility of 2 for catalytic asymmetric allylic transfer with
achiral aldehydes promoted by a chiral Lewis acid catalyst.
Initial attempts at the addition of 2 to heptanal indicated that the
conversion into the corresponding alcohol could not be realized
with a variety of chiral Lewis acids including BINOL–TiIV
complex. We subsequently observed that synergistic reagents
could also be employed for this purpose.5–8 After surveying
various conditions, several key points emerged: 1) initial
experiments on the allylic transfer reaction of 2 with heptanal
promoted by (R)-BINOL–Ti(IV) (a 2 : 1 mixture of BINOL and
Ti(OPri)4, 20 mol%) along with Et2BSPri at 220 °C for 24 h in
CH2Cl2 afforded encouraging but only marginal results—
although the reaction proceeded, products were formed as an
isomeric mixture and in low chemical yield; 2) BINOL–
Ti(IV)[OCH(CF3)2]2 (5),1 which was previously utilized for
sequential allylic transfer reactions, in the presence of 4 Å
molecular sieves proved to be the most efficient catalyst; 3) the
utilization of Et2BSPri (6) as a synergistic reagent proved to be
essential for the catalytic process; 4) the reaction performed at
220 °C in PhCF3 always gave the best results in terms of
chemical yields and enantioselectivities; 5) the catalytic process
produced the 1,2-carbonyl addition adduct 4 as a single
regioisomer (E)-vinylsilane as judged by analysis of 500 MHz
1H NMR. This high regio- and stereoselectivity in the formation
of 4 from 2 is not common; structurally similar crotylstannane
is usually converted into 2-methylallyl alcohol by reaction with
aldehyde mediated by a Lewis acid catalyst in the absence of
equilibrating reagents such as BuSnCl3.10
)
During the course of our research program aimed at finding new
reagents and realizing useful and practical ways to expand the
scope of allylic transfer reactions, we became quite interested in
the utilization of 3-trimethylsilyl-2-propenyltributylstannane 2
as a bifunctional allylating reagent. The background to this
current study was the discovery that the sequential catalytic
allylic transfer reaction of 2-trimethylsilylmethyl-2-propenyl-
tributylstannane with two carbonyl functionalities resulted in
the formation of a variety of tetrahydropyrans with high levels
of enantio- and diastereoselectivity.1 The availability of effi-
cient synthetic methods for achieving absolute stereoselectivity
via a catalytic process in the production of enantiomerically
pure compounds is of considerable current interest in the field of
synthetic chemistry.2 In this regard, allylic transfer reactions
provide excellent stereoselective routes for converting alde-
hydes into the corresponding alcohols.3 In the light of
widespread advances in catalytic methods for the synthesis of
chiral substances, the allylic transfer reactions of carbonyl
functionalities using a chiral Lewis acid catalyst led to
significant developments in the area of asymmetric synthesis.4
In previous studies, we have disclosed that the utilization of
synergistic reagents for catalytic asymmetric reactions resulted
in a significantly increased catalytic ability as a consequence of
expediting regeneration of chiral catalyst to provide highly
catalytic versions of enantioselective allylic transfer reactions
of achiral aldehydes such as allylation,5 propargylation,6
allenylation,7 and dienylation.8 It was envisaged that the
realization of an efficient catalytic method for the synthesis of
3 from 2 by the usual SE2A pathway could be valuable because
product 3 can serve as a precursor for the synthesis of
dihydropyrans.9 However, we have observed that the catalytic
reaction did not proceed through the SE2A reaction pathway to
form 3. We report herein our discovery of the exclusive
formation of 4 with high levels of regio- and enantioselectivity
(Scheme 1).
Under optimal conditions, the catalytic allylic transfer
reaction was conducted by dropwise addition of Et2BSPri (6, 1.2
equiv.) in PhCF3 at 220 °C to the mixture of 1 (R = nC6H13,
1 equiv.) and 2 in the presence of (R)-BINOL–Ti(I-
V)[OCH(CF3)2]2 (5, 10 mol%) in PhCF3. After 18 h at 220 °C,
the reaction mixture was quenched by the addition of saturated
aqueous NaHCO3. Usual workup and silica gel chromatography
afforded 4a (89% isolated yield, 96% ee). The reliability of the
catalytic reaction was further examined with several aldehydes
as listed in Table 1.
Although the exact mechanistic aspects of this transformation
have not been rigorously elucidated, the following pathway
Table 1 Allylic transfer reaction of 2 with achiral aldehydesa
Entry
RCHO
4
t/h
Yield (%)b
ee (%)c
1
2
3
4
5
6
7
nC6H13
PhCH2CH2
Ph
a
b
c
d
e
f
18
18
18
18
24
24
36
89
74
83
81
84
71
47
96
93
91
93
91
88
83
4-Br-Ph
BuCO(CH2)3
PhC·C
PhCHNCH
g
a Reaction was run at 220 °C in PhCF3 with 10 mol% of 6. b Yields refer
to isolated and purified yield. c Enantiomeric excesses were determined by
preparation of (+)-MTPA ester derivatives, analysis by 500 MHz 1H NMR
(all entries), and by HPLC using a chiral column (Chiracel OD-H, 1–2%
PriOH in hexane, entries 2, 3, 4, 6).
Scheme 1 General pathways for the allylic transfer reaction of allylstannane
2.
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b3/b304356h/
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CHEM. COMMUN., 2003, 1744–1745
This journal is © The Royal Society of Chemistry 2003

could be a probable regio- and stereochemical route on the basis
of product formation. In general, allylic transfer of 2 to
aldehydes catalyzed by a Lewis acid catalyst might lead mainly
to 3 via an SE2A process. However, the sterically bulky
trimethylsilyl substituent on 2 would not allow appropriate
orientation between reagent and substrate–catalyst complex,
depicted as A in Fig. 1, to produce 3. Therefore the formation of
4 could be explained if the reaction produced products from the
equilibrium of 2 to 5 and 6 under the reaction conditions. Since
antiperiplanar attack would lead to the particular product 4 or 7
via stereochemical models B or C, the major reaction pathway
could be dependent on the stability in the transition state under
kinetic control such as orientations and steric factors without a
necessary link to stabilities of product and tin reagents. Thus,
we believe that the origin of the regiochemical and trans
geometry outcomes for this transformation might be a subtle
geometrical preference for orientation in the transition states
offered by this catalytic system. The stereochemical course of
this catalytic process is likely to be due to a geometrical
preference of B compared with C for a minimum allylic strain
with existing substituents in Fig. 1. Thus, this allylic transfer
reaction led to the formation of 4 with high levels of
stereoselectivity through the stereochemical model B.
yield.12 The absolute configuration of the predominating
enantiomer of the adducts 4 was unambiguously established,
after conversion of 8 into 9 under the reduction conditions
described in Scheme 2, by comparison of their specific rotations
with those of known alcohols.4 Dihydropyran-2-ylacetate 10
was obtained from 8 using the following two-step sequence in
61% yield: 1) synthesis of b-alkoxyacrylate from 8 with ethyl
propiolate in the presence of 4-methylmorpholine; 2) radical
cyclisation of b-alkoxyacrylate with a stannyl radical source.13
Synthesis of the 5,6-dihydropyran-2-one 11 was accomplished
by carbonylative cyclization of 8 with Ni(CO)2(PPh3)2 in the
presence of Et3N under reflux for 30 min in PhCF3 in 83%
yield.14
In summary, this communication describes a new and
efficient catalytic asymmetric allylic transfer reaction of 2 with
achiral aldehydes in the production of unusual 1,2-carbonyl
addition product 4 with high levels of regio- and enantiose-
lectivity, which promises to be widely applicable. We believe
this observation could be a useful example of a-addition
through the equilibrium of reagents regulated by an external
chiral Lewis acid catalyst based on the Curtin–Hammett
principle. We believe that the products can serve as synthetic
intermediates for useful substances.
The products 4 are readily amenable to further conversion
into useful synthetic intermediates by functional group trans-
formations of vinylsilane11 as demonstrated in Scheme 2.
For example, the (Z)-bromovinylic alcohol 8 was obtained by
the treatment of 4b with bromine followed by Bu4NF in 79%
Generous financial support by grants from the Korea
Ministry of Science and Technology through the National
Research Laboratory program (NRL) and the Center for
Molecular Design and Synthesis (CMDS: KOSEF SRC) at
KAIST is gratefully acknowledged.
Notes and references
1 C.-M. Yu, J.-Y. Lee, B. So and J. Hong, Angew. Chem., Int. Ed., 2002,
41, 161–163.
2 For general discussions, see: (a) Lewis Acids in Organic Synthesis, Vols.
I, II, ed. H. Yamamoto, Wiley-VCH, Weinheim, 2000; (b) Compre-
hensive Asymmetric Catalysis, Vols. I, II, III, eds. E. N. Jacobsen, A.
Pfaltz and H. Yamamoto, Springer-Verlag, Berlin, 1999; (c) Catalytic
Asymmetric Synthesis, 2nd Edn., ed. I. Ojima, Wiley-VCH, New York,
2000.
3 For recent reviews of allylations, see: (a) S. E. Denmark and N. G.
Almstead, in Modern Carbonyl Chemistry, ed. J. Otera, Wiley-VCH,
Weinheim, 2000, pp. 299–402; (b) S. R. Chemler and W. R. Roush, in
Modern Carbonyl Chemistry, ed. J. Otera, Wiley-VCH, Weinheim,
2000, pp. 403–490.
4 For a recent review, see: A. Yanagisawa, in Comprehensive Asymmetric
Catalysis, Vol. II, eds. E. N. Jacobsen, A. Pfaltz and H. Yamamoto,
Springer-Verlag, Berlin, 1999, pp. 965–979.
5 (a) C.-M. Yu, H.-S. Choi, W.-H. Jung, H.-J. Kim and J. Shin, Chem.
Commun., 1997, 761–762; (b) C.-M. Yu, H.-S. Choi, W.-H. Jung and S.-
S. Lee, Tetrahedron Lett., 1996, 37, 7095–7098.
6 (a) C.-M. Yu, S.-K. Yoon, H.-S. Choi and K. Baek, Chem. Commun.,
1997, 763–764; (b) C.-M. Yu, H.-S. Choi, S.-K. Yoon and W.-H. Jung,
Synlett, 1997, 889–890.
7 C.-M. Yu, S.-K. Yoon, K. Baek and J.-Y. Lee, Angew. Chem., Int. Ed.,
1998, 37, 2392–2395.
Fig. 1 Plausible stereochemical pathway for the formation of 4 from the
equilibrium of 2 to 5 and 6.
8 (a) C.-M. Yu, S.-K. Yoon, S.-J. Lee, J.-Y. Lee and S. S. Kim, Chem.
Commun., 1998, 2749–2750; (b) C.-M. Yu, S.-J. Lee and M. Jeon, J.
Chem. Soc., Perkin Trans. 1, 1999, 3557–3558; (c) C.-M. Yu, M. Jeon,
J.-Y. Lee and J. Jeon, Eur. J. Org. Chem., 2001, 6, 1143–1148.
9 (a) G. C. Micalizio and W. R. Roush, Org. Lett., 2000, 2, 461–464; (b)
W. R. Roush and G. J. Dilley, Synlett, 2001, 955–959.
10 J. A. Marshall, Chem. Rev., 1996, 96, 31–47 and references therein.
11 For an excellent discussion, see: I. Fleming, A. Barbero and D. Walter,
Chem. Rev., 1997, 97, 2063–2192.
12 F. Bjorkling, T. Norin, C. R. Unelius and R. B. Miller, J. Org. Chem.,
1987, 52, 292–294.
13 (a) E. Lee, C. M. Park and J. S. Yun, J. Am. Chem. Soc., 1995, 117,
8017–8018; (b) E. Lee, E. J. Jeong, E. J. Kang, L. T. Sung and S. K.
Hong, J. Am. Chem. Soc., 2001, 123, 10131–10132; (c) E. Lee, H. Y.
Song, J. W. Kang, D.-S. Kim, C.-K. Jung and J. M. Cho, J. Am. Chem.
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Scheme 2 Reagents and conditions: i. Br2, 278 °C, CH2Cl2, then Bu4NF,
THF, 79%; ii. AIBN(cat.), Bu3SnH, reflux, benzene, 63%; iii. ethyl
propiolate, 4-methylmorpholine, 23 °C, and then AIBN(cat.), Bu3SnH,
reflux, benzene, 61%; iv. Ni(CO)2(PPh3)2, Et3N, reflux, 30 min, PhCF3,
83%.
14 M. F. Semmelhack and S. J. Brickner, J. Org. Chem., 1981, 46,
1723–1725.
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