Chiral crotyl geminal bis(silane): a useful reagent for asymmetric Sakurai allylation by selective desilylation-enabled chirality transfer

Article abstract of DOI:10.1039/c7cc00632b

Enantioselective synthesis of SiMe₃/SiPh₂Me-substituted crotyl geminal bis(silane) has been developed. This compound is a useful reagent for Ph₃C⁺B(C₆F₅)₄^--catalyzed asymmetric Sakurai allylation and one-pot Sakurai allylation/Prins cyclization processes. Chemoselective desilylation of SiPh₂Me leads to efficient chirality transfer.

Full text of DOI:10.1039/c7cc00632b

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Reactivity differences of Sc
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Chiral Crotyl Geminal Bis(silane): A Useful Reagent for
Asymmetric Sakurai Allylation by Selective Desilylation-Enabled
Chirality Transfer†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a, b
DOI: 10.1039/x0xx00000x
Zhiwen Chu, Kai Wang, Lu Gao, and Zhenlei Song*
www.rsc.org/
3 2
Enantioselective synthesis of SiMe /SiPh Me-substituted crotyl become more challenging than that using hetero-bimetallic reagent,
geminal bis(silane) has been developed. This compound is a useful because chemoselective cleaving one of the C−Si bonds is difficult
+
-
reagent for Ph
3 6 5 4
C B(C F ) -catalyzed asymmetric Sakurai allylation by differentiation of the substituents on silicon. This is probably one
and one-pot Sakurai allylation/Prins cyclization process. of the reasons that little is known about the selectivity issues (i. e.
Chemoselective desilylation of SiPh
chirality transfer.
2
Me leads to the efficient chemo-, stereo- and enantioselectivity) in the allylation using allyl
geminal bis(silanes), except for Wetter’s (optically active,
5
6
SiMe
3
/SiMe
2
F), Lautens’ (racemic, SiMe
3
/SiMe
2
t-Bu) and our
7
a
Allylboranes, silanes and stannanes are very important allyl metallic
works (racemic, SiMe
3
/SiMe Ph). Their potential values are still
2
1
reagents in both academia and industry. Allylation of aldehydes
unexplored in organic synthesis.
using these reagents has emerged as one of the most powerful
methods for controlling the stereochemistry in acyclic structures.
Compared with allyl mono-metallic reagents, bimetallic species
appear to be more powerful for rapid increasing the structural
complexity. These reagents typically play as a linchpin to assemble
Herein, we report enantioselective synthesis of crotyl geminal
bis(silane) (S)-1a featuring a SiMe
3 2
/SiPh Me-substituted quaternary
carbon center. This compound proves to be a useful allylsilane
+
-
8
9
3 6 5 4
reagent for Ph C B(C F ) -catalyzed Sakurai allylation to give 2, or
one-pot Sakurai allylation/Prins cyclization process to give pyran 3
1
2
two electrophiles by sequential functionalization of C−M and C−M
with excellent stereo- and enantioselectivity.
1
2
bonds. Based on the position of M and M on the allyl moiety, allyl
bimetallic reagents can be divided into 1, 1-, 1, 2- and 1, 3-types
a) Typical allyl bimetallic reagents
(
Scheme 1a). Among them, 1, 1-type is structurally distinct from
allyl 1, 1-bimetallic
allyl 1, 2-bimetallic
allyl 1, 3-bimetallic
M2
M2
E1
E1
E2
E1
other two, as two metals share with the same allylic double bond.
When two metal moieties are different, the carbon they attached
would be a chiral center. The efficiency of chirality transfer would
largely depend on the chemoselectivity during the C−M bond
cleavage. To this end, most studies have been mainly focused on
hetero-bimetallic compounds containing two different metal
centers (Scheme 1b, left). The representative combinations include
M1
M2
E2
M1
M1
E2
b) Previously reported chiral allyl 1,1-bimetallic reagents
type-II (X = Si)[5-7a]
type-I
X1
X
X2
Y
(most studied)
(very few examples)
H
H
a[2]: X = B; Y = Si
b[3]: X = B; Y = Sn
chemoselectivity?
stereoselectivity?
enantioselectivity?
R
R
hetero-bimetallic
c[4]: X = Sn; Y = Si
homo-bimetallic
2
3
4
B/Si, B/Sn and Sn/Si, in which one of the C−M bonds (e.g. C−B
bond) is weaker than another (e.g. C−Si bond). Thus,
chemoselective eliminating the weaker C−M bond could be
achieved, leading to effective enantioselective control in allylation.
Another type of chiral allyl 1, 1-bimetallic reagents is homo-
bimetallic species (Scheme 1b, right). These reagents contain two
identical metal centers such as silicon, but with different
c) This work: chiral crotyl geminal bis(silyl) reagent (S)-1a
CO2Et
MePh2Si SiMe3
Me3Si
Me
1
2
R CHO
R CHO
R1
Me
R1
Me
Ph3C B(C6F5)4
CH2Cl2, rt
EtO2C
(0.02 equiv) EtO2C
OH
[one-pot]
R2
O
CHCl3, rt
up to 65% yield
(S)-1a
97% ee)
2 (Sakurai)
3 (Prins)
up to 90% yield
(
(dr 95:5; 87-97% ee)
(dr 95:5; 94-96% ee)
substituents on them. Imaginably, the chirality transfer would Scheme 1. a) Typical allyl bimetallic reagents; b) previously reported chiral
allyl 1, 1-bimetallic reagents; c) chiral crotyl geminal bis(silyl) reagent (S)-1a.
a.
The racemic 1a was initially synthesized by the protocol shown in
Scheme 2. Reduction of propargyl alcohol 4 with Red-Al followed by
Key Laboratory of Drug-Targeting and Drug Delivery System of the Education
Ministry, West China School of Pharmacy, Sichuan University, Chengdu, 610041
1
0
(
China). E-mail: zhenleisong@scu.edu.cn
trapping with iodine and silylation of the hydroxyl group afforded
vinyl iodide 5 in 85% overall yield. t-BuLi-promoted lithium-iodide
exchange provided the corresponding vinyl anion, which underwent
b.
State Key Laboratory of Biotherapy, West China Hospital, Sichuan University,
Chengdu, 610041(China)
Electronic supplementary information (ESI) available: Experimental procedures,
†
characterisation data for new compounds. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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a
Table 2 Scope of Asymmetric Sakurai Allylation of (S)-1a with Aldehydes.
Si1
Me
OH
S) Me
Si2 Si1
4
steps
77%
RCHO
(S)
R
(S)
(
(S)
Me
Ph3C B(C6F5)4
(0.02 equiv)
Si1
gram scale
EtO2C
OH
2c (dr 95:5)d
Yield e ee f
EtO2C
CHCl3, rt, 2 h
(S)-1a (97% ee) b
(
S)-4
Scheme 2. Reaction conditions of synthesizing racemic (±)-1a: a. i) Red-Al,
Entry
R
Product
o
Et
2
O, then I
2
; ii) Ph
2
MeSiCl, DMAP, Et
3
N, CH
2
Cl
2
, 0 C to rt, 85% (2 steps). b. t-
Br
F
Si1
Si1
Me
Me
o
BuLi, THF, 95%. c) CH
3
C(OEt) , CH CH
3
3
2
CO
2
H, toluene, 140 C, 15 h, 95%.
1
2
3
4
5
6
7
8
9
-C6H4-Br-p
-C6H4-F-p
2a
2b
2c
2d
2e
2f
75%
59%
70%
90%
70%
N.R.
72%
71%
60%
60%
97%
95%
97%
96%
94%
N.D.
96%
87%
93%
96%
EtO2C
EtO2C
OH
OH
retro-[1, 4]-Brook rearrangement to deliver allylic alcohol 6 in 95%
1
1
yield. The subsequent Johson-Claisen rearrangement of 6 with
1
2
CH C(OEt) gave rise to 1a cleanly in 95% yield. This procedure
3
3
was also applied successfully to synthesizing analogous crotyl
silanes 1b-1e, which were used for screening the Sakurai allylation
conditions (Table 1).
Si1
Me
Me
Cl
-C6H4-Cl-m
-C6H4-CN-p
-C6H4-CF3-p
Sakurai allylation of (±)-1a with p-Br-C H CHO was first
6
4
EtO2C
EtO2C
OH
OH
examined in CH Cl using stoichiometric amount of Lewis acids such
2
2
CN
Si1
as TMSOTf and SnCl4, or Brønsted acids such as HBF4.
Unfortunately, no desired allylation occurred even at room
temperature (Table 1, entry 1). However, we were delighted to find
+
-
that 0.02 equiv of Ph C B(C F ) appeared to be an effective
CF3
3
6
5 4
Si1
Me
catalyst. Interestingly, chemoselective desilylation of SiPh Me
2
rather than the generally more reactive SiMe was observed to give
3
EtO2C
OH
Z-anti-homoallylic alcohol (±)-2a in 36% yield as a single isomer. The
major by-product was pyran (±)-3a, which was generated by
sequential Prins cyclization of (±)-2a with the second aldehyde
OMe
Si1
Me
Me
-C6H4-OMe-p
(
entry 2). This side pathway could be inhibited by decreasing the
EtO2C
OH
OH
loading of aldehyde from 1.2 to 1.0 equiv, and switching the solvent
Si1
from CH Cl to CHCl . The yield of (±)-2a was improved to 75%
CO Et
2
2
3
2
CO2Et
SiEt3
+
-
2g
2h
2i
(
entry 3). In contrast to Ph C B(C F ) , trityl cation catalyst
3 6 5 4
2
EtO C
+
-
+
-
Ph C PF or Ph C SbCl was either ineffective or partially effective
3
6
3
6
Si1
Me
Me
Me
SiEt3
to form (±)-2a (entries 4 and 5). The combination mode of silyl
groups also has great impact on the allylation efficiency. As shown
in entry 6, a lower yield of 46% was obtained in the reaction of
silane 1b featuring a SiMe /SiMe Ph combination. In addition, no
EtO2C
OH
OH
OH
3
2
Si1
reaction
Table 1 Screening of Sakurai Allylation Conditions and Silyl Combinations.
-Cy
a
EtO2C
EtO2C
Si1
CO2Et
10
-CO2Et
2j
1
2
d
d
a
Entry (±)-1 (Si /Si )
ArCHO
equiv)
catalyst
solvent (±)-2a (±)-3a
Reaction conditions: (S)-1a (0.05 mmol), aldehdye (0.05 mmol) and
+
-
b
(
(equiv)
3 6 5 4 3
Ph C B(C F ) (0.02 equiv) in CHCl (0.5 mL) at room temperature for 2 h.
+
c
1
2
3
4
5
6
7
8
1a (SiMe
1a (SiMe
1a (SiMe
1a (SiMe
1a (SiMe
3
3
3
3
3
/ SiPh
/ SiPh
/ SiPh
/ SiPh
/ SiPh
2
2
2
2
2
Me) 1.2
Me) 1.2
Me) 1.0
Me) 1.0
Me) 1.0
L.A. or H
CH
CH
2
Cl
Cl
2
2
N.D. N.D.
36% 36%
The (S)-configuration was determined by circular dichroism spectroscopy.
The Z-(S, S)-stereochemistry was assigned by X-ray analysis of the diol of
+
[C ]-1 (0.02)
2
+
c
16
d
1
[C ]-1 (0.02)
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
CHCl
3
75%
N.R
31%
46%
N.R
N.R
N.R
0%
N.R.
0%
2a.
products.
The ratios were determined by H NMR spectroscopy of the crude
+
c
e
[C ]-2 (0.02)
3
3
3
3
3
3
Isolated yields after purification by silica gel column
+
c
f
[C ]-3 (0.02)
chromatography. The ee values were determined by HPLC analysis using a
chiral stationary phase.
+
1b (SiMe
3
/ SiMe
/ SiEt
/ SiMe
/ SiPh
2
Ph) 1.0
1.0
[C ]-1 (0.02)
0%
+
1c (SiMe
1d (SiMe
3
3
)
[C ]-1 (0.02)
N.R.
N.R.
N.R.
+
3
2
t-Bu) 1.0
[C ]-1 (0.02)
occurred when using other analogous crotyl silanes 1c (SiMe
d (SiMe /SiMe t-Bu) or 1e (SiEt /SiPh Me).
Enantioselective synthesis of (S)-1a (97% ee)
3
/SiEt ),
3
+
9
1e (SiEt
3
2
Me)
1.0
[C ]-1 (0.02)
1
3
2
3
2
a
1
3, 14
Reaction conditions: 1a (0.05 mmol), p-Br-C
6
H
4
CHO (0.05mmol) and
The stereochemistry was assigned using
was achieved on
gram scale from optically pure (S)-4 using the same procedure as
+
-
b
Ph
3
C B(C
6
F
5
)
4
(0.02 equiv).
15
optically pure compounds in Tables 2 and 3. The ratios were determined by
+
-
1
c
+
+
-
+
3 6 5 4
that used for racemic 1a. Ph C B(C F ) -catalyzed Sakurai allylation
3 6 5 4
H NMR spectroscopy of the crude products. [C ]-1: Ph C B(C F ) ; [C ]-2:
+
-
+
+
- d
.
Ph
3
C PF
6
; [C ]-3: Ph
3
C SbCl
6
Isolated yields after purification by silica gel of (S)-1a was tested next with a range of aldehydes. The approach
column chromatography.
was suitable for electron-withdrawing group-substituted aryl
2
| J. Name., 2012, 00, 1-3
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1
7
aldehydes (Table 2). Chemoselective desilylation of SiPh
2
Me was should be also capable to catalyze the Prins cyclization of 2a with
Cl as solvent. As expected, the approach
observed in all cases, leading to reliable chirality transfer from aldehydes if using CH
2
2
geminal bis(silyl) moiety to the products. 2a-2e were obtained in tolerated to alkyl, aryl, and α, β-unsaturated aldehydes (Table 3).
good yield as a single Z-anti-isomer with (S, S)-enantioselectivity Pyrans 3 were obtained as a single diastereomer in high yields and
(entries 1-5). However, no reaction occurred with p-anisaldehyde enantioselectivity. This clean Prins cyclizaton allowed us to realize a
containing an electron-donating MeO group (entry 6). Reaction of one-pot Sakurai allylation/Prins cyclization process (Scheme 3).
1
enal provided a higher ee (2g, 96% ee) than that (2h, 87% ee) Once the Sakurai allylation of (S)-1a with R CHO was complete and
2
obtained using less sterically demanding propargyl aldehyde had formed 2a, a solution of the second aldehdye R CHO in CH
2
Cl
2
(entries 7 and 8). While allylation of cyclohexyl aldehyde and ethyl was added to the same reaction tube. In this way, a range of pyrans
glyoxylate gave 2i and 2j, respectively, in good yield with high ee, 3k-3m were generated in 50-65% yields with high dr and ee.
using unbranched alkyl aldehydes led to pyran as the major product
(not shown).
1
R CHO
Si2 Si1
Ph C B(C F ) (0.02 equiv)
3
6 5 4
a
Me
R1
Table 3 Scope of Prins Cyclization of 2a with Aldehdyes.
CHCl3, rt, 2 h
CO2Et
Me
then R2CHO, CH Cl , rt, 20 h
2
2
R2
Si1
O
Me
2
EtO C
[one-pot]
2
Me
R1
R CHO
(
S)
R1
CO2Et
(S)-1a
(R1 = -C H -Br-p)
3 (dr 95:5
6 4
(R)
Ph3C B(C6F5)4 (0.02 equiv)
O
R2
EtO2C
OH
CH2Cl2, rt, 20 h
(R1 = -C6H4-Br-p)
Me
R1
Me
R1
Me
R1
3 (dr 95:5 b
2
a
CO Et
CO Et
2
CO Et
2
2
Entry
1
R2
Product
Yield c
ee d
O
O
O
SiEt3
Me
CO2Et
-C2H4Br
-C2H4Ph
i-Bu
3b
92%
70%
98%
94%
3k (55%, 95% ee
3l (50%, 94% ee
3m (65%, 94% ee
R1
Me
R1
O
O
O
Br
Scheme 3. One-pot Sakurai allylation/Prins cyclization to form 3k-3m.
CO2Et
Ph
2
3
3c
3d
95%
95%
Sakurai allylation of (S)-1a with aldehydes shows four different
selectivities in a single transformation: chemoselective desilylation
Me
R1
2 3
of SiMe Ph over SiMe , forming Z- over E-vinyl silane, anti over syn-
CO2Et
diastereoselectivity and S/S over R/S-enantioselectivity. We
proposed a model in Scheme 5 to rationalize the above selectivity
outcomes. The reactive conformation of (S)-1a should be that, in
i-Bu
Me
R1
CO2Et
4
-C6H4-F-p
3e
90%
96%
which the most sterically demanding SiPh
perpendicular to the alkene and the CH CO
orientation at the allylic position. This conformation would
minimize the allylic strain, and also benefits from
hyperconjugation effect between the C−SiPh Me bond and alkene.
2
Me group is positioned
O
2
2
Et group adopts an exo-
F
Me
R1
CO2Et
CO2Et
a
OMe
5
6
7
8
9
-C6H4-OMe-m
3f
3g
3h
3i
95%
98%
90%
80%
90%
95%
94%
96%
95%
96%
2
O
O
Thus, chemoselective elimination of SiPh
2
Me would generate
+
+
SiMe -substituted Z-vinyl silane. Despite the real catalyst (C , Si or
3
Me
R1
+
18
19
H ) is not clear currently, the classical anti-S
E
' mechanism could
3-thienyl
apply to each of them, giving unified allylation model:
a
S
antiperiplanar transition states 7-syn and 7-anti. 7-anti appears
being more favorable than 7-syn, which suffers from a severe
Me
R1
CO2Et
3
gauche interaction between R and SiMe groups. Thus, reaction
SiEt3
O
O
SiEt3
would proceed through 7-anti to generate Z-anti-(S, S)-2 diastereo-
and enantioselectively.
Me
R1
CO2Et
CO2Et
CO2Et
Me
R1
CO2Et
C6H4-NO2-p
3j
O
NO2
a
Reaction conditions: 2a (0.05 mmol), aldehyde (0.05 mmol) and
+
-
b
3 6 5 4 3
Ph C B(C F ) (0.02 equiv) in CHCl (0.5 mL) at room temperature for 20 h.
The stereochemistry was determined based on NOE experiments of 3d. The
1
c
ratios were determined by H NMR spectroscopy of the crude products.
d
Isolated yields after purification by silica gel column chromatography. The
ee values were determined by HPLC analysis using a chiral stationary phase.
Scheme 5. Model analysis to explain Z-anti-(S, S)-selectivity outcome.
Functionalization of the endo-cyclic olefin in 3 allowed us to
+
-
3 6 5 4
Formation of the by-product (±)-3a implies that Ph C B(C F )
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obtain fully substituted pyran with defined stereochemistry. As
see: For selected advances of geminal bis(silane) chemistry, see: (b)
Q. Luo, C. Wang, Y. X. Li, K. B. Ouyang, L. Gu, M. Uchiyama, Z. F. Xi,
Chem. Sci. 2011, 2, 2271; (c) K. Groll, S. M. Manolikakes, X. M. du
Jourdin, M. Jaric, A. Bredihhin,; K. Karaghiosoff, T. Carell, P. Knochel,
Angew. Chem. Int. Ed. 2013, 52, 6776; (d) H. Y. Cui, J. Y. Zhang, C. M.
Cui, Organometallics 2013, 32, 1; (e) X. F. Bai, W. H. Deng, Z. Xu, F. W.
shown in Scheme 6, treatment of 3d with NBS in CHCl
sequential bromination/lactonization. Pyran 8 was generated in
2% yield with excellent stereochemical control. Interestingly,
switching the solvent from CHCl to CH CN favored a
3
led to a
9
3
3
bromination/hydroxylation process, giving 9 in 90% yield as a single
Li, Y. Deng, C. G. Xia, L. W. Xu, Chem. Asian J. 2014, 9, 1108; (f) Z. X.
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0 (a) E. J. Corey, J. A. Katzenellenbogen, G. H. Posner, J. Am. Chem.
Soc., 1967, 89, 4245; (b) S. E. Denmark, T. K. Jones, J. Org. Chem.,
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4
Scheme 6. Bromination of 3d to form fully substituted pyrans 8 and 9
In summary, we have achieved an enantioselective synthesis of
SiMe /SiPh Me-substituted crotyl geminal bis(silane). This
compound is
asymmetric Sakurai allylation and one-pot Sakurai allylation/Prins
cyclization process. Chemoselective desilylation of SiPh Me leads to
.
3
2
+
-
a
3 6 5 4
useful reagent for Ph C B(C F ) -catalyzed
9
1
2
the efficient chirality transfer, giving Z-anti-(S, S)-selectivity.
Applications of this methodology in organic synthesis are underway.
We are grateful for financial support from the NSFC
(21622202, 21290180, 21502125). The authors thank Prof. Qin
6
137.
Ouyang in Third Military Medical University for his invaluable help
in computational works. The authors also thank Mr. Zhendong Chen
for his invaluable help in editing the supplementary materials.
1
1
1 K. D. Kim, P. A. Wagtiotis, Tetrahedron Lett., 1990, 131, 6137.
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1
1
3 The S-configuration of (S)-1a was assigned by comparing its circular
dichroism spectroscopy with the computational results of both (S)-
1a and (R)-1a. See Supporting Information for details.
Notes and references
4 Claisen rearrangement of mono-3-Z and -E-silyl allylic alcohols
usually proceeds by a classical chair-like transition state. For a
reference, see: J. S. Panek, T. D. Clark, J. Org. Chem., 1992, 57, 4323.
According to Panek’s work, the rearrangement of (S)-6a in our case
by a chair-like transition state should give 1a with the R- rather than
S-configuration. Thus, formation of (S)-1a implies that the reaction
might proceed by a boat transition state. More detailed studies are
underway on this interesting observation.
1
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6 CCDC 1528458 [diol of 2a
]
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1
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