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Catalytic Enantioselective Allyl- and Crotylboration ofAldehydes Using Chiral Diol•SnCl4 Complexes.
Optimization, Substrate Scope and Mechanistic Investigations
Vivek Rauniyar, Huimin Zhai, and Dennis G. HallJ. Am. Chem. Soc. 2008, 130, 8481-8490.
Current lit.06/28/08Akira Nakamura
BOR
OH
RCHO +R
OR
Akira Nakamura @ Wipf Group Page 1 of 15 7/3/2008
Chem2320 P. Wipf
Akira Nakamura @ Wipf Group Page 2 of 15 7/3/2008
Chem2320 P. Wipf
The Type I Reagents Mechanism
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syn
Chem2320 P. Wipf
The Type II Reagents Mechanism
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Acid Catalyzed Addition of Allylboronates
Dennis G. Hall. JACS. 2002, 124, 11586.
CO2Et
BO
O
Ph
CO2Et(RO)2BO
PhCHOO
O
Ph
+
none 12 days at r.t. 84%10% Sc(OTf)3 6 h at r.t 93%
10% Sc(OTf)3 16 h at 0 oC < 5%
10% TFA 16 h at 0 oC 96%
Dennis G. Hall. JACS. 2005, 127, 12808.
Akira Nakamura @ Wipf Group Page 5 of 15 7/3/2008
Miyaura. N. JACS. 2002, 124, 12414.
None traceAlCl3 92% anti = 99%Sc(OTf)3 94% anti = 99%
LA
LA
LA
PhCHO Bpin Ph
HO10 mol% cat.
toluene / -78oC
+
Acceleration Effect of Lewis Acid in Allylboration of Aldehydes
?
Akira Nakamura @ Wipf Group Page 6 of 15 7/3/2008
LA
LA
LA
Suppress no-pB overlap
Increase the acidic character of B
Double coordination
“superactivate” aldehyde
Investigation of Mechanism (Hypothsis)
Akira Nakamura @ Wipf Group Page 7 of 15 7/3/2008
Dennis G. Hall. JACS. 2004, 126, 4518.
B+PhCHO Ph
OH
DCM , -78 o
C
i) without LAii) 10% Sc(OTf)3 accelerate >100 times
+PhCHO B
O
O
Ph
OH
DCM , -78 o
C
no acceleration observed
Investigation of Mechanism (Result)
LA
LA
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Yamamoto’s Lewis acid assisted Brønsted acid system
Proposed TS based on calculation Yamamoto, H. JACS. 1999, 121, 4906. Yamamoto, H. JACS. 2000, 123, 8120.
(R)-BinaphtolSnCl4
toluene , -78
oC, 1 h
>95%, 96%ee
Ph
TBSO O
Ph
Yamamoto, H. JACS. 1994, 116, 11179.
HOOH
SnCl
ClClCl
HOOSiR3
SnCl
ClClCl
R
OR
SiR3
R
O
R H
+
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Yamamoto, H. JACS. 2003, 125, 24.
TMSO
TMSO
HO OMe
SnCl4 HO
O
F3C F3C
CF3F3C
toluene, -78 oC, >95%, 90%ee
Investigation of Mechanism
Linear π*- O-H bond interaction
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PhCHO
OH
BO
O
Ph
HO OH
85%, 78%ee
HO OH
99%, 95%ee
10% SnCl4 Na2CO3
toluene, -78 oC
ACIE. 2006, 45, 2426. Title paperVivol ligand
Background reaction
2% of product was observed -78 oC after 5 h
A maximum ee wuold be approximately 96%
Digest of Title Paper
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The enantioselectivity shown here is superior tothat of the most popular stoichimetric reagents.
Substrate Scope of Vivol Ligand
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Investigation of Mechanism
Truly Yamamoto’s LBA system?
PhCHO
OH
BO
O
Ph
MeO OMe
10% SnCl4 Na2CO3
toluene, -78 oC
HO OMe HO OH
4 h <10% conversion, racemic 3 h 50% conversion, 85%ee 4h 99%, 95%ee
HO OH HO OBn
12 h, 37%ee 12 h, 62%ee
ACIE. 2006, 45, 2426.
Yamamoto reportedmono-protected diol ligandsgave higher ee.
Yamamoto, H. JACS. 2000, 122, 8120.
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Farther Investigation of Mechanism
X-ray
Stacked structure
Activate protons are pointing pseudoequatirial
Rigid position
Hold chiral information (?)
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Conclusion
Vivol ligand – SnCl4 system is
- efficient catalyst for enantioselective allyl and crotylboration of aliphatic aldehyde.
- superior to well-established stoichiometric allylboration methods.
- hard to know the precise mechanism.
Akira Nakamura @ Wipf Group Page 15 of 15 7/3/2008
