Chemistry and Biochemistry Catalytic Asymmetric Synthesis · 2017. 8. 4. · NPTEL – Chemistry...
Transcript of Chemistry and Biochemistry Catalytic Asymmetric Synthesis · 2017. 8. 4. · NPTEL – Chemistry...
NPTEL – Chemistry and Biochemistry – Catalytic Asymmetric Synthesis
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Module 8 Asymmetric Hydrosilylation and
Related Reactions
Lecture 28
8.1 Hydrosilylation of Alkenes I
Asymmetric hydrosilylation and hydroboration of carbon-carbon double bonds
followed by oxidative cleavage of the C-Si and C-B bonds give effective
methods for the construction of optically active alcohols (Scheme 1).
C C H M C
C
H M
C
C
H OH
[O]Chiral cat
M = Si, B, Al, Sn
Ojima, I. In The Chemistry of Organic Silicon Compounds (Eds. Patai,S.; Rappoport, Z.).Chichester: John Wiley, 1989; p. 1479.
Scheme 1
Asymmetric hydrosilylation of carbon-carbon unsaturated substrates provides
effective methods for the synthesis of optically active organosilanes, which are
versatile intermediates in organic synthesis. Chiral alkyl and aryl silanes can be
converted into optically active alcohols with retention configuration by
oxidative cleavage of a carbon-silicon bond into carbon-oxygen bond, while the
diastereoselective reaction of chiral allyl- and allenyl silanes with C=O bond
can give homoallylic and homopropargylic alcohols.
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8.1.1 Reactions of Styrene and its Derivatives
Chiral Palladium-catalyzed asymmetric hydrosilylation of styrene with
trichlorosilane has been extensively studied. The reaction proceeds with
excellent regioselectivity to give 1-phenyl-1-silylethane via -benzyl
palladium intermediate (Scheme 2).
PhHSiCl3 Ph
SiCl3
Me Ph
OH
Me
PdSiCl3L*
[O]Pd/L*
Ojima, I. In The Chemistry of Organic Silicon Compounds (Eds. Patai,S.; Rappoport, Z.).Chichester: John Wiley, 1989; p. 1479.
Scheme 2
Scheme 3 illustrates the possible mechanism. Deuterium-labeling studies
suggest that the -hydrogen elimination is found to be much faster compared to
the reductive elimination from the intermediate II. The involvement
hydropalladation in the catalytic cycle has been revealed by the side product
analysis from the reaction of o-allylstyrene.
HSiCl3Ph
SiCl3
Pd L*
Ph
Pd
H
L*
SiCl3PdL*
SiCl3
H
Ph
Reductive elimination Oxidative addition
Hydropalladation
-Hydrogen elimination
Ph
IIIUozumi, Y. et al. J. Org. Chem. 1998, 63, 6137
Scheme 3
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The reaction has been utilized in the synthesis of 1-aryl-1,2-diols from
arylacetylenes (Scheme 4). Platinum-catalyzed hydrosilylation of arylacetylene
gives (E)-1-aryl-2-(trichlorosilyl)ethanes that could be further reacted with
trichlorosilane in the presence of chiral palladium complex to afford optically
active 1-aryl-1,2-bis(trichlorosilyl)ethanes. The latter could be transformed into
optically active 1,2-diol via oxidative cleavage of the carbon-silicon bond into
carbon oxygen bond.
H
P
F3C
F3C2
L* =
Ar H ArSiCl3
ArSiCl3
SiCl3
ArOH
OH
HSiCl3
[PdCl2(C2H4)]2(0.01 mol %)
20°C, 24 h
[PdCl(-C3H5)]2L* (0.3 mol %)
20°C, 48 h(excess)
H2O2, KF, KHCO3
Shimada, T. et al., J. Am. Chem. Soc. 2002, 124, 1584
OH
OH
OH
OH
Me
OH
OH
F3C
OH
OH
O2N
95% ee (R) 95% ee (R)
96% ee (R) 98% ee (R)
Examples:
(R)
Scheme 4
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Other chiral catalysts have also been employed for the asymmetric
hydrosilylation of alkenes. The chiral bis(oxazolinyl)phenylrhodium complex
catalyzes the asymmetric hydrosilylation of styrenes with hydro(alkoxy)silanes
in high enantioselectivity, although the regioselectivity is found to be somewhat
moderate (Scheme 5).
Ar Ar
SiMe(OEt)2
ArSiMe(OEt)2
Ar
OH
ArOH
Rh
N
O
O
N
i-Pr
i-Pr
OH2
Cl
Cl
Rh-phebox cat.(1 mol %)
Toluene
H2O2KF, KHCO3
Rh(phebox-ip)Cl2(H2O)
HSiMe(OEt)2
Tsuchiya, Y. et al., Synlett 2004, 2099.
OH OH
MeO
OH
Cl
89% yield95% ee (S)
92% yield92% ee (S)
92% yield95% ee (S)
Examples:
Scheme 5
-Substituted styrenes proceed reaction with phenylsilane to afford benzylic
tert-alkylsilanes in the presence chiral organolanthanide as catalyst in moderate
enantioselectivity (Scheme 6).
Ln
Si(Me3Si)2HCSm R* =
PhSiH3
SmLn* (70% de), 0.5 mol %
23°C
68% ee
SiH2Ph
SmLn*
Molander, G. A. et al., Organometallics 1998, 17, 3754.
Scheme 6
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8.1.2 Reactions of 1,3-Dienes
The reaction of 1,3-dienes with hydrosilanes having electron-withdrawing
groups on silicon affords synthetically useful optically active silanes in the
presence of chiral palladium complex (Scheme 7). The reaction proceeds in a
1,4-fashion providing chiral allylsilanes that could be converted into
homoallylic alcohols on the reaction with aldehydes.
SiRCl2
Ph
HOPhCHO/DMF
0°CHSiRCl2
Pd/L*
Masse, C. E. et al., Chem. Rev. 1995, 95, 1293.
Scheme 7
The use of ferrocenylphosphine and mop-phen ligands has been demonstrated
for the hydrosilylation of cyclo-1,3-hexadiene in the presence of palladium salts
(Scheme 8). The reaction with phenyldifluorosilane afforded the highest
enantioselectivity compared to that with trichlorsilane or methyldichlorosilane.
Based on the reaction of with deuterium-labeled sil -
allylpalladium intermediate and 1,4-cis-addition has been proposed.
SiR3
Pd/L*
(R)-(S)-ppfa: Y = NMe2
(R)-(S)-ppfOAc: Y =OAc(R)-(S)-ppfOMe: Y = OMe
Pd
F2PhSi L*
DDF2PhSi
Pd/(R)-mop-phen 0.1 mol %
DSiPhF2, 20°C
FeY
H Me
PPh2
HSiMeCl2HSiCl3HSiF2PhHSiF2PhHSiCl3 MeO
PPh2
(R)-mop-phen
Hiyama, T. et al., Organometallics 1996, 15, 5762.
HSiR3
Ligand HSiR3 Yield (%) ee (%)
(R)-(S)-ppfa (R)-(S)-ppfOAc(R)-(S)-ppfOAc (R)-(S)-ppfOMe (R)-mop-phen
9544585080
2 (S)38 (S)77 (S)54 (S)72 (S)
Scheme 8
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In case of linear 1,3-dienes, the regioselectivity has become an issue. In the
reaction of 1-phenyl-1,3-butadiene using ferrocenyl ligand, (R)-(S)-ppfa, the
formation of a mixture of regioisomeric allylsilanes is observed (Scheme 9).
However, in the reaction of alkyl substituted 1,3-dienes, 1,3-hexadiene and 1,3-
decadiene, a single regioisomer is obtained with moderate enantioselectivity.
Improvement in the enantioselectivity is observed employing the
bis(ferrocenyl)monophophine ligands a-d having two planar chiral ferrocenyl
moieties on phosphorus atom.
Pd/L*
Fe
P
Ph Ph
SiCl3 SiCl3
Ph
R
HSiCl3
a: Ar = 4-MeOC6H4b: Ar = Phc: Ar = 4-CF3C6H4d: Ar = 3,4-(CF3)2C6H3
(S)-(R)-bippfOMe
OMe
FeAr
MeO
HSiCl3
Pd/L* R
SiCl3
Hatanaka, Y. et al., Tetrahedron Lett. 1994, 35, 7981
FeNMe2
H Me
PPh2
94% (64% ee) 6% (30% ee)(R)-(S)-ppfa
L*
Ligand Yield (%) ee (%)
L*
a
b
c
d
62
43
78
89
91
64
68
76
78
87
R = Et, n-C6H13
Scheme 9
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Problems
Write the major products for the following reactions.
1. Ph + HSiCl3
[PtCl2(C2H4)2]2
2. + HSiRCl2
PdLn*
3. HSiCl3+ PdLn*
SiCl3
KF/NBS4.
SiCl3
KF/MCPBA5,
Reference/Text Book
I. Ojima, Catalytic Asymmetric Synthesis, 3rd
ed., Wiley, New Jersey, 2010.
M. B. Smith, Organic Synthesis, 2nd
edition, McGraw Hill, New Delhi, 2004.
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Lecture 29
8.1 Hydrosilylation of Alkenes II
8.1.2 Reactions of 1,3-Dienes
The reaction of 1-buten-3-ynes substituted with bulky groups at the alkyne
terminus affords enantiomerically enriched allenylsilanes in the presence of
palladium complex (Scheme 1). For example, the reaction of 5,5-dimethyl-1-
hexen-3-yne using (S)-(R)-bisppfOMea proceeds in a 1,4-fashion to give
allenyl(trichloro)silanes in high regio- and enantioselectivity. Further
enhancement in the enantioselectivity is shown employing chiral
phosphametallocene b having a sterically demanding η5-C5Me5 moiety.
Fe
P
(S)-(R)-bisppfOMe (a)
OMe
FeAr
Ph
HSiCl3t-Bu •
H
MeCl3Si
t-Bu
P
Fe
R**R
OMeR* =
L* = a: 90% ee at 0°C b: 92% ee at 0°C
[PdCl(-C3H5)]2/L*(1 mol % Pd)
b
Han. J. W. et al., J. Am. Chem. Soc. 2001, 123, 12915.
•
H
MeCl3Si
Me
Me
Me •
H
MeCl3Si
t-BuMe2Si
L* = a: 77% ee at 20°C 68% ee at 20°C
Examples:(S)
Scheme 1
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8.1.3 Reactions of Alkyl Substituted Alkenes
Hydrosilylation of simple terminal alkenes give branched products with high
regioselectivity. The palladium systems show exceptional catalytic system
compared to Pt, Ni and Rh based systems. For example, the hydrosilylation of
1-octene with trichlorosilane using palladium-(S)—MeO-mop gives a 93:7
mixture of 1-octylsilane and 2-octylsilane with 95% ee (Scheme 2).
The above catalytic system is also effective for the hydrosilylation of cyclic
alkenes, such as norbornene and bicyclo[2.2.2]octane, 2,5-dihydrofuran and
norbornadiene. For example, the reaction of norbornene gives exo adduct
exclusively (Scheme 3). The hydrosilylated product can be transformed into
exo-2-norbornanol or endo-2-bromonorbornane via the corresponding
pentafluorosilicate. In addition, chiral ferrocenylmonophosphines a-d are too
found to be effective for this process with excellent enantioselectivity.
R R
SiCl3
RCl3Si
Ph2P
OMe
R
OH
Pd/(S)-MeO-mop
40°C
>90% yield
1) EtOH/Et3N2) H2O2, KF/KHCO3
(S)-MeO-mop
93:7
HSiCl3
95% ee
R = n-C6H13
Uozumi, Y. et al., J. Am. Chem. Soc. 1991, 113, 9887.
n-C4H9
OH
n-C10H21
OH
OH
Ph
OH
70% yield, 94% ee 75% yield, 95% ee
68% yield, 97% ee 45% yield, 96% ee
Examples:
Scheme 2
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H
SiCl3
H
SiF5K2
H
OH
Br
H
Me
N
NFe PAr2
a: R = MeS, Ar = Phb: R = 9-Anthryl, Ar = Phc: R = 2,4,6-(MeO)3C6H4, Ar = Phd: R = MeS, Ar =3,5-(CF3)2C6H4
(1R, 2R, 4S)
KFPd/L*HSiCl3
MCPBA
or NBS
or
Uozumi, Y. et al., Tetrahedron Lett. 1992, 33, 7185.
R
Ligand
(R)-Meo-mop
a
b
c
d
Yield (%) ee (%)
99
56
54
30
59
96
91
81
92
99.5
Scheme 3
Chiral yttrium hydride complex (d0
metal complex) bearing non-Cp ligand
catalyzes the hydrosilylation of norbornene with phenylsilane to produce exo-
adduct with 90% ee (Scheme 4). More recently, the first chirality transfer from
silicon to carbon in a reagent-controlled reaction of norbornene is reported in
the presence of achiral palladium complex. The hydrosilylation of norbornene
with chiral silane A having 85% ee is found to form the hydrosilylated product
B with 93% ee exhibiting asymmetric amplification (Scheme 5).
N
N
Y
SiR3
SiR3
Me
OC4H8
OC4H8N
N
Y
SiR3
SiR3
OC4H8
C4H8ON
N
Y
R3Si
R3Si
H
H
H
SiH2PhPhSiH3
YLn* (3 mol %)rt, 48 h,
100% yield
SiR3 = SiMe2Bu-t
90% ee
PhSiH2CH3
PhSiH3
Gountchev, T. I. et al., Organometallics 1999, 18, 5661.
SiH2Ph Ph SiH2Ph
92% yield 76% yield
Examples:
YLn*
Scheme 4
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SiH
SiN
NCF3
CF3B 4
PdCH3
OEt2
A 85% ee
PdLn: [Pd(Me)(phen)(OEt2)]+[BAr4]-
PdLn
Oestreich, M. et al., Angew. Chem. Int. Ed. 2005, 44, 1661.
Si Si
B 93% ee
65% ee not isolated(no reaction)
Examples:
Scheme 5
8.1.4 Intramolecular Hydrosilylation
Synthesis of optically active polyols from allylic alcohols can be achieved using
chiral Rh-catalyzed intramolecular hydrosilylation followed by oxidation
ofallyloxy hydrosilanes (Scheme 6). For example, hydrosilyl ether of di(2-
propenyl)methanol can be converted into optically active 1,3-diol using
intramolecular hydrosilylation in the presence of chiral rhodium-(R,R)-diop
followed by oxidation. Rh-BINAP is also found to be effective catalyst for the
intramolecular hydrosilylation of hydrosilyl ethers of allyl alcohols.
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OSiR2H
O SiR2 OH OH
PPh2
PPh2
OSi
Ph
H
Ph
R2Si O
Ph
OH OH
Rh/(R,R)-diop(2 mol %)
30°C
Rh/(S)-binap(2 mol %)
25°C
Up to 93% ee
97% ee
PPh2
PPh2O
O
H
H
(R,R)-diop
(S)-BINAP
Bergens, S. E. et al., J. Am. Chem. Soc. 1992, 114, 2121
75% Yield96% ee
100% yield90% ee
75% yield96% ee
90% yield97% ee
Examples:
OSi
H
Ph OSi
HPh
OSi
H
Ph
OSi
H
OMe
MeO
OSi
H
75% yield94% ee
Tamao, K. et al., Tetrahedron Lett. 1990, 31, 7333.
Scheme 6
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t-BuO2C
t-BuO2CN
NO
i-Pr
PdMe
ClCH3
t-BuO2C
t-BuO2C
SiEt3PdLn*, NaBAr4
PdLn*
-40°CHSiEt3
Perch, N. S. et al., J. Am. Chem. Soc. 1999, 121, 6960.
98% de, 90% ee
CH3
BnO2C
BnO2C
SiEt3
95% de, 86% ee
CH3
i-PrO2C
i-PrO2C
SiEt3
CH3
BnO
BnO
SiEt3
CH3
MeOC
MeOC
SiEt3
CH3
MeO2C
MeO2S
SiEt3
CH3
MeO2C
Ph
SiEt3
98% de, 85% ee 95% de, 79% ee
98% de, 86% ee 44% de, 82% ee 47% de, 89% ee
Examples:
Scheme 7
8.1.5 Cyclization/Hydrosilylation
Asymmetric cyclization and hydrosilylation of ,ω-diunsaturated compounds
such as 1,6-dienes and 1,6-enynes affords powerful tool for the construction of
optically active functionalized carbocycles. For example, the tandem reaction of
diallylmalonate in the presence of cationic Pd complex bearing a chiral
pyridine-oxazoline proceeds with high diastereoselectivity to yield the
corresponding trans-substituted cyclopentane with 90% ee (Scheme 7).
The reactions of 1,6-diynes using cationic Rh complexes bearing chiral
bisphosphine gives the hydrosilylated alkylidenecyclopentanes with high
enantioselectivity. For example, the 1,6-enyne proceeds reaction with
triethylsilane in the presence of cationic Rh and (R)-biphemp to give
hydrosilylated alkylidene cyclopentane in 92% ee (Scheme 8). Subsequently,
chiral Rh complex containing spiro diphosphine (R)-sdp is found to be effective
for this process.
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MeMeO2C
MeO2C
PPh2
PPh2
Me
Me
SiEt3
Me
MeO2C
MeO2C
[Rh(cod)2]SbF6biphemp (5 mol %)
70°C
92% ee(R)-biphemp
HSiEt3
PPh2
PPh2
N
H
SiEt3Ts
H
NTs
[Rh(cod)2]BF4sdp (5 mol %)
(R)-sdp
HSiEt370°C
Fan, B-M. et al., Angew. Chem. Int. Ed. 2007, 46, 1275.
98% ee
N
H
SiEt3PhSO2
59% yield, 92% ee
N
H
SiEt3Ns
67% yield, 95% ee
N
H
SiEt3Ms
56% yield, 92% ee
SiEt3
Me
EtO2C
EtO2C
48% yield, 95% ee
Examples:
Scheme 8
The synthesis of carbocycles can also be accomplished by the cyclization of ω-
formyl-1,3-dienes in the presence of hydrosilanes and chiral nickel complex
(Scheme 9). For example, zerovalent nickel complex of (2R,5R)-2,5-dimethyl-
1-phenylphospholane catalyzes the cyclization of 1,3-dienes with a tethered
formyl group in the presence of triethoxysilane to give five-membered
carbocycle with 73% ee.
CHOMeO2C
MeO2C
P Ph
OSi(OEt)3MeO2C
MeO2C
OSi(OEt)3MeO2C
MeO2C
Ni(cod)2(R,R)-L*
(10 mol %)
-30°C
73% ee
HSi(OEt)3
(R,R)-L*
Sato, Y. et al., J. Am. Chem. Soc. 2000, 122, 2371.
OSi(OEt)3O
O
80% yield, 64% ee
TsN
OSi(OEt)3
60% yield, 48% ee
Examples:
Scheme 9
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Problems
Complete the following reactions.
1. + HSiEt3
MeO2C
MeO2C
RhLn*
2. CHOMeO2C
MeO2C+ HSi(OEt)3
NiLn*
3. + HSiEt3
MeO2C
MeO2C
PdLn*
Reference/Text Book
I. Ojima, Catalytic Asymmetric Synthesis, 3rd
ed., Wiley, New Jersey, 2010.
M. B. Smith, Organic Synthesis, 2nd
edition, McGraw Hill, New Delhi, 2004.
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Lecture 30
8.3 Hydroboration, Hydroalumination and Hydrostannation of
Alkenes
8.3.1 Hydroboration of Alkenes
Chiral Rh catalyzed hydroboration of alkenes provides effective method for the
synthesis of optically active organoboranes, which are versatile intermediates in
organic synthesis. The carbon-boron bond can be converted into several
functional group by subsequent carbon-carbon, carbon-oxygen, boron-carbon or
carbon-nitrogen bond-forming reactions with retention of stereochemistry
(Scheme 1).
ArO
BO
Ar
OB
O
H
OH
Ar
CO2H
Ar
CH2OH
Ar
NH2
Ar
chiral Rh cat.
Carroll, A.-M. et al., Adv. Synth. Catal. 2005, 347, 609.Fernandez, E. et al., Chem. Eur. J. 2000, 6, 1840.
Scheme 1
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t-Bu
Me
OB
OH
H
OH
t-Bu
Me
OH
PPh2
PPh2O
O
H
H
(R,R)-diop1) [RhCl(cod)]2, L*, -25°C
2) H2O2, NaOH
L* =
Burgess, K. et al., J. Org. Chem. 1988, 53, 5178.
1) [RhCl(cod)]2, L*, -25°C
2) H2O2, NaOH
OB
OH
>99% yield 57% ee
>99% yield 69% ee
HO
OHMe
Ph
27% ee 76% ee
Examples:
Scheme 2
The first catalytic asymmetric hydroboration of norbornene and 2-tert-
butylpropene with catecholborane appeared in the presence of Rh-(R,R)-diop
complex (Scheme 2). The products, 2-hydroxynorbornane and 2,3,3-
trimethylbutanol are obtained after the treatment with alkaline hydrogen
peroxide solution.
The use of the combination of chiral borane and achiral catalyst has been
demonstrated for the asymmetric hydroboration. For example, the
hydroboration of 4-methoxystyrene proceeds with chiral borane derived from
pseudoephedrine in the presence of achiral rhodium complex to the
corresponding secondary alcohol with 76% ee after the oxidation (Scheme 3).
MeO
+ OBH
NMe
MePh
i. [Rh(nbd)/(dppf]OTf
ii. H2O2, NaOH MeO
OH
+
MeO
OH
76% ee
+
MeO
82% 14% 4%
J. M. Brown, G.C. Lloyd-Jones, Tetrahedron: Asymmetry 1990, 1, 869.
Scheme 3
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The reaction of vinylarenes with catecholborane has been extensively studied
using chiral Rh complex. For example, the cationic Rh-(R)-BIANP catalyzes
the hydroboration of styrene with complete branch selectivity to afford 1-
phenylethanol with 96% ee after oxidation. The regioselectivity is opposite to
that observed with uncatalyzed reactions (Scheme 4).
Ph
OB
OH
PPh2
PPh2Ph
OH1) Rh(cod)2]BF4, (R)-BINAP
2) H2O2, NaOH
94% yield96% ee
(R)-BINAP
Hayashi, T. et al., J. Am. Chem. Soc. 1989, 111, 3426.
OH
Me
OH
Cl
OH
Cl
OH
MeO
OH
77% yield94% ee
98% yield91% ee
99% yield85% ee
84% yield82% ee
74% yield85% ee
Examples:
OMe
Scheme 4 Asymmetric desymmetrization of meso-bicyclic hydrazines has been shown
with catecholborane using chiral Rh and Ir-based complexes (Scheme 5). A
reversal of enantioselectivity is observed between the Rh and Ir catalysts.
NN
CO2Bn
CO2Bn+
OHB
O
i. [MCl(COD)]2 (2 mol%), L*
ii. H2O2, NaOH NN
CO2Bn
CO2Bn
HO
PPh2 PPh2
(S,S)-bdpp
O
OPPh2
PPh2
H
H(S,S)-diop
PPh2
(R,R)-L
L* =
L*
(S,S)-bdpp Rh
(S,S)-bdpp Ir
(S,S)-diop Rh
(S,S)diop Ir
(R,R)-L Ir
M Yield (%) ee (%)
91
30
46
40
76
84
32
54
44
71Luna et al., J. Am. Chem. Soc. 2002, 124, 12098.
Scheme 5
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HMe
MeO2C +O
HBO
[RhCl(COD)]2 (3 mol%), L*H
Me
MeO2C B
O
O
PPh2
PPh2
H
H(S,S)-norphos
PPh2
PPh2
(R)-phanephos
L*
(S,S)-norphos
(R)-phanephos
Yield ee (%)
86% 99% ee (1S, 2R)89% 97% ee (1R, 2S)
Rubina et al., J. Am. Chem. Soc. 2003, 125, 7198.
Scheme 6
PhO
BO
HPh
OH
O
P
OO
O
Ph Ph
PhPh
O Ph
O
P
OO
O
Ph Ph
PhPh
N
Ph
Bn
L* L1*
yield % eeL* 75% 95 (R)L1* 79% 96 (R)
1) [RhCl(nbd)]2 (2 mol % Rh), L*/L1*, DME, rt, 17 h
2) H2O2, NaOH
Moteki, S. A. et al., Org. Lett. 2006, 8, 3097.
Examples:
L*
96% yield
93% ee
96% yield
93% ee
96% yield
93% ee
96% yield
93% ee
96% yield
93% ee
96% yield
93% ee
96% yield
93% ee
96% yield
93% ee
96% yield
93% ee
96% yield
93% ee
Scheme 7
The reaction of cyclopropene is studied with pinacolborane as a new
hydroborating agent in the presence of a series of chiral Rh phosphine
complexes (Scheme 6). The reaction using pinacolborane showed enhanced
selectivity compared to that with catecholborane due to steric control between
the substrates and the hydroborating agent.
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Rh complexes with chiral monodentate phosphate and phosphoramidite derived
from taddol are studied for the hydroboration of vinylarenes with pinacolborane
(Scheme 7). The reactions of a series of vinylarenes having electron
withdrawing- and donating substituted proceed with high enantioselectivity
8.3.2 Hydroalumination and Hydrostannation of Alkenes
OMe
OMeO
OMe
OMe
OH
H Al(Bu-i)2
Ni(COD)2 (14 mol %),(R)-BINAP
Toluene, rt, 1 h
97% yield, 97% ee
PPh2
PPh2
(R)-BINAP
Lautens, M. et al., J. Am. Chem. Soc. 1995, 117, 532.
A B
Scheme 8
O
NH
Ph
HN
O
Ph2PPPh2
Ph
Me H
MeO2C
Me SnMe3
MeO2C HH SnMe3
[RhCl(cod)]2 (3 mol %), L*
THF, -30°C, 45 min
L*
90% yield, 94% ee
Rubina, M. et al., J. Am. Chem. Soc. 2004, 126, 3688.
Me SnMe3
Ph H
SnMe3
Ph H
87% yield90% ee
AcO
88% yield95% ee
SnMe3
p-Cl-Ph H
AcO
SnMe3
TMS H
AcO
83% yield96% ee
73% yield96% ee
SnMe3
TMS H
OO
87% yield94% ee
Examples:
Scheme 9
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While the catalytic asymmetric hydrosilylation and hydroboration reactions are
well known, the catalytic hydroalumination and hydrostannation of alkenes are
rare. Chiral nickel complex is used for the asymmetric hydroalumination of
oxabicyclic alkenes. For example, Ni-(R)-BINAP catalyzes the reaction of A
with iso-Bu2AlH to give B with 97% ee (Scheme 8).
The first example for the asymmetric hydrostannation of cyclopropenes is
appeared using Rh-complex bearing chiral diphenylphosphinobenzoic acid-
derived L* (Scheme 9). The product trans-cyclopropylstannane is obtained
with 94% ee. The procedure is general and the reaction a series of substituted
cyclopropenes is demonstrated.
Problems
A. Predict the major product for the following reactions.
1. SO2Ph +
OHB
O
i. RhLn*
ii.
2. Ph +O
HBO
H2O2/NaOH
RhLn*
Me H
PhH SnMe3
RhLn*3.
B. How will you prepare the following hydroborating agents?
OHB
O
OHB
O
A B
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Reference/Text Book
I. Ojima, Catalytic Asymmetric Synthesis, 3rd
ed., Wiley, New Jersey, 2010.
M. B. Smith, Organic Synthesis, 2nd
edition, McGraw Hill, New Delhi, 2004.