Hydrometallation

1
Hydrometallation
BH3 (borane) ? 2s22p1 electrons in 2s2px2py2pz
orbitals ? electron deficient compound (Lewis
acid) ? ready complexation with THF, Me2S, or
NR3 ? electronegativity 2.0 lt H (2.1) ? CB
bond length 1.57 Å (90 covalent) the carbon
has no nucleophilicity ? BO or BN bond
highly polar to be hydrolyzed ? BO
(1.36?1.47Å)bond length much shorter than
Li?O (1.92?2.00Å)
1.33 Å
dimer sp3 three-center, two-electron bond
1.19 Å
2
hydroboration
p-complex
syn
fourcenter transition state
Ionic Associative Mechanism for Borane
Transformations

vacant p-orbital


R stereochemistry retention
3
Borane Reagents
crystalline easy to handle
borane reagent for asymmetric synthesis
dissociation of B-C bond
important reactivity of borane

dissociative hydride transfer
4
HydrosilationOxidation Olefin Oxygenation
Method Developed by Prof. K. Tamao
For the route A the more the R' on silicon
becomes electrondonating, the more the reaction
become feasible.
For the route B the more the R' on silicon
becomes electronwithdrawing (electronegative),
the more the reaction become feasible.
Typical example for route A Hosomi-Sakurai
reaction
Typical example for route B Tamao reaction
5
Conceptually Possible Mechanism for Oxidative
Cleavage of SiC Bonds
O
R?"Si "
realization
H2O2
R?SiX3
R?OH
SiX3 hydro, fluoro, chloro, amino,
alkoxysilyl
retention of configuration at sp3carbon
Si SiF52, SiF3, SiMe(OEt)2, Si(OEt)3
6
oxidative cleavage examples
7
H
?
H
8
30 H2O2/NaHCO3/MeOH/THF or mCPBA/KHF2/DMF
mCPBA (1 eq)/CH2Cl2/0?/5 h
30 H2O2/KHF2/KHCO3/MeOH /THF/rt, 3 h
F
Base F
H2O
9
Diastereoselectivity for Hydrosilation of
Bis(2-propenyl)methanol
H2PtCl66H2O (0.1 mol) 20 ?/1 h
30 H2O2/NaHCO3/MeOH-THF/ 60 ?, 1248 h
H2PtCl66H2O (0.1 mol) 20 ?/1 h
30 H2O2/KF/KHCO3 MeOH-THF/rt, 10 h
10
Enantiosynthesis of Prelog-Djerassi Lactone
P.-D. lactone
11
compensative
Hydrosilation vs Hydroboration

syn
anti
major
major
minor
inside
outside
Pt
gt stable
outside
Pt
inside
anti
anti
Hydrosilation
Hydroboration

12
hydroalumination
Two unavoidable elementary reactions
1) Dissociation
2) Displacement
13
hydroalumination
Terminal acetylenes complicated by (1)
substitution of the methine hydrogen or other
heterosubstituents (Br, SnR3) and (2)
carbometallation
Internal acetylenes resulted in non-selective
stereochemical and/or regiochemical outcomes
Hydroalumination with hydroaluminates
14
Stereoelectronic Effect
Bicyclic orthoester A leads to only hydroxy ester
B on treatment with acid and never affords
bicyclic lactone C Deslongchamps, 1975
Mechanism
H2O -EtOH
H

15
Anomeric Effect
a-D-Glucopyranose 36
b-D-Glucopyranose 64
DG RTlnK 0.346 kcal/mol
(Ha-Oa) 2 0.45 2 0.9 kcal/mol
0.9 0.346 0.554 kcal/mol anomeric effect
(a) much more favorable orbital overlapping for
antiperiplanar (a) than for synclinal (b)
(b) banomer should be much more destabilized in
terms of dipolar-dipolar interaction
Nevertheless, why the banomer becomes more
stable than the aanomer ?
16
OR-axial 0.9
cis 57
trans 43
(80 ?)
0.17 kcal/mol stable
one gauche interaction 0.8
one gauche interaction 0.8 OR-axial 0.9 DS
0.42
DS 0.42
anomeric effect 0.42 0.8 0.9 0.17
anomeric effect 1.45 kcal/mol
17
Spiroacetalization of 5-Oxanonane-1,9-diol (D)
A two anomeric effect 1.45 2 2.9
B one anomeric effect 1.45
C no anomeric effect
D
1,3-diaxial interact. (Oa-Ha) x 2 x 2 0.45 x 2
x 2 1.8
1,3-diaxial interact. (Oa-Ha) x 2 0.45 x 2
0.9 (Ha-(CH3)a) x 2 0.9 x 2 1.8 1.8 0.9
2.7
1,3-diaxial interact. (Ha-(CH3)a) x 2 x 2 0.9
x 2 x 2 3.6
Magnitudes of non-bonded Interactions
(kcal/mol)
Ha-(CH3)a 0.90 Oa-Oa 1.5 Oa-(CH3)a
2.5 O1-O2 0.35 O1-(CH3)2 0.45
Energy difference between A and C DGA DGC
2.9 (3.6 1.8) 4.7 kcal/mol
Energy difference between A and B DGA DGB
1.45 (2.7 1.8) 2.35 kcal/mol
18
Stereoelectronic Effect on Acidity
orthogonal
kb / ka 5 x 105
parallel
19
Stereoelectronic Requirement for E2
Stereospecific (sterecenters)
Stereoelectronic Requirement for SN2
HOMO
LUMO
inversion
Linear T.S. sp2
Stereospecific (sterecenters)
20
SN2 Opportunities (1)
no intramolecular reaction
only intermolecular proces is allowed

Nucleophilic Ring Opening of Epoxides (1)
180
21
Nucleophilic Ring Opening of Epoxides (2)
LiAlH4
22
Stereoelectronic Requirement for Enolization
base
Br2
equatorial "H" easy to enolize
NaOEt
axial "H" difficult to enolize
Br2
23
Trajectory of Nucleophile Attacking onto CY
Double Bond
109
sp3
Base
TsOH
H
NaOMe
N.R.
24
Stereoelectronic requirement for
effective neighboring group participation
Opportunities (1)
Antiperiplanar arrangements
25
Opportunities (2)
Fragmentation reaction
P. A. Wender et al., JACS (1997)
"antiperiplanar (CaCCX)"
MCPBA
1. DABCO 2. TIPS-Cl
26
Y. Kita et al., JOC (1997)
BF3
BF3
BF3
BF3