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Hydrometallation. BH 3 (borane) ◆ 2s 2 2p 1 electrons in 2s 2 p x 2 p y 2p z orbitals ◆ electron deficient compound (Lewis acid) ◆ ready complexation with THF, Me 2 S, or NR 3 ◆ electronegativity: 2.0 < H (2.1) ◆ C––B bond length = 1.57 Å (90% covalent);

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Presentation Transcript
slide1

Hydrometallation

BH3 (borane)

◆2s22p1 electrons in 2s2px2py2pzorbitals

◆electron deficient compound (Lewis acid)

◆ready complexation with THF, Me2S, or NR3

◆electronegativity: 2.0 < H (2.1)

◆C––B bond length = 1.57 Å(90% covalent);

the carbon has no nucleophilicity

◆B––O or B––N bond: highly polar to be hydrolyzed

◆B––O (1.36ー1.47Å)bond length: much shorter than

LiーO (1.92ー2.00Å)

1.33 Å

dimer = sp3

three-center,

two-electron

bond

1.19 Å

slide2

hydroboration

π-complex

syn

four–center

transition state

Ionic Associative Mechanism for Borane Transformations

+

vacant p-orbital

+

+

R* stereochemistry:

retention

slide3

Borane Reagents

crystalline: easy to handle

borane reagent

for

asymmetric

synthesis

dissociation

of B-C bond

important reactivity

of borane

+

dissociative hydride transfer

slide4

Hydrosilation–Oxidation:

Olefin Oxygenation Method Developed by Prof. K. Tamao

For the route A: the more the R\' on silicon becomes electron–donating, the more

the reaction become feasible.

For the route B: the more the R\' on silicon becomes electron–withdrawing

(electronegative), the more the reaction become feasible.

Typical example for route A: Hosomi-Sakurai reaction

Typical example for route B: Tamao reaction

slide5

Conceptually Possible Mechanism for Oxidative

Cleavage of Si––C Bonds

[O]

Rー"Si "

realization

H2O2

RーSiX3

RーOH

[SiX3 = hydro–, fluoro–, chloro–,

amino–, alkoxy–silyl]

retention of configuration at sp3–carbon

[Si = SiF52–, SiF3, SiMe(OEt)2, Si(OEt)3]

slide7

H+

?

H+

slide8

30% H2O2/NaHCO3/MeOH/THFor

mCPBA/KHF2/DMF

mCPBA (1 eq)/CH2Cl2/0℃/5 h

30% H2O2/KHF2/KHCO3/MeOH

/THF/rt, 3 h

F–

Base: F–

H2O

slide9

Diastereoselectivity for Hydrosilation of Bis(2-propenyl)methanol

H2PtCl6・6H2O

(0.1 mol%)

20 ℃/1 h

30% H2O2/NaHCO3/MeOH-THF/

60 ℃, 12~48 h

H2PtCl6・6H2O

(0.1 mol%)

20 ℃/1 h

30% H2O2/KF/KHCO3

MeOH-THF/rt, 10 h

slide11

compensative

Hydrosilation vs Hydroboration

+

syn

anti

major

major

minor

inside

outside

Pt

>

stable

outside

Pt

inside

anti

anti

Hydrosilation

Hydroboration

+

slide12

hydroalumination

Two unavoidable elementary reactions:

1) Dissociation:

2) Displacement:

slide13

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

slide14

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+

+

slide15

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 anti–periplanar (a)

than for synclinal (b)

(b)b–anomer should be much more destabilized in terms of dipolar-dipolar

interaction

Nevertheless, why the b–anomer becomes more stable than the a–anomer ?

slide16

OR-axial: 0.9

cis : 57%

(80 ℃)

trans : 43%

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

slide17

Spiroacetalization of 5-Oxanonane-1,9-diol (D)

C

no anomeric effect

A

two anomeric effect

= 1.45 × 2 = 2.9

B

one anomeric effect

= 1.45

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

slide18

Stereoelectronic Effect on Acidity

orthogonal

kb/ ka= 5 x 105

parallel

slide19

Stereoelectronic Requirement for E2

Stereospecific

(sterecenters)

Stereoelectronic Requirement for SN2

HOMO

LUMO

Linear T.S.: sp2

inversion

Stereospecific

(sterecenters)

slide20

SN2 Opportunities (1)

no

intramolecular

reaction

only

intermolecular

proces is

allowed

+

Nucleophilic Ring Opening of Epoxides (1)

180°

slide22

Stereoelectronic Requirement for Enolization

base

Br2

equatorial "H"

easy to enolize

NaOEt

axial "H"

difficult to enolize

Br2

slide24

Stereoelectronic requirement for effective

neighboring group participation

Opportunities (1)

Antiperiplanar arrangements

slide25

Opportunities (2)

Fragmentation reaction

P. A. Wender et al., JACS (1997)

"anti–periplanar

(Ca––C—C—X)"

MCPBA

1. DABCO

2. TIPS-Cl

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