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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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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


hydroboration

π-complex

syn

four–center

transition state

Ionic Associative Mechanism for Borane Transformations

+

vacant p-orbital

+

+

R* stereochemistry:

retention


Borane Reagents

crystalline: easy to handle

borane reagent

for

asymmetric

synthesis

dissociation

of B-C bond

important reactivity

of borane

+

dissociative hydride transfer


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


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]



H+

?

H+


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


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


Enantiosynthesis of Prelog-Djerassi Lactone Bis(2-propenyl)methanol

P.-D. lactone


compensative Bis(2-propenyl)methanol

Hydrosilation vs Hydroboration

+

syn

anti

major

major

minor

inside

outside

Pt

>

stable

outside

Pt

inside

anti

anti

Hydrosilation

Hydroboration

+


hydroalumination Bis(2-propenyl)methanol

Two unavoidable elementary reactions:

1) Dissociation:

2) Displacement:


hydroalumination Bis(2-propenyl)methanol

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


Stereoelectronic Effect Bis(2-propenyl)methanol

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+

+


Anomeric Effect Bis(2-propenyl)methanol

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 ?


OR-axial: 0.9 Bis(2-propenyl)methanol

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


Spiroacetalization of 5-Oxanonane-1,9-diol (D) Bis(2-propenyl)methanol

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


Stereoelectronic Effect on Acidity Bis(2-propenyl)methanol

orthogonal

kb/ ka= 5 x 105

parallel


Stereoelectronic Requirement for E2 Bis(2-propenyl)methanol

Stereospecific

(sterecenters)

Stereoelectronic Requirement for SN2

HOMO

LUMO

Linear T.S.: sp2

inversion

Stereospecific

(sterecenters)


S Bis(2-propenyl)methanolN2 Opportunities (1)

no

intramolecular

reaction

only

intermolecular

proces is

allowed

+

Nucleophilic Ring Opening of Epoxides (1)

180°


Nucleophilic Ring Opening of Epoxides (2) Bis(2-propenyl)methanol

LiAlH4


Stereoelectronic Requirement for Enolization Bis(2-propenyl)methanol

base

Br2

equatorial "H"

easy to enolize

NaOEt

axial "H"

difficult to enolize

Br2


Trajectory of Nucleophile Attacking onto C=Y Double Bond Bis(2-propenyl)methanol

109°

sp3

Base

TsOH

H+

NaOMe

N.R.


Stereoelectronic requirement for effective Bis(2-propenyl)methanol

neighboring group participation

Opportunities (1)

Antiperiplanar arrangements


Opportunities (2) Bis(2-propenyl)methanol

Fragmentation reaction

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

"anti–periplanar

(Ca––C—C—X)"

MCPBA

1. DABCO

2. TIPS-Cl


Y. Kita et al., Bis(2-propenyl)methanolJOC (1997)

BF3

BF3

BF3

BF3


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