1 / 26

Hydrometallation

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);

kami
Télécharger la présentation

Hydrometallation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 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

  2. hydroboration π-complex syn four–center 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. 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

  5. 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]

  6. oxidative cleavage ––– examples

  7. H+ ? H+

  8. 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

  9. 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

  10. Enantiosynthesis of Prelog-Djerassi Lactone P.-D. lactone

  11. compensative Hydrosilation vs Hydroboration + syn anti major major minor inside outside Pt > 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 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 ?

  16. 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

  17. 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

  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 Linear T.S.: sp2 inversion 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 C=Y 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) "anti–periplanar (Ca––C—C—X)" MCPBA 1. DABCO 2. TIPS-Cl

  26. Y. Kita et al., JOC (1997) BF3 BF3 BF3 BF3

More Related