User:Benjah-bmm27/degree/3/CLW

S, Se, Si, P, CLW

Organosulfur electrophiles and sulfoxides

• Organosulfur compounds, organoselenium chemistry, organophosphorus, organosilicon
• Sulfur electrophiles: sulfenyl chlorides (RSCl) and organodisulfides (RSSR), e.g. diphenyl disulfide
• Sulfoxide elimination
  • Carbonyl group (with α and β hydrogens) + LDA → enolate
  • Enolate + PhSSPh → α-thiophenylcarbonyl
  • α-thiophenylcarbonyl + H₂O₂ → α-sulfinylcarbonyl (a sulfoxide group next to the carbonyl)
  • Sulfoxide + heat → α,β-unsaturated carbonyl (syn elimination of PhSOH)
• Selenoxide elimination
  • Analogous to sulfoxide elimination

Allylic oxidation

• Allylic oxidation using catalytic selenium dioxide: R-CH₂-CH=CH₂ → R-CH(OH)-CH=CH₂
  • Selenium dioxide is hydrated to selenous acid: SeO₂ + H₂O →O=Se(OH)₂
  • O=Se(OH)₂ + R-CH₂-CH=CH₂ (allyl compound) → R-CH=CH-CH₂-Se(OH)₃, a [4+2] cycloaddition like the ene reaction
  • R-CH=CH-CH₂-Se(OH)₃ eliminates H₂O to form the seleninic acid R-CH=CH-CH₂-Se(=O)OH
  • R-CH=CH-CH₂-Se(=O)OH undegoes a a [3,3]-sigmatropic rearrangement to R-CH(O-Se-OH)-CH=CH₂
  • R-CH(O-Se-OH)-CH=CH₂ hydrolyses to an allylic alcohol R-CH(OH)-CH=CH₂ and Se(OH)₂
  • Se(OH)₂ oxidised back to SeO₂ by tert-butyl hydroperoxide, tBuOOH

Julia olefination

• Julia reaction: trans-alkenes from sulfones
  • Sulfone + BuLi → sulfone-stabilised carbanion
  • Carbanion + aldehyde or ketone → alkoxide anion
  • Alkoxide ion + Ac₂O → acetoxy sulfone
  • Acetoxy sulfone + sodium amalgam (Na/Hg) → alkene
    • Reduction proceeds via long-lived anion formed by elimination of the sulfinate PhS(=O)O⁻

Sulfur ylids

• Ketone → epoxide by deprotonating trimethylsulfonium to Me₂S=CH₂
• Johnson-Corey-Chaykovsky reaction: carbonyl → epoxide, by a sulfonium ylide (sodium methylsulfinylmethylide (NaDMSO) and dimethyloxosulfonium methylide)

Swern oxidation

• Swern oxidation: alcohol → aldehyde or ketone, by DMSO, oxalyl chloride, triethylamine

Xanthates etc

• Dithianes (thioacetals) for carbonyl umpolung - nucleophilic carbonyl C
• Pummerer rearrangement
• Tschugaev elimination: preparation of alkenes from dithiocarbonates
• ROH → RH via reduction of the dithiocarbonate ROC(=S)SMe (AIBN, Bu₃SnH, )

Natural organosulfur compounds

• Penicillins and cephalosporins

Organophosphorus

• Macrolactonisation with triphenylphosphine and 2,2′-dipyridyl disulfide

Horner–Wadsworth–Emmons

• Horner–Wadsworth–Emmons reaction
  • Gives trans-alkenes
  • Greater reactivity of the ylid compared to Wittig allows use of stabilised ylids with ketones - not possible in the Wittig reaction
  • Phosphonates are easily made from phosphites by the Arbuzov reaction

Mitsunobu

• Mitsunobu reaction: ROH → RNu by DEAD, triphenylphosphine and nucleophile, with inversion of configuration (one-pot process)

Organosilicon

• Allylsilanes as nucleophiles