Q4(d). Stabilised and unstabilised ylids; E/Z selectivity in the Wittig reaction
What are stabilized and unstabilized ylids ? How does E-Z selectivity in Wittig reaction depend upon the nature of ylid ? Explain.
Answer in brief
Unstabilised ylid (R = alkyl) → Z-alkene ; stabilised ylid (R = CO₂Et, COR, CN) → E-alkene . The choice is made at the oxaphosphetane: kinetic cis ring for the reactive ylid, equilibrated trans ring for the stabilised one.
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CoversWittig reactiondelocalised onto O[2+2] cycloadditionoxaphosphetanesyn-cycloreversion
All parts of Question 4
- 4(a) Enamine bromination with ring contraction; Dakin reaction10 marks
- 4(b) Claisen ester condensation: why R₂CHCO₂Et fails10 marks
- 4(c) Fischer indole synthesis: 2-phenylindole10 marks
- 4(d) Stabilised and unstabilised ylids; E/Z selectivity in the Wittig reaction10 marks