More on Alkenes

Question 1

What is the Wittig reaction

Answer 1

1. Reaction of aldehyde or ketone with a phosphorus ylide

Question 2

How do you form the phosphorus ylide

Answer 2

1. Use a phosphonium salt and react with BuLi or another strong base

Question 3

What is the driving force of a Wittig reaction

Answer 3

1. Formation of a strong phosphorus-oxygen bond

Question 4

What does the Wittig reaction favour

Answer 4

1. The z-alkene (cis)

Question 5

What groups need to be protected when carrying out Wittig reactions

Answer 5

1. Acidic functional groups (alcohol)

Question 6

Give an example of a protecting group that can be used to protect alcohol and what can be used to remove it

Answer 6

1. OTBDMS

2. TBAF

Question 7

How can you make the phosphonium salt needed for the wittig reaction

Answer 7

1. Use an alkyl halide and triphenylphosphine

Question 8

What is a downside to the wittig reaction

Answer 8

1. The byproduct (triphenylphosphine oxide) is often a pain to remove

Question 9

What is a reaction that gets round the byproduct problem of Wittig

Answer 9

1. Horner-Wadsworth-Emmons

2. Produces an ionic byproduct that can be washed away in the aqueous phase

Question 10

What does the Horner-Wadsworth-Emmons require

Answer 10

1. An electron withdrawing group on the phosphorus reagent

Question 11

What type of alkenes does the Horner-Wadsworth-Emmons normally give

Answer 11

1. E-alkenes (trans)

Question 12

Give 3 examples of EWG

Answer 12

1. Ester
2. nitrile
3. ketone

Question 13

Describe reason for selectivity in Wittig reaction

Answer 13

1. Gives predominantly Z alkene
2. The phosphorus ylide is unstable and so the reaction to form the oxaphosphetane intermediate is irreversible
3. In the second step, a syn-oxaphosphetane intermediate will always give a z-alkene and an anti-oxaphosphetane intermediate always gives an E-alkene
4. So step which decides what oxaphosphetane is formed is the one which decides the alkene geometry- first step
5. In Wittig the first step normally gives a syn oxaphosphetane- reasons of orbital control the two reactants approach perpendicular to each other
6. They are aligned to keep the most bulky groups apart- reduce steric hindrance which gives a syn oxaphosphetane

Question 14

Describe reason for selectivity in HWE reaction

Answer 14

1. The phosphonate anion nucleophile is stabilised by the presence of the EWG.
2. This means oxaphosphetane formation is reversible
3. An equilibrium will be established which leads to formation of mostly the anti oxaphosphetane - E-alkene
4. Anti- more stable product as R groups are on opposite sides of the chain, syn is kinetic product and goes back to original in equilibrium
5. Fragmentation of the anti oxaphosphetane is faster due to a less sterically congested transition state

Question 15

How can a Wittig reaction be made to favour the formation of E-alkenes

Answer 15

1. Using stabilised phosphorus ylides- EWG

2. Resonance stabilises so equilibrium established - same reasons as HWE

Question 16

How can a HWE reaction be made to give Z- alkenes

Answer 16

1. Use of special substituents on the phosphonate side chains
2. Sill reagent and Ando reagent are two examples

Question 17

What are the Still-Gennari reagent and Ando reagent

Answer 17

1. Still-Gennari = CH2CF3

2. Ando reagent= Ph or -o-Tol