Question

a. How can the following alkene be prepared from propanal via a Wittig Reaction. Detail synthesis and an assessment of the stereochemical outcome are required for full points. (6 points)

Alkene is 4-methylhept-3-ene

Answer 1

From propanal, we can prepare 4-methylhept-3-ene via Wittig reaction.

Generally it gives E- and Z- alkenes (mixture).

1.Firstly from triphenyl phosphene, it reacted 2-halo pentane through SN2, gives alkyl triphenyl phosphonium halide

and then in the presence of strong base RLi, eliminated HX and form Wittig reagent.

2. This mehtyl substituted alkylidene triphenyl phosphene reacts with Propanal form betaine followed by formation of four membered ring, which further breaup due to P-O bond is more strong and gives alkene.

3. Here, if we take electron withdrawing group containing phosphrous ylide (stabilized ylide)then its stereoselectivity will give E- alkene.

4. Here, if we take simple alkylated  groups containing phosphrous ylide (unstabilized ylide)then its stereoselectivity will give Z- alkene.

In given reaction We have obtained Z- selective alkene.

Pho6 TaiplA Phosplne Mhe2 SN2 Ph f + CH Alleyd Taf pleg Phosphonin 2-halo pentane haliale +R-Li AK - R-H 2. Ph.p ropannl

+5 11 12c ch-CH I-Step cn -Ch-CHIC' Ph Ph Oxa phos phe tane APh) Betaine tats-

CMS Y Cen-CH phosphorao-Ox bronelo more tep 2 nc-Ch-CH=Efes C +Ph 4-menye hept-3-eme

Ph6 Ph unstabeliza Knutic controlled stereosperte decomporNs 2 t''s 2-alkene