Question

hi i need some guidance with this question please, can you explain it?

1) Propose a mechanism for the following vinylogous Wittig reaction. Do you expect high stereoselection for the illustrated product? NCCO Et O PBuz Ph. PhH CHCl3, rt CN CO Et

Answer 1

To understand the total mechanism, first PBu3 attacks at the more electron deficient position of allene(due to the presence of electron withdrawing ester group, that position is electron deficient) to give 2​​​​​​. Now compound 2 exist in two resonating form 3 and 4. Structure 4 is converted to the structure 5 to get more stability(stability given by electron withdrawing -CN group) via H-transfer. Structure 5 resonated to form structure 6 which is more stable and is an active vinylogous Wittig reagent which gives the final product in the second step.

NC- -cont Prva ph CN + Р Chel, rt CD of out the out out & WC- Mechanism NC - NC Ptu, Pouz Stepe 2 undey stand H-frame NC cout NC NC- PBy, 6 Buze (Vinylogous Wittig reagent Buz, co, lut Step-f : Ph the Pus Ph. h 6 cout (Betain ring Ly colet Target molecule