Question

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3. (20 points) Teruo Umemoto at the Zhejiang Jiuzhou Pharmaceutical Company in Taizhou City, China, developed the reagent shown below for the transfer of a trifluoromethyl group to a nucleophile. The X is a counterion and can be ignored for the purposes of this problem. 18-cFg Nu F3C-Nu Umemoto's reagent a. One mechanistic possibility for the reaction shown above would be an SN2 reaction. Push the arrows for such an S, 2 process and make a picture of the Sg2 transition state, indicating any partially formed and broken bonds with dotted lines and including any partial charges. b. It came as something of a surprise, but Umemoto and co-workers found that the trifluoromethyl transfer has significant Syl character. Write an Syl mechanism, including a geometrically accurate picture of the key intermediate. c. What factor might destabilize the key Sp1 intermediate from part b? As usual, a picture or pictures will be helpful here. d. On the other hand, what factor might stabilize the key Syl intermediate from part b? As usual, a picture or pictures will be helpful here.

Answer 1

a) Mechanism by SN pathway with transition state a bond 1 broken X II & I Nu CF3 / O I --- NU 0. - bon 2 Transition state (Trigonal bipyramidal shaped) NENAD I canut SNo mechanism is a concerted one step mechanism in which attacking nucleophile approaches from the oposite side of the leaving group TOP sho giving a TBT transition state. b) mechanism by SNI moe pathway / To CFB -Nu carbocation intermidate (Planar) SNI mechanism oceur by two step in step - I formation of carbocation take place with departure of the leaving group. Then attacking Nue attacks to this planar carbocation to give corresponding product. Itactoy destabilised SNI intermidiare la SNI intermidiate is the planar carbocation, we know -t has a strong - I effect the three f group. very strong - I effect exterts a which might destabilised the carbocation intermidiate. i t I effect of F. C destabirised the carbocation intermidiate)

Factor stabilised the SNI intermidiatel SNI intermidiate is CFB in which central carbon atom is spr hybridised. so it has a vacent p-orbital in its valance shell. 2s 2px 2py opz (spr) - Dunn spr hybrid unhydrid 212 orbital orbital. on the other hand F has lone pair of electrons in its 21 arbital. Therefore F-a tom can donates its lone pair of electron and stabilised the Locarbocation, intermidiate resonanance. A o FANS r 90 S905 wir i Nowo 2791 viviparistala vo