Question

QUESTION 4 [1,2- versus 1,4-Addition Reactions] 4.1 Propose a detailed mechanism for the reaction of 2-butenal with ethynylmagnesium bromide in dry THF followed by quenching with dilute aqueous acid. (5) 4.2 Propose a plausible mechanism for the reaction of 1,3-diphenyl-2-propen-1- one with potassium cyanide in ethanol followed by aqueous work-up. 1 (5) 4.3 Predict and draw the structure of the product of the reaction of 2-methyl-1,3- cyclohexanedione with methyl vinyl ketone (3-buten-2-one) in methanol in the presence of potassium hydroxide. (3) 4.4 Draw the structure of the product of the reaction o 4-methyl-2-cyclohexen-1- one with lithium dimethylcuprate in dry diethyl ether followed by aqueous work- up. [15]

Answer 1

Soft nucleophiles do the conjugate addition (1,4-addition).

Hard nucleophiles do the 1,2-addition

41 a o C2 Hq Mg Br Ethyl magnerium bromide 2-butenal. C2Hs mg BV is a hard nucleophile. — Hard electrophilic site soft eleckophilic site (2 H 5 Mg By reacts at the Being a hand nucleophile hand electrophilic Site THE is a solvent for Hq Mg By

N AV C2H5-MGBY THE LQuenching in aqueow aud Ktra am @ Ethanol ou pen-i-me 1,3-diphenyl-2-Propen-1-me -CN does conjugate 1,4-addition at higher femperaturer. At xoom temperature, en doel 1,2-addition Aque Waricu Hora Me 2-methy) -1,3-cyclohexanedione methyl vinyl Ketone 01 Ha Hatte These prolone are reen not removed by the base because they have higher pka than

- the protond on the 2 carbon... The negative change on E is Mabilized by the two carbonyl groups - As it is stabilized by two coubanyl groups it is considered al sot nucleophile. - It attaced in 1,4- addition React with e methyl vingl ketone . acts as a catalytic bale in this reaction KOH -4.4 me,cuhi Lithium dimethyl cuprate, 4-methyl-2-Cyclohexe-l-one

The electronegativity of cu is greater than mg. Therefore the cu-me bond is not completely polarized = (mel Cuhe is considered as soft nucleophile and it attacks at the _.80ft elecgrophilie lite of the conjugated keron p. 4 t imel tuthi diethylether t lcuni T soft me Aqueoly I coorkup electrophilic site. Hard elecirophilic Sith -me magor Aroduct for (melcuki Diethyl ether is solvent