Question

Propose two syntheses of 4-phenyl-2-pentanone, each involving conjugate addition of a lithium diorganocopper reagent.

Short answer

Question: Briefly describe two pathways to synthesize 4-phenyl-2-pentanone using conjugate addition of a lithium diorganocopper reagent. Answer: In Pathway 1, ethylmagnesium bromide reacts with copper(I) bromide to form lithium diethyl(phenyl)copper, which undergoes conjugate addition with ethyl vinyl ketone to form 4-phenyl-2-pentanone. In Pathway 2, phenylmagnesium bromide reacts with copper(I) bromide to form lithium diphenyl(ethyl)copper, which undergoes conjugate addition with methyl vinyl ketone to form 4-phenyl-2-pentanone.

Step-by-step solution

Pathway 1: Preparation of the lithium diorganocopper reagent

Ethylmagnesium bromide (EtMgBr) reacts with copper(I) bromide (CuBr) to form the lithium diorganocopper reagent Et_2CuLi, or lithium diethyl(phenyl)copper. The balanced equation for this step is: \(EtMgBr + CuBr \rightarrow Et_2CuLi\)

Pathway 1: Conjugate addition of the lithium diorganocopper reagent

Next, the prepared lithium diethyl(phenyl)copper reagent Et_2CuLi undergoes a conjugate addition reaction with ethyl vinyl ketone to form 4-phenyl-2-pentanone. The balanced equation for this step is: \(Et_2CuLi + CH_3CH=CHCOEt \rightarrow 4-phenyl-2-pentanone\)

Pathway 2: Preparation of the lithium diorganocopper reagent

Phenylmagnesium bromide (PhMgBr) reacts with copper(I) bromide (CuBr) to form the lithium diorganocopper reagent Ph_2CuLi, or lithium diphenyl(ethyl)copper. The balanced equation for this step is: \(PhMgBr + CuBr \rightarrow Ph_2CuLi\)

Pathway 2: Conjugate addition of the lithium diorganocopper reagent

Finally, the prepared lithium diphenyl(ethyl)copper reagent Ph_2CuLi undergoes a conjugate addition reaction with methyl vinyl ketone to form 4-phenyl-2-pentanone. The balanced equation for this step is: \(Ph_2CuLi + CH_3CH=CHCOCH_3 \rightarrow 4-phenyl-2-pentanone\) These are two possible pathways to synthesize 4-phenyl-2-pentanone using conjugate addition of a lithium diorganocopper reagent.