Chapter 25, Problem 25.58P

Interpretation Introduction

(a)

Interpretation:

How to produce the compound shown here from $\text{1-phenylprop-1-yne}$ is to be shown.

Concept introduction:

The structure of $\text{1-phenylprop-1-yne}$ is given as:

The product we see is a cis alkene. The $\text{C}\equiv \text{C}$ triple bond of an alkyne can undergo catalytic hydrogenation. The addition of ${\text{H}}_{\text{2}}$ removes a $\text{π}$ bond and causes the total number of bonds between the alkyne carbons to decrease by one. Thus, an alkyne is reduced to an alkene. Reduction of an alkyne can be stopped at the alkene stage by using a stoichiometric amount of ${\text{H}}_{\text{2}}$ and a poisoned catalyst. A poisoned catalyst is simply a metal catalyst that has been specially treated to decrease its catalytic ability, thus making possible a slower and more controlled reaction. One example is Lindlar catalyst — palladium adsorbed on $\text{calcium carbonate}$ ($\text{CaCO}$) and treated with a small amount of quinoline and a lead salt. The reduction of an alkyne to an alkene via catalytic hydrogenation favors production of the cis isomer over the trans.

Interpretation Introduction

(b)

Interpretation:

How to produce the compound shown here from $\text{1-phenylprop-1-yne}$ is to be shown.

Concept introduction:

The structure of $\text{1-phenylprop-1-yne}$ is given as:

The product we see is trans isomer. A dissolving metal reduction of an alkyne produces the E alkene exclusively. In a dissolving metal reduction, solvated electrons behave like the free radical species. When alkyne is treated with sodium metal dissolved in liquid ammonia ($\text{-78 °C}$), the triple bond is reduced to a double bond. This is an anti-hydrogenation reaction in which two $\text{H}$ atoms, overall, add to the triple bond in a trans fashion. It complements catalytic hydrogenation using a poisoned catalyst which exclusively forms the cis product. The reaction is highly stereoselective.