Chapter 18, Problem 18.15P

Interpretation Introduction

(a)

Interpretation:

The structures of the transition-metal complexes involved in each of the given mechanistic step are to be shown. The electron count and the metal oxidation state at each step is to be stated.

Concept introduction:

The Wilkinson’s Catalyst is a common name of coordination compound $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_{\text{3}}$. It is basically used for the catalytic hydrogenation of alkenes. The Wilkinson’s catalyst has square-planar structure.

Answer to Problem 18.15P

The structures of the transition-metal complexes involved in each of the given mechanistic step are shown below.

1. The oxidative addition reaction is shown below.

2. The ligand substitution of one ${\text{PPh}}_{\text{3}}$ by the alkene is shown below.

3. 1, 2-insertion of alkene into a $\text{Rh}-\text{H}$ bond and readdition of previously expelled ${\text{PPh}}_{\text{3}}$ ligand is shown below.

4. Reductive elimination of the alkane product to generate the catalyst is shown below.

Explanation of Solution

The general formula for the calculation of electron count in a given formula is shown below.

$\begin{array}{r}\text{Electron}\text{count}=\text{Number}\text{of}\text{valence}\text{electrons}\text{on}\text{central}\text{metal}\text{atom}+\left(\left(\text{Number}\text{of}\text{electrons}\text{donated}\text{by}\text{ligand}\right)\right)\text{×}\left(\text{Number}\text{of}\text{ligands}\right)\end{array}$

The number of valence electrons present in rhodium is $9$ and the phosphine donates $2$ electrons.

1. In oxidative addition reaction, the central metal atom gets oxidized with the addition of two ligands and there is an increase in electron count at the central metal atom.

The oxidative addition reaction is shown below.

Figure 1

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_{\text{3}}$ is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+2\times \text{(3)+1}\times \left(1\right)\\ & =& 9+6+1\\ & =& 16\end{array}$

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_{\text{3}}{\text{H}}_{\text{2}}$ is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+2\times \text{(3)+1}\times \left(1\right)+1\times \left(2\right)\\ & =& 9+6+1+2\\ & =& 18\end{array}$

2. The ligand substitution reaction of one ${\text{PPh}}_{\text{3}}$ by alkene is shown below.

Figure 2

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_{\text{3}}{\text{H}}_{\text{2}}$ is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+2\times \text{(3)+1}\times \left(1\right)+2\left(1\right)\\ & =& 9+6+1+2\\ & =& 18\end{array}$

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_2{\text{H}}_{\text{2}}-\text{alkene}$ complex is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+2\times \text{(2)+2}\times \left(1\right)+1\left(1\right)+1\left(2\right)\\ & =& 9+4+2+1+2\\ & =& 18\end{array}$

3. 1, 2-insertion of alkene into a $\text{Rh}-\text{H}$ bond and readdition of previously expelled ${\text{PPh}}_{\text{3}}$ ligand is shown below.

Figure 3

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_2{\text{H}}_{\text{2}}-\text{alkene}$ complex is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+2\times \text{(2)+2}\times \left(1\right)+1\left(1\right)+1\left(2\right)\\ & =& 9+4+2+1+2\\ & =& 18\end{array}$

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_2\text{H}-\text{alkyl}$ complex is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+2\times \text{(2)+1}\times \left(1\right)+1\left(1\right)+1\left(1\right)\\ & =& 9+4+1+1+1\\ & =& 16\end{array}$

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_3\text{H}-\text{alkyl}$ complex is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+3\times \text{(2)+1}\times \left(1\right)+1\left(1\right)+1\left(1\right)\\ & =& 9+6+1+1+1\\ & =& 18\end{array}$

4. Reductive elimination of the alkane product to generate the catalyst is shown below.

Figure 4

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_3\text{H}-\text{alkyl}$ complex is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+3\times \text{(2)+1}\times \left(1\right)+1\left(1\right)+1\left(1\right)\\ & =& 9+6+1+1+1\\ & =& 18\end{array}$

The electron count in $\text{ClRh}{\left({\text{PPh}}_{\text{3}}\right)}_{\text{3}}$ is shown below.

$\begin{array}{rll}\text{Electron}\text{count}& =& 9+2\times \text{(3)+1}\times \left(1\right)\\ & =& 9+6+1\\ & =& 16\end{array}$

Conclusion

The catalytic hydrogenation of alkene by wilkinson’s catalyst and electron count is shown in Figure 1, 2, 3 and 4.

Interpretation Introduction

(b)

Interpretation:

The stereochemistry of the product if ${\text{D}}_{\text{2}}$ substitutes ${\text{H}}_{\text{2}}$ in the given reaction is to be stated.

Concept introduction:

The complex that follows $18$-electron rule are most stable. The electrons are donated by the ligand to central metal atom. These $18$ electrons occupy the bonding molecular orbitals making the compound stable. The catalyst accelerate the rate of reaction. There is always syn addition of hydrogen on alkene takes place. It means addition from same side, it can be from top or from bottom.

Answer to Problem 18.15P

The reaction between cis-alkene with the deuterium substituted wilkinson’s catalyst is shown below.

This shows syn-addition of deuterium on alkene.

Explanation of Solution

Reduction of cis-alkene with the deuterium substituted Wilkinson’s catalyst.

Figure 5

In the reduction of cis-alkene with the deuterium substituted Wilkinson’s catalyst. Hydrogen added on Wilkinson’s catalyst for hydrogenation of alkene is replaced by deuterium. The alkene is present in the plane of the paper, the deuterium can attack the alkene either from below the plane or above the plane.

Therefore, this shows that syn-addition of deuterium also takes place.

Conclusion

The reaction in Figure 5 shows that syn addition of hydrogen takes place even when it is substituted by deuterium.