Reaction (1a): Free Radical Addition: Reaction and Mechanism

Solution to Q1(a): Intermolecular Radical Conjugate Addition

Reaction Overview: The Giese Reaction

Key Takeaways

• Announce: This is a carbon-carbon bond-forming process that adds an alkyl group to an electron-deficient alkene using radical intermediates.
• State: Treatment of an alkyl iodide with $\ce{Bu3SnH}$ and $\ce{Et3B}$ (in the presence of air) generates an alkyl radical that regioselectively attacks an $\alpha,\beta$-unsaturated system.
• Define: $\ce{Et3B/O2}$ acts as a low-temperature initiator (allowing the reaction to proceed at room temp). The tin hydride acts as the chain carrier and hydrogen donor.
• Apply: The nucleophilic phenethyl radical ($\ce{PhCH2CH2^\bullet}$) selectively attacks the terminal $\beta$-position of the acrylic acid moiety. The electron-rich, sterically hindered endocyclic double bond of the decalin core remains completely unreactive.

Bicyclic Substrate
($\alpha$-substituted acrylic acid)

$+$

1-Iodo-2-phenylethane
(Radical Precursor)

$\ce{->[\text{Et}_3\text{B, air (O}_2\text{)}][\text{Bu}_3\text{SnH, rt}]}$

Conjugate Addition Product

Mechanism: Initiation & Radical Generation

The reaction avoids heating by utilizing the auto-oxidation of triethylborane ($\ce{Et3B}$) to trigger the radical cascade at room temperature.

1. Initiation:
$\ce{Et3B + O2 -> Et^{.} + EtOO^{.}}$

2. Tin Radical Formation:
$\ce{Et^{.} + Bu3SnH -> EtH + Bu3Sn^{.}}$

3. Carbon Radical Generation:
$\ce{Bu3Sn^{.} + PhCH2CH2-I -> Bu3SnI + PhCH2CH2^{.}}$

Mechanism: Addition & Propagation

The highly nucleophilic phenethyl radical attacks the electrophilic $\beta$-carbon of the double bond. This generates a stable, tertiary $\alpha$-carboxy radical intermediate, which then abstracts a hydrogen atom from $\ce{Bu3SnH}$ to terminate the sequence.

$\ce{Substrate + PhCH2CH2^{.}}$

$\ce{->[\text{Addition}]}$

Resonance-Stabilized $\alpha$-Radical
(Tertiary & Capto-dative stabilized)

$\ce{->[\text{Bu}_3\text{SnH}][\text{- Bu}_3\text{Sn}^{.}]}$

$\ce{Final Product}$

4. Hydrogen Atom Transfer (Chain Propagation):
$\ce{R-C^{.}(COOH)-CH2CH2CH2Ph + Bu3SnH -> R-CH(COOH)-CH2CH2CH2Ph + Bu3Sn^{.}}$