Electrophilic addition of hydroiodic acid to (3S)-3-tert-butoxypent-l-ene gives

Question

Electrophilic addition of hydroiodic acid to (3S)-3-tert-butoxypent-l-ene gives an unequal mixture of two products [Reaction 2, 99:1 ratio] .Å The same reaction carried out on (3S)-3-methoxy-pent-l-ene also gave two stereoisomers albeit in a much lower ratio [Reaction 1, 55:45 ratio]. Answer the following questions related to this process: Explain why is the product ratio for Reaction 2 so much lower than the product ratio for Reaction 1? 55:45 HI H ,c CH, o" OCH3 H3CYCH; TH3CYCH3 OCH3 OCH3 Product 1 Product 2 Reaction 1 OtBu = HTCH3 3 CH3 99:1 OCH3 = CH3 na terenie - Reaction 2 HI H3c 1 CH2 E H3c 1 CH3 * H3c 1 CH OtBu OtBu OtBu Product 3 Product 4 O The size of the OtBu group is so much smaller O The size of the OtBu group is much closer to the reactive site O The size of the OCH3 group is much smaller O The OCH3 group is further away from the reactive site

Explanation / Answer

The electrophilic addition reaction across the double bond here results in two stereoisomers as the transition state involves attack of iodide ion either from the same face as the methoxy or t-butoxy group or from the opposite face as that of the alkoxy group.

In the first case, the methoxy group is relatively small and through the C-O-C bond rotation in the ether group, the most transition state can have the methyl group facing away from the iodide nucleophile in its approach resulting in an equal opportunity for iodide attack from both faces. However in case of the t-butyl group, the attack from the same face as the t-butoxy group will result in a highly sterically hindered transition state due to the extensive repulsion faced by the incoming iodide through the t-butoxy group. This results in a greater product formation from the most favoured transition state in which iodide attack from the opposite face relative to the t-butoxy group. Thus the much smaller size of methoxy results in lower enantioselectivity while the larger size of t-butoxy results in a higher enantioselectivity.

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