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Chapter 10 Suggested solutions for Chapter 10 67 Problem 10 en we discussed reduction of cyclopentenone to cyclopentanol, we suggested that ugate addition of borohydride must occur before direct addition of borohydride; in other this scheme must be followed: 0 0 OH NaBH₄ NaBH₄ cyclopentenone intermediate cyclopentanol not isolated What is the alternative scheme? Why is the scheme shown above definitely correct? Purpose of the problem Serious thinking on mechanisms is an advantage when reactions get more complex. Suggested solution The alternative scheme would be to reduce the ketone first and the alkene second. 0 oH OH NaBH₄ NaBH4 cyclopentenone intermediate cyclopentanol not isolated This scheme is wrong because simple alkenes are not reduced with NaBH₄, which is a cleophilic reducing agent and attacks alkenes only if they are conjugated with an electron- thdrawing group. The conjugate addition must always be done first while the carbonyl group is See Problem 2 for a simpler example. Problem 11 Suggest a mechanism for this reaction. Why does conjugate addition occur rather than direct addition? Why is the product shown as a cation? If it is indeed a salt, what is the anion? 0 OSiMe₃ Ph₃P PPh₃ Purpose of the problem Extension of simple ideas to more exotic reagents with trapping of the intermediate. Suggested solution The phosphine is a soft reversible nucleophile and is good at conjugate addition while unsaturated ketones are good at both types of reaction. On balance, conjugate addition wins. The intermediate enolate is trapped by silicon - an excellent electrophile for oxygen. The product is indeed a cation. The nucleophile was neutral and has lost electrons in the addition step. The counterion is chloride ES all the atoms in the reagents are incorporated into the product. 0 0 CI :PPh₃ PPh₃ PPh₃ CI