Problem Set 6

Due in class Friday, November 21

A.  Problems from Carey & Sundberg:

6.2, 6.4, 6.8, 6.9, 6.17, 6.21

B.  Additional problem.

Lithium hexamethyldisilazide (LiN(SiMe3)2, LiHMDS) is a common base used to make enolates. Consider the following data:  

  1. When labeled with 6Li and 15N (both NMR-active forms of the respective nuclei with spin=1/2), LiHMDS shows a triplet in the 6Li NMR.
  2. Reaction of LiHMDS with 2-methylcyclohexanone in toluene is slow. Addition of triethylamine accelerates the reaction, up to the point where approximately 20 equivalents has been added.
  3. A similar effect is seen with THF as an added cosolvent, but the peak effect is only about 1/100th as large as that of triethylamine.
  4. Use of 2, 6, 6-trideuterio-2-methylcyclohexanone reveals a primary KIE (kH/kD) = 10 at -78° C.

Suggest a mechanism for triethylamine-accelerated deprotonation that accounts for the data.

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Last updated: 9/18/2008