There are four diastereomers (A – D, margin) of (4S)-2-bromo-4-phenylcyclohexanol. As a team, formulate their structures and draw each diastereomer in the most stable chair conformation (see Table 4-3; the DG8 value for axial versus equatorial C₆H₅ is 2.9 kcal mol^-1). Divide your team into equal groups to consider the outcome of the reaction of each isomer with base (-OH).

(a) Using the curved-arrow formalism (Section 6-3), show the fl ow of electrons in the attack of the base on the various cyclohexane conformers. Reconvene and present your mechanisms to your teammates, justifying the structural assignments of A – D. Find an explanation for the qualitative rate differences and the divergent course of the reactions of A and B versus C and D.

(b) When compounds A – D are exposed to conditions favoring bromide dissociation in the presence of Ag1 salts (to accelerate heterolysis with formation of insoluble AgBr), A, C, and D give the same products as those obtained on treatment with base. Discuss the mechanism as a group.

(c) Curiously, compound B traverses another pathway under the conditions described in (b); that is, rearrangement to the aldehyde E. Discuss a possible mechanism for this ring contraction. (Hint: Keep in mind the principles outlined in Section 9-3. The mechanism proceeds through a hydroxycation. What is the driving force for its formation?)

Transcribed Image Text:

OH OH A B Fast Slow H;C H;C [Note: Enols are unstable with respect to isomerization to the corresponding ketone (Chapters 13 and 18).] [HO OH Fast C,H C,H Enol OH Slow