Answers to Problem Set 6.

7.2 
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7.4
a.  Deprotonation (and the resonance delocalization necessary to allow this) requires the enolate pi bond be placed in an already-strained bicyclic ring.  The [2.2.1] system can accommodate this more readily than the [2.1.1] system.

b.

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7.8.This should (and does) correlate best with sigma, because the structural relationship of the developing charge to the substituents on the ring is identical to that in benzoate ions.

7.9.  a.  Attack of H-; collapse of the tetrahedral intermediate with expulsion of Ph3C- (a highly resonance-stabilized carbanion).  The benzaldehyde is further reduced.
b.  Pyridine derotonates OH; the hemiacetal opens.  Repeat to form two molecules pf PhCH(OH)CHO.  Enolization allows isomerization to PhC(O)CH2OH.
c.  Deprotonation of OH, then ring opening to the benzylic carbanion.
d.  Deprotonation of the hemiacetal; ring opening of the two acetal rings to form a dialdehyde.  This can enolize to racemize the final stereocenter.  Note carefully that all stereocenters must invert to racemize!
e.  Deprotonation of the vinylogous position; protonation to form the nonconjugated enone; deprotonation of the opposite side of the ketone; reprotonation to form the conjugated enone.

7.17.This is the "reverse sense" of the anomeric effect.  There is electron-electron destabilization when the lone pair on carbon is placed in an axial orientation.  (The fully correct statement is that placing the C-H axial is preferred because of the interaction of the C-H antibonding MO with the lone pairs on sulfur.)

7.21
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Section B.

The data tells you the following:
1.  Each Li is bound to 2 nitrogens.
2.  The triethylamine can coordinate to Li, and addition of this electron density makes the amide nitrogen a kinetically better base.
3.  THF is a better coordinator, but not as good an electron donor.  Therefore, a change in aggregation is unlikely to be the principle effect.
4.  This is a normal, primary KIE:  the proton is transferred in the RDS.

See http://dx.doi.org/10.1021/ja021284l (on-campus access required) for a diagram of the proposed TS.

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Last updated: 12/01/2003