Conformational analysis: Tutorial Problems

Question 1

Answer: All three molecules adopt chair conformations for all three rings. The anomeric effect is induced by anti-periplanar alignment of an oxygen lone pair with an adjacent C-O σ* orbital. This has the effect of contracting the length of the O-C donating bond (forming +O=C) and elongating the C-O σ* bond (forming +O=C....-O). In system 2 below, all four C-O σ* orbitals can align with four of the eight oxygen lone pairs. This mutual effect results in all four C-O bond lengths being more or less equal. In system 3, for one O/C-O pair this reciprocal arrangement is maintained, but for the other it is not. For this latter system, the two C-O bond lengths are no longer equal. For system 4, neither O/C-O alignment is reciprocal, and both pairs of bonds are now unequal in length.

You can inspect this further by identifying whether there is an antiperiplanar alignment of any given C-O bond with either of the two oxygen lone pairs on the other oxygen atom (the lone pairs are shown in purple). You should be able to correlate any such alignments with the assertions made above.

The stability of the overall molecule is of course related to how many lone pair....C-O σ* interactions can be achieved. This results in molecule 2 being the most stable with four alignments, and molecule 4 the least stable with only two.

Total energies (kcal/mol) based on AM1 calculations
2: -199.4 kcal/mol 3: -195.0 4: -194.9

Question 3

Answer: The diazonium ion is evicted from the system by one of two different sets of anti-periplanar interactions. One ( ), starts at the remote anomeric centre, and breaks this bond ) in a process which populates the C-N σ* region to induce its departure. Note the H-bond that can form with this pathway.

The other starts here: producing a carbonium/oxonium species which looks like a π complex. It can also be considered similar to the 3-centre-two electron non-classical carbonium ion found for the norbornyl cation. This π complex/non-classical ion inhibits free rotations and hence is the mechanism that ensures stereochemical integrity. This pathway is about 5.6 kcal/mol lower in energy that the first one.

The reaction concludes via a C-C bond forming transition state.

Total energies (kcal/mol) based on B3LYP/6-31G(d) calculations
Intermediate carbonium ion:
-613.24165 (0.0 kcal/mol)
Alternative carbonium ion:
5.6 kcal/mol
Transition state for C-C bond formation