Non perpendicular attack

V The Crucial Role of Non Perpendicular Attack in Asymmetric Induction 156... 

However, Felkin s hypothesis may be advantageously replaced by the assumption of non perpendicular attack. It is clear that the steric hindrance encountered by the nucleophile is much more serious in 13 than in 12. Furthermore non-perpendicular attack increases the Nu-R interaction at the expense of the Nu-0 interaction, thus accounting for the predominance of R over O. Finally, as R becomes bigger, the nucleophile is pushed toward the chiral carbon and can feel better the difference between S (in 12) and M (in 13), which should lead to an increased selectivity. 

This hypothesis would need to hold even for R=H, which is difficult to justify. It can be advantageously replaced by the assumption of non-perpendicular attack.41 The approaching nucleophile will obviously be much less hindered in 3 than in 4. Furthermore, as R becomes larger, it tends to push the nucleophile toward the chiral carbon. This causes the difference between M and S to be felt more strongly and the selectivity rises. 

Torsional effects and non-perpendicular attacks are second in importance. Computational estimates indicate that rotational barriers involving torsional interations with partially formed bonds may be as large as - 3 kcal mol-1.93 To the best of our knowledge, all recent models incorporate non-perpendicular attacks and torsional effects. 

The broken lines in the diagrams show the trace of the plane of the n orbitals. A reaction will occur readily if X lies on this plane and makes an obtuse angle with the C=Y bond. Molecular models show that the carbon backbone is long and flexible enough to satisfy both of these criteria for the exo reactions. The 6-endo reaction poses problems. If X lies in the n plane, the carbon skeleton has to adopt a boat conformation, leading to a perpendicular attack. However, if X moves slightly out of the n plane, an acceptable compromise can be achieved the attack trajectory becomes non-perpendicular, with a fair nucleophile-n overlap. However, neither condition can be satisfied for a 5-endo reaction. Note that a direct application of Baldwin s empirical rules would have masked these subtleties. 

The non-perpendicular Dunitz-Biirgi attack is due a negative frontier orbital overlap between the nucleophile and the oxygen. In the ate complex, the LUMO... 

The rearside attack model became equally important in the discussion of stereoselectivity in the aldol condensation. In 1981, Seebach and Golinski proposed a topological rule for aldol-like reactions which says that the H atom, the smaller substituent on the donor component, adopts an anti position with respect to the acceptor double bond C=A in the preferred transition state, cf. A and B in Scheme 6.28 [35]. This preference of A over B comes about as a result of the increases in steric hindrance when the donor substituent R2 is jammed against Rj and H of the acceptor in the non-perpendicular approach B. 

The access to different -regions is controlled by the vibration spectra. In particular, it is the anti-symmetric vibration mode that may shift the geometry towards the point of maximal mixing. The other way round, freezing this mode will trap the quantum system at the initial state, cis or trans as the case may be. Now, a vibration excitation along this anti-symmetric mode will be prompting the electronic activity of the system. For example, a perpendicular symmetric attack of carbene can only proceed if non-zero amplitude develops at the diradical states. This concept includes the elementary orbital considerations and overcome them. 

Flo. 7.10. Displays of — for the formamide molecule in the plane containing the nuclei and in a perpendicular plane containing the C-N axis. The values refer to the non-bonded maxima in the VSCCs of the oxygen and nitrogen atoms. The VSCC of carbon exhibits three bonded maxima in the upper figure. The Laplacian is positive over much of the VSCC of carbon in the plane illustrated in the lower diagram. The two points in this plane where is most positive are the holes in the VSCC of carbon, the points of nucleophilic attack (as indicated by... 

The usual procedure to generate at least a non-uniform distribution i (Ya) of the angle of attack consists in populating one single or a few j,m-states (alignment) the index p denotes the prepared state distribution. As the angular momentum vector of a IZ-molecule is always perpendicular to the axis the symmetry relation Ap(Ya) = Ap(180O-Ya) holds that is the molecular axis is not orientable in this case. 

---