Synperiplanar interactions

The selectivity for (/ ,/ )( ,S)-10 has been rationalized by invoking a synperiplanar enolate species whose conformation is enforced by a donor(enolate oxygen)- acceptor) peril uo-rophenyl) interaction depicted in structure N47. Infrared and variable temperature NMR spectroscopic studies of the neutral precursor complex 8 support the existence of such a donor-acceptor interaction. 

It remains to explain why the Felkin transition states are the most stable. During the reaction, the major interaction occurs between the nucleophile s HOMO and the substrate s LUMO. Therefore, the most reactive conformation of the substrate is that with the lowest LUMO. This corresponds to the geometry in which the C2-L bond is parallel to the tt system, as there is then a good overlap between the orbital and the lowlying ac2-L orbital, leading to a stabilization of the LUMO. The nucleophile may attack this conformer in an antiperiplanar or synperiplanar stereochemistry. The latter is disfavoured for two reasons ... 

Evidence against an inductive contribution comes from experimental IEs on amine basicity. According to Equation (28), there is no angle-independent term. This is the term that would arise from an electrostatic interaction between a positive charge on the N and a C-H or C-D bond dipole.3 Although Equation (28) is imperfect, and there are smaller IEs from synperiplanar C-D, we conclude that an inductive effect is too small to contribute to the observed IE. 

In principle a crx A — 0 Y-A charge-transfer interaction would also be possible when the two vicinal X-A and Y-A bonds adopt a synperiplanar conformation. However, in this latter conformation the overlap integral crx A — [Pg.17]

Scheme 23 shows how four possible diastereomers can arise from the combination of two sp -carbon centers C-1 and C-2 in a donor component 23-1 and an acceptor component 23-2. Species 23-3 and 23-4 are two diastereomers and 23-5 and 23-6 are their enantiomers.The problem of simple diastereoselection is the control of the diastereomer ratio 23-3-1-23-5/23-4-1-23-6. The enantiocontrol of 23-3 vs 23-5 or of 23-4 vs 23-6 cannot be achieved by simple diastereoselection in this case an external source of chirality has to be applied, for instance a chiral catalyst or the incorporation of stereogenic units in one of the components. Simple diastereoselection can be exerted in terms of closed and open transition states, depending on the mutual interaction of the termini X and Do, respectively. If these termini are linked via a six-membered chelate, a closed ( Zimmerman-Traxler ) transition state 23-7 with synperiplanar olefinic units is formed. On the other hand, if the termini have a repulsive interaction an open transition state 23-8 with an antiperiplanar arrangement of the olefinic units is adopted. Efficient stereocontrol via Zimmerman-Traxler transition states 24-1 to 24-4 is observed in aldol-type and allylborane carbonyl additions (Scheme 24). The crucial stereo differentiating interaction is the diaxial repulsion between Rax and R, which must be kept as low as possible. Only small substituents (nor-...

As expected, the G-Tj interaction responsible for vicinal delocalization is dependent on conformation. However, relevant calculations reveal that the vicinal delocalization involves the bonding orbital b of the intervening bond B. Inagaki et al. (266, 267) found in addition that the interactions are controlled by orbital phase continuity, namely a- b out of phase, b-rc and fl - c in phase. These requirements were found to be simultaneously satisfied for the ap arrangement, as shown in Figure 36a. In the synperiplanar conformation at least one requirement cannot be met, for example, orbitals a and c are out of phase (Figure 36b). 

The results of an i.e.r. study of the gas-phase interaction (i.e. no solvent participation) of the relatively strong base CDaO with a number of hydrofluoroethanes have been discu ssed in terms of a j3-eIimination mechanism (see Scheme 16) involving attack at the most acidic hydrogen with formation of a synperiplanar chemically... 

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