Donor-acceptor interaction geminal

Following the procedure for vicinal interactions illustrated in Example 1.4 and Section 3.3.1, we can associate each geminal ctab— obc donor-acceptor interaction with an arrow-pushing diagram and partial admixture of an alternative resonance diagram, as shown in Fig. 3.78. The formal two-electron transfer from gab to obc results in the NBO configuration... 

Figure 3.78 Generic arrow-pushing diagrams (left) and secondary resonance structures (right) for vicinal (upper) and geminal (lower) NBO donor-acceptor interactions.

There has been a long debate about the interactions between geminal donor and acceptor sites, such as that shown in Scheme 12.VIa. It was initially posmlated that in compounds of the type R3E-CH2-NR2, where E = Si, Ge or Sn, there was a direct donor-acceptor interaction between the atoms E and N. This was first concluded from the lower basicities of compounds such as Me3SiCH2NMe2 (12.VIb, X = Me),... 

When two functional groups are borne by the same atom, they interact to cause either stabilization or destabilization. This is the geminal effect.20 If the substituents are both strong o-acceptors and n-donors [for example a gem-dimethoxy-C(OMe)2-], a stabilization occurs.21 This is easy to understand. Oxygen, being electronegative, is a o-acceptor and o co is low in energy and is 71-donor thanks to its lone pair. Therefore, we have here a double anomeric effect ... 

Fig. 10 Orbital description of the McConnell superexchange (through-bond) mechanism applied to a system comprising a donor and an acceptor covalently linked to a pentamethylene chain. The donor and acceptor chromophores each contribute a single tt orbital to the interaction and each C-C bridge bond is assumed to contribute a single a or a MO, depicted as the former type in the figure. T is the interaction matrix element between a chromophore tt MO and the ally lie C-C a MO, and t is the interaction matrix element between two geminal C-C a MOs.

The steric interaction between the donor and acceptor parts is enhanced if also the acceptors are built into a ring. An example is given by 30a, for which the geminal methyl resonance remained a sharp doublet to 200 °C, corresponding to AG > 26.5 kcal mol . The dissymmetric analogue 30b could be resolved into diastereomers by chromatography, and by monitoring the thermal isomerization by HPLC a barrier of 27.9 kcalmol could be obtained . 

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