Charge affinity inversion

Notice that the final result is a 1,3 charge affinity inversion Umpolung) of an allylic derivative via a FGI of a functional group of type E by a group of type A, followed by a [2,3]-sigmatropic rearrangement. If the intermediate allyl anion reacts with a carbonyl compound as the electrophile the result is then a 1,4-D system, such as ... 

The methods developed for the synthesis of 1-alkynylphosphonates use synthetic concepts such as charge affinity inversion and positive halogen abstraction. Four main reaction categories allow the preparation of 1-alkynylphosphonates the apparent Michaelis-Arbuzov and Michaelis-Becker reactions, the propargyl phosphite-allenylphosphonate rearrangement, carbanionic displacements at quinquevalent phosphorus centers SNPfV), and conversions of vinyl- to alkynylphosphonates by addition-elimination reactions. 1-Alkynylphosphonates possess remarkable potential, and cycloaddition reactions appear to offer very attractive synthetic procedures. ... 

This represents an inversion of the charge affinity of the migrating alkyl group from what it is in (107), and raises many interesting speculations for future investigation. However, at the present time the viability of any migratory insertions in alkyl nitrosyls must first be established. 

The last factor to be discussed is polarisation effects. Polarisation may simply be considered as the degree of distortion of the electronic charge density around an ion, and can arise from many causes. The two extreme cases, i.e., negligible distortion and effective removal of an electron from one ion toward its neighbour, give rise to a purely covalent bond and a purely ionic bond, respectively. With regard to transition-metal ions in spinels, it is expected that only spherically symmetric ions (d and d ) can show a tendency for covalency. In this case, tetrahedral sites are preferred. Cations which show covalent affinity for tetrahedral environments are Fe ", Ga ", In " and, more strongly, and Cd. Spinels with the former cations tend therefore to be inverse, while those with the latter tend to be normal. 

---