Isostructural synthons

Another illustration of isostructural synthons with reversed polarity comes from the allyl halide and allylsilane pair. Allyl halide, in reactions with standard nucleophiles, is an electrophile, equivalent to the synthon CH2=CH-CH/. Allylsilane is a nucleophile, equivalent to the synthon CH2 = CH-CHf (in Lewis acid mediated reactions with electrophilic reagents, examples in Scheme 2.41). The availability of both types of C3 synthon permits the introduction of an allyl group to either a nucleophilic or electrophilic site and to treat the respective synthetic options as viable alternatives. 

The three synthons E(NSO)2 (E = S, Se, Te), although isostructural, differ significantly in their chemical behavior, due mainly to the varying stability of the intermediates formed in their reactions. Specifically, the compounds Se(NSO)2 and Te(NSO)2 have enabled new procedures for the synthesis of selenium and tellurium heterocycles to be developed. For compounds such as X2Te(NSO)2 (X = F, Cl), it would be interesting to know if these compounds are also able to eliminate S02 and if so, how, inter- or in-tramolecularly. As the reaction pathways are not fully understood, more significant experimental work is needed. 

Figure 5.20 (a) N-H- O urea tape synthon along the b-axis and / -ChN halogen bonding in the crystal structure of the p-iodo compound PNPU-I. (b) The p-ethynyl crystal structure is isostructural, being mediated by urea tape synthons and C=C-H O2N interactions... 

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