Positive conjugation and hyperconjugation in vinyl systems

The manifestations of positive hyperconjugation in these systems are nbiqnitons. Even in allenes, the functionality that generally displays a balanced mixture of a it and n-ra interactions in the ground state, positive hyperconjugation can grow dramatically in importance in the course of reaction with electron deficient species. Furthermore, the o It delocalization gains significance as the acceptor ability of the it orbital increases (i.e. heterocumulenes) 

An important structural feature that distinguishes aUenes from alkenes and ketenes is that the donor and acceptor orbitals in the cumulenic systems are closer in space and constrained to the perfect coplanar conformation. We will see illustrative consequences of these features in the subsequent sections. 

4 delocalization in allenes allenyl silanes in reactions with electrophiles 

Eigure 7.12 Effects of a rt delocalization on stability of ketenes and allenes. 

The instability of fluoroketene is associated with high reactivy. An increase in reactivity can be useful. In particular, it was suggested to play a role in an important enzyme-catalyzed process. Fluoroacetyl-CoA thi-oesterase (FIK) enzyme, present in the organofluorine-producing microbe Streptomyces catleya, has an 

---