Stereoelectronic chameleons donors masquerading as acceptors

The focus of the following section will be on stereoelectronic conversion of donors into acceptors caused by a conformational change. 

Transforming donor to acceptor via rotation An interesting possibility exists for the situations when both donor and acceptor orbitals are present at the same atom but are projected to different directions. For such atoms, rotation around bonds connecting such an atom with the rest of the molecule can turn the donor and acceptor effects on and off. In some cases, this rotation can invert the overall electronic nature of the substituent, converting an effective donor into an apparent acceptor and vice versa. 

Methoxy groups as stereoelectronic chameleons Rotation of the p-OMe substituent has profound stereo- 

Amides as stereoelectronic chameleons The higher stability of amides in the family of carboxylic acid derivatives stems from the large magnitude of amide resonance interaction). In this system, nitrogen 

Singlet carbenes - chameleonic reactivity in cycloadditions A very intetesling functional group with two orthogo- 

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