Anchimeric
Assistance
The direct interaction of the
reaction centre (usually, but not necessarily, an incipient carbenium centre)
with a lone pair of electrons of an atom or with the electrons of a σ- or
π-bond contained within the parent molecule but not conjugated with the
reaction centre. A distinction is sometimes made between n-, σ- and
π-participation. A rate increase due to neighbouring group participation is
known as 'anchimeric assistance'.
'Synartetic acceleration' is the
special case of anchimeric assistance ascribed to participation by electrons
binding a substituent to a carbon atom in a β-position relative to the leaving
group attached to the α-carbon atom. According to the underlying model, these
electrons then provide a three-centre bond (or 'bridge') 'fastening together'
(as the word 'synartetic' is intended to suggest) the α- and β-carbon atoms
between which the charge is divided in the intermediate bridged ion formed (and
in the transition state preceding its formation). The term synartetic
acceleration is not widely used.
No comments:
Post a Comment