Halogenation neighboring groups

In cases of more effective 7t-electron donor or tz-donor neighboring groups, as is the case in forming /3-phenylethyl (studied by Don Cram from UCLA Nobel Prize in chemistry, 1987) or /3-halogen bridged species, these have sufficient electrons to form 2e-2c bonds (with some intermediate delocalization). 

In some molecules, the loss of halogen is unexpectedly facile and may occur extensively even over platinum, as illustrated by reduction of the dihydro-2-benzazepine 4 to 5. The authors (5i) raised the possibility that such facile loss of halogen may involve neighboring-group assistance from the amide moiety. 

Certain protons of fused troponoids show remarkable low-field shifts (Table XII, Scheme 75 for comparison, see Tables IX and X). These protons are abnormally deshielded by the paramagnetic anisotropy effect of peri-positioned or other neighboring groups, especially the tropone carbonyl groups. Similar low-field shifts are caused by adjacent nitro or halogen substituents [e.g., 74YZ1445 80JCS(P1)2081],... 

The hydroxy function may originate from the reduction of an a-halogenated carbonyl compound or a Grignard reaction. The oxirane oxygen is not always incorporated directly from a hydroxy group for instance, in the case of a carboxylic acid ester, it is derived with neighboring-group participation from an ortho-monoester intermediate. 

The first-order hydrolysis of a-halogenated acids appears to be an example of the participation of neighboring groups in substitution reactions (p. 96). 

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