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A Radical and An Electron Transfer Process Are Compared in Their Regioselectivities Towards A Molecule with Two Different C−I Bonds: Effect of Steric Congestion
Author(s) -
Branchi Barbara,
Galli Carlo,
Gentili Patrizia,
Marinelli Manuela,
Mencarelli Paolo
Publication year - 2000
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200007)2000:14<2663::aid-ejoc2663>3.0.co;2-y
Subject(s) - chemistry , steric effects , regioselectivity , radical , molecule , radical nucleophilic aromatic substitution , bond cleavage , ketone , medicinal chemistry , single bond , stereochemistry , photochemistry , computational chemistry , organic chemistry , nucleophilic aromatic substitution , nucleophilic substitution , catalysis , alkyl
Steric compression in 1,4‐diiodo‐2,6‐dimethylbenzene ( 2a ) makes the C−I bond flanked by methyls substantially weaker (a buttressing effect) than the unhindered C−I bond. Calculations also confirm the weaker bonding interaction of the hindered C−I bond of 2a . This causes a remarkable regioselectivity toward the weaker bond in dehalogenation by stannyl radicals. Conversely, a much lower regioselectivity is found for a process −a photostimulated S RN 1 reaction with the enolate ion of a ketone − which requires the conversion of 2a into a radical anion. A calculation of the BDE of the C−I bond for aa ArI •− system is offered. Finally, the hindered aryl radical intermediate resulting from cleavage of the weaker C−I bond of 2a •− shows a modest but detectable discrimination between reduction or substitution, this once again being due to the steric congestion.