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Reaction pathways for ambident aryloxide O‐ and C‐nucleophiles in S N Ar displacement versus Meisenheimer complex formation with picryl halides. Stereoelectronic effects on regioselectivity
Author(s) -
Manderville Richard A.,
Dust Julian M.,
Buncel Erwin
Publication year - 1996
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(199608)9:8<515::aid-poc816>3.0.co;2-5
Subject(s) - chemistry , regioselectivity , nucleophile , halide , displacement (psychology) , medicinal chemistry , stereochemistry , organic chemistry , catalysis , psychology , psychotherapist
To probe regioselectivity in Meisenheimer complexation, the reaction of two picryl halides (PiX where X = F, Cl) with a series of aryloxide nucleophiles (phenoxide, 2,4,6‐trimethylphenoxide and 2,6‐di‐ t ‐butylphenoxide) were monitored by 1 H NMR spectroscopy in dimethyl sulphoxide at ambient temperature and in acetonitrile‐dimethoxyethane(MeCN‐DME) at low temperature (−40°C). The reactions of both picryl halides with the ambident (oxygen versus carbon) nucleophile, phenoxide ion (PhO − ), and 2,4,6‐trimethylphenoxide (mesitoxide, MesO − ) leads to clean S N Ar displacement of X via the oxygen site of the nucleophile to form the respective aryl picryl ethers, i.e. phenyl picryl ether (3a) and mesityl picryl ether (3b). Meisenheimer complex formation at C‐1 or C‐3 was not detected in these systems. With 2,‐6‐di‐ t ‐butylphenoxide (2,6‐DTBPhO − ), where oxygen attachment of the aryloxide is precluded by the bulky ortho t ‐butyl groups, para ‐carbon attachment was found to occur at C‐1 to give picryl 2,6‐di‐ t ‐butylphenol (3d) in competition with C‐attack at C‐3 to give the respective carbon‐bonded Meisenheimer complexes [X = Cl (4) and X = F (5)]. For both picryl halides, the ratio of 3d, the product of C‐1 attack, to the product of C‐3 attack, 4 or 5, was roughly 7:1. These findings are considered with regard to the nucleofugality of the halide, X, steric hindrance (F‐strain) to attack by the aryloxides at the various positions and stereoelectronic stabilization of C‐1 adducts afforded by n → σ * donation.