REACTION PATHWAYS FOR AMBIDENT ARYLOXIDE O-NUCLEOPHILES AND C-NUCLEOPHILES IN SNAR DISPLACEMENT VERSUS MEISENHEIMER COMPLEX-FORMATION WITH PICRYL HALIDES - STEREOELECTRONIC EFFECTS ON REGIOSELECTIVITY

To probe regioselectivity in Meisenheimer complexation, the reaction o f two picryl halides (PiX where X = F, Cl) with a series of aryloxide nucleophiles (phenoxide, 2,4,6-trimethylphenoxide and 2,6-di-t-butylph enoxide) were monitored by H-1 NMR spectroscopy in dimethyl sulphoxide at ambient temperature and in acetonitrile-dimethoxyethane(MeCN-DME) at low temperature (-40 degrees C). The reactions of both picryl halid es with the ambident (oxygen versus carbon) nucleophile, phenoxide ion (PhO(-)), and 2,4,6-trimethylphenoxide (mesitoxide, MesO(-)) leads to clean SNAr displacement of X via the oxygen site of the nucleophile t o 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-butylphenoxid e (2,6-DTBPhO(-)), where oxygen attachment of the aryloxide is preclud ed 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 competiti on with C-attack at C-3 to give the respective carbon-bonded Meisenhei mer 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 a ttack by the aryloxides at the various positions and stereoelectronic stabilization of C-l adducts afforded by n --> sigma donation.