CARBON VERSUS OXYGEN NUCLEOPHILIC SELECTIVITY IN THE REACTION OF THE ARYLOXIDE IONS, 2,6-DI-TERT-BUTYLPHENOXIDE AND 3,5-DI-TERT-BUTYLPHENOXIDE, WITH THE 2-[(NITRO)(X)ARYL]-4,6-DINITROBENZOTRIAZOLE 1-OXIDE SERIES OF SUPER-ELECTROPHILES - STEREOELECTRONIC FACTORS ON C-7 MEISENHEIMER COMPLEX-FORMATION VERSUS C-1' SNAR DISPLACEMENT

The 2-[(nitro)aryl]-4,6-dinitrobenzotriazole 1-oxides (1, Pi-DNBT (x = 3); 2, DNP-DNBT (x = 2); 3, NP-DNBT (x = 1)) are electron-deficient n itro-substituted heteroaromatic substrates that possess two sites for nucleophilic attachment: C-7 and C-1'. Generally, attack at the super- electrophilic C-7 site yields spectroscopically observable anionic sig ma-bonded adducts, whereas attack at C-1' leads to displacement produc ts in an overall process of nucleophilic aromatic substitution (SNAr). To gain an understanding of the factors affecting C-1' versus C-7 att ack by potentially ambident aryloxide (C- and O-)nucleophiles, we have monitored the reactions of 1-3 with 2,6-di-tert-butylphenoxide (2,6-A rO-) and 3,5-di-tert-butylphenoxide (3,5-ArO-) using 400 MHz H-1 NMR s pectroscopy (deuterated dimethyl sulfoxide solvent at ambient temperat ure). The results indicate that 2,6-ArO- acts only as a C-nucleophile with O-attack precluded, presumably by the sterically demanding tert-b utyl groups flanking the O-nucleophilic centre. Although 2,6-ArO- reac ts preferentially at C-7 of 1-3, the biphenyl derivative that arises f rom C-1' attack is also observed with I, the first time that C-nucleop hilic attack has been seen at this electrophilic site. In contrast, 3, 5-ArO- acts only as an O-nucleophile, also as a consequence of the ste ric hindrance to the C-4 position; this aryloxide reacts entirely at C -1' of Pi-DNBT but also exclusively at C-7 of 3. However, with DNP-DNB T, 2, both the C-7 O-adduct and C-1' displacement products are noted; attack at C-1' is dominant. The selectivity (C-7 versus C-1') found in these reactions is discussed with emphasis given to stereoelectronic factors that may stabilize the putative C-1' O-adducts.