LOW-VALENT RHENIUM-OXO COMPLEXES .11. REACTIONS OF THE LOW-VALENT RHENIUM OXO ANIONS NARE(O)(RCCR)2 WITH ALKYL AND ARYL HALIDES - EVIDENCE FOR BOTH SN2 AND RADICAL MECHANISMS

The low-valent monooxo compounds NaRe(O)(RC = CR)2 [R = Me (1a), Et (1b), Ph (1c)] are strong bases and potent reducing agents, highly unusual reactivity for terminal oxo compounds. They are all protonated by H2O in CD3CN, 1a and 1b even by stoichiometric acetone in this solvent, to form the Re(III) oxo hydride complexes Re(O)H(RC = CR)2. Compounds 1 react as nucleophiles with the Me+ sources MeI, MeOTs, and Me3O+BF4- and with the acyl sources acetic anhydride and acetyl chloride, forming Re(O)Me(RC = CR)2 and Re(O)[C(O)Me](RC = CR)2, respectively. While there is no evidence for radical intermediates with these reagents, reactions with primary to tertiary alkyl halides occur at least in part by an electron-transfer pathway. Primary and secondary alkyl halides react to form mixtures of Re(III) alkyls, Re(O)R'(RC = CR)2, and Re(II) dimers, Re2(O)2(RC = CR)4 (2). (t)BuI and 1 produce solely 2 and the organic radical disproportionation products isobutane and isobutylene. The intermediacy of organic radicals is confirmed by significant cyclization of the hexenyl radical clock in the reaction of 6-iodo-1-hexene with 1a. Aryl halides are also reactive with 1a and 1b to produce, in roughly equal yields, novel Re(O)Ph(RC = CR)2 compounds and 2 in C6D6 solvent. In CH3CN, however, trapping of aryl radicals by the solvent occurs to produce Re(O)CH2CN(RC = CR)2 compounds and arene (together with 2). These reactions appear to occur via initial electron transfer from rhenium to the organic halide. Compounds 1 are tightly ion paired and show reactivity that is dependent on the solvent and on the presence or absence of 15-crown-5. The diphenylacetylene derivative 1c is significantly less reactive than the dialkylacetylene complexes 1a and 1b because of the influence of the better pi-acceptor PhC = CPh on the HOMO in 1.