Nature of the Chain Propagation in the Photostimulated Reaction of 1-Bromonaphthalene with Sulfur Centered Nucleophiles

LUCIANA C. SCHMIDT; JUAN E. ARGÜELLO; ALICIA B. PEÑÉÑORY

JOURNAL OF ORGANIC CHEMISTRY

AMER CHEMICAL SOC

Año: 2007 vol. 72 p. 2936 - 2944

0022-3263

The reactivity of -SC(NH)NH2 (1), MeCOS- (2), and PhCOS- (3) toward 1-naphthyl radicals was studied in DMSO. The photostimulated reaction of anions 1, 2, and 3 with 1-bromonaphthalene (4) after quenching with MeI renders 1-(methylthio)naphthalene (6) as a main product together with bis(1-naphthyl) sulfide (7) and naphthalene (5). The thioacetate ion (2) and thiobenzoate ion (3) were unreactive toward 4 as electron-donor under photostimulation; however, in the presence of potassium tert-butoxide anion (entrainment conditions), they gave the mentioned products 5, 6, and 7, after the addition of MeI. Quenching of the triplet state of 4 was assigned as the photoinduced initiation step, with a rate constant value of (4.6 ( 0.5) _ 108 M-1 s-1 for tert-butoxide anion and a rough estimated value of (8 ( 7) _ 107 M-1 s-1 for anion 1. By using hydrogen abstraction from DMSO as the competitive reaction, the absolute rate constants for the addition of anions 1, 2, and 3 to 1-naphthyl radicals have been determined to be 1.0 _ 109, 1.2 _ 109, and 3.5 _ 109 M-1 s-1, respectively. This reactivity order is in agreement with the stability of the resulting radical anions (ArNu).- (10-12).-. The inhibition experiments of the photoinduced substitution reaction in the presence of radical scavengers and the global quantum yield higher than the unity are evidence of a radical chain mechanism for these substitution reactions by anions 1 and 2. Anion 3 adds to the 1-naphthyl radical, but is neither able to initiate nor to keep the propagation cycle. Evaluation of the electron-transfer driving forces for the reaction between (ArNu).- and 4 together with the absence of a chain reaction for the anion 3 indicate that the propagation in the proposed mechanism is given by an acid-base reaction between the radical .C(O)Me or .C(NH)NH2 (13) and a base