Nature of the Chain Propagation in the Photostimulated Reaction of 1-Bromonaphthalene with Sulfur-Centered Nucleophiles
作者:Luciana C. Schmidt、Juan E. Argüello、Alicia B. Peñéñory
DOI:10.1021/jo062569+
日期:2007.4.1
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) x 10(8) M-1 s(-1) for tert-butoxide anion and a rough estimated value of (8 +/- 7) x 10(7) 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 x 10(9), 1.2 x 10(9), and 3.5 x 10(9) M-1 s(-1), respectively. This reactivity order is in agreement with the stability of the resulting radical anions (ArNu)(center dot-) (10-12)(center dot-). 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)(center dot-) 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-center dot(O)Me or C-center dot(NH)NH2 (13) and a base.