Chain propagation in the photostimulated reaction of 1- bromonaphthalene with sulfur centered nucleophiles in DMSO

LUCIANA C. SCHMIDT, JUAN E. ARGÜELLO, VALENTINA REY AND ALICIA B. PEÑÉÑORY

La Plata

Congreso; VIII Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2004

The reactivity of -SCNH(NH2) (1), CH3COS- (2) and PhCOS- (3) in photoinduced substitution reactions with 1-bromonaphthalene (4) was studied in DMSO. The photostimulated reaction of ion 1 with 4 renders after quenching with MeI 1- (methylthio)naphthalene (50%), bis (1-naphthyl) sulfide (22%) and naphthalene (9%). This reaction does not occur in the dark and is inhibited in the presence of DTBN. The thioacetate ion (2) was unreactive towards 4 as electron donor under photostimulation, but in the presence of potassium ter-butoxide or the enolate anion of cyclohexenone under irradiation (entrainment conditions), it gave 1-(methylthio) naphthalene (40%), bis (1-naphthyl) sulfide (14%) and naphthalene (6%), after quenching with MeI. Similar results were obtained with the ion 3. The lack of reaction in the dark and the inhibition of the photoinduced one by the presence of di-tertbutylnitroxide (DTBN) and p-dinitrobenzene (p-DNB) are evidence of a radical chain mechanism for these substitution reactions. To account for the chain reaction, a radical anion intermediate 9 is possible from deprotonation of 7 which by electron transfer to 4 keeps the catalytic cycle, as evidenced by the quantum yield dependence with the base concentration (Scheme 1). By using hydrogen abstraction from DMSO as competitive reaction, the absolute rate constants for the addition of ions 1, 2 and benzene thiolate ions to 1-naphthyl radicals have been determined to be 1.0 x 109 M-1s-1, 1.2 x 109 M-1s-1 and 5.1 x 109 M-1s-1 respectively.