Abstract
The reactivity of S 2- (1), - SCN (2) and - SCNH(NH2) (3) in photoinduced substitution reactions with 1-bromonaphthalene (5) in DMSO was studied. The photostimulated reaction of ion 1 with 5 renders after quenching with MeI 1-(methylthio)naphthalene (16%), bis (1-naphthyl) sulfide (17.5%) and naphthalene (42%). The lack of reaction in the dark and the inhibition of the photoinduced one by the presence of di-tert-butylnitroxide (DTBN) and p-dinitrobenzene (pDNB) are evidence of a radical chain mechanism for these substitution reactions. On the other hand, ion 2 was unreactive towards 5, both as an electron donor under photostimulation and as a nucleophile. Finally the thiourea anion (3) affords after a photoinduced reaction with 5 and quenching with MeI, 1-methylthionaphthalene (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. By using hydrogen abstraction from DMSO as competitive reaction, the absolute rate constants for the addition of ions 1, 3 and benzene thiolate ions (4) to 1-naphthyl radicals have been determined to be 0.5 x 10 9 M -1 s -1 , 1.0 x 10 9 M -1 s -1 and 5.1 x 10 9 M -1 s -1 respectively.
Highlights
Radical nucleophilic substitutions involving electron transfer pathways or SRN1 is an area of wide interest and its most relevant results have many times been reviewed.[1]
Sulfur centered nucleophiles, such as thiolate ions have been shown to react with aryl halides under photostimulation to yield a new C-S bond.1a,b While arene thiolate ions (ArS-) afford good yields of substitution products, alkane thiolate ions (RS-) yield a mixture of products from fragmentation of the radical anion intermediates and straightforward substitution, making the latter reactions disadvantageous for synthetic purposes.[2]
We have previously studied the reactivity of thiourea anion (3) in photoinduced aromatic radical nucleophilic substitution as a “one pot” method for the synthesis of aryl thiols, alkyl aryl sulfides, symmetrical or unsymmetrical diaryl sulfides and diaryl disulfides from moderate to good yields.[9]
Summary
Radical nucleophilic substitutions involving electron transfer pathways or SRN1 is an area of wide interest and its most relevant results have many times been reviewed.[1]. We report the photoinduced reaction of S2- (1) and -SCN (2) with haloarenes in DMSO and the absolute rate constants for the coupling reactions of ions 1, 3 in competitive reactions with benzene thiolate ion (4) towards 1-naphthyl radicals.
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