Structure and C−S Bond Cleavage in Aryl 1-Methyl-1-arylethyl Sulfide Radical Cations

Abstract

Steady state and laser flash photolysis (LFP) of a series of p-X-cumyl phenyl sulfides (4-X-C(6)H(4)C(CH(3))(2)SC(6)H(5): 1, X = Br; 2, X = H; 3, X = CH(3); 4, X = OCH(3)) and p-X-cumyl p-methoxyphenyl sulfides (4-X-C(6)H(4)C(CH(3))(2)SC(6)H(4)OCH(3): 5, X = H; 6, X = CH(3); 7, X = OCH(3)) has been carried out in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP(+)PF(6)(-)) under nitrogen in MeCN. Steady state photolysis showed the formation of products deriving from the C-S bond cleavage in the radical cations 1(+•)-7(+•) (2-aryl-2-propanols and diaryl disulfides). Formation of 1(+•)-7(+•) was also demonstrated by LFP experiments evidencing the absorption bands of the radical cations 1(+•)-3(+•) (λ(max) = 530 nm) and 5(+•)-7(+•) (λ(max) = 570 nm) mainly localized in the arylsulfenyl group and radical cation 4(+•) (λ(max) = 410, 700 nm) probably mainly localized in the cumyl ring. The radical cations decayed by first-order kinetics with a process attributable to the C-S bond cleavage. On the basis of DFT calculations it has been suggested that the conformations most suitable for C-S bond cleavage in 1(+•)-4(+•) and 7(+•) are characterized by having the C-S bond almost collinear with the π system of the cumyl ring and by a significant charge and spin delocalization from the ArS ring to the cumyl ring. Such a delocalization is probably at the origin of the observation that the rates of C-S bond cleavage result in very little sensitivity to changes in the C-S bond dissociation free energy (BDFE). A quite large reorganization energy value (λ = 43.7 kcal mol(-1)) has been calculated for the C-S bond scission reaction in the radical cation. This value is much larger than that (λ = 12 kcal mol(-1)) found for the C-C bond cleavage in bicumyl radical cations, a reaction that also leads to cumyl carbocations.

Transition-metal mediated carbon–sulfur bond activation and transformations

“…Copper(I) reagents, e.g., zinc cuprates (256), derived from readily available vinyl halides, were reacted with 2-thioethyl nitrocyclohexene (257) via conjugate addition/elimination provided the desired nitrodienes (258) which could be utilized for the synthesis of nitroso acetals (eqn ( 105)). 200 Other methods such as steady state and laser flash photolysis (LFP), 201,202 addition of alkylsulfanyl radicals, 203 use of stoichiometric amounts of strong bases, 204 conversion to sulfur ylides 205 and bioinspired protocols 206,207 have also been employed to cleave C-S bonds for various purposes.…”

N‐Alkoxyheterocycles As Irreversible Photooxidants

“…a . The same approach, using 3 , has subsequently been used in studies of triphenylmethyl sulfide and other radical cations .…”

Electron Transfer Mechanism in the Oxidation of Aryl 1-Methyl-1-phenylethyl Sulfides Promoted by Nonheme Iron(IV)–Oxo Complexes: The Rate of the Oxygen Rebound Process

“…Because no cleavable benzylic C−H bonds are present in the α position with respect to the S atom, fragmentation of radical cations 1 +• −4 +• exclusively involves the cleavage of the C α −S bond, as observed in the photochemical oxidations of aryl 1-methyl-1arylethyl sulfides with MeOP + PF 6 − and 9,10-dicyanoanthracene/O 2 systems. 26,28 ■ RESULTS AND DISCUSSION For product analysis of the biomimetic oxidations, aryl 1methyl-1-phenylethyl sulfides (1−4) (50 μmol) were added to a solution of the iron(IV)−oxo complexes prepared by oxidation of [(N4Py)Fe II ] 2+ or [(Bn-TPEN)Fe II ] 2+ (2.5 μmol) with PhIO 18 (12.5 μmol) in CH 3 CN (0.5 mL) and stirred at 0 °C for 90 and 30 min, respectively. The exclusive reaction products, identified by GC, GC−MS, HPLC, and 1 H NMR analyses (comparison with authentic specimens), were 2phenyl-2-propanol, diaryl disulfides, and aryl 1-methyl-1phenylethyl sulfoxides.…”

“…22 The rates of C α −S bond cleavage of a series of aryl 1-methyl-1-arylethyl sulfide radical cations, including 1 +• and 3 +• , have been previously determined by laser flash photolysis (LFP) in the photochemical oxidation of the sulfides performed in the presence of MeOP + PF 6 − in CH 3 CN at 25 °C. 26,33 In the same way, we have now determined the rate constants for fragmentation of 2 +• −4 +• (k f ) at 0 °C by fitting the exponential decays at the maximum absorption wavelengths (see Experimental Section, Table 3, and Figures S5−S7).…”

“…1−4 promoted by [(N4Py)Fe IV O] 2+ and [(Bn-TPEN)Fe IV O] 2+ involves an ET process with formation of radical cations 1 +• −4 +• . C α −S bond cleavage of 1 +• −4 +• leads to the 2-phenyl-2-propyl cation and an arylsulfenyl radical as described in path a of Scheme 3.The cation leads to 2-phenyl-2-propanol by reaction with traces of water present in CH 3 CN, 29 while arylsulfenyl radicals dimerize to diaryl disulfides [26][27][28]…”