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Electron Transfer Mechanism in the Oxidation of Aryl 1Methyl-1-phenylethyl Sulfides Promoted by Nonheme Iron(IV)Oxo Complexes: The Rate of the Oxygen Rebound Process Alessia Barbieri, Tiziana Del Giacco, Stefano Di Stefano, Osvaldo Lanzalunga, Andrea Lapi, Marco Mazzonna, and Giorgio Olivo J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02434 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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The Journal of Organic Chemistry

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 Alessia Barbieri,a Tiziana Del Giacco,b Stefano Di Stefano,a Osvaldo Lanzalunga,*a Andrea Lapi,a Marco Mazzonna,a and Giorgio Olivoa

a

Dipartimento di Chimica and Istituto CNR di Metodologie Chimiche-IMC, Sezione Meccanismi di

Reazione c/o Dipartimento di Chimica, Università degli Studi di Roma “La Sapienza”, P.le A. Moro 5, 00185 Rome, Italy. b

Dipartimento di Chimica, Biologia e Biotecnologie and Centro di Eccellenza Materiali Innovativi

Nanostrutturati, Università di Perugia, via Elce di sotto 8, 06123 Perugia, Italy.

Email: [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

Abstract: The oxidation of aryl 1-methyl-1-phenylethyl sulfides promoted by the nonhemeiron(IV) oxo complexes [(N4Py)FeIV=O]2+ and [(Bn-TPEN)FeIV=O]2+ occurs by an electron 1 ACS Paragon Plus Environment


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transfer - oxygen rebound (ET-OT) mechanism leading to aryl 1-methyl-1-phenylethyl sulfoxides accompanied by products deriving from Cα-S fragmentation of sulfide radical cations (2-phenyl-2propanol and diaryl disulfides). For the first time the rate constants for the oxygen rebound process (kOT), which are comprised in the range 95%). No products were formed in the absence of PhIO while small amounts of aryl 1-methyl-1-phenylethyl sulfoxides (< 1 %, referred to the amount of oxidant) were observed in the oxidation with PhIO in the absence of the nonheme complexes [(N4Py)FeII]2+ or [(BnTPEN)FeII]2+.

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Products and yields, referred to the amount of oxidant, are reported in Table 1 and in Table 2 for the oxidations promoted by [(N4Py)FeIV(O)]2+ and [(Bn-TPEN)FeIV=O]2+, respectively.

Table 1. Products and Yields in the Oxidation of Aryl 1-Methyl-1-phenylethyl Sulfides (1-4) by [(N4Py)FeIV=O]2+ in CH3CN at 0°C.a Products (Yields %)b

Substrate

Ratio [OH]/[SO]

1 X=OCH3

31±2

14±1

64±5

0.48

2 X=CH3

39±2

15±1

44±4

0.89

3 X=H

40±4

16±2

25±2

1.6

4 X=CF3

48±5

22±2

8.4±0.5

5.7

a

II

Iodosylbenzene (12.5 µmol), [(N4Py)Fe (OTf)2] (2.5 µmol) and aryl 1-methyl-1-phenylethyl sulfides (50 µmol) in CH3CN (500 µL). b Yields (mol %) refer to the amount of oxidant PhIO.

Table 2. Products and Yields in the Oxidation of Aryl 1-Methyl-1-phenylethyl Sulfides (1-4) by [(Bn-TPEN)FeIV=O]2+ in CH3CN at 0°C.a Products (Yields %)b

Substrate

a

Ratio [OH]/[SO]

1 X=OCH3

56±4

27±2

44±4

1.3

2 X=CH3

62±5

25±2

33±2

1.9

3 X=H

64±6

28±4

18±2

3.6

4 X=CF3

70±5

27±3

10±1

7.0

II

Iodosylbenzene (12.5 µmol), [(Bn-TPEN)Fe (OTf)2] (2.5 µmol) and aryl 1-methyl-1-phenylethyl sulfides (50 µmol) in CH3CN (500 µL). b Yields (mol %) refer to the amount of oxidant PhIO.


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From the data reported in Tables 1-2, it can be readily noted that higher yields of oxidation products are observed in the oxidations promoted by [(Bn-TPEN)FeIV=O]2+ with respect to those promoted by [(N4Py)FeIV(O)]2+ in accordance with the higher oxidizing ability of the former iron(IV)-oxo complex as also found in the oxidation of para-substituted thioanisoles.18 Moreover a regular increase of the total yields of oxidation products are observed with both the iron-oxo complexes by increasing the electron releasing power of the aryl substituents, in accordance with the electrophilic nature of the oxidizing species [(N4Py)FeIV=O]2+ and [(Bn-TPEN)FeIV=O]2+.18 As previously observed in the oxidation of aryl diphenylmethyl sulfide by the [(N4Py)FeIV=O]2+,22 the Soxygenation products (sulfoxides) are accompanied by significant amounts of fragmentation products (2-phenyl-2-propanol and diaryl disulfides). Thus also the oxidation of 1-4 promoted by [(N4Py)FeIV=O]2+ and [(Bn-TPEN)FeIV=O]2+ involves an ET process with formation of radical cations 1+•-4+•. Cα-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 CH3CN,29 while arylsulfenyl radicals dimerize to diaryl disulfides.26-28,32



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CH 3 II 2+ X + [(L)Fe ]

S CH 3 O

(b) CH 3 S

k OT

CH 3

[(L)Fe IV=O] 2+

+.

X + [(L)FeIII=O]+

S

X

CH 3

CH 3 X= OCH3 , CH3 , H, CF3

(a)

kf .

CH3

L = N4Py, Bn-TPEN

S

+

CH3

X x2

+H 2O, -H+ CH 3

S

S

OH CH 3

X

X

Scheme 3. Fragmentation vs oxygen rebound processes in the oxidation of aryl 1-methyl-1phenylethyl sulfides (1-4) by [(N4Py)FeIV=O]2+ and [(Bn-TPEN)FeIV=O]2+ in CH3CN.

Fragmentation of radical cations 1+•-4+• occurs in competition with the oxygen rebound process from the reduced iron(III)-oxo complexes to the radical cations leading to sulfoxides (OT, Scheme 3, path b).22 In accordance with the presence of the two competitive decay pathways for 1+•-4+• (oxygen rebound and Cα-S fragmentation), a regular increase of the 2-phenyl-2-propanol / aryl 2phenyl-2-propyl sulfoxide product ratios (Tables 1-2) is observed on increasing the electron withdrawing (EW) effect of the aryl substituents as shown in Figure 2.


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Figure 2. Yields (% referred to the amount of oxidant) of aryl 1-methyl-1-phenylethyl sulfoxides and 2-phenyl-2-propanol in the oxidation of aryl 1-methyl-1-phenylethyl sulfides 1-4 by (a) [(N4Py)FeIV=O]2+ and (b) [(Bn-TPEN)FeIV=O]2+ in CH3CN at 0°C.

Such increase of the relative amount of the Cα-S fragmentation products observed by increasing the EW power of the X substituents, results from the enhancement of fragmentation rate constants of the radical cations on going from 1+• to 4+•, as already reported for the oxidation of aryl diphenylmethyl sulfides by the [(N4Py)FeIV=O]2+.22 The rate of the Cα-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 carried out in the presence of MeOP+PF6- in CH3CN at 25°C.26,33 In the same way we have now determined the rate constants for fragmentation of 2+•-4+• (kf) at 0°C by fitting the exponential decays at the maximum absorption wavelengths (see experimental section, Table 3 and Figures S5-S7 in the Supporting Information). 9 ACS Paragon Plus Environment


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Table 3 shows that fragmentation rates regularly increase regularly from 1+• to 4+• in the same way as the 2-phenyl-2-propanol / aryl 1-methyl-1-phenylethyl sulfoxide molar ratios increase going from 1 to 4 as reported in Tables 1 and 2.

Table 3. Maximum Absorption Wavelengths and Cα-S Fragmentation Rate Constants (kf) of Aryl 1Methyl-1-phenylethyl Sulfide Radical Cations (1+•-4+•) Generated by Photooxidation of 1-4 Sensitized by MeOP+PF6- (λexc=355nm) and Oxygen Rebound Rate Constants (kOT) for the Oxidation of 1-4 by [(N4Py)FeIV=O]2+ and [(Bn-TPEN)FeIV=O]2+ in CH3CN at 0°C. kOT(104 s-1)b

4 -1 a

λMAX

kf(10 s )

1 X=OCH3

570

2 X=CH3

[(N4Py)FeIV=O]2+

[(Bn-TPEN)FeIV=O]2+

Electron Transfer Mechanism in the Oxidation of ... - ACS Publications

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