Photochemical co-oxidation of sulfides and phosphines with tris(p

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Photochemical co-oxidation of sulfides and phosphines with tris(p-bromophenyl)amine. A mechanistic study. Sergio Mauricio Bonesi, Stefano Protti, and Angelo Albini J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00913 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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The Journal of Organic Chemistry

Photochemical co-oxidation of sulfides and phosphines with tris(p-bromophenyl)amine. A mechanistic study. Sergio M. Bonesi*,a,b, Stefano Prottib and Angelo Albinib a

Departamento de Química Orgánica,CIHIDECAR – CONICET, 3er Piso, Pabellón 2, Ciudad

Universitaria, FCEyN, University of Buenos Aires, Buenos Aires, 1428, Argentina. Phone/FAX:+541145763346. b

PhotoGreen Lab, Department of Chemistry, V.leTaramelli 12, 27100 Pavia, Italy.

E-mail: [email protected]

Table of Content. e

Br

Br Br

Ar3P-OO.+ N Ar

Ar3P Ar3P=O

O2

O2 e

hν ν

e Ar3P

Br Br

Co-oxidation of phosphines

H

H

HO O H

Br

N Ar oxidizing Br species

Br N Ar

N Ar

R 2S Products

R2S=O + H2 O

Co-oxidation of sulfides

one-electron oxidizing species

Abstract. The photochemistry of tris(p-bromophenyl)amine was investigated in nitrogen- and oxygen-flushed solution under laser flash photolysis conditions. Intermediates detected were the corresponding amine radical cation (‘Magic Blue’) and the N-phenyl-4a,4b-dihydrocarbazole radical cation that, under an oxygen atmosphere is converted to the corresponding hydroperoxyl radical. The role of the last species was supported by the smooth co-oxidation of sulfides to sulfoxides. On the other hand, co-oxidation of nucleophilic triarylphosphines to triarylphosphine oxides arose from an electron transfer process between the photogenerated "Magic Blue" and phosphine that prevented the amine

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cyclization. In this case, intermediate Ar3POO•+ was found to play a key-role in phosphine oxide formation.

Introduction. Single electron transfer processes (SET, Scheme 1a) have a key-role ranging from synthetic chemistry1 to material chemistry.2 However, since the key intermediate contains an unpaired electron, it is much more reactive than the starting compound, and therefore multi-electron processes, interference with oxygen and chain reactions often followed spontaneously SET.

Scheme 1. a) Single Electron Transfer (SET) processes; b) SET occurring on triarylamines and following pathways observed. In the case of SET-based oxidation processes, organic oxidants, such as the popular 'Magic Blue',3 are often preferred to metal ions for mechanistic investigations since the better control on the kinetics of the reaction that these reagents allow. In this context, the case of tris(4bromophenyl)amine (1, Chart 1), the precursor of Magic Blue would be of high interest, because in the past, structurally related, but less oxidizing reagents, di- and triaryl amines were reported to undergo smooth cyclization to the corresponding carbazoles upon irradiation. The reactions of such compounds in the presence of nucleophiles under photochemical conditions are also investigated in detail3 and the competition between different processes may be more informative. Furthermore, the


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photoinduced electrocyclization of stable triarylamine radical cations was deeply described by M. A. Fox and co-workers.4

Chart 1. Structure of tri(4-bromophenyl)amine.

For this aim, we thought thus worthwhile to examine in detail the photochemical behavior of the Magic Blue precursor, viz. tris(p-bromophenyl)amine (1), in the presence of nucleophilic additives such as sulfides and triarylphosphines, oxygen and in solvents of different nature.

Results. Time-resolved spectroscopic measurements of tris(p-bromophenyl)amine (1) in solution. In order to obtain mechanistic information on the photochemistry of amine 1 in MeCN at room temperature, laser flash photolysis studies were carried out. Under nitrogen atmosphere, the initial formation of a transient with three bands centered at 370, 590 and 695 nm was evidenced (see Fig 1). This .

absorption pattern was identical to that of Magic Blue (1 +),5 and evolved in 38 µs into a further transient (λmax = 750 nm, τ>200 µs) that has been assigned, on the basis of previous studies, to the dihydrocarbazole 2 (Fig 1).4a,6 A comparison between the absorption spectra of transients 1•+, 2 and commercial Magic Blue (Figure 2) pointed out that 1•+ is formed after the laser pulse. In Fig. 2a the decay of the intermediate and its corresponding conversion into 2 are clearly showed and Scheme 2 depicts the observed stepwise process.



Scheme 2. Formation of transients 1•+ and 2 in nitrogen equilibrated media.

On the other hand, in oxygen equilibrated solution the end-of-pulse spectrum showed an additional feature at 550 nm, then evolving to give the same end spectrum as under nitrogen, where the absorption spectra of transients 1•+ and 2 were detected (Fig 1b). Figure 2b shows the decay trace of 1•+ and the formation of trace of transient 2 recorded after the laser pulse under oxygen atmosphere. The quantum yield values for formation of the two intermediates along with their lifetime were measured under both atmospheres, by using benzophenone in benzene as the reference (Table 1).

0.020

0.016

(a)

(b)

0.012

∆ OD

0.015

∆ OD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.010

0.008

0.004

0.005

0.000

0.000 400

500

600

700

800

400

500

600

λ / nm

λ / nm


700

800

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2.0

(c)

Arel

1.5

1.0

0.5

0.0 400

500

600

700

800

λ / nm

Figure 1. Time-resolved absorption spectra of transients of tris-(p-bromophenyl)amine (1, 0.3 mM) recorded during 200 µs after laser pulse (355 nm) in MeCN under (a) nitrogen and (b) oxygen atmospheres. (c) Absorption spectra of commercial Magic Blue (solid line), photogenerated 1•+ recorded at 10 µs after the pulse (dashed line) and transient 2 recorded at 90 µs after the pulse (dotted line).

(a)

(b)

0.02

∆ OD

0.02

∆ OD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.01

0.01

695 nm 740 nm

0.00

0

50

100

150

705 nm 750 nm

0.00

200

0

t / µs

50

100

150

200

t / µs

Figure 2. Decay traces of transients from tris(p-bromophenyl)amine (0.3 mM) recorded after laser pulse (355 nm) in MeCN under (a) nitrogen and (b) oxygen atmospheres.



The fleeting band located at 550 nm in the time-resolved absorption spectrum of tris(pbromophenyl)amine 1 (see fig. 1(b)) was assigned to the charge-separated complex between 1 and molecular oxygen which is energetically favorable process in polar solvents as it was early proposed by Wilkinson7 and in recently reported literature.8 Scheme 3 shows the formation of the chargetransfer complex that is converted to the tris(p-bromophenyl)amine radical cation (1.+) and superoxide ion and Figure 3 depicts the decay trace of the charge-transfer complex in MeCN under O2 atmosphere.

3

*

Br

+

N Br

3

O2

δ O2

Nδ Br

Br

Charge-Transfer Complex λabs = 550 nm

φISC Br

3

τET = 17.9 µs kET = 5.6x104 s-1 N Br

+

O2.-

Br

(1.+; λabs = 695 nm)

O2

τB = 2.2 µs kB = 4.5x105 s-1

N Br

Br

Br

Br

Laser Pulse Solvent, O2

Br

1

Scheme 3. Formation of a charge-transfer complex and transient 1•+ in oxygen equilibrated media.

0.025

λ = 550 nm 0.020

0.015

OD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.010

0.005

0.000

-0.005 -10

0

10

20

30

40

t / µs


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Figure 3. Decay trace of charge transfer complex recorded at 550 nm after excitation with a laser pulse (355 nm) of a solution of tris-(p-bromophenyl)amine (0.3 mM) in MeCN under oxygen atmosphere.

The charge-transfer complex trace shown in fig. 3 fitted with a bi-exponential decay and two halflifetime decays were obtained, viz. 2.2 µs and 17.9 µs, respectively. The evolution of the chargetransfer complex involved the reversion pathway to tris(p-bromophenyl)amine (1) and molecular oxygen with a rate constant kB of 4.5x105 s-1 and an electron-transfer pathway with a rate constant kET of 5.6x104 s-1 to provide tris(p-bromophenyl)amine radical cation (1.+) and superoxide ion (see Scheme 3). Noteworthy, under O2 atmosphere the quantum yield values for the formation of 1•+ and 2 were halved highlighting the interaction between molecular oxygen and 1 in its triplet excited state to produce singlet oxygen. Therefore, this competitive pathway diminishes the formation of both of the ensuing intermediates, 1•+ and 2. A similar oxygen effect was observed during the irradiation of triphenylamine in different solvents and was attributed to the quenching of the triplet excited state of triphenylamine by molecular oxygen producing singlet oxygen with φ∆ = 0.63.6 In this case, the corresponding intermediate dihydrocarbazole, which is equivalent to intermediate 2 is formed in a lesser extent in oxygen atmosphere than in nitrogen atmosphere.

Table 1. Measured Quantum Yields (Φ) for the formation of intermediates 1•+ and 2 under the examined conditionsa

Transients

λabs / nm

Atmosphere

Conc. / M

φ

τ / µs

1•+

695

N2

1.24x10-6

0.158

39



5

a

750

O2

8.29x10-7

0.077

N2

3.53x10-7

0.066

O2

2.17x10-7

0.030

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> 200

Measured in MeCN after laser pulse (355 nm); ε(1•+) = 19000 M-1 s-1; ε(2) = 30000 M-1 s-1. The

initial absorbance of 1 was fixed at 0.40. Actinometer: solution of benzophenone in benzene under N2 atmosphere (A(355 nm) = 0.37); ε(ketyl radical) = 7220 M-1 cm-1.

Further, the decay traces of transient 1•+ at 695 nm were also measured in MeCN after a laser pulse (355 nm) under different conditions (Figure 4) with the aim to determine both the rate constants of cyclization (kcyc) and back-electron transfer (kBET, see Scheme 2). The obtained results were collected in Table 2. Noteworthy, the traces of 1•+ show a nicely bi-exponential decay fitting with r2 values higher than 0.998 independent from the atmosphere used. As can be seen in Table 2 τcyc and kcyc remain almost constant for all the atmospheres used and also in an air-equilibrated solution while a noticeable dependence on the atmosphere was seen for the back-electron transfer process (τBET and kBET), particularly in the cases of air-equilibrated and oxygen atmosphere. Likewise, a similar behavior was observed for the concentration of 1•+ at zero time after the laser pulse which was attributed to the quenching of the triplet excited state of 1 by molecular oxygen.


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0.14 0.12

N2 N2O

0.10

Air O2

0.08 OD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06 0.04 0.02 0.00 -0.02 0

50

100

150

200

t / µs

Figure 4. Normalized decay traces of transients from tris(p-bromophenyl)amine (1, 0.3 mM) recorded after laser pulse (355 nm) in MeCN under: N2 (blue line); N2O (black line); Air (red line) and O2 (violet line) atmospheres.

Table 2. Rate constants, lifetimes and concentration of tris(p-bromophenyl)amine radical cation (1•+) at the end-of-pulse.a

a

Atmosphere

τcyc / µs

kcyc / s-1

τBET / µs

kBET / M-1 s-1

Conc. / M

N2

40.0

2.5x104

3.7

2.2x1010

5.93x10-6

N2O

40.0

2.5x104

3.7

2.1x1010

5.93x10-6

Air

39.2

2.6x104

5.4

1.9x1010

4.39x10-6

O2

37.0

2.7x104

6.4

1.4x1010

3.29x10-6

Measured in MeCN after laser pulse (266 nm); ε(1+·) = 19000 M-1 s-1. The initial absorbance of the

solutions was fixed at 0.64. The error of τ1 is ± 0.2 and for τ2 is ± 0.1. Due to the bi-exponential decay fitting of 1.+, the consumption of transient 1.+ can be written as eq. 1 highlighting the unimolecular electrocyclization step and the bimolecular back-electron transfer process, respectively, where the one-ejected electron after the laser pulse is the solvated electron and is designed as e-. ACS Paragon Plus Environment


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The conversion of 1•+ to 2 (see Scheme 2) involves a first order decay, and the kcyc can be calculated straightforward from the reciprocal of the half-lifetime (τcyc-1) under all the conditions examined and the data thus obtained are shown in Table 2. On the other hand, back electron transfer (BET) of 1•+ to tris(p-bromophenyl)amine (see Scheme 2 and eq. (1)) can be represented by kBET. From the slopes of the linear regression of the reciprocal of the concentration of transient 1•+ vs. time plots the bimolecular rate constants, kBET, of 1010 M-1 s-1 were obtained under all the conditions examined (see Table 2).

Co-oxidation of sulfides and phosphines. The oxidation activity of the photogenerated intermediates, namely 1.+ and 2, was tested by irradiating solutions of 1 in the presence of some substrates as the electron donors such as sulfides and phosphines (see Scheme 4). Under such conditions, 1 was consumed, while the additives were co-oxidized selectively to the corresponding sulfoxides and phosphine oxides. In the case of benzyl ethyl sulfide, however, significant amounts of benzaldehyde were also formed. The obtained results have been summarized in Tables 3 and 4. Sulfides (including Ph2S that is inert towards singlet oxygen9) were co-oxidized with a rate value ranging from 0.003 and 0.244 µmol min-1 while the ratio sulfoxidation/electrocyclic cyclization was lower than unity (0.06 to 0.80) in all of the solvents investigated, with the only exception of benzyl ethyl sulfide in MeCN/H2O 9/1 mixture. Nucleophilic triarylphosphines were also co-oxidized with a phosphine oxidation/electrocyclic cyclization ratio ranging from ca. 26/1 to 5/1 under irradiation of amine 1 in acetonitrile.


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R2S Ph2S

R Ph2S(O)

PhSMe

p-MeOC6H4SMe PhCH2SEt

p-MeOC6H4S(O)Me

Br

Ar3P

hn (366 nm) Solvent, O2

N

Me Br P 3 Ph3P

O S

PhS(O)Me PhCH2SEt + PhCHO

Ar3P=O Me

Br 1

Me

R

P=O

Me

3 Me

P 3 Me

Ph3P=O

Me

P=O 3 Me

Scheme 4. Co-oxidation of sulfides (upper path) and triarylphosphines (lower path) in the presence of 1. Table 3. Rate of co-oxidation of sulfides upon irradiation in the presence of 1.a

Rate of reaction (µmol.min-1) Substrate

Solvent

Sulfoxide

1b

Ph2S

MeCN

0.003

0.051

MeCN/H2O 9/1

0.005

0.041

TFE

0.033

0.049

DCM

0.008

0.026

MeCN

0.024

0.20

MeCN/H2O 9/1

0.091

0.31

TFE

0.193

0.24

DCM

0.012

0.05

MeCN

0.026

0.085

MeCN/H2O 9/1

0.118

0.239

PhSMe

p-MeOC6H4SMe


Other products


PhCH2SEt

a

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TFE

0.240

0.234

DCM

0.005

0.030

MeCN

0.088

0.065

PhCHO, 0.010

MeCN/H2O 9/1

0.184

0.150

PhCHO, 0.024

TFE

0.244

0.320

PhCHO, 0.030

DCM

0.093

0.024

PhCHO, 0.013

5x10-3 M solution of 1 irradiated in the presence of the chosen sulfide (10-2 M) at 310 nm (10 x 15

W lamps).b Rate of consumption of 1.

Table 4. Rate of co-oxidation of triarylphosphines upon irradiation in the presence of 1 in MeCN.a Rate of co-oxidation (µmol.min-1)

a

Substrate

Phosphine oxide

1b

Ph3P

2.91

0.11

(o-MeC6H4)3P

2.75

0.31

(2,4,6-Me3C6H2)3P

4.13

0.91

5x10-3 M solution of 1 irradiated in the presence of the chosen phosphine (1.1x10-2 M) at 366 nm

(High pressure Hg lamp provided with an interference Schott filter at 366 nm). bRate of consumption of 1.

Blank experiments were also performed. Thus, irradiation of oxygenated solutions of sulfides and phosphines in all the solvents used in the absence of tris(p-bromophenyl)amine (1) did not provide any sulfoxides or phosphine oxides. Likewise, experiments performed under N2 atmosphere but in the presence of tris(p-bromophenyl)amine (1) did not give any oxide.


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Interaction of sulfides and triphenylphosphine with photogenerated transients. Time-resolved spectroscopy was applied to solutions of 1 in the presence of selected sulfides and triphenylphosphine. Diphenyl sulfide, thioanisole and benzyl ethyl sulfide caused no change on the previously observed transients. On the other hand, p-methoxythioanisole and triphenylphosphine quenched the radical cation 1•+ as shown in Fig. 5.

30000

25000

20000

kobs / s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15000

10000

5000

0 0.000

0.002

0.004

0.006

0.008

0.010

0.012

[Q] / M

Figure 5. Quenching of 1•+ with increasing concentration of p-methoxythioanisole (□) and of triphenyl phosphine (○). Straight lines are the best linear regression fittings.

In the case of p-methoxythioanisole and triphenylphosphine, the observed first-order decay constant (kobs) of 1•+ depends linearly on the quencher concentration as it is shown in eq. 3.

Plotting kobs vs. the quencher concentration [Q] resulted in a linear correlation (see Figure 5). The corresponding bimolecular rate constants were obtained from the slope of the linear regressions, viz kpMeO = 9.7x105 M-1s-1for p-methoxythioanisole and kPh3P = 2.1x106 M-1 s-1 for triphenylphosphine.

Discussion. ACS Paragon Plus Environment


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Analysis of the intermediates formed during irradiation of tris(p-bromophenyl)amine (1). The above data show that excitation of 1 with a laser pulse (355 nm) generates two key intermediates in both N2- and in O2-saturated solution. However, one of these species reacts efficiently with molecular oxygen, as is shown in Scheme 5. Photoionization of 1 forms radical cation 1•+ (path (a)). Under nitrogen atmosphere, electrocyclization to radical cation 2 occurs (kcyc = 2.5x104 s-1 in acetonitrile; path (b)) in competition with electron recombination (kBET = 2.2x1010 M-1.s-1 in acetonitrile; path (b’)). Ensuing an electron transfer pathway (path(c)) intermediate, 2 reverts to amine 1 in competition with formation of products (path (d)). Irradiation of di(tri)arylamines can be counted among the many mild conversions into carbazoles.6,10,11 Indeed, the photochemical preparation of carbazoles under visible light in the presence of a copper-based sensitizer has been reported7g and formation of triarylamine radical-cation followed by cyclization to dihydrocarbazole intermediate that in turn provides a substituted carbazole, was also proposed.11f However, when amine 1 was irradiated under visible light photoredox catalysis conditions, no tribromocarbazole was observed, and only N-phenylcarbazole was obtained in low yield (> 16 %).11f

O 3

Ar3N + 1O2 φ∆ 3O 2 e- (solvent) hν

3

φISC Ar3N (1)

H

H

O2 3

O2.H

kcyc Br Ar3N* Ar3N.+ (31*) path (a) (1.+) path (b) kd kBET path (b') krev path (c) e- (solvent)

N Ar

H

N Ar

Br

Br

Ο2

Br

O

path (e)

(I)

path (f) e HO

(2)

Ar3N

path (d)

H

Br

O Br

Products

Ar3N+ Ar3N = (p-BrC6H4)3N


N Ar

(II)

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Scheme 5. Proposed reaction mechanism of the photocyclization of 1. (Filled arrows: nitrogen atmosphere; dashed arrows: oxygen atmosphere).

On the other hand, under aerobic conditions amine 1 in its triplet excited state sensitizes singlet oxygen thus halving the formation of intermediates 1•+ and, as a consequence, 2, as can be judged from the quantum yields obtained under oxygen atmosphere (see Table 1 and compare these values with those obtained in nitrogen atmosphere). Photoionization of triplet amine (1) forms 1•+ and the ejected electron was trapped, at least in part, by molecular oxygen giving superoxide ion with a bimolecular rate constant of 2x1010 M-1 s-1.12 Additionally, a back-electron transfer process (path(b’) in Scheme 5) can occur between 1•+ and the ejected electron with a bimolecular rate constant (kBET) of 1.4x1010 M-1 s-1. Cyclization (path (b)) afforded 2 that reacted with molecular oxygen and provided 4-peroxyl radical (I) (path (e)). Ensuing intramolecular hydrogen atom transfer led to intermediate (II) (path (f)) and both intermediates are, at least in part, potential oxidizing agents that are formed during irradiation of amine 1 under oxygen atmosphere. Superoxide ion then further reacted with adventitious water present in the organic media and led to ROS species, including OH•, HOO• and H2O2. Under air-equilibrated conditions a similar photochemical behavior was observed for 1 in MeCN, where both 1.+ and 2 were detected. Transient 1•+ decays competitively by two pathways, backelectron transfer to amine 1 (path (b’) in Scheme 4, kBET = 1.9x1010 M-1 s-1) and electrocyclization to 2 (path (b) in Scheme 4, kcyc = 2.6 x 104 s-1). Nitrous oxide is a good quencher of solvated electron and has been often used under pulse radiolysis and laser flash photolysis.13 However, bubbling N2O to a solution of 1 in MeCN no effect on the transient intermediates was observed, the competitive rate constants kcyc and kBET exhibiting the same values under N2O as well as under N2 atmospheres (see Table 3). This is due to the fact that chemical reaction occurs at the geminate pair level (1•+….e-) and no solvated electron (e-solv) is ACS Paragon Plus Environment


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formed. Furthermore, comparison of the rate constant for the reaction of N2O with the electron (9x109 M-1 s-1)13g and the BET reaction of 1.+ (2.1 x 1010 M-1 s-1) account for the spectroscopic behavior observed. As concerning oxygen trapping by 2, it was surmised that some intermediates generated during the irradiation may similarly oxidize other compounds under mild conditions.

Photo co-oxidation of sulfides (Ph2S; PhSMe and PhCH2SEt) in the tris(p-bromophenyl)amine/O2 system. Steady-state experiments where amine 1 was irradiated in the presence of a range of sulfides, the latter are co-oxidized to sulfoxides at a rate dependent on their structure and the solvent used (rate values ranging from 0.003 to 0.244 µmol s-1) while amine 1 is consumed efficiently. Further, the rate ratio of sulfoxide versus consumption of 1 was

Photochemical co-oxidation of sulfides and phosphines with tris(p

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