Voltammetric and Electrochemical ESR Studies of Oxidation

Cassandre Quinton , Valérie Alain-Rizzo , Cécile Dumas-Verdes , Gilles Clavier ... Cassandre Quinton , Val?rie Alain-Rizzo , C?cile Dumas-Verdes , F...
0 downloads 0 Views 198KB Size
Download PDF
J. Phys. Chem. B 2006, 110, 2681-2691

2681

Voltammetric and Electrochemical ESR Studies of Oxidation Reactions Mediated by Tris(4-bromophenyl)amine in Acetonitrile Andrew J. Wain,† Ian Streeter,† Mary Thompson,† Nicole Fietkau,† Ludovic Drouin,‡ Antony J. Fairbanks,‡ and Richard G. Compton*,† Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford, OX1 3QZ, U.K., and Chemistry Research Laboratory, Oxford UniVersity, 12 Mansfield Road, Oxford, OX1 3TA, U.K. ReceiVed: June 28, 2005; In Final Form: September 13, 2005

The electrochemical oxidation of tris(4-bromophenyl)amine in the presence of 2,6-lutidine is examined in acetonitrile. Voltammetric and spectroscopic investigations suggest that the electrogenerated triaryl aminium radical cation oxidizes 2,6-lutidine in an EC′ mechanism, and an equilibrium constant for this homogeneous electron transfer is estimated. The mediated oxidation of a protected phenyl selenoglycoside by this reaction mixture is studied by the use of electrochemical ESR, employing a tubular flow cell, and signal intensity data is found to be consistent with the proposed mechanism, allowing the determination of kinetic parameters by computational simulation. Products of the mediated glycoside oxidation are determined by proton NMR and mass spectrometry.

1. Introduction Mediated electrochemical oxidations and reductions present an attractive alternative to direct anodic and cathodic processes in numerous circumstances, particularly in electroorganic syntheses, where advantages such as enhanced efficiency, improved selectivity, easier product isolation and reduced electrode passivation have been recognized.1-3 The principle behind indirect electrolysis involves a combination of heterogeneous and homogeneous electron transfers; a redox catalyst, or mediator, is oxidized, for example, at the electrode surface, and the product performs a chemical oxidation on the substrate in solution, the mediator being regenerated in its reduced form. An especially well-established branch of oxidative redox catalysts are the para-substituted triarylamines, which can be oxidized electrochemically in organic solvents to yield stable radical cations, the ease of which is dependent on the extent of substitution.1-9 In particular, a mediator used commonly is tris(4-bromophenyl)amine (TBPA, see Figure 1a), which can be oxidized at easily accessible electrode potentials and its radical cation is well-known as a relatively mild single electron oxidant.1,10-12 In systems where paramagnetic species are electrogenerated and then consumed by homogeneous chemical processes, electrochemical ESR has served as a very powerful tool with which to probe kinetics and mechanisms.13-16 In the current work, we focus on TBPA as a redox catalyst in acetonitrile and present an interesting case in which the electrogenerated species (TBPA•+) is not an effective enough oxidant to carry out the mediation alone at any appreciable rate but instead is thought to oxidize a secondary electron donor, 2,6-lutidine, the product of which carries out the substrate oxidation. The substrate molecule in question is a protected selenoglycoside, phenyl * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +44 (0) 1865 275 413. Fax: +44 (0) 1865 275 410. † Physical and Theoretical Chemistry Laboratory, Oxford University. ‡ Chemistry Research Laboratory, Oxford University.

Figure 1. (a) Structure of tris(4-bromophenyl)amine (TBPA). (b) Structure of phenyl 2,3,4,6-tetra-O-benzoyl-1-seleno-β-D-glucopyranoside (BzSePh).

2,3,4,6-tetra-O-benzoyl-1-seleno-β-D-glucopyranoside (BzSePh, Figure 1b). The direct and indirect single electron oxidation of such glycosides has been at the center of much research from the point of view of radical-cation-initiated glycosylations, and is therefore an interesting example system to study, with possible applications in synthetic electrochemistry.10-12,17-20 In this paper we first investigate the effect of 2,6-lutidine on the TBPA/TBPA•+ redox couple with the aid of a variety of electrochemical and spectroscopic techniques and once a mechanism is established, study the BzSePh oxidation using in situ ESR, employing a tubular flow cell. Simulation of the ESR signal intensity data provides further support for the proposed mechanism, and the kinetics of the substrate oxidation is determined. Finally, the products of the BzSePh oxidation are elucidated by NMR and mass spectrometry, allowing a mechanism for the final stage of the reaction to be posed.

10.1021/jp0535137 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/21/2006

2682 J. Phys. Chem. B, Vol. 110, No. 6, 2006

Wain et al.

Figure 2. Schematic of the tubular flow cell and parabolic flow profile, including relative position of electrode within the ESR cavity.

2. Theory 2.1. Introduction. Numerical simulation of the concentration profiles of each species was used to elucidate the likely mechanism involved in the BzSePh oxidation. Several mechanisms are proposed, and these are detailed in Appendix 1. To model the concentration profiles, from which both limiting current and ESR signal data can be found, the mass transport equations combined with the appropriate kinetic terms associated with the proposed mechanism must be solved for each species. The full mass transport equation for flow in a tubular cell under laminar flow conditions21,22 is

(

)

∂c ∂2c 1 ∂c ∂2c ∂c )D 2+ + 2 - Vx ∂t r ∂r ∂x ∂r ∂x

(1)

where c and D are the concentration and diffusion coefficient of the species described, x and r are the coordinates in the axial and radial directions, respectively, and Vx is the axial flow velocity, given by the parabolic equation

( )

V x ) V0 1 -

r2 F2

(2)

where V0 is the maximum flow velocity found at the tube center and F is the radius of the tube. The general theory and simulation method for the tubular flow cell has been reported in detail in previous publications23,24 and will not be repeated here. Instead, for each mechanism tested just the normalized forms of the kinetic mass transport equations are given together with relevant boundary conditions. The results obtained are discussed in Section 4.5. 2.2. Numerical Simulation. As with the previous work,15,24 the experimental results are given by the ESR signal data as well as the electrochemical (limiting current) results. This means that the concentration profile must be simulated for a much longer tube length than just the electrode length (Figure 2), so even in the Le´veˆque regime at the electrode23,24 it may be necessary to take radial diffusion into account further downstream as the diffusion layer widens; this radial term is therefore always included in the simulations. However, comparison with previous work15,23,24 shows that the experimental flow rates are above the limit where axial diffusion starts to become important, so this term can be legitimately neglected in the simulations. The limiting current, ilim, is found from the concentration profile of electroactive species A (vide infra) over the electrode length, yielding the Levich expression

The signal intensity can be calculated by integrating the total number of spins in the ESR cavity, found from solution of the mass transport equation downstream of the electrode, and convoluting this with the sin2 x sensitivity dependence across the cavity length. 2.3. Proposed Mechanisms. 2.3.1. Mechanism 1. The first mechanism outlined in Appendix 1 involves oxidation of TBPA (A) to the radical cation (B). This radical cation then oxidizes the 2,6-lutidine and it is the 2,6-lutidine radical cation which then reacts with the BzSePh to form the products. It is shown in the appendix how the mechanism may be simplified to give a reaction with an effective rate constant, and a rate which is first order in both TBPA•+ and BzSePh. For simulation purposes, the scheme can therefore be simplified to

A-efB keff

B + C 98 A + products The normalized kinetic mass transport equations are

(2p - Y) ∂a ∂a 1 ∂a ∂ 2a )0) 2- PeY + Keff bc (4) ∂τ p Y ∂Y 2p ∂X ∂Y

(

)

(

)

DB ∂2b (2p - Y) ∂b ∂b 1 ∂b - PeY )0) - Keff bc ∂τ DA ∂Y2 p - Y ∂Y 2p ∂X (5) D C ∂ 2c (2p - Y) ∂c 1 ∂c ∂c - PeY )0) - Keff bc ∂τ DA ∂Y2 p - Y ∂Y 2p ∂X (6) where Pe ) 4Vf x2e /πDAF3, p ) F/xe, Y ) y/xe, and Keff ) keff[A]bulkx2e /DA. The initial conditions are

a)1 b)0 c)

[C]bulk [A]bulk

and the boundary conditions used are as follows:

Y ) 0 (tube center)

∂a ∂b ∂c ) ) )0 ∂Y ∂Y ∂Y

Y ) p (electrode surface) a ) 0; b ) 1;

∂c )0 ∂Y

2.3.2. Mechanisms 2 and 3. It is seen from Appendix 1 that both mechanisms 2 and 3 can be simplified to the rate of loss of the sugar being first order in sugar and second order in TBPA•+, with an effective rate constant, keff. For the simulation we simplify the scheme to

A-efB keff

2B + C 98 2A + products

(3)

and the normalized kinetic mass transport equations associated with this scheme are

which uses the Le´veˆque approximation21 assuming the diffusion layer is very thin in comparison to the tube radius.

(2p - Y) ∂a ∂a ∂ 2a 1 ∂a )0) 2- PeY + Keff b2c (7) ∂τ p - Y ∂Y 2p ∂X ∂Y

2/3 1/3 ilim ) 5.24 × 105nFAbulkx2/3 e DA Vf

Reactions Mediated by Tris(4-bromophenyl)amine

(

)

DB ∂2b 1 ∂b ∂b )0) ∂τ DA ∂Y2 p - Y ∂Y PeY

(

(2p - Y) ∂b - Keff b2c (8) 2p ∂X

)

D C ∂ 2c (2p - Y) ∂c ∂c 1 ∂c )0) - PeY ∂τ DA ∂Y2 p - Y ∂Y 2p ∂X Keff b2c (9) 2 2 keff[A]bulkxe . where Keff ) DA The initial and boundary conditions are as detailed above. 3. Experimental Section 3.1. Chemical Reagents. All voltammetric experiments were carried out in acetonitrile (Fisher) with 0.1 M tetrabutylammonium perchlorate (TBAP, Fluka, puriss. electrochemical grade) as supporting electrolyte. Tris(4-bromophenyl)amine (TBPA), tris(4-bromophenyl)aminium hexachloroantimonate, 2,6-lutidine (Lu), and 2,6-di-tert-butyl-4-methylpyridine (DTBMP) were purchased from Aldrich. All reagents used were of the highest commercially available grade and were used without further purification. The synthesis of the protected selenoglycoside, phenyl 2,3,4,6-tetra-O-benzoyl-1-seleno-β-D-glucopyranoside (BzSePh), has been reported previously.17 3.2. Apparatus. Ex-situ voltammetry was carried out on a variety of macro- and microdisk working electrodes including a 0.5-mm-diameter platinum disk, a 3-mm-diameter glassy carbon disk, a 0.75-mm gold disk, and platinum and gold microelectrodes, nominally 10 µm in diameter. A typical threeelectrode cell was completed with a platinum coil counter and a silver wire quasi-reference electrode (Goodfellow, Cambridge, U.K.). In cases where a more stable reference potential was necessary, the quasi-reference electrode was replaced with a Ag/ AgNO3 reference, constructed from a discarded SCE containing a silver wire and 10 mM AgNO3 with 0.1 M TBAP in acetonitrile, and separated from the bulk solution by a porous frit. Potential control was achieved using a computer-controlled PGSTAT30 potentiostat (Autolab, Eco Chemie, Utrecht, The Netherlands). The working electrodes were polished using 1 µm and 0.3 µm alumina (Kemet International Ltd., U.K.). All solutions were degassed with oxygen-free nitrogen (BOC Gases, Guildford, Surrey, U.K.) for at least 15 min prior to experimentation. In situ electrochemical ESR experiments were carried out using a platinum tube flow cell, the construction and characterization of which has been described previously.24 A platinum gauze counter electrode was positioned downstream from the platinum working electrode and a silver wire reference was placed upstream. As a result of the highly resistive nature of the cell design, it was necessary to attach a 0.1 µF capacitor between the reference and counter electrodes in order to ensure electronic stability under steady-state potentiostat control. Fluid motion was maintained using a gravity flow system employing glass capillaries to achieve slower flow rates. The volume flow rates achieved were in the range of 0.1 × 10-3 to 200 × 10-3 cm3 s-1. The tube electrode surface was cleaned between experiments with 3 micron diamond spray (Kemet) using a tungsten rod to polish. The electrode length was calibrated at 1.7 mm using a solution of 1 mM ferrocene (Aldrich) in supported acetonitrile. The tube diameter (2F) was measured at 1 mm using callipers.

J. Phys. Chem. B, Vol. 110, No. 6, 2006 2683 ESR spectra were obtained using a JEOL JES-FA100 X-band spectrometer with a cylindrical (TE011) cavity resonator.25 The cell was positioned in this cavity such that xgap, the distance between the upstream edge of the electrode and the end of the sensitive region of the cavity (see Figure 2), measured 2 mm. Cavity tuning was achieved using the JEOL spectrometer software (A-SYSTEM v.1.100, FA-MANAGER v.1.01). To account for variations in cavity Q, signal intensities were measured relative to a standard marker consisting of solid MgO dispersed with Mn2+, inserted into the cavity at the time of measurement. In all of the experiments, a microwave power of 1 mW was used and regular tests were carried out to check that increasing the microwave power increased the ESR signal. Doing so ensured that the system was not power saturated such that the ESR signal intensity gave a direct measure of the number of electrogenerated spins in the cavity.25 All experiments were conducted at room temperature, measured at approximately 20 ( 2 °C. UV-visible spectroscopic measurements were made using a solution-filled transparent cuvette with a 1-cm path length in a Unicam UV2 series UV-visible spectrophotometer (Unicam, Cambridge, U.K.). 4. Results and Discussion 4.1. Preliminary Studies. Preliminary voltammetry of the protected selenoglycoside, phenyl 2,3,4,6-tetra-O-benzoyl-1seleno-β-D-glucopyranoside (BzSePh), was carried out on a 0.5mm-diameter platinum disk electrode in supported acetonitrile, and the response is shown in Figure 3a. At 50 mV s-1, a chemically and electrochemically irreversible oxidation wave is observed at 1.23 V versus Ag/Ag+, in good agreement with voltammetry reported by Yamago et. al. for this oxidation on glassy carbon.20 Upon successive scanning, however, the oxidation wave was found to gradually diminish, shifting to more positive potentials, suggesting surface passivation. This appeared to be even more evident when the direct oxidation was attempted on a gold working electrode, to the extent that reproducibility was poor and voltammetry difficult to interpret. As has been demonstrated in previous studies, such electrode fouling can be avoided by use of a suitable mediator,1-3 a popular choice being tris(4-bromophenyl)amine (TBPA), the radical cation of which is a well-known mild, and therefore selective, single electron oxidant.1-3,8,10-13 For comparison, Figure 3a depicts the single electron oxidation of 2 mM TBPA, which displays an electrochemically reversible couple with a formal potential of 0.78 V versus Ag/Ag+, well removed from the oxidation potential of BzSePh (the oxidative peak potential of TBPA/TBPA•+ is close to 400 mV negative of the BzSePh oxidation wave at this scan rate). Note that this does not suggest that the oxidation of BzSePh by TBPA•+ is thermodynamically unfavorable because the oxidation of BzSePh is electrochemically irreversible; numerous studies have shown that single electron-transfer reagents such as TBPA are capable of effecting the oxidation of compounds with irreversible half-wave potentials that are several hundred millivolts more positive than that of the mediator.1,11,26 The electrochemical irreversibility of the selenoglycoside oxidation is perhaps not unexpected in the light of the slow electrode kinetics observed with the analogous oxygen- and sulfur-containing species. Indeed, the high overpotentials for the direct oxidation of sugars is part of the basis of their being notoriously challenging electroanalytical targets. These slow kinetics can probably be rationalized in the context of the significant bond length and bond angle changes concomitant on single electron transfer, consistent with the notions of Marcus theory.

2684 J. Phys. Chem. B, Vol. 110, No. 6, 2006

Wain et al. reverse sweep (Figure 3b), these features together being characteristic of an EC′ mechanism, for example

TBPA - e- f TBPA•+ k

TBPA•+ + BzSePh 9 8 TBPA + BzSePh•+ lutidine

Figure 3. (a) Voltammetric oxidation of 1 mM BzSePh on 0.5 mm Pt in acetonitrile at 50 mV s-1 (solid line). Shown also is the response from 2 mM TBPA under the same conditions (dotted line). (b) Oxidation of 2 mM TBPA at 100 mV s-1 (solid line), in the presence of 1 mM BzSePh (dotted line) and in the presence of 1 mM BzSePh and 2 mM 2,6-lutidine (dashed line). (c) Oxidation of 2 mM TBPA at 100 mV s-1 (solid line), in the presence of 20 mM (dotted line) and 40 mM 2,6-lutidine (dashed line).

To study the possibility of a mediated oxidation, the voltammetry of TBPA was investigated on a platinum electrode in the presence and absence of BzSePh, and the results are shown in Figure 3b for 2 mM TBPA with and without 1 mM BzSePh. It is clear from the voltammetry that at 100 mV s-1, the addition of the selenoglycoside has little to no effect on the electrochemical behavior of TBPA, suggesting that on this time scale, no mediated oxidation of BzSePh takes place and the electrogenerated TBPA•+ does not undergo homogeneous reaction (this was found to be the case, within error, over numerous experiments). However, in the presence of 1 mM BzSePh and 2 mM 2,6-lutidine, the oxidative peak current was found to increase, with a subsequent decrease in the reductive peak current on the

It should be noted that at this concentration the 2,6-lutidine alone (BzSePh absent) had no significant effect on the TBPA oxidation. The extent of the observed catalysis was found to fall with increasing scan rate, such that little mediation was observed at scan rates above 500 mV s-1, as the homogeneous kinetics become “out-run” on the voltammetric time scale. Slow scan rate voltammetry might therefore be informative in terms of reaction kinetics, but poor reproducibility was encountered at scan rates below 50 mV s-1, possibly as a result of natural convection and minor passivation effects due to the BzSePh becoming significant over extensive periods of electrolysis. Nevertheless, it can be concluded from the above that the homogeneous kinetics must be taking place approximately on the second time scale. Similar behavior was observed under the same conditions with a glassy carbon electrode. At this point, the role of the 2,6-lutidine in the catalysis is unclear. In previous studies into mediated oxidations, the addition of such a nonnucleophilic base has served to promote the homogeneous reaction either by deprotonating the product formed and enhancing the kinetics of the rate determining step or simply by preventing the inhibiting effect of any acid building up during the reaction.3,27,28 However, BzSePh does not appear to have any acidic protons because all of the glycoside hydroxyl groups are protected, and no other proton sources are apparent; so such a rationalization cannot be substantiated. Before the mediated oxidation of BzSePh could be investigated any further, it seemed important that the interaction between TBPA and 2,6lutidine first be fully understood and so experiments in the absence of the selenoglycoside were conducted accordingly. 4.2. TBPA/TBPA•+ Voltammetry in the Presence of 2,6Lutidine. In the preliminary electrochemistry described above, it was noted that 2,6-lutidine alone had little effect on the voltammetric oxidation of TBPA at the concentrations used. However, at much higher 2,6-lutidine concentrations a change in the voltammetric response was observed, which is depicted in Figure 3c for a 100 mV s-1 sweep rate. Although the difference is small, it can be seen that upon addition of 20 and 40 mM 2,6-lutidine to 2 mM TBPA (10 and 20 times that used in the preliminary voltammetry), the oxidation wave becomes enhanced and the reverse sweep reduction wave is diminished, which, as before, implies a catalytic mechanism in which the TBPA is regenerated in homogeneous solution from its radical cation, with the apparent oxidation of 2,6-lutidine. Because there is little to no literature surrounding the electrochemical oxidation of 2,6-lutidine, the plausibility of its anodic oxidation was investigated briefly to verify the possibility of the above catalytic effect. To maximize the solvent window, this was carried out on a 10-µm platinum disk electrode, the oxidative voltammetry being depicted in Figure 4. At a scan rate of 10 mV s-1 the oxidation appears as a stripping peak, indicating that the electroactive species is surface bound, a fact that is further supported by the linear variation of peak current with scan rate (see inset). The anodic peak occurs at a potential of approximately 2.3 V versus Ag, over a volt more positive than that of TBPA, but because the wave is irreversible it is impossible to draw any conclusions as to the thermodynamics of its oxidation and is it not apparent what follow up chemistry

Reactions Mediated by Tris(4-bromophenyl)amine

J. Phys. Chem. B, Vol. 110, No. 6, 2006 2685

Figure 4. Voltammetric oxidation at 10 mV s-1 of 5 mM 2,6-lutidine in acetonitrile on 10 µm Pt. Inset: Variation of peak current with scan rate.

ensues, such that any mechanism proposed is speculative. Nevertheless, the above has served to prove that the homogeneous oxidation of 2,6-lutidine by TBPA•+ is a possibility. To investigate the TBPA-lutidine system further, we envisaged that the electrochemistry of TBPA•+ (commercially available as the hexachloroantimonate salt) might provide additional insight into the reaction. Macrodisk voltammetry of solutions of TBPA•+ in acetonitrile was found to be complicated by a surface-confined process at potentials close to that of the TBPA/TBPA•+ redox couple, possibly as a result of the hexachloroantimonate counterion, but employing the 10-µm platinum disk appeared to eliminate this problem so further voltammetry was conducted using this electrode. As expected, the steady-state reduction of TBPA•+ (1 mM) was observed as a sigmoidal curve, beginning with zero current and approaching a cathodic limiting current as the potential was swept more negatively (Figure 5a). Upon addition of 2 mM 2,6-lutidine, however, the whole voltammetric curve quickly shifted toward more positive currents, with an anodic limiting current now registering at the beginning of the reduction sweep and with a drop in the cathodic limiting current, as depicted in Figure 5a. This shifting of the sigmoid was found to continue further over time, though at a decreasing rate, until the anodic limiting current reached a maximum and the cathodic limiting current was zero, the two plateaus being permanently separated by approximately the same current throughout. The effect can be seen more clearly in Figure 5b where the magnitudes of the oxidative and reductive limiting currents have been plotted as a function of time after 2,6-lutidine addition, wherein the two currents can be seen to mirror each other. When this experiment was repeated at a 2,6lutidine/TBPA•+ ratio of 0.5, similar behavior was observed, but the voltammetric curve only shifted such that the cathodic limiting current approached half of its original value, confirming that the reaction is stoichiometric (i.e., one TBPA•+ radical cation is consumed per 2,6-lutidine molecule). Interestingly, for reasons to be discussed below, the above effect was observed to occur in the absence of 2,6-lutidine, only at a very much slower rate (over the course of a day, for example), suggesting that the age of the prepared TBPA•+ solutions was important in terms of accurately knowing their concentration, and this is taken into account in the experiments below. To probe the initial stages of the above-observed process, the transient response was investigated using chronoamperometry; the micro-disk electrode was held at a potential at which the radical cation is reduced to the neutral parent, and the current allowed to reach a constant mass transport limited value, and then 2,6-lutidine was introduced as a single addition (carried

Figure 5. (a) Steady-state voltammetry on 10 µm Pt of 1 mM TBPA•+ SbCl6- with the addition of 2 mM 2,6-lutidine. Before addition (dashed line), 30 s after (dash-dot-dash), 34 min after (dotted line), and 270 min after (solid line) addition. (b) Anodic (b) and cathodic (4) limiting current magnitudes as a function of time after 2,6-lutidine addition. (c) Cathodic current transients for additions of 2,6-lutidine to 1 mM TBPA•+ SbCl6-. 2 mM (dashed line), 20 mM (dotted line) and 40 mM (solid line) 2,6-lutidine.

out from a 2,6-lutidine stock solution using a micropipet). The result can be seen in Figure 5c, which depicts the transient response after the addition of 2, 20, and 40 mM 2,6-lutidine to 1 mM TBPA•+. Attempts were made to fit the curves to simple first- and second-order kinetics, but none were successful, deviation from these being particularly apparent in the initial stages of the reaction. It would appear from the transients that the process involved is more complex; the limiting current seems to drop by a large amount immediately on addition, and then tails off more slowly, implying the occurrence of two different processes, either taking place consecutively at different rates or in parallel. It could be the case that the TBPA•+ and 2,6lutidine form a quickly equilibrating complex (with equilibrium constant Kcomplex), which decomposes slowly

2686 J. Phys. Chem. B, Vol. 110, No. 6, 2006 slow

TBPA•+ + Lu h {Lu-TBPA}•+ 98 products or, as the macro- and microdisk voltammetry suggest, the two species may be involved in a full electron-transfer equilibrium (with equilibrium constant KET), with an additional slow process consuming the remaining TBPA•+ at a slower (but 2,6-lutidine catalyzed) rate.

TBPA•+ + Lu h TBPA + Lu•+ slow

TBPA•+ + Lu 98 products (The 2,6-lutidine oxidation has been written as a single electron transfer, though it may be much more complex than this). In any case, the transients suggest that there is a fast initial equilibration, which can be interrogated by studying the initial “titration”. The magnitude of the immediate current jump, which we defined as the current increase within the first 10 s after 2,6-lutidine addition, was found to increase with increasing 2,6lutidine concentration, and can be seen in Table 1. Note that the limiting currents before the addition of 2,6-lutidine (t ) 0 s) are also shown in light of the gradual decay of TBPA•+ in the absence of 2,6-lutidine, as mentioned above. To determine the TBPA•+ concentrations accurately before and after the addition of 2,6-lutidine, the limiting current of a freshly prepared 1 mM solution was measured as a calibration point from which the experimental limiting currents could be measured relative to, and these calculated concentrations are shown in Table 1. Using these data it was possible to estimate the equilibrium constants for the above two reaction mechanisms:

Kcomplex )

[{Lu-TBPA}•+] [TBPA•+][Lu]

and KET )

[TBPA][Lu•+] [TBPA•+][Lu]

In the case of the complexation, the concentration of {LuTBPA}•+ was determined from the decrease in cathodic current upon the addition of 2,6-lutidine and, in the case of full electron transfer, it has been assumed that TBPA•+ and 2,6-lutidine react with a 1:1 stoichiometry such that [TBPA] and [Lu•+] are approximately equal. The equilibrium constants determined at each concentration of 2,6-lutidine are shown in Table 1. At 2,6lutidine concentrations above 5 mM (vs 1 mM TBPA•+), both sets of equilibrium constants appear to be relatively invariant, the discrepancies at low 2,6-lutidine concentrations probably being a result of competing processes such as the abovementioned “slow” reactions, which have been ignored in this calculation. Because both equilibrium constants appear to be relatively self-consistent, it is not easy to rule either out as a plausible mechanism. As such, further experimental evidence was sought through the spectroscopic properties of TBPA•+ solutions. 4.3. UV-Visible Spectroscopic Measurements. During the voltammetric experiments, it was noted that the TBPA•+ solutions were an intense blue color, but decolorization occurred upon addition of 2,6-lutidine, reflecting the consumption of the radical cation. To monitor this quantitatively, the UV-visible absorption spectra of TBPA•+ solutions were recorded with various concentrations of added 2,6-lutidine, as a function of time after addition. Solutions of TBPA•+ in acetonitrile have an absorption maximum at 705 nm,4,8 although it was noted that 0.1 mM solutions were found to age over the course of a few hours with a decrease in the maximum absorbance and a small wavelength shift. Such behavior has been reported by Eberson and Larsson for the tetrafluoroborate salt of TBPA•+ and was attributed to the slow dimerization of the radical cation,

Wain et al. TABLE 1: Cathodic Limiting Current Data and Calculated Equilibrium Constants at Various Additions of 2,6-Lutidine at 25 °C [TBPA•+], [TBPA•+], ired, ired, nA, 10-3 M, nA, 10-3 M, Kcomplex, [Lu], t ) 10 s t ) 10 s 10-3 M t ) 0 s t)0s M-1 1.0 2.0 5.1 10.2 20.4 30.5 33.6 40.7

-2.23 -2.19 -2.39 -2.37 -2.34 -2.34 -2.20 -2.34

0.75 0.74 0.81 0.80 0.79 0.79 0.75 0.79

-1.63 -1.69 -1.92 -1.78 -1.57 -1.31 -1.24 -1.24

0.55 0.57 0.65 0.60 0.53 0.44 0.42 0.42

464 159 49 33 25 26 24 22

KET 0.094 0.027 0.008 0.007 0.006 0.009 0.008 0.008

with loss of molecular bromine.4 An alternative possibility is that the radical cation is reduced slowly by water, present in trace amounts in the acetonitrile solvent, as reported by Pokhodenko et. al.,29 which accounts for the gradual shift in the microdisk voltammetry with time discussed in section 4.2. In any case, to account for the variation in the TBPA•+ concentration before the addition of 2,6-lutidine, for use in later calculations, the absorbance at 705 nm of a 0.1 mM solution was measured immediately after preparation, and the starting concentrations of all solutions were extrapolated from this. Figure 6 shows the variation of absorbance with time after addition of 0.1, 0.5, and 2.0 mM 2,6-lutidine to (nominally) 0.1 mM TBPA•+. As was found with the microdisk transients, in each case it appears that there is an immediate drop in absorbance after addition of 2,6-lutidine, followed by a more gradual tail-off (eventually, the absorbance reached the baseline value, although for some of the low 2,6-lutidine concentrations, this took several hours). Again, attempts to fit the data to simple first and second-order kinetics failed, and so the same analysis as that with the electrochemical data was employed, in which the initial drop was measured. Because of experimental difficulties, the time period between the 2,6-lutidine addition and the initial scan was longer than the 10-s “induction” period defined for the electrochemical procedure, the delay here being approximately 40 s (a difference that needs to be considered when analyzing the calculated equilibrium constants). Table 2 shows typical absorbance data, along with the calculated concentrations and equilibrium constants for the complexation and electron-transfer mechanisms. As with the electrochemical data, variations in the determined equilibrium constants are only apparent at the low 2,6-lutidine concentrations, particularly for the complexation mechanism, suggesting that the models used are an oversimplification under these conditions. On comparison with the electrochemically determined equilibrium constants, these data support the electron-transfer mechanism. Although

Figure 6. Absorbance at 705 nm for 0.1 mM TBPA•+ SbCl6- as a function of time, after additions of 0.1 mM (9), 0.5 mM (3), and 2.0 mM (b) 2,6-lutidine.

Reactions Mediated by Tris(4-bromophenyl)amine

J. Phys. Chem. B, Vol. 110, No. 6, 2006 2687

TABLE 2: UV-Visible Absorption Data at 705 nm and Calculated Equilibrium Constants at Various Additions of 2,6-Lutidine abs, [Lu], 10-3 M t ) 0 s 0.1 0.2 0.5 1.0 2.0 4.0

1.90 2.57 1.78 2.23 2.18 1.83

[TBPA•+], [TBPA•+], 10-3 M, 10-3 M, Kcomplex, abs, t)0s t ) 40 s t ) 40 s M-1 0.069 0.096 0.065 0.082 0.081 0.067

1.44 2.12 1.28 1.49 1.21 0.73

0.052 0.079 0.046 0.054 0.045 0.025

4100 1200 860 530 410 420

KET 0.070 0.020 0.016 0.015 0.015 0.017

KET varies by about a factor of 2 between the two sets of data, Kcomplex varies by at least an order of magnitude, suggesting that this mechanism is not an accurate description of the system. The small inconsistency between KET obtained by the UVvis and the electrochemical data can be rationalized by the likely effect of trace water in the acetonitrile solvent because TBPA•+ has been reported to oxidize water (a process that could be catalyzed by the base, 2,6-lutidine, and may well be responsible for the “slow” stage of the TBPA•+ decay as seen in Figures 5c and 6).29 In the UV-vis experiments, it was necessary to use a low concentration of TBPA•+ (0.1 mM) such that the relative concentration of water was much higher than that in the electrochemical experiments (1 mM TBPA•+), and hence its affect on the determination of the equilibrium constant is much more significant. In the calculation of the KET, the affect of the parallel “slow” process has been neglected, but if this does involve a reaction with water, it is likely to become important when the relative concentration of the TBPA•+ radical cation is much lower, and therefore the calculated equilibrium constant is higher. Additionally, as discussed above, the induction period defined for the UV-vis data was longer than that used for the voltammetric analysis, suggesting that the effect of the slow process will be more apparent. The experimental results presented so far have all supported the notion that TBPA•+ undergoes an electron-transfer reaction with 2,6-lutidine in acetonitrile. An alternative mechanism still to be considered is the dimerization of TBPA•+, which could in some way be facilitated by the addition of 2,6-lutidine. If two radical cations were to couple in the ortho position of the aromatic ring with the loss of two protons (removed by the 2,6lutidine), then the dimeric product may be further oxidized, possibly by two more electrons, which may lead to an enhanced current in the TBPA voltammetry in an ECE mechanism. In the thin layer extreme, two electrons per molecule would be transferred on the forward sweep, but only one on the reverse scan, giving the impression of a catalytic mechanism. A mechanism similar to this is known to proceed in the wellstudied oxidation of triphenylamine in acetonitrile, where radical cations couple in the unblocked para position.6-9,30,31 However, although a single oxidation peak is observed in this system, two well-separated reduction waves occur on the reverse sweep and an additional oxidative feature is observed upon further scanning, neither of which are present in the TBPA voltammetry, suggesting that this is an unlikely course of events. Also, if TBPA•+ were to undergo a dimerization reaction, one might expect to see a change in its steady-state voltammetry. An important consideration is the separation between the anodic and cathodic limiting currents in Figure 5a and the fact that this remains approximately invariant throughout the reaction with 2,6-lutidine, suggesting that the electroactive species have very similar diffusion coefficients. Using spectroelectrochemical measurements, Wang and co-workers reported values of 1.64 ( 0.02 × 10-5 cm2 s-1 and 1.57 ( 0.03 × 10-5 cm2 s-1 for

the diffusion coefficients of TBPA and TBPA•+ in acetonitrile, respectively,32 which is consistent with the notion that 2,6lutidine reduces TBPA•+ to TBPA. At this point the TBPA/TBPA•+-lutidine system is believed to be understood to the extent that we can now consider the mediated oxidation of the selenoglycoside discussed in section 4.1. Because no catalysis appears to occur in the presence of the sugar alone, and at low concentrations of 2,6-lutidine (e.g., 2 mM) the effect of the above reaction on the TBPA voltammetry is negligible, it seems logical that the oxidation product of 2,6-lutidine, considered to be the 2,6-lutidine radical cation, must carry out the mediated oxidation of BzSePh observed in Figure 3b. We therefore propose the following mechanism

TBPA - e- 98 TBPA•+ TBPA•+ + Lu h TBPA + Lu•+ k1

Lu•+ + BzSePh 98 products where k1 is the second-order rate constant for the oxidation of BzSePh by the 2,6-lutidine radical cation. To further verify the arguments presented above and to study the kinetics of this suggested mechanism, electrochemical ESR was used next in order to probe the affect of 2,6-lutidine and BzSePh on the TBPA/TBPA•+ system in situ. 4.4. Electrochemical ESR. Electrochemical ESR was carried out using a tubular flow cell in which species electrolyzed at a platinum tube working electrode are transported into an ESR cavity resonator. Electrolysis of 2 mM solutions of TBPA at a potential of 1.4 V versus Ag yielded single line ESR spectra as reported in previous studies,13 and variable flow rate experiments in which signal intensities and limiting currents were measured revealed that plots of log(S/ilim) against log(Vf) had a slope close to -2/3 (Figure 7a). As has been shown in previous studies, this indicates that the TBPA•+ radical cation is stable on the experimental time scale.16,24 The addition of 2 mM and 20 mM 2,6-lutidine to the solution had an interesting effect on the loglog plot, also depicted in Figure 7a. Although the current normalized signal, S/ilim, drops with increasing 2,6-lutidine concentration, it does so by approximately the same amount at each flow rate, such that the plot remains roughly linear, with the same -2/3 slope. This shifting of the plot suggests that a given proportion of TBPA•+ radical cations have been “titrated” out of the system by the 2,6-lutidine, and those that remain behave as stable as before, which supports the fast homogeneous electron-transfer equilibrium mechanism proposed above. For this to be the case, it is required that equilibration be achieved before the solution reaches the sensitive region of the ESR cavity, otherwise curvature would appear on the log-log plots at high flow rates as the titration becomes “out-run”. The addition of 0.2, 0.4, and 2.0 mM BzSePh to the 2 mM TBPA appeared to have very little effect on the signal intensity behavior, as can be seen in Figure 7b, although a small deviation from linearity at the slowest flow rate is an indication of a small degree of chemical reactivity between TBPA•+ and BzSePh alone (i.e., it would appear that TBPA•+ does in fact oxidize BzSePh but at a rate not visible on any conventional voltammetric time scale). Next, signal intensity measurements were carried out in the presence of various concentrations of BzSePh and 2 mM 2,6-lutidine, and the results can be seen in Figure 7c. It is clear that even at the lowest concentration studied there is significant deviation from linearity at slow flow rates, an effect that becomes even more evident at higher concentrations, reflecting the chemical reactivity of the otherwise stable TBPA•+

2688 J. Phys. Chem. B, Vol. 110, No. 6, 2006

Figure 7. Log-log plots of current normalized ESR signal (S/ilim) as a function of flow rate (Vf) for 2 mM TBPA. In each case, 9 represents the TBPA only case. (a) 2 mM (4) and 20 mM 2,6-lutidine (b). (b) 0.2 mM (4), 0.4 mM (b), and 2.0 mM BzSePh (Η). (c) 2 mM 2,6lutidine plus 0.1 mM (4), 0.2 mM (b), 0.4 mM (Η), and 0.8 mM (×) BzSePh. (d) 0.4 mM BzSePh plus 2 mM (4) and 8 mM (Η) 2,6-lutidine, and 2 mM DTBMP (×).

radical cation under these conditions. Also, at the high flow rate extreme, although the plots tend to the expected slope of -2/3, they do not coincide as a result of the titration of TBPA•+

Wain et al.

Figure 8. Plots of log(S/ilim) against log Vf; simulations using the second-order model with a range of rate constants, keff, compared to experimental data. (a) 0.1 mM, (b) 0.2 mM, (c) 0.4 mM, and (d) 0.8 mM BzSePh.

with 2,6-lutidine as found in the absence of BzSePh (Figure 7a). Experiments were also conducted with 0.4 mM BzSePh at a higher 2,6-lutidine concentration of 8 mM, and in the presence of an alternative, more sterically hindered pyridine derivative, 2,6-di-tert-butyl-4-methylpyridine (DTBMP), the results being

Reactions Mediated by Tris(4-bromophenyl)amine depicted in Figure 7d. It is evident that at the higher 2,6-lutidine concentration, the maximum in the log-log plot is diminished, signifying the enhanced rate of consumption of TBPA•+ as the electron-transfer equilibrium with 2,6-lutidine is driven to the right-hand side. It is interesting that the bulky 2,6-lutidine substitute, DTBMP, has much less of an effect on the ESR signal. On comparison with the TBPA only case, there is evidence of both an overall downward shift due to the titration of TBPA•+ with DTBMP and also curvature at the low flow rate limit due to the reaction with BzSePh, but neither are as significant as in the equivalent 2,6-lutidine experiment, indicating the reduced reactivity of the sterically hindered base (i.e., KET for DTBMP is probably much lower than that for 2,6-lutidine). Having discussed the electrochemical ESR results on a qualitative level, the simulated fitting of the experimental data will now be presented to provide further support of the proposed mechanism, and to determine the rate constant, k1. It is important to stress that, throughout all electrochemical ESR experiments, only a single broad line (2.8 mT) was ever observed, even at low magnetic field modulation widths, and no additional signals were detected upon addition of 2,6-lutidine and BzSePh, suggesting that products of the homogeneous chemistry are either invisible to ESR or react too quickly to be detected. This is in contrast to the work by Waller et. al. in which the TBPA mediated oxidation of acetate led to an additional signal, superimposed on the TBPA•+ line, due to paramagnetic products.13 This is an important consideration because in the following modeling TBPA•+ is assumed to be the sole contributor to the ESR signal. 4.5. Simulation of Electrochemical ESR Data. If we take the mechanism proposed at the end of section 4.3, then it can be shown that for a given 2,6-lutidine concentration

-

dS k [TBPA•+][BzSePh] dt eff

The details of this derivation are outlined in Appendix 1 (mechanism 1). Because this is 2,6-lutidine independent, it is possible to simplify the scheme and model it as a simple secondorder decomposition of electrogenerated TBPA•+. The simulations using mechanism 1 were compared to experimental data for a range of BzSePh concentrations. Simulations were carried out over the range of flow rates used experimentally (∼1 to 1 × 10-4 cm3 s-1), and best fit plots of log(S/ilim) versus logVf were found by changing the rate constant, keff, used in the simulations. The results are presented in Figure 8 for each BzSePh concentration used. It is seen that the results for keff ≈ 1 × 105 agree well for the lower concentrations of BzSePh, although there is some disagreement at the low flow rate end for the higher BzSePh concentrations. However, if a much higher keff is used then the slope of -2/3 observed at the high flow rate end is lost because the reaction is fast enough to deplete the TBPA•+ radical even under these fast flow rate conditions. It is also possible, however, that the 2,6-lutidine•+ radical cation disproportionates into the neutral parent and the dication, and that the latter is responsible for the BzSePh oxidation

TBPA - e- f TBPA•+ TBPA•+ + Lu h TBPA + Lu•+ 2Lu•+ h Lu2+ + Lu•+ k2

Lu2+ + BzSePh 98 products

J. Phys. Chem. B, Vol. 110, No. 6, 2006 2689 Alternatively, the 2,6-lutidine dication could be generated by reaction with another TBPA•+ radical cation

Lu•+ + TBPA•+ h Lu2+ + TBPA k2

Lu2+ + BzSePh 98 products Both of these lead to kinetics that are second-order in TBPA•+ and first-order in BzSePh, for a constant concentration of 2,6lutidine (Appendix 1, mechanisms 2 and 3), and hence the scheme can be modeled with overall third-order kinetics. Simulations were carried out using these postulated mechanisms, as it was thought that the consumption of two TBPA•+ radicals per BzSePh molecule would provide better agreement at the low flow rate end. However, the results exhibit a similar problem to the above at the high flow rate end; the slope of -2/3 at high flow rate simply cannot be reached using the same rate constant, keff, as would fit the low flow rate end. The results are shown in Figure 9 and it can be seen that overall the simulated data fits with the experimental data less well than that from simulation of mechanism 1, and so mechanisms 2 and 3 are not considered further. It is seen from the details of mechanism 1 given in Appendix 1 that the rate constant, keff, is given by

keff )

k1KET[Lu] [TBPA]

so the value of keff that fits the experimental data should be proportional to the concentration of 2,6-lutidine used. The experimental results shown in Figure 8 all used 2 mM 2,6-lutidine, and the value of keff used to fit with these results was 1 ( 0.3 × 105 mol-1 dm3 s-1. As mentioned above, for a BzSePh concentration of 0.4 mM, the experiment was repeated using a 2,6-lutidine concentration of 8 mM (Figure 7d). The fitting of these experiments with simulations using mechanism 1 are shown in Figure 10, and it is seen that the rate constant required for fitting the 8 mM 2,6-lutidine case is roughly four times that used to fit the 2 mM 2,6-lutidine experiments for the whole range of BzSePh concentrations. These results therefore provide further support for the theory postulated in mechanism 1. 4.6. Final Mechanistic Considerations. Up to this point, no comment has been made as to the products of the selenoglycoside oxidation. To determine this and to elucidate the final step of the oxidation mechanism, a preparative synthesis was carried out and the products analyzed by mass spectrometry and proton NMR. 100 mg of BzSePh was dissolved in acetonitrile (0.142 mM) and 1.5 equivalents of TBPA•+ SbCl6and 2,6-lutidine were added and stirred. The acetonitrile solvent was removed from the product mixture using a rotary evaporator; a purification by flash column chromatography on silica allowed extraction of three components. These components were identified as the BzSePh starting material, its hydrolyzed derivative (formed via loss of the phenylselenyl group), and diphenyl diselenide (PhSeSePh), the latter having an interesting proton NMR spectrum in which two multiplets are present due to its optical activity.33 With this in mind, we propose the mechanism depicted in Scheme 1 in which the BzSePh•+ radical cation loses a phenylselenyl radical, the resultant oxocarbenium ion being quickly trapped by the nucleophilic attack of trace water, and with the fast dimerization of the ejected PhSe• radicals. This mechanism has been encountered in numerous studies into the oxidation of protected glycosides, in which water

2690 J. Phys. Chem. B, Vol. 110, No. 6, 2006

Wain et al.

Figure 10. Fitting of the experimental data with the second-order simulation model using different concentrations of 2,6-lutidine.

SCHEME 1

Figure 9. Plots of log(S/ilim) against log Vf; simulations using the thirdorder model with a range of rate constants, keff, compared to experimental data. (a) 0.2 mM, (b) 0.4 mM, and (c) 0.8 mM BzSePh.

has been rigorously excluded and other nucleophiles added, the aim being to generate new glycoside linkages.11,12,17,19 5. Conclusions An investigation into the mediated oxidation of 2,6-lutidine and BzSePh, by the redox catalyst TBPA, has been presented. In acetonitrile, electrochemical and spectroscopic analyses support a mechanism in which a homogeneous electron-transfer equilibrium exists between TBPA•+ and 2,6-lutidine, to form the parent TBPA and 2,6-lutidine in an oxidized form, possibly the radical cation. This is an interesting finding in itself, especially because TBPA might be employed as a redox catalyst in an electroorganic synthesis, for example, and 2,6-lutidine added as a base, without knowledge of these effects. Grosse Brinkhaus and co-workers reported that 2,6-lutidine is oxidized

by a more heavily brominated triaryl aminium radical cation, but interestingly, no mention was made of the analogous reaction with TBPA, probably as a result of the low concentrations used.3 The product of the 2,6-lutidine oxidation is thought to oxidize BzSePh, yielding the radical cation that is believed to be hydrolyzed by trace water present in the solvent. It remains to be known what chemistry may have been observed in thoroughly dry solvents, or in the presence of other nucleophiles, although this was not the purpose of the current study. The absolute fate of the 2,6-lutidine is also uncertain (i.e., whether it is fully regenerated, or consumed during the BzSePh oxidation), but unfortunately a more thorough study of this is difficult due to electrode passivation problems. Numerous methodologies have been employed in order to elucidate the mechanism of redox catalysis operating, and in particular, electrochemical ESR has proven to be powerful in probing different aspects of the multistage reaction. The simulation of signal intensity data has also been especially successful in the estimation of reaction rate constants. This particular system serves as an interesting example of a twostep mediation, such relayed redox mediation being reminiscent of “shuttling” electron transport chains in, for example, countless biological systems. Acknowledgment. We thank the EPSRC and JEOL for funding. Appendix 1 Mechanism 1.

TBPA - e- f TBPA•+ TBPA•+ + Lu h TBPA + Lu•+

Reactions Mediated by Tris(4-bromophenyl)amine k1

Lu•+ + BzSePh 98 products where the equilibrium constant for the charge transfer is given by

KET )

[TBPA][Lu•+]

J. Phys. Chem. B, Vol. 110, No. 6, 2006 2691 The rate of loss of the sugar is given by

-

dS ) k2[Lu2+][BzSePh] ) dt k2KETKdisp,2[TBPA•+]2[Lu][BzSePh] [TBPA]2

•+

[TBPA ][Lu]

Therefore, the rate of loss of the sugar is given by

k1KET[TBPA•+][Lu][BzSePh] dS •+ - ) k1[Lu ][BzSePh] ) dt [TBPA]

and at constant 2,6-lutidine and TBPA concentration

-

dS ) k′′eff[TBPA•+]2[BzSePh] dt

where S ) [BzSePh]. So at constant 2,6-lutidine concentration, and assuming a low fractional conversion of TBPA to its radical cation (i.e., constant [TBPA]),

It is clear from the above that mechanisms 2 and 3 are experimentally indistinguishable.

dS ) keff[TBPA•+][BzSePh] dt

(1) Steckhan, E. Angew. Chem., Int. Ed. Engl. 1986, 25, 683. (2) Pletcher, D.; Zappi, G. D. J. Electroanal. Chem. 1989, 265, 203. (3) Grosse Brinkhaus, K.-H.; Steckhan, E.; Schmidt, W. Acta Chem. Scand. Ser. B 1983, 37, 499. (4) Eberson, L.; Larsson, B. Acta Chem. Scand., Ser. B 1986, 40, 210. (5) Eberson, L.; Larsson, B. Acta Chem. Scand., Ser. B 1987, 41, 367. (6) Oyama, M.; Nozaki, K.; Okazaki, S. Anal. Chem. 1991, 63, 1387. (7) Oyama, M.; Higuchi, T.; Okazaki, S. J. Chem. Soc., Perkin Trans. 2 2001, 2, 1287. (8) Oyama, M.; Kambayashi, M. Electrochem. Commun. 2002, 4, 759. (9) Larumbe, D.; Gallardo, I.; Andrieux, C. P. J. Electroanal. Chem. 1991, 304, 241. (10) Marra, A.; Mallet, J.-M.; Amatore, C.; Sinay¨ , P. Synlett 1990, 572. (11) Metha, S.; Pinto, B. M. Carbohydr. Res. 1998, 310, 43. (12) Amatore, C.; Jutand, A.; Meyer, G.; Bourhis, P.; Machetto, F.; Mallet, J.-M.; Sinay¨ , P.; Tabeur, C.; Zhang, Y.-M. J. App. Electrochem. 1994, 24, 725. (13) Waller, A. M.; Northing, R. J.; Compton, R. G. J. Chem. Soc., Faraday Trans. 1990, 86, 335. (14) Streeter, I.; Wain, A. J.; Davis, J.; Compton, R. G. J. Phys. Chem. B, in press, 2005. (15) Streeter, I.; Wain, A. J.; Thompson, M.; Compton, R. G. J. Phys. Chem. B 2005, 109, 12636. (16) Wadhawan, J. D.; Compton, R. G. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds.; Wiley-VCH Verlag GmbH and Co.: Weinheim, Germany, 2003; Vol. 2, Chapter 3. (17) France, R. R.; Rees, N. V.; Wadhawan, J. D.; Fairbanks, A. J.; Compton, R. G. Org. Biomol. Chem 2004, 2, 2188. (18) Amatore, C.; Jutand, A.; Mallet, J.-M.; Meyer, G.; Sinay¨ , P. J. Chem. Soc., Chem. Commun. 1990, 718. (19) Mallet, J.-M.; Meyer, G.; Yvelin, F.; Jutand, A.; Amatore, C.; Sinay¨ , P. Carbohydr. Res. 1993, 244, 237. (20) Yamago, S.; Kokubo, K.; Hara, O.; Masuda, S.; Yoshida, J.-I. J. Org. Chem. 2002, 67, 8584. (21) Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall: Englewood Cliffs, NJ, 1962. (22) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; John Wiley and Sons: New York, 2001. (23) Thompson, M.; Klymenko, O. V.; Compton, R. G. J. Electroanal. Chem. 2005, 575, 329. (24) Wain, A. J.; Thompson, M.; Klymenko, O. V.; Compton, R. G. Phys. Chem. Chem. Phys 2004, 6, 4018. (25) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic Resonance, Elementary Theory and Practical Applications; John Wiley and Sons: New York, 1994. (26) Kamata, M.; Murayama, K.; Miyashi, T. Tetrahedron Lett. 1989, 30, 4129. (27) Shono, T.; Matsumura, Y.; Inoue, K. J. Am. Chem. Soc. 1984, 106, 6075. (28) Semmelhack, M. F.; Schmid, C. R. J. Am. Chem. Soc. 1983, 105, 6732. (29) Pokhodenko, V. D.; Koshechko, V. G.; Titov, V. E.; Golovatyi, V. G.; Shabel’nikov, V. P. Theor. Exp. Chem. 1984, 20, 25. (30) Nelson, R. F.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 3925. (31) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, D. W.; Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3498. (32) Wang, R. L.; Tam, K. Y.; Marken, F.; Compton, R. G. Electroanalysis 1997, 9, 284. (33) Shimizu, T.; Isono, H.; Yasui, M.; Iwasaki, F.; Kamigata, N. Org. Lett. 2001, 3, 3639.

Mechanism 2.

TBPA - e- f TBPA•+ TBPA•+ + Lu h TBPA + Lu•+ 2Lu•+ h Lu2+ + Lu•+ k2

Lu2+ + BzSePh 98 products where k2 is the second-order rate constant for the BzSePh oxidation by Lu2+ and the equilibrium constant for the 2,6lutidine radical cation disproportionation, Kdisp,1, is given by

Kdisp,1 )

[Lu2+][Lu] [Lu•+]2

Hence, the rate of loss of the sugar is now given by

-

dS ) k2[Lu2+][BzSePh] ) dt k2K2ETKdisp,1[TBPA•+]2[Lu][BzSePh] [TBPA]2

and, as above, at constant 2,6-lutidine and TBPA,concentration

-

dS ) k′eff[TBPA•+]2[BzSePh] dt

Mechanism 3.

TBPA - e- f TBPA•+ where the equilibrium constant for this disproportionation, Kdisp,2

TBPA•+ + Lu h TBPA + Lu•+ Lu•+ + TBPA•+ h Lu2+ + TBPA k2

Lu2+ + BzSePh 98 products is given by

Kdisp,2 )

[Lu2+][TBPA] [Lu•+][TBPA•+]

References and Notes