45

J. Elec!roanal. Chem., 210 (1986) 45-67

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

ELECTROCHEMISTRY OF THIOXANTHENE, THIOXANTHONE AND RELATED COMPOUNDS IN ACETONITRILE

SUBSTITUENT EFFECTS AND CORRELATION OF ELE~OCHEMICAL BEHAVIOR WITH MOLECULAR ORBITAL CALCULATIONS

E.W. TSAI, L. THROCKMORTON, R. McKELLAR, M. BAAR, M. KLUBA, D.S. MARYNICK l , K. RAJESHWAR l and A.L. TERNAY, Jr. *

Department of Chemistry, The University of Texas at Arlington. Arlington, TX 760194065 (U.S.A.)

(Received 14th October 1985; in revised form 28th April 1986)

ABSTRACT

This paper presents the synthesis, electrochemistry, and molecular orbital (MO) picture of a series of conformationally-restricted diary1 sulfur compounds. The primary electrooxidative and electroreductive

pathways of these compounds are compared with model systems including dibenxothiophene, thiochro-

man4one, and benxothiophene. The oxidation of these compounds is invariably irreversible (with the

exception of thianthrene) and involves rapid dismutation of the radical cation which is formed in the

primary electron transfer step. In the presence of “electrophoric” groups such as C=O (e.g., thio-

xanthone) and SO, (e.g., dibenxothiophene sulfone), characteristic reversible electrochemical reduction

responses are observed, which involve the radical anion in each case. The combined use. of cyclic

voltammetry and chronoamperometry permitted computation of the number of electrons involved in the electrochemical reaction and the diffusion coefficient for each compound.

A series of CZ-substituted thioxanthones was examined to probe the electronic influence of the

substituent on the electrooxidation and electroreduction sites (i.e., on the electron densities at the lO- and

9-positions) respectively. These substituent effects are presented in terms of correlations of oxidation (or

reduction) potentials with the substituent (Hammett) constants, and the highest occupied molecular

orbital (HOMO), or lowest unoccupied molecular orbital (LUMO) energies respectively. The orbital

eigenvalues were computed using the PRDDO method. Finally, the influence of varying bridging atoms

in the above structure is discussed in terms of the case of oxidation and the disposition of the HOMO orbital.

INTRODUCTION

Diary1 sulfur systems can exist in a multitude of conformations each having potential foi its own chemical and spectroscopic properties. In order to understand

l To whom correspondence should be addressed.

0022-0728/86/$03.50 0 1986 Elsevier Sequoia S.A.

46

better the effect of geometry upon these, and related, properties, one of us (A.L.T.) has devoted considerable effort [l] to studies of the chemistry of derivatives of thioxanthene (l), thioxanthone (2), thianthrene (3), phenotbiazine (4), and similar conformationally-restricted structures.

a:n

1: M=CHz 3: M=S

2: M=C=O 4: M=NH

These molecules exhibit varying degrees of folding about the imaginary line containing M and S (the “meso” positions). In folded structures, the non-bonding electrons on sulfur (or moieties covalently bonded to sulfur) exist in two distinctly different environments. One of these (pseudo-axial, a’) is parallel to the aryl ?T system and may be thought of as “truly” benzylic while the other (pseudo-equa- torial, e’) is essentially orthogonal to the aryl r network and is only “‘formally” benzylic.

In order to understand better differences between such a’ and e’ electron pairs (and substituents) and the interactions between ring substituents and sulfur, we have begun to study the electrochemistry of derivatives of such systems and to examine results in terms of theoretical calculations. Because of the near planarity of their central ring the study has begun with derivatives of thioxanthones, particularly those bearing a substitutent puru to sulfur (i.e., at C2). Infrared, as well as X-ray data, suggest that resonance similar to that shown below is significant in 2, consistent with the planarity of t~ox~tbones [2].

Q$JJ-& +

While of obvious interest because of these stereochemical ~onsid~ations, the diary1 sulfur system also has been of interest to electrochemists for a number of years. Several reviews exist of pertinent literature [3]. One motivation for much of the early electrochemical study was the clarification of reaction mechanisms involv- ing cation radicals. Thus 3 has been the subject of many electrochemical studies [4]. Derivatives of 4 also have been of particular interest 151, stemming, at least, in part from the biological activity of its N- and C-substituted derivatives. An additional impetus for study is the fact that these diary1 sulfur systems exhibit a host of valuable ph~a~uti~ activity including neuroleptic [6], ~ti~st~~c [7] and antiparasitic activity [8]. Finally, structures of the type considered in this paper are present in coal and other fossil-fuel matrices [9]. Electrochemical methodology is an obvious candidate in the search of alternatives to extant desulfurization procedures.

For comparison with the electrochemical behavior of 1 and 2 (and CZ-substituted derivatives), model compounds such as dibenzothiophene (5), thiochroman4-one (6), and benzothiophene (7) were also included in this study.

5 6 7

Previous electrochemical studies of 1, 2, 5-7 include those on the oxidation of 1 by Kissinger [lo], the reduction of 2 by Scott [ll], the reduction of 1, 3, and 5 by Holt [12], the oxidation of 5 by Bontempelli et al. [13] and Houghton and Humffray [14], the oxidation of 7 by Srogl et al. [15] and Mann [16] and the reduction of 7 by Mairanovskii et al. [17]. Furthermore, electrosynthesis of the sulfoxide of 3 has been also reported by Humffray’s group [18]. Finally, reduction of the sulfone derivative of 5 has been discussed by Drushel and Miller [19]. These studies are referenced at appropriate junctures in what follows.

Although mechanistic aspects are considered here at some length for some of the above compounds, we are concerned in this study mainly with the primary electron-transfer step involving oxidation or reduction. In particular, we probe how the nature of M, and the presence of substituents at C2 influence this step. To this end, we have carried out new calculations on some of these structures and present correlations between parameters from these calculations and the observed electro- chemical behavior.

EXPERIMENTAL

Syntheses

Thioxanthene (l), thioxanthone (2), dibenzothiophene (S), thiochroman-4-one (6), benzothiophene (7), dibenzothiophene-5,5-dioxide (8), and 2-chlorothioxanthone (12) are commerciahy available and were used after one recrystallization from ethanol. All compounds were homogeneous by their thin layer chromatography (TLC) after recrystallization (Eastman Kodak Silica plates; chloroform eluent; iodine visualization).

2-Methoxythioxanthone (9) Cont. sulfuric acid (160 ml) was cooled to 5°C in a 2 1 flask. To this was added

50 g (0.32 mol) of thiosalicylic acid in 5 g portions. To the resulting, stirred suspension was added 162 g (1.50 mol) of anisole, dropwise. After addition was complete the mixture was heated on a steam bath for 21 h. Upon cooling to room temperature the reaction mixture was poured on to 1 1 of ice. The resulting solution was neutralized with aqueous sodium bicarbonate and the neutral solution extracted with 10 x 100 ml of chloroform. The combined extracts were dried (anhydrous magnesium sulfate) and the solvent removed under reduced pressure to afford 30 g of crude product. TLC showed two yellow spots which fluoresced under UV. This is typical of thioxanthones.

This solid was recrystallized from chloroform. The mother liquor was con- centrated to dryness and the residue recrystallized from ethanol to afford 9.25 g of a solid, m.p. 100-109°C. Two subsequent recrystallizations from methanol afforded 5.16 (21.3 mmol; 6.5% yield) of the desired product as a yellow powder, m.p. 128-129°C (m.p. [20] 128-129°C). The poor recovery of water-insoluble product from the initial reaction mixture suggested that sulfonation might be a serious side reaction. By using 85% sulfuric acid, rather than ~n~ntrat~ acid, heating the reaction mixture on a steam bath for 3 h and then quenching with ice, the ultimate yield of 2-methoxythioxanthone was increased to about 40%. The use of 85% sulfuric acid did not improve the yield of several thioxanthones made by this procedure if the ring did not bear a strongly activating group.

2-Bromothioxanthone (13) Cone. sulfuric acid (80 ml) was cooled in an ice bath. To this was added 26.0 g

(0.169 md) of thiosalicyclic acid. Addition required 5 mm. To this was added, dropwise and with stirring, 129 g (0.822 mol) of bromobenzene. After addition was complete the reaction mixture was heated for 10 h on a steam bath. During this time it turned from orange to dark red. Upon cooling to room temperature the reaction mixture was poured over 1 1 of ice. After the ice had melted, the resulting mixture was extracted with a total of 700 ml of chloroform. The combined extracts were dried (magnesium sulfate) and all volatile materials removed under reduced pressure with warming (approx. 60%). There resulted 25 g of yellow solid, m.p. 147-149OC. Recrystallization from ethanol : benzene (9 : 1 v/v) afforded 17.2 g (0.0591 mol; 35.0% yield) of the desired compound, m.p. 154-156°C (m.p. 160°C [21]).

2-Carboxy-4’-methyldiphenyl sulfide Sodium (1.4 g, 0.061 mol) was reacted with 20 ml of anhydrous methanol and the

resulting solution heated to 90°C. To this hot solution was added potassium 2-chiorobenzoate (11.0 g, 0.0568 mol), p-thiocresol (19.0 g, 0.153 mol) and 0.4 g of freshly precipitated copper. The resulting mixture was carefully heated to 190°C for 40 min. During this time the contents of the flask became yellow. After cooling to lOO”C, the mixture was made alkaline using saturated, aqueous sodium bi- carbonate. The resulting mixture was heated to boiling and filtered hot. The filtrate, upon cooling, was extracted with ether and the extracts discarded. The filtrate was acidified with 6 M hydrochloric acid and the resulting precipitate removed by filtration. There resulted 12.2 g (0.0499 mol, 87.9%) of the desired product. This acid was used without further pu~fication in the next step.

2-Methylthioxanthone (IO) The acid described above was added to 100 ml of sulfuric acid (85%). The

resulting suspension was heated to 105’C for 30 min. (At this point all of the acid had dissolved.) Heating was continued for an additional 60 min at 105OC. After cooling to room temperature the reaction mixture was poured on to 800 ml of ice; a

49

yellow solid resulted. This solid was separated by filtration, washed with saturated aqueous sodium bicarbonate and then with water. Air drying afforded 8.90 g of the desired compound, m.p. llO-111°C. Recrystallization from benzene afforded 6.90 g (0.0305 mol, 61.1%) of 2-methylthioxanthone, m.p. lll-112°C. This synthesis illustrates one method of preparing ‘L-substituted thioxanthones which are free of the corresponding 4-substituted isomer.

2-Carboxy-4’-fluorodiphenyl sulfide Sodium metal (1.4 g; 0.0609 mol) was added, in pieces, to 20 ml of dried

(molecular sieves) methanol. When the sodium had reacted, o-iodobenzoic acid (12.0 g; 0.0484 mol), Qfluorothiophenol (17.0 g, 0.133 mol) and 200 mg of freshly-pre- cipitated copper were added. After refluxing for 30 mm, the methanol was removed by distillation and the reaction mixture was heated to 130°C for 30 min. Upon cooling the reaction mixture was treated with 100 ml of saturated aqueous sodium bicarbonate and heated to reflux. The hot solution was filtered and, upon cooling, the filtrate was extracted with ether (4 X 25 ml). The aqueous solution was acidified with 6 M hydrochloric acid and the resulting solid removed by filtration and washed with water. Air drying afforded 21.9 g of solid, m.p. 17%190°C. Recrys- tallization from acetone eventually afforded 7.6 g (0.0327 mol; 67.6%) of acid, m-p. 199-202’C. This acid was used, without further purification, in the reaction below (m.p. [22] 204-205°C).

2-Fhmothioxanthone (II) 2-C~boxy~‘-fluoro~phenyl sulfide (6.5 g, 0.0280 mol) was suspended in 50 ml

of concentrated sulfuric acid and the mixture stirred for 3.8 h. The reaction mixture then was poured onto 500 ml of ice. When the ice had melted the suspension was extracted with 4 x 50 ml of chloroform. After drying with magnesium sulfate, the solvent was removed under reduced pressure to afford 5.4 g (0.0235 mol; 83.9% yield) of the desired compound as a yellow powder, m-p. 167-168.5”C.

2-Car~xy*4’-acetyldiphenyl sulfide Thiosalicyclic acid (15.0 g, 0.097 mol), p-bromoacetophenone (20.0 g, 0.100 mol),

anhydrous sodium carbonate (12.0 g, 0.113 mol) and 0.2 g of freshly precipitated copper powder were combined in 200 ml of dry DMF and the resulting mixture refluxed for 4 h. The solvent was then removed by vacuum distillation to afford a light brown residue. The residue was slurried with 200 ml of water and the suspension filtered to afforded a green-brown residue. The filtrate was acidified and the resulting solid washed and then dried in vacua over potassium hydroxide. This afforded 24.6 g of crude product. Recrystallization from ethanol afforded 19.5 g (0.0717 mol, 73.9% yield) of the desired product, m.p. 207-209OC (m.p. [23] 205-206°C).

2-Acetytthioxanthone (14) Polyphosphoric acid (35 ml) was combined with 7.75 g (0.0285 mol) of 2-

carboxy-4’-acetyldiphenyl sulfide and the reaction mixture heated to 50°C and

stirred for 3.75 h. This was then diluted with 700 ml of water and the resulting solution neutralized with 10% aqueous sodium hydroxide. The resulting solid was removed by filtration and washed with water. Drying in vacua afforded 7.76 g, m.p. 159-165”C, of crude product. Recrystallization from ethanol : benzene (9 : 1 Y/V) afforded 4.70 g (0.0185 mol, 64.9% yield) of the desired compound as a yellow solid, m.p. 174-175°C (m-p. 1231 179-18OOC).

Cyclic volt~et~ (Cv), ~~ro~o~rn~rurnet~ (CA), and constant potential coulometry (CPC) were performed in “dry” acetonitrile. The latter was obtained by distillation of HPLC-grade solvent (Fisher Scientific) over PzOs. Tetrabutylam- monium perchlorate (Southwestern Analytical Chemicals) was used as supporting electrolyte (0.1 M). Three electrode cell geometry with positive feedback iR com- pensation was used in all cases; all me~urements pertain to ambient temperature. Potentials in this study are quoted wrt Ago”” reference in acetonitrile.

A Princeton Applied Research (EG&G) electrochemistry system assembled from Model 173 and 175 modules was used in conjunction with either an IBM Instru- ments Inc. or a Bioanalytical Systems Inc. electrochemical cell. The working electrode was either Pt or C and had a nominal geometric area of 0.20 cm’; the reference electrode was Ag/O.l M Ag+ in acetonitrile.

Pre-treatment of the working eleotrode surfaces prior to use followed eonven- tional procedures (cf. ref. 24). Frequent monitoring of cyclic voltammogram peak shapes and peak separation potentials, AE,, for the ferrocene/ferricenium redox couple (which was.used for calibration), provided a convenient means of assessing the efficacy of these pre-treatment procedures.

Pulse polaro~aphy was performed on a PAR Model 264 el~tr~he~st~ system which was used in conjunction with a Model 303 A Static Mercury Drop electrode. Nominal parameters for these experiments were the following: drop size: “small”; drop time: 0.5 s; potential scan rate: 5 mV/s; and pulse amplitude: 25 mV.

The electrochemical behavior of 1,2, and 5-7, was probed by cyclic voltammetry (CV) (and where possible, pulse polarography), chronoamperometry (CA) and constant potential coulometry (CPC), in “dry” acetonitrile (see Experimental). The CV scans were performed in the usual manner, by first moving the potential in the positive direction, and subsequently in the negative direction, starting from the rest potential. In what follows, the superscript “( +)” and “( -),’ (e.g., Eg), EL’) denote oxidation and reduction branches respectively for a given compound.

Figure la shows the CV behavior observed for I in dry acetonitrile. No reduction

51

Fig. 1. Comparison of the CV oxidation behavior on Pt of title compounds in acetonitrile. Curves labeled

a-e correspond to compounds, 1,2,!5-7, respectively. Potential scan rate: 0.1 V/s. The peak marked “A” in Fig. la does not appear in double-distilled acetonitrile (refer to text).

was observed for 1 out to - 2.50 V. The oxidation wave at 4 1.00 V is irreversible for CV scan rates up to at least 10 V/s. Another wave at ca. +1.35 V appears if water is present above the nominal levels present in doubly-distilled acetonitrile [25]. Cathodic features (at - 0.27 V and - 0.38 V) on the return sweep (which are not shown in Fig. la) are essentially analogous to those observed by previous authors [lla,b] except that the anodic wave reported at ca. +0.64 V by these authors on the second sweep cycle is not evidenced under the relative slower-sweep conditions of our experiments. The i,/~?/~ ratio (i, = CV peak current, u = potential scan rate) was constant for the anodic wave for 1 showing that the electrochemical reaction was diffusion-controlled without surface-filming complications that are often characteristic of organic sulfur compounds (see below).

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53

The CA profiles for potential steps past the first anodic wave were consistent with the overall CV behavior shown in Fig. la. Within the constraints imposed by irreversible electrochemical behavior, the Cottrell slopes were approximately twice that for the corresponding reduction response for 2 where an n value of 1 was established in the CA time regime (see below). Furthermore, CPC analyses at + 1.10 V reveal a value of n = 2.00 f 0.03 (from five replicate measurements). Values for n > 2 are obtained by systematic additions of water to acetonitrile. This increase in n parallels the monotonic growth of the CV wave at + 1.35 V.

Table 1 contains diffusion coefficient data for 1 obtained from the Cottrell expression:

it’/2 = nF’c*D’12/r’12 (1)

(All the above terms have their usual electrochemical significance.) To our knowl- edge, there is no literature value of D for 1 available for comparison with our measurements.

The major CV oxidation wave in Fig. la is explained by reaction (2):

&& -26a-H*, &

(2)

1 la

The electrochemical route leading to formation of the thioxanthylium cation (la), therefore mimics the chemical oxidation pathway [26]. The cation radical of 1 had been proposed [26] as an intermediate in reaction (2); its fleeting existence (d ms time scale), however, remains to be verified experimentally. To see if blocking of the 9-position via substitution would enhance cation radical stability, electrochemical oxidation of 9-isopropylidene thioxanthene was examined by CV. Oxidation, how- ever, remained irreversible at scan rates up to 10 V/s.

The presence of water in acetonitrile facilitates conversion of la to 2 via the 9-ol-intermediate. Thus, electrochemical oxidation of 2 obviously accounts for the second anodic feature at + 1.35 V, cf. Fig. lb; and for n values > 2.

Finally the reduction of protons generated in reaction (2) and in the conversion of la to the radical precursor of dithioxanthyl [lo] completes the accounting of features that are observed in the CV oxidation branch for 1.

Thioxanthone (2) Electrochemical oxidation of 2 is characterized by a single, irreversible CV wave

at + 1.34 V for the scan limit up +2.00 V (Fig. lb). As with 1, this wave is irreversible at scan rates up to at least 10 V/s. Both the CV i,/u1’2 and CA it’12 parameters are approximately twice those for the corresponding reduction process (see below). CPC at + 1.45 V yields an n value of 2.03 f 0.07 from six replicate measurements. We thus conclude that the oxidation of 2 proceeds cleanly to the sulfoxide.

-1 a -18 -20 -2 2

Potential/ V (ys Ag/Ag+)

Fig. 2. CV scans as a function of scan rate for the reduction of 2 in acetonitrile.

TABLE 2

Cyclic voltammetry parameters for reduction of tbioxanthone (2)

v/v s-1 Parameter

E’_‘/V a E;,)/V a W/V L/k b

PC

0.02 - 2.01 - 1.93 0.08 1.021 0.05 - 2.01 - 1.93 0.08 1.023 0.10 - 2.01 - 1.93 0.08 1.026 0.20 - 2.01 - 1.93 0.08 1.023

a As in Table 1, the superscript “( - ),, denotes the negative branch of the CV scan. b Ratio corrected for current at the switching potential; cf. ref. 27.

55

Figure 2 illustrates typical CV data for the reduction of 2. Parameters computed from such data are collected in Table 2. The relative insensitivity of AE,, and the peak current ratio to u is diagnostic of complete electrochemical reversibility down to u = 0.02 V/s. A measure of the coulometric n value is therefore obtainable via the procedure outlined by previous authors [28]. Use of eqn. (1) in conjunction with the CV peak current expression, eqn. (3); (k is the Randles-Sevcik constant equal to 2.69 x 105, cf. ref. 29) yields a value for n given by eqn. (4):

i, = kn3/2AD’/Qc*u’/2 (3)

n = (&vWsc,k’> (4)

In eqn. (4), S,, and S,, are the slopes of i vs. t-“2 and i, vs. d/’ plots and the constant k’ = 4.77 x lo5 (C/mol)3/2(J/mol)- “2. As pointed out previously [28], this procedure allows computation of n without a priori knowledge of A, c* or D. The results of such analyses for 2 are shown in Table 1. An n value close to 1 is obtained on the CA time scale. Bulk coulometry at - 2.15 V yields a blue solution with n = 1.98 * 0.04 from five replicate analyses. Continuous addition of further aliquots of 2 to the electrolysis solution followed by prolonged CPC yields values of n close to 3.

Finally, systematic addition of water to the acetonitrile is seen to degrade the peak current ratios from unity in the CV traces for 2. Ultimately, a completely irreversible reduction response is observed.

All of the above observations are consistent with the classical Hoijtink scheme 1301 represented by eqns. (5)-(8):

EP Th + e- + Th- (5)

Th.- + H,O2ThH.+ OH- (6)

ThII’+ e- SThH-

ThH- + H,O%hH, + OH- (8)

Here Th = 2 and Ep > EP. This mechanism is well-established for the cathodic electrochemistry of aromatic hydrocarbons [31]. Additionally, our long time-scale CPC data conform to a “nuance” reaction which can be incorporated into the above mechanism (cf. eqn. 9) [ll]:

Th.-+ThH’ k’ k~ Th+ThH- (9) 3

A shift of the above equilibrium to the left by increasing addition of Th would explain the above observation of n values higher than 2.

Computation of D for 2 is facilitated both from CA and CV data (cf. eqns. 1 and 3). Values were obtained from oxidation and reduction responses and are shown in

56

(a)

IOS

1 1 i I

-1.90 -1.90 -2.00 -2.10 -220

Pot.ntiJ/Vh Ag/Ae+)

1.0

0.6

id+ 0

-0.5

-1.0

W

-1.92 -1.99

slop* - 99.2 mv r =asss

Fig. 3. (a) Normal and differential pulse polarographic scans for 2 in acetonitrile (see “Experimental” for

scan parameters). (b) A typical log plot culled from the normal pulse polarography data.

Table 1. These values are in excellent agreement with that reported previously [ll] using chronocoulometry (D = 0.97 f 0.2 X 10e5 cm*/s).

The above electrochemical behavior of 2, pertains to solid electrode (Pt or C) surfaces. Figure 3 illustrates representative pulse polarographic data for 2 in dry acetonitrile. The polarographic reduction response (cf. inset, Fig. 3) is entirely consistent with the reversible electrochemical behavior seen in CV; and furthermore underlines the insensitivity of the electrochemical reaction to the nature of the electrode surface.

Dibenzothiophene (5) Only a brief discussion is warranted here since fairly detailed studies are

available in the literature [12-141. The CV oxidation behavior is shown in Fig. lc

57

8

I I I I 1 I I I -19 -2 t -23 -25

Potential/ V(upr Ag/Ag*)

Fig. 4. CV scans as a function of scan rate for the reduction of 8 in acetonitfile.

for this compound. We find that i,/~‘/~ and itlf2 parameters for 5 are comparable to those for the reduction of 2 (see above). This finding coupled with the n values from CPC (n = 1.21 f 0.01 from five replicate analyses) are in agreement with the mechanistic scheme proposed by previous authors [13]. A dimeric sulfonium ion was shown as the product after the initial electron transfer from 5.

TABLE 3

Parameters from CV for reduction of di~~t~~h~e sulfone (8)

v/v s-1 I$- ‘/V E;, ‘/V hE,/V ‘pa/i, a

0.02 - 2.21 -2.15 0.06 1.02

0.05 - 2.21 - 2.14 0.07 1.05

0.10 - 2.21 - 2.14 0.07 1.07

0.20 - 2.22 - 2.14 0.08 1.04

’ Ratio corrected for current at switching potential, cf. ref. 27.

58

A value for D computed from the CA oxidation response (cf. eqn. 1) is shown in Table 1 for 5. Values for D for comparison are not available in the literature to the best of our knowledge.

No reduction of 5 was observed in CV for scan limits up to - 2.50 V. Figure 4 illustrates typical CV data for reduction of ~be~ot~ophene sulfone,

(8). Analysis of these data are assembled in Table 3. As with the reduction of 2, all the criteria for electrochemical reversibility are satisfied by the CV parameters. Analyses of CV and CA data similar to those outlined above for 2 were also carried out (cf. equations 1, 3,4). An n value close to unity is consistent with reaction (10):

Q I ;‘- s

0

(10)

Follow-up reactions involving 8 should parallel the route outlined above for 2 (cf. eqns. 5-9).

Values of D for 8 are shown in Table 1. These were again obtained from analysis of the CA data for oxidation and from both CA and CV results for reduction.

It is pertinent to note that the electrochemical reduction is much better behaved on a Pt surface than on Hg - the electrode system studied by a previous author [12] (cf. Table 3). Follow-up reactions and rather irreversible behavior were noted by this author [12] for the Hg case; a behavior which contrasts with that observed here for 2 (see preceding section)_

This compound was chosen for comparison with 2. Figure Id shows typical CX data for the oxidation of 6. The overall voltammetric behavior is similar to that of 2 and 5, the first irreversible anodic wave at -I- 1.33 V and the cathodic feature at - 0.30 V (not shown in the figure) being accounted for by formation of the dimeric cation and reduction of protons generated in the anodic process, respectively. Consistent with this scheme, the CV peak amplitude is approximately that expected for a one-electron process. Analyses of iJo ‘I2 data are affected by complications from surface filming of the electrode; Cottrell plots, however, are relatively well behaved and permit computation of D (cf. Table 1).

An irreversible CV reduction response is observed for 6 at - 2.24 V.

Benzothiophene (7) Figure le shows typical CV data for the oxidation of this compound. The

proclivity of this compound to strong adsorption on the electrode surface is indicated by the “spiked” CV peak shape. As in the case of 1,5 and 6, no reduction is evident in CV up to -2.50 V.

No analyses of i, vs. u”~ and i vs. t -*‘2 plots were attempted for 7 because of adsorption complications.

59

Electrochemical behavior of substituted thioxanthones

We have examined the effect of ring substitution in the 2-position on the electrochemical behavior of 2.

Compound: 2 9 10 11 12 13 14

x: H OMe Me F Cl Br AC

Oxidation The question of whether the oxidation mechanism for 9-14 is similar to that of 2

was first addressed by examining the corresponding an values (a = transfer coeffi- cient) [32]. These values were culled from analyses of shifts in Ep,* ( Ep,z = half-peak potential) at varying scan rates, v, and v2 [33]:

Er+ (2) - Ep,* (1) = (0.012g/an ) ln( v2/v1 ) (11)

A graphical method was used to compute an; plots of E,,,(2) - E,,,(l) vs. In vJv, in all cases, yielded straight lines with correlation coefficients (r) close to unity. The an values thus obtained are assembled in Table 1, along with other parameters from analyses of CV and CA data. Note that the 2-methoxy derivative [9] reveals an an value which is significantly different from the other CZ-substituted compounds. We have also observed similar deviations in the oxidation mechanism for the methoxy derivative in our study of the N-aryl-P,P,P-triphenylphospha-X5- azene(R-C,H,-N=PPh,) series [34]. These complications will be discussed elsewhere.

(b) (a)

1.6 , -2.10

-2.05

E~/v (*J -2.00

-,.es

E’ffV

(0)

-0.11 -0.e -0.4 -0.2 0 0.2 0.4 0.6 -0.2 -a1 0 0.) 42 0.8 0.4

a; (Up) urn

Fig. 5. Correlation of I?$:) (Fig. Sa), I$;) and E1(;2), with of for 2 and its derivatives (refer to text). E(+) and E’-’ were obtained from CV (scan rate: 0.1 V/s) and E,‘;,’ from polarography (cf. Fig. 3). Tk substi&t constants were obtained from ref. 32. T’he dashed line in (a) pertains to the correlation in the absence of 9 (refer to text).

60

I I I 1 I I I I I

-I 4 -16 -18 -2 0 -2.2

Potential/ Vbs Ag/Ag)

Fig. 6. Typical CV scan for the reduction of 13 in acetonitrile. Potential scan rate: 0.1 V/s.

The systematic shift observed in the peak potentials for the primary oxidation wave for CZ-substituted thioxanthones has been analyzed in terms of Zuman plots (Fig. 5) [32]. Figure 5a illustrates the type of correlation observed for Ek:’ with the ,substituent constant, u. This correlation (r = 0.989, if 9 is omitted and r = 0.993 if 9 is included, see above) is presented in terms of u+ and uP (the latter for 11 and 14). The reaction coefficients, p from the slopes of the plot in Fig. 5a are + 0.22 V and 0.19 V respectively, with and without inclusion of 9.

The D values for 9-14 were calculated using the methodology outlined above for the title compounds. Such values are also assembled in Table 1 for these com- pounds. A value of D close to 1.00 X 10e5 cm*/6 is observed and is relatively insensitive to the nature of substituent in the 2-position.

Reduction The reduction response for the various derivatives of 2 was analogous to the

parent compound; major exceptions are 2-bromothioxanthone (13), and to a lesser extent, 2-chlorothioxanthone (12). For 13, a set of two reduction waves is observed in CV instead of the typical response illustrated for 2 in Fig. 3. The first of these waves is irreversible and the second is quasi-reversible, Fig. 6. For 12, similar anomalous behavior was observed only at very low scan rates (Q 0.01 V/s). Such behavior is best explained by the following reaction scheme:

Th-Br + e- 2 (Th-Br)‘- 02)

(Th-Br’-)%‘h+ Br- k4

Th’+ SH$Th (14)

Th+e-zTh.- (5)

In the above scheme, Et < ET and Th-Br = 13. The close correspondence of peak potentials for the second set of CV waves in Fig. 6 with the corresponding Ep values for 2 (cf. Fig. 3) lends credence to the above reaction scheme. Precedence for similar behavior is found in the CV behavior of halonitrobenzenes [35].

Plots analogous to Fig. 5a for shifts of E$,;)‘Ej/;’ with (I are illustrated in Fig. 5b. As expected, both Eg’ and E,$) values (measured by pulse polarography) vary in an analogous fashion with u. The departure from strict linearity for 12 is explainable in terms of the complicating effects alluded to above (13 was omitted for this reason from this correlation). The correlation as shown in Fig. 5b for EL) has a coefficient of 0.994 and yields a value of +0.28 V for p. The corresponding parameters for E&j are 0.979 and Ct.29 V respectively.

Table 1 also contains parameters from analyses of CV and CA data for the reduction branch. Derivatives of 2 (other than 13) yield n values close to 1. Values of D computed from the CV and CA data are also included in Table 1.

Molecular orbital (MO) calculations: methodology

All molecular orbital calculations were done with the PRDDO approximations [36] using standard geometries. The PRDDO method is an approximate MO treatment which yields wavefunctions of near ab initio quality in only a fraction of the computing time necessary for the corresponding ab initio calculation using the same basis set. This makes the method ideal for the large molecules studied here. A minimum basis set is used, with or without d orbitals on sulfur (the calculations reported here do not include d orbitals, although their inclusion does not alter significantly the correlations discussed here. Pople’s standard exponents were used ~rou~out 1371.

For 2 and its derivatives, the basic structural parameters were taken from the crystal structure of 12 [38], partially idealized to C, symmetry (all symmetry related C-C distances in the parent compound are equal to within f0.02 nm). The geometry of the ring system was not altered when the various substituents were added. The geometries of the substituents were derived from similar compounds. The methyl and methoxy group geometries were taken from the corresponding phenyl derivatives [39]. The geometry of the acetal group was derived from methyl acetate [39]. A C-F distance of 0.1278 nm was employed for the fluoro-substituted compound. The chloro- and bromo-derivatives (12 and 13) were not included in the theoretical study due to computational limitations. The current program is not able to do calculations on molecules containing bromine, and proper predictions of

62

chlorine substituent effects appear to require d orbitals on both S and Cl. At present, only one set of d orbitals in the basis set is allowed. The geometries of 3, 5, xanthene (15) and 9,10-dihydroanthracene (16) were taken directly from the litera- ture [40] except that C-H bond distances were idealized to 0.109 nm when necessary. All MO calculations were done on an IBM 4341-12 computer, while the contour maps shown below, were calculated on a DEC 2060.

Correlation of electrochemical behavior with MO calculations

Substituent effects Correlation of polarographic half-wave potentials with Hiickel MO parameters

has been presented for a variety of compounds; reviews of earlier work may be found in refs. 39 and 40. In general, Eiz) and EL’ should scale with the energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) respectively. Notwithstanding complications from electrochemical irreversi- bility (kinetic complications) and entropic/salvation contributions, such correla- tions (e.g., Ei,) vs. ionization potential) have been surprisingly good [43].

The typical correlations presented in Fig. 7 for Ei:) vs. HOMO energy (Fig. 7a) and E;) and E,‘jd vs. LUMO energy (Fig. 7b) show that 2 and its derivatives are no exceptions to this rule. The following expressions describe the variation of Eiz’, E(+) and E1’;2’

PC respectively with HOMO and LUMO energies for the various

compounds in this series:

E(+’ = - 0.88E,,,o - 3.39 Pa 05)

E’-’ = - 0.60E PC LUMO + o*88 (16)

E&Z) = - 0.60E LUMO + o.96 07)

A slope of unity is predicted in correlations of electrochemical potentials vs. orbital energies provided that (a) shifts in these potentials from either charge-trans- fer limitations or coupled chemical reactions are either negligible or constant across a compound series, and (b) solvation energy differences between the two redox

(b)

Fig. 7. Correlation of I$:’ with HOMO energies (Fig. 7a) and Eg-’ and II,‘;,) with LUMO energies for 2 and selected derivatives. The MO energies were computed by the PRDDO method (refer to text).

63

Fig. 8. Wavefunction plot for the highest occupied molecular orbital (HOMO) of thioxanthone. Contour values are *OS, 0.4, 0.3. 0.2, 0.1. 0.05, 0.02, and 0.01. The plane of the plot is perpendicular to the

molecular plane.

states are constant [44]. Both these conditions apparently are not fully satisfied with this compound series. Other complicating effects include geometric distortion accompanying electron transfer. However, the latter are expected to play only a minor role since both the HOMO and the LUMO are essentially pure B orbitals in these systems (see below).

Plots of the HOMO (Fig. 8) and the LUMO (Fig. 9) show that both orbitals are of nearly pure 7r character, as expected from the high degree of planarity in these systems. Both orbitals are highly delocalized into the 7r systems of the rings. The HOMO is dominated by sulfur (0.81e) with lesser contributions (0.050.16e) from the ring carbons and 0.17e from the carbonyl group. The LUMO is similarly dominated by the carbonyl group (0.77e) with 0.088e on the sulfur. A respectable linear correlation between the calculated Mulliken charge on sulfur and EA:) exists;

Fig. 9. Wavefunction plot for the lowest unoccupied molecular orbital (LUMO) of thioxanthone. The contour values and plane of the plot are the same as those of Fig. 8.

64

TABLE 4

Correlation of oxidation potentials (I$,: ‘) ’ with HOMO energy for selected compounds

Compound M

1 CH,

2 C=O 3 S 5 b

15 CH2

16 CH2

Y E;:‘/V

S 1.00

S 1.34

S 0.91

S 1.38

0 1.18

CH2 1.68

HOMO energy/eV

- 6.266

- 6.173 - 6.198 - 6.369 - 6.592

- 7.428

a Measured by CV at 0.10 V/s. b M is absent in the five-membered central ring system.

however, no reasonable correlations involving the charges on the carbonyl group are present for Ei,‘. Indeed, it is very difficult to analyze the substituent effects in terms of population analyses, since the populations change very little at the sites of oxidation and reduction as the substituent is varied. For instance, the Mu&ken charges on S range from 0.171 to 0.165 e for the derivatives considered here. The most sensitive probe of the bonding seems to be orbital eigenvalues, which change significantly and correlate well with the experimental results.

Other compounds To examine the generality of EP correlations with orbital energies for the other

heterocycles, similar calculations were performed on thianthrene (3) dibenzo- thiophene (5), and xanthene (15). The computed energies of the HOMO’s of these molecules, along with the measured EiB+) values, are presented in Table 4. The effect of the heteroatom was also probed by inclusion of 9,10-dihydroanthracene (16) for this correlation. Unlike the previous case (cf. Fig. 7) involving closely-re- lated members of a series, no simple correlation exists between HOMO energy and “oxidative stability” (the latter being expressed in terms of Ei:‘). The problem here is the rather disparate nature of the electrochemical reactions involving the various compounds. Entropic effects accompanying electron transfer are also likely to be different for these compounds; a problem which has been stressed before in correlations of this sort [45].

CONCLUDING REMARKS

Electrochemical oxidation of all compounds included in this study proceeded inevitably to the sulfonium salt. Dimer formation was especially facile with decreas- ing ring size and conjugation (e.g. 5-7). In this regard, these compounds provide a stark contrast to the oxidation of thianthrene wherein the cation radical exhibits

65

remarkable stability. It appears that effective electron delocalization into the ring system for this compound lowers the charge density on the sulfur and thereby precludes nucleophilic attack on the cation radical. Our electron density computa- tions by the PRDDO method bear out this expectation. The reduction response, on the other hand, becomes well-behaved on addition of oxygen to the 9-position (e.g., 2) or to the sulfur (e.g., 8). It is perhaps useful here to invoke the concept of an “electrophoric” group which was perhaps first coined in a different context by Miller [46]. Analogous to the definition of a functional group or a chromophore in optical spectroscopies, the presence of certain electrophoric groups in the molecule (e.g., C=O, S=O) can be viewed as giving rise to a characteristic (e.g., reversible) electrochemical behavior and/or CV morphology, which we suggest may be a useful “fingerprint”. A similar idea proved useful in the interpretation of the electrochem- ical behavior of the R-C,H,-N=PPh, system [34]. In the latter case, reversible electron transfer was observed in the oxidation and reduction branches of CV scans respectively for the NMe, and NO2 derivatives.

The combined use of CV and CA methodology has proved useful for computa- tion of n and D values for these compounds. Comparison with corresponding n values obtained from bulk electrolysis yield a measure of the importance of follow-up chemical reactions. A case in point is 2. The D values obtained from oxidation and reduction responses are internally consistent (cf. Table 1). Further- more, these D values are fairly insensitive to substituent effects but sensitive to molecular size (cf. Table 1).

Correlation of E,, and E,,, with substituent constants and MO energies have been presented for the first time for these sulfur heterocycles. These data constitute the first step in developing further correlations for more complex structures involv- ing these heterocycles (e.g., systems with long-chain substituents).

ACKNOWLEDGEMENTS

We thank the Robert A. Welch Foundation [Grants Y-743 (D.S.M.) and Y-484 (A.L.T.) and the Organized Research Fund of UTA (K.R.) for financial support. One of the authors (K.R.) also thanks Drs. Peter Kissinger and James Chambers for a critical reading of the manuscript. Finally, we thank one of the referees for directing our attention to previous work contained in refs. 15 and 17.

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67

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