Substituent and charge distribution effects on the

Substituent and charge distribution effects on the redox potentials of radicals. Thermodynamics for homolytic versus heterolytic cleavage in the 1-naphthylmethyl system1 PAULH. MILNEAND DANIAL D. M.

WAYNER~

Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ont., Canada KIA OR6 AND

DAYAL P. DECOSTAAND JAMESA.

PINCOCK'

Department of Chemistry, Dalhousie University, Halifax, N.S., Canada B3H 4J3

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Received May 22, 199 1 DAYALP. DECOSTA,and JAMESA. PINCOCK. Can. J. Chem. 70, 121 (1992). PAULH. MILNE,DANIALD. M. WAYNER, The electrochemical oxidation and reduction potentials of a number of substituted 1-methylnaphthalenes (la-1) and 1naphthylmethyl radicals (2a-1') as well as 2-methylnaphthalene (3) and the 2-naphthylmethyl radical (4') have been measured by cyclic voltammetry and photomodulation voltammetry. The oxidation potentials correlate with a+(p+ = -7.1 and -8.4 for 1 and 2' respectively) while the reduction potentials correlate with a (p- = 10.1 and 13.0 for 1 and 2' respectively). The relative magnitude of the p values can be rationalized when the charge density distribution in these systems is considered. This leads to the interesting conclusion that even though a full charge is placed in the IT-system of 1 when it is oxidized or reduced, the fraction of the charge that accumulates at C, is actually less than in 2+ or 2where only 50-70% of the charge is delocalized into the ring. A correlation between p for the redox reactions of 1 , 2', benzyl, diphenylmethyl, and cumyl and the calculated (AMI) charge density at C, is established, implying that the sensitivity of the corresponding ions to substituent effects increases as the fraction of charge at that site increases. The redox data have been used in thermochemical cycles in order to estimate the substituent effect on the homolytic, mesolytic, and heterolytic cleavage reactions of 1 and its corresponding radical ions. The implication of these results on the C-C cleavage versus deprotonation of radical cations and on the photochemical homolysis versus heterolysis of naphthylmethyl halides and-acetates is discussed. Key words: electrochemistry, homolysis, heterolysis, naphthylmethyl, substituent effect PAULH. MILNE,DANIAL D. M. WAYNER, DAYALP. DECOSTAet JAMESA. PINCOCK. Can. J. Chem. 70, 121 (1992). Faisant appel a la voltampCromCtrie cyclique et i la voltamperomttrie de photomodulation, on a mesure les potentiels d'oxydation et de reduction Clectrochimique d'un certain nombre de 1-mCthylnaphtalknes substitues (la-I) et de radicaux I-naphtylmethyles (2a-1') ainsi que du 2-methylnaphtalbne (3) et du radical 2-naphtylmethyle (4'). On peut etablir a (p' = -7,l et -8,4 respectivement pour 1 et 2'). On peut raune correlation entre les potentiels d'oxydation et les ' tionaliser l'amplitude relative des valeurs de p en considerant la distribution de la densite de charge dans ces systemes. Ceci conduit B la conclusion interessante que, meme si une charge complete est placee dans le systeme IT du compose 1 (qu'il soit oxyde ou rkduit), la fraction de la charge qui s'accumule en C, est en fait moindre que dans les entitCs 2 + et 2' dans lesquelles seulement 50-70% de la charge est dClocalisCe dans le cycle. On a Ctabli une correlation entre les valeurs de p des reactions rCdox des entitCs 1 , 2', benzyle, diphenylmethyle et cumyle et la densite de charge calculte (AM I) pour la position C,; elle implique que la sensibilite des ions corespondants aux effets des substituants croit avec une augmentation de la fraction de la charge dans cette position. Les donnees redox ont CtC utilistes dans des cycles thermodynamiques dans le but d1Cvaluer I'effet de substituant sur les reactions de clivage homolytique, mesolytique et hCtCrolytique du compose 1 et des ions radicaux correspondants. On discute de l'implication de ces rksultats sur le clivage des liaisons C-C versus la deprotonation des cations radicaux et sur I'homolyse photochirnique versus I'heterolyse des halogenures et des acetates de naphtylmethyle. Mots clks : electrochimie, homolyse, heterolyse, naphtylmethyle, effet de substituant. [Traduit par la redaction ]

Introduction There has been continually growing interest in the use of electrochemical data in thermochemical cycles since Breslow and co-workers used this method to estimate the pK, of a number of weak carbon acids (1-5). Recently, Arnold and co-workers (6-8) and Bordwell et al. (9- 12) used this approach for the determination of pK,'s of radical cations and (or) the energies for the homolytic cleavage of C-H bonds. More recently, Arnett et al. (13- 15) combined calorimetric and electrochemical measurements to obtain data for the heterolytic C-C cleavage reactions. In essentially all of these cases the electrochemical mea'NRCC No. 3327 l . 2 ~ u t h o rto s whom correspondence may be addressed.

surements were made in solutions of very long-lived molecules or ions. We have shown that reliable electrochemical measurements can actually be made in solutions of transient radicals using the photomodulation voltammetry technique (16). We also have recently pointed out that all of the thermochemical cycles describing the homolytic and heterolytic cleavage reactions of molecules and their radical ions can be conveniently represented in a thermochemical mnemonic (Scheme 1, eqs. 111-[7]) (17). In this mnemonic the vertical arrows represent the loss of R' as a radical (i.e., the homolytic bond dissociation energy). The horizontal arrows represent the addition or removal of an electron while the diagonal arrows represent the loss of R'+ (up to the right) or R1- (up to the left). For the case where R' = H, the reactions along the diagonals are representations of more familiar thermodynamic parameters; reactions [9] and [ l I] are


122

CAN. J. CHEM.

deprotonation reactions (pKals) while reactions [lo] and [12] are hydride affinities. The latter two reactions are of particular importance to the chemistry of NADH/NAD' and its analogs (18-20).

ical characterization of the parent methylnaphthalenes more straightforward than it is for the toluenes. Recent results on the photochemical bond cleavage reactions of I-naphthylmethyl derivatives (N-CH2-X where N is a substituted naphthalene ring) (22-24) have indicated the importance of the 1-naphthylmethyl radical as an intermediate. Moreover, the rate of oxidation of this radical to the corresponding 1naphthylmethyl cation seems to be critical in controlling the final product composition of these photoreactions (22). To understand these results in a quantitative sense, values for the oxidation potentials of the 1-naphthylmethyl radicals are needed. In this paper we report redox potential measurements for 12 substituted 1-methylnaphthalenes (la-1) and the corresponding 1-naphthylmethyl radicals (2a-1') as well as similar values for 2-methylnaphthalene (3) and the 2-naphthylmethyl radical (4'). The implication of these results on the thermodynamic parameters in Scheme 1 and on the photochemistry of the 1-naphthylmethyl derivatives will be discussed. CH3 I

la, X lb, X lc, X Id, X le, X If, X lg, X lh, X li. X lj, X lk, X 11, X

The thermochemistry for all of the possible homolytic and heterolytic cleavage reactions can then be written simply as the sum of the homolytic bond dissociation energy (eq. [I]) with the difference between two potentials (eqs. [2]-[7]). It is important to keep in mind that an entropy correction must be applied to the bond dissociation enthalpies that are usually reported in the literature since the redox potentials are actually free energy differences. For R' = H, this correction is ca. -8 kcal mol-I (16). These free energy relationships are given in eqs. [8]-[13].

We have now applied this thermochemical approach to the naphthylmethyl system. This system has an advantage over the benzyl system that has been reported (21), as the relative ease of oxidation and reduction makes the electrochem-

CH2* I

-- 4-CN 3-CN 3-0CH3

" 4-C02CH2CH3

2'

"

'H

" 4-F

- 2-0CH3 ' 4-CH3

' 4-OCH3

4,s-di-0CH3 " 4,s-di-0CH3

4.7-di-0CH3

Results and discussion Substituent effects on redox properties The oxidation and reduction potentials of the substituted 1 (measured by cyclic voltarnmetry) and the 2' (measured by photomodulation voltammetry) are given in Table 1. Not surprisingly the series follow the trends expected; electrondonating groups make the oxidation of either 1 or 2' easier and electron-withdrawing groups make the reduction of either easier. This can be put on a more quantitative basis by using Hammett correlations. Only data for the 4-substituted derivatives of 1 and the 3- and 4-substituted derivatives of 2 were included in this analysis to avoid the complication of adding a values for the disubstituted cases. The usual benzene ring a+and a- values were used since only a limited number of these parameters are available for naphthalene rings (25). The Harnrnett plots for oxidation of 1 (p+ = -8.4 0.9) and 2' (p+ = -7.1 ? 0.7) are shown in Fig. 1 and for the reduction of 1 (p- = 13.0 1.2) and 2' (p- = 10.1 + 0.9) in Figure 2. Good fits (r > 0.95) were obtained in

*

*

MILNE ET AL.

123

TABLEI . Oxidation and reduction potentials of compounds 1-4 E,/?OX (V vs. SCE) Compound n

b c d e

f Can. J. Chem. Downloaded from www.nrcresearchpress.com by 190.207.173.157 on 06/05/13 For personal use only.

g h I

j k

1 3a,

4'

2'h

Substituent

la

4-C02CH,CH, 3-CN 4-CN 3-0CH3 H 4-F 4-CH3 2-OCH~ 4-0CH3 4,8-Di-0CH3 4,5-Di-0CH3 4,7-Di-OCH,

1.81 1.93 1.99 1.30 1.59 1.75 1.57 1.30 1.34 1.09' 1.09' 1.19'

0.81 0.79 0.72 0.52 0.47 0.46 0.35 0.13 0.04 -0.05 -0.05 -0.06 1.64 0.61

E , /?led (V vs. SCE)

1'

2'b

-2.00 -0.71 -2.04 -0.87 -1.96 -0.55 d -2.63 - 1.27 -2.55 - 1.33 -2.50 -2.62 -1.35 -2.62 -1.36 -2.61 -1.55 -2.80' -2.78' -2.68 - 1.55 -2.58' -1.25

"

"Measured by cyclic voltammetry at a glassy carbon electrode in acetonitrile with 0.1 M TBAP as supporting electrolyte at a scan rate of 200 mV/s. All potentials are irreversible and potentials are reported as E, - 30 mV. 'Measured by photomodulation voltammetry at a gold minigrid in acetonitrileldi-tert-butyl peroxide (9: 1 v/v) with 0.1 M TBAP as supporting electrolyte. 'Measured by cyclic voltammetry at a glassy carbon electrode in acetonitrile with 0.1 M TBAP as supporting electrolyte at a scan rate of 200 mV/s. All potentials are reversible and potentials are reported as the mid-point between the cathodic and anodic peak. dThe reduction wave was not observed due to high background. 'Quasireversible wave. %eversible wave.

=

FIG. 1. Hammett plot for the oxidation of 1 (0, p+ = -7.1, r 0.95) and 2' (m, p' = -8.4, r = 0.97).

all cases. While most of these redox reactions are irreversible (Table l), Bordwell et al. (9-12) have shown that for a series of structurally related species, the changes in redox properties closely approximate thermodynamic changes; i.e., AE, from cyclic voltammetry = AEO,the thermodynamically significant potential. The oxidation and reduction potentials of the radicals (measured by photomodulation vol-

FIG.2. Hammett plot for the reduction of 1 (0, p- = 10.1, r 0.97) and 2' (m, p- = 13.0, r = 0.98).

=

tammetry) also have been shown to be close to E O (16, 17, 21). It is interesting to note that for both the oxidations and the reductions, the Hammett p value was larger for the redox reaction of the radical than it was for the methylnaphthalene derivative (although the uncertainties in the p+ are large enough that the difference between these values may not be significant). Intuitively one might expect this order to be reversed since a full charge is delocalized in the naphthalene ring in the radical ions (I+' and I-') whereas only a fraction (ca. 0.5) of the charge is expected to be delocalized into the ring for the naphthylmethyl ions (2+ and 2 7 . It seemed reasonable to expect the sensitivity to substituent effects to be related to the extent of charge delocalization into the ring system. To investigate this possibility, the charge distributions in these ions were calculated using the AM1 molecular orbital theory (26). The 2pZcharge densities for the parent ions le+', le-', 2e+, and 2e-' as well as the related benzyl ions are shown in Fig. 3 (the total atomic charges give an essentially identical distribution when the charge on the neighboring hydrogens is included). These data show some interesting parallels with the experimental results. Firstly, the charge distribution in the benzyl and the 1-naphthylmethyl systems is similar. This fits with the observation that the Hammett p values for the redox potentials of these systems also are the same within the experimental error (27). Secondly, the difference between the 1-naphthylmethyl and the l-methylnaphthalene systems becomes clear. Because of the odd alternate nature of the arylmethyl ions (2+ or 2 7 , more of the charge density accumulates at C , and C, while very little accumulates at C, or in the fused benzo ring. For the even alternate radical ions (I+' or I-') the charge is distributed more evenly since there are no nodes in the SOMO. This result leads to the interesting conclusion that even though essentially a full charge is placed in the n-system of the naphthalene ring in the radical ions, the amount of that charge that ends up near the substituent is actually less than in those systems where only 50-70% of the charge in delocalized. This effect of charge distribution is clearly

CAN. J. CHEM. VOL. 70. 1992


benzy 1 anion

benzy l cation

FIG.3. Calculated (AMI) 2p, charge densities for the cations and anions generated by the one-electron oxidation or reduction of the benzyl radical, 2' and le.

-10.0 0.15

0.20

0.25

0.30

0.35

2p, Charge on C4

6.0 -0.35

-0.30 -0.25 -0.20 2p, Charge on C4

-0.15

FIG.4. Plot of p+ for the oxidation of 1, 2', benzyl, diphenylmethyl, and cumyl versus the calculated (AM1) 2p, charge density at C4 in the parent ion.

FIG.5. Plot of p- for the reduction of 1, 2', benzyl, diphenylmethyl, and cumyl versus the calculated (AM1) 2p, charge density at C4 in the parent ion.

seen when the Hammett p values for the oxidation and reduction of substituted benzyl, diphenylmethyl, and cumyl (27) radicals along with those for 1 and 2' are plotted versus the calculated (AM1) 2p, charge density at C4 (Figs. 4, 5 ) . Although there is some scatter in the plot (which contains electrochemical data for about 40 radicals), the trend is clear. These observations are important inasmuch as they reinforce the idea that substituent effects in ionic species should not be additive. This issue has been addressed in detail by Dubois and co-workers (28, 29) and is supported by work

on substituent effects on the redox properties of a,p-disubstituted benzyl radicals (30). In the latter work it was concluded that the effect of two "destabilizing" substituents on an ion is additive while the effect of two "stabilizing" substituents should be less than additive. The lack of additivity is usually rationalized as a saturation effect. However, it is equally well understood in terms of the effect that one substituent will have on the charge density at the site of the second. It is interesting to note that, in this case, the effect of two methoxy substituents on the redox potentials of 1 is essentially additive while the effect on 2 is less than addi-

TABLE 2. Heterolytic and mesolytic bond energies (kcal mol-I) for some naphthylmethyl systems from thermochemical cycles" Compound

Substituent

AGO8

ACo9

ACoIo ACol,

AGO^^

ACo13

la b c d e

4-C02Et 3-CN 4-CN 3-OCH3 H 4-F 4-CH3 2-OCH, 4-OCH3 4,8-Di-OCH, 4,5-Di-OCH, 4,7-Di-OCH,

54.9 51.7 48.7 60.0 52.1 48.2 49.8 51.0 48.0 51.7 51.7 49.4 54.2

-6.9 -9.7 -11.1 4.8 -1.8 -5.5 -1.4 4.8 3.9 9.7 9.7 7.4 -3.0

104.9 104.5 102.8 98.2 97.1 98.9 94.3 89.2 87.2 85.1 85.1 85.1 100.3

51.2 54.9 47.5

107.7 104.9 110.5

64.1 65.5 66.0 66.2 70.6

107.5 104.9 107.2 107.0 102.4

40.1 39.2 41.0 25.6 27.4 28.6 25.8 25.8 26.1 21.7 22.1 24.4 26.8

f g h


i

j k 1

3

"

I,

70.6 65.6

104.0 108.6

"See text for equations. 'Insufficient data to complete the cycle.

''

i I

tive. The configuration of the SOMO in the even alternate and odd alternate ions is consistent with this effect (Fig. 3). Substituent effects on derived thermochemical parameters The substituent effects on the C-H cleavage reactions of 1-methylnaphthalene and its radical ions (eqs. [8]-[13]) are given in Table 2. We have assumed that the bond energy for 1-methylnaphthalene derivatives is not dependent on ring substitution (AH0, = 85 kcal mol-'; AGO, = 77 kcal mol-I (31)). This assumption seems reasonable since there appears to be little effect of substitution on the C-H bond energy in toluenes (32, 33). The values of AGO6 and AGO, have been estimated as - 1.87 V and -0.36 V vs. SCE, respectively (17). 'The substituent effect on AGO, to AGO,, depends only on the redox properties of one of the substituted species. For example, changes in AGO, are determined only by changes in AGO,. Similarly AGOlo is determined by AGO,, AGO,, by AGO,, and AGO,, by AGOs. On the other hand, AGOs is determined by the difference between the oxidation potential of the naphthylmethyl radical (AGO,) and the oxidation potential of the methylnaphthalene (AGO,). In this case, since the Harnmett slopes for these two processes are similar, the substituent effects partially cancel so the net effect is quite small. Although this cleavage reaction is not considered to be an important pathway for the substituted naphthalenes (I), the result has some important implications. Arnold and co-workers (7, 8) and Maslak et al. (34, 35) have studied the C-C cleavage reactions of radical cations in solution. Maslak has suggested that the term "rnesolytic" cleavage (34) be used to distinguish these reactions, which involve the cleavage of a formal one-electron bond, from those reactions that are truly hornolytic or heterolytic. For reactions of symmetrical 1,2-diarylethanes the loss of proton (R' = H, AGO,; eq. [14]) is expected to have a strong substituent effect while the competing pathway, C-C cleavage (R' = R, AGO8; eq. [15]), will have a significantly smaller effect. Although Arnold et al. (36) and Tolbert et al. (37) have pointed out that kinetics of these competing pathways is strongly influenced by stereoelectronic factors, the relative thermochemistry for these pathways will have a significant impact on the interpretation of these data. Proton

loss is usually more exoergonic than C-C cleavage. However, it may be possible to design a reaction, by using the appropriate substitution, in which the thermochemistry of these two pathways is reversed.

Photoinduced homolytic and heterolytic cleavage reactions The photocleavage of benzylic substrates with leaving groups (Ar-LG) is well known to give products that arise from both arylmethyl cations and radicals (38). However, the pathway for the formation of these critical intermediates is not well understood so that reliable predictions about product yields are still impossible. Early mechanisms proposed that the relative yield of cation and radical was controlled by the initial cleavage of the excited state and that the substituents on the aromatic ring could alter the ratio; i.e., the meta effect (39, 40). More recently, it has become obvious that electron transfer, converting the initially formed radical pair (Ars/LG') to an ion pair (Ar+/LG-), will be an important factor in determining these yields. Obviously, the data in Table 1 for the oxidation potentials of the substituted naphthylmethyl radicals as well as previous data for the substituted benzyl, cumyl, and diphenylmethyl radicals (16) are critical for understanding this electron transfer process. To this end, we have determined the free energy change (in acetonitrile) for the conversion of the radical pair NpCH,'/ LG') to the ion pair (N~CH,+/LG-) for LG = C1 (S), Br (6), I (7),and MeC(0)O (8) (Table 3).

CAN. J. CHEM. VOL. 70, 1992

TABLE 3. Homolytic and heterolytic bond energies for 1 and 5-8" NpCH2LG le (HI 5 (c1) 6 (Br) 7 (1) 8 (OAc)

-+


-+

(AGO,),

85.9 67.8 52.8 40.0 68.2

(AGOIO)g

234.5 151.0 141.6 136.1 163.8

(AGOIO)MeCN

105.1 30.3 24.5 23.2 43.3

(AGOET)gb

148.6 83.2 138.8 96.1 95.6

(AGOET)MeCNC

19.2 -37.5 -28.3 - 16.8 -24.9

"Subscript refers to the gas phase (g) or acetonitrile solution (MeCN). Values in kcal mol-'. "Free energy change for the conversion of a radical pair into an ion pair: NpCH,' + LG' NpCH,' + LG-. (AGoET),= (AG",,), -(AG"),. 'Free energy change for the conversion of a radical pair into an ion pair: NpCH?' LG' NpCH2' + LG-. (AGoET)M,cN = (AG"o)M,cN - (AG"),

The redox data for C1' Br', I., and MeC(O)OS in acetonitrile are easily derived from thermodynamic data in the literature (41). For acetate, a thermochemical cycle was used (based on eq. [ l 11) assuming a bond dissociation enthalpy (0-H) of 105 kcal mol-' (30) and literature values for the free energy change associated with the transfer of the relevant ions from water into acetonitrile (42). The values in Table 3 are only for the unsubstituted naphthalene ring. Changes in these values for the substituted cases will parallel the oxidation potential of the 1-naphthylmethyl radicals (Scheme 1) so they have been omitted. Note also, that now LG refers to R' in Scheme 1. It is interesting to point out that (except for H , which is not photochemically labile) the ion pair is always more stable than the radical pair in solution. This is in contrast to the relative stability in the gas phase where the ions are not solvated (Table 3), clearly demonstrating that solvation of the product ions is the driving force for all of these reactions. The reasonable assumption that (AGO,), = AGO^)^,,, is being used. The exoergonicity of these reactions spans a very wide range, from -9 kcal mo1-I for the reaction of 4-carboethoxy-1-naphthylmethyl iodide to -50 kcal mol-' for 4methoxy-1-naphthylmethylchloride. Because of this fact, it is perhaps intitially surprising that products derived from the radical pair are ever observed. However, the Marcus theory of electron transfer (43) gives an explanation, since an activation banier is provided by the reorganization energy that is necessary to convert the radical pair to an ion pair. These ideas have been confirmed (22) by results for the photolysis of 1-naphthylmethyl esters (NpCH2-02CR). The product analysis leads to the conclusion that the excited state cleaves exclusively to the radical pair (NpCH2'/'02CR) and the radical pair partitions between electron transfer (to form the ion pair) and decarboxylation of the acyloxy radical. The rates of the electron transfer process were shown to be rationalized by the Marcus equation even into the inverted region. A reorganization energy of ca. 9 kcal mol-I was estimated. Obviously application of these ideas to other cleavage reactions must be considered.

Conclusions The sensitivity of oxidation and reduction potentials of arylmethyl radicals to ring substitution is related not only to the extent of delocalization of the charge in the corresponding ions, but also to the charge distribution. Thus p+ and pfor the oxidation and reduction of 1-naphthylmethyl radicals are greater than those for the oxidation and reduction of the 1-methylnaphthalenes since the odd alternate nature of

+

the former allows a greater fraction of the charge in the ions to accumulate at C2 and C, even though only 50-70% of the charge is in the aryl ring (as calculated by AM1 molecular orbital theory).The SOMO of even alternate l-methylnaphthalene radical ions leads to a more even distribution of the charge density at all of the sites. A correlation between the experimental p value and the fraction of charge in the 2p, orbital at C, has been established. The redox data lead to estimates of differences in bond energies for homolysis versus heterolysis of l-naphthylmethyl halides and acetates. In all cases the heterolysis is the energetically favourable process. The electrochemical data complement observed photochemistry of these compounds in which electron transfer of an initially formed radical pair (to generate an ion pair) is found to compete with the intersystem crossing process.

Experimental Methylnaphthalenes

1-Methyl, 2-methyl, and 1,4-dimethylnaphthalene were purchased from the Aldrich Chemical Company and purified by bulbto-bulb distillation before use. 3-Cyano and 4-cyanomethyl naphthalene were prepared previously in our laboratory (23). All others were prepared as described below. 3-Methoxy-I-methylnaphthalene: prepared by the method of Bartoli and co-workers (44) and recrystallized from ethanol to give colourless crystals: mp 47-48°C (lit. (44) mp 48-49°C). 4.8-Dimethoxy-I-methylnaphthalene: prepared by the method of Buu-Hoi and Lavit (45). Recrystallization from ethanol gave colourless plates: mp 104-105°C (lit. (45) mp 105°C); 'H nmr (CDCI,) 6:8.0(d, I H , J = 8.5Hz),7.4(t, 1 H , J = 7.8Hz),7.15 (d, lH, J = 8.2 Hz), 6.8 (d, lH, J = 8.0 Hz), 6.6 (d, lH, J = 8.0 Hz), 3.9(~,3H),3.8(~,3H),2.8(~,3H). 4.7-Dimethoxy-I-methylnaphthalene: prepared by the method of Buu-Hoi and Lavit (46). Recrystallization from ethanol gave colourless plates: mp 54-55°C (lit. (46) mp 54°C); 'H nmr (CDCI,) 6: 8.2 (d, lH, J = 8.5 Hz), 7.2-7.0 (m, 3H), 6.7 (d, lH, J = 8.0 Hz), 3.95 (s, 3H), 3.9 (s, 3H), 3.9 (s, 3H), 2.7 (s, 3H). 4.5-Dimethoxy-I-methylnaphthalene: prepared by the method of Buu-Hoi and Lavit (47). Recrystallization from ethanol gave colourless plates: mp 64-65°C (lit (47) mp 65°C); 'H nmr (CDC13)6: 7.44-7.2 (m, 2H), 7.1 (d, lH, J = 8.0Hz), 6.8 (d, lH, J = 8.0 Hz), 6.7 (d, lH, J = 8.0 Hz), 3.9 (s, 3H), 3.85 (s, 3H), 2.6 (s, 3H). 4-fluoro-I-methylnaphthalene: prepared by the method of Sauer et al. (48): bp 64-66°C at 1 Torr (133.3 Pa) (lit. (48) bp 107.5109.5"C at 12 Torr). 2-Methoxy-I-methylnaphthalene: prepared by the method of Buu-Hoi and Lavit (49). Recrystallization from ethanol gave colourless plates: mp 41-42°C (lit. (50) mp 39°C); 'H nmr (CDC13)

MILNE ET AL.


8: 8.1-7.5 (m, 3H), 7.4-7.2 (m, 2H), 7.1 (d, l H , J = 8.0 Hz), 3.9 (s, 3H), 2.6 (s, 3H). 4-Carbomethoq-I-methylnaphthalene: prepared by esterification of the acid with methanol: bp 80-82°C at 0.05 Torr. Crystallization from hexane gave colourless plates: mp 30-3 1°C (lit. (51) bp 192-194°C at 12 Torr); 'H nmr (CDC1,); 8: 9.1-8.9 (m, lH), 8.0 (d, lH, J = 8.0 Hz), 7.9-7.8 (m, lH), 7.60 (t, lH, J = 8.0 Hz), 7.50 (t, l H , J = 8.0 Hz), 7.22 (d, lH, J = 8.0 Hz), 3.91 (s, 3H), 2.60 (s, 3H). The acid, 4-methylnaphthoic acid, was prepared by hydrolysis of 4-cyano-1-methylnaphthalene (23) with sodium hydroxide in ethanol/water. Recrystallization of the crude acid from ethanolwater gave colourless needles: mp 178-179°C (lit. (52) mp 180°C). Electrochemical measurements Solvents and electrolyte were purified as described previously (16). Radicals for photomodulation voltammetric measurements were generated by photolysis of a solution of acetonitrileldi-tertbutyl peroxide (9: 1 v/v; 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte) in the presence of the appropriate methylnaphthalene using a 1000-W Hg/Xe arc lamp (Oriel Corp). The output of the lamp was modulated as a sine wave with a mechanical light chopper that operated at frequencies between 20 and 250 Hz. The reference signal for the lock-in amplifier (PAR model 124A) was taken from the output of the light chopper. In electrochemical experiments the ac component of the electrolysis current was demodulated with the lock-in amplifier (Ithaco model 391A) interfaced to a microcomputer. The electrochemical cell was constructed from Teflon@ and had a volume of 0.5 mL. The electrode, a gold minigrid (1000 wires/in.; Buckbee-Mears) was placed in the cell so that the exposed surface area was a circle with a radius of 2.5 mm. Currents in the range of 0-500 nA were measured with a PAR model 174A polarograph. Cyclic voltammetric measurements were performed in acetonitrile/O. 1 M tetrabutylammonium perchlorate at a glassy carbon electrode using an EG&G model 173 potentiostat and an EG&G model 175 universal cell programmer. All potentials are reported with respect to the saturated calomel electrode (SCE). 1. R. Breslow and K. Balasubramanian. J. Am. Chem. Soc. 91, 5182 (1969). 2. R. Breslow and W. Chu. J. Am. Chem. Soc. 95,411 (1973). 3. M. R. Wasielewski and R. Breslow. J. Am. Chem. Soc. 98, 4222 (1976). 4. R. Breslow and R. Goodin. J. Am. Chem. Soc. 98, 6076 (1976). 5. B. Jaun, J. Schwarz, and R. Breslow. J. Am. Chem. Soc. 102, 5741 (1980). 6. A. M. deP. Nicholas and D. R. Arnold. Can. J. Chem. 60, 2165 (1982). 7. A. Okamoto, M. S. Snow, and D. R. Arnold. Tetrahedron, 22, 6175 (1986). 8. R. Popielarz and D. R. Arnold. J. Am. Chem. Soc. 112, 3068 (1990). 9. F. G. Bordwell and M. J. Bausch. J. Am. Chem. Soc. 108, 2473 (1986). 10. F. G. Bordwell, J.-P. Cheng, and J. A. Harrelson, Jr. J. Am. Chem. Soc. 110, 1229 (1988). 11. F. G. Bordwell and J.-P. Cheng. J. Am. Chem. Soc. 111, 1792 (1989). 12. F. G. Bordwell and J. A. Harrelson, Jr. Can. J. Chem. 68, 1714 (1990). 13. E. M. Amett, K. Amamath, N. G. Harvey, and J.-P. Cheng. J. Am. Chem. Soc. 112, 344 (1990). 14. E. M. Amett, K. Amarnath, N. G. Harvey, and S. Venimadhavan. J. Am. Chem. Soc. 112, 7346 (1990). 15. E. M. Amett, K. Amamath, N. G. Harvey, and J.-P. Cheng. Science, 247, 423 (1990).

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