Intramolecular reactions of aromatic radical anion with halides. Electron-transfer and SN2 processes

Rodiaf. Phys. Chem. Vol. Printed in Grear Britain.

21.

No.

I-2.

pp.

233-238.

OM-5724/83/010233M$03.00/0 Pergamon Press Ltd.

1983

INTRAMOLECULAR REACTIONS OF AROMATIC RADICAL ANION WITH HALIDES. ELECTRON-TRANSFER AND SN2 PROCESSES SETSUO TAKAMUKU, HITOSHI KIGAWA, SUSUMU TOKI and HIROS+II SAKURAI The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567, Japan Abstract-One-electron reduction of I-(4-biphenylyl)-o-chloroalkane (BPa-n) by electrons and the intramolecular reactions of the radical anion thus formed have been investigated1 by pulse radiolysis at room temperature and 77 K matrix y-irradiation of the 2-methyltetrahydrofuran solutions. An intramolecular electron transfer from a biphenyl radical anion to a C-Cl bon was observed for BP&l and BPc,-2 while an intramolecular nucleophilic displacement of the biphe1 yl radical anion on the chlorine center, SN2 reaction, has been presumed for BPc,-3 and BP&. 04 the other hand, in 77 K matrix y-irradiation an intramolecular electron transfer was observed for all these BP,+, and SN2 reaction could not be detected in BPc,-3 and BP&. The characteristic i feature of the intramolecular reactivity of the BPcrn radical anion in both systems has been discksed. INTRODUCTION

compare the results obtaiaed by a pulse radiolysis with those of 77 K matrix y-irradiation of 1-(4biphenylyl)-o-chloroalkane (BPc,-n).

ONE OF the typical reactions of solvated electrons

with alkyl halides is a dissociative electron capture. Aromatic radical anions also induce the cleavage of a C-X bond. For example, the reactions of sodium naphthalene with alkyl halides in 1,Zdimethoxyethane at room temperature yield substantial quantities of hydrocarbon products such as alkyl- and dialkyldihydronaphthalenes.“’ The currently accepted mechanism includes an initial electron transfer, producing an alkyl radical, and immediately following competitive reactions of the alkyl radical such as reduction to carbanions, addition to aromatic radical anions, and radical-radical reactions.“’ The interpretation of the kinetics for such a system where an endergonic and irreversible electron-transfer step is followed by a rapid chemical reaction has been carried out by Walling,‘3’ and Scandola, Balzani and Schuster.(‘) However, the details of the primary electrontransfer steps have not been clarified yet, and in some cases, a nucleophilic substiution, SN2 was supported rather than an electron-transfer mechanism.‘” Therefore, it seems essential to investigate the initial step of respective reaction system, in order to clarify the characteristics of both the mechanisms. In a previous paper.‘@ we have investigated intramolecular reactions of aromatic radical anions with a terminal alkyl halide which is linked to the aromatic moiety by a methylene chain and observed that an intramolecular electron transfer and &2 reaction are competitively taking place in some reaction systems. The present purpose is to

] mCH2)“-X $- “0 t,

234 I,

BP,-n

EXPERIMBNTAL Materials

4-Chlorobiphenyl (BP&) (Tokyo Kasei) and 4chloromethylbiphenyl (BPrrll) (Aldrich) were recrvstalked fro& ethanol and dr?ed under vacuum. 4-Methylbiphenyl (BP-Me) (Aldrich) iwas used without further purification. Other I-(4-biphenylyl)-w-chloroalkanes (BPc,-n. n = 2,3,4) were spthesized according to methods described previously. 6, Hexamethylphosphoric triabide (HMPA) used as a solvent for the pulse radiolys$ was distilled over CaH, twice. For 77 K matrix v-itidiation. 2-methvltetrahvdrofuran (MTHF), purchased rom Wake Pure -Chemi&l Industry, was fractionally disti led over LiAlH4 and then stored over CaH2 in a degasse9, vessel under vacuum. Apparatus and procedures The L-band accelerator

at &aka University was used The details of the here.“’ Briefly, the Linac was operated at about 28 MeV,lproviding single pulses of electron with pulse length ofl 40~s and 3-10 ns. The charge of the firmer w& IO& anh the peak current of the latter was 7 A. The beam WCSfocused to about 4 mm dieter at a rectangular Spe+trosil quartz optical cell (10X 10mm). The absorbed d se, measured by using Fricke solutioas. was 10-20 kra! per pulse depending on the pulse kngth and the beam diameter.

233

S. TAKAMUKU et al.

234

Conk 01

Room

ExP

Photomulli-

k?JiQr

[

lnstrumenlal Synchronous Shutler

Xe-Lamp

FIG. 1. Pulse radiolysis facility at Osaka University. A. Block diagram; B. The time sequence.

As the detection system, a 450 W xenon pulse lamp (OPG-450, Osram), a monocbrometer (Nikon G-250), a photomultiplier (lP28, R843). and a programmable transient digit&r (Tektronix 7912AD) were used. A block diagram of thepulse radiolysis facility at Osaka University and the time sequence are shown in Fi8. 1. This system allows for computer control of kinetic spectrometry experiments with time response up to 7OOps.

RESULTS AND DISCUSSION Pulse radiolysis at room temperature Hcxamethylphosphoric triamide (HMPA) solutions of BPc,-n and Cmethylbiphenyl (BP-Mel were irradiated with 3 ns electron pulses and the transient spectra were recorded at various times after the pulse. The spectrum observed immediately after the pulse has an absorption maximum at 410 nm except for BPc,- 1. The 410-nm band is assigned to the corresponding biphenyl radical anion. In the case of BP=,-1, the 410-nm band was weakly observed only during a 10ns pulse with a high dose. The 33O-nm band formed immediately after the pulse is assigned to Cphenylbenzyl radical. Such a short lifetime of the radical anion of

BP=,-1 may be due to the very fast C-Cl bond cleavage. The results obtained with BP& and BP=,-4 are shown in Fig. 2, together with those of BP-Me. The 410-nm band in BPc,-3 and BP& decayed

rather slowly by first-order kinetics and were characterized by a simultaneous formation of a 330-nm band, which coincides with that of a phenylcyclohexadienyl radical produced by protonation of a biphenyl radical anion in the shape and A,., of the spectrum.‘” On the other hand, the 410-nm band observed for BP-Me decayed by second-order kinetics and the 330-nm band did not build up simultaneously. The results obtained by the pulse radiolysis are summarized in Table 1, together with the product yields by the y-radiolysis. The main product, which was analyzed by GLC after y-irradiation of HMPA solutions of BPcrn, was the corresponding Calkylbiphenyl. BP&l and BP=,-2 gave rather high G values which are close to that of the solvated electrons, G = 2.3 in HMPA.“’ A low value observed in BPcl-1 is reasonably explained by the formation of dimer on the analogy that

Intramolecular reactions of aromatic radical anion with halides TABLE. 1. PULSE RADIOLYSIS’ AND y-RADIOLYSI~ OF BPc,-n

AND

BP-ME

235

IN HMPA

AT ROOM TEM-

PERATURE EIPc,-" 0

DO410

k ( 5-l )

0.22

7.5~10~

&OD33D G(BP-alkaneJb _mD4,D 0

r(A)C

D(C-Cl)d

2.1Dh

5.5

BBe

0.36h

6.2

66f

1

-

2

0.13

1.7~10~

0.05

2.08:

7.6

819

3

0.40

5.5x105

0.45

0.97

8.5

819

4

0.35

1.2~10~

0.92

0.05

9.9

Big

BP-Me

0.39

i

2.06

0

a Pulse width was 3 ns and the absorbed dose was 19 krad per pulse. Substrate concentration b "Co-y-radiolysis

;

Substrate concentration '@@+;l

l.l~lD-~ M.

:

Dose

: :

150 krad.

Products were analyzed by glc.

5.0X10m2 M.

d kcal/mol e Ph-Cl

f PhCH2-Cl' g n-C3H7-Cl

h MTHF was used as the solvent because the reaction products from BPc,-D and BPC,-1 were not clearly isolated from HRPA on glc.

i

O-y

Second -order rate constant

:

l.2x1D1D M-'s-',

BP-3

0.3 0.2 0.1 0 0.3 0.2 0.1 0 0.3 0.2 0.1 0

300

350 400 Wavelength

450

FIG. 2. Transient absorption spectra obtained at various times after a 3-ns puke irradiation of HMPA solutions of BPW3, BP& and BP-Me (1.1 x lo-* M) at room tem-

perature. RPC Vol. 21,No. 1-2-p

benzyl chloride yields bibenzyl as a main product. While a pronounced decrease in the G value was observed in the case of BPc,-3, especially in BP,-,4, and appeared to correspond to the formation of the 330-nm band which is indicated as a relative value in the table. On the basis of these observations, the reaction mechanism will be discussed in some detail. With BP&l and BPcl-2, which result in large rate constants and also large product G values, an intramolecular electron wnsfer from a biphenyl radical anion into a C-Cl bond is presumed. Such an electron transfer would be affected by the difference in the reduction potential between electron donor and electron acceptor, and also by the distance required to the electron transfer(r). It has been shown that such an electron transfer proceeds very slowly when n is less than or equal to 2, that is, t is less than 8 A. However, the rate constants of the radical anions decrease in the order, n; 1 > 2 > 0 * 3, which is roughly parallel to the C-Cl bond energy of these compounds and does not correlate with r as shown in Table 1. Thus, in Scheme I the cleavage of the C-Cl bond seems to be rate-determining. On the other hand, in the case of BPc,-3 and BP& the decay rates of the biphenyl radical anions decreased about two orders of magnitude. The formation of the 330&m band which is assig-

236

S. TAKAMUKUet al.

~(cH2)n-cl =

-

w(CH2)nzc,

w(cy,;,

+cc

(HI

n = O-2

I mCH2)nH Scheme I. 1OmM

RI-0 I

150krad ---

-

Imm. 0.5hr.

Imm.

-_-

47hr.

,

700 500 Wavelength

500 700 Wavc\cngth(nm)

300 I

1OmM

BP,-1

L

1OmM 140krad

BP&

45kmd ---

Imm. 1OOhr.

3

s

D-

b

n

9

I

300

350 ZOO Wavc(cngth(nm) llmhi

BPu-2

-

I

300

I

I

I

I

8P

l

I

I

I

I

1

900

n-BuCt

1OmM 13Okrad

4COkrad

Imm.

-

3.30

I

100 500 Wavelength

t

700 500 Wavelength

I

Imm.

I

900 Wavdcngth(nml

FIG. 3. Absorption spectra of y-irradiated MTHF solutions of BPc,-n and biphenyl-butyl chloride mixture at 77 IL

237

Intramolecular reactions of aromatic radical anion with halides

ned to the phenylcyclohexadienyl radical is significant, while the G value of Calkylbiphenyl formation becomes low; instead of this, dimen were obtained as main products. On the basis of these observations intramolecular nucleophilic displacement of the biphenyl radical anion on the chlorine center, that is, an intramolecular SN2 reaction is presumed as shown in Scheme II.

&KH21n-Cl -

the terminal alkyl chloride come close to each other. In order to clarify the effect of the chain length on the electron-transfer rate constant, 77 K matrix y-irradiation of BPc,-n in MTHF has been carried out. The results are presented in Fig. 3. Similarly to those of pulse radiolysis, the absorption band at 410nm which is assigned to the corresponding biphenyl radical anion was obser-

mCH2)n

+ cl-

R n=3,4

4 Dimers Scheme 2.

By considering the spectral data and the G value of the product, BP& seems to undergo the intramolecular SN2 reaction preferentially while in the case of BP=,-3, the SN2 and electron transfer are competitive. It is reasonable that the SN2 reaction of BP& is favored over BPc,-3 because the strain energy of the cyclic intermediate is higher in BPc,-3. 77 K matrix y-irradiation of BPc,-n in MTHF An intramolecular electron transfer from a bipheayl radical anion to a C-Cl bond is expected to be affected by the chain length, n which corresponds to the distance required for the electron transfer. However, the experimental results indicated that the rate constant is roughly parallel to the C-Cl bond energy of these compounds and does not correlate with the distance, r. This is simply explained by the fact that the rate of electron transfer is far slow compared with that of the molecular motion by which the biphenyl group and

ved except for BP=,-1. The decay behavior of all these 410-nm bands followed first-order kinetics and the rates were dependent on the chain length while the equimolar mixture of biphenyl with butyl chloride (10 mM, respectively) did not indicate any decrease in the 410-nm band for the time-range investigated at 77 K. The$e observations suggest that the intramolecular electron-transfer is the main decay path. The characteristic feature of the 77 K matrix irradiation is the fact that the simultaneous formation of the 330-nm band reflecting the occurence of the intramolecular SN2 was not observed in BP=,-3 and even in BP&. This is attributable to suppression of molecular motion required for the formation of the cyclic transition state for the SN~ reaction. The main product analyzed after warming to room temperature was the corresponding dalkylbiphenyl. The decay behavior of BP=,-3 and BP,& is shown in Fig. 4. TABLE

2. Low

BPC,+l

1.0

TEMPERATURE

n-BuCl

IN

MATRIX OF BP,-,+ MTHF AT 77 K” kf( s-' )b

am

AND

BP +

r(ii)c

1.0L-, 0

1.55

1.8x1o-4

1

5.5 6.2

2

fast

fast

7.6

3

160

1.7x10-6

a.5

4

300

9.3x1o-7

9.9

BP+n-BuCl

No Decay

Substrate concentrations and dose are presented in

a2 I

0

50

Figure 3.

1

100 150 Time(h1

200

250

Fro. 4. Decay plots of the 410-nm band of BPc,-3 (0) and BP& (0) in MTHF at 77 K.

Lifetime(r) and first-order rate constants are determined from the decay at 410 nm. The distance (r) is shown in Table 1.

238

S. TAKAMUKU et al.

The decay followed rather complex kinetics at first, and then, first-order kinetics. The distance required for the electron transfer has a distribution depending on the molecular cornformation when frozen in the matrix. The distance is shorter, the rate constant of the electron transfer higher. Thus, as the reaction proceeds, the linear molecule with a longest, therefore, constant distance rO, is left, providing the simple first-order decay kinetics. The first-order decay constants are summarized in Table 2. The difference in the rate constant between BPc,-3 and BP& caused by the different distance, r. because in both compounds the dissociation energy of the C-Cl bond is the same. Acknowledgemenrs-The present work was supported in Dart bv a Grant-in-Aid for Scientific Research (555334) irom ihe Ministry of Education of Japan. The ‘authors are greatly indebted to the members of the Radiation Laboratory of the Institute of Scientific and Industrial Research of Osaka University for their running the linear accelerator.

REFERENCES 1. J. F. GARST,Act. Chem. Res. 1971,4, 400. 2. S. BANKand D. A. JUCKETT,J. Am. Chem. Sot. 1975, 97, 567. 3. C. WALLING,J. Am. Chem. Sot. 1980,102,6855. 4. F. SCANDOLA. V. BALZANIand G. B. SCHUSTER,J. Am. Chem. St&. 1981,103,2519.

5. W. H. SMITHand A. J. BARD,J. Am. Chem. Sot. 1975. 97, 6491. 6. (a) H. KIGAWA,S. TAKAMUKU. S. TOKI, N. KIMURA, S. TAKEDA,K. TSUMORIand H. SAKURAI,1. Am. Chem. Sot. 1981, 103, 5176; (b) S. TAKAMUKU, H. KIGAWA,S. TOKI, K. TSUMORIand H. SAKURAI,Bull.

Chem. Sot. Jrm. 1981.54.3688. 7. H. SAKURAI, h. KAW~NISHI.K. HAYASHI.K. TSUMORI and S. TAKEDA,Memoirs Inst. Sci. Ind. Res., Osaka Univ. 1981,38, 51. 8. T. SAWAIand W. H. HAMILL.J. Am. Chem. Sot. 1966. 88, 3689. 9. E. A. SHAEDE,L. M. DORFMAN, G. F. FLvNMand D.

C. WALKER,Can. J. Chem. 1973,51,3905. DISCUSSIONS N. GETOFF:We observed the same absorption spectra with a pulse radiolysis of naphthalene. Absorption at 410nm is exactly the same radical anion, that is an electron adduct of naphthalene. But we also studied these electron adducts can react with an electron accepter such as chlorinated and halogenated compounds. Could the thud reaction occur that the electron adduct reacts with a solute as a starting compound and shows pseudo-first reaction? S. TAKAMUKU: I did not indicate the effects of the solute concentration on the decay rate of the 410 nm band, but the rate was independent on the solute concentration under the present reaction condition (less than 0.1 M). This means that an intermolecular reaction between BPc,-n radical anion with another neutral molecule is negligibly small.