H. E.
4274
BACHOFNER,
I n the article by Beringer and co-workers?on this reaction a sequence was proposed CUCL f C1-
+
Rnl CUCln-
anion. KochiZ5and Dickerman and co-morkers?6 have proposed that the initial step is electron transfer froin anion to cation.
&2CUCl?-
+ cuc1?-
(free ion$) RI:
iwr+
R%?Cucl2- (ion pair)
+RC1 f R I
+ CUCI
I t is now proposed that this last step proceeds as
ClCOCl
RIR
+ClCUCl - I
CIICI~
-+ r c i 2 . c1cii
Further, both Kochi'Od and Reutschi and Trumplerl'Id have shown coulometrically that the diazonium ion can accept one electron. I t is not clear whether the subsequent decomposition RXz. CLIC12 +RCl
i
RIR
1~01.so
F. lf.BERIKGER AKD LOUISMEITES
R IR c 1 CUCl
As electron-transfer reactions not involving bondbreaking generally have only a low energy of activation, the formation of diphenyliodine would be expected to proceed rapidly. The diphenyliodine and the cupric chloride may react in their solvent cage ur on collision after separation. Alternatively, if phenyl free radicals are formed from diphenyliodine, they might be converted to chlorobenzene by cupric chloride. For the ideas expressed above we are in part indebted to investigators of the closely related Sandineyer and Meerwein reaction of diazonium salts. Cowdrey and D a ~ i e shave ~ ~ reviewed the Sandmeyer reaction and have concluded that the effective catalytic species is the dichlorocuprate (I) (24) W. A Comdrey and D S navies, J. Chem. SOL,Supplements. 548 (1949); Quart. Revs, 6, 358 (1952).
4- 1-2
+ CUCl
occurs in one or two steps. However, some phenyl radicals add to double bonds or abstract hydrogen from solvent and have thus apparently escaped from the solvent cage. From the above i t may be predicted that a n addition reaction or a displacement reaction will be catalyzed by cuprous ion when the nucleophilic reagent complexes with the cuprous ion and when the electrophilic reagent easily can accept one electron. Acknowledgments.-The authors wish to acknowledge the helpful discussions with Dr. Joseph Steigman and Dr. Milton Allen during the early stages of this work and with Dr. Jay K. .Kochi and Dr. S. Carleton Dickerman on details of the mechanism of copper catalysis. (25) J. K. Kochi, TIIISJ O U R N A L , 7 7 , 5090 (1955); 78, 1228 (1956); 79, 2942 (1937). (26) S. C. Dickerman, K. Weiss and A. K. Ingberman, J . Org. Chem., 21, 380 (1956); THISJOURNAL, 80, 1904 (19.58).
BROOKLYN 1, N. Y .
-[CONTRIBUTION FROX
THE
DEPART*MENTOF CHEMISTRY O F
THE
POLYTECHNIC INSTITUTE
OF
BROOKLYN]
Diaryliodonium Salts. VI. Polarography of Substituted Diphenyliodonium Salts's2 BY H. ELIZABETH BACHOFNER, F.
MARSHALL
BERINGERh K D LOUISAfEITES
RECEIVED AUGLTST 9, 1957 Polarograms of substituted iodonium salts showed the effects of the size, electronegativity, charge and reducibility of the substituents on the three reduction waves.
The preceding paper2 of this series on the chemistry of diaryliodonium salts surveyed the literature on the electroreduction of 'onium salts and showed that very probably the polarographic reduction of the diphenyliodonium cation proceeded as A
Wave I : U'aveII:
RIR
+
4-e-*
RIR
RIR + Z e - + H + + R H +RI + slow Wave 111: RIR + 4 e - + H + +R H R-
+
+ I-
Substituted diphenyliodonium salts were prepared by known methods,' though in some cases the individual salts have not been reported as yet.4 Cyclic iodonium salts in which the 2- and 2'-positions of the diphenyliodonium cation are joined by a methylene bridge or -(CH2)3- were kindly made available by Dr. Reuben Sandin of the University of Alberta, Edmonton, Canada.6 Polarograms were recorded with a Leeds and Northrup Type E Electrochemograph, as described.2 Unless otherwise specified polarograms were run with 0.4 m M iodonium salt and 0.1 M tetraethylammonium phosphate in 1: 1 ethanol-water of apparent PH 8.6. Half-wave potentials are referred to the saturated calomel electrode (S.C.E.).
Results and Discussion Table I lists the three half-wave potentials of a I n this paper the effects of substituents on these number of substituted diphenyliodonium salts. three waves are reported. These values are plotted against the values of Experimental Hammett's sigma constant in Fig. 1. For either the unsubstituted iodonium cation or Solvents, buffers, supporting electrolytes and unsubstituted diphenyliodonium salts were prepared as described.2 substituted cations containing strongly electron R-
fast
+ H C --+
RH
(1) Taken from the doctoral dissertation of Miss Hilde Elizabeth Bachofner, submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy, June, 1957. (2) Preceding paper (on the electroreduction of unsubstituted diphenyliodonium salts) : H. Elizabeth Bachofner, F. Marshall Beringer and Louis Meites, THISJOURNAL, 80, 4269 (1958).
(3) F. M. Beringer, M. Drexler, E. M. Gindler and C. C. Lumpkin, ibid., 71, 2705 (1953). (4) F. M. Beringer, ef al., an article on the synthesis of substituted diphenyliodonium salts has been submitted to THISJOURNAL. (5) J. D. Collette. D. McGreer, R. Crawford, F. Chubb and R. B. Sandin, ibid.. 78, 3819 (1956).
POLAROGRAPHY OF SUBSTITUTED DIPHENYLIODONIUM SALTS
Bug. 20, 1958
TABLE I SUBSTITUTED DIPHENYLIODONIUM SALTS Half-wave potentials,O Wave I11 Wave I Wave TI -7
Substituents
-0.200 - ,186 - ,205 - .093* - ,292‘ - .300“ - .243 - ,180‘ - .192 - ,193
‘
-
.200 - ,200 - .190 - ,202 - ,185
- ,190
-
-1.120 -1.120 -1.122 - 1.20Od
-1.695 -1.660 -1.650 -1.676
- 1. 160d
-1.676
-1.68 -1.64 -160
-1.132
0
h
.20 .21 .512’
h
h
-1.672 - 1.640‘ -1.672 -1.645 -1.615 -1.615 -1.550 -1.575 -1.557 -1.410 -1.645 1
-1.69’ -1.72’ -1.70i
0.
I
0
-
0
*
I
I
I
I
1 *
~l -1.56 ’
2 -1.48
-
Jp
.
1 1 :
2
-1.52
+J 0
4Z
-1.44
*~ I
1
1
*
- l.lti
I
1
-114[ -1.12
,
- 0.22 -0.20 -0.18
~
I
1
-1.40
l WAVEm
I
I
c
-1.175 - 1.065‘ -1.131 -1.142 -1.128 -1.136 -1.132 -1.148 -1.150 -1.138
.
4275
c
’
1
,
,‘I
,
! I1
,I
**
I
e
-1,
!!
.‘ . 1 I WALE1
0
.
-1.3’
- 0.2 0.0 +0.2 t0.4 Hammett’s Sigma Constant. - .45 -1.13 -1.588 In 1 : 1 ethanol-water with 0.4 m M iodonium salt and Fig. 1.-Variation of half-wave potential of substituted di0.1 M tetraethylammonium phosphate supporting electro- phenyliodonium cations with Hammett’s sigma constant. Adsorption wave. True wave. lyte of apparent PH 8.6. ography of i o d o b e n ~ e n e s ,a~ ,marked ~ influence of The half-wave potential may be influenced by adsorption on wave I. “Obscured by the anodic wave of the iodide substituent on the half-wave potential of wave ion. f The wave was a composite of those for iodobenzene I11 was demonstrated. Electron-releasing subOb- stituents hindered addition of electrons, as shown and p-diiodobenzene. a Varies with concentration. scured by the reduction of the nitro group. i Because of the reduction of the nitro,group to the amino group, these by a more negative half-wave potential; electronvalues correspond to the half-wave potentials for the 2- attracting substituents facilitated reduction, as NH2,3-NHz, and 4-NH2 groups, respectively. The2,4,6,- shown by the less negative half-wave potential. 2’,4’,6’-hexamethyldiphenyliodonium cation. I o-SubstituAdsorption.-Adsorption was demonstrated for ents frequently give anomalous values for half-wave potendiphenyliodines with bulky aliphatic substituents, tials; see ref. 8. -0.955‘
- 1.645’
namely, 4,4‘-dicyclohexyl- and for 4,4’-di-t-buty releasing or withdrawing substituents, the half- diphenyliodine (Table 11, Fig. 2). wave potentials for wave I fell within the narrow range of -0.185 to -0.205 v. It is apparent that the ease of the first step in the reduction of the di9.4 phenyliodonium cation is not strongly influenced by the nature of the substituent in the benzene 2.0 ring. Iodonium cations with bulky hydrocarbon + c1 substituents are discussed separately below since 5 1.6 adsorption of the substituted diphenyliodine proc duced by reduction of the iodonium cation caused 3 wave I to consist of two portions, an adsorption g 1.2 0 pre-wave and a diffusion-controlled normal wave. The half-wave potential of wave I1 is largely 0.8 unaffected by the nature of the substituents. Since the nitro group is reducible a t the dropping mercury 0.4 electrode a t potentials close to those for wave 11, the nitro-substituted iodonium cations are disU.10 0.20 0.30 1.05 1.15 1.65 1.70 cussed separately. Half-wave potentials for both V S . S.C.E., V. waves I and I1 for the 2-carboxy- and the 33’dicarboxydiphenyliodoniuni cations differ markedly Fig. 2.-Variation of half-wave potential of 4,4’-di-tfrom those for most of the substituted diphenylbutyldiphenyliodonium cation with concentration. iodonium cations. Therefore these two cations At concentrations up to 0.4 m M , normal a d also are discussed separately. I n agreeement with our suggestion that wave sorption phenomena were observed. When a d . I11 corresponds to the reduction of iodobenzene (F) E. Gergely and T.Iredale, J . Chem. Soc.. 3226 (1953). and with previously reported work on the polar(7) E. L. Colichman a n d S. K. Liu, THISJOURNAL, 76, 913 (1954)
4276
H. E. BACHOFNER, F. 11. BERINGER AND LOUISMEITES
1'01. 80
i < i K -+ K.
-
KI
K. I€*-'r e - -> IiII Although similar behavior would be expected with the unsubstituted diI)henyliodoniuln cation, this could not be demonstrated. However, the same splitting of wave I1 was iioted with a diphenyliodoniuni cation iu which the two ortho positions are linked with a methylene bridge (see below). .AS this cornpouiicl also exhibits adsorption, i t is possible that the heh:i\,ior is related to adsorption. 'i'Afii,E
0 . 09C
0 . 20
.12 .21 . 36 .48 .60
.24
- (J. 180 --
2.08 2.00 2.25 2 . 14
,130
- ,120 (1.13 --11.30t; ,411 - , 110 .87 ,293 .58 - - ~ ,300 2,(J-4 .A0 - . 110 .10 - . 110 .90 --- ,295 2.17 a Concentration is in niillitnoles/liter; current is iri microamperes; potential is in volts i's. the saturated calomel electrode. ..'%I
\-ARIATION OF
I\-
UIFFUSIOSCCRKENT OF P K E - ~ . A V o sELI'AVE
cm.
5iJ. .? 45. 3
10.6 3.5 5 30. ,5
.4s
1.20
1 . (j8
;1 . 4,s
1 . OCI i ) . $14
1 , 3;
50
82
(1
,
1.42
0 , ?:
