Electron Transfer from a Cyclic Diaryliodine Species to Arenediazonium Salts. Evidence for the Intermediacy of 9-I-2 Structure in Inodine Atom Transfer Reactions

Article abstract of DOI:10.1021/jo980113p

2,2′-Diiodobiphenyl (1) reacts with arenediazonium hexafluorophosphates (ArN₂+PF₆^- ) in acetonitrile to form biphenyleneiodonium salt (4) and iodoarenes (ArI) as major products. The reaction follows a free-radical-chain mechanism and involves a cyclic diaryliodine, 9-I-2 species (3), which is trapped as the iodonium salt (4) by a single-electron transfer (SET) to the diazonium salt. The results show that diaryliodine intermediates formed by the addition of phenyl radicals to iodoarenes can have sufficient lifetimes to allow trapping bv bimolecular processes.

Full text of DOI:10.1021/jo980113p

J . Org. Chem. 1998, 63, 6141-6145
6141
Electr on Tr a n sfer fr om a Cyclic Dia r yliod in e Sp ecies to
Ar en ed ia zon iu m Sa lts. Evid en ce for th e In ter m ed ia cy of 9-I-2
Str u ctu r es in Iod in e Atom Tr a n sfer Rea ction s1
Karen B. Geahigan, Richard J . Taintor, Bindu M. George, and Denise A. Meagher
Division of Natural Sciences, Purchase College, State University of New York, Purchase, New York 10577
Thomas W. Nalli*
Chemistry Department, Winona State University, Winona, Minnesota 55987
Received J anuary 22, 1998
2
,2′-Diiodobiphenyl (1) reacts with arenediazonium hexafluorophosphates (ArN
2 6
+PF -) in acetonitrile
to form biphenyleneiodonium salt (4) and iodoarenes (ArI) as major products. The reaction follows
a free-radical-chain mechanism and involves a cyclic diaryliodine, 9-I-2 species (3), which is
trapped as the iodonium salt (4) by a single-electron transfer (SET) to the diazonium salt. The
results show that diaryliodine intermediates formed by the addition of phenyl radicals to iodoarenes
can have sufficient lifetimes to allow trapping by bimolecular processes.
In tr od u ction
a series of iodoarenes versus abstraction of chlorine from
CCl A good Hammett correlation (r ) 0.95) was
observed, consistent with both the concerted mechanism
eq 2) and with an irreversible stepwise process (eq 1, k
4
.
The field of free-radical chemistry has attained greater
significance in recent years due, in part, to its increasing
application to complex organic syntheses. Halogen atom
transfer reactions are particularly important, forming the
basis for several important synthetic methodologies, and
are interesting mechanistically because of the possibility
that they may proceed through an energy minimum
corresponding to a hypervalent 9-X-2 intermediate,
(
2
2
5
. k-1). However, Tanner and co-workers subsequently
found an unsatisfactory Hammett correlation (r ) 0.82),
suggesting that the iodine atom transfer must involve
the reversible formation of a 9-I-2 intermediate (eq 1,
2
k
2
≈ k-1). Unfortunately, the later workers’ well-
conceived attempt to generate the intermediates, mea-
sure k /k-1, and extract the relative rates of phenylarylio-
dine formation (k /kCl) from the kinetic data served
ultimately to only confuse the issue.
•
3
R-X -R′.
2
The transfer of iodine from an iodoarene to a phenyl
1
7
8
-1
-1 4-6
radical is a fast reaction (k ≈ 10 -10
M
s ).
5
b
Mechanistically, either the reaction involves the inter-
vention of a 9-I-2, diaryliodine intermediate (eq 1) or
it is a concerted process (eq 2).
7
Sahyun and co-workers detected a short-lived (<0.2
•
2
ns) absorption assigned to diphenyliodine (Ph I ) in the
laser flash photolysis of diphenyliodonium iodide, and
several groups have provided CIDNP evidence for diphe-
k1
k2
9
•
•
8
nyliodine in sensitized diphenyliodonium photolyses.
Ph + I-Ar y z [Ph- I˙ -Ar]
8 Ph-I + Ar (1)
k-1
However, the detection of a diphenyliodine intermediate
formed by phenyl radical addition to an iodoarene has
not been reported in the literature.
9
-I-2
kabs
•
•
Ph + I-Ar
8 Ph-I + Ar
(2)
9
Recently, Scaiano and co-workers found a UV absorp-
tion with a lifetime of 4.4 ( 0.3 µs in the laser flash
photolysis of 1,5-diiodo-1,5-diphenylpentane that was
attributed to a cyclic dibenzyliodine 9-I-2 structure.
However, the much longer lifetime of the dibenzyliodine
4
In an early study, Danen and Saunders measured the
relative rates (kabs/kCl) of reaction of phenyl radicals with
(
1) This work was taken, in part, from the senior theses of K.B.G.,
R.J .T., B.M.G., and D.A.M. (a) Lake, K. B. B.S. Thesis, Purchase
College, State University of New York (SUNY), 1990. (b) Taintor, R.
J . B.A. Thesis, Purchase College, SUNY, 1992. (c) George, B. M. B.A.
Thesis, Purchase College, SUNY, 1993. (d) Wenzel, D. A. B.A. Thesis,
Purchase College, SUNY, 1993. Some of our results were presented
previously: (e) Lake, K. B.; O’Donoghue, J .; Taintor, R. J .; Nalli, T.
W. Book of Abstracts, J oint 44th Southeastern-26th Middle Atlantic
Regional Meeting of the American Chemical Society, Alexandria, VA,
December 1992; American Chemical Society: Washington, DC, 1992;
Abstract 224. (f) Lake, K. B.; Taintor, R. J .; George, B. M.; Wenzel, D.
A.; O’Donoghue, J .; Nalli, T. W. Abtracts of Papers, Part 2, 212th
National Meeting of the American Chemical Society, Orlando, FL, Aug
(4) Danen, W. C.; Saunders: D. G. J . Am. Chem. Soc. 1969, 91,
5924-5925. Also see: Danen, W. C. In Methods in Free-Radical
Chemistry; Huyser, E. S., Ed.; Marcell Dekker: New York, 1974; Vol.
5, Chapter 1.
(5) (a) Tanner, D. D.; Reed, D. W.; Setiloane, B. P. J . Am. Chem.
Soc. 1982, 104, 3917-3923. (b) Tanner, D. D.; Reed, D. W.; Setiloane,
B. P. J . Am. Chem. Soc. 1983, 105, 6768.
(6) Weldon, D.; Holland, S.; Scaiano, J . C. J . Org. Chem. 1996, 61,
8544-8546.
(7) Devoe, R. J .; Sahyun, M. R. V.; Serpone, N.; Sharma, D. K. Can.
J . Chem. 1987, 65, 2342-2349.
2
5-29, 1996; American Chemical Society: Washington, DC, 1996;
(8) (a) Devoe, R. J .; Sahyun, M. R. V.; Schmidt E.; Serpone, N.;
Sharma, D. K. Can. J . Chem. 1988, 66, 319-324. (b) Wang, X. Z.;
Wang, E. J . Chin. Chem. Lett. 1994, 5, 831-834. (c) Eckert, G.; Goez,
M.; Maiwald, B.; Mueller, U. Ber. Bunsen-Ges. Phys. Chem. 1996, 100,
1191-1198. Also see: Pappas, S. P.; Gatechair, L. R.; J ilek, J . H. J .
Polym. Sci., Polym. Chem. Ed. 1984, 22, 77-84.
ORGN 366.
(2) For a recent review see; J asperse, C. P.; Curran, D. P.; Fevig, T.
L. Chem. Rev. 1991, 91, 1237-1286.
(
3) The 9-X-2 designation refers to nine formal valence electrons
about a halogen atom with two ligands. See: Perkins, C. W.; Martin,
J . C.; Arduengo, A. J .; Lau, W.; Algeria, A.; Kochi, J . K. J . Am. Chem.
Soc. 1980, 102, 7754-7759.
(9) Banks, J . T.; Garcia, H.; Miranda, M. A.; Perez-Prieto, J .;
Scaiano, J . C. J . Am. Chem. Soc. 1995, 117, 5049-5054.
S0022-3263(98)00113-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/24/1998

6
142 J . Org. Chem., Vol. 63, No. 18, 1998
Geahigan et al.
Sch em e 1
which abstract iodine from the starting diiodobiphenyl
(
(
1) to give iodobenzene and the iodoaryl radical (2)
eq 3).
•
Ph + 1 f PhI + 2
(3)
Although arenediazonium salts are known to some-
times react via phenyl cation intermediates, 4-methylac-
etanilide, ArNHCOCH , the ultimate product of reaction
3
of 4-methylphenyl cation with acetonitrile,1 was not
formed in these reactions. Thus, we rule out the possible
role of aryl cation chemistry in the formation of any of
the observed products. Instead, the formation of 4-iodo-
toluene (ArI) clearly points to the presence of 4-methyl-
1
7
structure compared to that of diphenyliodine raises the
question of the applicability of these findings to the
mechanism of iodine atom transfer between phenyl
radicals and iodoarenes.
•
phenyl radical (Ar ) intermediates in the reacting solu-
tions.12 Accordingly, the iodotoluene is formed chiefly by
the reaction of the p-tolyl radical with the diiodobiphenyl
reactant (1) (first step, Scheme 1).
Bimolecular reactions of free divalent iodine radicals
have not been identified in the literature. We thought
that an exoergic single-electron transfer from such a
radical to a suitable acceptor could be fast enough to
compete with C-I bond cleavage. Because the observa-
tion of such a reaction would provide strong support for
-I-2 intermediates in iodine atom abstractions, we set
out to generate iodoaryl radical, 2, in the presence of
arenediazonium salts as one-electron acceptors (Scheme
). We report in this paper that the 9-I-2 species, 3,
The formation of the diazene (10), the bitolyls (6 and
7), and the iodobitolyls (8 and 9) is also consistent with
the involvement of p-tolyl radicals, which react with the
diazonium salt in the manner recently identified by
Minisci and co-workers.13 Attack on the diazonium group
leads to the diazo compound (10) while attack on the ring
of the arenediazonium salt (preferentially at the 2-posi-
tion) leads, after loss of N and a proton, to bitolylyl
2
radicals (11 and 12), which abstract hydrogen (see later
discussion) to give the observed bitolyl products (6 and
9
1
does, in fact, have a sufficient lifetime to allow it to be
trapped by bimolecular electron transfer.
7
) or abstract iodine from 1 to form the iodobitolyls (8
Resu lts a n d Discu ssion
and 9).
Visible-light irradiation of an acetonitrile solution of
2
,2′-diiodobiphenyl (1) (0.05 M), p-toluenediazonium
hexafluorophosphate (0.20 M), and an initiating quantity
0.1 equiv based on 1) of the visible-light-sensitive radical
source, phenylazoisobutyronitrile (PAIBN ) PhNdNC-
CH CN), gives biphenyleneiodonium salt (4, isolated
as the iodide) and 4-iodotoluene (ArI) as major products
experiment 1, Table 1). Other products are 2-iodobi-
(
(
3 2
)
(
The presence of free aryl radicals in the reacting
phenyl (5, PhPhI), biphenyl (PhPh), toluene (ArH), io-
dobenzene (PhI), bitolyls (2,4′-dimethylbiphenyl (6) and
solutions indicates that the arenediazonium salt under-
goes a one-electron reduction to give an aryl radical and
3
,4′-dimethylbiphenyl (7)), iodobitolyls (5-iodo-2,4′-dim-
1
4
N
2
.
In addition, the production of the iodonium salt (4)
ethylbiphenyl (8) and 2-iodo-5,4′-dimethylbiphenyl) (9),
and bis(4-methylphenyl)diazene (10) (Scheme 2).
Mass balances on biphenyl-containing compounds (sum
implicates the diaryliodine radical (3) as a one-electron
reductant. Accordingly, we propose a single-electron
transfer from the 9-I-2 structure (3) to the arenedia-
zonium salt (third step, Scheme 1) as a key step in the
of yields of 1, 4, 5, and PhPh) for experiments 1-3 of
7
5-80% show that most of the starting material (1) was
1
5
mechanism of the reactions summarized in Table 1. The
accounted for by our analyses. Only trace amounts of
-fluorotoluene were formed in the product mixtures
Table 1), showing that thermal decomposition of the
9
2
-I-2 intermediate (3) is formed by cyclization of the
-iodobiphenyl-2′-yl radical (2) (second step, Scheme 1).
The sequence amounts to a free-radical-chain reaction
4
(
diazonium hexafluorophosphate was minimal under our
reaction conditions.
When the radical source, PAIBN, was not included in
the reaction solution, the reaction was considerably
slower (ca. 8 times slower based on the yield of 4-iodo-
toluene; compare experiments 3 and 5, Table 1). This
observation, along with the formation of trace amounts
of iodobenzene in the PAIBN-containing experiments,
shows that the function of the azo compound is simply
to initiate a free-radical-chain reaction. It does so
through its photochemical conversion to phenyl radicals,10
(11) Dektar, J . L.; Hacker, N. P. J . Org. Chem. 1990, 55, 639-647.
(
12) Wassmundt, F. W.; Kiesman, W. F. J . Org. Chem. 1997, 62,
8
304-8308.
(13) Minisci, F.; Coppa, F.; Fontana, F.; Pianese, G.; Zhao, L. J . Org.
Chem. 1992, 57, 3929-3933.
(
14) The initially formed diazenyl radical (ArN
2
•) quickly fragments
to give the aryl radical and N2. For a review of single-electron transfer
to arenediazonium salts to give aryl radicals, see; Galli, C. Chem. Rev.
1
988, 88, 765-792.
15) The proposed electron transfer is substantially exoergic. This
is based on half-wave reduction potentials (vs SCE) reported in the
(
7
literature of E1/2 ) -0.7 V for diphenyliodonium salts ) and E1/2
)
1
6
+
0.25 V for p-toluenediazonium salts corresponding to ∆G° ) -22
kcal/mol.
(16) Elofson, R. M.; Gadallah, F. F. J . Org. Chem. 1969, 34, 854-
857.
(10) (a) Ford, M. C.; Rust, R. A. J . Chem. Soc. 1958, 1297-1298. (b)
Bleha, M.; Lim, D. J . Polym. Sci., Part C 1968, 23, 15-20.

Electron Transfer from Diaryliodine to ArN +
PF
-
J . Org. Chem., Vol. 63, No. 18, 1998 6143
2
6
Ta ble 1. Rea ction s of 2,2′-Diiod obip h en yl (1) w ith p-Tolu en ed ia zon iu m Hexa flu or op h osp h a te (Ar N2+)a
GC yieldsb
expt
[1], M
[ArN2+], M
[PAIBN], mM
solvent
ArI
PhI
5
PhPh
ArH
7
ArF
1c
14
76
55
7.0
66
4d
1
2
3
4
5
0.05
0.05
0.10
0.05
0.10
0.20
0.05
0.10
0.20
0.10
6
6
6
6
0
CH3CN
CH3CN
CH3CN
CD3CN
CH3CN
94
6.7
40
98
5.3
0.60
0.27
0.53
0.54
nd
9.6
19
nd
6.0
5.0f
nd
21
6.0
nd
0.50
5.1f
nd
na
42
0
6
na
0
0.95
9.7
1.6
8.6
8.8f
1.1
0.07
0.14
0.26
0.15
2.8f
0.48
e
a
Reaction solutions contained the photoinitiator, phenylazoisobutyronitrile (PAIBN), and were irradiated with a 150-W flood lamp at
b
c
approximately 15 °C for 16.5 h. Percent yields based on 1; “nd” means not detected, and “na” means not analyzed. Unchanged starting
material. Percent yield of iodonium salt, 4, isolated as the iodide. e A different workup procedure was used and was unsuccessful. These
d
f
products showed significant deuterium incorporation by GC/MS. See Table 2.
Sch em e 2
of diiodobiphenyl (1) with ArN +
(
Ta ble 2. Deu ter iu m Con ten t of P r od u cts of Rea ction of
2
to form iodonium salt
1
w ith p-Tolu en ed ia zon iu m Sa lt in CD3CN (exp t 4)
4), iodoarene (ArI), and N
2
(eq 4).17
product
ArI
PhI
%da
product
%da
+
-2.2 ( 5.5
-0.5 ( 2.8
3,4′-ArAr (7)
3,4′-ArArI (9)
ArNdNAr (10)
25.4 ( 2.7
-0.3 ( 1.8
4.6 ( 3.4
7.6 ( 9.0
1
+ ArN2 f 4 + ArI + N2
(4)
PhPhI (5) 18.1 ( 1.5
The formation of toluene (ArH) and 2-iodobiphenyl (5),
PhPh
ArH
38.8 ( 0.5 (32.7% ArF
the respective products of hydrogen donation to p-tolyl
d1, 3.0% d2)
41.7 ( 0.9 (39.5%
d1, 1.1% d2)
•
radical (Ar ) and radical 2 (eqs 5 and 6), highlights a
competitive pathway available to intermediate free radi-
cals in the reaction solution. The use of acetonitrile-d
as solvent (experiment 4, Table 1) led to significantly
decreased yields of these reduced products (compare
experiment 4 to experiment 1, Table 1). Thus, acetoni-
trile is an important source of hydrogen in these reactions
a
Deuterium incorporation calculated from relative abundances
in molecular ion clusters of mass spectra averaged over the
duration of GC peaks. A variety of calculation methods was used,
and the averages of those most appropriate for each compound
are given in the table. Errors given are 1 standard deviation of
the mean.
3
with eqs 5 and 6 being slower for CD
primary deuterium isotope effect.
3
CN because of the
The mass spectra of the reduction products in CD
revealed significant deuterium incorporations of 42% for
toluene and 18% for 5 (Table 2). Assuming k /k
leads to the conclusion that ca. 80% of the toluene formed
in the nondeuteriated solvent (experiment 1) resulted
from the reaction of tolyl radicals with the solvent.
Therefore, it can generally be concluded that the bulk of
hydrogen donation observed in the nondeuteriated sol-
vent was from the solvent molecules. In addition, the
3
CN
•
•
Ar + Don-H f ArH + Don
(5)
(6)
H
D
≈ 5
•
2
+ Don-H f 5 + Don
(17) We also subjected 2,2′-diiododiphenylmethane (13) to reaction
conditions which led to the transformation of 1 to 4 (i.e. same conditions
as experiment 1, Table 1) but, interestingly, found no evidence for the
formation of the six-member ring iodonium salt analogous to 4. This
may be due to a failure of the 2-iododiphenylmethane-2′-yl radical to
cyclize or to a failure of the electron transfer from the cyclized
diaryliodine to the diazonium salt. The first possibility seems more
likely because abstraction of the doubly benzylic hydrogens of the
diiodide (13) is an additional competitive pathway available to the
uncyclized iodoaryl radical in this system. Another factor may be that
2
5% deuterium incorporation observed for the 3,4′-biaryl
product (7) in experiment 4 demonstrates its formation
from the 5,4′-dimethylbiphenyl-2-yl radical (12), as dis-
cussed previously.
The rate constant for the reaction of phenyl radicals
with acetonitrile has been reported in the literature,
4
is more stable than the corresponding six-member ring iodonium
salt because of the potential participation of the iodine atom in the
delocalized bonding of a 14-electron aromatic ring system. We thank
a reviewer for pointing out the potential aromaticity of 4.
5
-1
-1 18
k
MeCN ) 1.0 × 10
M
s
.
We were able to use a
competitive method to measure the rate of reaction of

6
144 J . Org. Chem., Vol. 63, No. 18, 1998
Geahigan et al.
+
a
Ta ble 3. Rea ction s of 2,2′-Diiod obip h en yl (1) w ith p-Nitr oben zen ed ia zon iu m Hexa flu or op h osp h a te (Ar ′N2 ).
GC yieldsb
+
PAIBNc
1c
4d
expt
[1], M
[Ar′N2 ], M
[PAIBN], mM
conditions
Ar′I
PhI
5
PhPh
Ar′H
e
6
7
8
9
0.05
0.05
0.05
0.05
0.10
0.10
0.05
0.05
6
0
0
0
vis, 17 h
74
62
40
nd
0.1
nd
nd
nd
2.6
2.4
3.8
nd
0.9
0.4
0.7
nd
28
27
15
nd
1.7
nd
nd
nd
21
21
43
90
100
15
e
vis, 17 h
f
hν(UV), 2 h
dark, 48 h
107
na
a
Acetonitrile solutions. b Percent yields based on 1; “nd” means not detected, and “na” means not analyzed. Unchanged starting material.
c
d
f
e
Percent yield of iodonium salt, 4, isolated as the iodide. Reaction mixture was irradiated with a 500-W halogen flood lamp at 27 °C.
Irradiation with a 450-W Hg arc.
phenyl radicals with 2,2′-diiodobiphenyl (1), kabs ) 1.25
zenediazonium salt than to p-toluenediazonium salt.22
Presumably, radicals 2 and 3 rapidly interconvert, and
the relative yields of reduced biphenyls and the 9-I-2
trapping product (4) are, therefore, simply determined
by the relative rates of hydrogen abstraction by 2 and
electron transfer from 3 to diazonium salt (Curtin-
Hammett Principle).
8
-1
-1 19
×
10
M
s
.
Assuming tolyl radicals undergo a
simple competition between iodine abstraction from 1 and
3
hydrogen abstraction from CH CN (and using [CH -
3
CN]neat ) 19 M and [1] ) 0.05 M), one could predict a
product ratio of [ArI]:[ArH] ≈ 3.3 for experiments 1 and
2
0
2
.
The observed product ratios, experiment 1 [ArI]:
[
ArH] ) 4.5 and experiment 2 [ArI]:[ArH] ) 4.1 agree
well with the predicted value, lending further support
Con clu sion
for the role of p-tolyl radicals in these reactions and for
the role of CH
As the reaction proceeds and the concentration of the
side product, 2-iodobiphenyl (5), builds up, it begins to
3
CN as the primary hydrogen donor.
The observations reported in this paper are consistent
with the central role of a redox radical chain reaction of
diiodobiphenyl (1) with arenediazonium salts as repre-
sented in Scheme 1. By far, the most important feature
of this chemistry is the ability of the cyclic diaryliodine
species (3) to act as a single-electron reductant of the
diazonium salts. This conclusion means that diaryliodine
intermediates formed by the addition of phenyl radicals
to iodoarenes can have sufficient lifetimes to be trapped
by bimolecular processes. Hence, our work provides
support for the role of diaryliodine intermediates in iodine
atom transfer reactions.
2
1
serve as an iodine donor with intermediate radicals (i.e.
•
•
Ar (eq 7), Ph , 11, 12) forming the biphenyl-2-yl radical
•
•
Ar + 5a f ArI + Ph-Ph
(7)
(8)
•
•
Ph-Ph + CH CN f PhPh + CH CN
3
2
•
(
Ph-Ph ), which abstracts hydrogen (eq 8) to form
biphenyl. Accordingly, the biphenyl product formed
when CD CN was the solvent (experiment 4, Table 1) was
3% d and 3% d (Table 2), exactly the statistical
3
Exp er im en ta l Section
3
1
2
distribution expected for sequential deuteriation steps
proceeding with 18% efficiency (the percent deuteriation
of 2-iodobiphenyl (5)).
1H NMR spectral data are reported in parts per million
relative to tetramethylsilane as an internal standard. Gas
chromatography was carried out with helium as the carrier
gas and the column temperature ramped from 60 to 300 °C at
a rate of 10 °C/min. For quantitation of reaction solutions,
flame ionization detection and split injection mode were used.
The column was a standard nonpolar (HP-1 or DB-1) capillary
column, and peak areas were corrected using response factors
generated from mixtures of pure compounds. GC-MS analysis
used an HP Model 5890/5971 system, splitless injection mode,
and a 25-m × 0.32-mm HP-5 column. Melting points are
uncorrected.
We finally note that p-nitrobenzenediazonium hexa-
+
fluorophosphate (Ar′N
2
) undergoes a radical-chain reac-
tion with 1 in a fashion similar to the methyl-substituted
diazonium salt whose reactions have been the main focus
of this paper (Table 3). Moreover, the nitrodiazonium
salt gives a much cleaner 9-I-2 trapping reaction, with
high yields of cyclic iodonium salt (4) and only trace
amounts of the reduction products, biphenyl and 2-io-
dobiphenyl (5) (experiments 6 and 7, Table 3). In this
case the presence of the photoinitiator (PAIBN) does not
affect the product yields (compare experiment 6 to
experiment 7, Table 3), presumably because of the ability
of the diazonium salt to absorb visible light. The low
yield of reduced products might reflect a faster electron
transfer from the 9-I-2 structure (3) to the p-nitroben-
All compounds not described below were obtained from
commercial sources and not purified. p-Toluenediazonium
23
10
hexafluorophosphate and phenylazoisobutyronitrile (PAIBN)
were prepared by literature procedures.
2,2′-Diiodobiphenyl (1) and 2,2′-diiododiphenylmethane (13)
were prepared by thermolysis of the corresponding cyclic
iodonium iodides24 and were purified to >99.9% GC purity by
recrystallization from ethanol.
Com p ou n d 1: Mp 106.3-106.5 °C (lit. mp 109-110 °C).25
1
H NMR (60 MHz, CCl
406, 279, 152, 127.
4
): δ 7.9 (d, 2H), 7.2 (m, 6H). MS: m/z
(
18) Scaiano, J . C.; Stewart, L. C. J . Am. Chem. Soc. 1983, 105,
609-3614.
19) This was accomplished by allowing phenyl radicals from the
visible-light photolysis of PAIBN to react with 1 in CCl solution and
3
(
4
(22) This is not surprising since the electron transfer to p-nitroben-
zenediazonium salt is 5 kcal/mol more exoergic than electron transfer
to p-toluenediazonium salt. This is based on the half-wave reduction
potential (vs SCE) reported in the literature of +0.45 V for p-
measuring the product ratio, [PhI]:[PhCl]. The relative rate constant
that we measured, kabs/kCl ) 59 at 20 °C, together with the value for
6
6
-1
k
s
Cl recently reported by Scaiano and co-workers (kCl ) 2.3 × 10
M
-
1
8
-1 -1
16
), gives kabs ) 1.25 × 10
M
s
.
nitrobenzenediazonium salts corresponding to ∆G° ≈ -27 kcal/mol.
(
20) We also assumed that rate constants for tolyl radical reactions
(See ref 15.)
are essentially the same as those for the corresponding phenyl radical
reactions; see: Pryor, W. A.; Echols, J . T., J r.; Smith, K. J . Am. Chem.
Soc. 1966, 88, 1189-1199.
(23) Rutherford, K.; Redmond, W.; Rigamonti, J . J . Org. Chem. 1961,
26, 5149-5152.
(24) Sato, T.; Shimizu, K.; Moriya, H. J . Chem. Soc., Perkin Trans.
2 1974, 1537-1539.
(
21) We also measured kabs/kCl for this iodoarene. The value obtained,
7
-1 -1
k
abs/kCl ) 41, corresponds to kabs ) 8.0 × 10
M
s
at 20 °C.
(25) Sandin, R. B. J . Org. Chem. 1969, 34, 456-457.

Electron Transfer from Diaryliodine to ArN +
PF
-
J . Org. Chem., Vol. 63, No. 18, 1998 6145
2
6
Com p ou n d 13: mp 79-81 °C (lit. mp 79-80 °C).26 1H NMR
Gen er a l Exp er im en ta l P r oced u r es. Dry Pyrex tubes
sealed with rubber septa were used as reaction vessels.
Reaction tubes were placed inside a Pyrex circulating-water
cooling jacket and placed 1.5 cm from a 150-W incandescent
flood lamp (Table 1) or 6 cm from a 500-W halogen lamp (Table
3). After irradiation, solutions were poured into water and
extracted with hexane. Octadecane internal standard was
added to the extracts, which were then dried and GC analyzed.
The aqueous layers were treated with KI(aq) or NaI(aq) in
order to precipitate the product iodonium salt (4) as the iodide.
The solid was washed thoroughly with acetone and air-dried:
(
3
60 MHz, CDCl ): δ 8.0 (d, 2H), 7.4 (m, 6H), 4.2 (s, 2H). MS:
m/z 420, 293, 166, 165.
The iodonium iodide precursors to 1 and 13 were prepared
from 2-iodobiphenyl (5) and 2-iododiphenylmethane (14) re-
2
7
spectively using a literature procedure. Iodoarenes 5 and
4 were prepared by diazotization of the corresponding ami-
1
noarenes followed by KI treatment. Hexane solutions of the
crude iodoarenes were washed with 3 M HCl(aq) and 6 M
NaOH(aq), the solvent was removed, and the residue was
vacuum distilled.
mp 208 °C (dec.) (lit. mp 210 °C (dec.),24 215 °C (dec.) ).
30
1
H
Com p ou n d 5: Bp 110 °C (0.25 Torr) (lit. bp 125-126 °C (1
2
8
29 1
Torr), 108-115 °C (0.8 Torr)).
H NMR (60 MHz, CH
2
Cl
2
):
NMR (270 MHz, Me SO-d ): δ 8.47 (d, 2H, J ) 9 Hz), 8.30 (d,
2
6
δ 8.4 (d, 1H, J ) 7 Hz), 7.0-8.0 (m, 8H). MS: m/z 280, 153,
52, 127.
2H, J ) 8.5 Hz), 7.85 (t, 2H, J ) 8 Hz), 7.70 (t, 2H, J ) 8 Hz).
The thermal decomposition of the solid gave 2,2′-diiodobiphe-
nyl as the only product detectable by GC-MS.24
1
Com p ou n d 14: bp 94-114 °C (0.1 Torr) (lit. bp 182 °C (11
2
7
1
Torr)).
3
H NMR (60 MHz, CDCl ): δ 7.8 (dd, 1H, J ) 8 Hz,
Ack n ow led gm en t. We thank Mr. J ohn O’Donoghue
for the preparation of PAIBN. We also thank Dr. J ack
A. Kampmeier for helpful discussions and encourage-
ment. This research was supported, in part, by Ciba-
Geigy Undergraduate Research Awards to K.B.G.,
R.J .T., B.M.G., and D.A.M.
J ′ ≈ 1 Hz), 6.8-7.6 (m, 8H), 4.3 (s, 2H). MS: m/z 294, 167,
1
65, 152, 127.
(
26) Sato, T.; Uno K. J . Chem. Soc., Perkin Trans. 1 1973, 895-
00.
27) Collette, J .; McGreer, D.; Crawford, R.; Chubb, F.; Sandin, R.
B. J . Am. Chem. Soc. 1956, 78, 3819-3820.
28) Parrack, J . D.; Swan, G. A.; Wright, D. J . Chem. Soc. 1962,
11.
9
(
(
J O980113P
9
(
29) Stillings, M. R.; Welbourn, A. P.; Walter, D. S. J . Med. Chem.
(30) Koser, G. F.; Wettach, R. H.; Smith, C. S. J . Org. Chem. 1980,
45, 1544-1546.
1
986, 29, 2280-2284.