Radical cation ester cleavage in solution. Mechanism of the mesolytic O-CO bond scission

Article abstract of DOI:10.1021/jo0057093

A wide range of enol carbonate, carbamate, and ester radical cations is characterized in solution by cyclic voltammetry and EPR spectroscopy. Preparative transformations using one-electron oxidants or anodic oxidation yield benzofurans after O-CO bond cleavage. Mechanistic investigations and direct detection of radical intermediates reveal that all enol radical cations undergo exclusively O-CO bond cleavage to provide α-carbonyl cations and acyl (or alternatively, alkoxyacyl and aminoacyl) radicals, respectively. The kinetics of the mesolytic fragmentation and the influence of nucleophilic additives have been determined using fast-scan cyclic voltammetry.

Full text of DOI:10.1021/jo0057093

J . Org. Chem. 2001, 66, 3265-3276
3265
Ra d ica l Ca tion Ester Clea va ge in Solu tion . Mech a n ism of th e
Mesolytic O-CO Bon d Scission †
Michael Schmittel,*,‡ Karl Peters,§ Eva-Maria Peters,§ Andreas Haeuseler,‡ and
Holger Trenkle|
FB 8 - OC1 (Chemie-Biologie) der Universita¨t-GH Siegen, Adolf-Reichwein-Strasse, D-57068 Siegen,
Germany, Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstr. 1, D-70506 Stuttgart, Germany,
and Institut fu¨r Organische Chemie der Universita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany
schmittel@chemie.uni-siegen.de
Received October 31, 2000
A wide range of enol carbonate, carbamate, and ester radical cations is characterized in solution
by cyclic voltammetry and EPR spectroscopy. Preparative transformations using one-electron
oxidants or anodic oxidation yield benzofurans after O-CO bond cleavage. Mechanistic investiga-
tions and direct detection of radical intermediates reveal that all enol radical cations undergo
exclusively O-CO bond cleavage to provide R-carbonyl cations and acyl (or alternatively, alkoxyacyl
and aminoacyl) radicals, respectively. The kinetics of the mesolytic fragmentation and the influence
of nucleophilic additives have been determined using fast-scan cyclic voltammetry.
In tr od u ction
a HOMO with high n character. Hence, we have to
construct a system whose π (or n) HOMO shows some
sizable overlap with the σ (O-C) orbital. While such
systems may undergo efficient bond cleavage when the
Homolytic and heterolytic bond dissociation processes
are rather well understood and have contributed in a
fundamental way to our present understanding of organic
chemistry. In contrast, much less is known about the
thermochemistry and in particular the kinetics of bond
cleavage processes of radical ions, although bond cleavage
in odd-electron species1 has become very important in
synthetically useful reactions.2
Radical cations of almost any organic substrate can be
conveniently prepared through chemical, anodic, or pho-
tochemical one-electron oxidation, and hence, it would
be desirable to understand the thermodynamic and
kinetic aspects of their bond cleavage pathways in
solution. While at present there is extensive data avail-
able on C-H,3 C-C,4 and C-Si4l,5 bond cleavage, other
processes have hardly been investigated, e.g., C-O6 and
C-N7 bond scissions and equally those involving two
heteroatoms, such as O-Si (e.g., enol and phenol silyl
ethers),8 O-Sn,9 O-Al,10 O-P,11 O-Ti,12 and O-Zr.13
Because of the importance of ester cleavage in organic
chemistry occurring along the traditional heterolytic
pathways, we have recently started to investigate the
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O-C(dO) bond cleavage in ester radical cations,6
a
process basically unknown in solution14 and gas-phase
oxidations.15 But how can one devise a system that
undergoes clean O-C(dO) scission? It is obvious that
oxidation of a pure σ (O-C) donor will lead to rather
efficient bond weakening, but simple esters usually have
† This paper is dedicated to Prof. S. Hu¨nig on the occasion of his
80th birthday.
‡ Universita¨t-GH Siegen.
§ Max-Planck-Institut fu¨r Festko¨rperforschung.
| Universita¨t Wu¨rzburg.
(1) Schmittel, M.; Ghorai, M. K. Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 2, Chapter 2.1.1.
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10.1021/jo0057093 CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/21/2001

3266 J . Org. Chem., Vol. 66, No. 10, 2001
Schmittel et al.
mesolytic16 bond strength is low, they will get involved
in reactions typical of π (or n) radical cations once the
mesolytic O-C(dO) bond strength is high. To avoid the
latter processes, we have decided to study radical cations
derived from 2,2-dimesitylenol esters, as the steric shield-
ing of the two bulky mesityl groups effectively prevents
disturbing side reactions, such as deprotonation in
â-position and nucleophilic attack at the unsaturated
system. This approach was successfully used to monitor
slow mesolytic processes, as in O-P11 and O-Ti12 bond
cleavage. Moreover, this approach worked for enol ac-
etate6 and enol trifluoroacetate17 radical cations providing
the first indication of a clean ester cleavage. In this paper,
we now wish to present a comprehensive study18 on the
mesolytic O-CO bond scission in various radical cations
including electron-poor as well as electron-rich esters,
carbonates, and carbamates establishing such a process
as a general reaction pathway as opposed to the acid and
base-catalyzed ester cleavage. In addition, we have
examined two bisenol ester derivatives to evaluate the
concept for multibond cleavage processes.
Ta ble 1
R1
R2
yield (%)
R1
R2
yield (%)
1a But NH-But
1b But O-But
1c But O-CH2Ph
1d But p-An
38
25
11
63
2a Ph NH-But
2b Ph O-But
69
73
1e But p-PhN(CH3)2
1f But Fc
43
31
1g But CH3
626
7717
2g Ph CH3
2h Ph CF3
776
1h But CF3
8017
drides, or acyl chlorides. In most cases, yields were not
too high because of the steric shielding about the OH
group. The steric hindrance in the model compounds is
Resu lts
1
apparent from the H NMR spectra that exhibit signifi-
Syn t h esis a n d Ch a r a ct er iza t ion of t h e Mod el
Com p ou n d s. To properly measure the kinetics of O-CO
mesolytic cleavage, we synthesized ester systems derived
from 2,2-dimesitylenols, for reasons outlined above.8b,19a
For comparison, we have included data for 1g,h and
2g,h .6,17 All systems offer decisive advantages from a
practical point of view. They are oxidized at rather low
potentials because of the electron-rich dimesitylenol
moiety, and the products formed after O-C bond cleavage
are well-known.
cant peak broadening because of coalescence. Such a
behavior has been observed in many related systems
caused by a two- or three blade propeller conformation
in which rotation of the mesityl groups is hindered.19b,20
In the bisenol carbonate 3a , the rotation is completely
frozen on the time scale of the NMR experiment, and
thus, a double set of signals is obtained for the o-mes-
CH3 and the m-mes-H protons.
The X-ray analysis of 1a (P21/c, a ) 806.6(1) pm, b )
2297.1(2) pm, c ) 1390.9(1) pm, â ) 92.630°) and 1b
(Fdd2, a ) 3483.4(2) pm, b ) 3660.7(2) pm, c ) 835.0(1)
pm) uncovers the steric bulk caused mainly by the
mesityl groups (Figure 1). Both mesityl rings are severely
twisted out of the CdC-O plane by dihedral angles æ1
) 59.5-62.5° (cis to OCOR) and æ2 ) 63.5-65.0°. This
leads to a very efficient shielding of the â-carbons of the
enol moiety thereby preventing any attack on the stage
of the radical cation at this position. The situation for
1g21 and 1h 22 (see Table 2) and for the corresponding
simple enol (æ1 ) 66.0, æ2 ) 63.7)23 is very similar. The
CdC and C-O bond lengths are within the expected
magnitude.21-23 Interestingly, the dihedral angles for Cd
C-O-C are about 70° for 1a ,b and about 77° for 1g,h ,
thus allowing for convenient overlap of the scissile O-CO
bond with the enol π system.
The enol ester derivatives 1a -f, 2a ,b, and 3a ,b (Table
1) could be easily prepared by reaction of the correspond-
ing lithium or sodium enolates with suitable acylation
reagents, such as the corresponding isocyanates, anhy-
(6) Schmittel, M.; Heinze, J .; Trenkle, H. J . Org. Chem. 1995, 60,
2726.
(7) Adam, W.; Kammel, T.; Toubartz, M.; Steenken, S. J . Am. Chem.
Soc. 1997, 119, 10673.
(8) a) Schmittel, M.; Keller, M.; Burghart, A. J . Chem. Soc., Perkin
Trans 2 1995, 2327. (b) Bockman, T. M.; Kochi, J . K. J . Chem. Soc.
Perlin Trans. 2 1996, 1633. (c) Schmittel, M.; Burghart, A.; Werner,
H.; Laubender, M.; So¨llner, R. J . Org. Chem. 1999, 64, 3077.
(9) Kohno, Y.; Narasaka, K. Bull. Chem. Soc. J pn. 1995, 68, 322.
(10) Sartori, G.; Maggi, R.; Bigi, F.; Arienti, G.; Casnati, G.; Bocelli,
G.; Mori, G. Tetrahedron 1992, 48, 9483.
(11) Schmittel, M.; Steffen, J .-P.; Burghart, A. Chem. Commun.
1996, 2349; Schmittel, M.; Steffen, J .-P.; Burghart, A. Acta Chem.
Scand. 1999, 53, 781.
On e-Electr on Oxid a tion . Preparative one-electron
oxidation of the carbamates 1a ,2a , carbonates 1b,2b, and
enol esters 1d -e by tris(1,10-phenanthroline)iron(III)-
hexafluorophosphate ([Fe(phen)3]3+, E1/2 ) 0.76 VFc24) or
thianthrenium perchlorate (TPC, E1/2 ) 0.95 VFc24) in
acetonitrile provided the benzofuran B1 or B2 as the
exclusive product (Scheme 1, Table 3).25 Only in the case
of enol ester 1f, containing a ferrocene unit, conversion
(12) (a) Schmittel, M.; So¨llner, R. Angew. Chem., Int. Ed. Engl. 1996,
35, 2107. (b) Schmittel, M.; So¨llner, R. Chem. Ber./ Recueil 1997, 130,
771.
(13) Schmittel, M.; So¨llner, R. Chem. Commun. 1998, 565. Schmittel,
M.; So¨llner, R. J . Chem. Soc., Perkin Trans. 2 1999, 515.
(14) Ogibin, Y. N.; Terent’ev, A. O.; Ilovaisky, A. I.; Nikishin, G. I.
Mendeleev Commun. 1998, 239. Fry, A. J .; Little, R. D.; Leonetti, J . J .
Org. Chem. 1994, 59, 5017. Peek, R.; Streukens, M.; Thomas, H. G.;
Vanderfuhr, A.; Wellen, U. Chem. Ber. 1994, 127, 1257.
(15) Grossert, J . S.; Yhard, G. B.; Pincock, J . A.; Curtis, J . M. J .
Mass Spectrosc. 1996, 31, 761. Suh D.; Burgers, P. C., Terlouw, J . K.
Rapid Commun. Mass Spectrosc. 1995, 9, 862.
(16) The expression mesolytic has been coined by Maslak and will
be used throughout this paper: Maslak, P.; Narvaez, J . N. Angew.
Chem., Int. Ed. Engl. 1990, 29, 283.
(17) Schmittel, M.; Trenkle, H. J . Chem. Soc., Perkin Trans 2 1996,
2401.
(20) Nadler, E. B.; Rappoport, Z. J . Am. Chem. Soc. 1987, 109, 2122.
(21) Peters, K.; Peters, E.-M.; Schmittel M.; Trenkle H. Z. Kristal-
logr. 1998, 213, 105.
(22) Peters, K.; Peters, E.-M.; Schmittel, M.; Trenkle, H. Z. Kristal-
logr. 1997, 212, 467.
(18) Some preliminary results have been published: Schmittel, M.;
Trenkle, H. Chem. Lett. 1997, 299.
(19) (a) Schmittel, M.; Ro¨ck, M. Chem. Ber. 1992, 125, 1611. (b)
Schmittel, M.; Keller, M.; Burghart, A.; Rappoport, Z.; Langels, A.
Perkin Trans. 2 1998, 869.
(23) Kaftory, M.; Nugiel, D. A.; Biali, S. E.; Rappoport, Z. J . Am.
Chem. Soc. 1989, 111, 8181.
(24) All potentials are referred to the ferrocene/ferrocenium (Fc)
couple unless otherwise noted. To obtain values vs SCE, simply add
+0.39 V.

Radical Cation Ester Cleavage in Solution
J . Org. Chem., Vol. 66, No. 10, 2001 3267
F igu r e 1. X-ray structure of 1a (left) and 1b (right).
Ta ble 2. Selected Bon d Len gth s, Bon d An gles, a n d
Dih ed r a l An gles of Som e Mod el Com p ou n d s
Ta ble 3. P r ep a r a tive On e-Electr on Oxid a tion
oxidant
oxidant (equiv) benzofurana mass balanceb
1a
1b
1ga
1h b
1a [Fe(phen)3]3+
1a [Fe(phen)3]3+
2a [Fe(phen)3]3+
2a [Fe(phen)3]3+
1b [Fe(phen)3]3+
1b [Fe(phen)3]3+
1b TPC
0.46
0.92
0.44
0.90
0.39
0.73
0.51
1.10
0.43
0.87
0.48
0.97
0.50
1.00
1.96
53
55
45
41
43
47
42
33
44
47
57
55
84
69
92
86
71
73
69
66
80
98
75
95
82
73
77
O-CO (Å)
1.376
1.323
1.421
1.364
1.343
1.428
1.345
1.329
1.451
1.296
1.331
1.443
CdC (Å)
CdC-O (Å)
C-O-C (deg)
CdC-O (deg)
O-CdO (deg)
CdC-O-Cc (deg)
Mescis-CdCd (deg)
Mestrans-CdCe (deg)
116.8
116.6
117.3
119.9
116.5
123.4
70.6
62.3
65.0
116.1
125.8
69.5
59.5
63.5
116.4
124.0
77.8
63.7
60.5
115.6
129.9
76.3
63.0
63.0
1b TPC
2b [Fe(phen)3]3+
2b [Fe(phen)3]3+
1d [Fe(phen)3]3+
1d [Fe(phen)3]3+
1e [Fe(phen)3]3+
1e [Fe(phen)3]3+
1e [Fe(phen)3]3+
a
b
Taken from ref 21. Taken from ref 22. c Dihedral angle
d
between the O-CO bond and the enol system. Dihedral angle
of the enol system and the cis-mesityl moiety. e Dihedral angle of
the enol system and the trans-mesityl moiety.
22
38
a
Based on oxidation equivalents. b Based on starting enol ester.
Sch em e 1. P r od u cts of th e On e-Electr on
Oxid a tion (Mesityl ) 2,4,6-Tr im eth ylp h en yl)
methane or bromotrichloromethane as radical trapping
agents had no sizable effect on the yields. The bisenol
carbonate 3a also provided benzofuran B1 upon anodic
oxidation (62% based on converted 3a and considering
two enol units in the oxidative transformation), while in
the reaction with [Fe(phen)3]3+ or Tl(CF3COO)3, only the
unreacted substrate was recovered. The low solubility of
1c and 3b prevented preparative one-electron oxidation
studies.
Cyclic Volta m m etr y. The oxidation potentials of the
carbamates and carbonates have been determined by
cyclic voltammetry, providing irreversible oxidation waves
at normal scan rates (20-500 mV s-1), but reversible
waves for all monoenol systems at high scan rates (e5000
V s-1) in acetonitrile or dichloromethane, allowing for the
first time to determine the thermodynamic half-wave
potentials of enol carbamates and carbonates (Table 4).
The electron-rich enol esters 1e and 1f exhibit partially
reversible oxidation waves even at low scan rates (>20
mV s-1). Their oxidation potentials are much lower than
for systems as 1d or 1g. All oxidation potentials are
within an interesting range where chemical one-electron
oxidation, photoinduced electron transfer as well as
to B1 was not detected and the unreacted starting
material was recovered. Aside from B1,2 no other prod-
ucts, e.g., derived from a nucleophilic attack in the
â-position of the enol fragment or from direct cyclization
reactions of the radical cation, as demonstrated earlier
for the corresponding enol trifluoroacetate with R1 ) Ph
(2h ),17 could be detected. The addition of tetrabromo-
(25) (a) Ro¨ck, M.; Schmittel, M. J . Prakt. Chem. 1994, 336, 325. (b)
Schmittel, M.; Baumann, U. Angew. Chem., Int. Ed. Engl. 1990, 29,
541.

3268 J . Org. Chem., Vol. 66, No. 10, 2001
Schmittel et al.
Ta ble 4. Ha lf-Wa ve P oten tia lsa of Dim esitylen ol
Der iva tives
a
b
a
b
Epa
system (VFc
E1/2
∆Ep
Epa
E1/2
∆Ep
)
(VFc
)
(mV) system (VFc)
(VFcV) (mV)
1a
1b
1c
1d
1e
1f
1g
1h
3a
3b
1.02
1.21
1.32
1.13
1.00
1.21
1.31
1.21
125
125
175
125
68
62
100
85
2a
2b
0.99
1.06
0.99
1.06
170
190
0.67c
0.35c
1.16d 1.16d
2g
2h
1.04d
1.26e
1.08d
155
190
1.51e
1.24
1.31
a
Measured at 100 mV s-1 in CH3CN/0.1 M Bu4NPF6 and
referenced to the ferrocene/ferrocenium couple; accuracy (0.01 V.
b
In dichloromethane at scan rates v ) 500-5000 V s-1; accuracy
(0.02 V. E1/2 (100 mV s-1 in CH3CN/0.1 M Bu4NPF6); accuracy
c
( 0.01 V. Taken from ref 6. e Taken from ref 17.
d
F igu r e 2. Multisweep cyclic voltammogram of 1a in aceto-
nitrile: left, experiment; right, digital simulation (details see
Supporting Information).
anodic oxidation can be realized in the presence of many
other functional groups.
The oxidation potentials provide a clear trend depend-
ent on the electron-withdrawing nature of R2 in the enol
derivative Mes2CdC(R1)OC(O)R2. Interestingly, in series
2a -h (R1 ) Ph) the influence of R2 on the oxidation
potentials is much lower than in series 1a -d and 1g,h
(R1 ) But). While in series 1a -d and 1g,h the electro-
phore is depicted by the enolate Mes2CdC-OC(O)R2,
apparently there is a gradual change in 2a -h from the
Mes2CdC-OC(O)R2 to the Mes2CdC-Ph electrophore
with increasing electron-withdrawing influence of R2.
Accordingly, in the electron-rich enol carbamate 2a the
electrophore is best represented by the Mes2CdC-O
moiety, whereas in trifluoroacetate 2h it rather resides
on the Mes2CdC-Ph substructure. A different situation
is met in 1e,f since groups R2 (p-C6H4NMe2, ferrocenyl)
are the best donor sites.
In more detailed cyclic voltammetric investigations,
enol carbamate 1a as well as carbonates 1b, 2b, and 1c,
p-methoxybenzoate 1d as well as both bisenol derivatives
showed intriguing curve crossings in the reverse scan at
slow scan rates (20-50 mV s-1) caused by the oxidation
of electroactive follow-up products which are formed in
a homogeneous reaction on the time scale of the experi-
ment. In multisweep experiments, the complete revers-
ible oxidation waves of these products could be observed
allowing for the determination of their half-wave poten-
tials. The potentials of the follow-up products of 1a -c,
3a -b and respectively those of 2a and 2b, were identical
to the independently measured oxidation potentials of
F igu r e 3. Multisweep-cv of 1e at 500 mV s-1 in acetonitrile.
s-1) a partially reversible oxidation wave emerged at low
potential due to the dimethylaniline (E1/2(1e) ) +0.67
VFc24) and the ferrocene donor (E1/2(1f) ) +0.35 VFc24).
This assignment was tested for 1e using ethyl p-di-
methylaminobenzoate (E1/2 ) +0.64 VFc24) as a reference.
With both substrates a second irreversible wave (1e:
Epa ) 1.32 VFc,24 1f: Epa ) 1.70 VFc24) showed up at higher
potential. While oxidation of 1e at v ) 20 mV s-1 yielded
an irreversible first oxidation wave at Epa ) 1.32 VFc,24
completely reversible wave was afforded at v g 2 V s-1
At these scan rates the ratio Ipa (at 1.32 V)/Ipa (at 0.67 V)
reached a limiting value of 1.0. At v ) 500 mV s-1
a
.
benzofurans B119a (E1/2 ) 0.93 VFc24) and B225a (E1/2
)
a
0.87 VFc24), respectively. Digital simulation of the mul-
tisweep cyclic voltammograms of 1a , based on a EC-
CECDISP mechanism,26,27 showed reasonable agreement
with the experimental trace when using the same pa-
rameter set for benzofuran formation as described earlier
(Figure 2).6 Only the oxidation potentials and the first-
order rate constant of the bond cleavage had to be
adapted to the enol carbamate radical cation.
reduction wave emerged, which in a multisweep experi-
ment translated into a reversible wave assigned to
benzofuran B1 (Figure 3). In cv measurements of 1f no
formation of B1 was registered, independent of whether
the switching point was set after the first or second
oxidation wave.
Enol carbonate 2b showed already a partially revers-
ible oxidation wave at scan rates >100 mV s-1 in
dichloromethane. After the first oxidation wave, a second,
irreversible wave occurred whose intensity increased at
faster scan rates in the same manner as the reversibility
of the first wave increased (Figure 4). As the reversibility
of the first oxidation wave was decreased with increasing
substrate concentration, this behavior is indicative of a
second-order process limiting the rate of the follow-up
The enol esters 1e and 1f showed different behavior
in cv experiments. Already at normal scan rates (100 mV
(26) “E” stands for electrochemical, “C” for chemical reaction step.
For this type of reactions, the expression “DISP” has been established
although this is no disproportionation reaction but an homogeneous
electron-transfer equilibrium between the employed redox-active spe-
cies.
(27) Heinze J . Angew. Chem., Int. Ed. Engl. 1984, 23, 831.

Radical Cation Ester Cleavage in Solution
J . Org. Chem., Vol. 66, No. 10, 2001 3269
F igu r e 4. Cv of 2b in dichloromethane: left, 50 mV s-1; right, 500 mV s-1
.
Ta ble 5. Ra te Con sta n ts for th e Mesolytic Clea va ge of En ol Ester Ra d ica l Ca tion s a s Mea su r ed by F a st Sca n Cyclic
Volta m m etr y a t Room Tem p er a tu r e
1a •+
2a •+
1b•+
2b•+
<2c
<1500c
1d •+
1e•+
1g•+ a
2g•+
1h •+ b
k (CH2Cl2) (s-1
)
)
502 ( 9
2870 ( 20
228 ( 2
1650 ( 13
104 ( 2
2020 ( 12
ca. 200
ca. 5000
1.2 ( 0.01
0.4 ( 0.01
110 ( 0.5a
ca. 5000
ca. 50
k (CH3CN) )s-1
1.4 ( 0.02
a
b
Taken from ref 6. Taken from ref 17. c The exact rate constant could not be determined because the disproportionation reaction 2
2b+• f 2b2+ + 2b becomes the rate-determining step as shown by concentration dependent cv experiments.
reaction of 2b•+. As a consequence, we assign the second
wave to the oxidation of the persistent radical cation to
the corresponding dication 2b•+f 2b2+ (Epa ) 1.13 VFc,24
500 mV s-1, CH2Cl2). Because of the small difference
between the first and the second oxidation potential, the
rate constant given in Table 5 can only provide an upper
limit for the O-CO bond scission, as the disproportion-
ation reaction of 2b•+ competes with the bond cleavage
and restricts the lifetime of the radical cation.
In single sweep experiments, the bisenol carbonate 3a
exhibited the same qualitative cv behavior as 2b. How-
ever, the slope of the cyclic voltammograms was rather
small, presumably caused by a slow heterogeneous
electron transfer which even prevented the occurrence
of a pronounced reduction wave. In the cathodic back
sweep, a slight crossing effect showed up and a reduction
24
wave appeared at Epc ) 0.90 VFc whose intensity
depended on the scan rate and which could be assigned
to the reduction of the benzofuran radical cation B1. In
F igu r e 5. Multisweep-cv of 3a in acetonitrile.
multisweep-experiments, the completely reversible oxi-
a typical EC-mechanism, i.e., a decreasing ratio Ipa/v1/2
with increasing scan rate, an anodic shift of the peak
potential with increasing v, and an increasing ratio Ipc/
Ipa with increasing v that approached asymptotically the
theoretical value 1 for a reversible one-electron oxidation.
These data allow to determine the rate constants of the
dation wave of the follow-up product B119a (E1/2 ) 0.93
VFc24) emerged (Figure 5), and the cv exhibited four
isopotential points indicative of a clean conversion of 3a
into B1. In the case of bisenol oxalate 3b, the waves for
the first and the second oxidation step were not sepa-
rated, attesting that the two enol electrophores are
radical cation cleavage using the Nicholson-Shain for-
electronically insulated and the oxidation of one electro-
malism,29 assuming a first-order process for systems 1a ,
2a , 1b, 2b, 1d , and 1e (Table 5). Precise values for 1c
could not be obtained because of its low solubility and a
strong electrode covering. Oxidation of 1f did not reveal
phore system has a rather small Coulombic influence on
the second oxidation potential. The low solubility of 3b
prevented further investigations.
F a st Sca n Cyclic Volta m m etr y. The kinetics of the
formation of B1 so that a kinetic analysis was not
mesolytic cleavage of the radical cations have been
meaningful.
determined by fast scan cyclic voltammetry in acetonitrile
Notably, the lifetime of the radical cations increased
and dichloromethane (Figure 6). When the scan rates
by a factor 5-50 when changing the solvent from di-
were increased from 100 to 5000 V s-1, the irreversible
chloromethane to acetonitrile. To check for nucleophile-
oxidation waves of all monoenol compounds turned
induced bond cleavage30 the effect of methanol was
reversible.28 In the intermediate range, partially revers-
probed; addition of less than 400 mol-% methanol to the
ible waves were obtained showing the characteristics of
acetonitrile solution had no significant effect on the
(28) Except for 1d . In this case, only partially reversible oxidation
waves could be obtained in acetonitrile.
(29) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 6.

3270 J . Org. Chem., Vol. 66, No. 10, 2001
Schmittel et al.
F igu r e 6. Fast scan cv of 1a in dichloromethane (left) and acetonitrile (right).
F igu r e 7. Change in the EPR spectrum after the oxidation of 2a in the course of 4 h at 140 K (for details, see text).
lifetime of 1a •+. At higher concentrations of methanol the
oxidation wave 1a •+ f 1a 2+ experienced a kinetic ca-
benzofuran B2.32 The formation of benzofuran radical
cations under conditions of the EPR experiment can be
easily explained by the fact that the benzofuran possesses
a lower oxidation potential than the model systems.
Therefore, once the benzofuran is formed, it will be
oxidized by the original 2a •+. Interestingly, when the dark
green solution of 2a •+ was warmed to ca. 200 K for a few
seconds a sudden change of color to dark violet occurred.
The EPR spectrum of this solution measured again at
140 K showed a broad triplet with a coupling constant
of aN ) 22.9 G and a g value of g ) 2.0017 (Figure 8),
which could be assigned to the tert-butylaminoacyl radi-
cal,33 generated by mesolytic O-CO bond scission from
2a •+ (Scheme 2). The large coupling constant and the
absence of an additional coupling to the N-H proton
indicates that the radical adopts the Z-conformation from
the parent enol carbamate 2a . In line with former
investigations on this radical,33 no signs of interconver-
thodic shift merging with the original wave 1a f 1a •+
.
Hence, it is the dication that is trapped rapidly by
methanol not the radical cation.
EP R of th e Ra d ica l Ca tion s a n d F r a gm en ta tion
P r od u cts. At a high vacuum-line, the radical cations
were generated by reaction of the enol derivatives with
dioxygenyl hexafluoroarsenate31 in chlorodifluoromethane
at low temperature. In the oxidation of 2a at 140 K a
green solution resulted that afforded a signal without
hyperfine splittings at g ) 2.0023 (Figure 7). In the
course of 4 h the spectrum converted slowly into a highly
resolved signal with g ) 2.0026, which could undoubtedly
be assigned to the radical cation of the corresponding
(30) Dinnocenzo, J . P.; Farid, S.; Goodman, J . L.; Gould, I. R.; Todd,
W. P.; Mattes, S. L. J . Am. Chem. Soc. 1989, 111, 8973. Baciocchi, E.;
Bernini, R.; Lanzalunga, O. J . Chem. Soc., Chem. Commun. 1993, 1691.
Todd, W. P.; Dinnocenzo, J . P.; Farid, S.; Goodman, J . L.; Gould, I. R.
Tetrahedron Lett. 1993, 34, 2863. Herbertz, T.; Roth, H. D. J . Am.
Chem. Soc. 1998, 120, 11904. Herbertz, T.; Roth, H. D. J . Org. Chem.
1999, 64, 3708.
(32) The EPR signals of the benzofuran radical cations including
the hyperfine splittings are known from independent measurements.
Schmittel, M.; Gescheidt, G.; Eberson, L.; Trenkle, H. Perkin Trans. 2
1997, 2145.
(31) Dinnocenzo, J . P.; Banach, T. E. J . Am. Chem. Soc. 1986, 108,
6063.
(33) Sutcliffe, R.; Ingold, K. U. J . Am. Chem. Soc. 1981, 103, 7686.

Radical Cation Ester Cleavage in Solution
J . Org. Chem., Vol. 66, No. 10, 2001 3271
Sch em e 3. F or m a tion of Ben zofu r a n by O-X
Bon d Dissocia tion of th e En ol Ra d ica l Ca tion 11-13
F igu r e 8. Detection of the butylaminoacyl radical (aN ) 22.9
G) from 2a +•
.
Sch em e 4
Sch em e 2
must take place. As such the situation is reminiscent of
the chemistry of enols and different enol derivatives Cd
C-O-X after one-electron oxidation with X ) SiR3,
TiCp2Cl, P(OEt)2, and others.11-13 With those substitu-
ents, the mesolytic bond cleavage at the stage of the enol
radical cation could be established and the formation of
the benzofurans traced down to the mechanism depicted
in Scheme 3. In all bond cleavage processes, the group X
was cleaved off as cationic fragment thus resulting in the
formation of an R-carbonyl radical which was further
oxidized under the reaction conditions. Cyclization of the
R-carbonyl cation, [1,2]-methyl shift and deprotonation
finally lead to the benzofuran. This mechanistic scheme
could additionally be confirmed by preparative and cyclic
voltammetry studies indicating that benzofuran forma-
tion can equally be triggered by oxidation of enolates.
Ta ble 6. EP R of En ol Ca r ba m a te a n d En ol Ca r bon a te
Ra d ica l Ca tion s
1a +•
2a +•
1b+•
2b+•
g value
2.0015
2.0023
2.0014
2.0023
sion to the E-isomer could be detected on the time scale
of the EPR experiment. The triplet signal is superim-
posed by an additional singlet signal, derived from the
radical cation 2a •+ with g ) 2.0023.
Enol carbonates 1b and 2b as well as enol carbamate
1a provided also unresolved EPR spectra at 140 K in
chlorodifluoromethane solution upon oxidation with O2-
AsF6. The g values are given in Table 6.
Discu ssion
Th e Na tu r e of th e En ol Ra d ica l Ca tion s. From the
X-ray structure of 1a and 1b one can derive that the enol
π system and the O-CO bond are almost orthogonal (70
( 1° for 1a and 1b), which should allow for convenient
overlap of the scissile σ(O-CO) and the π(CdC) system.
While these data do not reveal details about the corre-
sponding odd-electron cations, some important informa-
tion can be derived from the unresolved EPR spectra of
the enol carbonate and carbamate radical cations. Al-
though no statement about the spin density distribution
is possible, the absence of couplings to the nitrogen in
the enol carbamate radical cations indicates that the
positive charge and the spin density are delocalized in
the π-system of the enol moiety rather than in the σ(O-
C) bond. As the g values depend only on the nature of
the substituent in 1-position and show equal values for
1b•+ and 1a •+, respectively for 2b•+ and 2a •+, one can
suppose that the nature of the SOMO is similar for most
model systems (a -d ). Decisively different is the situation
with 1e,f. The oxidation potentials indicate that for the
latter systems the key electrophore is now the electron-
rich acyl group.
For the various enol ester radical cations of this study
two distinct reaction pathways to the benzofurans need
to be considered: (a) O-C bond dissociation of the enol
ester radical cations in analogy to the examples above
or (b) cyclization on the stage of the radical cation
followed by O-C bond cleavage at a later stage of the
reaction (Scheme 4). Dissociative electron transfer, a
process well-known for radical anion bond cleavage
reactions,34 can easily be discarded since the radical
cations could be observed both by cyclic voltammetry and
EPR spectroscopy.
Three arguments strongly favor mechanism (a) over
(b): (1) the steric crowding in the transition state and in
the distonic radical cation along route (b) is prohibitive35
according to AM136 (when R1 ) But and R2 ) OMe), and
(2) one would expect that the formed distonic radical
cation should easily be oxidized at the radical site. The
oxidation potential of the cyclohexadienyl radical has
(34) Save´ant, J .-M. Acc. Chem. Res. 1993, 26, 455. Donkers, R. L.;
Maran, F.; Wayner, D. D. M.; Workentin, M. S. J . Am. Chem. Soc.
1999, 121, 7239.
Mesolytic O-CO Bon d Clea va ge. The formation of
benzofurans B1 and B2, respectively as one-electron
oxidation products of the enol systems implicates that
at some stage of the overall reaction O-CO bond cleavage
(35) According to our AM1 calculations, 4 is not local minimum
structure but readily opens to 1•+
.
(36) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P.
J . Am. Chem. Soc. 1985, 107, 3902.

3272 J . Org. Chem., Vol. 66, No. 10, 2001
Sch em e 5. Mech a n istic Alter n a tives for th e O-C Bon d Clea va ge
Schmittel et al.
Ta ble 7. Ion iza tion a n d Solu tion Oxid a tion P oten tia ls of Va r iou s Ra d ica ls
5•
6•
tBuNHCO•
tBuOCO•
pAnCO•
CH3CO•
CF3CO•
IPaa/eV
E
E
E
6.87
0.12
-0.19
0.14
6.87
0.12
-0.19
0.24
7.45
0.56
0.41
8.22
1.14
1.21
7.01
0.23
-0.05
7.72
0.76
0.69
9.16
1.86
2.19
1/2/VFc (calcd)b
1/2/VFc (calcd)c
pa/VFc d (expt)
a
b
Adiabatic ionization potential calculated by AM1. Solution oxidation potential calculated using Gassman’s40 correlation. c Solution
oxidation potential calculated using Wayner’s correlation.41 Irreversible anodic peak potentials.
d
been determined by Wayner and Griller37 to be E1/2
)
corresponding enolates, the oxidation potentials of the
-0.04 V vs SCE (-0.43 VFc24). Even when we consider a
strong anodic shift in 4 because of the electron-withdraw-
ing oxonium ion it is hard to conceive that 4 will exhibit
R-carbonyl radicals39 are known to be E1/2 (5•) ) 0.14 VFc
24
and E1/2 (6•) ) 0.24 VFc.24
Unfortunately, the solution oxidation potentials of the
acyl radicals with R2 ) Me, CF3, OBut, and NHBut are
not known. As a consequence, we have resorted to
adiabatic ionization potentials IPa calculated by the AM1
method36 which obviously reflect the gas-phase situation.
To extract solution data we have used well-established
correlations of oxidation potentials in solution and ion-
ization energies (i.e., those of Gassman:40 E1/2 [VFc] ) 0.76
IPa - 5.10 and of Wayner:41 E1/2 [VFc] ) 1.043 IPa - 7.36).
Using the experimental data from 5• and 6• as a
reference (Table 7) it becomes evident that for our class
of intermediates Gassman’s correlation provides reason-
able solution oxidation potentials in combination with
AM1 calculations. As the oxidation potentials of all acyl
radicals (except for pAnCO•) are decisively higher than
that of 5• and 6• it is save to predict that the cleavage of
enol ester radical cations 1a -c,2a ,b•+ will directly lead
to R-carbonyl cations and acyl radicals. In former inves-
tigations, this cleavage selectivity could be confirmed
an E1/2 g 1 VFc.24 Accordingly, 4 should be more easily
ox
oxidized than the parent enol ester. The cyclic volta-
mmmetry experiments, however, indicate clearly that the
irreversible wave in the enol ester oxidation represents
only the transfer of one, but not of two electrons, thus
eliminating pathway (b) as a plausible mechanistic
alternative. (3) In the dissociation of 1a •+ we find the
aminoacyl radical as product whose formation cannot be
explained with 4 being an intermediate.
Selectivity of Bon d Clea va ge. For systems a -d the
donor site is clearly the enol moiety. Hence, the mecha-
nism of the O-C bond cleavage according to pathway (a)
may now be classified along three distinct mechanistic
alternatives: Either it involves (a1) a heterolytic, (a2) a
homolytic, or (a3) a special type of nucleophile induced
bond cleavage as represented in Scheme 5. While for the
first two reaction modes there are well-characterized
examples in the literature,4,11-13,38 pathway (a3) would
involve a two-step process that has not been established
in the literature in contrast to one-step nucleophile-
induced bond cleavage reactions.30 A concerted mecha-
nism (a4), however, does not make sense in light of the
well-known addition elimination chemistry at carbonyl
sites.
A bimolecular cleavage process along route (a3) can be
rather convincingly ruled out on the basis of our kinetic
data in comparison with systems for which a nucleophile-
assisted bond-cleavage process has been established (vide
infra). The outcome of a monomolecular bond cleavage
either according to (a1) or (a2) should easily be predict-
able from a simple thermochemical cycle calculation since
it ought to depend only on the redox potentials of the
two radicals formed in the putative homolytic cleavage
of the neutral compound. From cv investigations of the
(37) Wayner, D. D. M.; McPhee, D. J .; Griller, D. J . Am. Chem. Soc.
1988, 110, 132.
(38) Mariano, P. S. Acc. Chem. Res. 1983, 16, 130; Lan, A. J . Y.;
Quillen, S. L.; Heuckeroth, R. O.; Mariano, P. S. J . Am. Chem. Soc.
1984, 106, 6439. Yoshiyama, T.; Fuchigami, T. Chem. Lett. 1992, 1995.
Akaba, R.; Niimura, Y.; Fukushima, T.; Kawai, Y.; Tajima, T.;
Kuragami, T.; Negishi, A.; Kamata, M.; Sakuragi, H.; Tokumaru, K.
J . Am. Chem. Soc. 1992, 114, 4460. Zhang, X.; Yeh, S.-R.; Hong, S.;
Freccero, M.; Albini, A.; Falvey, D. E.; Mariano, P. S. J . Am. Chem.
Soc. 1994, 116, 4211. Fuchigami, T.; Fujita, T. J . Org. Chem. 1994,
59, 7190. Mizuno, K.; Tamai, T.; Hashida, I.; Otsuji, Y. J . Org. Chem.
1995, 60, 2935. Fukuzumi, S.; Yasui, K.; Itoh, S. Chem. Lett. 1997,
161. Baciocchi, E.; Crescenzi, C.; Lanzalunga, O. Tetrahedron 1997,
53, 4469. Su, Z.; Mariano, P. S.; Falvey, D. E.; Yoon, U. C.; Oh, S. W.
J . Am. Chem. Soc. 1998, 120, 10676.
(39) Ro¨ck, M.; Schmittel, M. Chem. Commun. 1993, 1739.
(40) Gassman, P. G.; Yamaguchi, R. J . Am. Chem. Soc. 1979, 101,
1308.
(41) Wayner, D. D. M.; Sim, B. A.; Dannenberg, J . J . J . Org Chem.
1991, 56, 4853.

Radical Cation Ester Cleavage in Solution
J . Org. Chem., Vol. 66, No. 10, 2001 3273
experimentally for the enol acetate 1g•+.6 The detection
of the tert-butylaminoacyl radical in the EPR experiments
of 1a •+ shows that the oxidative cleavage of the enol
carbamates follows the same selectivity. Therefore, on
the basis of the semiempirical calculations one can
conclude that model systems 1a -d ,2a ,b•+ undergo a
homolytic bond scission according to (a2).
Sch em e 6. Th er m och em ica l Cycle for th e
Eva lu a tion of ∆GMeso (CR ) r-ca r bon yl Ra d ica l)
Mesolytic Bon d Clea va ge of Sp ecia l System s. A
different situation is met in the oxidative decomposition
of 1e. The preparative oxidation and the cv data, in
particular the concentration independence of the kinetics,
indicate that 1e•+ undergoes slow O-CO bond cleavage.
Thermodynamic arguments propose that monomolecular
fragmentation should result in formation of the amino-
benzoyl cation (IPa ) 6.47 eV, E1/2 (calcd) ) -0.18 VFc)
and the R-carbonyl radical. Since the charge in 1e•+ is
originally located on the aminobenzoyl group, this cleav-
age occurs in the homolytic fashion. The positive charge
on the aminobenzoyl residue both in 1e•+ and after
cleavage in the acyl cation nurtured our hope that 1e•+
might undergo O-CO bond cleavage in the presence of
a nucleophile via pathway a3 (see Scheme 5). However,
no change in reversibility was registered upon addition
of up to 10 equivalents of methanol. In contrast, a drastic
change was detected in the cv with pyridine, but this is
due to deprotonation at the methyl groups as demon-
strated by control experiments with ethyl p-dimethyl-
aminobenzoate. With good nucleophiles that are less
basic, i.e., trimethylsilyl azide, as nucleophiles again
reversibility over the a scan range 50-500 mV s-1 is the
same as in pure acetonitrile.
Table 5 are analyzed the deceleration of the mesolytic
bond cleavage in the case of 2b•+ and 1h •+ is most
surprising.
For 2b, an additional important observation stems
from a comparison of the mesolytic cleavage of 1a,b,2a,b•+
.
While for enol carbamates (kf(1a •+)/kf(2a •+) ) 2.2) the
ratio of the rates is very small, it is markedly increased
for the analogous enol carbonates (kf(1b•+)/kf(2b•+) )
52).42 To understand this difference, we have analyzed
the thermodynamics of the mesolytic bond cleavage
(Scheme 6) in detail.
According to thermochemical cycle calculations,43 the
mesolytic bond energy ∆GMeso can be derived from the
homolytic bond energy and the oxidation potentials of
reactant and the R-carbonyl radical (CR): ∆GMeso
∆GHomo - nF[E1/2(enol) - E1/2(CR)].
)
Because of the cross-conjugation of the phenyl group,
∆GHomo in both the carbamates and the carbonates is
expected to be roughly 8 kJ mol-1 lower for series 2 (R1
) Ph) molecules as compared to those of series 1 (R1 )
But).44 However, this difference in homolytic bond ener-
gies ∆∆GHomo ) ∆GHomo(2) - ∆GHomo(1) is basically
compensated by the difference in the oxidation potentials
of the R-carbonyl radicals ∆E1/2(CR) ) E1/2(6•) - E1/2(5•)
) 0.09 V (8 kJ mol-1).45 As a consequence, the differences
No cleavage occurred after oxidation of 1f. The corre-
sponding radical cation is apparently a persistent ferro-
cinium type species. Intramolecular activation of the
scission with ferrocene as redox relay is not operative.
While oxidative cleavage of 3a is observed in the cv
experiments and in anodic oxidation (furnishing two
equivalents of benzofuran), the failure to oxidatively
cleave 3a using one-electron oxidants indicates that the
bond scission is very slow and cannot compete with back
electron transfer to the original oxidant. Presumably,
bond cleavage is sluggish because of stereoelectronic
reasons, as the dihedral angle between σ(O-CO) and π-
(CdC)O assumes Φ ) 50°.
Kin etics a n d Th er m od yn a m ics of th e Mesolytic
O-CO Bon d Clea va ge. Table 5 shows the rate con-
stants for the mesolytic cleavage of the O-CO bond in
various enol ester type radical cations. Most of them fall
in a narrow range, except those of enol carbonate 2b, enol
ester 1e, and trifluoroacetate 1h . The small variation in
the rate constants of enol acetates, carbonates, and
carbamates once again confirms the homolytic cleavage
mode, as one would expect greater variations in kf for
the heterolytic mode because of the vastly different
oxidation potentials of the acyl radicals. Furthermore,
the bimolecular cleavage process, e.g., a new nucleophile-
assisted bond cleavage (a3), can also be ruled out by the
kinetic data, as the activation entropy of the mesolytic
cleavage of 1g•+ is positive6 (+11 cal mol-1 K-1) in line
with a dissociative process in the rate-determining step.
in mesolytic bond cleavage energies ∆∆GMeso ) ∆GMeso
-
(2) - ∆GMeso(1) are determined only by nF[E1/2(2) -
E1/2(1)]. The small difference in the oxidation potentials
of the enol carbamates leads to similar values of ∆GMeso
for 1a •+ and 2a •+, whereas ∆GMeso is increased in enol
carbonate 2b•+ with respect to 1b•+ by 14.5 kJ mol-1
because of ∆E1/2 ) 0.15 V. Hence, the rate difference can
readily be explained by the thermodynamics of the
mesolytic bond cleavage.
A different effect is responsible for the drastic change
of rate between 1h •+ and 1g•+, but that has been
discussed previously.17
Bon d Clea va ge a t th e Dica tion Sta ge. For 2b, a
third mechanistic possibility has to be discussed since
the rate-determining step proved to be dependent on the
concentration of 2b . Both at higher scan rates and at
lower concentration the first oxidation wave turns re-
versible and an irreversible wave shows up at more
anodic potential. According to the cv of 2b in dichloro-
methane (Figure 4), the potential difference between the
first and the second oxidation is as small as 140 mV.
Because mesolytic cleavage in dichloromethane is slow,
(42) The same trend is seen in acetonitrile, but it is not possible to
quantify the effect since the observed rate constants represent only
an upper limit for the mesolytic cleavage because of disproportionation
reactions.
(43) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287.
(44) Based on the AM1 calculated homolytic bond energies.
(45) Ro¨ck, M.; Schmittel, M. Chem. Commun. 1993, 1739.
What is the reason for the slow oxidative bond dis-
sociation of 2b, 1e, and 1h ? For 1e the situation is clear,
as the low oxidation potential (Table 4) decreases the
thermochemical driving force ∆GMeso for the bond cleav-
age according to Scheme 6. Hence, when the data in

3274 J . Org. Chem., Vol. 66, No. 10, 2001
Schmittel et al.
the main reaction pathway of 2b•+ is now disproportion-
ation into neutral 2b and 2b2+. This interpretation is
supported by the concentration dependence of the reduc-
tion wave of 2b•+, which decreases with increasing
concentration.
For enol carbamate 2a , a similar effect was observed
in the presence of higher concentrations of methanol. The
more methanol was added, the smaller was the potential
difference between the first reversible wave of the car-
bamate and a second irreversible wave. This behavior can
readily be rationalized if the second wave is assigned to
the oxidation of 2a •+ f 2a 2+ as the addition of methanol
increases the rate of the follow-up reaction of the dication
by nucleophilic attack. This leads to a cathodic shift of
the oxidation wave, which is typical for ECirr-mecha-
nisms.
inorganic chemistry, University of Wu¨rzburg. Melting points
were taken on a Bu¨chi Smp-20 apparatus and are uncorrected.
Theoretical studies were performed with the help of the AM1
molecular orbital method.36 The geometries of the radical
cations were optimized by using the unrestricted Hartree-
Fock (UHF) formalism. Heats of formation were calculated
from the UHF-optimized structures by utilizing the half-
electron (HE) method. Adiabatic ionization potentials (IPa)
were obtained as the difference in the heat of formation
between the species involved in the redox process.
Ma ter ia ls. Commercial reagents were purchased from
standard chemical suppliers and were used without further
purification. Acetonitrile for cv and one-electron oxidations was
purchased in HPLC quality from Riedel-de-Hae¨n, distilled over
calcium hydride and filtered through basic alumina (ICN). The
preparation of the corresponding enols has been reported
elsewhere.19a,25a
2-(ter t-Bu t yla m in oca r b on yloxy)-1,1-d im esit yl-3,3-d i-
m eth yl-1-bu ten e (1a ). A suspension of 60% NaH in paraffin
oil (1.80 g, 45.0 mmol) was washed under nitrogen atmosphere
with 25 mL of dry n-pentane, suspended in 10 mL of dry THF,
and then added to a solution of 1,1-dimesityl-3,3-dimethylbut-
1-en-2-ol (1.01 g, 3.00 mmol) in dry THF (30 mL). The mixture
was stirred for 20 min at room temperature and excess NaH
was filtered off under nitrogen atmosphere. tert-Butylisocy-
anate (3.0 mL, 30.0 mmol) was added, and the mixture was
allowed to stir for 24 h. After hydrolysis, the aqueous layer
was extracted three times with ether. The combined organic
layers were first washed with saturated NaHCO3 and water
and then dried over Na2SO4. Removal of the solvent, filtration
over silica gel (using dichloromethane/cyclohexane 1:2), and
recrystallization from ethanol afforded 0.50 g (1.15 mmol, 38%)
of pure 1a as a white solid: mp 140 °C; IR (KBr) ) 3644, 3495,
3447, 3348, 1739, 1611 cm-1; 1H NMR δ 0.95 (s, 9 H), 1.02 (s,
9 H), 1.88 (br s, 6 H), 2.16 (s, 3 H), 2.20 (s, 3 H), 2.40 (br s, 3
H), 2.75 (br s, 3 H), 4.04 (s, 1 H), 6.60 (br s, 2 H), 6.80 (br s, 2
H); 13C NMR δ 20.49, 20.94, 28.22, 28.38, 38.72, 49.59, 124.06,
127.57, 127.77, 128.73, 130.24, 134.95, 135.26, 135.39, 135.83,
137.5, 138.19, 138.95, 140.18, 152.79 (C2), 153.03. Anal. Calcd
for C29H41NO2: C, 79.95; H, 9.49; N, 3.22. Found: C, 79.67;
H, 9.45; N, 3.13.
Additional irreversible oxidation waves at higher an-
24
odic potentials are also registered for 1e (Epa ) 1.32 VFc
)
and 1f (Epa ) 1.70 VFc). While the first oxidation occurs
at the p-dimethylaniline and ferrocene site, respectively,
the second oxidation is due to removal of an electron from
the enol moiety. This assignment is supported by the
oxidation potential of the protonated enolester 1e-H+ (Epa
) 1.28 VFc), which should have an electronic structure
similar to that of 1e•+
.
Quantitative formation of B1 is only detected in cv
experiments, when the dication 1e2+ is formed, which
obviously cleaves to yield two cationic fragments. In
contrast to the latter case, no B1 is detected in prepara-
tive oxidations or cv measurements from either 1f•+ or
1f2+. While the oxidation of the ferrocene moiety in 1f•+
is completely reversible even at low scan rates (v ) 20
mV s-1), the second oxidation is expected to result in bond
cleavage as indicated by the irreversible cv wave. Due
to the high potential, however, benzofuran B1 is formed
46
as an unstable dication at Epa > 1.70 VFc that decom-
poses and precludes the detection of the B1•+ f B1 redox
1,1-Dim esityl-2-ter t-bu tyllith iu m En ola te. Under nitro-
gen atmosphere, 14 mL (21 mmol) of a 1.5 M solution of t-BuLi
in pentane was added to 200 mL of an ethereal solution of
dimesitylketene (7.82 g, 28.1 mmol) during 1 h at -40 °C. After
being stirred for 3 h, the mixture was allowed to warm to room
temperature.
process.
Con clu sion
We have investigated the O-CO bond scission in
different enol ester, carbamate and carbonate radical
cations and probed various mechanistic possibilities. It
could be shown by EPR experiments and thermochemical
cycle considerations that the cleavage selectivity of the
O-CO bond scission follows a monomolecular, homolytic
pathway both for electron-rich and electron-poor deriva-
tives. The rate constants of the mesolytic bond cleavage
are for most investigated compounds in a very similar
range (k (CH3CN) ) (2-5) × 103 s-1). All attempts to find
a nucleophile-induced pathway via a addition-elimina-
tion mechanism were met with failure.
1,1-Dim esityl-3,3-d im eth yl-2-ter t-bu toxyca r bon yloxy-
1-bu ten e (1b). To a solution of 1,1-dimesityl-2-tert-butyllithi-
umenolate (12.4 mmol) in 80 mL of dry ether was added di-
tert-butyl carbonate (4.10 g, 18.6 mmol), and the mixture was
stirred for 20 min. After addition of additional di-tert-butyl
carbonate (2.70 g, 12.4 mmol), the mixture was stirred
overnight and the white precipitate was filtered off. The filtrate
was washed with 5% NaOH, water, saturated NaHCO3, and
again with water. After drying, it was concentrated to a red
oil that was purified by column chromatography on silica gel
(hexane/dichloromethane 1:1) to yield 1.33 g (3.05 mmol, 25%)
of pure 1b as white crystals: mp 127 °C; IR (KBr) 1748, 1610
1
cm-1; H NMR δ 1.03 (s, 9 H), 1.08 (s, 9 H), 1.87 (br s, 6 H),
2.15 (s, 3 H), 2.20 (s, 3 H), 2.40 (s, 3 H), 2.72 (br s, 3 H), 6.61
(d, 2 H), 6.82 (s, 2 H); 13C NMR δ 20.58, 20.82, 27.19, 28.45,
38.70, 81.05, 124.26, 127.66, 128.23, 128.92, 129.96, 134.42,
134.75, 135.65, 136.07, 151.81, 152.68. Anal. Calcd for
Exp er im en ta l Section
Gen er a l Asp ects. 250 MHz 1H NMR and 150 MHz 13C
NMR spectra were recorded in CDCl3 on a Bruker A250 or a
Bruker DMX600. Chemical shifts are reported in ppm down-
field vs the internal standard TMS. The abbreviation br.
designates signals broadened by coalescence. IR spectra were
recorded in KBr using a Perkin-Elmer FT-IR1605 infrared
spectrometer. Mass spectra were recorded on a Finnigan MAT
8200 Mass spectrometer using electron ionization (EI) at 70
eV. Elemental analyses were performed at the institute of
C
29H40O3: C, 79.77; H, 9.10 Found: C, 79.83; H, 9.23.
1,1-Dim esityl-3,3-d im eth yl-2-ben zyloxyca r bon yloxy-1-
bu ten e (1c). To a solution of 1,1-dimesityl-2-tert-butyllithium
enolate (12.4 mmol) in 80 mL of dry ether was added a solution
of carbobenzoxychloride (4.00 g, 23.4 mmol) in 10 mL of dry
ether, and the mixture was stirred for 20 min. After addition
of another 2.70 g (15.6 mmol) of carbobenzoxy chloride, the
mixture was stirred overnight and the white precipitate was
filtered off. The filtrate was washed with 5% NaOH, water,
saturated NaHCO3, and water. After drying, it was concen-
trated to leave a red oil which was purified by column
(46) B1•+ f B12+: Epa ) +1.66 VFc (at 100 mV s-1 in acetonitrile).
Schmittel, M.; Langels, A. J . Org. Chem., 1998, 63, 7328.

Radical Cation Ester Cleavage in Solution
J . Org. Chem., Vol. 66, No. 10, 2001 3275
chromatography on silica gel (hexane/dichloromethane 1:1) to
yield 0.83 g (1.76 mmol, 11%) of 1c: mp 160 °C; IR (KBr) 1749,
1608 cm-1; 1H NMR δ 1.07 (s, 9 H), 1.89 (br s, 6 H), 2.20 (s, 3
H), 2.22 (s, 3 H), 2.44 (s, 3 H), 2.73 (s, 3 H), 4.80 (s, 1 H), 4.90
(s, 1 H), 6.57 (s, 1 H), 6.64 (s, 1 H), 6.83 (s, 2 H), 6.95 (m, 2 H),
7.27 (m, 3 H); 13C NMR δ 20.78, 28.42, 38.81, 69.15, 125.38,
127.57, 127.95, 128.27, 128.96, 129.94, 133.94, 134.38, 135.30,
135.85, 136.29, 152.81, 153.63. Anal. Calcd for C32H38O2: C,
81.66; H, 8.14. Found: C, 81.36; H, 7.87.
2.18 (s, 3 H), 2.21 (s, 3 H,), 2.46 (br, 3 H), 6.67 (br s, 2 H),
6.7-6.9 (br, 2 H), 7.14 (m, 5 H); 13C NMR δ 20.75, 21.00, 27.18,
82.00, 127.27, 127.70, 128.09, 128.55, 129.30, 133.70, 135.91,
136.33, 136.58, 138.19, 138.61, 146.28, 151.68. Anal. Calcd for
C31H36O3: C, 81.54; H, 7.95. Found: C, 81.11; H, 7.79.
Bis(1,1-d im esit yl-3,3-d im et h ylb u t -1-en -2-yl)ca r b on -
a te (3a ) Reaction of 1,1-dimesityl-3,3-dimethylbuten-2-ol (0.80
g, 2.38 mmol) and sodium hydride (60% suspension in paraffin;
0.40 g, 10.0 mmol) with bis(trichloromethyl)carbonate (0.23
g, 0.75 mmol) by the method described before yielded 420 mg
(600 µmol, 50%) of pure 3a as a white solid: mp 148 °C; IR
(KBr) 1756, 1611; 1H NMR δ 0.56 (s, 18 H), 1.81 (s, 6 H), 1.83
(s, 6 H), 2.13 (s, 6 H), 2.18 (s, 6 H), 2.39 (s, 6 H), 2.60 (s, 6 H),
6.52 (s, 2 H), 6.57 (s, 2 H), 6.78 (s, 2 H), 6.83 (s, 2 H); 13C NMR
δ 20.57, 20.78, 21.63, 22.39, 27.70, 38.37, 124.85, 127.54,
128.27, 128.85, 130.34, 134.67, 135.85, 136.07, 137.61, 138.64,
139.98, 152.53; HRMS calcd for C49H62O3 698.4699, found
698.4697.
1,1-Dim esityl-2-(p-m eth oxyben zoyloxy)-3,3-d im eth yl-
1-bu ten e (1d ). Reaction of 1,1-dimesityl-3,3-dimethylbuten-
2-ol (1.80 g, 5.35 mmol) and sodium hydride (138 mg, 5.75
mmol) with 4-methoxybenzoyl chloride (1.00 g, 5.86 mmol) by
the method described before yielded 1.59 g (3.38 mmol, 63%)
of pure 1d after chromatography (silica gel, toluene) as a white
1
solid: mp 157 °C; IR (KBr) 1730, 1610 cm-1; H NMR δ 1.10
(s, 9 H), 1.94 (br s, 6 H), 2.03 (s, 3 H), 2.23 (s, 3 H), 2.54 (br s,
3 H), 2.85 (br s, 3 H), 3.82 (s, 3 H), 6.5-6.8 (br.m, 4 H), 6.84
3
3
(d, J ) 7.0 Hz, 2 H), 7.83 (d, J ) 7.0 Hz, 2 H); 13C NMR δ
20.57, 20.81, 28.64, 39.28, 55.32,113.4, 122.61, 125.67, 128.65,
131.38, 134.92, 135.41, 136.08,138.05, 151.75, 163.06, 164.46.
Anal. Calcd for C32H38O3: C, 81.66; H, 8.14. Found: C, 81.37;
H, 8.18.
Bis(1,1-dim esityl-3,3-dim eth ylbu t-1-en -2-yl)oxalate (3b)
Reaction of 1,1-dimesityl-3,3-dimethylbuten-2-ol (0.80 g, 2.38
mmol) and of sodium hydride (60% suspension in paraffin; 0.40
g, 10.0 mmol) with bis(trichloromethyl)carbonate (0.30 g, 2.36
mmol) by the method described above yielded 577 mg (794
µmol, 67%) of pure 3b as a white solid: mp 263 °C; IR (KBr)
1,1-Dim esit yl-2-(p -(N,N-d im et h yla m in o)b en zoyloxy)-
3,3-d im eth yl-1-bu ten e (1e). Reaction of 1,1-dimesityl-3,3-
dimethylbuten-2-ol (1.80 g, 5.35 mmol) and sodium hydride
(60% suspension in paraffin; 230 mg, 5.74 mmol) with 4-(N,N-
dimethylamino)benzoyl chloride (2.16 g, 11.7 mmol) by the
method described before yielded 1.12 g (2.32 mmol, 43%) of
pure 1e after chromatography (silica gel, toluene) and recrys-
tallization from pentane as a white solid: mp 183 °C; IR (KBr)
2913, 1718, 1604, 1259 cm-1; 1H NMR δ 1.10 (s, 9 H), 1.94 (br
s, 6 H), 2.03 (s, 3 H), 2.23 (s, 3 H), 2.54 (br s, 3 H), 2.85 (br s,
1
1756, 1610; H NMR δ 0.93 (s, 18 H), 1.81 (s, 6 H), 1.91 (s, 6
H), 2.12 (s, 6 H), 2.20 (s, 6 H), 2.52 (br s, 6 H), 2.66 (br s, 6 H),
6.5-6.9 (br. 8 H); 13C NMR 20.57, 20.70, 20-22 (br), 28.07,
38.50, 125.98, 127-130 (br), 133.83, 133.95, 135.98, 136.41,
137-139 (br), 155.03, 198.44; HRMS calcd for
726.4648, found 726.4630.
C50H62O4
Gen er a l P r oced u r e for th e On e-Electr on Oxid a tion s
of th e En ol Ca r ba m a tes a n d En ol Ca r bon a tes. In a
glovebox the desired amount of the one-electron oxidant and
50 µmol of the enol derivative were placed into two separate
test tubes equipped with stirring rods. At a high purity argon
line acetonitrile (2 mL) was added in each test tube to dissolve
the reactants. The solution of the one-electron oxidant was
added dropwise via syringe to the solution of the enol and the
resulting mixture was stirred overnight. The mixture was
quenched with 2 mL of saturated aqueous NaHCO3 and
diluted with 10 mL of CH2Cl2. The aqueous layer was extracted
three times with CH2Cl2, the combined organic layers were
washed with saturated aqueous NaCl and water and dried over
Na2SO4. Removal of the solvent afforded the crude product.
In the reactions using [Fe(phen)3](PF6)3 as oxidant the residue
was extracted with ether and filtered off to separate insoluble
iron salts. Product analysis was performed by GC, GC-MS and
1H NMR. The benzofurans were identified by comparison with
data of authentic samples. For the detailed characterization
of benzofurans B1 and B2, see ref 25.
3
3 H), 3.82 (s, 3 H), 6.5-6.8 (br.m, 4 H), 6.84 (d, J ) 7.0 Hz, 2
3
H), 7.83 (d, J ) 7.0 Hz, 2 H); 13C NMR δ 20.61, 20.81, 28.68,
39.35, 40.00, 110.62, 125.55, 131.27, 133.47, 135.12, 135.27,
135.97, 138.12, 151.72, 153.04, 164.94. Anal. Calcd for C33H41
-
NO2: C, 81.94; H, 8.54; N, 2.90. Found: C, 81.59; H, 8.75; N,
2.65.
1,1-Dim esit yl-2-(fer r ocen oyloxy)-3,3-d im et h yl-1-b u -
ten e (1f). Reaction of 1,1-dimesityl-3,3-dimethylbuten-2-ol
(1.80 g, 5.35 mmol) and sodium hydride (60% suspension in
paraffin; 230 mg, 5.74 mmol) with ferrocenoyl chloride (2.16
g, 11.7 mmol) by the method described above yielded 910 mg
(1.66 mmol, 31%) of pure 1f after chromatography (silica gel,
toluene) and recrystallization from pentane as a brown solid:
mp 110 °C; IR (KBr) 2959, 1728, 1609, 1454 cm-1; 1H NMR δ
1.05 (s, 9 H), 1.89 (br s, 3 H), 1.99 (br s, 3 H), 2.11 (s, 3 H),
2.23 (s, 3 H), 2.58 (br s, 3 H), 2.83 (br s, 3 H), 3.85 (s, 5 H),
4.32 (s, 2 H), 4.61 (br s, 1 H), 4.75 (br s, 1 H), 6.73 (br.d, 4 H);
13C NMR δ 21.11, 21.25, 29.24, 39.88, 70.13, 71.42, 72.56,
126.60, 129.2 (br.), 135.63, 136.46, 136.57, 138.80, 151.89,
170.04. Anal. Calcd for C35H40FeO2: C, 76.64; H, 7.35. Found:
C, 76.30; H, 7.65.
1-(ter t-Bu tyla m in oca r bon yloxy)-2,2-d im esityl-1-p h en -
yleth en e (2a ). Reaction of 2,2-dimesityl-1-phenylethenol (1.00
g, 2.81 mmol) and NaH (60% suspension in paraffin; 0.40 g,
10.0 mmol) with tert-butylisocyanate (4.96 g, 50.1 mmol) by
the method described before yielded 0.80 g (1.93 mmol, 69%)
of pure 2a as a white solid: mp 151 °C; IR (KBr) 3444, 1750,
1610 cm-1; 1H NMR δ 1.02 (s, 9 H), 1.82 (br s, 3 H), 2.01 (s, 6
H), 2.18 (s, 3 H), 2.22 (s, 3 H), 2.48 (br s, 3 H), 4.30 (s, 1 H),
6.66 (br. s, 2 H), 6.7-6.9 (br, 2 H), 7.12 (m, 2 H), 7.20 (m, 3
H); 13C NMR δ 20.72, 20.81, 21.05, 21.32, 28.30, 29.85, 50.01,
127.16, 127.64, 127.92, 128.76, 129.28, 129.80, 130.40, 134.50,
134.65, 136.16, 136.38, 136.86, 138.29, 146.63, 152.84. Anal.
Calcd for C31H37NO2: C, 81.72; H, 8.18; N, 3.07. Found: C,
81.80; H, 8.08; N, 2.86.
Cyclic Volta m m etr y. In a glovebox tetra(n-butyl)ammo-
nium hexafluorophosphate (232 mg, 600 µmol) and the elec-
troactive species (6 µmol) were placed into a thoroughly dried
CV cell. At a high purity argon line acetonitrile or dichloro-
methane (6.0 mL) was added through a gastight syringe, a 1
mm platinum disk electrode as working electrode and a Pt wire
counter electrode as well as an Ag reference electrode were
placed into the solution. The cyclic voltammograms were
recorded at various scan rates using various starting and
switching potentials. For determination of the oxidation
potentials ferrocene (6 µmol) was added as the internal
standard. For fast scan cyclic voltammetry, 385 mg (1.00
mmol) of supporting electrolyte, 50 µmol of substrate and 5
mL of solvent were employed as described before. Fast scan
cyclic voltammograms were carried out at 25 µm Au ultramicro
electrodes, a Pt wire serving as counter electrode and a Ag
wire as reference electrode. Cvs were recorded using a Prin-
ceton Applied Research Model 362 potentiostat with an Philips
model PM 8271 XYt-recorder for scan rates <1 V s-1. For fast
scan cyclic voltammetry, a Hewlett-Packard Model 331A4
Function Generator was used, connected to a three-electrode
potentiostat developed by C. Amatore.47 Data were recorded
by a HP 54510 A Digitizing Oscilloscope linked to a 486DX33
1-(t -B u t o x y c a r b o n y lo x y )-2,2-d im e s it y l-1-p h e n y l-
eth en e (2b). Reaction of 2,2-dimesityl-1-phenylethenol (1.00
g, 2.81 mmol) and NaH (60% suspension in paraffin; 0.40 g,
10.0 mmol) with di-tert-butyl dicarbonate (2.20 g, 10.1 mmol)
by the method described above yielded 0.94 g (2.06 mmol, 73%)
of pure 2b as a white solid: mp 141 °C; IR (KBr) 1753, 1609
cm-1; 1H NMR δ 1.16 (s, 9 H), 1.81 (br, 3 H), 2.00 (br. s, 6 H),

3276 J . Org. Chem., Vol. 66, No. 10, 2001
Schmittel et al.
computer using HP data transfer program Scopelink. The
ratios Ipc/Ipa were determined according to the equation of
Nicholson.48
Digita l Sim u la tion s of th e Cyclic Volta m m ogr a m s. The
computer simulation of the redox chemistry of the examined
reaction steps were varied to achieve the best possible agree-
ment with the experimental curves.
EP R. Radical cations were generated by reaction of dioxy-
genyl hexafluoroarsenate with the neutral compound in de-
gassed chlorodifluoromethane at 150 K at a high vacuum line.
The mixture was degassed at <10-5 mbar and poured into a
test tube, which was sealed off. The EPR spectra were recorded
on a Bruker ESP300 EPR spectrometer.
compounds was carried out on
a PC using the Crank-
Nicholson technique49 and DigiSim.50 The simulation of a full-
cycle voltammogram consisted of 1000 data points, allowing
for an acceptable resolution of the cyclic voltammograms. All
chemical reactions were assumed to be reversible except the
ET equilibria. With a standard heterogeneous electron-transfer
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support by the Deutsche Forschungsgemein-
schaft (SFB 347 and Schwerpunkt “Radikale in der
enzymatischen Katalyse). In addition, we are most
indebted to the Fonds der Chemischen Industrie for the
continuous support for our research. We thank the
Degussa for a generous gift of electrode materials.
constant of k0
) 1 cm‚s-1 and standard diffusion coef-
hetero
ficients of D ) 10-5 cm2‚s-1, the rate constants for the chemical
(47) Amatore, C.; Lefrou, C.; Pflu¨gler, F. J . Electroanal. Chem.
Interfacial 1989, 270, 43.
(48) Nicholson, R. S. Anal. Chem. 1966, 38, 6.
(49) (a) Lasia, A. J . Electroanal. Chem. Interfacial Electrochem.
1983, 146, 397. (b) Heinze, J .; Sto¨rzbach, M.; Mortensen, J . J .
Electroanal. Chem. Interfacial Electrochem. 1984, 165, 61. (c) Heinze,
J .; Sto¨rzbach, M. J . Electroanal. Chem. Interfacial Electrochem. 1993,
346, 1.
(50) (a) Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem.
1994, 66, 589A. (b) Rudolph, M. J . Electroanal. Chem. Interfacial
Electrochem. 1992, 338, 86.
Su p p or tin g In for m a tion Ava ila ble: Details of digital
simulation of cyclic voltammograms and crystallographic
information files are available free of charge via the Internet
at http://pubs.acs.org.
J O0057093