The role of stereoelectronic effects on the side-chain fragmentation of alkylaromatic radical cations. The reactivity of 5-methoxy-2,2-dimethylindan-1-ol radical cation

Article abstract of DOI:10.1016/S0040-4020(02)00468-4

A kinetic and products study of the reaction of 2,2-dimethyl-5-methoxyindan-1-ol (1) radical cation, in acidic aqueous solution (pH≤4) has been carried out. 1·⁺ undergoes C-H deprotonation as the exclusive reaction with k=4.6×10⁴s^-1. The kinetic data have been compared with those obtained for the radical cations of 1-(4-methoxyphenyl)ethanol (2) and 1-(4-methoxyphenyl)-2,2-dimethyl-1-propanol (3), suggesting that the deprotonation rate increases when the C-H bond is forced into a conformation where it is almost aligned with the π-system. The conclusion that overlap between the scissile bond and the π-system is an important requisite for the occurrence of bond cleavage is also supported by the results of DFT calculations carried out for 1·⁺ and 3·⁺.

Full text of DOI:10.1016/S0040-4020(02)00468-4

TETRAHEDRON
Pergamon
Tetrahedron 58 '2002) 5039±5044
The role of stereoelectronic effects on the side-chain
fragmentation of alkylaromatic radical cations. The reactivity
of 5-methoxy-2,2-dimethylindan-1-ol radical cation
Monica Bellanova, Massimo Bietti,p Gianfranco Ercolani and Michela Salamone
Á
Dipartimento di Scienze e Tecnologie Chimiche, Universita `Tor Vergata', Via della Ricerca Scienti®ca, I-00133 Rome, Italy
Received 25 February 2002; revised 4 April 2002; accepted 2 May 2002
AbstractÐAkinetic and products study of the reaction of 2,2-dimethyl-5-methoxyindan-1-ol ' 1) radical cation, in acidic aqueous solution
'pH#4) has been carried out. 1z1 undergoes C±H deprotonation as the exclusive reaction with kˆ4.6£104 s21. The kinetic data have been
compared with those obtained for the radical cations of 1-'4-methoxyphenyl)ethanol '2) and 1-'4-methoxyphenyl)-2,2-dimethyl-1-propanol
'3), suggesting that the deprotonation rate increases when the C±H bond is forced into a conformation where it is almost aligned with the
p-system. The conclusion that overlap between the scissile bond and the p-system is an important requisite for the occurrence of bond
cleavage is also supported by the results of DFT calculations carried out for 1z1 and 3z1. q 2002 Elsevier Science Ltd. All rights reserved.
Side-chain reactions of alkylaromatic radical cations mostly
involve the cleavage of bonds between the a and b atoms,
C±H and C±C bond cleavage being the most common frag-
mentation patterns of these reactive intermediates.1 These
reactions play a key role in synthetically useful processes2
as well as in the chemical and enzymatic oxidative degrada-
tion of lignin and lignin model compounds to give lower
molecular weight aromatic compounds.3
data have been obtained, based on the results of product
studies 'inter- and intramolecular selectivities).
Evidence in favor of stereoelectronic control of fragmen-
tation in C±C bond cleavage reactions of alkylaromatic
radical cations has been provided by Arnold for a variety
of systems.7,8 For example, product studies have shown that
while 2,2-diphenyl-1-methoxyethane radical cation under-
goes C±C bond cleavage,9 no products deriving from this
pathway but only from a reversible C±H deprotonation
reaction are observed with the radical cations of the cyclic
analogues 3-phenyl-2,3-dihydrobenzofuran, 5-methyl-3-
phenyl-2,3-dihydrobenzofuran, cis- and trans-2-methoxy-
1-phenylindane; all incorporating the b,b-diphenylethyl
methyl ether moiety, where alignment between the scissile
C±C bond and the p-system is prevented by the presence of
the ®ve-membered ring.8
Avery important aspect of the reactivity of alkylaromatic
radical cations is that the rate of side-chain fragmentation
can be affected by the relative orientation between the scis-
sile bond and the aromatic p-system 'stereoelectronic
effect). As illustrated in Fig. 1, the most suitable orientation
for cleavage is the one where the dihedral angle between the
plane of the aromatic ring and the plane de®ned by the
scissile bond and the atom of the aromatic ring to which
this bond is connected is 908. In this conformation the
scissile bond is aligned with the p-system bearing the
unpaired electron, and the best orbital overlap for bond
cleavage can be achieved.4
Along these lines, in order to obtain more information on the
role of stereoelectronic effects on the side-chain fragmen-
tation of alkylaromatic radical cations, and in view of the
lack of absolute kinetic data for these processes, we have
The importance of stereoelectronic effects on the side-chain
deprotonation of alkylaromatic radical cations has been
pointed out by the studies of Tolbert and Baciocchi, dealing
with 9-alkyl- and 9,10-dialkylanthracene radical cations,5
and with alkylbenzene, 1,4-dialkylbenzene, 5,6-dimethyl-
and 2,2,5,6-tetramethylindane radical cations.6 Quantitative
Figure 1. The most suitable orientation for C±H bond cleavage in an
alkylaromatic radical cation. For the sake of clarity, in the former structure
the two additional groups bonded to the benzylic carbon have been omitted.
In the latter structure, the bold line represents the plane of the aromatic ring.
Keywords: stereoelectronic effect; radical cation; pulse radiolysis; deproto-
nation; DFT calculations.
Corresponding author. Fax: 139-06-72594328; e-mail: bietti@uniroma2.it
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020'02)00468-4

5040
M. Bellanova et al. / Tetrahedron 58 .2002) 5039±5044
Scheme 1.
carried out a kinetic and product study of the reactivity of
2,2-dimethyl-5-methoxyindan-1-ol '1) radical cation, a
substrate where again the presence of the ®ve-membered
ring prevents a favorable alignment between the scissile
C±C bond and the p-system, favoring instead that with
the C±H bond.
is a well-known one-electron chemical oxidant able to
oxidize methoxybenzene derivatives via outer-sphere
electron transfer.11±13 SO4z2 is a strong oxidant which is
able to react with aromatic substrates via electron transfer
to yield the corresponding radical cations with
k<109 M21 s21 'Eq. '1)).14±16
SO4 z2 1ArCHꢀX†R ! SO242 1 Ar z1 CHꢀX†R
ꢀ1†
SO4z2 has been generated either photochemically by steady-
state irradiation 'lˆ254 nm) of aqueous K2S2O8 solutions
'Eq. '2));
hn
S2O282 ! 2SO4z2
ꢀ2†
254 nm
or by chemical radiation 'pulse and steady-state g-radio-
lysis) according to Eqs. '3)±'5).
H2O
H1; zOH; e2aq
ꢀ3†
_
zOH 1 CH3CꢀCH3†2OH ! H2O 1 CH2CꢀCH3†2OH
ꢀ4†
ꢀ5†
e2aq 1 S2O822 ! SO422 1 SO4z2
Moreover, to provide a quantitative evaluation of these
effects, the kinetic data have been compared with those
obtained analogously for the radical cations of 1-'4-meth-
oxyphenyl)ethanol '2) and 1-'4-methoxyphenyl)-2,2-di-
methyl-1-propanol '3),10 substrates structurally related to
1, lacking however the rigidity imposed by the presence
of the ®ve-membered ring. Additional information comes
from the results of DFT calculations for the determination of
the equilibrium geometries of 1z1 and 3z1, employing the
method UB3LYP/6-31G'd).
Radiolysis of water leads to the formation of the hydroxyl
radical 'zOH) and the hydrated electron 'eaq2) 'Eq. '3)). The
former is scavenged by 2-methyl-2-propanol 'Eq. '4);
2
kˆ6£108 M21 s21),14 while eaq reacts with the peroxy-
disulfate anion leading to the formation of SO4z2 'Eq. '5);
kˆ1.2£1010 M21 s21).14
1.2. Product studies
The oxidation of 1 was carried out at pH 3.1 employing
Co'III)W or SO4z2 as the oxidant, showing in all cases the
exclusive formation of 5-methoxy-2,2-dimethylindan-1-
one.
1. Results and discussion
1.1. Generation of the radical cation
Ketone formation can be readily explained on the basis of
the formation and deprotonation of an intermediate radical
cation leading to a 1-hydroxyindanyl radical, which is then
oxidized to give 5-methoxy-2,2-dimethylindan-1-one
'Scheme 1), in analogy with one-electron oxidations of
1z1 was generated in aqueous solution employing either
potassium 12-tungstocobalt'III)ate 'from now on simply
indicated as Co'III)W) or SO4z2 as the oxidant. Co'III)W

M. Bellanova et al. / Tetrahedron 58 .2002) 5039±5044
5041
Figure 2. Adiabatic potential energy curve for the internal rotation about the benzylic bond of 3z1 calculated by the UB3LYP/6-31G'd) method. The dihedral
angle w is measured between the benzylic C±H bond and the aromatic ring plane. The points refer to the Newman projection formulae shown in Scheme 2.
arylalkanols.17 Thus, the exclusive reaction of 1z1 is
deprotonation from C-1.
cleavage with kˆ1.5£105 s21 10
. This behavior is attributed
to a stereoelectronic effect since the presence of the bulky
tert-butyl group disfavors the most suitable conformation
for C±H bond cleavage, where the scissile C±H bond is
aligned with the p-system due to its interaction with the
ortho-hydrogens. Unfortunately, because the radical cations
1z1 and 3z1 are relative short lived intermediates, this
hypothesis cannot be easily con®rmed by direct observation
'ESR) of the radical cations. It is however nicely supported
by the results of DFT calculations.
1.3. Kinetic studies
These were carried out using the pulse radiolysis technique.
The time-resolved absorption spectra of 1z1 at pH 3.5 and
room temperature show the characteristic UV and visible
absorption bands of anisole-type radical cations centered at
290 and 460 nm.10,15,18 The decay rate of the radical cation
was measured spectrophotometrically following the
decrease in optical density at 460 nm. Under these con-
ditions, 1z1 decays with ®rst-order kinetics with kˆ
4.6£104 s21, in a reaction which, on the basis of product
analysis results, is assigned to C±H deprotonation.
The adiabatic potential energy curve for the rotation about
the benzylic bond of 3z1 has been calculated by the
UB3LYP/6-31G'd) method with a resolution of 208
'Fig. 2). It shows that the two conformations where the
scissile C±H bond is aligned with the p-system
'Scheme 2, conformations I and IV) are characterized by
signi®cantly higher energies '<21±27 kJ mol21) than those
most suitable for C±C bond cleavage 'conformations II and
V) where is the C±C'CH3)3 bond to be aligned with the
p-system.
The rate constant for deprotonation from 1z1,
kˆ4.6£104 s21, can be compared with that measured
under analogous conditions for 2z1, which also undergoes
exclusive deprotonation with kˆ7.0£103 s21 10
.
These
results indicate that when the C±H bond is forced into a
conformation where it is almost aligned with the p-system
as in 1z1, the deprotonation rate increases signi®cantly 'ca.
6.5 times) as compared to 2z1, where all conformations are
instead accessible for the scissile C±H bond.19
In particular, conformations II and V are very close in energy
to the two minima at wˆ2348 and 144.58, respectively.
With 1z1, the results of DFT calculations show that the
most stable structure is that in which the scissile C±H
bond is almost aligned with the p-system 'pseudo axial),
It is also very interesting to compare the reactivity of 1z1
with that of 3z1 which undergoes exclusive C±C bond
Scheme 2.

5042
M. Bellanova et al. / Tetrahedron 58 .2002) 5039±5044
unpaired electron is an important requisite for the occur-
rence of bond cleavage, a result which is nicely supported
by DFT calculations carried out for 1z1 and 3z1.
However, Gilbert pointed out that there could be exceptions
to this behavior when speci®c structural features favor
C±C bond cleavage without stereoelectronic assistance.
Accordingly, 2-indanol radical cation undergoes C±C
bond cleavage yielding the 2-'2-oxoethyl)benzyl radical,21
a result which has been explained in terms of the loss of ring
strain resulting from ring-opening and the stability of the
radical and cation fragments formed.
An alternative explanation able to account for this behavior
might also be that under the experimental conditions
employed by Gilbert, the 2-indanol radical cation undergoes
a-OH deprotonation, followed by C±C bond cleavage in an
intermediate 2-indanyloxyl radical and/or 2-indanol radical
zwitterion 'Scheme 3), as described for 1-'4-methoxy-
phenyl)-2-alkanol radical cations.22,23
Figure 3. Most stable conformation for 1z1, optimized by the UB3LYP/
6-31G'd) method. All hydrogen atoms except the benzylic one bonded to
C-1 have been omitted for clarity.
characterized by a dihedral angle between the benzylic C±H
bond and the aromatic ring plane of wˆ92.48 'Fig. 3).
Aconformation where the C±C bond is aligned with the
p-system is impossible to reach in this case because of
the very high strain energy involved, and accordingly
deprotonation is the exclusive reaction pathway for 1z1.
By assuming that the amount of products deriving from
C±H bond cleavage in 3z1 is #0.5%,10,18 it is possible
to set an upper limit for the deprotonation rate as
k#7.5£102 s21, a value which indicates that the deproto-
nation rate increases at least 60 times on going from 3z1
to 1z1.
2. Experimental
2.1. Reagents
Potassium peroxydisulfate, perchloric acid and 2-methyl-2-
propanol were of the highest commercial quality available.
Milli-Q-®ltered 'Millipore) water was used for all solu-
tions. Potassium 12-tungstocobalt'III)ate was prepared as
described previously.13
In conclusion, this work provides evidence for the impor-
tance of stereoelectronic effects in the deprotonation of
alkylaromatic radical cations, in full agreement with the
studies of Tolbert5 and Baciocchi.6 In the radical cations
of indan-1-ol derivatives, an increase in deprotonation rate
results from the alignment between the scissile C±H bond
and the p-system imposed by the presence of the ®ve-
membered ring. The absolute rate constant for deproto-
nation of 1z1 has been measured, providing a quantitative
evaluation of this effect. In contrast, C±C bond cleavage
appears to be strongly depressed in these systems, as
shown by the comparison between two structurally related
radical cations 1z1 and 3z1, undergoing exclusive C±H
and C±C bond cleavage, respectively. This result clearly
indicates that in alkylaromatic radical cations, overlap
between the scissile bond and the p-system containing the
2,2-Dimethyl-5-methoxyindan-1-ol '1) was prepared by
reduction of 2,2-dimethyl-5-methoxyindan-1-one with
1
NaBH4 in 2-propanol. H NMR 'CDCl3), d 'ppm): 7.29±
6.73 'm, 3H, ArH); 4.60 's, 1H, ArCHOH); 3.79 's, 3H,
OCH3); 2.81±2.56 'm, 2H, CH2); 1.14 's, 3H, CH3); 1.06
's, 3H, CH3).24
2,2-Dimethyl-5-methoxyindan-1-one was prepared by reac-
tion of 5-methoxyindan-1-one with sodium hydride and
excess methyl iodide in anhydrous tetrahydrofuran. 1H
NMR 'CDCl3), d 'ppm): 7.71±6.86 'm, 3H, ArH); 3.88 's,
3H, OCH3); 2.95 's, 2H, CH2); 1.22 's, 6H, CH3). 13C NMR
'CDCl3), d 'ppm): 209.68 'CO); 165.44 'Ar C-6); 155.14
'Ar C-9); 128.45 'Ar C-5); 126.07 'Ar C-4); 115.38 'Ar
C-7); 109.7 'Ar C-8); 55.6 'OCH3); 45.62 'C-2); 42.91
'2CH3); 25.38 'C-3).24
Scheme 3.

M. Bellanova et al. / Tetrahedron 58 .2002) 5039±5044
5043
2.2. Product analysis
coordinates in steps of 208. The energy results reported in
Fig. 2 are relative to the global minimum structure corre-
sponding to wˆ144.58. The calculations relative to 1z1
indicated the conformation with the benzylic hydrogen in
pseudo axial position 'wˆ92.48) as the most stable 'Fig. 3),
whereas that with the benzylic hydrogen in pseudo-
equatorial position 'wˆ45.78) resulted to be 3.5 kcal mol21
higher in energy.
All reactions were carried out in a 50 mM citrate buffer
solution 'pH 3.1). Reaction products were identi®ed
and quantitatively determined by GC 'comparison with
authentic samples) and GC±MS analysis. In order to
check the stability of the substrates to reaction conditions,
blank experiments were performed under every condition
showing, in the reactions with K2S2O8, the presence of
negligible amounts of oxidation products.
Acknowledgements
Oxidations induced by Co'III)W were performed at
Tˆ508C. In a typical experiment, 5 mL of an argon-satu-
rated solution containing the substrate '1±2 mM) and
Co'III)W '1±2 mM) were stirred until complete conversion
of the oxidant. Workup was performed as described
previously.12
This work was carried out into the framework of the
EU project `Towards Ef®cient Oxygen Deligni®cation'
'Contract No. QLK5-CT-1999-01277). We thank Dr
Pedatsur Neta 'NIST Gaithersburg, MD) for assistance
on the pulse radiolysis experiments, Professor Enrico
Baciocchi for helpful discussion, and Mr Luigi Gastaldo
'Helios Italquartz s.r.l.) for providing us with a photo-
chemical reactor.
Steady-state photolysis was carried out employing a Photo-
chemical Multirays Reactor 'Helios Italquartz) equipped
with 10£15 W 254 nm lamps. Five milliliters of an argon-
saturated solution containing 1 '2 mM) and K2S2O8 '0.1 M)
was irradiated at room temperature for 1 min showing
quantitative conversion of 1 in 2,2-dimethyl-5-methoxy-
indan-1-one.
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