Kinetic solvent effects on hydrogen abstraction reactions from carbon by the cumyloxyl radical. the importance of solvent hydrogen-bond interactions with the substrate and the abstracting radical

Article abstract of DOI:10.1021/jo200660d

A kinetic study of the hydrogen atom abstraction reactions from propanal (PA) and 2,2-dimethylpropanal (DMPA) by the cumyloxyl radical (CumO ^?) has been carried out in different solvents (benzene, PhCl, MeCN, t-BuOH, MeOH, and TFE). The corresponding reactions of the benzyloxyl radical (BnO^?) have been studied in MeCN. The reaction of CumO^? with 1,4-cyclohexadiene (CHD) also has been investigated in TFE solution. With CHD a 3-fold increase in rate constant (k_H) has been observed on going from benzene, PhCl, and MeCN to TFE. This represents the first observation of a sizable kinetic solvent effect for hydrogen atom abstraction reactions from hydrocarbons by alkoxyl radicals and indicates that strong HBD solvents influence the hydrogen abstraction reactivity of CumO ^?. With PA and DMPA a significant decrease in k_H has been observed on going from benzene and PhCl to MeOH and TFE, indicative of hydrogen-bond interactions between the carbonyl lone pair and the solvent in the transition state. The similar k_H values observed for the reactions of the aldehydes in MeOH and TFE point toward differential hydrogen bond interactions of the latter solvent with the substrate and the radical in the transition state. The small reactivity ratios observed for the reactions of CumO^? and BnO^? with PA and DMPA (k _H(BnO^?)/k_H(CumO^?) = 1.2 and 1.6, respectively) indicate that with these substrates alkoxyl radical sterics play a minor role.

Full text of DOI:10.1021/jo200660d

ARTICLE
pubs.acs.org/joc
Kinetic Solvent Effects on Hydrogen Abstraction Reactions
from Carbon by the Cumyloxyl Radical. The Importance
of Solvent Hydrogen-Bond Interactions with the Substrate
and the Abstracting Radical
Michela Salamone, Ilaria Giammarioli, and Massimo Bietti*
Dipartimento di Scienze e Tecnologie Chimiche, Universitꢀa “Tor Vergata”, Via della Ricerca Scientifica, 1 I-00133 Rome, Italy
S Supporting Information
b
ABSTRACT: A kinetic study of the hydrogen atom abstraction
reactions from propanal (PA) and 2,2-dimethylpropanal
(DMPA) by the cumyloxyl radical (CumO•) has been carried
out in different solvents (benzene, PhCl, MeCN, t-BuOH,
MeOH, and TFE). The corresponding reactions of the benzy-
loxyl radical (BnO•) have been studied in MeCN. The reaction
of CumO• with 1,4-cyclohexadiene (CHD) also has been
investigated in TFE solution. With CHD a 3-fold increase in
rate constant (kH) has been observed on going from benzene,
PhCl, and MeCN to TFE. This represents the first observation
of a sizable kinetic solvent effect for hydrogen atom abstraction reactions from hydrocarbons by alkoxyl radicals and indicates that
strong HBD solvents influence the hydrogen abstraction reactivity of CumO•. With PA and DMPA a significant decrease in kH has
been observed on going from benzene and PhCl to MeOH and TFE, indicative of hydrogen-bond interactions between the carbonyl
lone pair and the solvent in the transition state. The similar kH values observed for the reactions of the aldehydes in MeOH and TFE
point toward differential hydrogen bond interactions of the latter solvent with the substrate and the radical in the transition state.
The small reactivity ratios observed for the reactions of CumO• and BnO• with PA and DMPA (kH(BnO•)/kH(CumO•) = 1.2 and
1.6, respectively) indicate that with these substrates alkoxyl radical sterics play a minor role.
’ INTRODUCTION
negligible KSEs. With substrates that are not solvated to a
significantly greater extent in polar relative to nonpolar solvents
(e.g., cyclohexane and 1,4-cyclohexadiene), this behavior has
been explained on the basis of the lack of solvent effects on the
reactivity of the abstracting alkoxyl radical.19,20 In this context, we
have recently observed very similar rate constants for hydrogen
abstraction (kH) from 1,4-cyclohexadiene (CHD) by the cumy-
loxyl radical (CumO•) when the reaction was studied in MeCN,
benzene, or chlorobenzene.16 However, a small (∼20%) and
reproducible increase in kH was observed when the same reaction
was studied in MeOH and t-BuOH, suggesting that in these
solvents hydrogen-bond interactions with the alkoxyl radical may
play a role.
With substrates that bear polar groups in proximity of the
abstractable hydrogen atom such as triethylamine (TEA), we
have recently observed a 7-fold decrease in rate constant for
R-CꢀH abstraction by CumO• on going from apolar solvents
(benzene, chlorobenzene) to MeOH.16 This behavior has been
explained in terms of a hydrogen-bond interaction between the
nitrogen lone pair and the solvent. This interaction results in
Hydrogen atom abstraction by alkoxyl radicals has attracted
considerable interest as these reactions play a key role in a variety of
important chemical and biological processes such as the oxida-
tive damage to biomolecules and polymers,1ꢀ8 the radical scaven-
ging activity of natural and synthetic antioxidants,9ꢀ11 and the
degradation of volatile organic compounds in the atmosphere.12
One aspect that has received particular attention is the study of
solvent effects on these reactions.9,13ꢀ20 For example, Ingold and
co-workers observed dramatic kinetic solvent effects (KSEs) for
hydrogen atom abstractions from phenols by alkoxyl radicals,9,13
where a decrease in rate constant (kH) was observed with
increasing the solvent hydrogen bond acceptor (HBA) ability,
and good correlations were obtained between log kH and the
solvent HBA parameter β2 .
H 21 This effect was explained on the
basis of a hydrogen bond interaction between the phenolic OH
group and the solvent. Accordingly, in relatively strong HBA
solvents the substrate must experience desolvation in order to
undergo hydrogen atom abstraction, and consequently, a de-
crease in reactivity is observed as compared to weaker or non-
HBA solvents.
On the other hand, when dealing with CꢀH bonds it is
Received: March 29, 2011
Published: April 28, 2011
generally assumed that abstractions by alkoxyl radicals display
r
2011 American Chemical Society
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Scheme 1
constants for hydrogen abstraction from PA and DMPA by the
benzyloxyl radical (BnO•) have also been determined, limited,
however, to MeCN solvent.29
’ RESULTS
The reactions of CumO• and BnO• with the substrates shown
above have been studied using the laser flash photolysis (LFP)
technique. CumO• and BnO• have been generated by 266 nm
LFP of nitrogen-saturated MeCN and, for the former radical,
TFE solutions (T = 25 °C) containing dicumyl and dibenzyl
peroxide, respectively (eq 2). CumO• has been also generated by
355 nm LFP of nitrogen-saturated MeCN, benzene, PhCl,
t-BuOH, and MeOH solutions (T = 25 °C) containing dicumyl
peroxide (eq 2).
a decrease in the degree of overlap between the R-CꢀH bond
and the nitrogen lone pair in the transition state for hydrogen
atom abstraction, thus lowering the reactivity. To our knowledge,
this represents the largest KSE observed for hydrogen atom
abstraction from carbon by an alkoxyl radical.
Aldehydes are known to react efficiently with alkoxyl radicals
via hydrogen atom abstraction from the carbonyl CꢀH bond to
give acyl radicals (eq 1).23ꢀ28
Second-order rate constants (kH) between 107 and 108 Mꢀ1 sꢀ1
have been measured for the reaction of aromatic and aliphatic
aldehydes with the tert-butoxyl radical (t-BuO•).26 In this study,
comparable rate constants have been measured for propanal and
In MeCN solution, CumO• is characterized by a broad
absorption band in the visible region of the spectrum centered
at 485 nm,31,32 whose position is red-shifted in MeOH and TFE
benzaldehyde (kH = 8.9 ꢁ 107 and 6.8 ꢁ 107 Mꢀ1 sꢀ1
,
(λmax = 50033 and 515 nm,34 respectively). For BnO• λmax
=
460 nm in acetonitrile.31,35 Under these conditions, CumO•
decays mainly by CꢀCH3 β-scission,20,32 with the exception of
the experiments carried out in MeOH solution where hydrogen
atom abstraction from the solvent is instead the predominant
reaction.33 The decay of BnO• can be mainly attributed to
hydrogen atom abstraction from the solvent.30
respectively), whereas a significant decrease in rate constant
has been observed when the alkyl or aryl group has been replaced
by an electron-withdrawing substituent such as dimethylamino in
N,N-dimethylformamide and ethoxy in ethyl formate. The ob-
served reduction in rate constant has been explained in terms of
the greater contribution of the polar structure (Scheme 1,
structure B) to the transition state for hydrogen atom abstraction
from XCHO by t-BuO• for benzaldehyde and propanal (X = Ph
and Et, respectively) as compared to N,N-dimethylformamide
and ethyl formate (X = NMe2 and OEt, respectively).26
Quite interestingly, this explanation points toward a relatively
polar transition state for hydrogen atom abstraction from
aldehydes and suggests in particular that these reactions may
experience sizable KSEs.
The time-resolved spectra observed after reaction of CumO•
with DMPA in MeCN solution are reported in the Supporting
Information (Figure S2).
The reactions of CumO• and BnO• with PA and DMPA have
been studied by LFP. As mentioned above, it is well-established
that these reactions proceed by hydrogen atom abstraction from
the carbonyl CꢀH bond by the alkoxyl radical, as described in
eq 1.23ꢀ28
Along this line, in order to probe this issue and to provide a
deeper and more general understanding of the role of solvent
effects on hydrogen atom abstraction reactions from carbon by
alkoxyl radicals, we have carried out a detailed time-resolved
kinetic study in different solvents (benzene, chlorobenzene
(PhCl), acetonitrile (MeCN), 2-methyl-2-propanol (t-BuOH),
methanol (MeOH), and 2,2,2-trifluoroethanol (TFE)) for the
reactions of CumO• with propanal (PA) and 2,2-dimethylpro-
panal (DMPA), whose structures are displayed below.
The kinetic studies have been carried out by LFP following the
decay of the CumO• and BnO• visible absorption bands at
490ꢀ515 and 460 nm, respectively, as a function of the aldehyde
concentration. The observed rate constants (kobs) gave excellent
linear relationships when plotted against substrate concentration
and provided the second-order rate constants for hydrogen atom
abstraction from the substrates by CumO• and BnO• (kH) from
the slopes of these plots. As an example, the plots of kobs vs
[DMPA] and of kobs vs [CHD] for the reactions between
CumO• and both DMPA (filled circles) and CHD (empty
circles) carried out in benzene (a) and TFE (b) solution are
shown in Figure 1. In these plots, the 1 order of magnitude
increase in the intercept value observed on going from benzene
to TFE reflects the significantly faster β-scission reaction of
CumO• in the latter solvent as compared to the former
one.20,32,34 Additional plots for hydrogen atom abstraction by
CumO• and BnO• from the two aldehydes in the different
solvents are displayed in the Supporting Information (Figures
S3ꢀS16). All the kinetic data thus obtained for the reactions
of CumO• are collected in Table 1. The hydrogen atom abstrac-
tion reactivity of CumO• and BnO• in MeCN solution is
compared in Table 2, where the kH(BnO•)/kH(CumO•) ratios
are also displayed. Also included in the two Tables are the rate
constants obtained previously under analogous experimental
The results obtained are discussed and compared with those
previously obtained for the corresponding reactions of CumO•
with 1,4-cyclohexadiene (CHD) and triethylamine (TEA),16
where however, with the former substrate, in order to probe
the possible role of hydrogen bond interactions between the
solvent and the alkoxyl radical, the rate constant for reaction in
TFE has also been measured. In order to gain information on the
role of alkoxyl radical structure on these processes, the rate
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Figure 1. Plots of the observed rate constant (kobs) against [substrate] for the reactions of the cumyloxyl radical (CumO•) with 2,2-dimethylpropanal
(filled circles) and 1,4-cyclohexadiene (empty circles), measured in nitrogen-saturated benzene (a) and TFE (b) solutions at T = 25 °C, following the decay
of CumO• at 490 and 515 nm. From the linear regression analysis: (a) CumO• þ DMPA in benzene: intercept = 5.97 ꢁ 105 sꢀ1, kH = 5.09 ꢁ 107 Mꢀ1 sꢀ1
,
r2 = 0.9972; CumO• þ CHD in benzene: intercept = 6.05 ꢁ 105 sꢀ1, kH = 6.81 ꢁ 107 Mꢀ1 sꢀ1, r2 = 0.9974. (b) CumO• þ DMPA in TFE: intercept =
6.39 ꢁ 106 sꢀ1, kH = 1.09 ꢁ 107 Mꢀ1 sꢀ1, r2 = 0.9960; CumO• þ CHD in TFE: intercept =6.23 ꢁ 106 sꢀ1, kH = 1.86 ꢁ 108 Mꢀ1 sꢀ1, r2 = 0.9972.
Table 1. Second-Order Rate Constants (kH) for Reaction of the Cumyloxyl (CumO•) Radical with Hydrogen Atom Donors
Measured in Different Solvents at T = 25 °C.a
kH/Mꢀ1 sꢀ1c
solvent
λexb (nm)
PA
DMPA
CHD
TEA
benzene
PhCl
355
355
266
355
355
355
266
(4.27 ( 0.07) ꢁ 107
(4.89 ( 0.02) ꢁ 107
(1.76 ( 0.02) ꢁ 107
(2.23 ( 0.05) ꢁ 107
(3.58 ( 0.02) ꢁ 107
(9.55 ( 0.03) ꢁ 106
(9.3 ( 0.3) ꢁ 106
(5.0 ( 0.3) ꢁ 107
(4.47 ( 0.02) ꢁ 107
(2.33 ( 0.07) ꢁ 107
(2.69 ( 0.05) ꢁ 107
(3.7 ( 0.2) ꢁ 107
(1.7 ( 0.1) ꢁ 107
(1.04 ( 0.05) ꢁ 107
(6.79 ( 0.02) ꢁ 107d
(6.90 ( 0.05) ꢁ 107d
(6.65 ( 0.02) ꢁ 107d
(6.56 ( 0.01) ꢁ 107d
(8.3 ( 0.1) ꢁ 107d
(2.8 ( 0.1) ꢁ 108d
(2.7 ( 0.1) ꢁ 108d
(2.0 ( 0.1) ꢁ 108d
(2.19 ( 0.05) ꢁ 108d
(1.61 ( 0.03) ꢁ 108d
(3.8 ( 0.1) ꢁ 107d
MeCN
t-BuOH
MeOH
TFE
(8.25 ( 0.03) ꢁ 107d
(1.89 ( 0.03) ꢁ 108
a 266 nm LFP, N2-saturated, [dicumyl peroxide] = 10 mM. 355 nm LFP, N2-saturated, [dicumyl peroxide] = 0.7ꢀ1.0 M. b Laser excitation wavelength.
c Determined from the slope of the kobs vs [substrate] plots, where in turn kobs values have been measured following the decay of the CumO• visible
absorption band at 490ꢀ515 nm. Average of at least two determinations. d Reference 16.
Table 2. Second-Order Rate Constants (kH) for Reaction of the
Cumyloxyl (CumO•) and Benzyloxyl (BnO•) Radicals with
Hydrogen Atom Donors Measured in MeCN at T = 25 °Ca
’ DISCUSSION
A first and very important observation is represented by the
3-fold increase in reactivity measured for the reaction between
CumO• and CHD on going from benzene, PhCl, and MeCN to
TFE (Table 1). This represents the first observation of a sizable
KSE for hydrogen atom abstraction reactions from hydrocarbons
by alkoxyl radicals and clearly indicates that strong hydrogen
bond donor (HBD) solvents can influence the reactivity of
alkoxyl radicals in these processes, in contrast with the results
of previous studies, that were, however, limited to weaker HBD
or non-HBD solvents.13,19,20,36 In the transition state of the
CumO• þ CHD reaction, electron density is moved toward the
oxygen of the alkoxyl radical, leading to a certain degree of charge
separation with the development of a partial negative charge on
the oxygen atom and a partial positive charge on the incipient
carbon centered radical (Scheme 2, X = PhC(CH3)2, where, for
the sake of simplicity, CHD is represented as RꢀH).
kH/Mꢀ1 sꢀ1b
kH(BnO•)/
kH(CumO•)
substrate
CumO•
BnO•
PA
(1.76 ( 0.05) ꢁ 107
(2.33 ( 0.07) ꢁ 107
(6.65 ( 0.02) ꢁ 107c
(2.0 ( 0.1) ꢁ 108c
(2.19 ( 0.02) ꢁ 107
(3.76 ( 0.06) ꢁ 107
(1.29 ( 0.02) ꢁ 108d
(4.3 ( 0.1) ꢁ 109d
1.2
1.6
DMPA
CHD
TEA
1.9
21.5
a 266 nm LFP, N2-saturated, [dicumyl peroxide] = 10 mM, [dibenzyl
peroxide] = 8 mM. b Determined from the slope of the kobs vs [substrate]
plots, where in turn kobs values have been measured following the decay of the
CumO• or BnO• visible absorption band at 490 and 460 nm, respectively.
Average of at least two determinations. c Reference 16. d Reference 35.
TFE can engage in hydrogen bonding with a CumO• oxygen
lone pair, and, on approaching the transition state, as a partial
negative charge develops on the oxygen atom the strength of the
conditions for the reactions of CumO• and BnO• with CHD and
TEA.16,35
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hydrogen bond interaction increases.38 This interaction results in
a greater extent of stabilization for the transition state as
compared to the reactants and in a corresponding increase in
hydrogen atom abstraction reactivity as compared to weaker or
non-HBD solvents. The slight increase in reactivity observed on
going from benzene, PhCl, and MeCN to MeOH and t-BuOH16
can be explained analogously in terms of a hydrogen-bond
interaction between the radical and the protic solvent, where
the significantly larger increase in kH observed on going from the
former solvents to TFE is in line with the greater HBD ability of
TFE as compared to MeOH and t-BuOH.37,40,41
A similar explanation in terms of the importance of polar
contributions to the transition state for hydrogen atom abstrac-
tion has been recently put forward by Tanko and co-workers to
account for the increase in reactivity observed for hydrogen atom
abstraction reactions from hydrocarbons (RꢀH) by the hydroxyl
radical (•OH) on going from MeCN to MeCN/H2O 9:1
(Scheme 2, X = H).42 As compared to MeCN, the presence of
H2•O, that can engage in hydrogen bonding with the oxygen atom
of OH, leads to a stabilization of the transition state and to a
corresponding increase in reactivity. Taken together, this me-
chanistic picture provides a rationale for the KSEs observed on
going from apolar solvents to strong HBD solvents for hydrogen
atom abstraction reactions from hydrocarbons by both alkoxyl
and hydroxyl radicals.
pointed out previously,26 the rate constant for hydrogen atom
abstraction reactions from aldehydes and formyl derivatives
XCHO by alkoxyl radicals is influenced by the nature of the X
group, decreasing by increasing the electron-withdrawing char-
acter of X. Along this line, the observed differences in reactivity
between PA and DMPA can be explained in terms of the slightly
higher electron-releasing effect of a tert-butyl group as compared
to an ethyl group.44
Very interestingly, the rate constants measured for reaction of
CumO• with PA and DMPA in benzene and chlorobenzene are
about 5 times higher than those measured in MeOH and TFE.
This observation points toward a hydrogen-bond interaction
between a carbonyl lone pair and the latter solvents, similar to the
one described previously for MeOH on the CumO• þ TEA
reaction.16 The KSEs observed in the reaction between CumO•
and TEA were explained on the basis of a hydrogen-bond
interaction between the nitrogen lone pair and the solvent, an
interaction that decreases the degree of overlap between the R-
CꢀH bond and the nitrogen lone pair in the transition state for
hydrogen atom abstraction. The acyl radicals formed following
hydrogen atom abstraction from both PA and DMPA are σ
radicals and their structure prevents efficient overlap between the
aldehydic CꢀH bond and an oxygen lone pair in the transition
state for hydrogen atom abstraction.
A more general description of the KSEs observed for these
reactions can be obtained by representing the transition states for
hydrogen atom abstraction from the aldehydes and from TEA by
CumO• in valence-bond terms,26,45 as the hybrids of structures
AꢀC and DꢀG, respectively (Scheme 3).
An implication of these findings is that when studying KSEs on
hydrogen atom abstraction reactions by oxygen-centered radi-
cals, in addition to solvent/substrate interactions, solvent/radical
interactions should also be considered whenever strong HBD
solvents are employed (see below).
On the basis of Scheme 3, the decrease in kH observed for PA
and DMPA on going from aprotic solvents to MeOH and TFE is
in line with the importance of polar contributions to the
transition state for hydrogen atom abstraction mentioned
above.26 Accordingly, when the solvent engages in hydrogen
bonding with the carbonyl oxygen atom, this interaction results
in a decrease in electron density at the incipient acyl radical
center and thus reduces the importance of the polar contribution
(structure B) to the transition state for hydrogen atom abstrac-
tion. A similar explanation can be also extended to the CumO• þ
TEA reaction where interaction of the solvent with the nitrogen
lone pair reduces the importance of structures E and F. Structure
G would instead account for the previously mentioned impor-
tance of orbital overlap between the R-CꢀH bond and the
nitrogen lone pair in the transition state.16,46ꢀ48
The kH value measured for hydrogen atom abstraction from
PA by CumO• in benzene solution is in reasonable agreement
with the one measured previously in benzene/di-tert-butyl per-
oxide 1:2, for the corresponding reaction of t-BuO•.26 With both
PA and DMPA, a slight decrease in kH has been observed in
MeCN on going from 355 to 266 nm LFP for CumO• generation.
As mentioned previously,43 these differences can be attributed to
variations in solution composition (and polarity) determined by
the significantly higher concentration of dicumyl peroxide em-
ployed in the 355 nmLFP experiments as compared to the 266nm
LFP ones ([dicumyl peroxide] = 1.0 and 0.01 M, respectively).
The data displayed in Table 1 show that in CꢀH abstraction
reactions from the formyl group by CumO• a slight increase in
reactivity is generally observed on going from PA to DMPA. As
Quite interestingly, a relatively small decrease in reactivity has
been observed for both PA and DMPA on going from MeOH to
TFE, despite the significantly greater HBD ability of TFE as
compared to MeOH.41 We propose that this behavior is the
result of differential hydrogen-bond interactions of the solvent
TFE with both the substrate and the alkoxyl radical. As discussed
above, the interaction of TFE with CumO• results in an increase
Scheme 2
Scheme 3
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in hydrogen atom abstraction reactivity for this radical, as
evidenced by the 3-fold increase in kH measured for the CumO• þ
CHD reaction on going from benzene, PhCl, and MeCN to
TFE. This difference can be reasonably taken as a quantitative
measure of the effect of the solvent (TFE) on the reactivity of
CumO•. The 3- to 5-fold decrease in reactivity observed for the
reactions between CumO• and both PA and DMPA on going
from benzene and PhCl to MeOH mostly reflects the effect of the
latter solvent on the reactivity of the aldehydes. This assumption
is based on the observation of a relatively small increase in kH
(∼20%) for the CumO• þ CHD reaction on going from
benzene, PhCl, and MeCN to MeOH (Table 1).16 In other
words, in TFE solution the decrease in reactivity determined by
hydrogen bonding between the solvent and the carbonyl
group is compensated by the increase in reactivity due to
the interaction of the solvent with the alkoxyl radical, resulting
in kH values that are very similar to those measured in MeOH,
a solvent where only the former interaction (solvent/sub-
strate) appears to play an important role. On the basis of the
transition-state representation of Scheme 3, while as described
above, MeOH destabilizes structure B via preferential hydro-
gen bonding with the aldehyde oxygen lone pair, the stronger
HBD solvent TFE can interact efficiently with the oxygen
atom of both the aldehyde and the radical. These differential
interactions result in an overall effect that is comparable to
that observed for MeOH.
In contrast with the results obtained for TEA, where a decrease
in reactivity was observed on going from MeCN to t-BuOH, an
opposite effect has been instead observed with PA and DMPA,
with kH that increases on going from MeCN to t-BuOH. At
present we do not have any clear-cut explanation for these subtle
solvent effects.49,50 Another point of interest is represented by
the decrease in reactivity observed for all four substrates on going
from benzene and PhCl to MeCN, with the effect that is
significantly more pronounced for TEA, PA, and DMPA as
compared to CHD and exceeds in particular a factor of 2 for
the aldehydes. This effect can be reasonably associated to the
weak HBD character of MeCN.52
In conclusion, by means of detailed time-resolved kinetic
studies, new information on the role of solvent effects on
hydrogen atom abstraction reactions from carbon by the cumy-
loxyl radical have been provided. The KSEs observed for the
reactions of the cumyloxyl radical with aldehydes and amines
have been explained on the basis of a general mechanistic scheme
where the importance of hydrogen-bond interactions of the
solvent with the heteroatom lone pair and, in strong HBD
solvents, the radical oxygen atom is highlighted. A 3-fold
increase in reactivity has been observed for the reaction
between the cumyloxyl radical and CHD on going from aprotic
solvents to TFE. This represents the first example of a sig-
nificant KSE for hydrogen atom abstraction from hydrocarbons
by alkoxyl radicals and is indicative of a hydrogen bond
interaction between the solvent and the alkoxyl radical. A
consequence of this finding is that in strong HBD solvents
solvent/radical interactions can play an important role in the
kinetics of hydrogen atom abstraction reactions by oxygen-
centered radicals.
’ EXPERIMENTAL SECTION
Materials. Acetonitrile, benzene, chlorobenzene, methanol, 2-methyl-
2-propanol, and 2,2,2-trifluoroethanol used in the laser flash photolysis
experiments were spectroscopic grade solvents.
1,4-Cyclohexadiene was of the highest commercial quality available
and was further purified prior to use by filtration over neutral aluminum
oxide. Commercial samples of propanal and 2,2-dimethylpropanal of the
highest commercial quality available were further purified prior to use by
distillation. The purity of the substrates employed in the kinetic studies
was checked by GC prior to the kinetic experiments and was in all cases
>99.5%.
Dicumyl peroxide was of the highest commercial quality available and
was used as received. Dibenzyl peroxide was prepared in small portions
by reaction of KO2 with benzyl bromide in dry benzene, in the presence
of 18-crown-6 ether, according to a previously described procedure.30,54
The product was purified by column chromatography (silica gel, eluent
hexane/dichloromethane 1:1) and identified by 1H NMR (see the
Supporting Information, Figure S1). 1H NMR (CDCl3): δ 7.34
(s, 10H, ArH), 4.95 (s, 4H, CH2).
Laser Flash Photolysis Studies. Laser flash photolysis (LFP)
experiments were carried out with a laser kinetic spectrometer using the
third harmonic (355 nm) or the fourth harmonic (266 nm) of a
Q-switched Nd:YAG laser, delivering 8 ns pulses. The laser energy
was adjusted to e10 mJ/pulse by the use of the appropriate filter.
A 3.5 mL Suprasil quartz cell (10 mm ꢁ 10 mm) was used for all
experiments. Nitrogen-saturated solutions of dicumyl peroxide
(0.01ꢀ1.0 M) or dibenzyl peroxide (0.008 M) were used. In the
266 nm LFP experiments a 10 mM concentration of dicumyl peroxide or
a 8 mM concentration of dibenzyl peroxide was employed. In the
355 nm LFP experiments, concentrations of dicumyl peroxide between
0.7 and 1.0 M were instead employed. In all experiments the conditions
were arranged in such a way as to ensure predominant light absorption
by the peroxide.
The hydrogen atom abstraction reactivity of CumO• and
BnO• toward PA and DMPA in MeCN solution has been also
compared by means of the kH(BnO•)/kH(CumO•) ratios. The
data displayed in Table 2 clearly show that the reactions of the
two radicals with PA and DMPA are characterized by relatively
small and comparable reactivity ratios (kH(BnO•)/kH(CumO•) =
1.2 and 1.6, respectively), indicating that with these substrates
the sterics of the alkoxyl radical play a minor role. The increase in
reactivity ratio observed on going from PA to DMPA reasonably
reflects the increased steric hindrance in proximity of the
carbonyl group in DMPA determined by the presence of the
two R-methyl groups. A similar ratio (kH(BnO•)/kH(CumO•) =
1.9) has been previously determined by us for the reactions of
CHD with the two radicals.35 We have recently shown that in
MeCN the reaction of BnO• with TEA proceeds through the
formation of a hydrogen bonded complex between the BnO•
R-CꢀH and the amine lone pair, wherein hydrogen atom
transfer occurs.35 Accordingly, with TEA a large decrease in kH
was observed on going from BnO• to CumO• (kH(BnO•)/
kH(CumO•) = 21.5). The large difference in kH(BnO•)/
kH(CumO•) observed between the aldehydes and TEA clearly
indicates that with the former substrates no such interaction is
operating. This observation is in full agreement with the lower
HBA ability of aldehydes as compared to trialkylamines.53
All the experiments were carried out at T = 25 ( 0.5 °C under
magnetic stirring. The observed rate constants were obtained by
averaging 3ꢀ6 individual values that were reproducible to within 5%.
Second-order rate constants for the reactions of the cumyloxyl
(CumO•) and benzyloxyl (BnO•) radicals with the hydrogen atom
donors were obtained from the slopes of the kobs (measured following
the decay of the CumO• and BnO• visible absorption bands at 490ꢀ515
and 460 nm, respectively) vs [substrate] plots. Fresh solutions were used
for every substrate concentration. Correlation coefficients were in all
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The Journal of Organic Chemistry
ARTICLE
cases >0.99. The given rate constants are the average of at least two
independent experiments, typical errors being <5%.
(7) Small, R. D., Jr.; Scaiano, J. C.; Patterson, L. K. Photochem.
Photobiol. 1979, 29, 49–51.
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Bꢁeviꢀere, S. D. J. Chem. Soc., Perkin Trans. 2 2000, 1193–1198.
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In order to assess the possible formation of hemiacetals following the
reaction of the aldehyde with the alcoholic solvent, the stability of
propanal and 2,2-dimethylpropanal in alcoholic solution was checked by
UVꢀvis spectroscopy monitoring the intensity of the aldehyde nfπ*
absorption band (in MeOH λmax = 285 and 290 nm, respectively) as a
function of time. In TFE no significant decrease in intensity was
observed after 1 h. In methanol and 2-methyl-2-propanol a e30%
decrease in intensity was instead observed after 1 h, a behavior that can
be reasonably associated to the formation of an hemiacetal. The decrease
in intensity was e3% after 5 min. Accordingly, in order to minimize
hemiacetal formation, in the kinetic experiments propanal and 2,2-
dimethylpropanal were added in pure form to thermostated cuvettes
containing dicumyl peroxide in methanol or 2-methyl-2-propanol, and
the solutions were photolyzed immediately after mixing. No significant
decrease in kobs was observed on kinetic traces obtained from successive
laser shots on the same solution, indicating negligible substrate con-
sumption (hemiacetal formation) during the experiment time. As a
matter of comparison, a significantly lower value of the rate constant for
hydrogen atom abstraction (kH = 4.2 ꢁ 106 Mꢀ1 sꢀ1 as compared to
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9.55 ꢁ 106 Mꢀ1
s
ꢀ1, see Table 1) was instead obtained when the
experiment was performed by successive additions of portions of a
solution of propanal in methanol to thermostatted cuvettes containing
dicumyl peroxide in methanol. This observation suggests that under
these conditions significant hemiacetal formation has occurred in the
parent methanolic propanal solution prior to addition.
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’ ASSOCIATED CONTENT
(20) Avila, D. V.; Brown, C. E.; Ingold, K. U.; Lusztyk, J. J. Am. Chem.
Soc. 1993, 115, 466–470.
(21) The solvent's HBA ability can be expressed in terms of the β2
H
S
Supporting Information.
Time-resolved spectra ob-
b
served after reaction of CumO• with DMPA. Plots of kobs vs
[substrate]. This material is available free of charge via the
Internet at http://pubs.acs.org.
parameter, which represents a general, thermodynamically related scale
of solute hydrogen-bond basicities in CCl4 and ranges in magnitude
from 0.00 for a non-HBA solvent such as an alkane to 1.00 for HMPA.22
(22) Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Morris, J. J.;
Taylor, P. J. J. Chem. Soc., Perkin Trans. 2 1990, 521–529.
(23) Chatgilialoglu, C.; Crich, D.; Komatsu, M.; Ryu, I. Chem. Rev.
1999, 99, 1991–2069.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: bietti@uniroma2.it.
(24) Brown, C. E.; Neville, A. G.; Rayner, D. M.; Ingold, K. U.;
Lusztyk, J. Aust. J. Chem. 1995, 48, 363–379.
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1980, 819–824.
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(29) BnO• undergoes a rapid 1,2-H-atom shift reaction in water and
alcohols (see ref 30).
’ ACKNOWLEDGMENT
Financial support from the Ministero dell0Istruzione
dell0Universitꢀa e della Ricerca (MIUR) is gratefully acknowl-
edged. We thank Lorenzo Stella for the use of a LFP equipment,
Keith U. Ingold for helpful discussions on the effect of fluorinated
alcohols on the hydrogen atom abstraction reactivity of the
cumyloxyl radical, and Gino A. DiLabio for helpful discussions
on solvent hydrogen-bonding interactions.
(30) Konya, K. G.; Paul, T.; Lin, S.; Lusztyk, J.; Ingold, K. U. J. Am.
Chem. Soc. 2000, 122, 7518–7527.
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(34) Bietti, M.; Gente, G.; Salamone, M. J. Org. Chem. 2005,
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(36) It is important to point out that the reaction between cyclo-
hexane and the cumyloxyl radical has been also studied in acetic acid
where no KSEs was observed.20 This solvent is characterized by a HBD
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4651
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