Nucleophilicities and nucleofugalities of organic carbonates

Article abstract of DOI:10.1002/ejoc.201000414

The kinetics of the reactions of the methyl carbonate ion with benzhydrylium ions in acetonitrile have been studied by UV/ Vis spectrophotometry. Substitution of the resulting secondorder rate constants and the electrophilicity parameters E of the benzhydrylium ions into the linear free energy relationship logic = s(N + E) yielded the nucleophilicity parameters N₂₅ = 16.03 and s₂₅ = 0.64 for methyl carbonate in acetonitrile. The kinetics of the reverse reactions, i.e., of the solvolyses of ring-substituted benzhydryl alkyl carbonates in different aqueous solvents were followed by conductimetry. The obtained first-order rate constants and the known electrofugality parameters E_f of benzhydrylium ions were used to determine the nucleofugality parameters N_f and s_f of the ROCO₂^- groups by using the linear free energy relationship logic = S_f(N_f + E_f). The leaving group abilities of carbonates decrease by a factor of about 300 from PhOCO ₂^- over MeOCO₂^- and iBuOCO ₂^- to tBuOCO₂^- in various alcoholic and aqueous solvents. feri-Butyl carbonates (iBocO-R) are, thus, considerably more stable with respect to heterolytic cleavage of the O-R bond than other organic carbonates.

Full text of DOI:10.1002/ejoc.201000414

FULL PAPER
DOI: 10.1002/ejoc.201000414
Nucleophilicities and Nucleofugalities of Organic Carbonates
Nicolas Streidl,[a] Ramona Branzan,[a] and Herbert Mayr*[a]
Dedicated to Professor Dieter Lenoir on the occasion of his 75th birthday
Keywords: Kinetics / Linear free energy relationships / Solvolysis / Correlation analysis / Leaving groups
The kinetics of the reactions of the methyl carbonate ion with
benzhydrylium ions in acetonitrile have been studied by UV/
Vis spectrophotometry. Substitution of the resulting second-
order rate constants and the electrophilicity parameters E of
the benzhydrylium ions into the linear free energy relation-
ship logk = s(N + E) yielded the nucleophilicity parameters
N25 = 16.03 and s25 = 0.64 for methyl carbonate in acetoni-
trile. The kinetics of the reverse reactions, i.e., of the solvo-
lyses of ring-substituted benzhydryl alkyl carbonates in dif-
ferent aqueous solvents were followed by conductimetry. The
obtained first-order rate constants and the known electro-
fugality parameters Ef of benzhydrylium ions were used to
determine the nucleofugality parameters Nf and sf of the
–
ROCO2 groups by using the linear free energy relationship
logk = sf(Nf + Ef). The leaving group abilities of carbonates
–
decrease by a factor of about 300 from PhOCO2 over Me-
–
–
–
OCO2 and iBuOCO2 to tBuOCO2 in various alcoholic and
aqueous solvents. tert-Butyl carbonates (tBocO-R) are, thus,
considerably more stable with respect to heterolytic cleavage
of the O–R bond than other organic carbonates.
Introduction
Alkoxycarbonyl groups are widely used as protecting
groups of alcohols and phenols, because they can easily be
introduced and removed.[1] Under basic conditions organic
carbonates, i.e., the diesters of carbonic acid, are typically
more stable than the corresponding esters of carboxylic ac-
ids and they have found wide use in organic synthesis.[2,3]
Recently, Denegri and Kronja have studied the nucleofugal-
ity (leaving group ability) of phenyl and methyl carbonate
(k1 in Scheme 1) in different solvents.[4] tert-Butyl carbon-
ates (tBocO–R), the most prominent carbonates, were not
included in this study. As these data are of particular inter-
est for synthetically working chemists, we now report on
the nucleofugality of the tBocO group. In order to charac-
terize the electrofugalities of benzhydrylium ions which are
better stabilized than the 4,4Ј-dimethoxybenzhydrylium ion,
we have furthermore studied the nucleofugality of isobutyl
carbonate and extended the work of Denegri and Kronja
on the leaving group abilities of methyl carbonate; these
data are needed for the construction of comprehensive nu-
cleofugality and electrofugality scales.[5]
Scheme 1. Simplified solvolysis scheme.
In previous work we compared the nucleophilic reac-
tivities of halide[6] and carboxylate[7] ions toward benzhy-
drylium ions and demonstrated that the relative nucleophi-
licities are not the inverse of the relative nucleofugalities.[7]
In order to extend this comparison to organic carbonates,
we have now investigated the nucleophilic reactivity of the
methyl carbonate ion by studying the rates of its reactions
with benzhydrylium ions, which allows us to include the
methyl carbonate ion in the comprehensive reactivity scales
based on Equation (1),[8]
logk = s(N + E)
(1)
where k is a second-order rate constant, E is an electro-
philicity parameter, and N and s are nucleophile-specific pa-
rameters.
Results and Discussion
[a] Department Chemie, Ludwig-Maximilians-Universität
München,
Determination of the Rates of the Combinations of Benz-
hydrylium Ions with the Methyl Carbonate Ion (k–1 in
Scheme 2): The reactions of tetra-n-butylammonium methyl
carbonate with the colored benzhydrylium ions gave color-
Butenandtstrasse 5-13 (Haus F), 81377 München, Germany
Fax: +49-89-2180-77717
E-mail: Herbert.Mayr@cup.uni-muenchen.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000414.
Eur. J. Org. Chem. 2010, 4205–4210
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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N. Streidl, R. Branzan, H. Mayr
FULL PAPER
Scheme 2. Combination reaction of benzhydrylium ions with methyl carbonate and subsequent decarboxylation.
less products in acetonitrile. 4-Methoxybenzhydryl methyl
carbonate (6-OCO2Me) was formed in quantitative yield
and isolated as a stable compound. However, we were not
able to isolate the corresponding products when using
highly stabilized benzhydrylium ions (Table 1), as the prod-
ucts rapidly decarboxylate with formation of the corre-
sponding benzhydryl methyl ethers (1–5)-OMe (Scheme 2).
The mechanism of this decarboxylation reaction is not
clear, because a solution of tetra-n-butylammonium methyl
carbonate in acetonitrile did not decompose noticeably at
25 °C during the time of the kinetic investigations. Possibly
ionization of (1–5)-OCO2Me and recombination of the ini-
tially generated ion-pair does not exclusively regenerate the
Figure 1. Exponential decay of the absorbance at 616 nm during
carbonates Ar2CH-OCO2Me; occasionally electrophilic at-
tack at the methoxy oxygen of H3CO-CO2– yields a zwitter-
ion which decomposes with formation of CO2 and the ob-
served benzhydryl methyl ethers.
–
the reaction of (ind)2CH+ (1+) with MeOCO2 in CH3CN at 25 °C
([(ind)2CH+] = 2.13ϫ10–5 ; [MeOCO2 ] = 1.25ϫ10–3 ; kobs
=
–
57.1 s–1). Insert: determination of the second-order rate constant
k–1 (3.83ϫ104 –1 s–1) as the slope of the correlation of the first-
–
order rate constants kobs vs. [MeOCO2 ].
Table 1. Benzhydrylium ions Ar2CH+ and their electrophilicity pa-
rameters E.
As depicted in Figure 1 the absorbance of the electro-
phile does not fade completely during the monitored reac-
tion time (0.1 s), indicating an equilibrium between covalent
carbonates and ionic starting materials. However, when the
reaction shown in Figure 1 was monitored for 2 seconds,
complete disappearance of the absorbance of the benz-
hydrylium ion was observed, which is explained by the sub-
sequent irreversible formation of the benzhydryl methyl
ether (Scheme 2).
As illustrated in the insert of Figure 1, kobs increases
–
linearly with the concentration of MeOCO2 . The second-
order rate constants k–1 listed in Table 2 were obtained as
the slopes of these plots [Equation (2)].
–
kobs = k–1[MeOCO2 ]
(2)
[a] Electrophilicity parameter as defined by Equation (1) (from
ref.[8]).
Table 2. Second-order rate constants k–1 for the reactions of benz-
–
hydrylium ions with Bu4N+MeOCO2 in acetonitrile at 25 °C.
No
Ar2CH+
E[a]
k–1 [–1 s–1]
Most reactions were followed photometrically at the ab-
sorption maxima of Ar2CH+ by using a stopped-flow in-
strument. For studying reactions on the microsecond time-
scale, the benzhydrylium ions were generated by laser pulse
irradiation of benzhydryl tri-n-butylphosphonium tetrafluo-
roborates[9] in acetonitrile solution in the presence of tetra-
n-butylammonium methyl carbonate. All reactions were
performed under pseudo-first-order conditions (high excess
1+
2+
3+
4+
5+
(ind)2CH+
(thq)2CH+
(pyr)2CH+
(mor)2CH+
(mfa)2CH+
–8.76
–8.22
–7.69
–5.53
–3.85
3.83ϫ104
9.69ϫ104
2.57ϫ105
5.21ϫ106
5.77ϫ107
[a] Electrophilicity parameter as defined by Equation (1) (from
ref.[8]).
–
of n-Bu4N+MeOCO2 ) at 25 °C in acetonitrile. The first-
When the logarithms of the second-order rate constants
order rate constants kobs were obtained from the ex- k–1 are plotted against the previously reported electrophilic-
ponential decays of the absorbances of the electrophiles ity parameters E of the benzhydrylium ions (Figure 2), a
(Figure 1). Details are given in the Supporting Information. linear correlation is obtained, which yields the nucleophilic-
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Nucleophilicities and Nucleofugalities of Organic Carbonates
ity parameter[10] N25 = 16.03 as the negative intercept on plified by consideration of Cl–, which is a far better nucleo-
the abscissa (E axis) and the nucleophile-specific slope pa- fuge than carboxylate ions but has a similar nucleophilicity
rameter[10] s25 = 0.64.
as PNB.[6]
Determination of the Nucleofugalities of Organic
Carbonates
Synthesis of Precursors: The benzhydryl methyl carbon-
ates 10-OCO2Me and 11-OCO2Me were prepared by com-
bining the corresponding benzhydrols (see structures of
benzhydryl moieties in Table 3) with methyl chloroformate
in the presence of pyridine as described previously for
(6–9)-OCO2Me.[4] The same method has been used for the
synthesis of the benzhydryl isobutyl carbonates (6–11)-
OCO2iBu from the corresponding benzhydrols and isobutyl
chloroformate. The crude products were obtained as yellow
oils which sometimes contained starting material (benz-
hydrol) and (particularly in case of the donor substituted
benzhydryl systems 10+ and 11+) the aforementioned de-
carboxylation products (benzhydryl methyl or isobutyl
ether). Attempts to purify the crude carbonates by crystalli-
zation or column chromatography resulted in decomposi-
tion. Therefore, the crude products have been used for the
kinetic studies without further purification. As the side
products are stable in aqueous organic solution and do not
react with the benzhydryl carbonates, they do not interfere
with the conductimetric measurements of the solvolysis re-
actions.
Figure 2. Plot of the second-order rate constants logk–1 (25 °C, ace-
tonitrile) against the electrophilicity parameters E[8] of the reference
electrophiles for the reactions of Bu4N+MeOCO2 with benzhy-
–
drylium ions.
As depicted in Figure 3, the nucleophilicity parameter[10]
N25 for methyl carbonate is in between those of benzoate
and 4-nitrobenzoate (PNB).
Table 3. Electrofugality parameters Ef of benzhydrylium ions.
Figure 3. Comparison of the nucleophilicity parameters[7,10] N25 of
common leaving groups in acetonitrile at 25 °C, s25 parameters in
parentheses.
The nucleophilicity parameters N as defined by Equa-
tion (1) reflect the relative reactivities of anions towards
such electrophiles, which react with logk = 0. As the reac-
tions of methyl carbonate and of carboxylates (N Ն15) with
such weak electrophiles (E Ͻ –15) are thermodynamically
unfavorable and do not give rise to detectable equilibrium
concentrations of covalent esters, it seems to be more ap-
propriate to compare the relative nucleophilic reactivities of
these anions towards stronger electrophiles. When (thq)2-
CH+ (2+, E = –8.22) is used as reference electrophile, the
nucleophilicity order in acetonitrile is:
[a] These parameters revise previously published values from ref.[5]
[b] Previously unpublished electrofugality parameters, based on this
work and additional unpublished data.
The benzhydryl tert-butyl carbonates (8–11)-OCO2tBu
were prepared by deprotonation of the corresponding
benzhydrols with BuLi, followed by treatment with Boc2O
–
AcO– Ͼ BzO– Ͼ PNB Ͼ MeOCO2 Ͼ DNB
This sequence corresponds to the inverse order of the at –78 °C. Pure compounds were obtained by crystallization
nucleofugalities of these compounds. However, nucleofugal- [in case of (9–11)-OCO2tBu] or column chromatography (in
ity is not generally the reverse of nucleophilicity, as exem- case of 8-OCO2tBu).
Eur. J. Org. Chem. 2010, 4205–4210
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N. Streidl, R. Branzan, H. Mayr
FULL PAPER
Determination of Ionization Rates: The solvolysis rates of obviously does not occur. This is in line with previous
the benzhydryl carbonates were monitored by following the work[7] where we have shown that acetate and benzoate
increase of the conductivity of the reaction mixtures. Be- ions, which are stronger nucleophiles than methyl carbonate
cause calibration experiments, i.e., portionwise addition of ions, also do not show common ion return in solvolysis re-
the rapidly solvolyzing benzhydryl methyl carbonate (10– actions under similar conditions. The conductimetrically
OCO2Me) into 50A50W and 60E40W and determination measured rate constants listed in Table 4, therefore, corre-
of the conductivity after completion of the solvolysis, spond to the ionization rate constants k1 defined in
showed a proportionality between conductivity (G) and the Scheme 1.
concentration of the substrates in the concentration range
Correlation Analyses: From the plots of logk1 vs. Σσ+,
investigated (see Supporting Information), we were able to one derives Hammett reaction constants of –2.9 Ͻ ρ Ͻ –2.4
obtain first-order rate constants k1 by fitting the time de- for the solvolysis reactions of benzhydryl carbonates in
pendent conductivities G to the monoexponentional func- 60E40W (Figure 4). The magnitudes of the reaction con-
tion (3).
stants ρ suggest transition states, which correspond to the
carbocations.
G = Gmax [1 – exp(–k1t)] + const.
(3)
Because all solvolyses studied in this work follow first-
order rate laws, common ion return[11] (k–1 in Scheme 1)
Table 4. Conductimetrically measured solvolysis rate constants
(25 °C) of the benzhydryl alkyl carbonates (6–11)-LG in different
solvents.
Solvent[a]
50A50W
LG
Electrofuge
k1 [s–1]
OCO2Me
10
11
6
4.87ϫ10–2
1.24ϫ10–1 [b]
1.09ϫ10–4
4.81ϫ10–4
1.20ϫ10–3
7.16ϫ10–3
2.13ϫ10–2
5.97ϫ10–2
1.91ϫ10–2
4.66ϫ10–2 [b]
6.06ϫ10–5
3.89ϫ10–4
1.51ϫ10–3
3.92ϫ10–3
6.87ϫ10–3
2.45ϫ10–2 [b]
2.31ϫ10–4
1.00ϫ10–3
2.24ϫ10–3
1.66ϫ10–2
5.40ϫ10–2
3.94ϫ10–4
3.05ϫ10–3
8.54ϫ10–3
2.15ϫ10–2
4.72ϫ10–2
1.50ϫ10–4
1.15ϫ10–3
3.96ϫ10–3
1.07ϫ10–2
2.33ϫ10–2
1.81ϫ10–4
5.80ϫ10–4
4.53ϫ10–3
1.45ϫ10–2
4.14ϫ10–2
9.87ϫ10–5
8.28ϫ10–4
2.76ϫ10–3
7.13ϫ10–3
OCO2iBu
7
8
9
10
11
10
11
8
60A40W
OCO2Me
Figure 4. Plots of logk1 of the solvolysis reactions of the benzhydryl
alkyl carbonates in 60% aqueous ethanol (60E40W) vs. Hammett’s
OCO2tBu
9
substituent constants Σσ+ (k1 for tBuOCO2 and iBuOCO2 from
–
–
10
11
10
11
6
–
Table 4, k1 for MeOCO2 from ref.[4], σ+ from ref.[12]).
70A30W
60E40W
OCO2Me
In previous work[5] we have demonstrated that Equa-
tion (4) can be used for the calculation of the heterolysis
rate constants k1 of benzhydryl derivatives in various sol-
vents.
OCO2iBu
7
8
9
10
8
OCO2tBu
logk1 (25 °C) = sf (Nf + Ef)
(4)
9
10
11
10
8
In Equation (4) k1 is a first-order rate constant (s–1), sf
and Nf are nucleofuge-specific parameters (referring to
combinations of leaving groups and solvents), and Ef is a
carbocation-specific electrofugality parameter.
80E20W
90E10W
OCO2Me
OCO2tBu
9
10
11
10
7
Plots of logk1 (from this work and ref.[4]) for the solvoly-
sis reactions of various substituted benzhydryl carbonates
vs. the electrofugality parameters Ef of the benzhydrylium
ions (see Table 3) are linear as exemplified in Figure 5 for
the solvolyses of benzhydryl tert-butyl carbonates in dif-
ferent aqueous solvents (Table 5). For analogous corre-
lations of isobutyl and methyl carbonates see Supporting
Information. From these correlations one can extract the
nucleofugality parameters Nf as the negative intercepts on
the abscissa (Ef axis) and the sf parameters as the slopes of
OCO2Me
OCO2iBu
8
9
10
11
8
9
10
11
60AN40W
OCO2tBu
[a] Mixtures of solvents are given as (v/v); solvents: A = acetone, AN
= acetonitrile, E = ethanol, W = water. [b] Stopped-flow kinetics. the correlations. The nucleofugality parameters for methyl
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Nucleophilicities and Nucleofugalities of Organic Carbonates
carbonate are in very good agreement with those published
by Denegri and Kronja,[4] which were based on a smaller
data set.
Figure 6. Comparison of the nucleofugalities of leaving groups in
various solvents (DNB = 3,5-dinitrobenzoate, PNB = 4-nitroben-
zoate from ref.[5]).
Figure 5. Plots of the first-order rate constants logk1 of the solvo-
lyses of substituted benzhydryl tert-butyl carbonates in different
solvent mixtures against the electrofugality parameters Ef [mixtures
of solvents are given (v/v); W = water, E = ethanol, AN = acetoni-
trile, A = acetone].
reactive), i.e., the half-life of 4-methoxybenzhydryl phenyl
carbonate in 80% aqueous ethanol at room temperature is
about 4 min, whereas the half-life of the corresponding tert-
butyl carbonate is about 19 h. Figure 6 furthermore shows
that tBocO has a smaller leaving group ability than p-nitro-
benzoate (PNB) or 3,5-dinitrobenzoate (DNB) in various
solvents.[7]
Table 5. Nucleofugality parameters Nf and sf of organic carbonates
in different solvent mixtures.
Nf / sf
OCO2Me
Solvent[a]
OCO2iBu
–2.51/0.87
OCO2tBu
–3.62/0.94
Conclusions
50A50W
60A40W
70A30W
60E40W
80E20W
90E10W
60AN40W
–2.13/0.86[b]
–2.56/0.88[b]
–2.83/0.94[b]
–1.59/0.89[b]
–1.96/0.95[b]
–2.20/0.98[b]
The great synthetic value of the tBoc protecting group
has generally been assigned to the ease of deprotection un-
der acidic conditions. This work has demonstrated an ad-
ditional advantage of the tBoc group. The tert-butoxy-
–2.04/0.89
–2.46/0.98
–2.91/0.89
–3.12/0.96
–
–3.28/0.96
carbonyloxy group tBuOCO2 is a considerably weaker
nucleofuge than phenyl carbonate or other alkyl carbonates
with the result that the tert-butyl carbonates tBuOCO2-R
are much less sensitive toward heterolytic cleavage of the
O–R bond than other carbonates. The higher lability of
tert-butyl carbonates under acidic conditions is thus com-
bined with greater stability under neutral and basic condi-
tions.
[a] Mixtures of solvents are given (v/v); W = water, E = ethanol,
AN = acetonitrile, A = acetone. [b] These parameters differ slightly
from previously published values which were based on a smaller
data set for the correlation (ref.[4]).
With these nucleofugality parameters it now becomes
possible to directly compare the leaving group abilities of
various carbonates with other commonly used leaving
groups. As the slope parameters sf of different leaving
groups differ only slightly (∆sf Յ0.08), it appears justified
to compare the nucleofugalities of these leaving groups on
the basis of Nf (Figure 6).
Experimental Section
General: Acetonitrile (VWR, Ն 99.9%) and acetone (VWR,
As depicted in Figure 6, phenyl carbonate which has pre- Ն 99.8%) were used as purchased. Water was purified by using
viously been studied by Denegri and Kronja[4] is the most
reactive carbonate investigated so far, ionizing about one
order of magnitude faster than methyl carbonate. This dif-
a Millipore MilliQ (final specific resistance Ն 18.2 MΩ/cm). The
benzhydrylium tetrafluoroborates were synthesized according to
literature procedures.[8a]
ference in reactivity has been explained by the more Tetra-n-butylammonium methyl carbonate was synthesized by bub-
bling CO2 through a tetra-n-butylammonium methoxide solution
in methanol as reported in ref.[13] As the crude product still con-
tained some methoxide, complete conversion of methoxide to
methyl carbonate was achieved by treatment of a solution of the
crude material in toluene with a stream of CO2. The benzhydryl
methyl and isobutyl carbonates were prepared by esterification of
the corresponding benzhydrols with methyl or isobutyl chloro-
formate, whereas the benzhydryl tert-butyl carbonates were pre-
pared by reaction of the corresponding lithium diarylmethoxides
with Boc2O. In some cases the crude products were contaminated
efficient delocalization of the negative charge in phenyl
carbonate.[4] Replacement of the methyl group by an iso-
butyl group reduces the reactivity by half an order of mag-
nitude, and tert-butyl carbonate (tBocO) is the weakest nu-
cleofuge of this series, approximately 10 times less reactive
than isobutyl carbonate. Sterically hindered solvation of the
tBocO– anion can be assumed to account for its low nucleo-
fugality. In conclusion the carbonate reactivity can be al-
tered by about 2.5 orders of magnitude by changing the
substituent from phenyl (most reactive) to tert-butyl (least with starting material (benzhydrol) or decarboxylation product
Eur. J. Org. Chem. 2010, 4205–4210
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N. Streidl, R. Branzan, H. Mayr
FULL PAPER
(benzhydryl alkyl ether). Details of the syntheses and characteriza-
tion of the benzhydryl alkyl carbonates are described in the Sup-
porting Information.
during preparation of this manuscript and the Deutsche For-
schungsgemeinschaft (DFG) (Ma 673/20-3) and the Fonds der
Chemischen Industrie for financial support.
Determination of the Rates of the Combination of Benzhydrylium
–
Ions with MeOCO2 : Solutions of benzhydrylium tetrafluoro-
–
borates in CH3CN were mixed with solutions of nBu4N+MeOCO2
[1]
a) P. J. Kocienski, Protecting Groups, 3rd ed., Thieme, Stutt-
gart, 2005; b) P. G. M. Wuts, T. W. Greene, GreeneЈs Protective
Groups in Organic Synthesis, 4th ed., Wiley, Hoboken, NJ,
2007.
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in CH3CN in a stopped-flow instrument (Applied Photophysics
SX.18MV-R), and the decay of the benzhydrylium absorbance was
followed by UV/Vis spectrometry. All experiments were performed
under pseudo-first-order conditions (excess of MeOCO2 ). The
temperature was kept constant at 25 °C in all experiments by using
–
[2]
[3]
a circulating water bath.
[4]
[5]
B. Denegri, O. Kronja, J. Org. Chem. 2007, 72, 8427–8433.
Laser-Flash Photolysis: For determining the rates of the reactions
of Ar2CH+ with MeOCO2 with reaction times below 10 ms, benz-
–
B. Denegri, A. Streiter, S. Juric, A. R. Ofial, O. Kronja, H.
Mayr, Chem. Eur. J. 2006, 12, 1648–1656; B. Denegri, A. Stre-
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2006, 12, 5415.
S. Minegishi, R. Loos, S. Kobayashi, H. Mayr, J. Am. Chem.
Soc. 2005, 127, 2641–2649.
hydrylium ions were generated by laser-flash photolysis of benz-
hydryl tri-n-butylphosphonium tetrafluoroborates in acetonitrile at
25 °C with an Innolas SpitLight 600 Nd:YAG laser (fourth har-
monic at λ = 266 nm, power/pulse of 40–60 mJ, pulse length 6.5 ns)
in a quartz cell.[9] The rate constants were determined by observing
the time-dependent decay of the UV/Vis absorptions of the benz-
hydrylium ions.
[6]
[7]
H. F. Schaller, A. A. Tishkov, X. Feng, H. Mayr, J. Am. Chem.
Soc. 2008, 130, 3012–3022.
[8] a) H. Mayr, T. Bug, M. F. Gotta, N. Hering, B. Irrgang, B.
Janker, B. Kempf, R. Loos, A. R. Ofial, G. Remennikov, H.
Schimmel, J. Am. Chem. Soc. 2001, 123, 9500–9512; b) H. Mayr,
B. Kempf, A. R. Ofial, Acc. Chem. Res. 2003, 36, 66–77; c) H.
Mayr, A. R. Ofial, Pure Appl. Chem. 2005, 77, 1807–1821; d)
H. Mayr, A. R. Ofial, J. Phys. Org. Chem. 2008, 21, 584–595;
e) For a comprehensive data base for nucleophilicity and elec-
trophilicity parameters, see: www.cup.lmu.de/oc/mayr/.
[9] J. Ammer, H. Mayr, Macromolecules 2010, 43, 1719–1723.
[10] The index 25 is added to the N and s parameters, as all rate
constants k–1 in this work were measured at 25 °C for the sake
of compatibility with ionization rate constants k1 determined
at 25 °C.
Determination of Ionization Rates: The ionization rates of the benz-
hydryl alkyl carbonates were monitored by following the increase
of the conductivity of the reaction mixtures using a conventional
conductimeter (Tacussel CD 810, Pt electrode: WTW LTA 1/NS).
Typically, each run was repeated at least once; the reported rate
constants are the arithmetic means. For faster reactions, a stopped-
flow conductimeter (Hi-Tech Scientific SF-61 DX2, platinum elec-
trodes, cell volume 21 µL, cell constant 4.24 cm–1, minimum dead
time 2.2 ms) was used. In order to achieve a complete ionization
of the liberated weak acid, 2 to 40 equiv. of 1,8-bis(dimethylamino)-
naphthalene or piperidine was used as additive. The temperature
was kept constant at 25 °C in all experiments using a circulating
water bath.
[11] For common ion rate depression, see: a) S. Winstein, E. Clip-
pinger, A. H. Fainberg, R. Heck, G. C. Robinson, J. Am.
Chem. Soc. 1956, 78, 328–335; b) D. J. Raber, J. M. Harris,
P. v. R. Schleyer, in: Ions and Ion Pairs in Organic Reactions
(Ed.: M. Szwarc), Wiley, New York, 1974, vol. 2.
Supporting Information (see also the footnote on the first page of
this article): Preparative procedures, product characterization, de-
tails of the kinetic experiments are available.
[12] a) C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165–
195; b) O. Exner, Correlation Analysis of Chemical Data, Ple-
num Press, New York, 1988.
Acknowledgments
[13] A. Berkessel, M. Brandenburg, Org. Lett. 2006, 8, 4401–4404.
Received: March 26, 2010
Published Online: June 16, 2010
We thank Johannes Ammer for help during the laser-flash photoly-
sis experiments, Dr. Armin R. Ofial and Tanja Kanzian for help
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