Electrophilicities of benzaldehyde-derived iminium ions: Quantification of the electrophilic activation of aldehydes by iminium formation

Article abstract of DOI:10.1021/ja401106x

Rate constants for the reactions of benzaldehyde-derived iminium ions with C-nucleophiles (enamines, silylated ketene acetals, and enol ethers) have been determined photometrically in CH₃CN solution and used to determine the electrophilicity pa

Full text of DOI:10.1021/ja401106x

Article
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Electrophilicities of Benzaldehyde-Derived Iminium Ions:
Quantification of the Electrophilic Activation of Aldehydes by
Iminium Formation
Roland Appel,§ Saloua Chelli,§ Takahiro Tokuyasu,§ Konstantin Troshin, and Herbert Mayr*
Department Chemie, Ludwig-Maximilians-Universitat Munchen, Butenandtstrasse 5-13, 81377 Munchen, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Rate constants for the reactions of benzaldehyde-
derived iminium ions with C-nucleophiles (enamines, silylated
ketene acetals, and enol ethers) have been determined photo-
metrically in CH3CN solution and used to determine the
electrophilicity parameters E of the cations defined by the
correlation log k20°C = sN(E + N) (Mayr, H.; et al. J. Am. Chem.
Soc. 2001, 123, 9500−9512). With electrophilicity parameters
from E = −10.69 (Ar = p-MeOC6H4) to E = −8.34 (Ar = p-CF3), the iminium ions Ar−CHNMe2 have almost the same
reactivities as analogously substituted arylidenemalononitriles Ar−CHC(CN)2 and are 10 orders of magnitude more reactive
than the corresponding aldehydes. The rate constants for the reactions of iminium ions with amines and water in acetonitrile are
103−105 times faster than predicted by the quoted correlation, which is explained by the transition states which already
experience the anomeric stabilization of the resulting N,N- and O,N-acetals.
+
iminium species increases the electrophilicity of the carbonyl
compound for the subsequent attack of nucleophiles. Related
are Brønsted acid-catalyzed transformations of imines,4 though
in these cases the protonation of the imine may be incomplete,
and the effective electrophile may be either an iminium ion pair
or a hydrogen-bond-activated imine.5
In recent years, we have shown that the rates of the reactions
of carbocations and Michael acceptors with n-, π-, and σ-
nucleophiles can be described by eq 1, where k20°C is the
INTRODUCTION
■
Aminoalkylations of arenes and CH-acidic compounds
(Mannich reaction),1,2 and iminium-activation of unsaturated
carbonyl compounds in organocatalytic asymmetric trans-
formations,3 are the most prominent examples for the use of
iminium ions in organic synthesis (Scheme 1). In both types of
reactions, the transformation of the carbonyl compound into an
Scheme 1. Electrophilic Activation of Carbonyl Compounds
in Aminoalkylations1 and Iminium-Activated Nucleophilic
Additions to α,β-Unsaturated Aldehydes3
log k20°C = sN(N + E)
(1)
second-order rate constant in M−1 s−1, sN is a nucleophile-
specific sensitivity parameter, N is a nucleophilicity parameter,
and E is an electrophilicity parameter.
On the basis of this linear free-energy relationship, we have
developed the most comprehensive nucleophilicity and electro-
philicity scales presently available.6 Recently, the electrophilicity
parameters for several aldehydes have been derived from the
rates of their reactions with sulfur ylides in DMSO solution.7
Furthermore, α,β-unsaturated iminium ions derived from
cinnamaldehyde have been integrated into our electrophilicity
scales by studying the rates of their conjugate addition reactions
with silyl enol ethers, pyrroles, amines, and phosphanes.8
Though the E-parameters of formaldehyde-derived iminium
ions have previously been estimated,9 a direct comparison of
the electrophilicities of iminium ions with their carbonyl
analogues has so far not been achieved (Scheme 2).
Received: January 31, 2013
© XXXX American Chemical Society
A
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Scheme 2. Electrophilic Reactivities of Aldehydes and
Iminium Ions7,8a
Table 1. Reference Nucleophiles Used in This Work and
Their Nucleophilicity Parameters N and sN in CH2Cl2 and
CH3CN
We now report on the kinetics of the reactions of several
iminium ions, derived from different benzaldehydes (Scheme
3), with enamines, silylated ketene acetals, and silyl enol ethers
Scheme 3. Iminium Ions 1a−f Investigated in This Work
(Table 1) which are used as reference nucleophiles.
Comparison of these data with previously reported carbonyl
reactivities will provide a quantification of the iminium
activation of aldehydes.
a
Nucleophilicity parameters N and sN for 2a were taken from ref 10,
b
for 2b,c,e,i−l from ref 6b, and for 2d,f−h from ref 11. Nucleophilicity
parameters N and sN in CH3CN for 2a were taken from ref 12. The N
c
RESULTS AND DISCUSSION
and sN parameters of the enamine 2b and the ketene acetals 2c,e,j in
acetonitrile have not been reported previously and were determined by
studying the rates of their reactions with benzhydrylium ions following
previously reported procedures; for details, see Supporting Informa-
tion.
■
Product Analysis. In order to establish the course of the
reactions, which were investigated kinetically, we have studied
the products of representative combinations of the iminium
ions 1 with the nucleophiles 2. The reactions of enamines (such
as 2a,b) with preformed iminium salts in various solvents have
previously been reported to give β-amino ketones after aqueous
workup.13
unambiguously characterized by NMR, the initial products were
not hydrolyzed but methylated (2c, 2l) or oxidized (2e) and
subsequently transferred into the α,β-unsaturated carbonyl
compounds 5 (Scheme 5) by Hofmann or amine oxide
elimination. The characterization of all products is given in the
Supporting Information.
As shown in Table 2, the reactions of the silyl ketene acetals
2d,f−j and of 2k with the iminium triflates (1a−c)·OTf yielded
the β-amino esters 3a,d−h or the β-amino acids 3b,c after
aqueous workup. The silyl ketene acetal 2d yielded only one of
the two conceivable diastereomers. A moderate degree of
diastereoselectivity was also observed in the reactions of 2f with
(1a,c)·OTf, which yielded 3b,c with anti:syn ratios of 4:1 to 5:1
(NMR). The preferential formation of the anti-diastereomer,
which has been confirmed by X-ray analysis14 (p S10 of the
Supporting Information), can be explained with Zimmerman−
Traxler-like transition states as depicted in Scheme 4.
The product from Danishefsky’s diene 2k and the iminium
triflate 1b·OTf could not be isolated due to its very low
stability. For this reason, the reaction was performed in CDCl3
in an NMR tube; the NMR spectra taken after addition of a
trace amount of KOH in D2O (0.1 M in D2O) showed
exclusive formation of the β-aminoketone 4. Detailed
descriptions of the experimental procedures and the character-
izations of the isolated compounds are given in the Supporting
Information.
The imine 6 was obtained in 87% yield by the reaction of
benzylamine with iminium triflate 1d·OTf (Scheme 6).
Kinetic Investigations. The kinetics of the reactions of the
nucleophiles 2 with the iminium triflates (1a−f)·OTf (and in
some cases the iminium tetrafluoroborates (1a,b)·BF4) were
performed in CH2Cl2 or CH3CN solution at 20 °C. All
investigated reactions were monitored photometrically by
following the disappearance of the iminium ions 1 at or close
to their absorption maxima (265−330 nm). In order to achieve
first-order conditions, the iminium ions 1 were combined with
at least 10 equiv of the nucleophiles 2. In case of the ketene
acetals 2c,e, stock solutions in CH2Cl2 were used, as these
compounds slowly decompose in CH3CN. Small portions of
these stock solutions were dissolved in CH3CN before each
kinetic experiment.
From the resulting exponential decays of the UV−vis
absorbances of the iminium ions 1 (Figure 1), the first-order
rate constants kobs were obtained. Plots of kobs (s−1) against the
concentrations of the nucleophiles were linear with negligible
intercepts as required by the relation kobs = k2[2]0 (eq 4 and
When the reactions of the cyclic silyl ketene acetals 2c,e and
of the silyl enol ether 2l with iminium triflates were performed
analogously (Scheme 5), mixtures of diastereomers were
obtained. As these mixtures could neither be separated nor
B
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Table 2. Reactions of Iminium Triflates (1a−f)·OTf with the Silyl Ketene Acetals 2d,f−j and Danishefsky’s Diene 2k in CH2Cl2
Followed by Aqueous Workup (pH 7)
a
b
c
d
Only one diastereomer was obtained. Aqueous workup at pH 5. Based on the 1H NMR spectrum of the crude product 1H NMR of the crude
e
product isolated after the reaction in CH3CN showed the same dr as in CH2Cl2. Product could not be isolated; reaction was performed in an NMR
tube in CDCl3 with subsequent addition of trace amounts of KOH in D2O.
Figure 1). Only in case of the reaction of the enamine 2b with
iminium ion 1b, the plot of kobs vs [2b]0 showed a significant
positive intercept, indicating the high reversibility of the
addition. However, in all cases the second-order rate constants,
C
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Scheme 4. Zimmerman−Traxler Transition States for the
Reactions of 2f with Iminium Ions 2
Table 3. Second-Order Rate Constants (M−1 s−1) for the
Reactions of the Iminium Triflates (1a−d)·OTf with the
Nucleophiles 2 in CH2Cl2 at 20 °C
Scheme 5. Reactions of the Silyl Ketene Acetals 2c,e and the
Silyl Enol Ether 2l with the Iminium Ions 1a,b and
Subsequent Elimination Reactions to the α,β-Unsaturated
Carbonyl Compounds 5
a
As treatment with MeI in Et2O and K2CO3 in EtOH/H2O gave poor
yields, steps 3) and 4) were replaced by treatment with H2O2 (30% aq
solution) in i-PrOH.
Scheme 6. Reaction of 1d·OTf with Benzylamine
a
Figure 1. UV−vis spectroscopic monitoring of the reaction of the
iminium ion 1d (2.16 × 10−5 mol L−1) with the silyl ketene acetal 2h
(3.80 × 10−3 mol L−1) at 328 nm in CH2Cl2 at 20 °C. Inset:
Determination of the second-order rate constant k2 = 1.58 × 102 M−1
s−1 from the dependence of the first-order rate constant kobs on the
concentration of 2h.
A significant positive intercept of the kobs vs [2]0 plot was observed,
indicating a considerable degree of reversibility of the reaction.
b
Iminium tetrafluoroborate 1b·BF4 was employed for the kinetic
measurement.
As shown in Table 3, the rate constants of the reactions of
iminium triflate 1b·OTf with the nucleophiles 2e,i,k varied by
which are summarized in Table 3, could be derived from the
slopes of the linear correlations of kobs versus [2]0.
−
less than a factor of 1.2 when TfO− was replaced by BF4 as a
counterion. Therefore, we can conclude that the counterions
(TfO− or BF4 ) are not involved in the rate-determining step
−
−d[1]/dt = k2[2][1]
(2)
under these reaction conditions. When the second-order rate
constants for the reactions of the iminium ions 1a−d with the
nucleophiles 2 in CH2Cl2 (Table 3) were analyzed by eq 1,
poor correlations of (log k2)/sN vs N were observed (Figure 2
[2]0 ≫ [1]0 ⇒ [2] ≈ [2]0
(3)
−d[1]/dt = kobs[1], kobs = k2[2]0
(4)
D
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Table 4. Second-Order Rate Constants (M−1 s−1) for the
Reactions of the Iminium Triflates 1 with the Nucleophiles
2a−c,e,j in CH3CN at 20 °C
Figure 2. Poor correlation of the second-order rate constants (log k2/
sN) for the reactions of the iminium ion 1b with the nucleophiles 2b−k
in CH2Cl2 at 20 °C versus the corresponding nucleophilicity
parameters N of 2b−k. The slope is fixed to 1.0 as required by eq 1.
and Tables S8, S22, S32, and S39 of the Supporting
Information), indicating that eq 1 does not provide reliable
predictions for the reactions of the benzaldehyde-derived
iminium ions 1 with nucleophiles 2 in CH2Cl2.
Second-order rate constants of the reactions of the iminium
ions 1 with the enamines 2a,b and the ketene acetals 2c,e,j,
which have been determined in CH3CN, are summarized in
Table 4. As for the reactions in CH2Cl2, significant counterion
effects were also absent in CH3CN solution, and with one
exception (reaction with 2b, factor 1.3) the reactivities of
iminium triflates and tetrafluoroborates differed by less than
10% (Table 5).
Comparison of Tables 3 and 4 shows that the reactions of
iminium ions with π-nucleophiles are generally faster in CH2Cl2
than in CH3CN, and, as specified for the reactions of 1b·OTf in
Table 6, the reactivity ratio depends strongly on the nature of
the nucleophile. While the reaction with the silyl ketene acetal
2j proceeds only 1.55 times faster in CH2Cl2 than in CH3CN, a
much stronger acceleration was found for the reactions of
compounds 2b,c,e (factors of 7.25−56.0). A rationalization for
this behavior will be attempted below.
In contrast to the poor correlation of the reactivities in
CH2Cl2 (Figure 2), Figure 3 shows a good linear correlation
between (log k2/sN) and the corresponding nucleophilicity
parameters N for the reactions of the C-nucleophiles 2a−c,e,j
with the iminium ions 1 in CH3CN. Now eq 1 is applicable,
and the electrophilicity parameters E of the iminium ions 1
(Table 4) were derived by least-squares minimizations, while
fixing the slope at 1.0 as required by eq 1. For the sake of clarity
only correlation lines for the iminium ions 1b,d are shown in
Figure 3. The correlations for 1a,c,e,f are of similar quality and
are shown in the Supporting Information (Tables S70, S82,
S92, and S97).
a
For determination see text as well as Tables S70, S76, S82, S87, S92,
and S97 of the Supporting Information.
Table 5. Second-Order Rate Constants (M−1 s−1) for the
Reactions of the Iminium Salts 1b·OTf and 1b·BF4 with
Various Nucleophiles in CH3CN at 20 °C
a
b
Nucleophile
k2 (1b·OTf)
k2 (1b·BF4)
krel
2a
2b
2c
2e
7.93 × 104
1.49 × 103
3.31 × 102
1.31 × 101
8.06 × 104
1.96 × 103
3.66 × 102
1.27 × 101
1.02
1.32
1.10
0.97
As shown in Figure 4, the electrophilic reactivities of the
benzaldehyde-derived iminium ions 1b and 1d are almost the
same as those of the analogously substituted benzylidene
malononitriles, i.e., the Me2N+ group has a similar activating
effect as a (NC)2C group.
a
b
From Table 4. krel = k2(BF4)/k2(OTf).
Figure 4 furthermore shows that the electrophilicities of
benzaldehyde-derived iminium ions are similar to those of the
conjugate positions of cinnamaldehyde-derived iminium ions.
Nevertheless, α,β-unsaturated iminium ions undergo preferen-
tially Michael additions with cyclic silyl ketene acetals,8a which
implies that the reactivity of the α-position of cinnamaldehyde-
derived iminium ions is lower than that of their benzaldehyde-
E
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less affected by the variation of the substituents on the nitrogen
center than the reactivities of the vinylogous cinnamaldehyde-
derived iminium ions.
In the parametrization of eq 1, solvent effects are included in
the nucleophile-specific parameters N and sN, while the
electrophilicity parameters E of carbocations and Michael
acceptors are treated as solvent-independent. Though limi-
tations of this approximation have been mentioned,15 this
procedure has been working reliably for a large number of
electrophile−nucleophile combinations.
Our observation that the rate constants of the reactions of
the iminium ions 1 with the nucleophiles 2 in acetonitrile
follow eq 1 (Figure 3), while the same reactions in CH2Cl2 do
not (Figure 2), is atypical and may be indicative of different
stabilizing interactions in the transition states in these two
solvents.
Table 6. Comparison of the Second-Order Rate Constants k2
(M−1 s−1) for the Reactions of Iminium Ion 1b·OTf with
Enamine 2b and the Silyl Ketene Acetals 2c,e,j in CH2Cl2
and CH3CN
a
b
c
Nucleophile
k2 (CH2Cl2)
k2 (CH3CN)
krel
7.25
35.0
56.0
1.55
2b
2c
2e
2j
1.08 × 104
1.16 × 104
7.33 × 102
1.39
1.49 × 103
3.31 × 102
1.31 × 101
8.98 × 10−1
a
b
c
From Table 3. From Table 4. krel = k2(CH2Cl2)/k2(CH3CN)
As the nucleophilicity parameters for compounds 2 were
derived from their reactions with benzhydrylium ions in
CH3CN, the good correlations shown in Figure 3 indicate
similar transition states for both reaction series in this solvent;
i.e., the rates of the reactions of the π-nucleophiles 2 with the
iminium ions 1, like those of their reactions with benzhy-
drylium ions, are controlled by the interactions of only two
reaction centers. The interaction between N and Si in the
transition state depicted in Scheme 4 must be so weak in
acetonitrile that it is not noticeable in the kinetics.
Assuming that the electrophilicity parameters E of the
iminium ions 1 (Table 4) are also solvent-independent, one
might combine them with the dichloromethane-specific N and
sN parameters of 2 (Table 1) to calculate the corresponding
second-order rate constants by eq 1. The fact that the
experimental rate constants determined in dichloromethane
are 1.8−1800 times higher than the calculated ones shows that
eq 1 does not hold. The large scatter of the deviations
furthermore implies that the use of solvent-dependent E
parameters for iminium ions would not solve the problem, as
illustrated above by the poor correlation in Figure 2. A reason
for the failure of eq 1 to describe reactions of iminium ions 1
with silyl enol ethers and ketene acetals 2c−k in CH2Cl2 might
be the interaction between the nitrogen of the iminium ion and
the silicon of the silyl enol ether or ketene acetal (incipient
metallo-ene reaction16,17) in the Zimmerman−Traxler tran-
sition states (Scheme 4). This interaction, which does not exist
in the reference reactions with benzhydrylium ions, may
become strong enough in the less polar solvent CH2Cl2 that its
influence on the kinetics is no longer negligible as it is in
CH3CN.
Figure 3. Correlation of the second-order rate constants (log k2/sN)
for the reactions of the iminium ions 1b,d with the nucleophiles 2a−
c,e,j in CH3CN at 20 °C versus the corresponding nucleophilicity
parameters N.
Reactions of O- and N-Nucleophiles with Iminium
Ions in CH3CN. In previous work, we have emphasized that eq
1 can only be applied when at least one of the reaction centers
is carbon, because the N and sN parameters of various
nucleophiles have been derived from reactions with carbon
electrophiles, and the E parameters of electrophiles are based
on reactivities toward carbon nucleophiles. As a consequence,
heteroatom−heteroatom bond formations are not covered by
eq 1.18 Deviations from the predictions by eq 1 have also been
expected7 for reactions which produce anomerically stabilized
Figure 4. Comparison of the electrophilic reactivities E of the iminium
ions 1 with those of analogously substituted α,β-unsaturated iminium
ions, Michael acceptors, and benzhydrylium ions. E parameters were
taken from Table 4 and refs 8a, 6b, and 6g.
2
products, e.g., reactions of O- or N-substituted electrophilic Csp
centers with O- or N-nucleophiles, because the extra
stabilization of the resulting acetals or aminals is not taken
into account when the electrophilicities E are derived from the
rates of the reactions with C-nucleophiles.
derived analogues. Figure 4 also illustrates that the electro-
philicities of the benzaldehyde-derived iminium ions 1b,e,f are
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In order to examine whether the anomeric stabilization in
O,N-acetals or N,N-aminals already affects the transition states
of their formation, we have now investigated the kinetics of the
reactions of the iminium ions 1 with water and with primary
amines in acetonitrile as well as with hydroxide in aqueous
acetonitrile. The kinetics were studied photometrically with the
same experimental setup as described above. In case of the
reactions with aqueous acetonitrile the expected first-order rate
law and in case of the reactions with OH− the expected second-
order rate law were found. However, the reactions of the
iminium ions 1 with primary amines, which were used in high
excess (>10 equiv), showed an upward curvature of the plots of
the pseudo-first-order rate constants kobs against the amine
concentrations, indicating the participation of two molecules of
amines in the rate-determining step. A similar observation was
previously reported for the reactions of secondary amines with
quinone methides in CH3CN solution.19 The evaluation of
these kinetics, which follows previously reported procedures,19
is described in the Supporting Information. By using this
formalism, the second-order rate constants for the initial attack
of the amines at the iminium ions 1 can be extracted (Tables 7
and S111−S125).
Table 7. Experimental and Calculated Rate Constants for the
Reactions of the Iminium Ions 1 with O- and N-
Nucleophiles in CH3CN at 20 °C
Using the electrophilicity parameters E of the iminium ions 1
listed in Table 4 and the previously published nucleophilicity
parameters N and sN of the O- and N-nucleophiles, which were
derived from their reactivities toward benzhydrylium ions,19−21
we have now calculated the rate constants of their combinations
by eq 1. As anomeric effects are not included in the reactivity
parameters N, sN, and E, the reactions of O- and N-nucleophiles
with iminium ions 1 turned out to proceed 103−105 times
faster than calculated by eq 1 (Table 7), indicating that the
anomeric stabilization22 of the resulting products already affects
the corresponding transition states.
Table 7 furthermore shows that the magnitude of the
anomeric acceleration of the reactions of O- and N-
nucleophiles with iminium ions depends on the nature of the
electrophile as well as on the nature of the nucleophile. Thus,
an anomeric rate acceleration of 3 orders of magnitude was
found for the reactions of water/acetonitrile mixtures and of
primary amines with the dimethylamino-derived iminium ions
1a−d, while the reactions of OH− with 1c,d are 5 orders of
magnitude faster than calculated. The substituents at the
iminium nitrogen also affect the degree of anomeric
acceleration. While the reactions with the piperidine-derived
iminium ion 1f are similarly accelerated as those of the
dimethylamine-derived iminium ions 1a−d, the reactions of the
pyrrolidine-derived iminium ion 1e with water/acetonitrile
mixtures and primary amines are only 1−2 orders of magnitude
faster than calculated by eq 1. The E-parameters for iminium
ions 1 are thus restricted to their reactions with carbon
nucleophiles, unlike the electrophilicity parameters E of
ordinary carbocations and Michael acceptors, which can be
employed for reactions with all types of nucleophiles (C-, N-,
O-, P-, S-, etc.).6
a
Hydrolysis in H2O/CH3CN (v/v) mixtures (W = water, AN =
b
acetonitrile). First-order rate constants k1/s−1 for hydrolysis reactions.
c
Second-order rate constants k2/L mol−1 s−1 were determined by eq
d
CONCLUSION
S4 (see Supporting Information) and are less precise. Reaction was
performed in a 9:91 (v/v) H2O/CH3CN mixture. Calculated from eq
■
e
We have shown that the reactions of the benzaldehyde-derived
iminium ions 1 with electron-rich π-systems (enamines,
silylated ketene acetals, and enol ethers) in acetonitrile can
be described by eq 1, using the N and sN parameters derived
from the reactions of these π-nucleophiles with benzhydrylium
ions. One can, therefore, conclude that the reactions of iminium
1 with the electrophilicity parameters E for the iminium ions 1a−f
(Table 4) and the N (sN) parameters for the O- and N-nucleophiles in
CH3CN; for H2O/CH3CN (v/v) mixtures (50W50AN, 5.05 (0.89);
33W67AN, 5.02 (0.90); 20W80AN, 5.02 (0.89); 10W90AN, 4.56
(0.94)) from ref 20; for OH− (10.19 (0.62) in a 50/50 (v/v) H2O/
CH3CN mixture from ref 21; and for primary amines (Bn-NH2, 14.29
(0.67); iPr-NH2, 13.77 (0.70); tBu-NH2, 12.35 (0.72)) from ref 19.
+
ions Ar−CHNMe2 and benzhydrylium ions Ar2CH+ with
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C-nucleophiles in CH3CN have analogous transition states, in
which only the interactions of two reaction centers are of
importance. Equation 1 does not properly describe the
reactions of iminium ions with silylated enol ethers and ketene
acetals in CH2Cl2, which may be explained by the greater
importance of secondary N−Si interactions in Zimmerman−
Traxler-like transition states (incipient sila-ene reaction) in the
less polar solvent CH2Cl2.
AUTHOR INFORMATION
Corresponding Author
herbert.mayr@cup.uni-muenchen.de
■
Author Contributions
§R.A., S.C., and T.T. contributed equally.
Notes
The authors declare no competing financial interest.
As illustrated in Figure 5, the N,N-dimethyl-substituted
iminium ion derived from benzaldehyde is 10 orders of
ACKNOWLEDGMENTS
■
Dedicated to Professor Scott Denmark on the occasion of his
60th birthday. We thank the Deutsche Forschungsgemeinschaft
(SFB 749) for financial support. We are grateful to Dr. Peter
Mayer for performing the X-ray diffraction experiments and to
Dr. Armin R. Ofial for help during preparation of this
manuscript.
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ASSOCIATED CONTENT
* Supporting Information
Synthetic procedures, product characterization, and details of
the kinetic experiments. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
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