An efficient and practical system for the catalytic oxidation of alcohols, aldehydes, and α,β-unsaturated carboxylic acids

Article abstract of DOI:10.1021/jo0612574

(Chemical Equation Presented) Upon exposure to commercial bleach (～5% aqueous sodium hypochlorite), nickel(II) chloride or nickel(II) acetate is transformed quantitatively into an insoluble nickel species, nickel oxide hydroxide. This material consists of high surface area nanoparticles (ca. 4 nm) and is a useful heterogeneous catalyst for the oxidation of many organic compounds. The oxidation of primary alcohols to carboxylic acids, secondary alcohols to ketones, aldehydes to carboxylic acids, and α,β- unsaturated carboxylic acids to epoxy acids is demonstrated using 2.5 mol % of nickel catalyst and commercial bleach as the terminal oxidant. We demonstrate the controlled and selective oxidation of several organic substrates using this system affording 70-95% isolated yields and 90-100% purity. In most cases, the oxidations can be performed without an organic solvent, making this approach attractive as a "greener" alternative to conventional oxidations.

Full text of DOI:10.1021/jo0612574

An Efficient and Practical System for the Catalytic Oxidation of
Alcohols, Aldehydes, and r,â-Unsaturated Carboxylic Acids
Joseph M. Grill, James W. Ogle, and Stephen A. Miller*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843-3255
samiller@mail.chem.tamu.edu
ReceiVed June 19, 2006
Upon exposure to commercial bleach (∼5% aqueous sodium hypochlorite), nickel(II) chloride or nickel-
(II) acetate is transformed quantitatively into an insoluble nickel species, nickel oxide hydroxide. This
material consists of high surface area nanoparticles (ca. 4 nm) and is a useful heterogeneous catalyst for
the oxidation of many organic compounds. The oxidation of primary alcohols to carboxylic acids, secondary
alcohols to ketones, aldehydes to carboxylic acids, and R,â-unsaturated carboxylic acids to epoxy acids
is demonstrated using 2.5 mol % of nickel catalyst and commercial bleach as the terminal oxidant. We
demonstrate the controlled and selective oxidation of several organic substrates using this system affording
70-95% isolated yields and 90-100% purity. In most cases, the oxidations can be performed without
an organic solvent, making this approach attractive as a “greener” alternative to conventional oxidations.
Introduction
bleach would cleave the diols in several sugars to give
carboxylic acids.4
Nickel oxide hydroxide has been known to oxidize a host of
organic substrates. One of the first examples appeared in a patent
from 1901.1 Among the substrates that can be oxidized using
nickel oxide hydroxide are alcohols, aldehydes, phenols, amines,
and oximes. However, early reports demonstrated oxidations
of organic compounds using 1-1.5 equiv of nickel. In 1975,
George and Balachandran published an extensive review cover-
ing the major reactions that can be carried out using stoichio-
metric nickel oxide hydroxide.2 Only two reports exist that
demonstrate the catalytic application of nickel oxide hydroxide
on organic substrates with excess bleach (∼5% aqueous sodium
hypochlorite) as the oxidant. Weijlard reported the first example
in 1945; nicotine and vitamin C were oxidized using a catalytic
amount of a nickel(II) salt (3-5 mol %) and excess sodium
hypochlorite.3 In 1996, Bekkum et al. reported that larger
amounts of nickel oxide hydroxide (15-120 mol %) and excess
We reasoned that many of the reactions carried out using
stoichiometric nickel oxide hydroxide2 could be carried out
instead with a catalytic amount of nickel in the presence of
excess sodium hypochlorite. The known stoichiometric reactions
offer considerable literature precedent, but their catalytic
analogues3,4 are essentially unexplored. This reasoning led to
our recent finding of the only other catalytic example with this
oxidation system. We utilized nickel(II) salts (2.5 mol %) and
excess bleach to oxidize several R,â-unsaturated carboxylic acids
to epoxy acids (Scheme 1a).5 Methylene chloride was employed
to dissolve the reactant.
Of particular interest to our group is the facile oxidation of
alcohols using relatively benign and inexpensive catalysts.
Typically, oxidations can be accomplished in the lab through
(4) Mombarg, E. J. M.; Abbadi, A.; van Rantwijk, F.; van Bekkum, H.
J. Carbohydr. Chem. 1996, 15, 513-522.
(1) Verfahren zur Oxydation Von Methylgruppen aromatischer Kohlen-
wasserstoffe; German Patent 127388, issued December 24, 1901.
(2) George, M. V.; Balachandran, K. S. Chem. ReV. 1975, 75, 491-
519.
(5) Grill, J. M.; Ogle, J. W.; Miller, S. A., submitted.
(6) Collins, J. C.; Hess, W. W.; Frank, F. J. Tetrahedron Lett. 1968, 9,
3363-3366.
(7) Bowden, K; Heilbron, I. M.; Jones, E. R. H.; Weedon, B. C. L. J.
Chem. Soc. 1946, 39-45.
(3) Weijlard, J. J. Am. Chem. Soc. 1945, 67, 1031-1032.
10.1021/jo0612574 CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/11/2006
J. Org. Chem. 2006, 71, 9291-9296
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Grill et al.
SCHEME 1. Oxidation of Organic Substrates with
Catalytic Nickel(II) Salts and Aqueous Bleach
various nickel(II) salts and excess bleach under ambient
conditions and appears to be quite general, giving predictable
products with high yields (70-95%) and high purities (90-
100%) in most cases (Scheme 1). With a few exceptions, these
oxidations can be performed without the use of an organic
solvent. For the oxidation of water-insoluble organics, a small
amount of added dichloromethane (or diethyl ether in certain
cases) greatly facilitates the reaction.
Results and Discussion
In order to obtain relevant data for activity and selectivity
comparisons, we decided to perform each oxidation reaction
under our standard conditionsswhich called for a small amount
of dichloromethane to dissolve the organicsand then perform
a separate set of experiments excluding all organic solvent from
the reaction. The results could then be compared to determine
the impact, if any, of the organic solvent. Since the active form
of the nickel oxide hydroxide catalyst is insoluble under our
reaction conditions,5 it is reasonable to assume that the oxidation
is occurring on the catalyst surface. Thus, we surmised that
liquid alcohols could be oxidized without the use of an organic
solvent, whereas water-insoluble solid alcohols would require
dissolution in an organic solvent. In most cases, this prediction
holds true. An organic substrate that is either a liquid or water-
soluble can be oxidized without the use of organic solvents.
This is clearly advantageous because it reduces the amount of
waste generated and it eliminates the need for a potentially toxic
organic solvent. This reaction appears to be quite general,
oxidizing both aliphatic and aromatic alcohols quite cleanly.
Additionally, aldehydes are readily oxidized to carboxylic acids
with this catalytic system.
SCHEME 2. Oxidation of 3-Butenoic Acid to Fumaric Acid
and Proposed Mechanism
SCHEME 3. Oxidation of Acrylic Acid
Oxidation of Primary Alcohols. The oxidation of primary
alcohols proceeds smoothly under the standard reaction condi-
tions to give the corresponding carboxylic acids following acidic
workup. Isolated yields for these reactions were generally good
ranging from 68 to 89%, and initial purities were high (80-
98% by gas chromatography, GC), indicating that the reaction
generates few byproducts (Table 1). Since these alcohols are
all liquids and/or water soluble, similar results were expected
when the organic solvent was excluded from the reaction.
Indeed, the different reaction conditions resulted in very similar
isolated yields and GC purities. These five tested substrates are
readily oxidized to the carboxylic acids without the need for
organic solvent.
Oxidation of Secondary Alcohols. Secondary alcohols are
oxidized to ketones in high yields and purities in most cases
(Table 2). Again, in the case of liquid alcohols or water-soluble
alcohols, no organic solvent is required, with one exception.
The oxidation of racemic sec-phenethyl alcohol reached only
33% conversion after the 4 h reaction time in the absence of
organic solvent (Table 2, entry 4). The balance of the material
was determined to be starting material by GC. Curiously,
running this reaction for longer periods of time did not improve
the yield or the conversion. With organic solvent, the GC purity
is high, but a low isolated yield of the corresponding ketone is
obtained. Also, the substrate in Table 2 entry 8 (a mixture of
diastereomers) did not react under either of the reaction
conditions, suggesting that excessive steric bulk severely
impedes oxidation. Finally, the cyclic vicinal diol 1,2-dihy-
droxycyclohexane (Table 2, entry 9, a mixture of diastereomers)
undergoes carbon-carbon bond cleavage to afford the R,ω-
dicarboxylic acid in high yield (90% isolated).
the stoichiometric use of Collins reagent,6 Jones’ reagent,7
pyridinium chlorochromate (PCC),8 or potassium permangan-
ate.9 These routes are generally undesirable due to the large
amounts of toxic-metal-containing waste that they produce in
addition to the fact that the reagents are always used in excess
to ensure reaction completion. They do, however, tend to be
selective and very reliable. Many industrial organic oxidations
can be accomplished with oxygen or hydrogen peroxide using
heterogeneous catalysts, but the reaction conditions can often
lead to over-oxidation of the product to give carbon dioxide
and water.10 Nevertheless, heterogeneous systems are industrially
useful for synthesizing commodity chemicals from abundant
feedstocks because modest selectivity is often compensated by
the low cost of the oxidant, usually oxygen gas. However, with
complex and expensive alcohols (fine chemical synthesis), a
more selective route is desirable.
Herein we report the facile oxidation of primary alcohols,
secondary alcohols, aldehydes, and R,â-unsaturated acids to give
carboxylic acids, ketones, carboxylic acids, and epoxy acids,
respectively, using rather inexpensive and commonly available
reagents. The oxidation proceeds rapidly in the presence of
(8) Corey, E. J.; Suggs, J. W. Tetrahedron Lett. 1975, 16, 2647-2650.
(9) (a) Fatiadi, A. J. Synthesis 1987, 2, 85-127. (b) Shaabani, A.;
Tavasoli-Rad, F. Synth. Commun. 2005, 35, 571-580.
(10) Bartholomew, C. H.; Farrauto, R. J. Fundamentals of Industrial
Catalytic Processes; Wiley: Hoboken, NJ, 2006; pp 579-581.
9292 J. Org. Chem., Vol. 71, No. 25, 2006

Efficient and Practical Catalytic Oxidation
TABLE 1. Oxidation of Primary Alcohols under Standard Reaction Conditions and without Organic Solvent
a Standard reaction conditions: 2.5 mol % of NiCl2, 45 mmol alcohol, 15 mL of CH2Cl2, 300 mL of ∼5% aq. NaOCl, 2 h at 0 °C, then 2 h at 20 °C.
b Organic solvent-free conditions: same, except no CH2Cl2. c The isolated products were at least 97% pure. GC ) gas chromatography; n.d. ) not determined.
Oxidation of Aldehydes. As expected, aldehydes are readily
oxidized to the corresponding carboxylic acids under the
standard reaction conditions. The four entries in Table 3 aptly
demonstrate this conversion, which affords the carboxylic acids
in high yield and high purity. The oxidation also proceeds rather
well without the use of organic solvent. Notably, even very
bulky aldehydes (Table 3, entry 4) are oxidized with facility.
The same oxidation of pivaldehyde with traditional oxidants
(Scheme 2). To our surprise, we obtained fumaric acid in high
yield (75%, >99% trans isomer by H NMR) rather than the
1
expected epoxide. The fumaric acid was not further oxidized
to the epoxide. This unusual transformation formally involves
isomerization of the double bond and an oxidation of carbon 4.
In independent experiments, we attempted to epoxidize fumaric
acid and maleic acid. Consistent with the results obtained from
the oxidation of 3-butenoic acid, these attempts failed to yield
epoxide and gave only starting material. If 3-butenoic acid were
first isomerized to 2-butenoic acid (cis or trans), then it should
simply be epoxidized as in Table 4, entry 2; this is not observed.
An alternative mechanism calls for direct epoxidation and
subsequent tautomerization. Ring opening of the epoxide/ene-
diol tautomer generates an allylic alcohol, which under the
reaction conditions should be oxidized to yield fumaric acid
(Scheme 2). Support for this mechanism is found in our
observation that 3-butenoic acid epoxide, prepared independently
with m-chloroperbenzoic acid, is converted with 69% yield to
fumaric acid under our standard conditions. Additionally, since
the oxidation of crotonic acid yields no fumaric acid, we must
reason that 3-butenoic acid is not isomerized to crotonic acid
prior to being oxidized.
12
oxone11 or H2O2 occurs with lower yields: 45 or 40%,
respectively.
Epoxidation of R,â-Unsaturated Carboxylic Acids. We
recently observed several examples of the epoxidation of R,â-
unsaturated carboxylic acids using our standard reaction condi-
tions.5 Thereafter, we decided to explore the possibility of
epoxidizing these olefins without the use of organic solvent.
All of our substrates dissolved readily in our aqueous reaction
medium, and as targeted, the R,â-unsaturated carboxylic acids
were readily epoxidized in yields and purities comparable to
those synthesized in the presence of dichloromethane (Table
4). Note that the methallyl alcohol in entry 5 is most likely
first oxidized to the R,â-unsaturated acid (methacrylic acid),
which is subsequently epoxidized.
Other Oxidations Using Nickel Oxide Hydroxide/
NaOCl: 3-Butenoic acid. While exploring the scope of this
reaction, we observed several unusual reactions. Initially, our
principal research interests were in applying nickel oxide
hydroxide/NaOCl toward epoxidation catalysis. This catalyst
system does not react with most standard olefins such as styrene
and 1-hexene. However, as previously discussed, R,â-unsatur-
ated carboxylic acids are readily epoxidized. We reasoned that
the acid functional group was binding to the surface of the
heterogeneous catalyst and thus positioning the olefin to accept
an oxygen atom. The requirement of a directing group would
explain the inertness of standard olefins. During the exploration
of this hypothesis, we attempted to epoxidize 3-butenoic acid
Other Oxidations Using Nickel Oxide Hydroxide/
NaOCl: Acrylic Acid. Payne reported the sodium tungstate
catalyzed epoxidation of R,â-unsaturated carboxylic acids with
hydrogen peroxide in 1959,13 and Sharpless later optimized this
reaction.14 However, it is curious that neither Payne nor
Sharpless reported the epoxidation of acrylic acid with the
tungsten-based system. The omission of this simple substrate
led us to believe that its reaction with the tungsten system is
poorly selective or altogether unfruitful. Our nickel system also
(13) Payne, G. B.; Williams, P. H. J. Org. Chem. 1959, 24, 54-55.
(14) Kirshenbaum, K. S.; Sharpless, K. B. J. Org. Chem. 1985, 50, 1979-
1982.
(15) Konaka, R.; Terabe, S.; Kurama, K J. Org. Chem. 1969, 34, 1334-
1337.
(11) Travis, B. R.; Sivakumar, M.; Hollist, G. O.; Borhan, B. Org. Lett.
2003, 5, 1031-1034.
(12) Sato, K.; Hyodo, M.; Takagi, J.; Aoki, M.; Noyori, R. Tetrahedron
Lett. 2000, 41, 1439-1442.
(16) Konaka, R.; Kurama, K. J. Org. Chem. 1971, 36, 1703-1704.
(17) Terabe, S.; Konaka, R. J. Am. Chem. Soc. 1969, 91, 5655-5657.
(18) Terabe, S.; Konaka, R. J. Chem. Soc., Perkin Trans. 1972, 2, 2163-
2172.
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Grill et al.
TABLE 2. Oxidation of Secondary Alcohols under Standard Reaction Conditionsa
a Standard reaction conditions: 2.5 mol % of NiCl2, 45 mmol alcohol, 15 mL of CH2Cl2, 300 mL of ∼5% aq. NaOCl, 2 h at 0 °C, then 2 h at 20 °C.
b Organic solvent-free conditions: same, except no CH2Cl2. c The isolated products were at least 97% pure. GC ) gas chromatography; n.d. ) not determined;
n.r. ) no reaction, starting material recovered. d The organic solvent employed for 2-adamantanol was diethyl ether.
TABLE 3. Oxidation of Aldehydes under Standard Reaction Conditions and without Organic Solvent
a Standard reaction conditions: 2.5 mol % of NiCl2, 45 mmol aldehyde, 15 mL of CH2Cl2, 300 mL of ∼5% aq. NaOCl, 2 h at 0 °C, then 2 h at 20 °C.
b Organic solvent-free conditions: same, except no CH2Cl2. c The isolated products were at least 94% pure. GC ) gas chromatography.
fails to epoxidize this R,â-unsaturated carboxylic acid. Instead,
it is fragmented and chlorinated, quite possibly via radical
pathways known to operate with nickel oxide hydroxide.15-18
A 1:1 mixture of chloroacetic acid and dichloroacetic acid was
obtained from this reaction (overall yield is 78%), demonstrating
the cleavage of carbon-carbon bonds along with chlorination
(Scheme 3). In some casessunder seemingly identical reaction
conditionssthe chloroacetic acid/dichloroacetic acid ratio is as
9294 J. Org. Chem., Vol. 71, No. 25, 2006

Efficient and Practical Catalytic Oxidation
TABLE 4. Epoxidation of r,â-Unsaturated Carboxylic Acids
under Standard Reaction Conditions5 and without Organic Solvent
FIGURE 1. Distribution versus time in the oxidation of benzyl alcohol
with 2.5 mol % of NiCl2/aqueous NaOCl at 20 ( 1 °C.
a Standard reaction conditions: 2.5 mol % of NiOAc2, 45 mmol olefin,
15 mL of CH2Cl2, 300 mL of ∼5% aq. NaOCl, 2 h at 0 °C, then 2 h at 20
°C. See ref 5. b Organic solvent-free conditions: same, except no CH2Cl2.
GC ) gas chromatography.
high as 5:1. We were unable to determine the fate of the C1
fragment that was apparently generated and lost during workup.
On the basis of the behavior of acrylic acid, it seems that a
vinylic methyl substituent is required for selective epoxidation
of R,â-unsaturated carboxylic acids. Moreover, greater selectiv-
ity is effected when this methyl group is in the R position, as
evidenced by the modest conversion (42% by GC) observed in
the epoxidation of â-methyl acrylic acid (crotonic acid) versus
that of R-methyl acrylic acids (85-95% by GC). For crotonic
acid epoxidation, the radical fragmentation/chlorination pathway
is partially operative, and the balance of the substrate is largely
converted to chloroacetic acid and dichloroacetic acid.
Preliminary Kinetic Studies. In an effort to gain a better
understanding of this catalyst system, the oxidation of benzyl
alcohol was explored in greater detail. Gas chromatography
(GC) was employed to determine the distribution of organic
components present as a function of time at 5 min intervals for
a reaction at a temperature of 20 ( 1 °C. In the case of benzyl
alcohol, the reaction proceeds cleanly through a benzaldehyde
intermediate, which is subsequently oxidized to benzoic acid.
The concentration of benzoic acid is initially very low, while
the concentration of benzaldehyde steadily increases, reaching
its peak abundance around 40-45 min (Figure 1). The benzyl
alcohol is completely oxidized to benzoic acid in about 70 min
at room temperature.
FIGURE 2. Distribution versus time in the oxidation of benzaldehyde
with 2.5 mol % of NiCl2/aqueous NaOCl at 20 ( 1 °C.
Separately, the oxidation of benzaldehyde was also explored
under these same reaction conditions. Benzaldehyde is oxidized
rapidly by the catalyst, achieving complete conversion to
benzaldehyde in about 20 min (Figure 2). This time frame is
consistent with data from the oxidation of benzyl alcohol (Figure
1), wherein the reaction is complete about 25 min following
the peak concentration of benzaldehyde.
Analysis of Figures 1 and 2 should reveal the relative
reactivity of benzyl alcohol versus that of benzaldehyde. The
first 30 min of Figure 1 shows [benzyl alcohol] versus time to
be linear. Since 45 mmol of substrate was employed and 33.8%
was consumed during this time period, the initial rate can be
calculated as kalcohol ) (0.045 mol)(33.8% conversion)/(30 min)
) 5.1 × 10-4 mol/min. Similarly, the first 15 min of Figure 2
shows [benzaldehyde] versus time to be linear, and the initial
rate can be calculated as kaldehyde ) (0.045 mol)(93.6% conver-
FIGURE 3. Distribution versus time in the oxidation of 1-propanol
with 2.5 mol % of NiCl2/aqueous NaOCl at 20 ( 1 °C. Propionaldehyde
concentration was not determined.
sion)/(15 min) ) 2.8 × 10-3 mol/min. Thus, the oxidation of
benzaldehyde is somewhat faster than the oxidation of benzyl
alcohol under these conditions and kaldehyde/kalcohol ) 5.5.
Our initial kinetic studies indicate that aliphatic alcohols are
oxidized more rapidly than benzylic alcohols. 1-Propanol is
J. Org. Chem, Vol. 71, No. 25, 2006 9295

Grill et al.
product. The purities could generally be improved to >98% by
distillation of the crude product or by recrystallization in the case
of solids.
quantitatively converted to propionic acid in just 15 min (Figure
3), compared to 70 min for benzyl alcohol. Unfortunately, we
were unable to monitor the formation of the presumed propi-
onaldehyde intermediate, as it is coincident with the solvent
peak in the gas chromatogram. An initial rate analysis of Figure
3 suggests that kpropanol ) (0.045 mol)(100% conversion)/(15
min) ) 3.0 × 10-3 mol/min, which is a similar to that of
benzaldehyde and faster than that of benzyl alcohol by a factor
of 5.9.
Standard Procedure for the Oxidation of Secondary Alcohols.
A 500 mL flask was charged with NiCl2 hexahydrate (0.27 g, 1.14
mmol) and water (5 mL). A secondary alcohol (45 mmol) was
added followed by (optional) dichloromethane (15 mL). The
reaction was cooled in an ice bath, and cold bleach (300 mL) was
added in a steady stream over 5 min. A fine black precipitate formed
immediately. The resulting slurry was stirred for 2 h at 0 °C and 2
h at room temperature. The aqueous layer was extracted with diethyl
ether (3 × 100 mL). The combined organic extracts were dried
over anhydrous MgSO4 and filtered. Removal of the solvent by
rotary evaporation and brief high vacuum gave the crude product.
The purities could generally be improved to >98% by distillation
of the crude product or by crystallization in the case of solids.
Standard Procedure for the Epoxidation of R,â-Unsaturated
Carboxylic Acids. In a 500 mL round-bottom flask, sodium
hydroxide (1.8 g, 45 mmol) was dissolved in water (10 mL), and
an R,â-unsaturated carboxylic acid (45 mmol) was added followed
by (optional) dichloromethane (15 mL). The flask was cooled in
an ice bath, and a solution of nickel(acetate)2 tetrahydrate (0.28 g,
1.14 mmol) in water (5 mL) was added. Cold bleach (300 mL)
was added in a steady stream over 5 min. A fine black precipitate
formed immediately. The resulting slurry was stirred for 2 h at 0
°C and 2 h at room temperature. The slurry was then acidified with
2 M hydrochloric acid until the aqueous layer was strongly acidic
by pH paper. The aqueous layer was extracted with diethyl ether
(3 × 100 mL). The combined organic extracts were dried over
anhydrous MgSO4 and filtered. Each product was then analyzed
by GC.
Conclusions
We have demonstrated a very effective, practical, and
inexpensive method for oxidizing alcohols, aldehydes, and R,â-
unsaturated carboxylic acids with readily available nickel(II)
salts and commercial bleach. The nickel oxide hydroxide formed
serves as a selective and high-yielding heterogeneous catalyst,
converting both aliphatic and aromatic substrates to the expected
product in most cases. These reactions can often be performed
without organic solvents, thus improving the environmental
impact of these oxidative transformations, which traditionally
generate much organic and toxic metal waste. Future research
will target the catalytic oxidation of other functional groups.
Experimental Section
Standard Procedure for the Oxidation of Primary Alcohols
and Aldehydes. A 500 mL flask was charged with NiCl2 hexahy-
drate (0.27 g, 1.14 mmol) and water (5 mL). A primary alcohol or
an aldehyde (45 mmol) was added followed by (optional) dichlo-
romethane (15 mL). The reaction was cooled in an ice bath, and
cold bleach (300 mL) was added in a steady stream over 5 min. A
fine black precipitate formed immediately. The resulting slurry was
stirred for 2 h at 0 °C and 2 h at room temperature. The slurry was
then acidified with 2 M hydrochloric acid until the aqueous layer
was strongly acidic by pH paper. The aqueous layer was extracted
with diethyl ether (3 × 100 mL). The combined organic extracts
were dried over anhydrous MgSO4 and filtered. Removal of the
solvent by rotary evaporation and brief high vacuum gave the crude
Acknowledgment. This research is supported by The Robert
A. Welch Foundation (No. A-1537) and the National Science
Foundation (CAREER CHE-0548197).
Supporting Information Available: Detailed experimental
procedures and characterization data, along with details of the
kinetic studies. This material is available free of charge via the
Internet at http://pubs.acs.org.
JO0612574
9296 J. Org. Chem., Vol. 71, No. 25, 2006