High pressure mechanistic diagnosis in Baeyer-Villiger oxidation of aliphatic ketones

Article abstract of DOI:10.1016/S0040-4039(01)01955-4

The pressure effect is examined in Baeyer-Villiger oxidation of aliphatic ketones. This effect is small, reflected in slightly negative activation volumes (-2 to -8 cm³ mol^-1). These values allow the picturing of the volume profile. They refer to a late transition step and give support for a rate-determining migration step experiencing full concertedness.

Full text of DOI:10.1016/S0040-4039(01)01955-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8969–8971
High pressure mechanistic diagnosis in Baeyer–Villiger
oxidation of aliphatic ketones
Ge´rard Jenner*
Laboratoire de Pie´zochimie Organique (UMR 7509), Faculte´ de Chimie, Universite´ Louis Pasteur, 67008 Strasbourg, France
Received 8 August 2001; revised 16 October 2001; accepted 17 October 2001
Abstract—The pressure effect is examined in Baeyer–Villiger oxidation of aliphatic ketones. This effect is small, reflected in slightly
negative activation volumes (−2 to −8 cm3 mol−1). These values allow the picturing of the volume profile. They refer to a late
transition step and give support for a rate-determining migration step experiencing full concertedness. © 2001 Elsevier Science
Ltd. All rights reserved.
In former papers, we reported on high pressure reac-
tions involving aliphatic ketones of increasing steric
complexity, e.g. Knoevenagel reactions1 and nucleo-
philic additions.2 We disclosed an important result as
sterically hindered reactions are prone to increased
sensitivity to pressure compared to their unhindered
analogs. There are many other name reactions using
ketones as main substrates. As an example, the venera-
ble Baeyer–Villiger oxidation of ketones to esters has,
to our knowledge, never been examined under pressure
(Scheme 1). Our original objective was attempted gen-
eralization of the pressure versus steric hindrance rela-
tionship. To that purpose, we examined the oxidation
of aliphatic ketones by m-chloroperbenzoic acid
(MCPA) under ambient and high pressure (300 MPa)
conditions (Table 1). The reaction proceeded cleanly
with full chemoselectivity.
The alleged mechanism rests on a multi-step ionic
process proposed by Criegee5 consisting of the acid-cat-
alyzed addition of MCPA to the protonated carbonyl
bond followed by migration within the transient inter-
mediate (A) (Scheme 2). The mechanism of the Baeyer–
Villiger reaction has received extensive attention since
Criegee’s proposal.6 Most authors consider the migra-
tion step as rate determining on the basis of stereo-
chemical and isotopic labeling results, kinetic studies
and theoretical calculations.7 Interestingly, it was found
that migration is synchronous with the departure of the
leaving group.8 A recent paper reported on unusual
Table 1. Effect of pressure on yields in Baeyer–Villiger
oxidation of ketonesa
R
R%
Yields (%)
300 MPa
0.1 MPa
These results are indicative of a small pressure effect, as
the yields are increased by a factor of 2–3 when pres-
sure is raised to 300 MPa. There is no clear relation
between pressure steric acceleration and steric bulk of
R and R% if any, at variance with our previous
results.1,2. This apparent singularity prompted us to
examine kinetically the Baeyer–Villiger reaction in the
pressure range 0–100 MPa in order to determine the
volume of activation DV".3,4
CH3
CH3
CH3
CH3
nC4H9
C3H7
n-C4H9
12
58
37
5
21
28
37
89
88
16
60
47
sec-C4H9
tert-C4H9
CH2-tert-C4H9
n-C4H9
i-C3H7
a In CHCl3 at 25.0°C for 6 h.
Cl
Cl
R
R'
O
O
OOH
RCOOR' +
+
O
OH
Scheme 1.
Keywords: Baeyer–Villiger reactions; mechanism; pressure; activation volume.
* Corresponding author. Tel.: (33) 03.90.24.16.79; fax: (33) 03.90.24.17.39; e-mail: jenner@chimie.u-strasbg.fr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01955-4

8970
G. Jenner / Tetrahedron Letters 42 (2001) 8969–8971
O
O
R
H
+
H
R
Cl
Cl
Cl
+
HO
O
O
O
O
O
R'
R'
O
O
R'
O
R
O
C
O
R'
Cl
R
O
H
O
(A)
+
OH
H
O
O
R
R'
Cl
+
HO
O
Scheme 2.
These observations are in full harmony with the B–V
mechanism advocated by other ways.6–8 Particularly, it
was found that with peroxidic nucleophiles a steric
requirement is not important.6 This is also corrobo-
rated by the k-values determined at ambient pressure
showing only slight variations even with increasing size
of R and R%. In fact, k-values are even higher when R%
is tert-C4H9 or sec-C4H9 than the corresponding value
in the reaction involving R%=C4H9. The result is in line
with earlier observations6a and can be accounted for by
steric assistance of migration of the more bulky
group.12
migration modes in the Baeyer–Villiger oxidation of
a-fluorocyclohexanones to lactones.9
Considering that the addition step involves formation
of a CꢀO bond whereas migration implies formation
and cleavage of various bonds, the activation volume
could be a decisive argument to determine which step is
rate determining since bond making and bond breaking
are clearly reflected in the volume. We therefore, fol-
lowed the kinetics at different pressures of the Baeyer–
Villiger oxidation of some aliphatic ketones where R
and R% have variable bulkiness.10 All reactions satisfied
clear second-order kinetics (the reaction is first-order in
both ketone and peracid and is acid-catalyzed).6,11
Conclusion
If the RDS is migration it involves a major rearrange-
ment process with several events consisting of equally
matched bond-making and bond-breaking except the
cleavage of the OꢀO bond. Intuitively, based on mere
structural modifications, the RDS should feature
The present piezokinetic study nicely confirms in every
aspect the mechanism of the Baeyer–Villiger reaction
previously determined by a number of authors also in
agreement with recent semi-empirical studies.13 Impor-
tantly, it should be emphasized that high pressure
kinetics is a complementary tool to assess mechanisms14
supported by other lines of evidence, despite recent
slightly positive DV"-values meaning
a pressure
induced deceleration of the rate in B–V oxidations.
Considering the DV"-values listed in Table 2, this is
clearly not the case although it is obvious that the first
step is not rate-determining since it would require val-
ues around −20 cm3 mol−1. The results underscore the
remarkable uniformity of DV" values which are all less
negative or around zero (from −2 to −8 cm3 mol−1).
Table 2. Kinetic data for Baeyer–Villiger oxidation of
ketonesa
R
R%
105 k (M s−1
)
DV" (92 cm3
mol−1
)
CH3
CH3
CH3
CH3
CH3
C3H7
i-C4H9
n-C5H11
n-C4H9
sec-C4H9
tert-C4H9
CH2-tert-C4H9
i-C3H7
3.89
3.36
10.93
9.34
1.56
9.03
1.98
−8.0
−6.5
−5.0
−5.0
−8.0
−2.0
−6.0
Comparison with reaction volumes (+9–10 cm3 mol−1)
suggests that the transition state is late but not close to
products (Scheme 3). We are, therefore, inclined to
consider that the slightly negative DV"-values reflect a
perfectly concerted process involving simultaneous elec-
tron flow with concomittant migration and OꢀO cleav-
age. Interestingly, they do not reflect any steric
contribution, contrasting with our preceeding results.1,2
i-C4H9
a The reported k-values refer to reactions carried out in CHCl3 at
49.7°C and 0.1 MPa.

G. Jenner / Tetrahedron Letters 42 (2001) 8969–8971
8971
Scheme 3. A possible volume profile for the Baeyer–Villiger reaction (TS: transition state). The relative positions of the different
volume levels are arbitrary.
allegations denying any value for mechanistic deline-
ation by this method.15
M. F.; Emmons, W. D. J. Am. Chem. Soc. 1958, 80,
3698–6404.
7. Krow, G. R. Org. React. 1993, 43, 251–798.
8. Winnik, M. A.; Stoute, V.; Fitzgerald, P. J. Am. Chem.
Soc. 1974, 96, 1977–1979.
References
9. Crudden, C. M.; Chen, A. C.; Calhoun, L. A. Angew.
1. Jenner, G. Tetrahedron Lett. 2001, 42, 243–245.
2. Jenner, G. Tetrahedron Lett. 2001, 42, 4807–4810.
3. Standard runs (Table 1): In a typical run, MCPA (1.0
mmol), ketone (0.5 mmol) and bibenzyl (internal stan-
dard about 0.12 mmol) are placed in a flexible 2.5 mL
PTFE tube. The volume is adjusted with CHCl3. The
tube is shaken and rapidly introduced into the high
pressure vessel. After release of pressure, the solution is
washed with an aqueous solution of NaHCO3 (20%). The
organic layer is extracted with ether washed first with the
NaHCO3 solution, then with water. The volatile com-
pounds are removed in vacuo. The residue is directly
analyzed by 1H NMR and the yield determined from
relative intensities of characteristic protons versus meth-
ylene protons of the internal standard.
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10. Kinetics. A stock chloroform solution of weighed MCPA
and bibenzyl (standard) is prepared and kept at 5°C. For
a kinetic run 60 ml of ketone is placed in a PTFE tube
and weighed. An adjusted volume (600 ml of stock solu-
tion is added and weighed. The tube is then submitted to
the desired pressure. Working up and analysis as above.
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12. We thank the referee for his pertinent comments.
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stone’s arguments were refuted in: le Noble, W. J.;
Asano, T. J. Phys. Chem. A 2001, 105, 3428–3429.
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J. Am. Chem. Soc. 1958, 80, 3693–6398; (b) Hawthorne,