C O M M U N I C A T I O N S
Figure 1. Plot of logarithm of 4:5 ratios log (kR/kH) vs Taft’s polar
substituent constants σ* for aliphatic groups.
was reported for classical BVO of substituted acetophenones using
trifluoroperacetic acid in dichloroethane, in which decomposition
of the Criegee adduct constitutes the rate-limiting step.16 Further-
more, these two relative rate factors for the λ3-bromane-induced
and the classical peracid-induced BV reactions correlate very well
with each other (see Figure S2). These results indicate that our
BVO of benzaldehydes probably involves a rate-limiting migration
of aryl groups and that a similar kind of electronic effect of
substituents operates in the transition states of both classical and
modern BVO.
Figure 2. Energy profile for the Me and H migration in BVO of
acetaldehyde based on ab initio calculations. Br (large dark purple), O (red),
C (gray), and F (blue purple).
calculated for the 1,2-shift of the methyl group in λ3-iodane-derived
Criegee adducts (see Figure S4) agrees with our experimental
findings (Table 1, entry 9). Thus, calculations strongly suggest that
the reaction proceeds easily and reasonably to give the products
via the protonated species such as 11 and 12, although the solvent
effect other than the protonation is not considered in the calculations.
Further calculations for BVO of aldehydes RCHO (R ) Et, i-Pr,
t-Bu) revealed that the activation energy for the concerted 1,2-shift
of an alkyl group in λ3-bromane-derived Criegee adducts decreases
considerably with an increasing degree of alkyl substitution in the
order (kJ mol-1) Me (48.7), Et (24.5), i-Pr (16.2), and t-Bu migration
(9.4). Differences in these estimated activation barriers are correlated
well (r ) 0.95) with Taft’s σ* values of alkyl groups (see Figure
S5), again suggesting that the electronic nature of alkyl groups is
of paramount importance in the BV rearrangement.18
Scheme 3
Scheme 3 clearly demonstrates that the essential feature of the
classical BVO holds for our unique BVO technology: oxidation of
chiral (S)-aldehyde 6 with difluorobromane 2 takes place with
extensive retention of stereochemistry at the migrating center.
Scheme 3 also depicts BVO of cyclohexanones, producing seven-
membered lactones in high yields. Preferential migration of the more
substituted methine group of 2-methylcyclohexanone yielding
lactone 8b as a major product was observed, as in the case of the
classical BVO.2
Thus, we have developed a conceptually distinct, new strategy
for the BVO. The method involves a hydration-ligand exchange
sequence to Criegee intermediates using difluorobromane(III). Our
method makes it possible to selectively induce the normal BVO of
straight chain aliphatic as well as aromatic aldehydes, which is
missing in the classical BVO.
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research (B) (JSPS). We thank Central Glass Co.
Ltd., Japan for a generous gift of BrF3.
Calculated structures and the energy profile for the methyl and
hydrogen migration of the activated (protonated) intermediate,
R-hydroxyethoxy(phenyl)-λ3-bromane 11, in the bromane-induced
BVO of acetaldehyde at the MP2/6-311G(d) method of the Gaussian
03 program are illustrated in Figure 2.17 The leaving phenyl-λ3-
bromanyl and the migrating methyl groups in transition state 13
occupy an anti periplanar arrangement with a dihedral angle of
-169.9° about the O-C bond, along which the migration takes
place. A similar anti arrangement (-176.8°) in 10 was evaluated
for the hydrogen migration (see also Figure S3). Transition state
10 for hydrogen shift is predicted to lie 25.3 kJ mol-1 higher in
energy than 13 for the methyl shift, which is not compatible with
the experimental observations of the preferred formation of acetic
acid over methyl formate production (Table 2, entry 1). Therefore,
it seems reasonable to assume that the acetic acid formation
probably involves a ꢀ-elimination process, but not a 1,2-H shift of
Supporting Information Available: Experimental procedures,
supplementary figures, and computational results. This material is
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9238 J. AM. CHEM. SOC. VOL. 132, NO. 27, 2010