1624 Bull. Chem. Soc. Jpn. Vol. 81, No. 12 (2008)
Oxidation with Bismuthonium Salts
was observed for the alcohol oxidation by Ar3BiCl2. For in-
stance, both mesityl and 2,6-xylyl ligands abstract the ꢀ-hy-
drogen >300 times faster than the phenyl ligand. The substitu-
ent effects on the reaction rates will be discussed later.
the forward rate of the first step. (3) The second step is suscep-
tible to the steric nature of the substituents. The introduction of
methyl groups at the ortho positions destabilizes the alkoxy-
tetraarylbismuth(V) intermediate A due to the steric conges-
tion around the bismuth center.17 As a consequence, the rate
of ꢀ-hydrogen abstraction is accelerated significantly by the
introduction of the mesityl and 2,6-xyly ligand. Hence, to con-
struct highly efficient onium-type BiV oxidants, it is desirable
to introduce an electron-withdrawing substituent at the para
position and/or moderately bulky substituents at the ortho
positions by considering a balance between these two effects.
In this context, triphenyl(2,6-xylyl)bismuthonium salt 1h and
tris(p-chlorophenyl)(mesityl)bismuthonium salt 1i are promis-
ing oxidants similar to mesityltriphenylbismuthonium salt 1g.
With new efficient organobismuth(V) oxidants 1g–1i in
hand, we examined the oxidation of some alcohols. As summa-
rized in Table 5, primary and secondary alcohols were oxi-
dized by 1g–1i/TMG to aldehydes and ketones, respectively,
at room temperature. The carbonyl compounds were easily
separated from the bismuthane and the guanidinium salt by
silica gel column chromatography. In the reaction of primary
alcohols, overoxidation to carboxylic acids did not occur at
all (Entries 1, 2, and 6). As expected, 1h and 1i oxidized the
alcohols more rapidly than 1g. In particular, the remarkable
rate acceleration was observed for the reaction of secondary
benzylic alcohols (Entries 3 and 4) and saturated alcohols
(Entries 6 and 7) without noticeable side reactions. It is also
noteworthy that 1g–1i converted 2,2,2-trifluoro-1-phenyletha-
nol to 2,2,2-trifluoro-1-phenylethanone within 0.5–1.5 h at
room temperature (Entry 5).18 The efficiency of this oxidation
is appreciably higher than that of the Dess–Martin oxidation,
which was reported to require excess oxidant (3.7 equiv) to ob-
tain a good yield of the ketone.18a The above results clearly
demonstrate the synthetic utility of tetraarylbismuthonium
salts 1g–1i as stoichiometric oxidants.19
To get some insight into the rate-determining step of the
present oxidation system, we also measured intermolecular
and intramolecular H/D kinetic isotope effects for the
ꢀ-hydrogen abstraction. The intermolecular competition
reaction of 1g with a mixture of p-BrC6H4CH2OH and p-
BrC6H4CD2OH (each 3 equiv) afforded the corresponding p-
bromobenzaldehydes (eq 1). Similarly, the intramolecular
competitive reaction of 1g with p-BrC6H4CH(D)OH was con-
ducted (eq 2). In these reactions, mesitylenes (MesH/MesD)
and triphenylbismuthane were formed in >99% yields. The
kinetic isotope effects were determined by 1H NMR spec-
troscopy based on the relative ratio of p-BrC6H4CHO/
p-BrC6H4CDO. As listed in eqs 1 and 2, the observed intermo-
lecular kinetic isotope effect (kH/D ¼ 3:9 ꢅ 0:6) is almost
identical to the intramolecular kinetic isotope effect (k0H/D
¼
3:9 ꢅ 0:5), indicating that the alcohol oxidation using 1 con-
sists of fast pre-equilibrium (first step) and irreversible ꢀ-hy-
drogen abstraction (second step).12 Therefore, it can be con-
cluded that the rate-determining step involves the ꢀ-hydrogen
abstraction by the aryl ligand.
1g + TMG
RCHO + RCDO
(– ArH) (– ArD)
RCH2OH + RCD2OH
CDCl3, 24 1 °C
H/D = 3.9 0.6
ð1Þ
(R = p-BrC6H4)
k
1g + TMG
CDCl3, 24 1 °C
kH/D = 3.9 0.5
RCDO + RCHO
(– ArH) (– ArD)
RCH(D)OH
ð2Þ
(R = p-BrC6H4)
It is well known that the primary kinetic isotope effect on
the concerted hydrogen abstraction reaction is related to the
Finally, we examined the competitive oxidation reactions
between primary (1ꢃ) and secondary (2ꢃ) alcohols using tetra-
arylbismuthonium salts 1d and 1f–1i, tris(2-methylphenyl)-
bismuth dichloride, and Dess–Martin periodinane (DMP)
(Table 6). When a mixture of benzyl alcohol (PhCH2OH)
and benzylic secondary alcohol (PhCH(R)OH; R ¼ Me, Et,
and i-Pr) was treated with 1g/TMG in CDCl3 at 24 ꢅ 1 ꢃC,
benzaldehyde (PhCHO) was produced predominantly over
phenyl alkyl ketone (PhCOR; R ¼ Me, Et, and i-Pr). In this
series, the 1ꢃ/2ꢃ (PhCHO/PhCOR) selectivity increases from
81/19 to 94/6 with increasing the size of ꢀ-substituents (R)
of the secondary benzylic alcohols. It should be emphasized
that all of these alcohols are oxidized quantitatively by 1g/
TMG under the appropriate conditions. In the competitive
reactions between benzyl alcohol and 1-phenyl-1-propanol
(Entries 2 and 4–7), the mesityl and 2,6-xylyl-substituted de-
rivatives 1g–1i exhibit higher 1ꢃ/2ꢃ selectivities (91/9–92/8)
than do the 4-methylphenyl and 2-methylphenyl derivatives
1d and 1f (74/26 and 77/23). These results imply that the first
step is sensitive to bulkiness of the substrates as well as steric
environment around the cationic bismuth center. Thus, not on-
ly the ortho-methyl groups of the oxidants but also the ꢀ-sub-
stituents of the alcohols retard the Bi–O bond-forming reaction
(nucleophilic attack of the alcohol to the bismuth center) kinet-
linearity of the X H–C bond at the transition state (X = hy-
ꢁꢁꢁ
drogen-abstracting atom).13 The four-fold preference for loss
of H relative to D observed in the present intramolecular com-
petition experiments suggests that the ꢀ-hydrogen is abstracted
via a weakly polarized, cyclic transition state.14 Thus, the hy-
drogen-abstracting aptitude of the aryl ligands should depend
on bond dissociation energies of the Bi–C bonds of alkoxybis-
muth(V) intermediate A. Presumably, the Bi–C bonds of the
mesityl and 2,6-xyly ligands in A are weakened because of
steric congestion around the bismuth center (vide supra).15,16
The substituent effects of the aryl ligands observed in the
present study are summarized as follows. (1) The electronic ef-
fect observed for the overall step is appreciably larger than that
observed for the second step. In other words, the first step is
sensitive to the electronic character of the aryl ligands. With
increasing electron-withdrawing ability of the para substitu-
ents, the electrophilicity of the bismuth center increases to ac-
celerate the forward rate of the first step (Bi–O bond forma-
tion), and as a result, the pre-equilibrium is put forward to
the right side. (2) The steric effect observed for the overall
steps is considerably smaller than that observed for the second
step. It is likely that the bulky aryl ligands, such as mesityl and
2,6-xylyl, kinetically protect the bismuth center to decelerate