and 2d with the alcohol for 3 h resulted in the formation of the
corresponding aldehyde only in 12 and 16% yields, respectively
(runs 3 and 4). The reason for the enhanced oxidizing ability of the
bulky telluroxides is not clear at the present stage. We assume that
the telluroxides 2a and 2b can exist as a monomeric form due to the
bulky substituents, and, accordingly, the inherent reactivity of
telluroxide itself, not as a hydrate or molecular aggregates, would
be expressed. In fact, the 125Te NMR spectra of 2a and 2b in CDCl3
at room temperature show signals at 1308 and 1314 ppm (relative
to Me2Te), respectively, which are far downfield as compared with
those of diphenyl telluroxide 2d (1035 ppm),10 bis(pentafluor-
ophenyl) telluroxide (1010 ppm)4c and tellurathiocin derivative
(1154 ppm).10 With the “true” telluroxide in hand, we examined the
oxidation of a variety of alcohols. The representative results are
also compiled in Table 1.
which undergoes intramolecular dehydration to afford the corre-
sponding carbonyl compound and diaryl telluride 1. The bulky
substituents may also contribute to acceleration of the final
reductive elimination due to the steric congestion around the
tellurium(IV) atom. This is the first example of chalcogen oxide
which can oxidize simple alcohols via a formal dehydrogenation
reaction. An azeotropic removal of the liberated water by refluxing
in xylene or toluene is essential to complete the reaction. When the
oxidation of 4-bromobenzyl alcohol with 2a was performed in a
sealed tube, the yield of 4-bromobenzaldehyde was lowered to 65%
1
even after 22 h. In all cases, the H NMR spectra of the reaction
mixture revealed the quantitative formation of diaryl telluride 1,
indicating a catalytic process is feasible if re-oxidation of the
recovered telluride can be performed in the same reaction system.
Studies on the singlet oxygen oxidation of organic substrates using
bulky diaryl telluroxides as a catalyst are currently under way.
Benzyl alcohol and its 4-substituted derivatives having electron-
donative and -withdrawing groups were efficiently oxidized with
telluroxide 2a to give the corresponding benzaldehydes in excellent
yields (runs 5–7); however, 2,6-dichlorobenzyl alcohol suffered
steric retardation to afford 2,6-dichlorobenzaldehyde in moderate
yield (60%, run 8). While the oxidation of benzhydrol gave
benzophenone in 92% yield (run 9), a-methylbenzyl alcohol was
somewhat less reactive to afford acetophenone in 32% yield even
after 17 h (run 10). Allylic alcohols such as trans-2-dodecenol (run
11) and cinnamyl alcohol (run 12) were also oxidized to the
corresponding aldehydes in 70 and 89% yields, respectively,
whereas saturated aliphatic alcohols such as 1-octanol (run 13) and
cyclododecanol were entirely unreactive. Other hydroxy com-
pounds such as methyl mandelate (run 14) and hydroquinone (run
15) were also converted to methyl benzoylformate and 1,4-benzo-
quinone in 98% and quantitative yields, respectively. With
1,2-benzenedimethanol, phthalide was obtained in 73% yield along
with phthalaldehyde (27%) in the presence of three equivalents of
2a (run 16), and the formation of phthalide can be explained by
further oxidation of the intermediary hemiacetal. In all reactions,
the bulkier telluroxide 2b was equally effective as an oxidizing
reagent and, in some cases, better yields were obtained by using the
telluroxide 2b (runs 8, 10, 11 and 14), where longer reaction time
was needed to secure acceptable yields by using 2a. For example,
the oxidation of trans-2-dodecenol with 2a afforded trans-
2-dodecen-1-al in 43% yield even after 19 h. We speculate that the
higher thermal stability of the telluroxide 2b due to bulkier aryl
substituents may be responsible for its enhanced reactivity.
In mechanistic consideration of the oxidation of alcohols, the
pathway is postulated to be shown in Scheme 2. Namely, the
alcohol attacks the tellurium–oxygen bond to give an adduct A
Notes and references
† Physical and spectral data for 2b: colorless solids, mp 181–183 °C. 1H
NMR (CDCl3) d 1.07 (d, J = 7 Hz, 12 H), 1.10 (d, J = 7 Hz, 12 H), 1.20
(d, J = 7 Hz, 12 H), 2.86 (m, 2 H), 3.30 (m, 4 H), 7.10 (s, 4 H). 13C NMR
(CDCl3) d 23.7, 24.3, 24.6, 33.0, 34.2, 123.7, 131.9 (JC–Te = 340 Hz),
152.3, 153.2. 125Te NMR (CDCl3) d 1314. HRMS (EI) m/z 552.2648 (M+,
calcd for C30H46OTe 552.2611). Anal. Calcd for C30H46OTe: C, 65.48; H,
8.43. Found: C, 65.41; H, 8.33.
‡ Only 13% yield of triphenylphosphine oxide was detected in the reaction
mixture after 2 h.
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M. R. Detty and P. B. Merkel, J. Am. Chem. Soc., 1990, 112,
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15, 1913–1919.
Scheme 2
C h e m . C o m m u n . , 2 0 0 4 , 1 6 7 2 – 1 6 7 3
1673