Table 1. Evaluation of Catalytic Conditions
entry
catalyst (equiv)
additive (equiv)
LiBF4 (1.5)
oxidant (equiv)
solvent
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
time (h)
yield (%)
1
2
3
4
5
6
7
8
9
[TEMPO+][BF4-] (1.1)
TEMPO (0.05)
TEMPO (0.05)
[TEMPO+][BF4-] (0.05)
[TEMPO+][BF4-] (0.05)
[TEMPO+][BF4-] (0.05)
[TEMPO+][BF4-] (0.05)
[TEMPO+][BF4-] (0.05)
[TEMPO+][BF4-] (0.05)
TEMPO (0.01)
0.1
24
1
0.1
2
7
1
2
0.5
2
94
5
Phl(OAc)2 (1.5)
Phl(OAc)2 (1.5)
Phl(OAc)2 (1.5)
Phl(OAc)2 (1.5)
Phl(OAc)2 (1.5)
Phl(TFA)2 (1.5)
NalO4 (1.5)
27
64
80
60
42
85
90
92
H2O (3.0)
H2O (10.0)
CH2Cl2-H2O
CH2Cl2
CH2Cl2
NalO4-SiO2 (2.0)
NalO4-SiO2 (2.0)
10
ketones and have reported two environmentally friendly
alternatives using IBX4 and oxoammonium salts.5-9
In discovering the markedly productive method using
in light of the satisfactory precedents12 in the catalytic
oxidation of alkenyl substrates using nitroxyl radical/oxoam-
monium ion as catalyst (Table 1).13,14
oxoammonium salts, such as [TEMPO+][BF4 ] and
-
Typical catalytic conditions with TEMPO/PhI(OAc)2 only
gave rise to a sluggish reaction, producing 1b in 5% after
24 h, along with recovered 1a (entry 2). Simple addition of
1.5 equiv of LiBF4 to this reaction slightly improved the
situation to give 1b in 27% yield (entry 3). Notably,
[TEMPO+][SbF6-], we identified that bulky, less nucleophilic
-
-
counteranions such as BF4 and SbF6 play indispensable
roles in this particular transformation (Scheme 2).5 With this
insight, we have begun investigations to find agents that make
oxoammonium ions work catalytically.10 We disclose herein
the novel catalytic oxidative 1,3-rearrangement of tertiary
allylic alcohols to ꢀ-substituted R,ꢀ-unsaturated ketones.
We started screening reagents capable of converting 1a
to 1b, where PhI(OAc)211 was employed as the bulk oxidant,
[TEMPO+][BF4 ] resulted in the better turnover with PhI-
-
(OAc)2 to convert 1a to 1b in 64% yield within 10 min,
supporting the important role of BF4- anion, although it was
accompanied by 20% yield of 1-acetoxy-3-butylcyclohex-
2-ene (1c) (entry 4). Addition of 3 equiv of H2O effectively
improved the catalytic performance of TEMPO+, yielding
1b in 85% yield, by suppressing production of 1c, while
addition of 10 equiv of H2O attenuated the yield to 60%
(entries 5 and 6). The use of PhI(TFA)2 as bulk oxidant
induced decomposition (entry 7). We next examined an
inorganic version of a hypervalent iodine reagent, NaIO4, to
determine its effect (entry 8).18 Use of NaIO4 on SiO2
brought about further improvement in the catalytic efficiency
(entry 9).19 Ultimately, it was found that NaIO4-SiO2
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-
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