A green oxidation protocol for the conversion of secondary alcohols into ketones using heterogeneous nanocrystalline titanium (IV) oxide in polyethylene glycol

Figures & data

Scheme 1.  Model reaction for oxidation using TiO₂ nanoparticles.

Table 1. Oxidation of alcohols using different nanocrystalline metal oxides.^a

Entry	Nanoparticles	Time (h)	Yield (%)^b
1	CuO	8	38
2	ZnO	8	40
3	MgO	8	50
4	TiO2	6	92
^aReaction conditions: diphenylcarbinol (1 mmol), hydrogen peroxide (1 mmol) and 10 mol% nanocrystals nm; PEG-400; temperature 70–75°C.
^bIsolated yields.

Figure 1.  Powder XRD pattern of nanocrystalline titanium (IV) oxide.

Table 2. Screening of solvent for oxidation reaction.

Entry	Solvent	Yield (%)^b
1	CH2Cl2	40
2	THF	37
3	MeOH	42
4	PEG-400	92
5	PEG-600	85
6	PEG-800	80
^aReaction conditions: diphenylcarbinol (1 mmol), hydrogen peroxide (1 mmol) and 10 mol% nano TiO2 (50±2) nm; solvent; temperature 70–75°C.
^bIsolated yields.

Figure 2.  Optimization of concentration of H₂O₂. Reaction conditions: diphenylcarbinol (1 mmol), hydrogen peroxide (1 mmol) and 10 mol% nano TiO₂ (50±2) nm; PEG-400; temperature 70–75°C. Isolated yields.

Table 3. Oxygen availability in various oxidants.

Oxidants	% Active O	By-product
H2O2	47	H2O
t-BuOOH	18	t-BuOH
CH3COOOH	22	CH3COOH
NaOCl	22	NaCl
KIO4	8	KIO3

Table 4. Nanocrystalline titanium (IV) oxide catalyzed oxidation of alcohol into ketones.^a

Entry	Alcohols	Products	Time (h)	Yield (%)^b
1	Diphenylcarbinol	Benzophenone	6	92
2	1,2,3,4-Tetrahydro-1-naphthol	3,4-Dihydro-2H-naphthalen-1-one	7	87
3	2-Methylbenzhydrol	2-Methylbenzophenone	8	90
4	9-Hydroxyfluoren	Fluoren-9-one	7	92
5	4,4′-Dimethoxybenzhydrol	4,4′-Dimethoxybenzophenone	8	94
6	α-Methyl-2-naphthalene-methanol	1-Naphthalen-2-yl-ethanone	6	90
7	4-Chlorobenzhydrol	4-Chlorobenzophenone	8	88
8	2-Phenylethanaol	Acetophenone	10	85
9	Cyclohexanol	Cyclohexanone	11	82
10	Cyclopentanol	Cyclopentanone	11	80
^aReaction conditions: Alcohol (1 mmol), hydrogen peroxide (1 mmol) and 10 mol% nano TiO2 (50±2) nm; PEG-400; temperature 70–75°C.
^bIsolated yields.

Scheme 2.  Proposed reaction mechanism for the oxidation.

Figure 3.  Recycling studies of nano-TiO₂ oxidation reaction. ^aReaction conditions: diphenylcarbinol (1 mmol), hydrogen peroxide (1 mmol) and 10 mol% nano TiO₂ (50±2) nm; PEG-400; temperature 70–75°C. Isolated yields.

Figure 4.  (a) TEM image of fresh nanoparticles. (b) TEM image of recycled nanoparticles.