Direct Cyanomethylation of Alkane and Aromatic Ring
Using Titanium Oxide Photocatalyst
○Emiko Wadaa, Tomoaki Takeuchia, and Hisao Yoshidaa,b
a Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan
b Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Kyoto, Japan
Introduction
Cyanomethylation of Benzene
・Formation of carbon-carbon bonds is an important reaction in organic
synthesis.
Yield/µmol
・The reaction without leaving group or harmful solvents is undeveloped.
+ CH3CN
hν
+ H2
M/TiO2
Pt/TiO2
0.08
0.5
n.d.
Pd/TiO2
3.5
0.3
n.d.
1.1
5.9
Pt/TiO2 + Pd/Al2O3
2.2
1.2
n.d.
n.d.
n.d.
benzyl cyanide
Pd/TiO2
3.5 mmol
>
H2
Catalyst
Pt/TiO2
0.09 mmol
Hybrid catalyst (Pd catalyst + TiO2 photocatalyst) was effective
for cyanomethylation of aromatic ring1)
Present study
・Cyanomethylation of aliphatic compounds
・Confirm the effect of Pd for the reaction
≪Isotopic effect : determination of the rate-limiting step≫
Product /μmol
Experimental
➣ Photodeposition was used for metal loading
-
Ar
C 6D 6
• Acetonitrile 3.9 mL,
Sampling
• Hexane or Hexene or Benzene 0.1 mL
•Wavelength 360 ± 20 nm
(benzene)
CD3CN (acetonitrile)
3.6
n.d.
0.4
-
-
3.6
n.d.
0.7
1.0
0.6
1.4
n.d.
0.7
2.6
0.6
Reaction conditions: acetonitrile 3.8 mL, benzene 0.1 mL, water 0.1 mL
Catalyst: Pd(0.1wt%)/TiO2 0.2 g
The light intensity was 27 mW/cm2 at wavelength of 402±20 nm
UV light
(27 mW cm-2 at 360 nm)
• Gas GC-TCD
• Liquid GC-MS
kH/kD
Isotope
Pre-irradiation • Pt(0.1wt%)/TiO2 or Pd(0.1wt%)/TiO2 0.2 g
• Argon purge
0.5 h
Analysis
27
・Only acetonitrile was oxidized to radical species.
・The yield of benzyl cyanide was improved in the presence of Pd
⇒Pd functioned as a catalyst
1) H. Yoshida, et al, Chem. Commun., 2013, 49, 3793
Reaction
1h
12
Xe lamp
kH/kD >1 ⇒Radical formation of acetonitrile was the rate-limiting step.
Mirror
Cyanomethylation of Aliphatic Compounds
+
+ CH3CN
Product /µmol
Catalyst
Pt/TiO2
Pd/TiO2
4.6
16
8.0
≪Isotopic effect for cyanomethylation of cyclohexane≫
Product /µmol
Product /µmol
H2
10
CH3CN
Catalyst
2.3
50
Pt/TiO2
0.71
34
Pd/TiO2
10
2.8
12
2.8 0.17
5.2
H2
Isotope
70
C6D12
CD3CN
5.0
・Pt/TiO2 and Pd/TiO2 gave dimeric compounds and cyanomethylated compounds.
⇒Reaction might be including radical coupling mechanism.
・Amount of products were increased by Pt/TiO2.
H
H2
H+
Pt
eh+
Rate-limiting step
CH2CN
H
e-
2.5
0.12
hν
e-
eh+
h+
Aromatic compounds
Addition-elimination mechanism
CH2CN
H+
e
eh+
0.84
5.0
TiO2
Aliphatic compounds
Radical coupling mechanism
H+
2.3
0.83
Pd
H
TiO2
2.3
0.90
19
kH / kD < 1 : Competitive reaction would be occurred between
cyclohexane and acetonitrile.
⇒ The oxidation of the former was easier than the latter.
Proposed Reaction Mechanism
H
16
19
3.2
kH / kD > 1 : The yield of cyclohexyl acetonitrile was decreased
⇒ Oxidation of cyclohexane was the rate-limiting step.
Pd catalyst was not effective for the cyanomethylation of aliphatic compounds.
H2
10
4.3
12
kH / kD
CH3CN
NC
H
Activate
CH3CN
H
H+
CH2CN
e-
Rate-limiting step
Conclusions
Pd catalyst was effective only for the
cyanomethylation of aromatic ring.
 Pd catalyst was important for cyanomethylation of benzene.
 Pt/TiO2 was effective more than Pd/TiO2 for the cyanomethylation of aliphatic compounds.
Acknowledgement: This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 25105723) from JSPS.