Recent Development of A New Class of Metal Containing Catalysts

Oxidation of cycloalkanes by H2O2 using a copper-hemicryptophane complex as a

catalyst

O. Perraud et al, Chem. Commun., 2013, 49, 1288.

2014/05/30 Isamu Katsuyama

Main Objective

An approach to biocatalysts (i.e. natural enzyme) by use of chemical catalysts

How can efficient biomimetic catalytic systems be developed?

IntroductionNatrural EnzymeAdvantageHigh reactivity, high selectivityMild conditionsDisadvantage Instability Limitation of substrate

IntroductionChemical CatalystsAdvantage

StabilityWide application

Disadvantage Low reactivity, low selectivity Sometimes severe conditions, toxicity

Direct and efficient oxidation of alkanes remains an ambitious goal.

In particular, oxidation of cyclohexane to cyclohexanol and cyclohexanone has industrial significance.

Solvent

OH O+

Cat. Oxidant

Biocatalyst (enzyme) viewed as promising candidates

The copper containing particulate methanemonooxygenase (pMMO)

Nature, 2010, 465, 115.

Cu

Attempts to develop efficient biomimetic catalytic systems The systems involving copper-based

catalysts using H2O2 as an oxidant

Adv. Syn. Catal., 2006, 348, 159.

However, conversions and yields for the oxidation are commonly low.

A further improvement using supramolecular copper complexs

Enhancement of catalytic activity However, these complexes often suffer from

product inhibition, thus require large loadings,

→ more than 1 equiv. of complexes.

Chem. Rev., 2011, 111, 5434.

A new class of catalystbased on hemicryptophane

Hemicryptophane complexes have been developed.

J. Catal., 2009, 267, 188.

A hemicryptophane copper complex 1

A new copper containing catalystbased on hemicryptophane

Preparation of the hemicryptophan complex 1

+ Cu(ClO4)2(H2O)6

1CH2Cl2 MeOH

Et3N rt. for 90min

Yield = 82%

H2O2

MeCN

OH O+

Cat. 1 etc.

r. t. for 6h

35 for 2h℃

Important findings

1. Best conditions: H2O2 10eq, Catalyst 1%, at 35 for 2h (entry 6)℃

2. The use of tBuOOH instead of H2O2 → much lower yield (<5%)

3. Addition of acids (HNO3, AcOH) → no effect

4. Catalytic activity: 1 > 2 >> Cu(ClO4)2 (entry 6-9)

H2O2MeCN

OH O+

1, 2 or Cu(ClO4)2(H2O)6

Important findings

The reaction rates: 1, 2 >> Cu(ClO4)2 Whereas initial reaction rates in 1 and 2

are similar, 1 provide 2-fold higher yield (28%) than 2 (14%) after one hour.

→ The cage structure prevents degradation of catalyst; self-oxidation between two catalyst molecules should be avoided.

H2O2

MeCN

OHCat. 1 or 2 OH

H2O2

MeCN

OHCat. 1 or 2Admantane Admantane-OH

C6 C8

C10C6

C6C8

C6 C10

35 for 4h℃

1. Competitive oxidation of C6 and C8 Catalyst 2; Similar conversion and yields Catalyst 1; Higher conversion in C8, but

higher yield in C6 (lower yield in C8)

(C8 can interact more strongly with cavity of 1, but C8 also remain in the cavity to undergo over-oxidation because of higher hydrophobicity of C8.)

Conversion and yields in 1 are higher than that in 2.

`→ Catalyst 1 can discriminate C6 from C8.

2. Competitive oxidation of C6 and C10 Yield are tripled with 1 (16+45=61%) compared

with 2 (5+15=20%). After 2 hours, conversion ratio C6 / C10

50 / 10 (= 5) for 1, 42 / 29 (= 1.7) for 2

→ Catalyst 1 can discriminate C6 from C10.

C10C6

oxidation

Consideration of oxidation mechanism using KIE (Kinetic

Isotope Effect)

KIE = kH / kD exp) ≈ AdOH-d2 / AdOH-d1

KIE = 1.52±0.05 for cat.1, 1.35±0.04 for cat.2

oxidant / catalyst

Mechanism of Cu-mediated oxidation based on KIE study

J. Am. Chem. Soc., 1993, 115, 7293.

O

Cu

C

H

O

Cu

C

HHO

Low values of KIE are in better agreement with mechanism involving bent transition state.

Conclusion

Cu-hemicryptophane complex 1 is an efficient catalyst for oxidation of cyclohexane (C6).

The stability of catalyst has been improved (yield was up to tripled compared to model complex 1).

Ability of 1 to discriminate C6 from C8 and C10 suggests that oxidation occur in catalyst cavity similarity to enzyme.