Photoactive Chiral Metal–Organic Frameworks for Light ...

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1 Ru(bpy) 3 2+ *Ru(bpy) 3 2+ Ru(bpy) 3 + Photon Source FG Br FG Br FG H R N FG H R N R H N NH FG H R O O H R Journal Club 2012.11.2 Takumi Yoshida Photoactive Chiral Metal–Organic Frameworks for Light-Driven Asymmetric α - Alkylation of Aldehydes Wu, P.; He, Chang.; Wang, J.; Peng, X.; Li, X.; An, Y.; Duan, C. J. Am. Chem. Soc. 2012, 134, 14991–14999. 1. Introduction 1.1 Toward green sustainable chemistry Goal: Convert basic compounds into useful products in a sustainable fashion For example… –Using more clean and cheap energy source –Reuse catalyst to decrease the waste and cost 1.2 Photoredox catalysts with organocatalysts Recently, combination of organometallic complexes such as [Ru(bpy) 3 ] 2+ , [Ir(ppy) 2 (dtbpy)] + and organocatalysts are intensively investigated as an asynmmetric photoredox catalysts (scheme 1). 1 Sunlight can be used as an inexhaustible light source ideally. Scheme 1. Photoredox catalyst with an organocatalyst 1.3 Metal–organic frameworks (MOFs) 2 MOFs are hybrid solids with infinite network structures built from organic ligands and metals. It can tune the size and the shape of pore. Used for gas storage, molecular sensing, separation, medical applications and heterogeneous (asymmetric) catalysts. MOFs can be reused for heterogeneous (asymmetric) catalyst. 1.4 This work Incorporation of an asymmetric organocatalyst within a photoactive MOF For constructing reusable photocatalytic systems Strategy for this work H R 1 O + Br R 3 R 2 NMe N H O Me t-Bu (20 mol%) N N N N N N Ru 2+ (0.5 mol%) 2,6-lutidine, DMF 15W fluorescent light bulb, rt H O R 2 R 3 R 1 up to 93% up to 99% (1) Photocatalyst + Metal salt + Organocatalyst MOF H R O + Br CO 2 Et CO 2 Et fluorescent lamp MOF cat. H R O CO 2 Et CO 2 Et ! (2) (3)

Transcript of Photoactive Chiral Metal–Organic Frameworks for Light ...

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Ru(bpy)32+

*Ru(bpy)32+Ru(bpy)3+

Photon Source

FG Br

FG Br

FG HR

N

FG HR

N

RH

NNH

FG HR

O

O

HR

Journal Club 2012.11.2 Takumi Yoshida

Photoactive Chiral Metal–Organic Frameworks for Light-Driven Asymmetric α-Alkylation of Aldehydes

Wu, P.; He, Chang.; Wang, J.; Peng, X.; Li, X.; An, Y.; Duan, C. J. Am. Chem. Soc. 2012, 134, 14991–14999.

1. Introduction 1.1 Toward green sustainable chemistry

Goal: Convert basic compounds into useful products in a sustainable fashion For example… –Using more clean and cheap energy source

–Reuse catalyst to decrease the waste and cost

1.2 Photoredox catalysts with organocatalysts • Recently, combination of organometallic complexes such as [Ru(bpy)3]2+,

[Ir(ppy)2(dtbpy)]+ and organocatalysts are intensively investigated as an asynmmetric photoredox catalysts (scheme 1).1

Sunlight can be used as an inexhaustible light source ideally. Scheme 1. Photoredox catalyst with an organocatalyst

1.3 Metal–organic frameworks (MOFs)2 • MOFs are hybrid solids with infinite network structures built from organic ligands and metals. • It can tune the size and the shape of pore. Used for gas storage, molecular sensing, separation, medical applications and heterogeneous (asymmetric) catalysts. • MOFs can be reused for heterogeneous (asymmetric) catalyst. 1.4 This work

Incorporation of an asymmetric organocatalyst within a photoactive MOF For constructing reusable photocatalytic systems

Strategy for this work

HR1

O+

Br R3

R2

NMe

NH

O

Me t-Bu

(20 mol%)

N

N

N

NN

NRu2+

(0.5 mol%)

2,6-lutidine, DMF15W fluorescent light bulb, rt

H

O R2

R3

R1

up to 93%up to 99% (1)

Photocatalyst + Metal salt + Organocatalyst MOF

HR

O+ Br CO2Et

CO2Et fluorescent lamp

MOF cat. HR

O CO2Et

CO2Et!!

(2)

(3)

2

Figure 2. (a) CD spectrum of Zn-BCIP1 and 2 (b) CD spectrum of Zn-PYI1 and 2 (c) CD spectrum of L-BCIP and D-BCIP (d) UV spectrum of Zn-BCIP1 and Zn-PYI1

N

C O

O Zn

OH2OHZnO

ON N

NBoc

O

Figure 1. Crystal structure of Zn-BCIP1

O

• Synthesizing MOF with photocatalyst, metal salt and asymmetric organocatalyst. • They also synthesized MOF with only photocatalyst and metal salt to confirm

installation of organocatalyst within a single MOF is crucial for enantioselectivity. 2. Results and Discussion 2.1 Synthesizing MOF (Zn-PYI1 and Zn-PYI2)

• Zn-BCIP1 and Zn-BCIP2 can be easily synthesized. Determined by elemental analysis and X-ray analysis (figure 1). • Thermogravimetric analysis (TGA) of Zn-BCIP1 exhibited a significant weight loss of 12.9% in the 100–200 °C. Boc group can be deprotected in 100–200 °C. • The expulsion of the Boc group was further verified by 1H NMR, IR and elemental analysis. 2.2 Nature of ZnPYI1 – Dye-uptake study 1. Soaking Zn-PYI1 and Zn-BCIP1 in a Fluorescein (2’,7’-dichlorofluorescein) 2. Washing with solvent several times 3. Digesting fluorescein with sodium ethylenediaminetetraacetic acid 4. Quantified by UV the amounts of released fluorescein • 14% of the framework weight of fluorescein was absorbed for Zn-PYI1, but fluorescein was not absorbed for Zn-BCIP1. These results implied the releasing of channels during deprotection process. – Circular dichroism (CD) spectrum • Zn-BCIP1 and Zn-PYI1 showed cotton effect in each wavelength (figure 2, (a) and (b)). • The peak around 340 nm was assignable to the absorbance corresponding to the π-π*

transition of the triphenylamine groups.

+ Zn(NO3)3•6H2O +

NN

NBoc

N NN

Boc

orDMF/EtOH100 °C, 3 days

(0.25 mmol) (1.00 mmol) (1.00 mmol)

Zn-BCIP1or

Zn-BCIP2 DMFMW, 165 °C, 4 h

Remove Boc

60 or 65% yield

NAr

Ar Ar

Ar = 4-HOOCC6H4

Zn-PYI1or

Zn-PYI2

>95% yield

(4)

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Figure 3. (a) Solid-state CV of Zn–BCIP1 and Zn–PYI1 with a scan rate of 50 mV s-1 in the scan range 0.5–1.1 V. (b) Normalized absorption (black line) and emission spectra (red line) of Zn–PYI1, excited at 350 nm.

Figure 4. (a) Solid-state emission spectra of Zn–PYI1 (black line), Zn–PYI1 upon absorbance of phenylpropylaldehyde (blue line) and diethyl 2-bromomalonate (red line), excited at 350 nm. (b) Transient emission spectra of solid Zn–PYI1 and Zn–PYI1 with diethyl 2-bromomalonate incorporated.

– Redox potential of MOF • The redox potential of the Zn–PYI1+/Zn–PYI1* couple was –2.12 V (figure 3). This potential was more negative than that of the diethyl 2-bromomalonate (E0 = –0.49 V). 2.3 Absorption experiment (figure 4). • The luminescence intensity of Zn-PYI1 was significantly reduced when it absorbed diethyl 2-bromomalonate molecules (figure 4, (a)). • Decrease of fluorescence lifetime of Zn-PYI1 absorbed diethyl 2-bromomalonate molecules was observed (figure 4, (b)). The quenching process was attributed to the photoinduced electron process from Zn-PYI1* to diethyl 2-bromomalonate. 2.4 Photocatalytic α-alkylation of Aliphatic Aldehydes

• Zn-PYI1 and Zn-PYI2 gave corresponding desired product in good yields with good

ee values (in parentheses) • When compound 1 whose size is larger than the pore size of Zn-PYI1 was used, the

desired reaction only gave the desired product in 7% yield. The desired reaction took place mostly in the channel of the catalyst. Control experiment for Zn-PYI1 • Reaction did not occur in the absence of fluorescent lamp. • Zn–BCIP1 instead of Zn-PYI1 did not afford the desired product. • The removal of Zn-PYI1 by filtration shut off the reaction. • Zn-PYI1 can be isolated by filtration and can be reused at least three times. • Zn-PYI1 maintained the framework after the reaction. Confirmed by dye uptake study. • Confirmation of heterogeneous and photocatalytic nature of the reactions.

HR

O+ Br CO2Et

CO2Et 0.5 mol% MOF

2,6-lutidine (2.0 eq)fluorescent lamprt, 24h

HR

O CO2Et

CO2Et!!

R = CH2Ph catalyst = ZnPYI1R = CH2Ph catalyst = ZnPYI2R = n-Hexyl catalyst = ZnPYI1R = n-Hexyl catalyst = ZnPYI2

74% (+92)73% (–81)65% (+86)61% (–78)

(1.0 eq)(2.0 eq)H

O

O

O

17.4 Å

13.8 Å

(5)

1

4

2.5 Control Experiment • To investigate the effect of combination of photocatalyst and organocatalyst within a

single MOF, they synthesized Ho-based MOF.

• Confirmed by elemental analysis and X-ray analysis. • Dye-uptake studies showed a 23% uptake of fluorescein. • Redox potential of the (Ho–TCA+/Ho–TCA*) is –2.20 V. This is more negative than

that of Zn-PYI1. It is enough for photocatalytic reaction.

• The yield of the desired product is higher than that of Zn–PYI1 and Zn–PYI2. • ee values (in parentheses) dramatically got worse. This result suggests that incorporation of organocatalyst and photocatalyst within single MOF is effective for high enantioselectivity. 3. Conclusion • MOF-based asymmetric photocatalyst through the cooperative combination of

triphenylamine photocatalysis and proline-based asymmetric organocatalysis within a single MOF was developed.

• Control experiment showed that integration of photocatalyst and asymmetric organocatalyst into a single MOF is superior in the view of enantioselectivity.

4. References 1) Nicewicz, D. A.; MacMillan, D. W. C. Science. 2008, 322, 77–80. 2) Ferey, G. Chem. Soc. Rev. 2008, 37, 191–214.

+ Ho(NO3)3•6H2ODMF/EtOH100 °C, 3 days

(0.25 mmol) (1.00 mmol)

Ho–TCA

70% yield

NAr

Ar Ar

Figure 5. Structure of Ho–TCA

(6)

HR

O+ Br CO2Et

CO2Et 0.5 mol% Ho–TCA20 mol% Organocatalyst

2,6-lutidine (2.0 eq)fluorescent lamprt, 24h

HR

O CO2Et

CO2Et!!

R = CH2Ph organocatalyst = L–PYIR = CH2Ph organocatalyst = D–PYIR = n-Hexyl organocatalyst = L–PYIR = n-Hexyl organocatalyst = D–PYI

86% (+20)85% (–21)90% (+21)90% (–20)

(1.0 eq)

(7)L–PYI =

D–PYI =

NHN

N

N NHN

(2.0 eq)

Yield is isolate yield. ee values are shown in the parentheses