A Cascade Cross-Coupling Hydrogen Evolution Reaction by Visible ...
A Cascade Cross-Coupling Hydrogen Evolution Reaction by Visible Light Catalysis Meng, Qing-Yuan et al. JACS 2013, 135, 19052-19055
Cross-Dehydrogenative Cross-Coupling (CDC) C1-H
+
C2-H
Ox
C1-C2 + H-Ox-H
• Forms C-C bonds directly from two C-H bonds • Reduces traditional prefunctionalization and defunctionalization. • Normally requires stoichiometric amounts of oxidizing agents.
Liu, Peng et. al. Chem. Commun., 2010, 46, 2739
Catalyst: iron terpyridine complex attached to SBA-15 (class of mol sieves).
Boess, E. et. al. JACS, 2012, 134, 5317
Meng, Quin-Yuan et. al. Org. Lett., 2012, 14, 5992
Graphenesupported RuO2 was best
No reaction in the absence of O2
New Method: Cross-coupling Hydrogen Evolution (CCHE) C1-H
+
C2-H
Ox hv, eosin Y, G-RuO2
C1-C2 + H2
• No oxidants required • Only byproduct is hydrogen gas
Graphene-supported RuO2 nanocomposite (G-RuO2) • Electrically conductive
Eosin Y • Excited by visible light • Easily reduced
Cross-coupling Hydrogen Evolution (CCHE)
• Eosin Y and G-RuO2 both in catalytic amounts. • Visible light also required.
Cross-coupling Hydrogen Evolution (CCHE)
Electrophile
Nucleophile
How does tetrahydroisoquinoline get oxidized? How does visible light act as the driving force for this reaction?
Mechanistic Studies KH / KD = 1.7:1
• The dissociation of the proton is the rate-determining step.
• The released protons are quickly exchanged with D2O.
Mechanistic Studies
• What is responsible for H2 evolution? – Without indole, H2 still evolved – Without G-RuO2, no H2 detected
• New system – Evolved H2 immediately – Turnover number = 832 over 8 hours
• G-RuO2 outcompeted Al2O3-supported RuO2 and RuO2*nH2O for H2 evolution.
Mechanistic Studies • Can G-RuO2 act as an electron acceptor? • An eosin Y solution was excited with 532 nm light, and then absorption was measured. Eosin Y alone
Eosin Y and TEOA
+ 560 nm absorption 3[eosin Y]*
560 nm absorption 3[eosin Y]* 400 nm absorption [eosin Y] radical anion
[eosin Y] radical anion lifetime = 94.3 µs
Mechanistic Studies • Can G-RuO2 act as an electron acceptor? • An eosin Y solution was excited with 532 nm light, and then absorption was measured. Eosin Y alone
Eosin Y and G-RuO2
+ 560 nm absorption 3[eosin Y]*
560 nm absorption 3[eosin Y]*
Absorption unchanged: Oxidative quenching of 3[eosin Y]* by G-RuO2 is negligible.
Mechanistic Studies • Can G-RuO2 act as an electron acceptor? • An eosin Y solution was excited with 532 nm light, and then absorption was measured. Eosin Y alone
Eosin Y, TEOA, and G-RuO2
+ 560 nm absorption 3[eosin Y]*
+ 560 nm absorption 3[eosin Y]* 400 nm absorption [eosin Y] radical anion
[eosin Y] radical anion half life = 54.6 µs
• Presence of G-RuO2 reduced the halflife of [eosin Y] radical anion, indicating electron transfer.
Proposed Mechanism ISC 3eosin
1eosin
Y*
Y*
hv
eosin Y -
eosin Y -e
-e H+ H2O
H+ + OH-
H2
Optimizing Reaction Conditions
2 equiv. indole 2.5 eq. indole 3.0 eq. indole 4.0 eq. indole
Scope of tetrahydroisoquinolines
Electron donating
Electron withdrawing
Scope of Indoles • Electronwithdrawing groups: detrimental to nucleophilicity. • Electron-donating groups: beneficial to nucleophilicty
Nucleophiles Other Than Indole
Conclusion • Development of cross-coupling hydrogen evolution (CCHE). – Formation of C-C bonds – No sacrificial oxidants required – Visible light catalysis – H2 is the only byproduct
Cross-Dehydrogenative Cross-Coupling (CDC) C1-H
+
C2-H
Ox
C1-C2 + H-Ox-H
• Forms C-C bonds directly from two C-H bonds • Reduces traditional prefunctionalization and defunctionalization. • Normally requires stoichiometric amounts of oxidizing agents.
Liu, Peng et. al. Chem. Commun., 2010, 46, 2739
Catalyst: iron terpyridine complex attached to SBA-15 (class of mol sieves).
Boess, E. et. al. JACS, 2012, 134, 5317
Meng, Quin-Yuan et. al. Org. Lett., 2012, 14, 5992
Graphenesupported RuO2 was best
No reaction in the absence of O2
New Method: Cross-coupling Hydrogen Evolution (CCHE) C1-H
+
C2-H
Ox hv, eosin Y, G-RuO2
C1-C2 + H2
• No oxidants required • Only byproduct is hydrogen gas
Graphene-supported RuO2 nanocomposite (G-RuO2) • Electrically conductive
Eosin Y • Excited by visible light • Easily reduced
Cross-coupling Hydrogen Evolution (CCHE)
• Eosin Y and G-RuO2 both in catalytic amounts. • Visible light also required.
Cross-coupling Hydrogen Evolution (CCHE)
Electrophile
Nucleophile
How does tetrahydroisoquinoline get oxidized? How does visible light act as the driving force for this reaction?
Mechanistic Studies KH / KD = 1.7:1
• The dissociation of the proton is the rate-determining step.
• The released protons are quickly exchanged with D2O.
Mechanistic Studies
• What is responsible for H2 evolution? – Without indole, H2 still evolved – Without G-RuO2, no H2 detected
• New system – Evolved H2 immediately – Turnover number = 832 over 8 hours
• G-RuO2 outcompeted Al2O3-supported RuO2 and RuO2*nH2O for H2 evolution.
Mechanistic Studies • Can G-RuO2 act as an electron acceptor? • An eosin Y solution was excited with 532 nm light, and then absorption was measured. Eosin Y alone
Eosin Y and TEOA
+ 560 nm absorption 3[eosin Y]*
560 nm absorption 3[eosin Y]* 400 nm absorption [eosin Y] radical anion
[eosin Y] radical anion lifetime = 94.3 µs
Mechanistic Studies • Can G-RuO2 act as an electron acceptor? • An eosin Y solution was excited with 532 nm light, and then absorption was measured. Eosin Y alone
Eosin Y and G-RuO2
+ 560 nm absorption 3[eosin Y]*
560 nm absorption 3[eosin Y]*
Absorption unchanged: Oxidative quenching of 3[eosin Y]* by G-RuO2 is negligible.
Mechanistic Studies • Can G-RuO2 act as an electron acceptor? • An eosin Y solution was excited with 532 nm light, and then absorption was measured. Eosin Y alone
Eosin Y, TEOA, and G-RuO2
+ 560 nm absorption 3[eosin Y]*
+ 560 nm absorption 3[eosin Y]* 400 nm absorption [eosin Y] radical anion
[eosin Y] radical anion half life = 54.6 µs
• Presence of G-RuO2 reduced the halflife of [eosin Y] radical anion, indicating electron transfer.
Proposed Mechanism ISC 3eosin
1eosin
Y*
Y*
hv
eosin Y -
eosin Y -e
-e H+ H2O
H+ + OH-
H2
Optimizing Reaction Conditions
2 equiv. indole 2.5 eq. indole 3.0 eq. indole 4.0 eq. indole
Scope of tetrahydroisoquinolines
Electron donating
Electron withdrawing
Scope of Indoles • Electronwithdrawing groups: detrimental to nucleophilicity. • Electron-donating groups: beneficial to nucleophilicty
Nucleophiles Other Than Indole
Conclusion • Development of cross-coupling hydrogen evolution (CCHE). – Formation of C-C bonds – No sacrificial oxidants required – Visible light catalysis – H2 is the only byproduct
