tqc switchablecatalysis 2018 upload - princeton...

Tiffany Chen

MacMillan Lab

June 5, 2018

Switchable Catalysis

N

O

O

NNt-Bu

t-Bu

Meh!1

h!2

N

O

O

NN

t-Bu

t-Bu

Me

regulated by >100 accessory proteins

initiation, chain-growth, and cross-linking

example: actin filament polymerization provides structure and allows cells to move

Pollard, T.D.; Cooper, J.A. Science 2009, 326, 1208.

Switchable Catalysis: Inspiration from Nature

the synthetic machinery of natural systems makes complex polymers with fine temporal and spatial control


the synthetic machinery of natural systems makes complex polymers with fine temporal and spatial control

operations must be controlled

to prevent parallel reactions in Nature from interfering with one another:

function must be switchable

switching must be reversible

enzymatic activity modulated through feedback loops and trigger-induced effects

basic requirements for switches:

bistability: occurrence of 2 forms of molecule that can be interconverted by an external stimulus

fast response times

thermal stability

fatigue resistance

toggling chemical reactivity of a catalyst between multiple distinct states

through application of external stimuli

switchable catalysis


basic requirements for switches:

bistability: occurrence of 2 forms of molecule that can be interconverted by an external stimulus

fast response times

thermal stability

fatigue resistance

toggling chemical reactivity of a catalyst between multiple distinct states

through application of external stimuli

switchable catalysis

Stoll, R.S.; Hecht, S. Angew. Chem. Int. Ed. 2010, 49, 5054.


switchable catalysis: biological inspiration

types of switchable external stimuli

pH

light

ion coordination

redox switching

mechanical forces

Light-Gated Catalysis

Stoll, R.S.; Hecht, S. Angew. Chem. Int. Ed. 2010, 49, 5054.Nelson, B.M.; Biewlawski, C.W. ACS Catal. 2013, 3, 1874.

types of photocontrol of catalytic activity

transient excited state is reactive

irreversible formation of active catalytic state

reversible toggling between active and inactive catalytic states

cat

cat*

cat

hvA

Bcat

cat*

hv

A

B

inactive reactiveinactive

reactive

cat

cat*

cat

hv1A

B

hv2A

B

k1 k2k1 ≠ k2

photocatalysis




photocaged catalysisphotoswitchable catalysis

light is an attractive external regulator: low-cost, ubiquitous, noninvasive, precise control

initiates and regulates complex processses with precision in Nature

(e.g., photosynthesis, vision)







cat

cat*

cat

hvA

Bcat

cat*

hv

A

B


reactive

cat

cat*

cat

hv1A

B

hv2A

B

k1 k2k1 ≠ k2

photocatalysis








catalytic activity from ground-state species

X XRR

R R

hv1

hv2X = S, NH, O

R ≠ H

X XR RR R


Nelson, B.M.; Biewlawski, C.W. ACS Catal. 2013, 3, 1874.

a photoswitchable catalyst involves a catalytically active species

that undergoes a reversible photochemical transformation altering its catalytic properties

XX

X Xhv1

hv2X = N, CH

azobenzene/stilbene

diarylethene

NMe

Me Me

NMe

Me Me

O O

hv1

hv2

spiropyran


Peters, M.V.; Stoll, R.S.; Kuhn, A.; Hecht, S. Angew. Chem. Int. Ed. 2008, 47, 5968.

N

O

O

NNt-Bu

t-Bu

> 400 nm

365 nmN

O

O

NN

t-Bu

t-Bu

Me Me

accessiblebasic site

photoswitchable base catalysis


basicity of deshielded Z isomer increases

by almost an order of magnitude


Peters, M.V.; Stoll, R.S.; Kuhn, A.; Hecht, S. Angew. Chem. Int. Ed. 2008, 47, 5968.

N

O

O

NNt-Bu

t-Bu

> 400 nm

365 nmN

O

O

NN

t-Bu

t-Bu

Me Me


photoswitchable base catalysis

O2N

H

O

Me NO2

O2N

OH

NO2

MeE- or Z-cat(10 mol%)

THF, rt

kZ/kE = 35.5

photoreversible steric shielding in a small molecule catalyst


Sud, D.; Norsten, T.B.; Branda, N.R. Angew. Chem. Int. Ed. 2005, 44, 2019.

chiral Cu chelationhigh enantioselectivity

no Cu chelationlow enantioselectivity

S SMe Me > 434 nm

313 nm

F FFF

FF

N

OO

N

i-Pr i-Pr

S SMe Me

F FFF FF

i-Pr

O

ON

i-Pr

N

A B

N2

CO2Et

Cu(L) (10 mol%)

H H

CO2Et

H CO2Et

H

enantiomer enantiomer

ligand trans cis d.r.

A

B

30 50 55:45

5 5 63:37

in situ photoinduced ring-openingfirst in situ photoswitching of stereoselective catalysis

large geometrical change induces change in orientation

of key sites on catalyst (cooperative effect)





S SMe Me > 434 nm

313 nm

F FFF

FF

N

OO

N

i-Pr i-Pr

S SMe Me

F FFF FF

i-Pr

O

ON

i-Pr

N

A B

N2

CO2Et

Cu(L) (10 mol%)

H H

CO2Et

H CO2Et

H



A

B

30 50 55:45

5 5 63:37

in situ photoinduced ring-openingfirst in situ photoswitching of stereoselective catalysis





S SMe Me > 434 nm

313 nm

F FFF

FF

N

OO

N

i-Pr i-Pr

S SMe Me

F FFF FF

i-Pr

O

ON

i-Pr

N

A B

N2

CO2Et

Cu(L) (10 mol%)

H H

CO2Et

H CO2Et

H



A

B

30 50 55:45

5 5 63:37

limitation: photoinduced ring-closing inefficient with Cu(I)only 23% of closed form at stationary state at 313 nm


Wilson, D.; Branda, N.R. Angew. Chem. Int. Ed. 2012, 51, 5431.

S SMe

Me

visible light S SMe MeN

MeO

H

NMe

H

O

UV

electronically insulated(inactive)

electronically connected (active)

photoinduced changes in electronic properties to alter catalytic activity

N

O H

ROH

MeH

R H

H2N CO2

N

N H

ROH

MeH

CH2O2RH

aldimine

N

N

ROH

MeH

CO2R

quinonoidPLP

mimic of bioactive form of vitamin B6 (pyridoxal 5’-phosphate, PLP)

proof-of-concept: photoinduced regulation of biomimetic, small molecule cofactors



S SMe

Me


MeO

H

NMe

H

O

UV




mimic of bioactive form of vitamin B6 (pyridoxal 5’-phosphate, PLP)

N

O H

ROH

MeH

R H

H2N CO2

N

N H

ROH

MeH

CH2O2RH

aldimine

N

N

ROH

MeH

CO2R

quinonoidPLPenzyme cofactor responsible for

amino acid metabolism

proof-of-concept: photoinduced regulation of biomimetic, small molecule cofactors



S SMe

Me


MeO

H

NMe

H

O

UV




D3N

HMe

CO2D

L-d4-alanine

D3N

DMe

CO2D

D3N

MeD

CO2D

L-d5-alanine

D-d5-alanine

catalyst (20 mol%)

CD3CO2D/D2O

catalytic activity for racemization of L-alanine

dependent on electronic connectivity

between functional groups

H/D-exchange to monitor catalytic racemization



S SMe

Me


MeO

H

NMe

H

O

UV




D3N

HMe

CO2D

L-d4-alanine

D3N

DMe

CO2D

D3N

MeD

CO2D

L-d5-alanine

D-d5-alanine

catalyst (20 mol%)

CD3CO2D/D2O

after 140 h at 40 oC:

< 3% conversion with ring-opened catalyst

30% conversion with ring-closed catalyst







cat

cat*

cat

hvA

Bcat

cat*

hv

A

B


reactive

cat

cat*

cat

hv1A

B

hv2A

B

k1 k2k1 ≠ k2

photocatalysis








catalytic activity from ground-state species







cat

cat*

cat

hvA

Bcat

cat*

hv

A

B


reactive

cat

cat*

cat

hv1A

B

hv2A

B

k1 k2k1 ≠ k2

photocatalysis








Fors, B.P.; Hawker, C.J. Angew. Chem. Int. Ed. 2012, 51, 8850.

Photocontrolled Living Radical Polymerization

EtO

OBr

Ph

O

MeOMe

initiator MMA monomer

EtO

O

Ph

Br

Me CO2Men

IrIII (0.005 mol%)

50 W lamp, DMF, rt

bromo-terminated chain end

*IrIII

IrIII

Pn–Br Pn RIrIV

IrIII = fac-[Ir(ppy)3]

light used to reversibly activate/deactivate living radical polymerization

why light? high degree of temporal control over chain growth

highly responsive external control

in absence of light, no excited state Ir

polymerization rests in bromo-capped dormant state

reactivation upon reexposure to light



ln([M0]/[Mt]) vs. time of light exposure linear relationship between Mn and conversion


polymerization stops immediately without light

no new chain ends initiated during polymerization

existing chain ends reactivated




EtO

OBr

Ph

O

MeOMe

EtO

O

Ph

Br

Me CO2Men

IrIII (0.005 mol%)

50 W lamp, DMF, rt

MW/Mn = 1.28

O

BnOMe

IrIII (0.01 mol%)

50 W lamp, DMF, rtEtO

O

Phn

Me

CO2Me

BrMe

CO2Bnm

“macroinitiator”

efficient block copolymer formation - LRP

little to no starting macroinitiator

minimal termination occurring during polymerization

a living polymerization should give efficient block copolymer formation




pH

light

ion coordination

redox switching

mechanical forces

N N

Me2N NMe2

Me

Me

Me

Me

RuClCl

OMe

Me

pH-Driven Switching

Balof, S.L.; P’Pool, S.J.; Berger, N.J.; Valente, E.J.; Shiller, A.M.; Schanz, H.-J. Dalton Trans. 2008, 5791.

N N

HMe2N NMe2H

Me

Me

Me

Me

RuClCl

OMe

Me

chemically driven processes based on changes in pH common in both Nature and molecular machines

H+

n

ROMP

80% yieldin 28 min

n

ROMP

in catalysis, pH-switching can modify catalyst solubility

benzene

catalyst removal after metathesis can be challengingafter protonation, efficient removal of Ru catalyst by filtration

Redox Switching

Gregson, K.A.G.; Gibson, V.C.; Long, N.J.; Marshall, E.L.; Oxford, P.J.; White, A.J.P. J. Am. Chem. Soc. 2006, 128, 7410.

Fe

O

N N

O

Fet-Bu t-Bu

Ti

Oi-Pr

Oi-Pr

Fe

O

N N

O

Fet-Bu t-Bu

Ti

Oi-Pr

Oi-Pr

Cp*2Fe

Cp*2Fe+AgOTf

Ag Cp*2Fe

Cp*2Fe+AgOTf

Ag

control of oxidation state of redox-active ligands to mediate catalytic activity

Redox Switching


Fe

O

N N

O

Fet-Bu t-Bu

Ti

Oi-Pr

Oi-Pr

Fe

O

N N

O

Fet-Bu t-Bu

Ti

Oi-Pr

Oi-Pr

OO

O

O

Me

Me

O

Men

– 2e- + 2e-

rapid

slow

kred/kox = ~ 30

lactide polymerization known to be slowfor e-deficient TiIV-salen catalysts


Redox Switching


Fe

O

N N

O

Fet-Bu t-Bu

Ti

Oi-Pr

Oi-Pr

Fe

O

N N

O

Fet-Bu t-Bu

Ti

Oi-Pr

Oi-Pr

OO

O

O

Me

Me

O

Men

– 2e- + 2e-

rapid

slow

reversible nature of redox eventdemonstrates switching effect

kapp = rate of propagation

before oxidation: kapp = 4.73 x 10-6 s-1

after (Cp*)2Fe added: kapp = 4.98 x 10-6 s-1


Redox Switching


Fe

O

N N

O

Fet-Bu t-Bu

Ti

Oi-Pr

Oi-Pr

Fe

O

N N

O

Fet-Bu t-Bu

Ti

Oi-Pr

Oi-Pr

OO

O

O

Me

Me

O

Men

– 2e- + 2e-

rapid

slow

reversible nature of redox eventdemonstrates switching effect

kapp = rate of propagation

before oxidation: kapp = 4.73 x 10-6 s-1

after (Cp*)2Fe added: kapp = 4.98 x 10-6 s-1


can redox-switching be used to make micro-block copolymers?

Redox Switching

Wang, X.; Thevenon, A.; Brosmer, J.L.; Yu, I.; Khan, S.I.; Mehrkhodavandi, P.; Diaconescu, P.L. J. Am. Chem. Soc. 2014, 136, 11264.

FeII Ti

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

FeIII Ti

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

Co(Cp)2AcFcBArF

block copolymer synthesis via redox switching of ligands

FeII Zr

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

FeIII Zr

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

AcFcBArF Co(Cp)2

Redox Switching


change in oxidation state changes binding profile of catalyst

FeII Zr

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

FeIII Zr

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

O

O

OO

O

O

Me

Me

Me

O

O

Me

O

O

O

n

n

– e- + e-and 93% conversion

92% conversion

or

Redox Switching


change in oxidation state changes binding profile of catalyst

FeII Zr

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

FeIII Zr

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

O

O

OO

O

O

Me

Me

Me

O

O

Me

O

O

O

n

n

– e- + e-and 93% conversion

92% conversion

or CL

LA

minimal change in rate of reaction

before/after changing oxidation state of Fe

Redox Switching


FeII Ti

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

FeIII Ti

N

N

O

O(Ot-Bu)2

t-Bu

t-Bu

t-Bu

t-Bu

Me

Me

O

O

OO

O

O

Me

Me

– e- + e-+n LAm CL

(LA)n (CL)m

in situ redox-switching to achieve a block copolymer

ox. Zr complex did not polymerize caprolactone in mixed monomer pool

more Lewis acidic Zr = increases bond strengths of all intermediates for ox. compound

Electrochemically Mediated Atom Transfer Radical Polymerization

Magenau, A.J.D.; Strandwitz, N.C.; Gennaro, A.; Matyjaszewski, K. Science 2011, 332, 81.

Pn–Br Pn

cathodic current

anodic current

CuIBr/Me6TREN CuIIBr2/Me6TREN

e-

e-

ka

kdakt

E1/2 = – 0.40 V vs Ag/Ag+

E1/2 = – 0.69 V vs Ag/Ag+

kt

termination

extent of reduction dictated by applied potential (Eapp)

shifting Eapp to more positive values deactivates polymerization

Me

BrOEt

O

initiator

OMe

O

monomer

in absence of potential, equilibrium favors starting materialse-

e-

redox control of activity of metal catalyst


KATRP = ka/kda

ATRP depends on active/dormant equilibriumbetween (lower oxidation state) activators and alkyl halides (Pn–Br)

and between (higher oxidation state) deactivators and radicals (Pn•)

Pn–Br CuIX(L) CuIIX(L)R•

Pn–X CuIX(L) CuIIX(L)P•

M

M

initiation

propagationequilibrium with dormant species

ka

kd

Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921.

favoring dormant state mediates polymerization, allowing for simultaneous growth of each polymer chain

concerted atom transfer mechanism via inner-sphere electron transfer

excess reducing agent used to regenerate CuIX activators from CuIIX2 deactivators (ARGET)




KATRP = ka/kda

ATRP depends on active/dormant equilibriumbetween (lower oxidation state) activators and alkyl halides (Pn–Br)

and between (higher oxidation state) deactivators and radicals (Pn•)

Pn–Br CuIX(L) CuIIX(L)R•

Pn–X CuIX(L) CuIIX(L)P•

M

M

initiation

propagationequilibrium with dormant species

ka

kd


favoring dormant state mediates polymerization, allowing for simultaneous growth of each polymer chain

concerted atom transfer mechanism via inner-sphere electron transfer

excess reducing agent used to regenerate CuIX activators from CuIIX2 deactivators (ARGET)




Pn–Br Pn

cathodic current

anodic current


e-

e-

ka

kdakt

E1/2 = – 0.40 V vs Ag/Ag+

E1/2 = – 0.69 V vs Ag/Ag+

kt

termination

applying potential of – 0.69 V: 80% conversion in 2 h

E1/2 = – 0.69 V vs Ag/Ag+

e-

e-e-

low dispersity at high conversion (Mw/Mn = 1.06)



Pn–Br Pn

cathodic current

anodic current


e-

e-

ka

kdakt

E1/2 = – 0.40 V vs Ag/Ag+

E1/2 = – 0.69 V vs Ag/Ag+

kt

termination

e-

e-E1/2 = – 0.69 V vs Ag/Ag+ e-

e-E1/2 = – 0.40 V vs Ag/Ag+

electrochemical switch to modulate Cu oxidation state

repetitive stepping of Eapp acts as

toggling between E1/2 = –0.69 V and E1/2 = –0.40 V exhibits responsive on/off switching of polymerization




pH

light

ion coordination

redox switching

mechanical forces

tqc switchablecatalysis 2018 upload - princeton...

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