Photodriven Sm(III)-to-Sm(II) Reduction for Catalytic Applications - johansen-et-al-2024-photodriven-sm(iii)-to-sm(ii)-reduction-for-catalytic-applications.pdf

Photodriven

Sm(III)-to-Sm(II)

Reduction

for Catalytic

Applications

Christian

M. Johansen,

‡

Emily

A. Boyd,

‡

Drew

E. Tarnopol,

and Jonas

C. Peters

Cite This:

J. Am. Chem.

Soc.

2024,

146, 25456−25461

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ABSTRACT:

The

selectivity

of SmI

as a one

electron-reductant

motivates

the development

of methods

for reductive

Sm-catalysis.

Photochemical

methods

for SmI

regeneration

are desired

for catalytic

transformations.

In particular,

returning

III

-alkoxides

is a crucial

step

for Sm-turnover

in many

potential

applications.

To this

end,

photochemical

conditions

for reduction

of both

SmI

and

a model

III

-alkoxide

to SmI

(THF)

are described

here.

The

Hantzsch

ester

can serve

either

as a direct

photoreductant

or as the reductive

quencher

for an Ir-based

photoredox

catalyst.

In contrast

to previous

III

reduction

methodologies,

no Lewis

acidic

additives

or byproducts

are involved,

facilitating

selective

ligand

coordination

to Sm.

Accordingly,

species

can

generated

photochemically

from

SmI

in the presence

of protic,

chiral,

and/or

Lewis

basic

additives.

Both

the photoreductant

and

photoredox

methods

for SmI

generation

translate

to intermolecular

ketone-acrylate

coupling

as a proof-of-concept

demonstration

a photodriven,

Sm-catalyzed

reductive

cross-coupling

reaction.

amarium

diiodide

(SmI

) is an exceptionally

versatile

single-electron

reductant.

The

large

and

labile

coordina-

tion

sphere

of Sm

can

recruit

one

or multiple

substrates

and

additives

to achieve

selectivity

in both

organic

synthesis

and

small-molecule

reductions

(Figure

1A).

−

However,

SmI

employed

stoichiometrically

in all but

a few

select

cases

−

because

its reactions

typically

terminate

in the

formation

highly

stable

III

−

alkoxide

species.

Catalytic

regeneration

the Sm

state

requires

abstraction

of OR

−

by a stoichiometric

oxophile

(EX)

to generate

a Sm

III

species

that

can be reduced

by a relatively

mild

reductant

(Figure

1A).

The

difficulty

associated

with

this

transformation

has

been

cited

as a

motivation

for the

development

of a variety

of alternative

photo-

and

electrochemically

driven

methods

for ketyl

radical

generation.

−

Early

strategies

for reductive

catalysis

relied

on harsh

combinations

of halosilane

oxophiles

SiX)

and

low

valent

metals

(Mg

for

X = Cl;

for

X = I) or an applied

electrochemical

potential

as the reductant.

−

In a collabo-

rative

effort

with

the Reisman

laboratory,

we recently

disclosed

comparatively

mild

silane-free

thermal

and

electrochemical

conditions

for catalytic

turnover

of SmI

in reductive

coupling

of ketones

and

acrylates

through

combination

of cationic

Brønsted

acids

with

either

or an applied

potential

−

1.55

V vs Fc

+/0

(Fc

+/0

= ferrocenium/ferrocene;

all potentials

referenced

to Fc

+/0

Given

the

growing

interest

in (metalla)photoredox

catal-

ysis,

photodriven

strategies

for

III/II

catalysis

remain

surprisingly

underexplored.

25,26

In a strategy

recently

show-

cased

by the

groups

of Borbas

and

Nemoto,

photo-

sensitizers

are incorporated

into

the

secondary

coordination

spheres

of Ln

III

complexes

(Ln

= Sm,

Eu;

Figure

1B).

Intramolecular

oxidative

quenching

of the excited

sensitizer

the Ln

III

center

produces

a potent

reductant

which

can

carry

out a variety

of transformations.

While

this

and

other

strategies

show

promise,

−

the

chelating

ligand

platforms

used

thus

far in photodriven

III/II

catalysis

(cryptands,

bidentate

phosphine

oxides)

restrict

the

coordination

sphere

and/or

shift

(Ln

III/II

) to strongly

negative

potentials,

belying

direct

translation

to the

rich

stoichiometric

chemistry

of SmI

(L)

as an inner

sphere

reductant

(L = solvent

molecule,

typically

THF).

Lewis

acidic

metal

ions

are

commonly

used

to template

substrates

in photodriven

reductive

coupling

reactions.

10,29,30

Recently,

in contrast

to the use of photocatalysts,

several

Lewis

acid-mediated

photoreductions

utilize

the blue-light

absorbing

Hantzsch

ester

(HEH

) as a photoreductant

(

(HEH

•

HEH

) =

−

2.5

V).

−

Photoexcited

HEH

(

HEH

) carries

out

III

reduction

in a catalytic-in-Cr

photodriven

Nozaki

−

Hiyama

−

Kishi

reaction

(Figure

1C).

Alternatively,

HEH

acts

as a photoreductant

in a Gd(OTf)

-mediated

Giese

addition

of an

N-

hydroxyphthalimide

(NHPI)

ester-derived

alkyl

radical

into

-unsaturated

ketones

or a lactone

(Figure

1C).

In the

latter

study,

an interaction

between

and

HEH

is observed,

but Gd

III

reduction

to Gd

is not accessible

even

HEH

Based

on these

precedents

we noted

that

HEH

should

capable

of reducing

III

-species

such

as SmI

(

(SmI

+ I

−

)) =

−

1.58

V; Figure

S35).

Because

Sm and

Gd are

similar

in size

and

oxophilicity,

we envisioned

that

photo-

excitation

of HEH

bound

to Sm

III

could

result

in intra-

molecular

oxidative

quenching

to produce

(Figure

1D).

Crucially,

however,

a more

dynamic

Sm-chromophore

interaction

might

allow

access

to coordinatively

unsaturated

Received:

July

23, 2024

Revised:

August

23, 2024

Accepted:

August

26, 2024

Published:

September

2024

Communication

pubs.acs.org/JACS

The Authors.

Published

American

Chemical

Society

25456

https://doi.org/10.1021/jacs.4c10053

J. Am. Chem.

Soc.

2024,

146, 25456

−

25461

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SmI

(L)

species

which

could

carry

out inner-sphere

reduction

in a photodriven

Sm-catalyzed

cross-coupling

reaction.

Importantly,

both

HEH

and

its 2H

/2e

−

oxidized

congener,

HE,

are

weak

bases

and

are

therefore

compatible

with

the

acidic

conditions

necessary

for recovery

of inactive

III

−

species

by protonolysis.

Gratifyingly,

HEH

proved

competent

as a photoreductant

for

III

-to-Sm

conversion.

Monitoring

the

−

visible

absorption

spectrum

of a solution

of SmI

(2 mM),

HEH

(60

mM)

and

2,6-lutidine

base

(Lut,

60 mM)

following

irradiation

at 440

for

5 min

in THF

reveals

the

characteristic

profile

of blue

SmI

(THF)

with

max

at 555

and

618

nm (Figure

2A, left panel).

Extended

irradiation

(120

min)

results

in increasing

SmI

generation,

with

maximum

yield

∼

25%.

Interestingly,

in the absence

of base

this

reaction

does

not

proceed

(Figure

S17),

likely

due

to rapid

back-

electron

transfer

(BET)

between

HEH

•

and

SmI

. However,

HEH

•

can

be deprotonated

in the

presence

of base,

circumventing

BET.

evaluated

conditions

for

photogeneration

SmI

(THF)

from

Sm(O

Pr)

as a model

III

-alkoxide.

Irradiation

of Sm(O

Pr)

(2 mM),

tetra-

-heptylammonium

iodide

(

Hep

NI, 6 mM),

and

HEH

(60

mM)

at 440

nm in

THF

shows

no evidence

of SmI

formation

(Figure

S19).

However,

upon

the addition

of only

1.5 equiv

of the acid

bis

trifluoromethylsulfonylimide

(HTFSI)

to Sm(O

Pr)

SmI

(THF)

is generated

upon

irradiation

with

Hep

NI and

HEH

(Figure

2A,

right

panel).

Parallel

studies

demonstrate

that

no SmI

is generated

from

Sm(O

Pr)

this

acid

(Figure

2B,

compare

light

and

dark

blue

traces),

and

current

attributable

to Sm

III

reduction

(presum-

ably

of an intermediate

mixture

of solvated

“SmI(O

Pr)

” and

“SmI

Pr”)

does

not onset

until

−

2.3

V. In contrast

to SmI

no external

base

is needed,

suggesting

that

the

Sm-bound

alkoxide

might

additionally

serve

the

role

of deprotonating

HEH

•

to avoid

BET.

−

vis

studies

reveal

that

addition

the colorless

III

−

Pr species

(gray

trace

in Figure

2A)

gives

rise

to a significantly

red-shifted

shoulder

in the

HEH

absorption

profile

(compare

light

and

dark

red

traces

Figure

2A),

consistent

with

preassociation.

The

modest

yields

and

rates

of these

reactions

motivated

the

study

of Sm

III

reduction

with

a photoredox

catalyst

Figure

Summary

of key

challenges

for Sm-turnover;

prior

studies

exploiting

III/II

photochemistry

and

photoreductions

with

HEH

and

Lewis

acidic

metals;

and

this

work

describing

photodriven

generation

of SmI

Figure

(A)

−

vis

spectra

following

photoreduction

of SmI

(left)

and

SmI

Pr)(L)

to form

SmI

. CVs

of Sm(O

Pr)

(2 mM)

in the

presence

of iodide

and

proton

sources

in THF.

Journal

of the American

Chemical

Society

pubs.acs.org/JACS

Communication

https://doi.org/10.1021/jacs.4c10053

J. Am. Chem.

Soc.

2024,

146, 25456

−

25461

25457

overcome

the

low

quantum

yield

and

excited

state

lifetime

(220

ps in MeCN)

of HEH

selected

[Ir(dtbbpy)(ppy)

]

([

III

]

)

as a photo-

sensitizer,

which

could

undergo

reductive

quenching

by a

sacrificial

electron

donor

to generate

is thermodynami-

cally

capable

of reducing

SmI

to SmI

(

III/II

) =

−

1.94

Figure

2B and

Figure

S36).

Irradiating

SmI

or SmI

Pr (2 mM)

with

[

III

]PF

(0.2

mM),

HEH

(60

mM)

as sacrificial

reductant,

and

Lut

(60

mM)

rapidly

generates

SmI

(80%

or 30%

conversion

in 2 min,

Figure

3A).

Again,

the

weak

base

Lut

enhances

the

process

(Figures

S20

−

S21).

The

accelerated

reduction

of SmI

Pr is curious,

electron

transfer

from

to this

III

species

is uphill

400

(Figure

2B).

A rationale

for these

observations

provided

in Figure

3B:

reductive

quenching

[

III

]

HEH

generates

not only

the strong

reductant

, but also

the

strong

acid

HEH

•

−

1 in MeCN),

39,40

the combination

of which

can carry

out net proton-coupled

electron

transfer

III

−

Pr.

Proton

transfer

from

HEH

•

to a Sm

III

−

species,

likely

via

proton

relay

mediated

by Lut,

liberates

PrOH

and

[SmI

]

The

latter

can then

be reduced

to SmI

Development

of Sm-catalysis

leveraging

diverse

ligand

coordination

to modulate

reactivity

is an attractive

goal.

Exploration

of Sm

generation

in the

presence

of potential

coligands

was

carried

out pursuant

to these

interests.

Satisfyingly,

is readily

photogenerated

from

SmI

[

III

]

and

quencher

(HEH

or Et

in the

presence

several

protic

additives

(ethylene

glycol,

N,N

-dimethylaminoe-

thanol,

Figures

S23

−

S24),

3,43

−

including

a chiral

aminediol

(Figure

4A,

Figure

S25)

that

has

been

utilized

in several

enantioselective

SmI

transformations.

−

The

reduction

potential

and

reactivity

of Sm

is highly

sensitive

to coordination

of Lewis-basic

additives

(HMPA,

−

;

Figure

4A).

While

[

] is insufficiently

reducing

to access

such

species,

the more

reducing

photocatalyst

3DPA2FBN,

when

paired

with

the

reducing

quencher

9,10-

dihydroacridine

and

N as base,

mediates

generation

both

SmBr

and

Sm(HMPA)

(Figure

4B).

3DPA2FBN

also

facilitates

III

reduction

and

binding

to the chiral

BINAPO

ligand

(Figures

4A and

S34).

51,52

Having

established

two

different

photochemical

approaches

to Sm

generation,

we targeted

an intermolecular

ketone-

acrylate

coupling

as a model

reaction

to benchmark

photo-

driven

Sm-catalysis

(Table

1). This

reaction

is representative

of the

qualities

that

set

SmI

apart

as a stoichiometric

reductant.

Inner-sphere

electron

transfer

to one

or both

of the

carbonyl

substrates

is obligatory

based

on comparison

of outer-

sphere

reduction

potentials.

Importantly,

a Sm-alkoxide

generated

as the

byproduct

of lactonization,

enabling

evaluation

of the

ability

of a set of conditions

to overcome

this

critical

barrier

to generalizable

Sm catalysis.

Irradiation

of ketone

(0.04

mmol),

phenyl

acrylate

equiv),

and

SmI

(THF)

(10

mol

%) in the presence

of HEH

(4.0

equiv)

in 2-MeTHF

(0.02

M) at 440

nm for 90 min

yields

lactone

in 76%

yield

(Table

1, entry

1, method

A). Addition

of a photoredox

catalyst

([

]PF

, 1 mol

%) with

pyridine

equiv)

results

in an increase

in yield

to 89%

(entry

1, method

B). Light

and

Sm were

required

for catalytic

formation

either

method

(entries

3 and

4). Sm(OTf)

is a competent

precatalyst

with

50 mol

% MgI

included

as an iodide

source

(entry

4). Substitution

of Gd(OTf)

for Sm(OTf)

results

trace

product

formation,

supporting

a key

role

for Sm

catalysis

(entry

5).

Figure

(A)

Photoreductions

of Sm

III

species

with

[

]PF

photocatalyst.

(B)

Rationale

for

net

photoinduced

proton-

and

electron-transfer

from

HEH

to [Sm

III

−

OR]

species.

Figure

(A)

Ligand

coordinated

Sm-species

generated

by a

photoredox

approach.

See

SI for

relevant

electrochemical

data.

Choice

of a sufficiently

reducing

photocatalyst

remains

crucial

observe

. (B)

−

vis

spectra

following

photogeneration

SmBr

and

Sm(HMPA)

Journal

of the American

Chemical

Society

pubs.acs.org/JACS

Communication

https://doi.org/10.1021/jacs.4c10053

J. Am. Chem.

Soc.

2024,

146, 25456

−

25461

25458

Both

methods

are

competent

in the

presence/absence

pyridine

(entries

1, 6, and

7), but yields

are greatly

diminished

in the

presence

of a stronger

base

(Et

N, entry

8).

This

suggests

that

the dynamics

of Sm-alkoxide

protonation

play

important

role

in turnover.

Interestingly,

the

use

of a

dihydropyridine

without

carbonyl

groups,

5,6-dihydrophenan-

thridine,

only

shows

product

formation

with

[

]

(entry

9). In

the

absence

of Ir, the

specific

interaction

between

and

HEH

appears

to be required.

The

Ir-catalyzed

reaction

is also

faster,

achieving

60%

conversion

in 15 min,

compared

to 29%

by method

A (entry

10).

Methods

A and

B were

tested

against

alternative

coupling

partners

to assess

their

relative

efficacies.

When

using

less

activated

substrate

pairs

(aliphatic

ketones

and

alkyl

acrylates,

entries

1, 11 and

12),

method

B is favored,

perhaps

because

these

slower

cross-couplings

require

rapid

III

-to-Sm

conversion.

Method

A is preferred

when

using

aryl

ketones

(entries

−

15),

as method

B gives

considerable

pinacol-

coupled

side-products

(Table

S3).

With

method

A, selective

inner-sphere

photogeneration

of Sm

by Sm

III

−

HEH

may

favor

-mediated

cross-coupling,

while

with

method

background

Ir-mediated

substrate

reduction

to homocoupled

products

can

dominate.

A proposed

mechanism

for this

photodriven

lactonization

reaction

(by

method

is presented

in Figure

5. The

mechanism

can

be divided

into

two

parts,

a photoreduction

side

in which

III

is reduced

to Sm

, and

a SmI

cross-

coupling

side

where

the

organic

substrates

are

coupled.

Starting

from

SmI

(OPh),

coordination

to HEH

(as

demonstrated

in Figure

2A)

followed

excitation

HEH

allows

for the

proton

and

electron

transfer

required

to generate

SmI

, with

PhOH

and

HEH

•

as additional

products.

Subsequently,

SmI

couples

the acrylate

and

ketone

to form

a radical

intermediate.

53,54

HEH

•

is capable

reducing

this

intermediate

as a potent

H atom

donor,

although

alternative

schemes

for reduction

of the radical

intermediate

can

be envisioned

(Figure

S45).

Following

reduction

and

lactonization,

is formed

along

with

SmI

(OPh).

With

[

]

, a similar

mechanism

is proposed,

differing

in the

regeneration

of Sm

, which

can

be regenerated

from

III

alkoxide

as depicted

in Figure

3B (see

Figure

S46

for full

scheme).

In summary,

we have

demonstrated

photodriven

generation

of SmI

(THF)

from

III

precursors

using

both

a photo-

reductant

and

a photoredox

catalyst.

These

conditions

translate

to proof-of-concept

photodriven

reductive

Sm-

catalyzed

ketone-acrylate

coupling.

Distinct

from

reported

methods,

photodriven

Sm-catalysis

occurs

in the

absence

competing

Lewis-acidic

metal

additives

and

byproducts

(e.g.,

and

salts),

−

which

may

be of utility

development

of Sm-catalysis

with

ligands.

3,18,43,46

−

These

findings

are anticipated

to facilitate

applications

of Sm-catalysis

beyond

the types

of thermally

driven

transformations

studied

thus

far.

■

ASSOCIATED

CONTENT

sı

Supporting

Information

The

Supporting

Information

is available

free

of charge

https://pubs.acs.org/doi/10.1021/jacs.4c10053.

Experimental

methods,

data

from

individual

catalysis

experiments,

and

additional

spectra

as referenced

in the

text.

(PDF)

■

AUTHOR

INFORMATION

Corresponding

Author

Jonas

C. Peters

−

Division

of Chemistry

and Chemical

Engineering,

California

Institute

of Technology

(Caltech),

Pasadena,

California

91125,

United

States;

orcid.org/

0000-0002-6610-4414;

Email:

jpeters@caltech.edu

Table

1. Photodriven

Sm-Catalyzed

Coupling

of Ketones

and Phenyl

Acrylate

to Form

Lactone

Products

Yields

were

determined

H NMR

analysis.

For additional

reaction

data,

see

Table

S2.

tert

-Butyl

acrylate

used

as coupling

partner;

lactonization

observed

only

upon

acidic

workup.

Figure

Proposed

mechanism

of Sm

cross-coupling

under

Ir-free

conditions

(method

A).

Journal

of the American

Chemical

Society

pubs.acs.org/JACS

Communication

https://doi.org/10.1021/jacs.4c10053

J. Am. Chem.

Soc.

2024,

146, 25456

−

25461

25459

Authors

Christian

M. Johansen

−

Division

of Chemistry

and Chemical

Engineering,

California

Institute

of Technology

(Caltech),

Pasadena,

California

91125,

United

States;

orcid.org/

0000-0003-0066-4424

Emily

A. Boyd

−

Division

of Chemistry

and Chemical

Engineering,

California

Institute

of Technology

(Caltech),

Pasadena,

California

91125,

United

States;

orcid.org/

0000-0003-0150-5396

Drew

E. Tarnopol

−

Division

of Chemistry

and Chemical

Engineering,

California

Institute

of Technology

(Caltech),

Pasadena,

California

91125,

United

States

Complete

contact

information

is available

at:

https://pubs.acs.org/10.1021/jacs.4c10053

Author

Contributions

‡

C.M.J.

and

E.A.B.

contributed

equally.

Notes

The

authors

declare

no competing

financial

interest.

■

ACKNOWLEDGMENTS

We thank

the National

Institutes

of Health

(R35GM153322).

E.A.B.

and

D.E.T.

thank

the National

Science

Foundation

for a

Graduate

Research

Fellowship

under

Grant

No.

DGE-1745301

and

2139433,

respectively.

C.M.J.

is grateful

for support

from

the

Aker

Scholarship

foundation.

also

acknowledge

the

Resnick

Sustainability

Institute

at Caltech

for

support

enabling

facilities.

thank

the

Reisman

laboratory

for

supplying

ligand

samples.

■

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