Photochemical generation of radical anions. An access to novel structures

Tetrahedron Letters,Vol.26,No.2,pp Printed in Great Britain

PHOTOCHEMICAL AN

181-184,1985

GENERATION

ACCESS

TO

OF RADICAL

NOVEL

lnstitut

der

Klingelbergstrasse CH-4056

The

preparation

fluorene

of

(FLH)

the

by

radical

ANIONS.

STRUCTURES

Huber

W. Physikalisch-Chemisches

Abstract:

oo40-4039/85 $3.00 + .OO a1985 Pergamon Press Ltd.

UniversitZt

Base1

80

Base1

anions

photochemical

of

tetrabenzocyclooctatetraene

methods

leads

to

the

(TBCOT)

observation

and

of

novel

structures.

Ejection with

of

(FI-) with

in

paramagnetic to

species

is one

of

the

structures

and

best

phase

exploited

for

ii)

of

a

by

the

compound.

The

of the

two alkali

documented

(11. the

species

preparation

(TBCOT=)

the

condensed

poorly

hydrocarbon

to a neutral new

the

is only

applications; richer

electron

carbanions

phenomenon new

an

It

This

radical light

induced

salts

181

quite

and

ESR

communication

anion

of the

metal

induced

is therefore

preparation

[2].

choice

light

i)

by

the

electron

substrates TBCOTr/K+,

has

processes surprising

spectroscopic focuses

the

photooxidation transfer proven FLH?Na+

from

which that

this

basic

investigation attention of

an

a stable

advantageous

.

occurs

on

of two

electron carbanion in dealing

182

Photooxidation the

neutral

literature a set

of TBCOTY compound

(31.

The

of eight

appears, vity

of

to the

the

reasons:

lent

two

formation

[S],

this The

spectrum

ESR

upon

spectrum

(0.171,

whereby

for

TBCOT-.

the

light

is cut

(I)

to the

right.

The

off

to

the

two

that

the

ESR

anion

each

of

of

of the

of the

belonging

a solution

spectrum,

to dis-

in the

the

ca-

hyperfine

the

yields

of four

two

immediately

values

usually

ob-

disappears

shifts

the

as four

equiva-

larger

0.139mT)

TBCOT=

following

same width

a set

of the

(0.020,

of

the

spectrum

with and

TBCOTT

solution

for

exhibits latter

associated

constants

spectrum

in

of TBCOTY

TBCOTT

of the

(0.018mT)

coupling

photolysis

ESR

of TBCOT

smaller

each

spectrum

of such

an

analysis

0.014mT).

of the

that

The

ESR

reduction

described

above.

radical

reduction

ii)

eight-proton

fact

indicates

again

upon

originally O.OZOmT),

the

described

the

upon

0.022,

averages

to

that

of TBCOT=.

0.087,

the

comparable

assigned

(0.139,

irradiation

observes

from

observed

photolysis

constants

are

is

During

observed

to that

reduction,

(41.

one

in MTHF

similar

constants

further

of TBCOT=

differs

is very

coupling

Upon

substantially

protons,

served

yields

protons.

of TBCOT’

mirror

spectrometer

i)

(0.129)

spectrum

ESR

obtained

coupling

ESR

of which

Nevertheless,

that

analysis

equivalent

due

structure

The

on a potassium

when

equilibrium

hv TBCOT’

Photoinduced

electron

transfer

anion

the

of

FI’

by

spectroscopic of

(FI’

of

FIH-

ejected

electron

served

might

The

FIH

from

low-temperature

spectrum

yields

five

which

similar

to those

spectrum

yields

four

two-proton

260K

ensures

The

average

observed compare

for

two

coupling

of these the

satisfactorely

two values

protons with

on the

(0.418,

(0.504, two

in

values

of

neutral

differ.

FIH’

0.424,

is close

[6].

of

0.129,

to the

analysis

remaining of FIH’

at

the

280K.

capture

spectrum

The

two

the

a The

of

of FI.

of

most

the

is ob-

single

remarkable

high-

the

0.395,

analysis

(260K)

of the

high

protons

to the

constant

two-proton

and

low temperature 0.123,

0.085mT) temperature

in

addition

Investigation

constants

coupling

[ 6 1. The

ESR

however,

even by

the

of a mixture

generates,

solvent

0.078mT).

coupling

diamagnetic

radical.

0.414,

The

the

impairs

irradiation

solvents.

The

of the a-protons

FIHT

The

no ESR

which

(0.505,

0.330mT)

one-proton

to

which

Fl’followed that

several for

for

reacts

spectrometer

fact

in

observed

FIHY

process

spectroscopically

of this

significantly

(1,

200K.

ESR

The

observed

(0.374mT)

C9

II).

constants

the

that

is this

detected

MTHF/benzene

constants

of

an

coupling

constants

assignment

It

photooxidation

was

(180K)

usually

be

dimerization

in

two-proton

coupling the

the

known

C9.

of

scheme

transfer

+ e’

approximately

can

to the

investigations

spectrum

on

cavity

which

reaction by

electron

is well

above

the

due

(see

be rationalized

result

are

FIH’

is certainly by

in

It atom

FIH’

salt) of

FIH:

hydrogen of

sodium

photoinduced

findings the

as

concentration

formation

to

an

characterization

FIH/FI’

stationary

loss

TBCOT:

_

of

protons

(0.414mT) coupling

at low temperature

(see

to

FIDon

at C9.

usually constants Table).

183

Table:

Coupling

constants

hv _

FI.

FIH + e’

-

FIH:

2FI.

-

(FI),

of TBCOTT

TBCOTrb TBCOT+

FI -

and

FIHY

in mT.

la

2a

0.020

0.139

0.022

0.014

0.171

0.087

Qa

ga

=a

%a

FIH-d

0.085

0.505

0.123

0.395

FIHye

0.078

0.504

0.129

0.424

a) b) Z’, e)

Positions TBCOT’ generated TBCOT 7 generated T = 180K T = 250 K

Structural

considerations:

is reduced

with

respect

a

deformation

caused

by

nylylene)-type view

of

TBCOT=

these are

the

framework

TBCOT;

This

“parent”

the by

0.418

data

0.330

the can

a polarization

be

are

TBCOT=)

are

for

spin

by

separated

the

the

the

symmetry

upon

data

this

analogously

density the

by

preparation an

on two

to

radical by

a

of

rings. the

anion

activation

the

methods barrier

In

dianion

reflect

a deformation

that

is

bis(biphe-

benzene

photolysis

(C,)

reduction

an unsymmetrically

assumption

different by

of

of TBCOT=

symmetry

“tub”-type

charge

TBCOT’

rationalized

includes

accessible

from of

hyperfine

of the

implicitly

that

(D2d ) 141. This

framework localization

observed

Obviously, and

previously compound

molecular

ESR

which of

reported neutral

a partial

dianion

and/or

TBCOT’

been

of the

interpretation

photooxidation

interconversion.

of

surprising.

molecular

of

has

findings,

of the

structures

It

to that

and

not

ga 0.414

direct reduction of TBCOT. photooxidation of TBCOT=.

structure

structure

counterion.

by by

+ e’

the

of the

positioned two

different

(reduction

impairing

of

a fast