The invention of radical reactions

Tetrahedrm~ Vol. 43, No. 19, pp. 4321 to 4328. 1987 in Great Bntain.

THE RADICAL

INVENTION

OF

DECARBOXYLATIVE

Derek

RADICAL

*a

Barton,

de

REACTIONS.

BROMINATION

H.R.

lnstitut

0040-4020/87 53.00+.00 1987 Pergamon Joumab Ltd.

0

rnnti

Chimie

Brigitte

des

91190

AND

XVI.

IODINATION

Lachera

Substances

PART

and

Samir

Naturelles,

Cif-sur-Yvette,

OF

AROMATIC

h

Zard

Z.

ACIDS

C.N.R.S.,

France.

(Received in Belgium 23 July 1987)

Pb5tfUCt: Thiohydroxamic esters of aromatic carbaxylic acids undergo clean decarbaxylative bromination or iodination an treatment with iadoform or diiodamethane in the presence of a bramatrichloromethane, radical initiator.

Decarboxylative has or

traditionally iodine.’

silver

been

The

but

but

yields

electrophillc expense

ring

include

silver

oxide

Other

alternate

achieve

visible

in and

situ

diacetate’

and

such

with

the

practical

of

the

silver

the

use

of

treating

of

a

the

halogen

useful

salt

of

by

heating

donor

acid

(cupric

lead

and

carboxylates

has

problems.

chloride

place been

of

tetraacetate-iodine or

N-halo

succinimide,

91128

of

Chemistry,

de

Palaiseau

Chimie Cedex,

Texas Synthese

AEM

Universlty,

Organique,

Ecole

France.

4321

College

Station,

Polytechnique,

Texas Route

77843,

to

strong and

iodobenzene

etc..).

“‘Department b) Departement

with silver.

exploited

involve

tetraacetate6

halides,

to

difficulties

acid

also

the

subject

these

in

of the primary

undergo are

invariably

with lead

the

salts

have

however,

wlth

silver obviate

of 4

thallium IRCO;)

carboxylic or

pure

treatment

These,

also acids The

which

bromine

purity

especially

groups

very

or

radicals

and

aromatic

with

to the

substrates.

variants

mercury’

acyloxy

hypoiodite5

withdrawing

dessication

halogenations.

as

not

and

aliphatic,

rich

reaction)

carboxylates

of water

electron

are

more

producing

t-butyl

presence

saturated

electron

and

other

decarboxylative

or .In

of

with

contrast,

Hunsdiecker-Borodin silver

presence

therefore

preparation

for

anhydrous

bearing

and

(the

to the

well

In

formation

methods

treatinq

works

acids

bromine’

conditions light

the

acids

by

variable.

development

similar

oxidising

in

the

These

are

carboxylic

sensitive

otherwise

bromination

inherent

encouraged

is very

Aromatic

acids.

reaction

of

accomplished

reaction

carhoxylate

carboxylic

halogenation

U.S.A.

de Saclay,

iodine

D. H. R. BARTONet al.

4322

In a recent radical

particular, for

aliphatlc

and

alicyclic

the various

neither

heavy

halogenation

Moreover,

found

that

if esters

or irradiated

loss of carbon

radical

propagates sulphide

by

a

synthetic

facets

metals nor strong was especially

judicious

the 2.

4

concomitant

the

carboxylic more recent

CHl3

chain

choice

of a novel

oxidants.

In

easy and efficient

of

being

whereas

the

1,

formation

by

far

The

the

experimental

of

Is conducted

another

(Scheme

1).

Thls

superior

(Y*

to the

of

1 to give

radical =

hypohalites

are

the

into

etc..

.I

halogenation’ aliphatic

of type

strongly

in

of a good halogen

reaction

esters

2,

ultimately

‘CH12

tertlary

Hunsdiecker-Borodin

Hunsdlecker-Borodin

We

carbon

is converted

.CCl3,

and

Is that

1.

resulting

the intermedlate

decarboxylative

secondary

for this superiority

reaction

in the presence

carbon

radical

chain

trap,

molecule

intermediate

in Scheme

N-hydroxy-2-thiopyridone

a radical

another

iodination.

is outlined

acids and

of an efficient

with

the

method

undergo

case of primary,

reason

normal

they

the reaction

etc...

in the

variants.

Ilght,

decarboxylative

novel

from carboxylic

reacting

propagator

efficient acids

by

If however,

halide

our

In the absence

chain

BrCC13,

as

derived

dioxide.

the corresponding

underlies

with visible

(Ccl,,.

exceptionally

which

1,

donor

with

concerning

process

overall

philic,

acids.’

results

chemical

are heated

serves

requires

were able to achieve the decarboxylative bromination of aromatic and a,610 acids. We would now llke to describe this latter work in detail as well as

more recent

The

pyridyl

we have described

which

we

unsaturated present

of papers,* reaction

we have shown that decarboxylatlve

conditions

have

series

decarboxylation

or

the

which proved

or alicyclic any

of

Its

1 are not electroelectrophilic

(see

below). RCOIH +

R’

+ CO2

etc..

3

. -

R

+

!i

,’ X-Y

Y’

+

R-X 4

X-Y

I

Cl-CC13,

I-ail2

BPcc13.

5

a)

R=

e-MOO-C,H,

a)

Y = ccl3

I-adamantyl

b)

Y = CH12

C)

Y =

bl

R =

cl

R = p-O,N-C,H,-

-(CF2)$F3

Scheme 1

Aromatic problematic. radical

in this

and The

a.t+unsaturated

difficulties

case ”

(104-10’

arise s-’

carboxylic from

the

for ArC02’

acids

somewhat

on

the

persistent

as compared

with

other

hand

nature

of

-10’

s-l

for

are

the

rather

carboxyllc

RC02’).

For

4323

The invention of radical reactions--XVI

example yield

irradiation

of p-methoxy

by the thiol.

of ester

2

benzoic

acid

In sharp

presence

produced

by

reaction

produced

-1 2 O s CT =\O/u

mercaptan

of the

acid could

only adamantane

R

Me0

of t-butyl

capture

no adamantanoic

contrast,

The

-lb even at -786C. (Scheme 2).

in the

afforded

intermedlate

an excellent

carboxylic

radical

be detected

in the case of ester 12 as the sole acid derived product

~o@co2” +a-,+

+5H

hv

Scheme 2

The

special

preliminary Whereas with

nature

series

aliphatic

visible

of esters

light,

(< 10%) of sulphide

we

gave

cleanly

from aromatic

examined

their

acids manifested

behaviour

the corresponding

sulphides

from p-nitrobenzoic -lc derived The &z in a rather unclean reaction.

its anhydride.

probably

ionic

origin

The

formation in

as shown

disproportionate

into

detected

reaction

enough

1 derived

ester

acid and

in the

of esters

experiments,

dipyridyl

disulphide

mixture.

of anhydride

Scheme

The

in this case to allow interference

3.

7 and

acid major

was found

The

Initial

fission

by unwanted

+

products

of the

were

N-O

a

Qs-m

+

general

of which

Scheme 3

and 5

of

may

have

been slow

J-0-i-h

Qs_SJQ Ilw be

yield

is probably

ionic processes.

a

7

low

disulphide

bond

in a light.

p-nitrobenzoic

6

I

and

or irradiating

only

to be quite

8 both

when,

heat

2 on heatlng produced

unsymmetrical

its di-N-oxide

homolytlc

itself

towards

D. H. R. BARTON ef al.

4324

To

favour

somewhat

high

suspension

of

heated and

ca

the

unimolecular

sodium

130°C

a-naphtoic

and to

to

the

some

acids

cases

example, entry

the 4)

produced amounts

a

not

yield

of

slight

so

of

clean

entries

trans

Yields

increase

in

the

2). were

from

thermal yield

of

(22%)

but

the also

the

was

resulted

these

formation

a

of was

poor

the

in

anhydride

In

lowered.

For

(Table

the

temperature

formation

of

1,

large

1

Bromo

Acid

derivative

Yield

Yield

P-Nitrobenzoic

Acid

4-Bromonitrobenzne

6gb)

of 5a c%)1 )

(%)

;;R,

Lit.

Yield (%)

95’5)

423d)

2

1-Naphthoic

acid

I-Bromo-naphthalene

a2

90 8Ob)

2-Naphthoic

acid

PhCHkHC02H

4.5’a) 60b)

Ola)

2-Bromo-naphthalene

05

78

PhCHkHBr

68

70

17.5’a)

12.5b)

39b)

Ola)

p-MeOC6H4C02H

p-MeOC6H4Br

04

87

3,4,5-Trimethoxy-

3,4,5-Trimethoxybromo 17a) benzene

62

68

26(j)

p-Bromotoluene

55c)

70

17la)

henzoic

p-Toluic

acid

acid

113d)

Br17b)

C02H

BnO

9

69

conditions

12-15%

Lowering in

45

55

benzene’7c

60

61

m-lodobromobenzne

81

81

BnO

OMe

OMe

Z,C-Dimethoxy-

2,6-Dimethoxy

benzoic

bromo

acid

m-lodobenzoic

acid

a

p-nltrobenzoic produced

of

temperature

acid

to

anhydride.

a)

1

of thus

extension

product.

at a

chloride

bromotrichloromethane

were

and

to operate

acid

brominatlon

variable when

compelled

and

However,

cinnamic

fragility

Table

Entry

thus

carboxylic

bromonaphthalene

especially

bromostyrene the

the

decarboxylative

1 and

suppressed

we were of

o-dlchlorobenzene

and

successful.

be easily

step addition

mixture

relatively

1,

perhaps

of cinnamic

a

slow

p-Bromonitrobenzene

not

reflecting a

in

[Table

was

could

salt

effected

acids.

80% respectively. other

decarboxylation Indeed

temperature.

C02H

a) Bramo derivatives with no reference are described in "Dictionary of Organic Compounds". b) Reactions performed without AIBN at Ca 13O'C. Sth Ed., Chapman and Hall. 1982. c)Low yield due to volatility of the product. All others at IOO'C with 15-30X AIBN.

The invention of radical reactions-XV1

Even

under

hampering with

the

the

the

consequent

number

initiator the

such

of

such

as

[eq.

side

radical

One

in

which

would

being

Indeed the

when

hot

decarboxylative temperature

acids

as shown This

the

by

give

hypobromide

in the

[Table

1,

the

of

entry success

the

pounds

(Table

cinnamic

acid

the

slow

derivative temperature

ester

Ieq 1)

3

to

more

could

addition

reproducible

be

run

of aromatic

methods

difficult

the

at

the

carboxylic

avoid

in

this

be

seen

donating

Hunsdlecker-Borodin

type.

of

the

of

66% as

compared

intermediate

the

No

cases.

dramatic

strongly

improvement

experimental with

acyl

Undesired

reactions.

these a

modlflcation to

aromatic

can

electron

related

Furthermore

rich As

with

nature

other

to

electron

procedure. acids

of

and

increased

as for

this

electrophilic

of

and

entries

AIBN

the by

thermal -5b. was

6,

reaction

is

is

procedure.

the

original

12.5%

thiosulphate.

We were

apparead

of to

be

which

poorly

of the

addition

of the

the

expected

are

not

yet

in

only

resulting

the

starting (Grignard

a few).

in view

suspension of

reaqents

but

slow

as

satisfactory

of a solution

sodium

salt

of 2 in

lodoaromatlc clear,

the

a

yield

low

comreaction of

the

10).

acquire

decomposition

increased.

proceeded

donor,

briefly

invaluable

organometallic

A similar

yields

reasons

us to examine are

to name

atom 9

to a refluxing

less

mixtures

iodine series.

For

2 ,’ entry

useful

derivatives

reasonable

8).

chloride

reaction

This

as the

alicyclic

encouraged compounds

very

CH13

afford

4,

of

palladium

in toluene

1,

iodo

or

and

did

(Table

the

a number

iodoform, aliphatic

bromination

Aromatic

mercury

iodoform

iododerlvative

is discharged

the

prior

and

variety

aromatic

method.

decarboxylative

lithium,

2,

Overall, Furthermore,

keep

‘CC1

chloride

reaction

as well of

2-71,

highly

using

is

iodination.

4-chloro-3-nitrobenzoyl

desired

poor

classical

is

+

acid

the

utility

(entries

our

by

1.

the

the

produced trichloromethyl

would

a cleaner

to a wider

electron

ring

of

of

chloride

containing

the

aromatic

the

BrCC13

Hunsdiecker-Borodin

in

in the

toluene

with

of

involved

tested

obtained acid

the

preparation

We initially

of the

the

with

organo-

results

the

bromostyrene

case

for

reagents,

1

with

external

4).

difficult

materials

Table

is

case

yield

The more

In

of

species

observed

in

results

to

In

in Table for

radicals

x

Furthermore,

vindicates

is a consequence

bromination

Thus,

made

2

to Increase an

to give

CN Br

of

was

incorporating

system,

seriously

concentration

pathway.

3

it applicable

collected

difficulty

by

in our

radical

was

of ester

isobutyronitrile

incorporated

salt

made

strongly

implicated

electrophilic

was

is effective

poor

The

desired

observed.

which

results

comparison

shortcoming

ionic

the

30%)

sodium

was

100°C

generality

substituents This

to

this

time

propagators

the

a buildup

bromotrichloromethane

RrCCl

the

decarboxylation

This

from

of

given

2; -

(up of

bromination

lower

acids.

AIBN

suspension

a

the

chain

favour

decarboxylation

causing

of circumventing at

with

efficient

%N=N+N-%

to

way

(AIBN).

react

therefore

sluggish

process

operation

azobislsobutyronitrile

low and

the

chain

reactions. chains

of AIBN

1)

concentration

conditions,

of the

radical

decomposition

radicals

drastic

efficienty

4325

the

efficient a dark thus

than

the

brown

colour

inclined

iodoform

corroborated

decarboxylative

to

ascrlhe

as

well

as

by

the

charaterlstic the of

poorer

modest

the yields

bromination. of

iodine

which

performance

coproduced observed

to

pyridine when

the

D. H. R. BARTQNet al.

4326

Table

Entry

Acid

ArCO2H

1 -Naphthoic

Iodine

acid

2

source

Yield

lodo derivative

I-Iodonaphthalene

CH13

II

(%) of

al

CF3fCF2)5-I

(69)

II

(57)

,I

(6’3)

CH2l2 CH13

3,4,5-(Me0)3C6H2C02H

laa)

3.4,5-fMe0)3C6H21 I,

II CH13

4-MeC6H41

II

(52)

8,

(54)

CH2’2 4-02NC6H4C02H

4-02NC6H41

CH13

(16)

*lab)

C02H 10

Cl

NO2

11

II

case.

sulphide

with and

no

2

Hall,

Using the

actual

produced

be

isolated in

are described

in

52% yield.

“Dictionary

of

Organic

5 th

Compounds”,

Ed.,

of the

would have probably

as unstable

substances.

l3

been ideal sources

We therefore

of io-

turned

to

perfluoroiodohexane.

the

sulphide

potential [Table

appearance

desired z

indeed

of the

reaction

iodonaphthalene

could

be

unstable

isolated and

was

mixture

was in

not

this

in

case

large

was indeed

improved

part

more

(Table

2,

in comparable the

source

pleasing

entry

yield of

the

2).

152%). iodine

experiments.

iodine donors

2, entries

Diiodomethane

were also examined.

3, 5, 7, 11) but neopentyl

was practically

iodide gave mediocre

results

as good (Table

9). Although

allow

yield

-5b was in the previous

Other

tive

reagent,

that

as iodoform

also

described

and iodotribromomethane they

but expensive,

this

corresponding

confirming

are

1982.

but unfortunately

the more stable,

could

reference

lodotrichloromethane

entry

(1’31

CH2I2

Chapman

The

NO2

II

“In this b) Products

but

(13)

CH13

Cl

dine

(54)

,I

II

(31) (49)

CH2G 4-MeC6H4C02H

ArIb)

lodination

on the whole the yield of an aromatic

an expedient

entry

into

acid

does

otherwise

of iodoaromatics not

seem

is only

to have

inaccessible

been

modest, reported.

the

radical This

decarboxyla-

method

could

compounds.

Experimental

NMR data are for deuterochloroform solutions Melting points are uncorrected. N-Hydroxy pyridine-2-thione 2 and its sodium tetramethylsilane as internal standard. Aromatic acid chlorides were prepared by standard methods are available commercially. thionyl chloride.

with salt using

2,

The invention of radical reactionsXV1 N-(p-Methoxy)-benzoyloxy-pyridine-2-thione (This

is

&.

sensitive

to

light

and moisture.

The reaction

should be corvered with aluminium foil). etc.. To an ice cold solution of I-hydroxypyridine-2-thione

column (400

compound

4327

mg)

in

dry, degassed After stirring

dichloromethane for 30 min.

5 mmoles). the yellow product eluted heating gave essentitllly ” (Nujol) 1775 cm * 6:&y-7.34

(2H.

Irradiation

m).

of

An

Eater

an

lamp under an chromatography

2

the

inert of

261 (M 1; J - 9 Hz). Presence

of

(Ca

ml)

&

of

and pyridlne (850 column

mg. and

Concentration under reduced pressure without was used without further purification:

8.13 (3H,

(2H. 8).

d.

J -

9 Hz),

7.55-7.80

(2H.

m),

(10

ml)

t-Butylmercaptan. (150

mg)

in

dry,

degassed

was

irradiated

t-butylmercaptan

until on

mp,, 5 nrmolea)

which

6 : 4!90

of

ester

0.5

atmosphere the residue

disulphide (115 rug. acetate:dichloromethane authentic

in

solution

excess

dichloromethane. (861+mg. 66%)

d.

(635

chrovtography

(10 ml) was added p-methoxybenzoylchloride the mixture was filtered through a silica

&

m/e:

6.95’(2H.

ice-cold

containing

with pure

2

vessel,

disappearance silica using

of the yellow dichloromeFpe

86%), identical with an authentic (1:l) afforded E-methoxybenzoic

dichloromethane with colour. gave

sample. acid (87

a

300

W tungsten

Concentration and t-butyl 2-pyridyl

Further elution with ethyl mg, 98%). identical with an

specimen.

Decarboxylative

Bromination

General

Procedures.

a) Procedure not involving AIBN. - To a suspension of the dry sodium salt of 2 (1.1 in a refluxing mixture of bromotrichloromethane and c-dichlorobenzene (2:3; 5ml) was dropwise (30 min) under an inert atmosphere a solution of the acid chloride (1 mmole) After a further heating period of 5 min. the solvents were the same mixture (10 ml).

mole) added

in evaporated

under

reduced

pressure

using dichloromethane-pentane sulphide -5a are known products. b)

Procedure

involving

mmole) in refluxing the acid chloride

and

the

mixtures. The yields the

use

of

bromotrichloromethane (1 mmole) and AIBN

residue was purified The bromoderivatives and references are

AIBN. (Ca

-

To a suspension

(5 ml) 25 mg)

After a further heating period of atmosphere. reduced pressure and the residue purified as above Decarboxylative

To a suspension under an inert

added tion tion

Iodination

lected

in

Table

(oven+temperature

150°C

*

the

sodium

by chromatography

salt

of

2

(1.1

(30 min) a solution of (5 ml) under an inert was evaporated under on silica.

salt of 2 (1.1 mmole) in iodine donor (1.1 mmole;

(1 mmole) and AIBN (Ca a further heating period

refluxing dry see Table 2)

25 mg) in the of 5 min the

toluene followed

(3 ml) was by a solu-

same solvent (5 ml). solvent was evaporated

The addiunder re-

by chromatography on silica using dichloromethane:penare known compounds. Yields and references are col-

2. n-hexyl

;

of

was added dropwise in the same solvent the solvent 5 min,

on silica co-produced 1.

Procedure.

and the residue purified All the iodo derivatives

Perf luoro perfluorohexyliodide (M-F) 1.24;

General

of the sodium atmosphere the

of the acid chloride took 30 min. After

duced pressure tane mixtures.

-

by chromatography as well as the collected in Table

as

8.60-8.72

N. t1.35;

S,

2-pyridyl the at

(1H. 7.18.

sulphide

source

of

3 mmHg); m),

iodine

v

was

(neat)

8.40-8.6~af2H.

Calc.

for

1580,

m).

ClIH4F,3NS:

which SC purified 1220.

7.20-7.45 C,

by

was bulb

1205 (lH,

30.78;

m);

H. 0.94;

isolated _y

cm

when

bulb ;

m/e

429

(Found: N.

using

distillation: C,

3.26;

S.

(H 1.

410

31.00;

H,

7.47%).

Acknowledgement We would helpful radicals the

like

discussion by

difficult

addition problem

to thank and of

for an

Prof. the.

E.

Vogel

suggestion

initiator.

of decarboxylative

Prof.

(Cologne to E.

Univ.,

increase Vogel

bromination

has

Federal the

successfully

of sensitive

Republic

concentration applied 14

trieneacids.

of of

Germany)

for

trichlomethyl this

method

to

D. H. R. BARTONef al.

4328

References 1.

a) R.G. 2,

2.

Johnson and R.K.

F.A.H.

Rice

discovered

and w. Morganoth. by

Borodin:

Chem. EnR. News, 65. 3.

Ingham. Chem.Rev.,

56,

219 (1956);

b) C.V. Wilson,

Org.React.,

332 (1957).

a)

Cristol

S.J.

Caston

L.K.

S. Carol1

see

T.

and J.

Firth

Tiedeman,

Jr.,

A. McKillop,

5.

D.H.R.

Bunds. c,

2,

_, 30

(1956).

Rae.

H.P.

Fare,

1279

I.

This

reaction

Solov’ev

_, 37

280

and

was first

C.

(1961);

b)

J.A.

Davies,

c)

d) N.J.

Steinberg,

Bunce,

e,

S.J.

Cristol.

J.

2,

Herynk.

664 (1972);

669 (1972). J.Org.Chem.,

Serebryakov

E.P.

2. (1964);

415 (1965);

D. Bromley and E.C. Taylor,

Barton,

1388

D.

J.Org.Chem.,

s,

e) J. Cason and D.M. Walba, c. 4.

21.

Kauffman,

28 (1987).

and W.J.

and

J.Org.Chem., C.B.

2.

and

1172 (1969).

N.F.

Woolsey,

J.Chem.Soc.,

2438

(1965). 6.

For a Review see:

7.

J.I.

Suarez.

a. For

a

D.H.R.

10.

D.H.R.

11.

J.K.

recent

J.R.

14.

B. Lather

in

16.

J.

Barton,

Vogel,

a)

T.M.

19,

279 (1972).

Hernandez,

J.A.

Salvazar

Barton

D.H.R.

and

S.Z.

Schieb,

729 (19B6). Fleming,

Lett.,

Vol.

2,

and

E.

Zard,

Pure

and

2,

2,

4779 (1983).

5939 (1985).

Wiley

Interscience.

2,

J.C.

Sci.. 2.

Zard,

413 (1958);

New York,

109,

897 (1987);

Bevington.

J.

Toole

and

863 (1958). unpublished

Somayajulu

Schulz.

observations.

and

B.J.

Schmidt,

K.

J.Org.Chem.,

Sharma and M.K.

P.K.

Kochi,

Lett.,

J. Am. Chem. Sot..

J.R.

W.H.

Tetrahedron

and K.U. Ingold,

Polym.

and S.Z.

98,

44.

Menen.

Zwolinski.

J.Chem.Rng.Data. and

Schmickler

H.

J.

Lese,

3405 (1979). Ind.J.Med.Res..

50,

750

(1962)

(cf

C-A,

and A.

Jahn.

196 ).

Kohn and L.

Chem.Ber.,

J.

T.

Dandija, 3222~.

R.

Tetrahedron

J.K.

Faraday Sot., Vickrey,

Meyers and H.P.

H.

see:

Zard, Ed.

J. Lusztyk

D. Bridon

E.

and S.Z.

Toole,

Trans.

251 (1979).

2, 17.

work

Radicals”,

“Free

Kennedy,

P.C.

this

and W.B. Motherwell.

Chateauneuf,

J.

2,

A.I.

of

Barton,

Angew.Chem.. 15.

Org.React.,

Freire,

402 (1986).

review

Bevington.

D.H.R.

Kochi.

R.

675 (1986).

L. Trossanelli.

13.

2.

D. Crich

(1973);

12.

Francisco,

Barton,

Kochi

J.C.

Sheldon and J.K.

C.G.

J.Org.Chem..

App.Chem.58, 9.

R.A.

Conception,

Steiner,

J.Org.Chem.,

85, 346 (1952);

c)

B.

g,

Riegel

30,

b)

(1947);

and H. Wittcoff,

H. Lethe

J.Am.Chem.Soc..

68.

1913.

(1946).

la. a)

R.G.R.

Bacon

and J. Miller, 19.

D.H.R.

Barton,

and

J.R.

Wright,

J.Am.Chem.Soc.. D. Crich

75.

J.Chem.Soc.(C),

1978

(1967);

b)

R.R.

4265 (1953).

and W.B. Motherwell.

Tetrahedron,

2,

3961 (1985).

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