Synthesis of anthracene photodimer incorporating a tropylium ring and its facile retro[4+4] cleavage

Tetrahedron Letters,Vol.30,No.35,pp Printed in Great Britain

SYNTHESIS

OF ANTHRACENE

A TROPYLIUM

Koichi

RING

Komatsu,*

Department

AND

Kyoto

PHOTODIMER

ITS FACILE

Ryotaro

of Hydrocarbon

Tsuji,

Sakyo-ku,

+ .OO

INCORPORATING

RETR0[4+4]

CLEAVAGE'

and Ken'ichi Faculty

Chemistry,

University,

oo40-4039/89 $3.00 Pergamon Press plc

4689-4692,1989

Kyoto

Takeuchi

of Engineering,

606, Japan

Abstract: The title cation 6 has been prepared and exhibits an intramolecular charge-transfer band at X,,, 440 nm in CH2Cl2; 6 undergoes a facile retro[4+4] cleavage owing to weakening of the central c bonds due to 0-r orbital mixing.

The tropylium molecular

ion has been

electronic

example,

clear

observed

in the cations

3

(381

nm),4

donor

intramolecular

and

successfully

interaction

between

utilized

closely

as a probe

located

4 (334 nm with

Amax

(CT) interaction has been 2 3 2 (392 nm), 354 nm), 5 In these cations, at 365-450 nm).

(CH2C12)

a shoulder

face-to-face

and acceptor (tropylium) rings are placed 6 The CT absorption is observed arrangement.

S,7 in which

the r-systems

to compare

properties separated

are these

separated cations,

apart

in an essentially even

by a dihedral

we synthesized

in the cation

angle

120'.

the

excessive

rings

of the cation

The cation and, amount

6 was

synthesized

therefore,

according

low concentration

of diazomethane

(9.8 mmol)

to Scheme

1.

Because

of the starting had to be used

reflux

4689

7,8b

for the ring-

6

8

and 5.

of poor

material

1.

7

of

and examined

6 (Scheme I), which has the donor and acceptor 8a inbetween those of the former cations by the dihedral angle

solubility

Scheme

with

For

charge-transfer

1 (CT band

(benzene)

In order

for intra-

n-systems.

an

4690

Table

1.

Spectral

Properties

Vis(CH2C12)

Cation

X/nm(log

(422 (3.34))a

10

6 and 10 13C NMR

'H NMR (CD3CN;gOMHz) 6 I wm

E)

440(3.21)

6

of the Cations

610 (3.20) (593 (3.17))a

(CDjCN;22.5MHz)

6 I wm 153.2(d). 151.9(d), 150.6(d):Trop+

8.81-8.49(m, 5H, Trap+)

175.8(s),

7.20~6.80(m, 12H, Benz) 5.39(d, 2H. J=ll.OHz. :CH-)

142.6(s). 142.2(s), 140.0(~),128.8(d).128.4(d),

4.92(d, 2H, J=ll.OHz, :CH-)

128.2(d). 128.1(d), 128.0(d),127.3(d): Benz 59.6(d), 53.4(d) : :CH-

9.86(d. 2H, J=lO.lHz. H-3)

164.2(d, C-3), 161.5(d, C-l),

9.62(s, 2H, H-5)

141.2(d. C-2hb

9.22(t, lH, J=9.5Hz. H-l)

137.7(s, C-6),

139.4(s, C-4).

8.64(dd,2H,J=lO.1,9.5Hz.H-2)

137.3(d. C-5)sb Numbering: 8.63(dd. 2H.J=6.7,3.2Hz.H-7) 134.l(d. C-7). 8.2O(dd.2H,J=6.7,3.2Hz.H-8) a)

b) Tentative

In CH3CN.

enlargement After

7

unchanged Ph3C+SbF6-

(60

of the solvent, % recovery)

spectral

in Table

10 which

will

spectrum

by extraction

properties

1 together be described

as compared

reference

cation 9"

the nearly spectrum

facing

with

on 8).

I

11

6 are

by a benze

arrangement.

absorption

hypsochromic polarity.

intramolecular tropylium

which

shift upon This

absorption

contrast

is thermally

unstable

even

spectral

change isolated this

at 75°C

in Fig. 2.

for

500

Wavelength I nm

the

Figure

rings.

1.

the cation

Visible

spectra

6 taken

to take

1.5 min).

The species

is formally purely

place

6 was

found

in the dark, as is demonstrated newly

of

at O°C.

also

formed

to be by the

1

H NMR

of consumed and naphtho[bltropylium ion 10. 11

as anthracene

cleavage

can proceed

found

I 84 450

to the

the precursor 7, 8c up to its mp, the cation

identified

facile

21, which was

stable

and

I

LOO

with

at room temperature shown

I,

I

the solvent

between

benzene

0.6-

a

is ascribed

CT interaction

In sharp

a longrange

exhibits

increasing

and the facing

Therefore,

6 shows

in the visible

440 nm in CH2C12),

heating

treated

(51 % based

apparently

(Fig. l), the cation

wavelength

cleavage

of the cation

and were

powder

from

with the

due to the shielding

(Scheme

chloroform,

as a brick-colored

(200 ml).

separated

of 6, the tropylium

shifted

6 were

with

8 were

below.

are 0.37 ppm upfield

which

1,2-dichloroethane

with

protons

(Amax

in refluxing

the cycloheptatrienes

to give the salt 6 SbF6-'

The given

assignments.

of 7 (0.432 g; 1.21 mmol)

evaporation

130.4(d, C-8)

expressed

thermally

without

in the solid

in place

as a retro[4+41 light

phase

reaction

irradiation.

(63 % cleavage

The after

4691

Benz. I

Solvent

Cation 6

0 min.

8 min.

18

min.

30 min.

Figure

2.

Scheme

2.

1 H NMR

(90 MHz)

spectral

change

of the cation

6 at 35.1°C in CD3CN.

11

6

The first-order rate constants of this cleavage were determined in CD3CN 1 Among the activation H NMR at Os35OC to give the results shown in Table 2. bY parameters, the value of AS* appears to be rather small for a bond cleavage yielding state

two

species.

structure

is tempting

to assume

simultaneously seven-membered rigorously

while

ruled

some

resembles that

be taken

as the indication

the structure

the two a-bonds

retaining

ring,

though

of the cation

are loosened

the positive

charge

the possibility

mainly

for stepwise

and

for a transition Thus,

6 itself. then

cleaved

delocalized cleavage

in the

cannot

be

out.

This weakening

ring.

This might

which

of the central

intramolecular Previously, Table

electrpnic

Temp. /"C 35.1 25.0 19.8 15.0 0.0

effect

the appreciable 2.

Kinetic

Data

105k, /s-l 100 32 16 6.7 0.8

u-bonds

is supposed

caused

elongation

to have originated

by introduction

of the tropylium

of the corresponding

and Activation Activation

Parameters Parameters

from

bonds

ICD?CNI (25'C)

Ea = 23.5 f 0.3 kcal/mol AH* = 23.0 f 0.3 kcallmol AS* = 2.4 + 0.5 cal/mol*K AG* = 22.3 + 0.1 kcal/mol

in 7

it

4692

(1.62

i,,8a

relative

to normal

al. as the results and a donation

lowered

resulting

cation 6, as is actually

with

interpreted

by Mislow

the o to TI* donation

weakening

of the u-bonds

in facile cleavage

grateful

at ambient

to Professor

T. Kawamura

and Professor

benzene,

in further

observed We are

Acknowledgment:

helpful

has been

bonds,

et

a transfer

as compared

enhanced

University

single

of a-electron density into r* orbitals 12 Since the LUMO of the tropylium ion is r to c* orbitals.

from

significantly considerably

of both

of Gifu

13

temperatures.

H. Fujimoto

University

be

would

in the

of Kyoto

for their

interest

and

discussions.

References: 1) "u-r Conjugated

and R. West,

2) J. G. O'Connor H. Horita,

Tetrahedron

Lett.,

5) K. Komatsu,

rings,

I. Murata, K. Okamoto,

J. Org.

less

M. Ehrenberg, "Elements

Chem.,

Acta

its mp

9) mp s143'C

R. Tsuji,

53,

and

3849

(1979).

K. Okamoto,

1979,

4747.

of the donor

K. Nakao,

and

T. Nakazawa,

K. Komatsu,

R. Fujiura,

and and

(1988). Sot.,

the facing

99,

rings

(1966).

1996

(1977).

is determined

b) D. 0. Cowan

Plenum,

dissociation

cleaved

Lett.,

arrangement

4999;

Photochemistry",

($260°C),

Tetrahedron

defined

J. Am. Chem.

between

(Completely

(1976):

Lett., =,3899.

T. Kinoshita,

K. Nakatsu,

1979,

Cryst., 20, 177

of Organic

8446

J. Org. Chem., 44, 2712

clearly

Lett.,

angle

98,

Tetrahedron

and K. Okamoto,

I. Murata,

8) a) The dihedral

Sot.,

and S. Misumi,

see; K. Yamamura,

and

Y. Jinbu,

2029.

99 (1989).

Tetrahedron

7) T. Nakazawa

c) Above

with

2, see K. Komatsu,

1988,

and K. Okamoto,

K. Komatsu,

K. Takahashi,

6) For the examples acceptor

Y. Sakata,

30,

For Part

Lett.,

J. Am. Chem.

N. Abe, K. Takahashi,

K. Takeuchi,

4) K. Ikai,

3".

Chem.

and P. M. Keehn,

T. Otsubo,

3) K. Komatsu,

Part

Molecules.

G. R. Gillette,

and R. L. Drisko,

New York, 1976, Chap. 2, p. 37.

occurs

to anthracene

as 46':

to give

anthracene.

and the cation

10 at this temp.);

IR u (KBr) 3010, 2920, 1620, 1590, 1475, 1450, 1145, 895, 815, 780, 765, 740, 655 cm-';

10)

Anal.

57.02;

H, 3.42

'H NMR

6 (CD3CN)

Calcd

for C2gH21SbF6:

green

W. Huber,

powder;

K. Mislow,

atomic

mp s155'C

7dec):

Helv.

Hickel

Lett.,

Found:

C,

of 6 also

population

at the central

connecting

the central

12 June

1989)

64,

synthesis, 2766

unpublished;

see

Lett., 1979, 4995. see F. Gerson,

(1981).

R. A. Bell,

and

3479.

bond

in Japan

Tetrahedron

H. B. Schlegel,

1976,

MO calculation

Acta,

the a-bonds (Received

and K. Okamoto,

As for the

Chim.

W. D. Hounshell,

Tetrahedron

13) The extended

K. Takahashi,

N. Abe, K. Kubo, and I. Murata,

and K. MGllen,

12) D. A. Dougherty,

8.

174.0(s), 153.1(d), 151.3(d), 150.8(d), 139.8(s), 128.5(d), 125.8

(d), 51.1(d), 25.0(t): K. Komatsu,

11) Dark

H, 3.50

9.02(m, 5H), 7.48(dd, 2H), 7.18(dd, 2H), 5.18(s, 2H), 1.93(s, 4H);

13C NMR 6 (CD3CN)

also T. Nakazawa,

C, 57.55;

%.

u-bonds

carbon

and

indicated as well

the decrease as

the increase

the tropylium

ring.

in at