13C NMR spectra of some symmetrically 2,2′-disubstituted stilbenes: conjugational and conformational effects

405

Journal of Molecular Structure, 267 (1992) 405-410 Elsevier Science Publishers B.V., Amsterdam

r3C NMR SPECTRA

OF SOME SYMMETRICALLY

2,2’-DISUBSTITUTED

STILBENES: CONJUGATIONAL

Draien

AND CONFORMATIONAL

Mladen Minta$ and Nenad Raos”

VikIC-Topie’,

(1Laboratory

of Analytical Chemistry,

University c Institute

National Institutes

of Organic Chemistry, of Zagreb,

Faculty of Technology,

41606 Zagreb,

for Medical

of

, MD 20892, USA

Health, Bethesda b Department

EFFECTS

Research

Republic

of Croatia

and Occupational

41006 Zagreb, Republic

Health,

of Croatia

Abstract- In the 13C NMR spectra of symmetrically

2,2’-disubstituted

of a-effects

with respect to m-isomers

However

and increase of y-effects

at m-carbons

the most significant differences

found for y-effects, olefinic carbons apparent

is greater

coupling

interactions molecules

at unsubstituted

constants

are assumed

were substantiated

Introduction-The

effect of &-substitution

time.

two aspects

&-substituted

molecules

&-substituents.‘” with different

considered:

and

direct

The binuclear

The &-substitution

should, therefore,

the planarity of molecule

due to enhanced

k

steric

in which two benzene x-delocalization

effects throughout

n-electron

steric hindrances.

address: Ruder BoSkoviC Institute,

0022-2860/92/$05.00

differences

of these

for a long

preference

of

interactions

of

rings are linked

&conjugation)

delocalization

0 1992 Elsevier Science Publishers

e&.

by changing

A planar conformation

41000 Zagreb, Republic

in

the system, which in turn

(chemical and isotope shifts, coupling constants influence

and

conjugational

the conformational space,

interest.4 The

of electronic

reflects upon 13C NMR spectral parameters

(1)

through

aromatic compounds

affects the transmission

and configurational

of

calculations.

have been (ii)

were

effects

in aromatic molecules has been attractive

groups are of our continuous

these molecules

*Permanent

mechanics

in substituent

mainly from the different

The conformational

by molecular

&isomers

The one bond C-H splitting

The differences

to originate

the decrease were observed.

effects between trans_and

and C-4 atoms.

in &- than m-stilbenes.

in es- and m-stilbenes.

Generally,

in substituent

m-C-6

cis_stilbenes

of Croatia

B.V. All rights reserved

is favoured

by x-delocalization, the

(C-a)-(C-1)

but when steric hindrance

is involved, a torsion of the benzene

rings about

single bond occurs and the molecule is not planar. As, due to different

environment,

the difference

be expected,

that

in planarity between

sterical difference

of that phenomenon

2,2’-disubstituted

m-

and &s-stilbenes

should be maniEasted in their 13C NMR spectra.

sterical should

The study

is the principal goal of this paper.

Methods- 13C NMR spectra were recorded

using a JEOL FX-100

NMR spectrometer

operating

in the pulse FT mode at 25.05 MHz. Samples were dissolved in CDCl, and chemical shifts were referred

to TMS as internal

widths were resolutions

standard.

were recorded

from 5000 to 800 Hz. FID’s were accumulated

conformational

V= 1/2Ct k,,i(bt-b,t)‘+

mechanics

calculations

non-bonded

for bond

lengths,

valence

Parent

function

programme

interactions

with parameters

and k, and 8, for

and non-bonded

V, and n (height

interactions

were computed

were performed

with

13C NMR spectra of m-

stilbenes

and & stilbene*

(1) and (2), respectively

(3) to (1O)8 were analysed (scheme).

The broadband

proton

decoupled

13C NMR spectra

of tSB (1)

and C-4 signals can be distinguished

and cSB(2) from C-2 and

The C-2 and C-3 signals were assigned on the

of C-H couplings and the multiplicities

of coupling patterns

and also by using

effects on aromatic chemical shifts.

Svmmetricallv

2,2’-disubstituted

The six signals correspond

stilbenes.

to the benzene

the olefinic carbon resonance. substituted

r is

with parameters

A, B and C. Calculations

C-3 signals, by taking into account their intensities.

substituent

respectively;

angles

for bond stretching

were determined

consist of five lines (Table 1). The C-l, C-a

basis of magnitudes

the

using Dashevsky force field.697

and Discussion-The

molecules-

and torsional

Kb and b, are empirical parameters

and multiplicity of the torsional barrier, respectively)

and 2,T-disubstituted

by minimizing

(1)

distance.

from a Buckingham

performed

to the

l/2 & k a~(0 j-6oj)2+ W&V”(l*cosn&)+

valence angle bending. Torsional

Results

The digital

(strain) energy:

b, 8, and I$ stand

Rasmussen

in 8K-4K addresses.

were

UArexp-Btrt)Ctrt?

where

at 25% and SO’C. The spectral

were from 1.22 Hz to 0.24 Hz. The pulse width of 4 us is applied corresponding

flip angle of 45’. The molecular molecular

Spectra

show seven-line

ring carbons and the seventh

tSB ,- trans-stilben;

C-l carbon atoms. The remaining cSB, &-stilben.

13C NMR spectra.

signal was attributed

Two signals of low intensity can be recognized

C-2 and quarternary

*Abbreviations:

These compounds

immediately

to as

five signals were assigned by

using the high temperature

measurements,

C-H coupling

chemical shifts (SCS)of aromatic compounds

R

Table 1. The differences

(1)

(3)

(3)

(4)

(5)

tram

cis

tram

cis

Pans cis

H

NO,

NO,

H

constants

(5)

Cl

Cl

and substituent

induced

in SCS between

(7)

(3)

(3)

(10)

Vans

cis

trans

cis

CH,

CH,

OCH,

OCH,

tSB’s and

Scheme

~93% are apparent. Qrtho- substituted &-carbons,

The signs are mostly preserved ~93% generally showed decrease

&at

C-2 and C-6

independent be related

carbons

on the &-substituent.

for the substituents of y-effect

However,

effect of substituents.

of y-effect

in decrease of a-effect

of a-effect

more than the a-effect

always shielded

against C-4 in related tSB’s. The shielding is less than in parent

that the slight removing of electron

The y-effect

at C-4 accompanying

and (6) the increase

of y-effect

trans-cis isomerization

was observed

(C-a) carbons of &-cSB’s

are deshielded

most

cSB’s are cSB (2) which

C-4 of &-

cSB’S .

can increase or decrease.

while for (8) and (10)the opposite

All effects at C-4 are positive except the one in (7). As for the parent molecules,

deshielding

the increase

at C-2. The C-4 of &-substituted

density occurs at position

may

the values of y-effect

at C-6 increase

indicates

decrease

Additionally,

against the

is slightly greater

at C-6 does not show any regularity in inductive effect of the substituents, distance from the substituent.

of

is virtually

of higher than of the lower inductive effect. On the contrary,

due to three-bond

at

at C-2 and decrease

of a-effect

slight variations The decrease

The

tSB’s (Table 1). As both

in cSB’s are still deshielded

in tSB’s and in parent tSB. The decrease

to the inductive

vary considerably.

and increase

to the increase of shielding

at C-6 atoms in cSB’s. However, m-carbons

corresponding

probably

of a-effect

with respect to corresponding

effects are usually positive, that corresponds shielding

but the magnitudes

For (3)

was found. the olefinic

against those of related tSB’s. However,

value is higher than in cSB (2) indicating the decrease

of shielding

at C-a by ortho

408 substitution.

The corresponding

y-effect

C-4 and C-6. As a consequence,

at C-a is always negative,

the decrease

ofy-effect

Table 1.13C chemical shifts (6 /ppm), substituent differences

) in c& (8 cl.$ -- 6mxI.V

and t rans stilbene

in contrast

to y-effects

at

at C-a was observed.

induced chemical shifts (SCWppm) (1) and (2) respectively

and their

and their derivatives

(3) to (lo)*. Compd ./

C-a

C-l

c-2

c-3

c-4

c-5

C-6

config. _________________-___^_________________~~~~~-------_______________________ (1) trans (2) cis

128.64 130.13

137.27 137.10

126.45 128.70

128.54 128.06

127.47 126.9

128.54

126.45

128.06

128.70

-0.57 -0-48 -0.17 2.25 -0.48 6& -6m l.4g _-__________________~~~~-~~~~~~~--~~~~~---~~~~~___----_______________

trans

2.25

128.78

132.28

147.89

124.60

128.59

133.26

128.68

scs

0.14

-4.99

21.44

-3.94

1.12

5.79

2.23

(4) cis

128.78

132.50

148.23

124.65

128.40

133.13

132.45

SC.9

-1.34

-4.60

19.53

-3.41

1.44

5.07

3.75

(3)

-0.19 -0.13 0.22 0.34 0.05 6& -6m O-O0 __________--_____-------___----___--~~~___----_______________________

3.77

128.59

126.64

126.54

0.92

1.12

-1.90

0.09

129.37

128.55

126.21

130.69

1.31

1.65

-1.85

1.99

-0.04 -0.43 0.32 0.46 -0.09 *a -6m -0.04 ---------________--_~~~~~-----____~~~~~-_----________________________

4.15

(5) tram

126.88

134.81

133.26

129.46

scs

-1.76

-2.46

6.81

(6) cis

128.84

135.13

133.72

SCS

-1.29

-1.97

5.02

(7) tram

127.61

136.42

135.40

129.99

127.17

125.81

125.18

scs

-1.03

-0.86

8.95

1.46

-0.30

-2.73

-1.27 129.98

(8) cis

129.33

136.39

136.00

129.82

126.89

125.28

scs

-0.80

-0.71

7.30

1.76

0.00

-2.78

1.28

-0.28 -0.53 -0.33 0.60 -0.17 6& -6* 1.72 --___----_____---___~~~~~~~~~~~~--~~~~~-~~~~~~~___-_--_______________

4.80

(9) trans

123.53

126.60

156.72

110.57

129.67

120.56

128.89

scs

-5.11

10.67

30.27

-17.97

2.20

-7.98

2.44

(IO) cis

125.53

126.31

157.05

110.47

128.26

120.02

129.91

SCS

-4.60

-10.79

28.35

-17.59

1.36

-8.04

1.21

0.00

-0.29

0.33

-0.10

-1.41

-0.54

1.02

6&

-6w

*a, p and y-effectscorrespond to the differencesin chemical shifts between ortho - cSB’s, ie tSB’s and parrent molecules (minus sign indicates shielding). These effects are referred with respect to the number of bonds from the substituent at C-2 atom.

409

The quartenary

C-l atoms of ortho_ cSB’s are deshielded

although

the effect is opposite

negative,

deshielding

for unsubstituted

results in decrease

molecules.

of p-effecfts

The C-3 and C-5 atoms, of ortho_cSB’s are shielded However,

shielding

at C-3 in &-&B’s

against

those of a-

As the p-effects

Decrease

remains constant.

of negative

p-effects

in both cases decrease

against corresponding

atoms in ortho_tSB’s.

is less than at C-3 of parent cSB (Z), while shielding

The p-effects

at C-3 atoms in &-cSB’s

of ortho_tSB’s and &B’s also show the differences.

apparent

one-bond

splittings

were always greater

&-tSB’s

couplings,

corresponding

Compd . / conf

ig .

(1)

trans

mechanics

of stilbenes

Torsion Ql

12.78

effects.

We found

are olefinic, that these

(from 155.3 Hz to 167.9 Hz) than in analogues aromatic C-H splittings are rather similar

the one at C-6 is always greater

in ortho_ cSB’s than in

calculations

(Table 2) should give some explanation

of the

(1) and (2) and their derivatives

(7) and (S).*

angles

Strain

Gibbs

Statistical

@2

energy

energy

w(V)

weight w(G)

-12.78

58.189

546.734

0.493

0.609

-14.79

-14.79

58.12

547.843

0.507

0.391

-47.96

64.625

554.777

1.000

1.000

52.33

74.248

682.902

0.466

0.421

(2)

cis

-47.96

(7)

trans

140.98 -147.16

52.71

75.856

684.397

0.247

0.230

55.26

76.853

684.637

0.165

0.209

-53.45

53.45

77.601

685.645

0.122

0.139

-125.87

64.14

72.717

681.669

0.673

0.845

60.30

74.502

685.880

0.327

0.155

55.26

cis

although

high-order

interesting

phenomena.

Table 2. Conformers

(8)

in &-&3’s

from

takes place. The C-H coupling Especially

tSB’s.

The results of molecular observed

resulting

(from 151.5 Hz to 162.0 Hz). The one-bond

in both series of stilbenes,

changes,

was true for positive effects. Therefore,

constants

C-H

only p-effect

at

are of both signs and the changes are of both kinds.

was found, while opposite

of shielding

are always

in ortho-cSB’s.

C-5 is virtually the same as in (2). It means that in trans -cis isomerization while i-effect

tSB’s,

60.30

‘Energy values are expressed in kJ mol.’ and torsion angles in degrees. Torsion angles & and & :orrespond to (C-2)-(C-l)-(C-a)-(C-a’) and (C-2’)-(C-l’)-(C-a’)-(C-a) angle, respectively (see scheme). Statical weights (t=25’C) were calculated from Boltzmann distribution either from strain [potential)energy. w(V), or from Gibbs energy, w(G), by taking into account also the vibrational :ontribution to Gibbs energy.

410 The calculated

average & angle, -z&2>, and chemical shift at

yielding an average increase of 0.2 ppmP for a declination be explained

the observed

A+&is)=15.58°;(cf.Table sensitive

position,

<&>(1)=34.39“;

difference

1) are correlated,

from planarity. In a similar way it can

in SCS at C-2 for (7) and (8) (A<&>(trans)=37.56”,

2). Such a simple correlation

but only for difference c&>(S)

C-2 (&Table

between

- c&>(7)=10.41°).

also holds for C-6, the other sterically

a(7)

and

n(8)

The observed phenomena,

isomer therefore,

(+&2)

-

call for further

studies. Acknowledgement. Republic

This study was supported

by grants from the Ministry for Scientific

Work of

of Croatia to whom we are grateful.

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(1977) 32 b, 181.

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Mass Suectrom.

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Analysis, Stringer,

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J. K. Jakobsons,

V. A. Gindin and J. A Gravitis, Zh. Organ. Khim.

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Zh. Strukt. Khim. (1970)

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Chem. Ber. (1895) 28, 1411, *C. Weygand and T. Siebenmark,

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