PPhotochemical and hotochemical and TThermal hermal TTransformations of ransformations of

,,’-aryl/heteroaryl/alkyl ’-aryl/heteroaryl/alkyl SSubstituted ubstituted BButadienesutadienes

Irena ŠkorićIrena Škorić,,11 Fabijan Pavošević, Fabijan Pavošević,11 Krešimir Molčanov, Krešimir Molčanov,22 Biserka Biserka Kojić-ProdićKojić-Prodić22 and Marija Šindler-Kulyk and Marija Šindler-Kulyk11**

1Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, 10000 Zagreb, Croatia

2Laboratory for Chemical and Biological Crystallography, Department of Physical Chemistry The Ruđer Bošković Institute, Bijenička cesta 54, 10 000 Zagreb, Croatia

iskoric@fkit.hr

References:References:[1] D. Vidaković, I. Škorić, M. Horvat, Ž. Marinić, M. Šindler-Kulyk, Tetrahedron 64 (2008) 3928-3934; I. Škorić, N. Basarić, Ž. Marinić, A. Višnjevac, B. Kojić-Prodić, M. Šindler-Kulyk, Chem. Eur. J. 11 (2005) 543-551 and the references cited therein.[2] M.-H. Filippini, J. Rodriguez, Chem. Rev. 99 (1999) 27-76.[3] A. D. Becke, J. Chem. Phys. 98 (1993) 1372; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B, 37 (1988) 785.

Fig. 2. ORTEP drawings of 3.

In order to prepare novel polycyclic structures by photochemical methodology, new o-substituted ,’-aryl/heteroaryl/alkyl-1,3-butadienes 1a-c were synthesized and the photochemistry of the prolonged conjugated system was studied. Our previous results [1] on photochemistry of different heteroaryl substituted o-divinylbenzenes showed interesting intramolecular cycloaddition reactions and formation of bicyclo[3.2.1]octadiene structures. By insertion of additional double bond into the stilbene-like moiety the obtained octatetraene conjugated system (1) might allow formation of new polycyclic structures with a double bond functionality for further transformations.Moreover bicyclo[3.2.1]octane skeleton is found in numerous important biologically active natural products [2].Upon irradiation of 1,3-butadiene derivatives 1a-c the benzobicyclo[3.2.1]octa-2,5-diene derivatives 2a-c are obtained in moderate to very good yield (40-90%) (Scheme 1). The photoreaction is stereoselective. In the case of 1a the formation of only one product is obtained whereas 1b and 1c give a mixture of endo- and exo-2b,c. Depending on the irradiation wavelength the formed benzobicyclo[3.2.1]octa-2,5-diene derivative 2a (Fig. 1) undergoes di--methane rearrangement to tricyclic structure 3 (Scheme 2; Fig. 2) while 2b and 2c do not react.

XXIInd IUPAC SYMPOSIUM ON PHOTOCHEMISTRY

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On the other hand, in the thermal reaction of cis,trans-

1a-c the new dihydronaphthalene-

cyclobutane structures 4a-c are isolated (Scheme 3) as a result

of octatetraene- followed by hexatriene-ring closure.

In the case of 1c the mixture of new endo- and exo-

benzobicyclic compound (yield 44%), while in the case of 1a,b

only endo-4a,b isomers were formed (yield 71 and 63%).

Scheme 1. Proposed mechanism of theintramolecular [2+2] photocycloaddition of 1a-c.

2-PS

3-PS

5-intermediate

4-intermediate

6-PS

7-PS122,0

Reaction coordinate

E

1-ct1-cc

endoexo

112,1

36,3 13,9

Fig. 5. B3LYP/6-31G(d)3 energies for electrocyclization

of cis,trans-1c.

Fig. 3. Part of the 1H NMR spectrum (in CDCl3)of photocycloadduct 2c.

2a

3

Scheme 4. Quantum-chemical Study of the Mechanism of Thermal Reaction of 1c.

3

Fig. 1. ORTEP drawings of 2a.

Scheme 3. Formation of new dihydronaphthalene-cyclobutane structures 4a-c.

Fig. 4. Part of the 1H NMR spectrum (in CDCl3) of the mixture of thermal products exo- and endo-4c.

h, 300 nm

2a

3

Scheme 2. di--methane rearrangement of 2a to tricyclic structure 3.

The novel structures were described spectroscopically. The NMR spectra of photoproduct of 1c and thermal product of 1c are presented in Fig. 3 and Fig.4, respectively. According to the NMR spectrum after thermal reaction of 1c the ratio of endo- and exo-4c is 3:1 (Fig. 5). Experimental investigations are supported by quantum-chemical calculations (B3LYP/6-31G(d)3 method). They pointed that the reaction goes through 8/6-electrocyclization mechanism (the radical mechanism is excluded) which is kinetically controlled (Scheme 4; Fig.5). The obtained results of quantum-chemical calculations correspond with the stereochemistry of the obtained products.

R

HH H

HR

H

HH

RH

HH

cis,trans-1a-c endo-4a-c exo-4c

+

R = Ph, 2-furyl, CH3

CH3

CH3

CH3

+

+

+

+

2-TS

6-electrocyclizationdisrotatory

8-electrocyclization

conrotatory

4-intermediate 5-intermediate

3-TS

6-TS 7-TS

endo-4c exo-4c

cis,trans-1c

CH3

HH

H

HH

H

HH

HH

HH

HH

H

H

H

H

H

H

H

CH3

CH3

CH3

CH3

H

H H

H

HH

H

CH3

R R

RR2a-c

(R = Ph, 2-furyl, CH3)

h, 300 nm (R = CH3)or 350 nm (R = Ph, 2-furyl)

1a-c

R

CH32c

PPM 6.2 6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2

A ' CH

DC

EF

CH3

B

G

D'C'

CH3'

F'

G'E'

B'

A

PPM 6.0 5.6 5.2 4.8 4.4 4.0 3.6 3.2 2.8 2.4 2.0 1.6 1.2

1

.00

0

0

.34

1

0

.34

4

1

.01

1

0

.33

6

1

.02

8

1

.01

5

1

.05

4

0

.33

7

0

.69

5

1

.16

9

0

.36

9

1

.11

3

1

.30

2

3

.29

0

A

A' B'

B C

C'

D E

D' E'

F'

F G

G'

CH3'

3