Flow-Vacuum Pyrolysis of Polycyclic Compounds. 26 [1] Pyrolysis of Some Acetates with Dibenzocycloalkane Skeletons

The syntheses and flow-vacuum pyrolyses of 10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-acetate ( 7 ), 10,11-dihydro-12H-dibenzo[a,f]cycloocten-5-acetate ( 8 ) and 5H-6,7-dihydrodibenzo[a,c]-cyclohepten-6-acetate ( 9 ) were presented. The products’ distributions were determined by GC/MS and a reaction mechanism involving radical species was suggested, in order to explain the formation of the main reaction products

The scarcity of literature data about thermal behaviour of the acetates with dibenzocycloalcane skeleton [7] was another reason for starting our study.

Experimental part
Melting points are uncorrected.IR spectra were registered on a Bruker Equinox 55 spectrometer.The NMR spectra were registered at 300 MHz ( 1 H-) and 75 MHz ( 13 C) on a Varian Gemini 300 apparatus using TMS as internal standard.The GC/MS analyses for pyrolyses of compounds 7 -9 were performed on a Varian 3400 gas-chromatograph with split/splitless injector, coupled with a Varian Saturn II mass-spectrometer provided with ion trap; a capillary DB-5 column (30 m length, 0.25 mm internal diameter) was used.The analysis conditions were: injector temperature

Synthesis of acetate 8
The synthesis of acetate 8 was performed in the same conditions as previous acetate, from corresponding alcohol 5.It was obtained a dark brown solid; after purification by column chromatography (silica, benzene as eluent) light brown crystals with m.p. 106-107 o C (η = 65%) were obtained.

Synthesis of acetate 9
1.5g (7.2 mmol) of alcohol 4 in 15 mL pyridine, at 0 o C, was treated with 5 mL of acetic anhydride, under magnetic stirring.The reaction mixture was maintained 24 hours at 50 o C.After vacuum evaporation of pyridine, 20 mL 5% hydrochloric acid solution were added.The mixture was extracted with chloroform.The organic layers were washed with 5% hydrochloric acid solution and then with water until neutral.The acetate 9 was obtained after solvent evaporation as white-yellow crystals (η = 65%; m.p. 96 o C).

General procedure of pyrolyses
For a good correlation of experimental data between the previous and present results we performed the flowvacuum pyrolyses of dibenzocycloalkane derivatives in the same described conditions [6]: the pyrolysis quartz tube (60 cm length, 10 mm internal diameter) was filled with quartz chips on 30 cm length; this zone was heated with a cylindrical electric oven.The temperature was continuously measured by a thermocouple and the pressure (1-2 mmHg) with a McLeod manometer.The sample (usually ~ 30 mg) was sublimed under argon flow (4 ml/min) in the pyrolysis tube.The reaction products were dissolved in dichloromethane, the solvent was evaporated in vacuum and the residue was GC/MS analysed (See Experimental).Analytical pyrolyses at optimal temperature were followed by preparative runs either in order to isolate the main products or for spectra registration.

Results and disscutions
The acetates 7 and 8 were obtained in a special way from the dibenzocycloalkanols 3 and 5 with acetic acid in the presence of malonic acid [8] (scheme 1).The usual conditions (acetic anhydride in pyridine) were unable to transform these alcohols into their corresponding acetates.
The acetate 9 was prepared from the corresponding alcohol 4 with acetic anhydride in pyridine, at room temperature (scheme 2) (part experimental): The syntheses of ketones 11 and 12 were performed using literature data [9] and [10] respectively.

Pyrolysis of 10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-acetate (7)
The compound 7 is very stable: at 600 o C it was scarcely converted (~ 1%) and only at 900 o C it was completely transformed.The main products (of the very complex mixture) resulted in these pyrolyses are presented in Scheme 3.
A rationalization of the experimental data is proposed in the mechanistic Schemes 6-8.
In the route a) a radical break of CH 2 -CH 2 bond in 7 affords the benzyl diradical 7A.By elimination of acetate radical the benzyl radical 7B is formed.It can be converted to spirane 24.A thermally allowed intramolecular [π 2s +σ 2s +σ 2s ] process in spirane compound 24 leads to dihydroanthracene (25) and anthracene (15).In the route b) by acetate radical elimination the benzyl radical 7D can be formed.It may be stabilised to alkene 13, which at high temperature decomposes and affords 9-methylanthracene (14) and anthracene (15) [11].A radical break (route c) of the O -COCH 3 bond followed by hydrogen atom elimination affords the ketone 10, which is decarbonilated In scheme 8 it is proposed the radical mechanism of the thermal decomposition in FVP conditions for 5H-6,7dihydrodibenzo[a,c]cycloheptene-6-acetate (9).
The break of the .acetate (OCOCH 3 ) radical generates the secondary radical 9A which is converted into alkene 21 by hydrogen atom elimination.At higher temperature, this alkene is transformed in stable aromatic hydrocarbons [12] the radical 9B, which may be stabilized with formation of the ketone 13; at high temperature, by decarbonilation of ketone 13 and dehydrogenation of dihydrophenanthrene (26), phenanthrene (23) is formed, as stable aromatic product.

Conclusions
In conclusion, in this work we have presented: -the synthesis and spectral characterization of three acetates (7-9) with annelated dibenzocycloalkane skeleton; -the thermal behaviour in flow-vacuum pyrolysis of these acetates; -the main reaction products and radical mechanisms of conversion were proposed; -the pyrolysis of the new acetates (7-9) with dibenzocycloalkane skeleton confirmed the previous results [7] involving acetic acid elimination.
-the mechanisms previously suggested involved the radicals' generation, which affords as final stable compounds, aromatic hydrocarbons, such as 9methylanthracene, anthracene and phenanthrene.
250 o C; split rate 1:50; temperature program 50 -250 o C at 5 o C/min and then 20 min at 250 o C; carrier gas helium (flowrate of 1 mL/min); temperature of transfer line 275 o C; trap temperature 170 o C; electron ionisation 70 eV.