1,1 ′ -Biazulene derivatives. Syntheses and reactions

3,3 ′ -Disubstituted 1,1 ′ -biazulene derivatives were obtained in good yields starting from thedimers* of azulene-1-azo aromatic compounds and from the corresponding Schiff bases coupled at 3 positions


Introduction
Several different synthetic approaches to the compounds with the general formulae 1a-c have been developed over the last fifty years. 1 However, the compounds attracted interest when it was found that some of them possess valuable non-linear optical properties. 2 Therefore, we have decided to develop more effective methods for the synthesis of these compounds and to study their structure and chemical properties.We have already reported the results for the compounds 1a and 1b. 3,4-A=B-Ar Az = substituted or unsubstituted 1-azulenyl * The terms dimerization and dimer are used for coupling with the elimination of two hydrogen atoms and for the products resulted in these reactions, respectively.
One of the most interesting reactions of 1 consists in the single electron oxidation.It is well known that the low oxidation potential of the azulenes 5 make them highly susceptible to oxidation which proceeds unselecively and in low yields. 6,7In contrast, Azulenes substituted with a double bond at C1 as in compound 1 display increased oxidation potential which allows for chemical 3b,c,e and electrochemical oxidation 3d without damage of the azulene skeleton.The dimers coupled at C1 were obtained as the main oxidation products.The almost complete conversion of the starting materials and the good yield of the dimers obtained inspired us to use this reaction for the syntheses of the interesting 1,1′-biazulenes which until now have been obtained by Ullmann type coupling.As the starting coupling reagent, Morita and Takase 8 have used ethyl 1-iodo-3-azulene carboxylate.In spite of the high coupling yield, the utility of the procedure for the syntheses of 1,1′-biazulene and their derivatives seems to be limited because of the difficult accessibile starting materials and by the fact that the carboxylate group offers rather few possibilities for subsequent transformations.
In the present paper, two types of starting materials were considered for 1,1′-biazulene synthesis, namely bis azo derivatives 3 and bis Schiff bases, 5 (Scheme 1 and, Scheme 2, respectively).The compounds 3 and 5 were obtained by oxidation of the corresponding monomers 2 3b and 4 3e with anhydrous FeCl 3 in benzene.The influence of the substituent R on the oxidation in both cases was discussed 3b,3e and OCH 3 was found to be most favorable for dimer generation.
However, the compound 6 could be quenched with methyl chloroformate to give the corresponding bis-methoxycarbamide 8 which was separated from the methoxycarbamide 7 (X = CO 2 CH 3 ) and completely characterized.
When the reduction medium (Zn and acetic acid) acetic anhydride or benzoyl chloride was added, acylated bis-amines 9a and 9b were obtained together with acylated para-anisidine, 7 (X = CH 3 CO or C 6 H 5 CO).The products were separated by column chromatography adding triethylamine to the eluent in order to avoid the destruction of the amides.
Our attempts to obtain bis-hydrazo compounds by reduction with hydrazine hydrate/Ni Raney failed and after treatment with methyl chloroformate only bis-methoxycarbamide 8 and 7 (X = CO 2 CH 3 ) could be detected in low yields in the reaction mixture.

Reactions of Schiff base dimer
By the chemical oxidation (FeCl 3 ) of 1-azulene carbaldehyde only 3-chlorinated product was obtained (33 % yield) instead of the dialdehyde 11.However, the generation of 11, in 100 % yield, was realized by the hydrolysis of Schiff base dimers, 5 (Scheme 2).It is interesting that although the hydrolysis of Schiff base monomers, 4, occurs fast, the corresponding dimers 5 react only slowly and in the presence of copper acetate as catalyst.A possible explanation for the observed difference is the low solubility of these dimers in the reaction medium.This convenient way One of the useful reactions of the CHO group is its replacement with hydrogen.The reported yields 9 for the decarbonylation of azulenic aldehydes in the presence of pyrrole under mild conditions were between 36 and 60%.By decarbonylation of dialdehyde 11 we obtained 1,1′-Biazulene 10 in only 10-20 % yields.However starting from the dimer Schiff base 5a the yield was above 40 %.The reaction sequence: azulene → 1-azulenecarbaldehyde → corresponding Schiff base → dimerization → reaction with pyrrole that occurs in a very good over-all yield, represents an excellent way for the generation of hydrocarbon 10 starting from azulene.The convenient access to 10 triggered a study of some reactions of this interesting hydrocarbon (Scheme 3).We have only studied reactions with electrophiles, which occur in positions 3 and 3′, some of which are reported below.Further investigations of reactions with nucleophiles or of redox and radical reactions are in progress.
Friedel Crafts acylation occurred in good yield in the presence of SnCl 4 as for the unsubstituted azulene. 10Reaction with acetyl chloride produced the diacetylated product 15 R = CH 3 in 58 % yield.
Vilsmeier acylation of 10 using the same protocol as for azulene 11 with equimolar amounts of dimethylformamide or dimethylacetamide gave the 3-substituted compound 17.With excess of formylating agent the 3,3′-disubstituted compound 15 was formed, The coupling reaction with diazonium salts reported in a previous paper deals with the oxidation of azulene-1azoaromatics.3b Also in this reaction for R = OCH 3 16a and 3a were generated, the major productbeing 16a.It is noteworthy that the same reaction with the diazonium salt derived from 4nitroaniline produced only monosubstituted product 16b was formed.
In conclusion, we have showed that the relatively easy procedure for the dimerization of azocompounds 1a 3a,b and their corresponding Schiff bases 1b 2d represents a very good starting point for the synthesis of many 1,1′-biazulene derivatives compounds otherwise difficult accessible.

3,3′-Bis(methoxycarbamide)-1,1′-biazulene (8).
To the solution of the bis-amine 6, and anisidine, 7 = H, cooled at 0 o C, 20 % aqueous sodium carbonate (1 mL) was added followed by an excess of methyl chloroformate (0.5 mL).The reaction mixture was stirred for 90 min at 0 o C and the work-up was the same as for compound 9b.The product collected as the first fraction represents probably a ureide because in the NMR spectrum methoxy group failed.The second green fraction represents the product 8 (

1 H-and 13 C
-NMR spectra: Bruker Avance DRX4 ( 1 H: 400 MHz,13 C: 100 MHz) and Gemini 300 ( 1 H: 300 MHz, 13 C: 75 MHz), TMS was used as internal standard; when necessary, unequivocal signal assignment was confirmed by the analysis of the corresponding COSY and HETCOR spectra (the numbering for the exemplified compounds was indicated in Schemes and is not always correlated with the IUPAC nomenclature; when symmetrical products are described only one number in molecule is specified for both identical positions).Mass spectra: Finnigan MAT 311-A/100 MS and Carlo Erba QMD 1000.Column chromatography: basic alumina (activity BII-III (Brockmann)) or silica [70-230 mesh (ASTM)].The dichloromethane (DCM) was distilled over calcium hydride.