Catalytic cycloaddition of diazoalkanes with heterocyclic substituents to fullerene C60

Abstract

Methanofullerenes were directly synthesized under mild conditions by cycloaddition to fullerene C 60 of diazoalkanes generated in situ by oxidation of ketone hydrazones containing a heterocyclic fragment with manganese(IV) oxide in the presence of the catalytic system Pd(acac) 2 -2 PPh 3 -4 Et 3 Al.Methanofullerenes available via Bingel-Hirsch reaction constitute now one of the most promising classes of fullerenes from the view point of practical applications. Methanofullerenes can be used in the manufacture of highly efficient solar batteries and in the preparation of drugs for the treatment of most dangerous human diseases [1,2]. Apart from the BingelHirsch reaction, methods based on cycloaddition of diazo compounds to carbon clusters have found wide application in synthetic practice; however, these procedures generally lead to the formation of mixtures of stereoisomeric homo-and methanofullerenes. The use of metal complex catalysts makes it possible to direct the cycloaddition of diazo compounds to fullerenes toward formation of individual methanofullerenes. Complexes based on palladium and rhodium were reported to successfully catalyze reactions of fullerene C 60 with diazomethanes [3], diazoacetates [4][5][6][7], and diazo ketones [8-10], and the corresponding cyclopropafullerenes were obtained with high selectivity.We recently showed that homofullerenes and spiro homofullerenes can be synthesized with high selectivity by reaction of C 60 with monosubstituted diazomethanes and cyclic diazoalkanes generated in situ by oxidation of the corresponding hydrazones in the presence of Pd(acac) 2 -PPh 3 -Et 3 Al as catalytic system [11][12][13][14][15][16]. We thus obtained pure [5,6]-open cycloadducts in which bulkier substituent was located above the five-membered fragment of C 60 . Analogous reactions with disubstituted diazomethanes generated in situ from unsymmetrical ketone hydrazones were characterized by lower selectivity, and mixtures of stereoisomeric homofullerenes were obtained [12, 13,15]. The proposed catalytic procedure is considerably more advantageous than known thermal cycloadditions of diazoalkanes to fullerene C 60 [17][18][19][20][21]. Reactions of carbon clusters with diazo compounds were reviewed in detail in [2].In continuation of our studies on cycloaddition of diazo compounds to fullerenes, in the present work we Me O I Scheme 1.

Synthesis and structure of new imidazo- and pyrazolo[5,1-d][1,2,3,5]thiatriazines based on the reaction of diazoazoles with acyl isothiocyanates controlled by S⋯O interaction

“…Now we have found that they also did not react with phenylsulfonyl isothiocyanate bearing a strong electron acceptor substituent. Indeed, 3-methyl-, 3-phenylsulfonyl-, and 3-phenylazolo [5,1-d] [1,2,3,5]tetrazine-4(3H)-thiones of type 6 could not be prepared in the same way as their oxygen analogs. In contrast to the reactions with methyl-and phenyl isothiocyanate the analogous reactions of diazoazoles 2bej with isothiocyanates containing carbonyl groups, such as ethoxycarbonyl isothiocyanate 4a, benzoyl isothiocyanate 4b and tetrafluorobenzoyl isothiocyanate 4c occur readily in solution of anhydrous ethyl acetate to give azolo [5,1-d] [1,2,3,5]thiatriazines 5 as the only products (Scheme 3, Table 1).…”

“…In summary, we have developed an efficient method for the synthesis of imidazo-and pyrazolo [5,4-b] [1,2,3,5]thiatriazines based on cycloaddition reaction of diazoazoles with acyl isothiocyanates. The prepared compounds are shown by both X-ray data for thiatriazines 5k,l and by calculations for the structures 5k,l,o to have relatively strong SeO nonbonded interactions.…”

“…All measurements were made on a Bruker SMART 6000 diffractometer equipped with a CCD detector using CuKa radiation (l¼1.54178 A). The crystal system of compound 5k (C 10 H 11 N 5 O 4 S, M¼297.30) is orthorhombic, space group Pbca, a¼8.6546(2), b¼7.8213(2), c¼37.2599(8) A, Z¼8, V¼2522.13(10) A3, D c ¼1.566 g/cm 3 , F(000)¼1232, crystal size: 0.10Â0.60Â0.60 mm, m(CuKa)¼2.524 mm À1 , T¼373 K, 11,654 reflections collected, 2356 unique reflections, R int ¼0.135. Final R indices: R 1 ¼0.0566 for 2123 reflections with I>2s(I) and R 1 ¼0.0500, wR 2 ¼0.1522 for all data.…”

“…At the same time, expanding the scope of the cycloaddition reaction of heterocyclic diazo compounds as an efficient approach to fused azoles is very important for medicinal chemistry. Reactions of 5diazoazoles with isocyanates are known to take place in very mild conditions to afford azolo [5,1-d] [1,2,3,5]tetrazines. Two compounds of the imidazole series 1a,b exhibited a high level of antitumor activity and have been used in both medicinal sciences and practice (Scheme 1).…”

“…Furthermore the structures of the products prepared by these authors 18 were confirmed by 1 H NMR spectra only, which may not be conclusive proof for such heteroatom-rich compounds. Therefore, for the better understanding of the direction of cyclization and with the aim of developing a general approach to azolo [1,2,3,5]thiatriazines we have further studied the reaction of a series of 5-diazoimidazoles and 5-diazopyrazoles, and 2-diazoimidazole-4,5-dicarbonitrile involving apart of benzoyl isothiocyanate, tetrafluorobenzoyl, phenylsulfonyl,-and ethoxycarbonyl isothiocyanates and have made a careful study of the structures of the reaction products by X-ray diffraction analysis, IR-, and NMR spectroscopy and theoretical calculations. Special attention has been paid to the factors that stabilize the reaction products because we have recently shown by calculations with HartreeeFock and DFT methods 19 that the reaction of diazo compounds 5 with acyl isothiocyanates is thermodynamically controlled.…”