Efficient access to β-vinylporphyrin derivatives via palladium cross coupling of β-bromoporphyrins with N-tosylhydrazones

Efficient access to β-vinylporphyrin derivatives via palladium cross coupling of β-bromoporphyrins with N-tosylhydrazones Vinicius R. Campos1,2, Ana T. P. C. Gomes*2, Anna C. Cunha*1, Maria da Graça P. M. S. Neves2, Vitor F. Ferreira1 and José A. S. Cavaleiro*2 Full Research Paper Address: 1Departamento de Química Orgânica, Instituto de Química, Universidade Federal Fluminense, 24020-150 Niterói, RJ, Brazil and 2QOPNA and Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal Open Access Beilstein J. Org. Chem. 2017, 13, 195–202. doi:10.3762/bjoc.13.22 Received: 20 October 2016 Accepted: 11 January 2017 Published: 30 January 2017 Email: Ana T. P. C. Gomes* - ; Anna C. Cunha* ; José A. S. Cavaleiro* - Dedicated to the memory of Professor José Barluenga, OviedoUniversity, Oviedo, Spain. * Corresponding author Associate Editor: B. Stoltz Keywords: N-tosylhydrazones; Pd-catalyzed cross coupling; porphyrins © 2017 Campos et al.; licensee Beilstein-Institut. License and terms: see end of document. Abstract This work describes a new approach to obtain new β-vinylporphyrin derivatives through palladium-catalyzed cross-coupling reaction of 2-bromo-5,10,15,20-tetraphenylporphyrinatozinc(II) with N-tosylhydrazones. This is the first report of the use of such synthetic methodology in porphyrin chemistry allowing the synthesis of new derivatives, containing β-arylvinyl substituents. Introduction Porphyrin-type heterocycles are well known for their special role in several scientific fields, including medicine [1-3] , supramolecular chemistry [4-6] and catalysis [7-10]; their functionalization became a target for several research groups [11-13]. In particular, the insertion of functional groups into the macrocyclic core merits considerable attention due to the diversity of structural features that can be induced in the porphyrin structure [14-16]. Alkenyl-type moieties are examples of versatile functional groups which are present in several biologically active natural porphyrin derivatives or are substituents that can be introduced into the macrocycles, affording in such way important intermediates for different synthetic strategies [4,17,18]. There are methods available allowing the insertion of alkenyl groups into porphyrin macrocycles (e.g., Heck reaction [4], metathesis [7,19-21], Wittig reaction [22,23]). However, palladium-catalyzed cross-coupling reaction between N-tosylhydrazones and aryl halides seems to be a powerful method to synthesize new alkene-type derivatives [24,25]. This methodology allowed the development of novel protocols enabling transformations that are difficult to achieve with other reactions and include other important features – stoichiometric organometal- 195 Beilstein J. Org. Chem. 2017, 13, 195–202. lic reagents are not required for the coupling reaction and diand trisubstituted olefins can be prepared with high stereoselectivity. In fact, N-tosylhydrazones used in this methodology can be easily prepared from carbonyl compounds (Scheme 1), as it has been well demonstrated by Barluenga and co-workers [11,26,27]. Scheme 1: Schematic representation of palladium-catalyzed crosscoupling reaction between aryl halides and N-tosylhydrazones. This methodology allowed the development of novel protocols enabling transformations that are difficult to achieve with other reactions and include other important features as: stoichiometric organometallic reagents are not required for the coupling reaction. Interestingly, this methodology has never been applied to the synthesis of porphyrin derivatives containing alkenyl groups at β-pyrrolic positions. Having this in mind and following our interest in the development of synthetic approaches to prepare novel porphyrin derivatives [28-30], we decided to explore the palladium-catalyzed cross-coupling reaction of mesotetraphenyl-β-bromoporphyrin using N-tosylhydrazone derivatives with different electronic features for the preparation of new β-alkenyl-type porphyrin derivatives. This approach can provide interesting compounds with potential biological properties or might become important templates for further functionalization procedures (Scheme 2). Results and Discussion For this work 2-bromo-5,10,15,20-tetraphenylporphyrinatozinc(II) (1) was used as the aryl halide and also the easily acces- sible N-tosylhydrazones 2a–c (Scheme 3). Both substrates were prepared by following literature procedures; the halide partner by controlled bromination of 5,10,15,20-tetraphenylporphyrin (TPP) with N-bromosuccinimide followed by metallation with Zn(AcO)2 [31,32] and the tosylhydrazones 2a–c by reaction of the adequate ketones with tosylhydrazines [33]. The use of the porphyrin zinc complex avoids the complexation of the tetrapyrrolic macrocycle with Pd(II) formed during the catalytic cycle, and in this way prevents the loss of the catalyst. In the first experiments, the reactions between 2-bromo5,10,15,20-tetraphenylporphyrinatozinc(II) (1) and the N-tosylhydrazone derivatives 2a–c were performed in the presence of Pd(PPh3)4 as catalyst, KOt-Bu as base and 2-dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl (Xphos) as ligand. All reactions were carried out in a sealed Schlenk tube in toluene and dioxane at 120 °C under conventional heating and anhydrous air-free conditions (Scheme 3) and stopped when the TLC controls showed no more evolution of the reaction progress. Having in mind the possible self-reaction of tosylhydrazone 2b, a process competing with the required reaction of 2b with bromoporphyrin, it was decided to use an excess of the tosylhydrazone. The results obtained after the work up and chromatographic purification are summarized in Scheme 3 and Table 1 (entries 1, 2 and 3). Under these conditions, the expected β-substituted vinylporphyrin derivatives 3a–c were obtained in 22–25% yields. No significant differences between the reactivity of the used N-tosylhydrazones 2a–c were observed. The major compounds isolated in these conditions were the starting porphyrin 1 and the zinc(II) complex of TPP (ZnTPP) resulting from the debromination of porphyrin 1. The reductive removal of the halo moiety has been already described in the literature for other palladium-catalyzed cross-coupling reactions [34-36], namely in some reactions involving Scheme 2: A retrosynthetic scheme for the synthesis of β-alkenyl-type porphyrin derivatives from the Zn(II) complex of β-bromo-meso-tetraphenylporphyrin and N-tosylhydrazones. 196 Beilstein J. Org. Chem. 2017, 13, 195–202. Scheme 3: Palladium catalysed cross-coupling reactions between β-brominated porphyrin 1 and N-tosylhydrazones 2a–c. halogenated porphyrin derivatives [37]. Presumably in this particular case the lack of efficiency of the catalytic system is probably responsible for this secondary reaction. Attempts to improve the efficiency of the process just by changing the cata- lyst to tris(dibenzylideneacetone)dipalladium(0), [Pd2(dba)3], and th (...truncated)