Microwave-assisted synthesis of new fluorescent indoline-based building blocks by ligand free Suzuki-Miyaura cross-coupling reaction in aqueous media  Zivil _ e  Zukauskait _ e a, b , Vida Buinauskait _ e a, b , Joana Solovjova a, b , Lina Malinauskait _ e a , Aist _ e Kveselyt _ e a , Aurimas Bieliauskas b , Greta Ragait _ e b , Algirdas  Sa  ckus a, b, * a Department of Organic Chemistry, Kaunas University of Technology, Radvil _ enu˛ pl. 19, LT-50254 Kaunas, Lithuania b Institute of Synthetic Chemistry, Kaunas University of Technology, K. Barsausko g. 59, LT-51423 Kaunas, Lithuania article info Article history: Received 21 February 2016 Received in revised form 21 March 2016 Accepted 5 April 2016 Available online 6 April 2016 Keywords: Indoline Suzuki-Miyaura cross-coupling Ligand-free Aqueous media Microwave-assisted synthesis Fluorescence abstract To obtain novel fluorescent building blocks, an efficient method for the preparation of an arylindoline unit containing heterocyclic derivatives was developed. The method is based on palladium-catalyzed, ligand- free, cross-coupling of 7-bromo- and 5,7-dibromoimidazo[1,2-a]indolone and 8-bromopyrimido[1,2-a] indolone derivatives with (hetero)aryl boronic acids. In the presence of the Pd(OAc) 2 catalyst and the Cs 2 CO 3 base, the microwave-assisted cross-coupling reactions of brominated indoline substrates with (hetero)aryl boronic acids proceeded smoothly in aqueous media. The required brominated substrates can be conveniently obtained with excellent yields by treatment of the corresponding fused heterocycles with NBS in acetonitrile. The optical properties of the novel prepared heterocyclic derivatives were studied by UVevis and fluorescence spectroscopy. The obtained functionalized arylindoline derivatives, exhibit in- tense fluorescence with significant Stokes shifts and are characterized by high quantum yields. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction N-alkylated derivatives of 2,3,3-trimethyl-3H-indole are im- portant intermediates for the preparation of organic materials with broad technical and biomedical applications. Quaternization of 2,3,3-trimethyl-3H-indole with haloalkanes and other similar alkylating agents affords N-substituted 2,3,3-trimethyl-3H-indo- lium salts, and condensation of these salts with electrophilic sub- strates produces organic dyes. 1 The reaction of 2,3,3-trimethyl-3H- indole with bifunctional alkylating agents, which possess an addi- tional reactive site with nucleophilic oxygen or nitrogen atoms, results into the formation of ring fused heterocyclic systems, which possess an annelated heterocycle at the N (1) eC (2) bond of the indoline nucleus, due to intramolecular nucleophilic addition of the nucleophilic heteroatom to the indole 2-C atom. For example, the alkylation of the aforementioned substrate with 2-haloethanol afforded 2,3,9,9a-tetrahydro[1,3]oxazolo[3,2-a]indoles, 2 with 2- chloromethyl-5-nitrophenold5a,6,6-trimethyl-5a,6-dihydro-12H- indolo[2,1-b][1,3]benzoxazines, 3 and the reaction with 2- haloacetamides led to 9,9a-dihydro-1H-imidazo[1,2-a]indol- 2(3H)-ones. 4 These adducts have been successfully applied for the preparation of various important materials including cyanine dyes, 5 water-, pressure- and heat-sensitive colour formers, 6 light- driven molecular switches 7 and chemosensors. 8 The target properties of these functional materials depend to a large extent on the electronic nature of the substituents on the aromatic portion of the indoline structural unit. 9 Moreover, the building blocks that possess a benzo[e]-annelated indoline moiety exhibited intense fluorescence and have been used in the de- velopment of fluorescence emitting dyes with broad biomedical and technical application. 10 The aim of the current study was to develop convenient methods for the preparation of arylated imidazo- and pyrimido [1,2-a]indolone derivatives as new fluorescent building blocks for the synthesis of functional materials, and investigate their optical properties. 2. Results and discussion 2.1. Synthesis Starting compound 9,9a-dihydro-1H-imidazo[1,2-a]indol- 2(3H)-one (3) was prepared by reacting 2,3,3-trimethyl-3H-indole * Corresponding author. Tel.: þ370 37451401; e-mail address: algirdas.sackus@ ktu.lt (A.  Sackus). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet Tetrahedron 72 (2016) 2955e2963 http://dx.doi.org/10.1016/j.tet.2016.04.010 0040-4020/Ó 2016 Elsevier Ltd. All rights reserved.