Thiosemicarbazone Salicylaldiminato‐Palladium(II)‐Catalyzed Mizoroki–Heck Reactions

Abstract

PPh 3 ] [saltsc = salicylaldehyde thiosemicarbazone; R = H (1), 3-tert-butyl (2), 3-methoxy (3), 5-chloro (4)], have been evaluated as catalyst precursors for the Mizoroki-Heck coupling reaction between a variety of electron-rich and electron-poor aryl halides and olefins. The palladium complexes (0.1-1 mol% loading) were found to effectively catalyze these reactions with high yields being obtained when aryl iodides and aryl bromides were utilized. The effects of base, catalyst loading, reaction temperature and reaction time on the catalytic activity of the most active complex were also investigated.Keywords: aryl halides; low metal loading; methyl acrylate; Mizoroki-Heck coupling; palladium(II) complexesThe synthesis and study of thiosemicarbazones as biological agents has received copious attention over the past few decades.[1-8] These Schiff base-type compounds contain several donor atoms and research into the coordination chemistry and biological applications of their metal complexes is fast becoming an area of prominent interest. [9][10][11][12][13][14] Their palladium(II) and platinum(II) complexes have been studied as anticancer agents and show great promise as chemotherapeutic agents. [12,13] While studies into the biological application of thiosemicarbazone metal complexes is prolific there are few reports on their use in catalysis. One of the first reports of their catalytic application was published in 1998.[15] Pelagatti and co-workers described chemoselective homogeneous hydrogenation reactions using tridentate [N,N,S]thiosemicarbazone palladium(II) complexes. [15] Palladium-catalyzed reactions have become essential synthetic tools; their applications including the syntheses of polymers, [16,17] natural products, [18] agrochemicals [19,20] and pharmaceuticals. [21,22] There are several well-known reactions that utilize palladiumcontaining compounds including the Mizoroki-Heck, Stille, Suzuki and Buchwald-Hartwig cross-couplings. [23,24] As with most of these coupling reactions, the Mizoroki-Heck reaction requires a palladium(0) source as catalyst and this is usually generated in situ by reduction of a palladium(II) complex.[25] Salicylaldehyde thiosemicarbazone Pd(II) complexes were recently applied as catalyst precursors for the Heck reaction [26] and the Suzuki-Miyuara [27,28] coupling reaction and were shown to effectively catalyze carboncarbon bond formation.In our group, we are currently investigating the synthesis and application of tridentate thiosemicarbazone transition metal complexes. We have prepared a series of palladium(II) complexes containing functionalized salicylaldimine thiosemicarbazones and evaluated them for activity as anticancer and antimalarial agents.[29] These complexes have shown promising biological activity and as part of our continuing study into the versatility of thiosemicarbazone metal complexes we chose to screen these complexes as catalyst precursors for the Mizoroki-Heck coupling reaction.

2‐(1‐Benzotriazolyl)pyridine: A Robust Bidentate Ligand for the Palladium‐Catalyzed CC (Suzuki, Heck, Fujiwara–Moritani, Sonogashira), CN and CS Coupling Reactions

“…( E )‐Butyl 3‐(4‐methoxyphenyl)acrylate (5a): 45 The product was obtained as a viscous colourless oil; yield: 219.9 mg (94%); 1 H NMR (300 MHz, CDCl 3 ): δ =7.64 (d, J =15.9 Hz, 1 H), 7.46 (d, J =8.7 Hz, 2 H), 6.90 (d, J =8.7 Hz, 2 H), 6.32 (d, J =15.9, Hz, 1 H), 4.19 (t, J =6.9 Hz, 2 H), 3.82 (s, 3 H), 1.73–1.64 (m, 2 H), 1.44 (q, J =7.5 Hz, 2 H), 0.96 (t, J =7.5 Hz, 3 H); 13 C NMR (75 MHz, CDCl 3 ): δ =167.4, 161.3, 144.2, 130.8, 127.2, 115.7, 113.3, 64.2, 55.3, 30.7, 19.2, 10.9; HR‐MS (ESI): m/z =234.1256, calcd. for C 14 H 18 O 3 (M+H + ): 234.1256.…”

Indole‐Based Aza[n]helicenes (n=5, 6) with Violet‐Blue Fluorescence and Two‐Photon Absorption (TPA)

“…[21] Then, HVI reacted with 2-Bromoanthracene, 2-Bromodibenzothiophene or 3-Bromoquinoline through Mizoroki-Heck reaction to afford AVHI, DTVHI and HIVQ, respectively. [22] At last, HTPI, HBTNI, HIPD and HPI were harvested through photocyclization reaction using the Hanovia high-pressure mercury lamp in 30 or 90 minutes (Scheme 1). 1 H NMR, 13 C NMR and HRMS of synthetic compounds, and single crystal structure data for HIVQ (CCDC 1961630) are given in Figures S1-S24, Table S7 and Figure S29 (see the Supporting Information).…”

Water‐Soluble Palladium(II) Sulfonated Thiosemicarbazone Complexes: Facile Synthesis and Preliminary Catalytic Studies in the Suzuki–Miyaura Cross‐Coupling Reaction in Water

“…[17] However, to the best of our knowledge, there are no known examples of water-soluble thiosemicarbazones li-gands shown to stabilise palladium as versatile catalysts for the Suzuki-Miyaura cross-coupling reaction in water. This motivated us to develop new water-soluble Pd II sulfonated thiosemicarbazone complexes, as an extension of our previous work, [18,19] and investigate their potential as SuzukiMiyaura cross-coupling catalysts in water.…”