Phosphonate functionalized N-heterocyclic carbene Pd(II) complexes as efficient catalysts for Suzuki-Miyaura cross coupling reaction
Graphical abstract
Introduction
Carbon-carbon bond formation reactions remains highly relevant in fine chemical as well as pharmaceutical industry and continuous efforts are being focused to address emerging challenges in this area [1]. In this regard, the advent of palladium catalyzed Suzuki-Miyaura cross coupling reaction of aryl halides with aryl boronic acid is a major breakthrough with diverse synthetic utility [2]. Comparatively mild and environmentally safer reaction conditions, easy access to diverse boronic acids, broad reaction scope and high selectivity of Suzuki-Miyaura cross coupling reaction favored wide proliferation of this reaction [3]. Initial studies established the superior catalytic efficacy of phosphine based palladium complexes in Suzuki-Miyaura cross coupling reaction [4]. Phosphine being a strong σ-donor imposes large electron density to the active metal centre and thus increases its reactivity [5]. However, in recent times N-heterocyclic carbene (NHC) based palladium catalysts have gained attention due to several advantages of these catalysts over the conventional palladium phosphine catalysts [6]. Strong σ-donating nature along with ability to easily tune the steric bulk promote both the oxidative addition as well as reductive elimination and thereby enhance catalytic efficacy. Apart from that, by modulating electronic characteristics of the N-pendant group linked to the NHC, the efficacy of Pd(II)-NHC based catalyst in cross coupling reactions can be drastically enhanced [7,8]. In particular, employing well-defined air and moisture stable Pd(II)-NHC complexes as pre-catalyst has emerged as an elegant strategy for Suzuki-Miyaura cross coupling reaction involving challenging substrates. In this context, the NHC-Pd-PEPPSI (pyridine enhanced pre-catalyst preparation stabilization and initiation) complexes developed by the Organ group [9] and the [PdCl(η3-R-allyl)(NHC)] complexes developed by the Nolan group [10,11] are notable due to their easy handling and considerably high catalytic efficacy. Systematic variation of the N-pendant group or the allyl moiety in respective complexes allowed optimization of catalytic efficacy as well as provided crucial insight into the mechanism involved [12,13].
Functionalization of NHC by incorporating side chains bearing donor groups has emerged as an efficient strategy to design hemilabile ligands [14,15]. Complexes of such donor functionalized NHC ligands are expected to show superior catalytic efficacy as the hemilabile arm is capable of reversible dissociation from the metal center. Apart from that, another advantage associated with donor functionalized NHC complexes is their improved water solubility when hydrophilic substituents are present [16,17]. In view of the above, plethora of donor functionalized NHC-Pd(II) complexes have been so far reported and many of these complexes show good catalytic efficacy [18]. In this regard it is surprising to note that phosphonate ester or phosphonic acid functionalized NHC or their complexes are yet to be reported. Due to the presence of three donor oxygen atoms in phosphonate group, it can show diverse coordination modes with a wide range of metal ions. Thus, phosphonate ester or phosphonic acid functionalized NHC complexes offer significant potential in devising efficient catalytic systems.
During this study, NHC-Pd(II)-PEPPSI and bis(NHC)-Pd(II) type complexes are prepared by employing a precursor imidazolium ligand containing N-pendant phosphonate group and their catalytic efficacies in Suzuki-Miyaura cross coupling reaction were explored. Starting from diethy-4-aminobenzylphosphonate, the precursor 4-imidazol-1-yl-benzyl-phosphonic acid diethyl ester was prepared. A bis(NHC)-Pd(II) complex and two NHC-Pd(II)-PEPPSI type complexes were prepared by the reaction of the precursor phosphonate functionalized imidazolium salt with Pd(OAc)2 and PdCl2 respectively. To the best of our knowledge, this set of complexes are the only examples of NHC-Pd(II) complexes having N-pendant phosphonate groups reported so far. The effect of the phosphonate group in the catalytic activity was also investigated during this study. The activities of the catalysts were explored in Suzuki coupling reaction of aryl chlorides and bromides with boronic acid.
Section snippets
Materials and methods
Starting materials were purchased from commercial sources and used without further purification. Ligand 4-imidazol-1-yl-benzyl-phosphonic acid diethyl ester and 3-[4-(diethoxy-phosphorylmethyl)-phenyl]-1-methyl-3H-imidazol-1-ium iodide [HL1]I, used for the synthesis of the three reported NHC-Pd(II) complexes were synthesised by following an unpublished procedure designed by our group with the help of a reported procedure [19]. Solvents were purified by conventional techniques and distilled
Result and discussion
Phosphonate ester functionalized imidazolium salt, 3-[4-(diethoxy-phosphorylmethyl)-phenyl]-1-methyl-3H-imidazol-1-ium iodide, [HL1]I was prepared by a multistep synthetic route recently developed in our laboratory [19]. Initially, a phosphonate ester functionalized imidazole was prepared by reaction of diethyl 4-aminobenzylphosphonate with glyoxal, NH4Cl and formaldehyde. Treatment of the imidazole with methyl iodide resulted the NHC precursor imidazolium salt [HL1]I in good yield. Reaction of
Conclusion
Thus, three new NHC-Pd(II) complexes, 1-3 are prepared by employing NHC ligand bearing N-pendent phosphonate ester or phosphonic acid functionality. Complexes, 1-3 are characterized using elemental analysis, FT-IR and 1H & 13C NMR studies while molecular structures of 1-2 are unambiguously established by single crystal X-ray diffraction technique. Structural analysis establish that the phosphonate ester groups in 1-2 are dangling and do not coordinate with Pd(II) centers. Complexes, 1-3 act as
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Generous financial support from SERB-DST (Grant No. EMR/2016/002178) is gratefully acknowledged. BB thanks CSIR for a research fellowship.
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Dedicated to Prof. Pradeep Mathur on the occasion of his 65th birth anniversary.