Atomically precise palladium nanocluster catalyzed tandem oxidation processes of alcohols and phosphorous ylides: Facile access to α,β-unsaturated esters
Graphical abstract
The first atomically precise palladium nanocluster catalyzed one-pot tandem oxidation-wittig reactions was reported.
Introduction
Atomically precise nanoclusters (APNCs) [1], as a new family of nanomaterials, have emerged as one of the most important branches of nanoscience and nanotechnology during the past decade and attracted significant attentions due to its applications in catalysis [2], biology [3] and sensing [4]. Owing to its high surface-to-volume ratio and the unique geometric structure and electronic structure, nanocluster show distinct catalytic activity in selective oxidation [5], reduction [6] and carbon–carbon coupling [7] reactions. Nanocluster with determined crystal structure and compositions make it possible to reveal the relationship between catalytic performance and their atomic level structure, which enable it have huge advantages over the uncertain surface structure of traditional metal nanoparticles [8]. Therefore, the development of new chemical processes promoted by atomically precise nanoclusters is a high desirable, yet challenging task.
One-pot tandem reactions [9], in which two or more transformation processes are combined into one synthetic operation, are great importance for efficient and practical organic synthesis and for green and sustainable chemistry, which avoid the isolation and purification of the intermediate. Thus allowing a reduction in time, labor and yield losses associated with the isolation and purification of intermediates. The tandem oxidation processes initiated by alcohol oxidation to the corresponding more reactive aldehydes, which can be easily trapped in situ by many nucleophilic agent leading to a variety of synthetically useful compounds such as esters, imines, amides and heterocycles [10].
The tandem oxidation-Wittig reaction, when the nucleophilie is stable phosphorous ylide, is an important reaction in organic synthesis because it provides a facile method to α,β-unsaturated ester, which is a very important 1,4-addition synthon in many organic reactions. Conventionally, various oxidants such as MnO2 [11], Dess-Martin agent [12], IBX [13], PCC [14] were studied for the one-pot tandem oxidation-Wittig reaction. However, these protocols require the use of stoichiometric or even large excess amounts of oxidant, which generated many unpredictable by-products and much effort is needed to remove the oxidant (Scheme 1(a)). Moreover, some of this oxidant themselves are highly toxic and hard to handle. To avoid the use of hazardous oxidants, a great deal of effort has been devoted to design of new catalysts with green oxidants (Scheme 1(b)). For example, Williams and co-works [15] reported the tetra-n-propylammonium perruthenate (TPAP) catalyzed tandem oxidation-process of alcohols with stable ylides to give the α,β-unsaturated esters using O2 as oxidant and N,N,N′,N′-Tetramethylethylenediamine dioxide (TMEDAO2) as co-oxidant. Park and co-works [16] reported porous aluminum oxyhydroxide embedded ruthenium nanoparticles catalyzed alcohol oxidation-Wittig reaction in toluene with O2 as oxygen source. However, this system suffers from high temperature and the poor recyclability of the catalysts. The Au/Pd bimetallic nanoparticles were also explored by Kobayashi and co-works to promote the tandem process of alcohol oxidation and Horner–Wadsworth–Emmons reaction [17]. Recently, using CuI/TEMPO as co-catalyst and air as the oxidant, Yu and co-works reported an efficient method to synthesis of α,β-unsaturated esters [18]. Despite this advances, the novel catalyst with excellent catalytic activity, easy recyclability and green oxidant are still highly demanding. So we developed and herein report the first mesoporous silica nanoparticles (MSNs) supported atomically precise palladium nanocluster [Pd3Cl(PPh3)3(PPh2)2]+Cl- (abbreviated Pd3Cl) catalyzed tandem oxidation-Wittig reaction of alcohol and phosphorous ylide with 1 atmosphere O2 (O2 balloon) as oxidant (Scheme 1(c)).
Section snippets
Results and discussions
The Pd3Cl nanocluster was synthesized following the previously reported method [19]. The as-prepared Pd3Cl nanocluster shows distinct peak at 418 nm and a weak absorption band at 485 nm in UV–vis absorption spectra (Fig. 1(A)), which is consistent with the literature. MALDI-MS analysis of the as-prepared Pd3Cl cluster with positive model in Fig. 1(B) shows mass peak at m/z = 1511.9 (assigned to molecular ion peak of [Pd3Cl(PPh3)3(PPh2)2]+, theoretical molecular weight: 1511.9). There are two
Conclusions
In summary, we have disclosed a novel, efficient and recyclable catalyst for the one-pot tandem oxidation-Wittig reaction of alcohol and phosphorous ylide. With this protocol, a wide range of α,β-unsaturated esters were produced in good to excellent yields. This also represents the first tandem reaction catalyzed by atomically precise nanocluster. This work is expected to open up a new frontier for atomically precise nanocluster in tandem reactions.
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
We are grateful to the Shandong Province Natural Science Foundation (ZR2018PB003).
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