Cobalt catalyzed synthesis of α, β-unsaturated esters from esters and alcohols via mild O2-interrupted hydrogen borrowing
Graphical abstract
Direct synthesis of α, β-unsaturated ester from unactivated ester and alcohol through cobalt catalyzed mild oxygen interrupted hydrogen borrowing.
Introduction
α, β-Unsaturated esters as important intermediates and products are widely used in the field of polymer materials production, adhesives and textiles synthesis, paints and coatings preparation, pharmaceutical and essences fabrication [1]. Traditional one-step approaches to such compounds include direct esterification [2], dehydrogenation [3] and aldol reaction [4], which are described in Scheme 1. The direct esterification method needs to prepare α, β-unsaturated acid firstly, but it is not feasible for the sterically-hindered t-BuOH. As for dehydrogenation, although it seems environmentally friendly and green sustainable, the non-recyclable and noble Pd catalysts are often required. Aldol condensation followed by dehydration is considered to be efficient for α, β-unsaturated esters fabrication from ester and aldehyde with the catalysis of solid oxide composites or ionic liquids. In addition, the Pd-catalyzed selective alkoxycarbonylation of terminal alkynes for synthesis of α, β-unsaturated esters was also developed in recent days [5].
The concept of hydrogen borrowing (HB) is an elegant strategy that used for direct alkylation of alcohol, ketone, ester, amide and amine [6], [7]. Typically, noble transition-metal catalysts, such as rhodium [8], ruthenium [9], [10] and iridium [11], [12] are used for such type reaction. Gradually the inexpensive and high efficient earth-abundant metal catalysts of manganese [13] and cobalt [14] were designed to replace these noble metals. The cobalt complexes stabilized by PN5P ligands (triazine backbone) are highly active to catalyze the dehydrogenation of alcohol into aldehyde [14]. Besides, such PN5P ligands and cobalt complexes can be easily prepared on milligram scale and these complexes are also stable to air and moisture.
Recently, the interrupted hydrogen borrowing (IHB) pathway was reported for synthesis of α-branched carbonyl compounds from corresponding ketone, methanol and nucleophile [15]. This ideal is aim to prevent hydrogen returning step of enone by subsequent conjugate addition of nucleophile onto CC bonds. Inspired by these above discoveries, the cobalt complex catalyzed O2-interrupted hydrogen borrowing (O2-IHB) system was designed to inhibit CC bonds further reduction by produced hydrogen for preparation of α, β-unsaturated ester from ester and alcohol (Scheme 1), which seems to be feasible. Herein, four types of PN5P ligands and corresponding cobalt complex catalysts were prepared, characterized and applied for our O2-IHB pathway. With the obtained optimal conditions in hand, the tolerance of such synthetic method on substrate scope and functional group was tested. Also, the catalytic dehydrogenation mechanism of alcohol was also determined after systematic investigation.
Section snippets
Materials and methods
Air- and moisture sensitive reactions were conducted under inert atmosphere (Ar or nitrogen) using Schlenk technologies or glove box. Dry solvents were obtained after re-distillation with sodium (or calcium hydride). Chemicals used for the reactions were provided by J&K, Acros and Aldich. NMR-spectra were collected at ambient temperature (25 °C) on Bruker Avance III (600 MHz for 1H and 13C) and JNM-ECA600 (600 MHz for 31P). Chemical shifts are reported in ppm relative to the residual solvent
Catalytic performance test of cobalt complexes
Four types of PN5P ligands and corresponding cobalt complexes were synthesized, characterized and applied for our O2-IHB pathway, which were presented in Table 1 and Supplementary Material. Among these cobalt complex catalysts, the crystals of Cat-1 and Cat-3 were obtained by liquid diffusion method at room temperature and each molecular structure was also respectively determined by X-ray crystalline diffraction technology (Supplementary Material). Although the remaining Cat-2 and Cat-4 were
Conclusions
In summary, the cobalt complex catalyzed synthesis of α, β-unsaturated esters directly from unactivated esters and alcohols via mild O2-interrupted hydrogen borrowing process was developed. Such cobalt complex is self-activating via double deprotonation of PN5P ligand and removal of dichloro ligands under the reaction condition that could accomplish the O2-interrupted hydrogen borrowing pathway. Sequentially, 2-fold alcohols could be dehydrogenized by the activated cobalt complex. The reaction
Acknowledgement
We thank the National Key Projects for Fundamental Research and Development of China (No. 2016YFB0601303), the Key Research Program of Frontier Sciences (No. QYZDB-SSW-SLH022), the International Cooperation and Exchange of the National Natural Science Foundation of China (No. 51561145020), the National Natural Science Fund for Distinguished Young Scholars (No. 21425625), Key Program of National Natural Science Foundation of China (No. 91434203), National Natural Science Foundation of China (No.
References (17)
- et al.
J. Catal.
(2002)et al.J. Catal.
(1990)et al.J. Catal.
(1996)et al.Tetrahedron Lett.
(2004)et al.J. Catal.
(1965)et al.J. Catal.
(1990) - et al.
Org. Lett.
(2009)et al.Beilstein J. Org. Chem.
(2012)et al.Angew. Chem. Int. Ed.
(1978)et al.Green Energy Environ.
(2016) - et al.
J. Am. Chem. Soc.
(2015) - et al.
J. Catal.
(2018) - et al.
Angew. Chem. Int. Ed.
(2007)et al.Science
(2010)et al.Adv. Synth. Catal.
(2010)et al.Chem. Soc. Rev.
(2015)et al.Angew. Chem. Int. Ed.
(2016) - et al.
Tetrahedron
(2014)et al.Adv. Synth. Catal.
(2016) - et al.
Angew. Chem. Int. Ed.
(2016) - et al.
Angew. Chem. Int. Ed.
(2018)et al.Angew. Chem. Int. Ed.
(2017)et al.Angew. Chem. Int. Ed.
(2017)et al.J. Am. Chem. Soc.
(2017)et al.J. Am. Chem. Soc.
(2017)et al.J. Am. Chem. Soc.
(2016)
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2020, Journal of Industrial and Engineering ChemistryCitation Excerpt :The α,β-unsaturated esters can also be developed by the Heck reaction which needs noble metal complex catalysts like Pd(OAc)2 [21], Pd/C [22], Pd(II) N-heterocycliccarbene [23], Ru compound [24] etc. Furthermore, several new methods have been developed to synthesis α,β-unsaturated esters use Ru complexes[25], Co complexes[26], enzyme[27] and so on as catalysts. Decarboxylative Doebner–Knoevenagel reaction of aldehydes and monoethyl malonate is another alternative to give the α,β-unsaturated esters [28].