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    Home > Sun Hongmei research group, Suzhou University: Ni catalyzed regioselective hydroaromatization of aryl ethylene and benzothiazole with N-heterocyclic carbene ligands

    Sun Hongmei research group, Suzhou University: Ni catalyzed regioselective hydroaromatization of aryl ethylene and benzothiazole with N-heterocyclic carbene ligands

    • Last Update: 2019-07-02
    • Source: Internet
    • Author: User
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    Author: with the increasingly serious environmental and resource problems, the development of green and efficient C-C bond formation method has become one of the most cutting-edge research fields in Organometallic Chemistry and organic synthesis chemistry The research and development of transition metal catalysts with high activity and selectivity is one of the core driving forces in this field Alkyl substituted heteroarenes are one of the most important members of the organic small molecule library, which are rich in varieties As structural units, they not only widely exist in drugs, pesticides, natural active products and other molecules, but also are the key building blocks for the construction of organic photoelectric materials, ligands and so on In recent years, transition metal catalyzed hydrogen aromatization of aryl vinyl compounds has attracted a lot of attention due to its advantages of 100% atomic economy, ideal step economy and wide substrate applicability, which has become one of the most ideal reactions for the synthesis of alkyl substituted heteroaromatics However, on the whole, the relevant research is still in a promising and challenging stage One of the key issues is how to flexibly regulate the regional selectivity of asymmetric carbon carbon double bond in the reaction and generate branched (b) or straight (L) products with high selectivity At present, although several successful cases have been reported (Fig 1a-c), the realization of these "regioselectivity switches" depends on sensitive alkylaluminum reagents, Grignard reagents or expensive and toxic large volume phosphine ligands, which still have great limitations Recently, sun Hongmei of Suzhou university designed and synthesized a class of N-heterocyclic carbene, The new mixed nickel (II) complexes of NHC ligands and phosphite [P (or) 3] ligands have been found to have good catalytic activity by the synergistic effect between the two ligands in the hydroformylation of aryl vinyl compounds and benzothiazole compounds The steric hindrance of ligands can realize the flexible regulation of regioselectivity, and a new strategy of regioselective conversion has been developed Related work was published on Org Lett (DOI: 10.1021/acs.orglett.9b01645) Fig 1 the regioselective regulation of transition metal catalyzed aryl vinyl hydroaromatization (source: org Lett.) firstly, based on the previous work of the research group (organometallics, 2015, 34, 5792; org Lett, 2016, 18, 2860), the mixed nickel (II) complex 1-3 containing NHC and P (or) 3 was synthesized by a simple ligand exchange method (Fig 2), they are all solid compounds with good air stability at room temperature Their single crystal structures were determined by X-ray diffraction In these mixed nickel (II) complexes, NHC ligands and P (or) 3 ligands are trans distributed According to the method developed by Cavallo et al In 2016 to calculate the percentage of buried volume of NHC ligands in metal complexes (% vbur), the authors calculated that the imes of complexes 1 and 2 and the% vbur of IPR * ome were 34.9 and 42.3, respectively, showing that the steric hindrance of the former was significantly smaller than that of the latter Fig 2 synthesis of mixed nickel (II) complexes (source: org Lett.) later, the author selected the hydroformylation of benzothiazoles and arylethenes rarely reported in the literature as the research object, and took benzothiazoles (4a) and styrene (5a) as model substrates for mixed nickel (II) complexes 1-3 The conditions of the catalytic hydrogen aromatization were selected (Table 1) The results show that in the presence of magnesium chips, the complex 1 with small steric hindrance imes can yield 93% gas phase yield and 98:2 (6Aa: 7aa) high selectivity to produce branched chain products (entry 1), while the complex 2 with large steric hindrance IPR * ome can yield 88% gas phase yield and 99:1 (7aa: 6Aa) high selectivity to produce straight chain products (entry 2) By comparing the catalytic activity of complexes 1 and 3, we found that the catalytic activity of mixed nickel (I I) complexes 1 with P (OET) 3 ligands was higher than that of mixed nickel (II) complexes 3 with P (OiPr) 3 ligands, indicating that P (OET) 3 ligands had an effect on improving the catalytic activity of nickel (II) complexes, but had no effect on the regioselectivity of the reaction (entries1 and 3) The effects of other metals, such as manganese powder and zinc powder, and alkali on the reaction were also investigated It was found that the effect of magnesium chips was the best (entries 1, 2 and 4-9) The results of the control experiment showed that the complexes Ni (imes) 2Br 2 and Ni [OET) 3] 2Br 2 could not catalyze the reaction (entries 10 and 11) If the same amount of P (OET) 3 was added to Ni (imes) 2Br 2, the gas phase yield of 51% and the selectivity of 99:1 (6Aa: 7aa) were obtained However, if the same amount of imes · HCl was added to Ni [OET) 3] 2Br 2, the reaction could hardly occur (entry 13) The author also compared the catalytic system composed of Ni (COD) 2, imes and P (OET) 3, and found that its catalytic performance is very similar to that of complex 1 (entry 14); however, for the catalytic system composed of Ni (COD) 2, IPR * OME and P (OET) 3, its catalytic activity is far lower than that of complex 2 (entry 15) These results fully show that the mixed nickel (II) complexes designed and synthesized by the authors not only have significant structure-activity relationship in this reaction, but also have good catalytic activity by the synergistic action between NHC ligands and P (or) 3 ligands The steric hindrance of ligands can flexibly control the regioselectivity of the reaction Table 1 reaction condition screening (source: org Lett.) was encouraged by the above results The author further investigated the applicability of the substrate and found that complexes 1 and 2 have good substrate applicability and regional selectivity (Fig 3 and Fig 4) In the presence of magnesium scraps, when complex 1 is used as catalyst, styrene containing electron donor or electron acceptor in the opposite position can successfully complete the reaction, with 31% - 91% separation yield and 97:3 or higher regioselectivity to form branched chain products It is worth mentioning that although the author's previous study found that the mixed nickel (II) complex Ni (NHC) (PR 3) Br 2 containing NHC and phosphine ligands can efficiently activate ar-cl, ar-f and Ar-O bonds (organotallics, 2015, 34, 5792; org Lett., 2016, 18, 2860), these substituents are well compatible in this reaction (Fig 3:6ad, 6ah and 6Ai) In addition, the reaction can generate some branched products with potential pharmacological activities (Fig 3:6ap, 6aq and 6aR) with 71% - 94% separation yield and more than 93:7 regioselectively It is also suitable for aromatic vinyl compounds containing heterocycles such as benzofuran, indole and carbazole (Fig 3:6av, 6aw and 6AX) When complex 2 was used as catalyst, the regioselectivity of the whole reaction was reversed immediately, and the selectivity of straight chain products was 95:5 or more, although the reaction rate was a little lower than that of complex 1 It should be noted that for 2-naphthylethylene compounds (Fig 4:7at, 7au), the regulatory strategy reported in the literature can only obtain a mixture of straight chain products and branched chain products with the ratio of 1:1, while the author uses the complex 2 containing IPR * ome as the catalyst to obtain the straight chain products with 95:5 regional selectivity Fig 3 hydrogen aromatization reaction catalyzed by complex 1 (source: org Lett.) Fig 4 hydrogen aromatization reaction catalyzed by complex 2 (source: org Lett.) finally, based on isotope labeling test, nuclear magnetic tracking reaction process and literature reports, the author proposed possible reaction mechanism (see Si: scheme S3) It is speculated that the mixed nickel (II) complex is transformed into the mixed zero valent nickel intermediate under the action of magnesium filings It is oxidized and added with benzothiazole c2-h bond to form the corresponding NIMH compound The reversible coordination and insertion reaction between the NIMH compound and styrene generates the alkylation compound of nickel, and finally the target product is eliminated by reduction In this catalytic cycle, a small volume of imes ligand is conducive to the formation of relatively stable benzyl nickel intermediates, which can generate branched products; a large volume of IPR * ome ligand results in the formation of straight chain products due to the increase of the steric hindrance around the nickel atom Summary: a new mixed nickel (II) complex, Ni (NHC) [P (or) 3] br 2 (1-3), was designed and synthesized The first regioselective hydroaromatization of aryl vinyl compounds and benzothiazole compounds in the presence of magnesium filings catalyzed by air stable bivalent nickel complexes was reported Through the proper match between NHC ligand and P (or) 3 ligand, not only the catalytic activity of Ni (II) complex is improved, but also the regional selectivity of the reaction can be flexibly controlled, providing a new strategy of "regioselective conversion" This achievement was recently published on organic letter (org Lett 2019, DOI: 10.1021/acs.orglett.9b01645) Master Li ruipeng is the first author of the article, and Professor Sun Hongmei is the corresponding author of the article I would like to celebrate Professor Shen Qi's 80th birthday with this article! Thanks to NSFC general program (21472134) and Jiangsu Key Laboratory of organic synthesis for their financial support.
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