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PdII/phosphine system, which was benefited from the N-tosylhydrazone chemistry (Scheme 17c) [101]. Gratifyingly, four novel bonds: two C-C, one C=C and one C-N formed simultaneously in the heterocycles. 2.5.2. O-Aryl carbamates from three-component reactions containing amines and CO2 Molecules 2019, 24, 182 19 of 41 A straightforward route to O-aryl carbamates was achieved through a three-component reaction of CO2, amines and diaryliodonium salts with an organic base (DBU) as the promoter Notably, Cheng et al. developed a strategy towards 4-aryl-2-quinolinones by a three-component (Scheme 18a) [102]. In this phosgene- and metal-free protocol, diaryliodonium salts was used as coupling reaction of N-tosylhydrazones, 2-iodoanilines and CO2 catalyzed by PdII/phosphine system, electrophilic arylating agents to trap the in situ-generated carbamate anions. However, this which was benefited from the N-tosylhydrazone chemistry (Scheme 17c) [101]. Gratifyingly, four novel transformation generated a stoichiometric amount of by-product, i.e., aryl iodides. In addition, the bonds: two C-C, one C=C and one C-N formed simultaneously in the heterocycles. preformation of diaryliodonium salts was required. Additionally, a more convenient route to O-aryl carbamates by a copper(I)-catalyzed oxidative 2.5.2. O-Aryl Carbamates from Three-Component Reactions Containing Amines and CO2 coupling reaction between arylboronic acids, amines and CO2 was developed (Scheme 18b) [103]. At A straightforward route to O-aryl carbamates was achieved through a three-component reaction of the same time, BF3·OEt2 was employed as the promoter and O2 as the oxidant. This approach CO , amines and diaryliodonium salts with an organic base (DBU) as the promoter (Scheme 18a) [102]. tole2rated a wide range of functionalized O-aryl carbamates. In this phosgene- and metal-free protocol, diaryliodonium salts was used as electrophilic arylating Ribas and Company et al. reported a one-pot procedure of a model pincer-like arene, amines agents to trap the in situ-generated carbamate anions. However, this transformation generated a and CO2 to synthesize O-arylcarbamates (Scheme 18c) [104]. This reaction performed smoothly at stoichiometric amount of by-product, i.e., aryl iodides. In addition, the preformation of diaryliodonium room temperature by an aerobic Cu(II)-based catalyst and a nucleophile of carbamic ammonium salts was required. salts in situ generated from amines and CO2 gas. X R3 I DBU (2 equiv.) CH3CN, 80 °C, 4 MPa, 12 h O R 3 trace-90% X=TfO,BF4,Br,TsO R3, R3 = H, Alkyl, Aryl, Cl, Br, CN, NO2 R4 1 R N O (eq.a) (eq. b) (eq. c) + CO2 + R1, R2 = H, Alkyl, Ph, Aryl RB(OH)2 N O R1 NH R1 R2 Cu2O (10 mol%), pyridine (3 equiv.) BF3 OEt2 (3equiv.) DCM, 80 °C, O2 (0.4 MPa) total pressure (4 MPa), 24h CuBr (10 mol%) HN CH3CN, RT, 0.4 MPa, 18 h, air O R2 R 25-92% X=TfO,BF4,Br,TsO R = Ph, Aryl DCM = dichloro-methane R2 HN H R1 HN N R2 trace-88% O NNO Scheme 18..SSyynnththeessisisoof fOO-a-rayrlyclacrbarabmaamteastebsybthyrethe-rceoem-copmonpeonnterneatcrteioanctsioconnstacoinitnaginaimnginaems ainedsCaOnd 2. CO2. Additionally, a more convenient route to O-aryl carbamates by a copper(I)-catalyzed oxidative c2o.5u.3p.lNin-gAreyalcatirobnambeatweefreonmarTyhlrbeoer-oCnoimcapcoindesn,tamReinacetsioansdCCoOntaiwniansgdAevmeilnoepseadn(dScChOem2e18b)[103]. At the same time, BF3·OEt2 was employed as the promoter and O2 as the oxidant. This approach N-Arylcarbamate synthesis directly from CO2, amines and organic halides often has negative tolerated a wide range of functionalized O-aryl carbamates. environmental effects [105]. Choi et al. reported the first successful example of a halogen-free Ribas and Company et al. reported a one-pot procedure of a model pincer-like arene, amines and process for producing N-phenylcarbamate from CO2, aniline and dibutyltin dialkoxide (Scheme 19a) CO2 to synthesize O-arylcarbamates (Scheme 18c) [104]. This reaction performed smoothly at room [106]. Gratifyingly, the corresponding methyl N-phenylcarbamate was formed in a yield of 80% temperature by an aerobic Cu(II)-based catalyst and a nucleophile of carbamic ammonium salts in situ along with N′-diphenylurea as by-product. Recently, they found that silicate ester (i.e., Si(OR)4) generated from amines and CO2 gas. could be incorporated with aliphatic or aromatic amines and CO2 to synthesize N-arylcarbamate (Scheme 19b) [107]. Notably, the corresponding carbamates of aromatic amines could be obtained in 2.5.3. N-Arylcarbamate from Three-Component Reactions Containing Amines and CO2 a yield of up to 96%. Besides, the reaction was chemoselective toward amine activation. The use of N-Arylcarbamate synthesis directly from CO2, amines and organic halides often has negative environmental effects [105]. Choi et al. reported the first successful example of a halogen-free process for producing N-phenylcarbamate from CO2, aniline and dibutyltin dialkoxide (Scheme 19a) [106]. Gratifyingly, the corresponding methyl N-phenylcarbamate was formed in a yield of 80% along with N′-diphenylurea as by-product. Recently, they found that silicate ester (i.e., Si(OR)4) could be incorporated with aliphatic or aromatic amines and CO2 to synthesize N-arylcarbamate (Scheme 19b) [107]. Notably, the corresponding carbamates of aromatic amines could be obtained in a yield of up to 96%. Besides, the reaction was chemoselective toward amine activation. The use of Zn(OAc)2/1,10-phenanthroline (phen) catalysts offered the best performance because of the carboxylate-assisted proton activation, as shown in Figure 7. Alternatively, adding the corresponding alcohols to the system including CO2 and amines was also an efficient route to obtain N-arylcarbamates. In 2018, the combination of CeO2 and 2-cyanopyridine was used in the direct synthesis of N-arylcarbamates from CO2, amines and alcohols HN 2PDF Image | Catalytic Conversion of Carbon Dioxide through C-N Bond
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