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On the other hand, trimethylsilyl alkynes react with carbon dioxide and organozinc compounds using a catalytic amount of a nickel complex in the presence of an excess of DBU as shown in Eq. (36). The regioselectivity caused by the introduction of CO2 into disubstituted alkyne is dependent on the electronic property of the substituents R and trimethylsilyl (TMS) group on the alkyne because the thermodynamic stability of oxanickelacycles A and B should be affected by the conjugation of the substituent R with the carbonyl group in A and B [152]. However, although simple olefins have a low reactivity, e.g.,ethylene reacts with carbon dioxide in the presence of Wilkinson’s catalyst ([RhCl(PPh3)3]), a promoter (HCl, HBr or HI) and distilled water as solvent. When the reaction was carried out at a pressure of 700 atm and 180 8C, propionic acid formed in 38.4% yield together with ethanol (24.3%) and ethylpropionate (11.2%), whereas the ethylene conversion reached 91.4% as shown in Eq. (37) [153]. But the electrochemical reactions of alkenes with carbon dioxide in the presence of nickel complexes yield the carboxylic acid in good yields [153a,153b]. Recently, with molybdenum phosphine catalysts, nickel bipyridine catalysts, etc., the coupling reactions of ethylene with carbon dioxide were reported together with the formation of the five-membered nickelacarboxylate complex toward the formation of acrylic acid in system [153c,153d]. Ni(acac)2 and arylphosphine catalyze the addition of CO2 and diorganozinc to bis-1,3-diene under very mild conditions as shown in Eq. (38). The reaction performed at a lower temperature resulted in a high enantioselectivity. For example,methylative cyclization of unsymmetrical bisdiene in the presence of bulky phosphine (S)-MeO-MOP, proceeds at 4 8C with regioseleictive introduction of CO2 into a less-substituted 1,3-diene moiety to afford the methyl ester in 88% yield and 96% ee as shown in Eq. (38) [154]. Recently, Olah et al. [155] reported the efficient chemoselective carboxylation of aromatics to arylcarboxylic acids with superelectrophilically activated carbon dioxide coordinated by Al and Cl atoms in AlCl3 molecules. Aromatic carboxylic acids are synthesized in high yields by the carboxylation of aromatics with carbon dioxide and AlCl3 at a moderate temperature. For example, the carbonylation of p-xylene affords benzene monocarboxylic acid derivatives as shown in Eq. (39) [155]. The electrochemical carboxylations of halides proceed in DMF solvent with a platinum cathode and magnesium anode under carbon dioxide atmosphere to give the carboxylic acid in good yields as shown in Eq. (40) [156]. These electrochemical reactions also proceed in the supercritical carbon dioxide by using a small amount of an organic solvent. For example, it was applied to the synthesis of antiinflammatory agent (S)-(+)ibuprofen, by the carbonylation of alkylalkenylbenzene bromide as shown in Eq. (41) [156,156a]. Polymerization involving carbon dioxide is one of the most important utilizations of carbon dioxide. Polycarbonate formation without using phosgene, an alternating copolymerization with an epoxide, a condensation with benzenedimethanol and an alternating copolymerization with diynes, etc., can be exemplified. Especially, the polycarbonate formation without using phosgene, and the alternating copolymerization with the epoxide were already industrially applied. Asahi Chemical Industry already started the 50,000 tons/ year commercial operation of a polycarbonate process without using phosgene and methylene chloride in June 2002 in Taiwan as a new environmentally benign process [157,158,158a]. The Asahi’s polycarbonate process was carried out by the following four production steps: (1) ethylenecarbonate (EC), (2) dimethylcarbonate (DMC) and ethylene glycol (EG) and (3) diphenylcarbonate (DPC) and polymerization, as shown in Scheme 8 [157]. The DPC is manufactured by the reaction of DMC with phenol in the presence of Pb(OPh)2 as a catalyst [158b]. The polymerization proceeds with two processes. The first process is the solid-state polymerization of amorphous polymers in three steps: (1) pre-polymerization; the reaction of the DPC with bisphenol A to produce a clear amorphous prepolymer; (2) crystallization; the molten prepolymer is converted to a porous, white, opaque material by treating it with a solvent such as acetone; (3) solid-state polymerization; the crystallized prepolymer is heated at 210–220 8C under a flow of heated nitrogen, or under a reduced pressure to produce a solid polymer. The second process is ‘‘self-mixing melt polymerization’’ utilizing gravity and without using a conventional twinscrew type reactor. The polymerization was carried out at 265 8C and 67 Pa. A prepolymer having a number average molecular weight of 6200 is polymerized to the molecular weight 11,700 [158b,158c] This process has four excellent characteristics: (1) actual raw materials are CO2, ethylene oxide and bisphenol A, and this process does not use toxic phosgene and the halogen solvent, CH2Cl2. (2) Products are polycarbonate and ethylene glycol. Both products are of a high quality because the products do not contain the halide materials from the phosgene orCH2Cl2. (3) All intermediate products, EC, DMC, MPC and DPC are produced in high yields and high selectivities. These intermediates and two raw materials (MeOH and PhOH) are completely recycled. (4) This process contributes to decrease CO2 by 1730 tons/PC10,000 tons because it usesCO2,which is a byproduct of ethylene oxide production, as the raw material. Hence, this process can decrease carbon dioxide by more than 450,000 tons per year if all of the polycarbonate production processes were carried out with this process in the world. |
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