《有机化学》课程教学资源（参考书籍）Activation of Small Molecules, Organometallic and Bioinorganic Perspectives, Edited by William B. Tolman

1.4 CO2 Conversion 21 R H 0.12 2 RH-C-CHO 01 3 RCCH 0.1 0 4 5 RCHO(RCOOH)2 0.2 6 0.2 carboxylation of olefins with that is either converted into the carbonate or isomerized to a terminal aldehyde (compound 2 in Scheme 1.13)or a ketone (compound 3 in Scheme 1.13). Using RhCl(PEt Ph)3 as catalyst a maximum TON of 2-4 towards the carbo- nate was observed [105a-d).A more detailed study [105e]showed that the reac- tion proceeds via formation of a peroxocarbonate that is the real oxidant and sinto the relevant Rh carbonate (Scheme 1.14).Such a pathway does not explain the obs e the carho nate active 6O,it has be en pos ssible to de R study and calculation of vibratio by accurat al frequenciesfor the peroxocarbonate com C02,*0L 0 OL' +P,CIRh IV PCIRh C=0 P:CIRh 0 C02 0一0 L PCIRh: C=0 Scheme 114 Catalytic ycle for the formation of carbonate via oxidative carboxylation of olefins

that is either converted into the carbonate or isomerized to a terminal aldehyde (compound 2 in Scheme 1.13) or a ketone (compound 3 in Scheme 1.13). Using RhCl(PEt2Ph)3 as catalyst a maximum TON of 2–4 towards the carbo￾nate was observed [105 a–d]. A more detailed study [105 e] showed that the reac￾tion proceeds via formation of a peroxocarbonate that is the real oxidant and converts into the relevant Rh carbonate (Scheme 1.14). Such a pathway does not explain the observed TON, as the carbonate should not be an active catalyst. Using 18O2, 13C18O2 or 13C16O2 it has been possible to demonstrate by accurate IR study and calculation of vibrational frequencies for the peroxocarbonate com- 1.4 CO2 Conversion 21 Scheme 1.13 Oxidative carboxylation of olefins with homogeneous and heterogeneous catalysts. Scheme 1.14 Catalytic cycle for the formation of carbonate via oxidative carboxylation of olefins

221 Carbon Dioxide Reduction and Uses asa Chemical Feedstock plex [107]that the formation of the peroxocarbonate occurs via insertion of COz into the O-O bond of the dioxygen-Rh complex.The formation of an asymme- trically labeled *-O peroxo bond allowed the subsequent oxygen transfer reac- tion [107]to an oxophile (like an olefin)to be followed.The resulting carbonate can be converted into a Rh(I)complex via deoxygenation of the carbonate [105el by a free phosphine (either added or released by the complex).The resulting Rh(I)complex can restart the process.The progressive release of phosphine causes the conversion of the original Rh()catalyst into a species bearing a phosphine oxide ligand that is no able to ote the re If exter nal phosphine nate is not as the free pho is pretere ntially oxidized with to the olefin turated Rh() f the fom phines or N-ligands) es not improve the yield st is deactivated via an intermolecular ligand exchange as depicted in Eq.(9).The resu "RhCl(L-L)2"or"RhCl(olefin)species are not able to promote the epoxidation of the olefin. 2"RhCl(L-L)"+n(olefin)-RhCl(L-L)2+RhCl(olefin) (9) Therefore,such Rh catalysts are not useful for practical applications.Using Co, Cr or Mn analogs,the yield in carbonate is always low with no real improve. ment of the TON. The use of Group 1 or 2 metal oxides [16]or of transition metal oxides gives catalysts with a lo nger life.It must be ized that in the oxidative lation.the oles:the oxidatio the olefin u and the ide akes the selectio e difficult. atalys For i t metal oxi ave as are not goo carboxylation ca 6 als ic do cleavage,suggestive of a radical reaction promoted by the metal oxide.A de tailed study has identified the role of Pco,and Po,,temperature,solvent,and cocatalyst in the double-bond cleavage reaction,enabling the reaction to be per- formed so that it is not the main process anymore and the carbonate can be synthesized with more than 50%selectivity [106c). 1.4.1.3.2 Linear Carbonates The most interesting route to linear carbonates is the direct carboxylation of al cohols (Eq.10) 2ROH+CO2(RO),CO+H2O (10) This reaction hasan atom efficiency higher than the phosgene route and is much and clea than the ENICh and UBE es that feature the low yield at cq rable use 1.15).Th to the exploita rium that ranges between 1 and

plex [107] that the formation of the peroxocarbonate occurs via insertion of CO2 into the O–O bond of the dioxygen–Rh complex. The formation of an asymme￾trically labeled *O–O peroxo bond allowed the subsequent oxygen transfer reac￾tion [107] to an oxophile (like an olefin) to be followed. The resulting carbonate can be converted into a Rh(I) complex via deoxygenation of the carbonate [105 e] by a free phosphine (either added or released by the complex). The resulting Rh(I) complex can restart the process. The progressive release of phosphine causes the conversion of the original Rh(I) catalyst into a species bearing a phosphine oxide ligand that is not able to promote the epoxidation of the olefin anymore. If external phosphine is added, the yield of carbonate is not improved as the free phosphine is preferentially oxidized with respect to the olefin. Using unsaturated Rh(I) complexes of the formula “RhCl(L-L)” (L-L = bidentate phos￾phines or N-ligands) does not improve the yield as the catalyst is deactivated via an intermolecular ligand exchange as depicted in Eq. (9). The resulting “RhCl(L-L)2” or “RhCl(olefin)n” species are not able to promote the epoxidation of the olefin. 2
RhClL-L

nolefin RhClL-L2
RhClolefinn 9 Therefore, such Rh catalysts are not useful for practical applications. Using Co, Cr or Mn analogs, the yield in carbonate is always low with no real improve￾ment of the TON. The use of Group 1 or 2 metal oxides [106] or of transition metal oxides gives catalysts with a longer life. It must be emphasized that in the oxidative carboxy￾lation, the catalyst must perform two roles: the oxidation of the olefin using O2 and the carboxylation of the epoxide. This makes the selection of the catalyst more difficult. For instance, Table 1.6 shows that metal oxides that behave as oxidants are not good carboxylation catalysts (see, e.g. Ag2O). Table 1.6 also shows that, in a nonoptimized system, the main reaction is olefinic double-bond cleavage, suggestive of a radical reaction promoted by the metal oxide. A de￾tailed study has identified the role of PCO2 and PO2 , temperature, solvent, and cocatalyst in the double-bond cleavage reaction, enabling the reaction to be per￾formed so that it is not the main process anymore and the carbonate can be synthesized with more than 50% selectivity [106 c]. 1.4.1.3.2 Linear Carbonates The most interesting route to linear carbonates is the direct carboxylation of al￾cohols (Eq. 10): 2 ROH
CO2 RO2CO
H2O 10 This reaction has an atom efficiency higher than the phosgene route, and is much safer and cleaner than the ENIChem and UBE processes that feature a comparable use of atoms (Scheme 1.15). The existing limitation to the exploita￾tion of the reaction is the low yield at equilibrium that ranges between 1 and 22 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock

Table 1.6 Products of the"oxidative styrene using several oxides Catalyst Styrene Selectivity towards conversion Benzoic acid carbonate (%oxide (dehyde (%)(% 31 31 11 17.3 241 Ag,O 164 24s 68 13.6 58.8 8.9 10.3 18 46.4 33.9 MoO 6.3 59 54.8 252 72674 2.7 2n0 12.5 142 41 ree tests.The action time 5 h b)Po.=5 atm:Pco.=45 atm (a)Phosgene Atom efficiency Ho OH COCl: CH.CI +2NaC1+2H,0 2 NaOH 0.5 (b)Oxidative carbonylation ENIChem 2CH,OH+hO2+2CuC☑→2 Cu(OCH)C1+H,0 0.8 2 Cu(OCH)CI +CO(CH:0)CO +2CuCI Ube 2CH:OH 2NO+02CH ONO H2o 0.8 2CHONO +CO (CH:O)CO +2NO fc)Carboxylation of alcohols 2ROH+CO2→(RO)CO+HO Scheme 1.15 Routes to linear carbonates and their atom efficiency

1.4 CO2 Conversion 23 Scheme 1.15 Routes to linear carbonates and their atom efficiency. Table 1.6 Products of the “oxidative carboxylation” of styrene using several oxides a) Catalyst Styrene conversion Selectivity towards Styrene carbonate (%) Styrene oxide (%) Benzal￾dehyde (%) Benzoic acid (%) Molecular sieves 5 Åb) 16 3.1 11.8 67.5 3.1 SiO2 anhydrous b) 23 9.1 15.2 45.6 19.1 SiO2 hydratedb) 22 1.1 17.3 50.9 24.1 Ag2Ob) 28 – 16.4 50.8 24.5 MgOb) 14 6.8 13.6 58.8 8.9 Fe2O3 b) 28 10.3 1.8 46.4 33.9 MoO3 b) 27 6.3 5.9 54.8 25.2 Ta2O5 b) 27 2.9 17 48.5 24.4 La2O3 b) 26 2.7 12.7 43 32.3 Nb2O5 b) 27 16.6 4.4 46.3 24.1 V2O5 b) 34 7.3 5 55.3 27 ZnO 12.5 1.3 14.2 41.1 36 a) Each entry is the average of three tests. The average devia￾tion is ± 5%. The operating conditions were the same in all tests. Catalyst: 710–4 mol; styrene: 1.7510–2 mol; N,N-di￾methylformamide as solvent: 10 mL; temperature: 393 K; reaction time: 5 h. b) PO2 = 5 atm; PCO2 = 45 atm.

241 Carbon Dioxide Reduction and Uses as a Chemical Feedstock Table 1.7 Thermodynamic properties for the direct carboxylation of alcohols Carbonate △H(k]mol-') (MeO)2CO allvi.-O).Co -3.91 [CHa(CH2)OCO (n>2) -4.17 (PhO)CO +12.06 Calculated according to Ref ous and het rogeneo c proper va d is disfavored with phenol and oth alcohols Nonetheless,the low equilibrium concentration may not be a drawback for pro cess development as the reagents can be recycled.Attempts have been made to use chemical water traps in order to displace the equilibrium to the right.Mo- lecular sieves cannot be used at the reaction temperature as the formed surface OH groups are acidic enough to protonate the carbonate and reverse the reac- tion.Organic water traps are better suited:aldols (Eq.11)[109,111].ketals (Eq.12)[110]and dicyclohexylcarbodiimide(DCC)[111]have been used as such. H CHO Ph CH.O +scCO2 cat. Ph Me(Et) cat. Me(Et) 0= Me(Et) (12) In particular,the dimethyl cyclohexanone ketal also has been reacted with ethy- leneglycol to afford a cyclic carbonate and cyclohexanone [112].plus methanol (Scheme 1.16).The use of DCC as water trap deserves comment.A detailed study has shown that it is a promoter of the carboxylation in addition to being a simple water removal agent.Combining experimental studies and DFT calcula- tions.the reaction mechanism has be ompletely elucidated,as shown in Scheme 1.17 113).Several carbonates have be roduced with very high vields is highly influ the as above 335 K the tion is the ation of carb heme 1.17A).With DCC.using metha h een possible to produce the mixed methyl-phenyl-carbonate,(MeO)(PhO)CO [113]

2% of converted alcohol, using both homogeneous and heterogeneous catalysts. Table 1.7 gives the thermodynamic properties of some carbonates (aliphatic and aromatic). The values of
H show that the process is not very favored for ali￾phatic alcohols and is disfavored with phenol and other aromatic alcohols. Nonetheless, the low equilibrium concentration may not be a drawback for pro￾cess development as the reagents can be recycled. Attempts have been made to use chemical water traps in order to displace the equilibrium to the right. Mo￾lecular sieves cannot be used at the reaction temperature as the formed surface OH groups are acidic enough to protonate the carbonate and reverse the reac￾tion. Organic water traps are better suited: aldols (Eq. 11) [109, 111], ketals (Eq. 12) [110] and dicyclohexylcarbodiimide (DCC) [111] have been used as such. 11 12 In particular, the dimethyl cyclohexanone ketal also has been reacted with ethy￾leneglycol to afford a cyclic carbonate and cyclohexanone [112], plus methanol (Scheme 1.16). The use of DCC as water trap deserves comment. A detailed study has shown that it is a promoter of the carboxylation in addition to being a simple water removal agent. Combining experimental studies and DFT calcula￾tions, the reaction mechanism has been completely elucidated, as shown in Scheme 1.17 [113]. Several carbonates have been produced with very high yields (90–96%) and selectivity (close to 100%). The latter is highly influenced by the temperature as above 335 K the favored reaction is the formation of carbamate (Scheme 1.17A). With DCC, using methanol and phenol it has been possible to produce the mixed methyl-phenyl-carbonate, (MeO)(PhO)CO [113]. 24 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock Table 1.7 Thermodynamic properties for the direct carboxylation of alcohols Carbonate
H (kJ mol–1) (MeO)2CO –4.00 (EtO)2CO –3.80 (allyl-O)2CO –3.91 [CH3(CH2)nO]2CO (n > 2) –4.17 (PhO)2CO +12.06 Calculated according to Ref. [108].

1.4 CO2 Comversion 25 H01 =0 HO OH. Scheme 1.16 Use of cyclohexanone-ketal in a two-step carbonate formation,avoiding the tor on mco um containing the CyN-C-NCy CH.OH CVHNC-NCY CHOHHCH CyHNCIOOCH+CYCH 0-c 8-c co: y CHQ =0 ON、H-pcH (b) Scheme 1.17 Reaction mechanism of carboxylation of alcohols promoted by DCC The carboxylation of alcohols is an interesting reaction for the synthesis of carbonates that requires a better understanding in order to avoid catalyst deacti vation by water.The reaction mechanism has been investigated for the Sn,Nb and DCC systems.Scheme 1.18 shows two different possible intra-and inter- molecular mechanisms.The intramolecular mechanism that operates with Sn and DCC is based on a "double base-activation of CHOH and produces an E=O double bond (E=C)that reduces the activity of the catalyst or generates an inert polymer(E-Sn).The intermolecular mechanism,that seems to be opera- tive with Nb systems,can follow two routes that differ with respect to the inter- mediacy of one or two alcohol molecules.In the latter case,the reaction follows a"base plus acid activation"of methanol,and the catalysts perform much better and do not lose activity over s eral cvcles.The wate r formed in the reac d in order to push the equilibrium to right and avoid the scatalysts also have been used [114c that do not show better

The carboxylation of alcohols is an interesting reaction for the synthesis of carbonates that requires a better understanding in order to avoid catalyst deacti￾vation by water. The reaction mechanism has been investigated for the Sn, Nb and DCC systems. Scheme 1.18 shows two different possible intra- and inter￾molecular mechanisms. The intramolecular mechanism that operates with Sn and DCC is based on a “double base-activation” of CH3OH and produces an E=O double bond (E =C) that reduces the activity of the catalyst or generates an inert polymer (E = Sn). The intermolecular mechanism, that seems to be opera￾tive with Nb systems, can follow two routes that differ with respect to the inter￾mediacy of one or two alcohol molecules. In the latter case, the reaction follows a “base plus acid activation” of methanol, and the catalysts perform much better and do not lose activity over several cycles [114]. The water formed in the reac￾tion must be eliminated in order to push the equilibrium to right and avoid the destruction of the catalysts. Heterogeneous catalysts also have been used [114 c] that do not show better performances than homogeneous ones. 1.4 CO2 Conversion 25 Scheme 1.16 Use of cyclohexanone-ketal in a two-step process for carbonate formation, avoiding the formation of water in the reaction medium containing the carbonate. Scheme 1.17 Reaction mechanism of carboxylation of alcohols promoted by DCC