16Carbon Dioide Reduide Chemicol Fedstock CI H aan+c学P RCIP+CO)P+P-O Scheme 1.8 Carboxylation of strained rings. conditions (Scheme 1.8).In the cases in Scheme 1.8(b and c).1 mol metal is consumed per mole of carboxylated product formed,which is a serious draw- back to exploitation of the process. 1.4.1.2 N-C Bond Formation The formation of the N-COz bond is relevant to industrial processes as it would allow the synthesis of carbamic acid derivatives avoiding the use of phosgene (Scheme 1.9). The synthesis of a labile carbamic acid has been achieved only very recently [77]using either benzylamine or PhP(OCH2CH2)2NH.Both carbamic acids have been isolated as solid dimers and characterized in the solid state by Xray dif mon feature is the existence of the dim ric moiety
Strained rings can be carboxylated by using transition metal catalysts [76]. The reactivity depends on the size of the ring, the metal used and the reaction conditions (Scheme 1.8). In the cases in Scheme 1.8 (b and c), 1 mol metal is consumed per mole of carboxylated product formed, which is a serious drawback to exploitation of the process. 1.4.1.2 N–C Bond Formation The formation of the N–CO2 bond is relevant to industrial processes as it would allow the synthesis of carbamic acid derivatives avoiding the use of phosgene (Scheme 1.9). The synthesis of a labile carbamic acid has been achieved only very recently [77] using either benzylamine or PhP(OCH2CH2)2NH. Both carbamic acids have been isolated as solid dimers and characterized in the solid state by X-ray diffraction. A common feature is the existence of the dimeric moiety represented below with an O–HO···O distance of 122 pm, very similar to that found in di- 16 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock Scheme 1.8 Carboxylation of strained rings
1.4 CO2 Conversion 17 (b)RR'NH +CO+R"X +B =RR'N-COOR"+BHX Scheme 1.9 Synthesis of carbamic acid(a)and carbamates(b)from amines and CO2. meric carboxylic acids.The monomer RR'N-COOH does not exist free,and de composes back to the free amine and CO2.Evidence for the formation of carba mic acid in solution has also been provided for amines like o(1-naphthyl)alky lamines [78]. ∠0.H-0 R'RN-C C-NRR 0-H.0 The reaction of amines with CO2 in the e (Sche me 1.9b)is impo on in the ch 9.pharmaceutic 80]and 81] The many at p to use tion meta s as catalyst in the 1970s [82]le ery that the metal-car to complexes I OC(O)-NRR'reacted with the alkylating agent RX to undergo an electrophilic attack by the alkyl group at the nitrogen rather than at the oxygen atom,with a net alkylation of the amine(Scheme 1.10a). More recently,carbamic esters have been synthesized successfully under very mild conditions by using either Group 1 metal or ammonium carbamates in the presence of a crown-ether [83](Scheme 1.10b).The latter interacts with the LnM-OC(ONRR”+R"X=LnMX+RR'NR”+CO2 (b) RNCOR'+MX (c)RR'NH +(R"O)CO RR'N-COOR"+R"OH (d)RR'NH (R"O)CO RR'R"N R"OH COz Scheme 1.10 Carboxy-alkylation of amines to carbamic esters
meric carboxylic acids. The monomer RRN-COOH does not exist free, and decomposes back to the free amine and CO2. Evidence for the formation of carbamic acid in solution has also been provided for amines like -(1-naphthyl)alkylamines [78]. R RN C O--H O CNRR O H--O The reaction of amines with CO2 in the presence of an alkylating agent and a base (Scheme 1.9 b) is important industrially as it produces organic carbamates that find large application in the chemical [79], pharmaceutical [80] and agrochemical industries [81]. The many attempts to use transition metals as catalysts in the 1970s [82] led to the discovery that the metal-carbamato complexes LnMOC(O)-NRR reacted with the alkylating agent RX to undergo an electrophilic attack by the alkyl group at the nitrogen rather than at the oxygen atom, with a net alkylation of the amine (Scheme 1.10 a). More recently, carbamic esters have been synthesized successfully under very mild conditions by using either Group 1 metal or ammonium carbamates in the presence of a crown-ether [83] (Scheme 1.10 b). The latter interacts with the 1.4 CO2 Conversion 17 Scheme 1.9 Synthesis of carbamic acid (a) and carbamates (b) from amines and CO2. Scheme 1.10 Carboxy-alkylation of amines to carbamic esters.
181 Carbon Dioxide Reduction and Uses as a Chemical Feedstock metal or ammonium cation,increases the nucleophilicity of the oxygen and pro motes the O-attack of the alkyl cation.Strong bases such as diazabicyclounde cene(DBU)[84]have also been used.As the crown-ether can be easily recovered and recycled,the process is an easy,selective and high-yield route to carbamates at room temperature by directly using amines,CO2 and alkylating agents.An alternative method for the carbamation of amines is the direct carboxy-alkylation by using carbonates(Scheme 1.10c).This reaction is of great industrial interest. It can be promoted by metal systems or other catalysts [851 with the maior drawback being the alkylation of the amine,a process that occurs at higher tem- 1.10d).Either homog neous [86] ger ous (871 catalysts have been devele d that work at lov ture and towards the ca oxy-al lkylation of amines This rou squite interest ent a pr e-a -fre ting as it route by any other phosgene-fre route. c catalyst has been developed for the carbamation of aromatic diamines 86h under mild conditions. 1.4.1.3 O-C Bond Formation The O-C bond formation is relevant to the synthesis of organic carbonates char- acterized by the o-c(o)o moiety.Both linear and cyclic carbonates are of indus. trial interest (Table 1.5).The semicarbonate species RO-C(O)OH is labile.as is the gous RRN-C(O)OH.CHO-CO)OH has ony re generated and characterized by IR [88a]and NMR spectroscopy [88b]. 1.4.1.3.1 Cyclic Carbonates The mo non rout to cyclic carbonates is the reaction of epoxides with pro ted by a var iety of homogeneous,h terogene ous and sup either cyclic carbonates or polym ers are obtaine (89].Mair group metal halides [90a]and metal complexes 9b].ammonium salts 91]and supported bases [92],phosphines [93].transition metal systems [88,94].metal oxides (951.and ionic liquids [96]have been shown to afford monomeric carbo nates.Al porphyrin complexes [97]and Zn salts [89,94,98]copolymerize ole. fins and CO2. The carboxylation of epoxides is strongly dependent on the reaction condi- tions such as temperature and solvent.The use of ionic liquids as reaction me. dium seems to accelerate the reaction with respect to any other organic solvent, most probably because ionic liguids promote the formation of and/or stabilize such as oxides 95]or sup ats92or exes 94]w ork well under the ns The solve ent can such actions.Amides methylfo etamide the car he
metal or ammonium cation, increases the nucleophilicity of the oxygen and promotes the O-attack of the alkyl cation. Strong bases such as diazabicycloundecene (DBU) [84] have also been used. As the crown-ether can be easily recovered and recycled, the process is an easy, selective and high-yield route to carbamates at room temperature by directly using amines, CO2 and alkylating agents. An alternative method for the carbamation of amines is the direct carboxy-alkylation by using carbonates (Scheme 1.10 c). This reaction is of great industrial interest. It can be promoted by metal systems or other catalysts [85] with the major drawback being the alkylation of the amine, a process that occurs at higher temperature than the carboxy-alkylation (Scheme 1.10 d). Either homogeneous [86] or heterogeneous [87] catalysts have been developed that work at low temperature and are very selective towards the carboxy-alkylation of amines. This route to organic carbamic esters is quite interesting as it may represent a phosgene-alkyl halides-free route if carbonates can be prepared from CO2 and alcohols or by any other phosgene-free route. A biomimetic catalyst has been developed for the carbamation of aromatic diamines [86 h] under mild conditions. 1.4.1.3 O–C Bond Formation The O–C bond formation is relevant to the synthesis of organic carbonates characterized by the O-C(O)O moiety. Both linear and cyclic carbonates are of industrial interest (Table 1.5). The semicarbonate species RO-C(O)OH is labile, as is the analogous RRN-C(O)OH. CH3O-C(O)OH has only recently been generated in solution and characterized by IR [88 a] and NMR spectroscopy [88 b]. 1.4.1.3.1 Cyclic Carbonates The most common route to cyclic carbonates is the reaction of epoxides with CO2, which is promoted by a variety of homogeneous, heterogeneous and supported catalysts; either cyclic carbonates or polymers are obtained [89]. Main group metal halides [90 a] and metal complexes [90 b], ammonium salts [91] and supported bases [92], phosphines [93], transition metal systems [88, 94], metal oxides [95], and ionic liquids [96] have been shown to afford monomeric carbonates. Al porphyrin complexes [97] and Zn salts [89, 94, 98] copolymerize olefins and CO2. The carboxylation of epoxides is strongly dependent on the reaction conditions such as temperature and solvent. The use of ionic liquids as reaction medium seems to accelerate the reaction with respect to any other organic solvent, most probably because ionic liquids promote the formation of and/or stabilize polar or ionic intermediates. Heterogeneous catalysts such as oxides [95] or supported ammonium salts [92] or metal complexes [94] work well under these conditions. The solvent can play a key role in such reactions. Amides such as dimethylformamides or dialkylacetamides can themselves promote the carboxylation of epoxides, albeit with a low TON [100]. Most likely this is due to the abil- 18 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock
1.4 CO2 Conversion 19 Table 1.5 Linear and cyclic carbonates and their market and use (total market 18 Mt year Carbonates Uses Linear (CH:O)2CO (CH2CH-CH2O)CO (EtO)2CO (PhO)CO solvents,reagents (for alkylation or DMC DEC DPC dimethyl diethyl carbonate diphenyl carbonate mers fo H.C Ph H HC- HC- hydroxyesters and EC PC sC cial materials PC Cc SC propylene carbonate cyclohe styrene carbonate onate ity of such species to activate either CO2 or the epoxide (Scheme 1.11).scCO2 also favors [99]the formation of the cyclic carbonate and copolymers. Metal oxides have also been used for the synthesis of optically active carbo. nates 1101l from pure enantiomers of the parent epoxide with total retention of configuration.The synthesis of optically active carbonates from a racemic mix. ture has been achieved with 22%ee in the best of cases,using Nb(IV)com nta scaused by the de-anchoring the ligand from the metal center with loss of induction of asymmetry. R、 R0 (a)R1 R- epoxide R H (b)R OH- Scheme 1.11 Activation of CO2 or the epoxide by an amide
ity of such species to activate either CO2 or the epoxide (Scheme 1.11). scCO2 also favors [99] the formation of the cyclic carbonate and copolymers. Metal oxides have also been used for the synthesis of optically active carbonates [101] from pure enantiomers of the parent epoxide with total retention of configuration. The synthesis of optically active carbonates from a racemic mixture has been achieved with 22% ee in the best of cases, using Nb(IV) complexes with optically active N, O or P donor atom ligands. An NMR study [101] has demonstrated that such low percentage ee is caused by the de-anchoring of the ligand from the metal center, with loss of induction of asymmetry. 1.4 CO2 Conversion 19 Table 1.5 Linear and cyclic carbonates and their market and use (total market 18 Mt year–1) Carbonates Uses Linear (CH3O)2CO (CH2CH=CH2O)CO (EtO)2CO (PhO)2CO solvents, reagents (for alkylation or acylation reactions), additive for gasoline DMC DAC DEC DPC dimethyl carbonate diallyl carbonate diethyl carbonate diphenyl carbonate Cyclic monomers for polymers, synthesis of hydroxyesters and hydroxyamines, component of special materials EC PC CC SC ethylene carbonate propylene carbonate cyclohexene carbonate styrene carbonate Scheme 1.11 Activation of CO2 or the epoxide by an amide
Carbon Dioide Redui e Chemical Feedtock 2n. 2n. Scheme 1.12 Mechanism of copolymerization of propene and CO with Zn complexes As noted above.when Al-porphyrin complexes 97orZn compounds]ar used as catalysts for the carboxylation of epoxides,the formation of polymers is observed.Al catalysts are now used in a plant in China.The mechanism of the polymerization reaction has been studied and the most credited mechanism when Zn compounds are used is shown in Scheme 1.12.The molecular mass of the polymers varies with the catalyst.Primarily propene oxide and styrene ox. ide have been used so far,with some interesting applications of cyclohexene ox- ide.It is wished to enlarge the use of substrates in order to discover new prop- erties of the polymers. The limiting factor in the commercial development of the carboxylation of ep oxides for the synthesis of monomeric or polymeric carbonates is the unavail- ability of large amounts of the parent epoxide.Such compounds are today pre pared by using se eral techni of which g pollution [1021.The e main nding a route carb that is decoupl ed fr O2 is of fund oping the large volume ir production of cyclic carbonates H +1202+C0 H ns,and a waste,such as Co into valuable compounds(Eq.8).Such a reaction has been performed using dif ferent catalysts on substrates such as propene and styrene.Little information is available on an early process that uses a complex catalytic mixture [104]. although in later studies the reaction mechanism has been elucidated for both homogeneous [105]and heterogeneous [106]catalysts.Two reactions are ob. served (Scheme 1.13):"two-oxygen"addition across the double bond that causes the splitting of the olefin to afford two aldehydes (compounds 5 and 6 in Scheme 1.13)and "one-oxygen"transfer to the olefin that produces the epoxide
As noted above, when Al-porphyrin complexes [97] or Zn compounds [98] are used as catalysts for the carboxylation of epoxides, the formation of polymers is observed. Al catalysts are now used in a plant in China. The mechanism of the polymerization reaction has been studied and the most credited mechanism when Zn compounds are used is shown in Scheme 1.12. The molecular mass of the polymers varies with the catalyst. Primarily propene oxide and styrene oxide have been used so far, with some interesting applications of cyclohexene oxide. It is wished to enlarge the use of substrates in order to discover new properties of the polymers. The limiting factor in the commercial development of the carboxylation of epoxides for the synthesis of monomeric or polymeric carbonates is the unavailability of large amounts of the parent epoxide. Such compounds are today prepared by using several techniques, some of which generate pollution [102]. The best route to epoxides is based on the use of H2O2 [103] that has as the main drawback the limited amount and the cost of H2O2. Finding a route to cyclic carbonates that is decoupled from H2O2 is of fundamental importance for developing the large volume industrial production of cyclic carbonates. 8 A reaction of great interest in this direction is the “oxidative carboxylation” of olefins that converts cheap products, such as olefins, and a waste, such as CO2, into valuable compounds (Eq. 8). Such a reaction has been performed using different catalysts on substrates such as propene and styrene. Little information is available on an early process that uses a complex catalytic mixture [104], although in later studies the reaction mechanism has been elucidated for both homogeneous [105] and heterogeneous [106] catalysts. Two reactions are observed (Scheme 1.13): “two-oxygen” addition across the double bond that causes the splitting of the olefin to afford two aldehydes (compounds 5 and 6 in Scheme 1.13) and “one-oxygen” transfer to the olefin that produces the epoxide, 20 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock Scheme 1.12 Mechanism of copolymerization of propene and CO2 with Zn complexes.