Cyclic carbonate formation

Lu has demonstrated that cyclic carbonate formation can dominate over polymer formation at much lower C02 pressures when the (salen)Co-OTs catalyst 24 and... 

During polymer chain growth, a back-biting process can lead to cyclic carbonate formation. In general, this process is more facile for aliphatic epoxides than for alicyclic epoxides and when the growing polymer chain dissociates from the metal center (Scheme 3). 

Scheme 3 Two modes of back-biting for cyclic carbonate formation...

Most often, the extent of completely alternating copolymer formation, expressed as 100% C02 linkages or 50% C02 content, is very high. With regards to the selectivity of the coupling reaction for copolymer versus cyclic carbonate production, two observations are consistently found, regardless of the catalyst. First, aliphatic epoxides are more prone to cyclic carbonate formation than alicyclic epoxides for example, PO affords propylene carbonate more readily than CHO provides cyclohexene carbonate. Second, in either instance, since it has been shown that the activation barriers for cyclic carbonate production are higher... 

Several novel modified salen derivatives of cobalt(III) have provided convincing evidence for the importance of the propagating copolymer chain staying in the vicinity of the metal center, so as to avoid the formation of cyclic carbonates this procedure is especially relevant to processes involving the PO monomer. Both, computational and experimental studies have shown that cyclic carbonate formation is enhanced relative to monomer enchainment under conditions where the growing polymer chain is outside the influence of the metal catalyst [50, 51]. To circumvent this issue, Nozaki and coworkers prepared a salen complex containing a piperidinium end-capping arm (Scheme 8.4) [52]. 

A1,2-0-isopropylidene acetal is formed in preference to a 1,2-carbonate. Where a primary alcohol group is present, preferential reaction with the pho ene occurs thereat, followed by cyclic carbonate formation (if participation with a neighboring hydroxyl group is sterically possible) the hydrogen chloride produced by these reactions catalyzes subsequent acetal formation at suitable positions. 

Chiral allylic cyclic carbonates such as 551 or 552 undergo excellent regioselective alkylation reactions with soft nucleophiles in the presence of palladium(O) in refluxing THF to provide ( )-allylic alcohols. The reaction of 4-0-benzyl-2,3-isopropylidene-L-threose (167) with the appropriate ylid, followed by deprotection of the isopropylidene ring with acidic resin and cyclic carbonate formation, provides a good overall yield of either 551 or 552. Ring opening with diethyl malonate in the presence of tetrakis(triphenylphosphine)palladium(0) provides in excellent yield the allylic alcohols 553 or 554, where the diastereoselectivity exceeds 99%. This reaction represents an efficient method of 1,3-chirality transfer [180] (Scheme 122). 

When 35 or 36 were reacted with polyols consisting of 1,2-diol substitution, cyclic carbonates were formed without unwanted side reactions or reaction with secondary amines (eqs 30 and 31). The cyclic carbonate formation most possibly proceeds via the selective reaction of 35 or 36 at the primary hydroxyl followed by cyclization involving the neighboring secondary hydroxyl. 

Darensbourg, D.J., Moncada, A.I., 2009. (Salen)Co(ll)/n-Bu 4 NX catalysts for the coupling of CO2 and oxetane selectivity for cyclic carbonate formation in the production of poly(trimethylene carbonate). Macromolecules 42, 4063—4070. 

Aluminium(iii)-based salens seem, however, used with a cocatalyst, to be limited in most cases to cyclic carbonate formation, the higher Lewis acidity of aluminium(iii) being probably partly responsible for this. The first successful aluminium-salen, Al(salen)Cl (Scheme 18.37), used in the formation of cyclic carbonates was tested by the group of Ren He in 2002, with the rather uncommon substrate, ethylene oxide and was further optimised by Xiao Bing Lu in 2004. ... 

Cyclic carbonates can also be obtained in good yields under mild conditions from oxiranes and carbon dioxide using an electrochemical procedure.The cyclic carbonate formation is catalyzed by Ni(cyclam)Br and is carried out in single-compartment cells fitted with a magnesium anode. 

Similarly, the copolymerization of TeMC with glycidyl phenyl ether initiated by CH3OSO2CF3 yielded polymers rich in cyclic carbonate-derived units when compared with the corresponding feed ratio and containing oxytetramethylene units as a result of five-membered cyclic carbonate formation. ... 

FIGURE 6 Illustrative and effective binary catalystic systems for cyclic carbonate formation including their respective process features. Note that only one representative catalyst combination is shown in each case [47-52]. MEK, methyl ethyl ketone. 

V. Cal6, A. Nacci, A. MonopoH, A. Fanizzi, Cyclic carbonate formation from carbon dioxide and oxiranes in tetrabutylammonium halides as solvents and catalysts, Org. Lett. 4 (2002) 2561-2564. 

The basic mechanism for the polymeric or cyclic carbonate formation involves initiation of the reaction by the opening of the epoxide to give an alkoxo linkage. This is shown in Figure 4.14 by the conversion of 4.58 to 4.59 where an attack by the nucleophile, e.g., a halide, opens the epoxide ring. The source of the nucleophile could be the metal complex itself or a cocatalyst such as a Lewis acid salt A X". 

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