Carbon dioxide lactone formation

The photoelimination of carbon dioxide from esters and lactones is a process that has been the subject of detailed investigations. Discussion here is limited to nitrogen containing systems. 3,4-Diphenylsydnone (464), on irradiation in benzene, is converted via the nitrile imine 465 into 2,4,5-triphenyl-1,2,3-triazole (466)388 initial bond formation between N-2 and C-4 followed by loss of carbon dioxide to give the diazirine 467 is proposed to account for the formation of the nitrile imine. Nitrile imines generated in this way have been trapped with alkenes and alkynes to give pyrazoles389... 

In most cases, uronic acids are liberated from acidic polysaccharides by hydrolysis leading to irreproducible concomitant formation of lactones. Several methods to circumvent this problem have been published describing conversion of the uronic acid into methyl esters followed by reduction with borohydride or borodeuteride reagents and subsequent hydrolysis and GC-MS detection [129]. Other techniques are based on the liberation and quantification of carbon dioxide. Direct determination of uronic acid residues in hydrolyzates has frequently been performed according to colorimetric assays, which are rather insensitive and have thus mostly been replaced by high-performance anion exchange chromatography (HP-AEC) methods [130-132]. 

The reactions contributing to this oxidation current are the formation of surface oxides on the carbon (quinones, lactones, carboxylic acids, etc.) and the evolution of carbon dioxide. Binder et al.51 had previously examined the oxidation of numerous carbons in KOH, H2S04, and HjP04 but not at temperatures as high as those in the work of Kinoshita and Bett. 

One remarkable group of homogeneous CO reactions 1 that of the conversions with unsaturated hydrocarbons using alkynes. alkcnes or dienes as the adducts, lactones, acids or esters are produced. The two most efficient catalyst metals are rhodium and palladium. This part of CO chemistry is a very fascinating one, because carbon dioxide is fixed in the organic compounds, through the formation of a new carbon-carbon bond. 

Dienes, especially butadiene, also react with carbon dioxide. Inotie and his co-workeis found that Pd(dppe)j catalyzes the telomerization of butadiene and CO to give the y-lactone 2 Cthylidcnc-5 hcpien-4-oUde in about a S% yield [182, 183]. The distribution of the products evidently depends on the solvent used and polar aproiic solvents such as DMF, DMSOand 1 mielhyl-2-pyrrolidone are most suitable for lactone formation. A temperature of 120 C is required. When the reaction is carried out at temperatures below lOO C and terminated before the complete conwrsion of butadiene, the free organic acids (the precursors of the >4actone) are isolated up to 10%. 

From a mechanistic point of view the first steps of the catalytic cycle should be similar to the telomerization of butadiene itself (Scheme 2). The catalytic precursor generates the Pd(0) species A that reacts to the bis-(ri -allyl) complex C. The C,C bond formation between two C4 units is followed by insertion of carbon dioxide into a Pd,C bond affording the carboxylate intermediate D. Different pathways have been discussed to describe the multiple product formation (refer to ). Interestingly, a bis-(carboxylato) complex may be prepared directly from the reaction of lactone 1, palladium acetate and P(i-Pr)3. This complex was structurally characterized by Behr and co-workers and shows good activity as catalyst. Reviewing the literature, there are some remarkable facts and open questions of theoretical and technical interest ... 

There has been a growing interest in the utilization of CO2 as a potential Cl source for chemicals and fuels to cope with the predictable oil shortage in the near future. Insertion reactions of CO2 into M-H, M-0, M-N, and M-C bonds are well documented, where these reactions are explained in terms of the electrophilicity of CO2 il, 2). Catalytic syntheses of lactones (3-9) and pyrones (10-16) are also established by incorporation of CO2 into dienes and alkynes activated on low-valent metal complexes. Carbon dioxide shows only an electrophilicity under usual reaction conditions, but it exhibits a nucleophilicity upon coordination to low-valent metals because of the intramolecular charge transfer from metals to CO2. Metal-C02 formation may be the key species in electro- and photochemical CO2 reductions. Since the first characterization of [Ni(PCy3)2(T) (C,0)-C02)] (17), a variety of metal... 

Dienes, allenes, and alkynes react with carbon dioxide to yield cyclic lactones—the catalysts include various Ni and Pd complexes.4 With certain diynes, alternating copolymerization with C02 results in the formation of poly (2-pyrones) (Scheme 6.16). 

The establishment of the structure of monocrotic acid as a, 3-dimethyl-levulinic acid (LXIV) and its formation from monocrotalic acid by loss of carbon dioxide limits very dehnitely the possible structures of the latter. Monocrotalic acid was known to contain one carboxyl group and the presence of a lactone group was indicated on back titration of the acid... 

The evidence so far has dealt solely with the nature of the lactone but has cast no light on the nature of the basic nucleus of this alkaloid. The experimental evidence necessary for complete elucidation of the structure of dioscorine is far from complete, yet the results of exhaustive methylation are sufficient to set up a working formula. Carbon dioxide as well as water is lost when dioscorine methohydroxide is distilled (dry) in vacuum with the formation of the triply unsaturated, oily base, methyldioscoridine (248). Methyldioscoridine no longer exhibits lactonic properties and molecular refraction measurements indicate that the three double bonds are... 

ABSTRACT The review covers particularly the synthesis of fine chemicals via the formation of C-C bonds between carbon dioxide and hydrocarbons. In the reactions of CO2 with alkenes, dienes and alkynes a great number of carboxylic acids, dicarboxylic acids, esters, lactones and pyrones are formed, whether in stoichiometric or catalytic reactions. In each chapter the reactions are considered in the order of the transition metals applied. In addition, some syntheses will be mentioned which are closely related to transition metal catalysis, for instance the electrocarboxylation ofolefinic hydrocarbons. 

The presumed mechanism of the formation of the 6-lactone is shown in Figure 26. Two molecules of butadiene combine to a Cs-chain and form a palladium-bis-n -allyl complex which is in equilibrium with a ri. ql-complex. Carbon dioxide inserts into the palladium-carbon bond yielding a carboxylate complex. The oxygen of the carboxylate group and the allyl group react and form the 6-lactone by a cycli-zation step. 

The proposed reaction mechanism for the formation of the two co-pro-ducts is shown in Equation 9. Butadiene, isoprene and the palladium complex form two different bis-n -allyl complexes, in which the isoprene unit is both head- and tail-connected to butadiene. Carbon dioxide inserts into both complexes to yield a carboxylate species, but the insertions occur only on the n -allyl part steming from the butadiene unit perhaps by electronic reasons. Finally, the two novel lactones are formed by reductive elimination. 

The interpretation of the formation of the Ci3-lactone requires a sequence of mechanistical pathways which are unknown so far in rhodium-catalysis. Two proposals for the mechanism were given in Equation 12. The mechanism of path B is similar to that shown for palladium catalysis. A rhodium Cg-carboxylate complex is formed which under further incorporation of butadiene could yield the lactone. In the mechanism of path A three molecules of butadiene react with the starting rhodium compound forming a C- 2 Chain, which is bound to the rhodium by two n -ally1 systems and one olefinic double bond. Carbon dioxide inserts into one of the rhodium allyl bonds thus forming a C- 3-carboxyl ate complex, which yields the new C-13-lactone. 

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