Alkyl formation

J-unsaturated ester is formed from a terminal alkyne by the reaction of alkyl formate and oxalate. The linear a, /J-unsaturated ester 5 is obtained from the terminal alkyne using dppb as a ligand by the reaction of alkyl formate under CO pressure. On the other hand, a branehed ester, t-butyl atropate (6), is obtained exclusively by the carbonylation of phenylacetylene in t-BuOH even by using dppb[10]. Reaction of alkynes and oxalate under CO pressure also gives linear a, /J-unsaturated esters 7 and dialkynes. The use of dppb is essen-tial[l 1]. Carbonylation of 1-octyne in the presence of oxalic acid or formic acid using PhiP-dppb (2 I) and Pd on carbon affords the branched q, /J-unsatu-rated acid 8 as the main product. Formic acid is regarded as a source of H and OH in the carboxylic acids[l2]. 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

Other mechanisms for the synthesis of alkylformates, not via formic acid esterification, are possible. Hydrogenation of C02 to CO, followed by catalytic car-bonylation of alcohol, would produce alkyl formate. This mechanism seems more likely for the anionic metal carbonyls because they are known catalysts for alcohol carbonylation. However, Darensbourg and colleagues [64, 74, 85] showed... 

Preparation of formamides from COz and a non-tertiary amine by homogeneous hydrogenation has been well studied and is extremely efficient (Eq. (12)). Essentially complete conversions and complete selectivity can be obtained (Table 17.3). This process seems more likely to be industrialized than the syntheses of formic acid or formate esters by C02 hydrogenation. The selectivity is excellent, in contrast to the case for alkyl formates, because the amine base which would stabilize the formic acid is used up in the synthesis of the formamide consequently little or no formic acid contaminates the product. The only byproducts that are likely to crop up in industrial application are the methylamines by overreduction of the formamide. This has been observed [96], but not with such high conversion that it could constitute a synthetic route to methylamines. 

Highly efficient catalysts have been developed for the hydrogenation of C02 to formic acid and formamides, to the point where industrialization could be considered. Researchers have been far less successful in developing efficient homogeneous catalysts and optimum conditions for the hydrogenation of C02 to alkyl formates, methanol, methane, and especially oxalic acid. These are the areas in which research efforts are most needed. 

Halothiophenes take part in Pd-catalyzed alkoxycarbonylations in the presence of CO, alcohol and base. In order to avoid the inconvenience of pressurized carbon monoxide, alkyl formate may be used as a safe surrogate [130], In one of the many examples, 2-Iodothiophene was carbonylated to the corresponding methyl ester using methylformate in place of CO. 

P. G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, Homogeneous Catalysis in Supercritical Fluids Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides ,J. Am Chem Soc 1996,118, 344-355. 

Formic acid (Ester of formic acid) Alkyl formate... 

Selected examples of alkyl formates, alkanols and diols... 

Z,Z)-Heptadeca-8,10-dien-l-ol Alkyl formates Undec-l-yl formate Dodec-l-yl formate Tridec-l-yl formate Tetradec-1 -yl formate Pentadec-l-yl formate Hexadec-1 -yl formate Heptadec-l-yl formate Octadec-l-yl formate Nonadec-l-yl formate Icos-l-yl formate Henicos-l-yl formate Docos-l-yl formate Tricos-l-yl formate Tetracos-l-yl formate Pentacos-l-yl formate Alkenyl formates (Z)-Tridec-7-en-l-yl formate (Z)-Tetradec-7-en-l-yl formate (Z)-Tetradec-8-en-l-yl formate (Z)-Pentadec-6-en-l-yl formate (Z)-Pentadec-8-en-l-yl formate (Z)-Hexadec-7-en-l-yl formate... 

Useful sources of these radicals were the hydrogen abstraction from alkyl formate and especially the decomposition by ferrous salts of mixtures of hydrogen peroxide and a-keto esters. The alkoxy-carbonyl radicals appear to be less nucleophilic than carbamoyl radicals. 

Formic acid is formed by the reaction of H2 and CO catalyzed by the dppe complex of Pd[96], In alcohol, alkyl formates are obtained[97]. DMF is obtained by the reaction of C02 (40 atm) and Me2NH under a high pressure of H2 (80 atm) in the presence of a base in methyl Cellosolve[98], The formate formation is explained by the insertion of C02 into a Pd—H bond to form Pd-formate species. Tetraethylurea (110) and diethylformamide (111) are obtained by the reaction of Et2NH and C02[99]. 

Figure 8.1. Relation of solubility parameters (solpars or Hildebrand 8 values) and carbon numbers in various homologous series of solvents. (4) Normal alkanes, (B) normal chloroalkanes, (C) methyl esters, (D) alkyl formates and acetates, (E) methyl ketones, (F) alkyl nitriles, ) normal alkanols, (H) alkyl benzenes, and (I) dialkyl phthalates.

Alkyl formates or formic acid and its esters can be converted to trialkyl orthothioformates [74-77] which in turn can be converted to trialkyl orthoformates in good yields [78, 79], It has been reported that acid chlorides of higher carboxylic acids can also be converted to trialkyl orthothioformates [80], but thus far no reports appear in the literature on attempts to convert them to trialkyl ortho esters. 

HydroxymcthyIcnecycIohcxanonc has been prepared by the reaction of cyclohexanone and alkyl formates.2 4... 

If no asymmetric induction takes place in the diastereomeric alkyls formation, the chiral aldehyde resulting from the three olefins must have the same chirality and the same optical purity. The experiments indicate (Table IV) the opposite result. Using the same chiral ligand [( — )-DIOP] the aldehyde obtained from 1-butene has prevailing [(R)] chirality while the same aldehyde arising from the two 2-butenes has prevailing [ (S) ] chirality. Furthermore, the two aldehydes obtained from cis-butene and trans-butene under the same reaction conditions have different optical purity (8.1 and 3.2% respectively). These results imply that the diastereomeric composition of the mixture IX(S) + IX,R) depends on the type of the starting C4 olefin and that for at least two of the olefins used the asymmetric induction occurs, at least in part, in the alkyl-rhodium complex formation. 

The results do not prove that in the reaction conditions used the alkyl formation is not reversible, but only that, if it is reversible, the carbon monoxide insertion on both diastereomeric rhodium-alkyls must be much faster than the rhodium-alkyls decomposition. Restricting this analysis of the asymmetric induction phenomena to the rhodium-alkyl complexes formation, two 7r-olefin complexes are possible for each diastereomer of the catalytic rhodium complex (see Scheme 11). The induction can take place in the 7r-olefin complexes formation (I — II(S) or I — II(R)) or in the equilibrium between the diastereomeric 7r-olefin complexes (II(r) and... 

So far the intermediates of the hydrogenation of olefins have been estimated by a hydrogen exchanged between D2 and olefins during the hydrogenation reaction, where the steps of the alkyl formation in Scheme 14 are assumed to be reversible and to be the main route of the hydrogen-exchange reaction. A historical reaction of olefins with D2 on nickel catalyst... 

Several reports have described the formation of alkyl formates or form-amides from C02, hydrogen, and an alcohol or amine. The earlier catalytic preparations have been reviewed (108). A recent paper describes the production of alkyl formates catalyzed by several transition metal complexes and tertiary amines under 25 atm each of C02 and H2 at 140°C, (78) (159). 

It should be noted that the production of formic acid from C02 and H2 proceeds with a net increase in free energy under ambient conditions, but that an increased pressure of H2 and C02 will shift the equilibrium favorably. When alkyl formates or formamides are produced from C02, H2, and the amine or alcohol, the stability of the water formed in the reaction provides a powerful driving force, and the thermodynamics of these reaction are favorable under ambient conditions. 

More recently another report on the catalytic conversion of C02, H2, and alcohols into formate esters has appeared (160a). This work uses as catalysts the anionic iron carbonyls HFe(CO)4" and HFe CO), and reports modest conversions to alkyl formates under conditions of elevated pressure and temperature. 

Esters are far less reactive as electrophiles when compared to aldehydes and ketones. Successful tandem vicinal dialkylations are possible using alkyl formates,67 but most esters lack the needed reactivity. More reactive thioesters can serve as electrophiles in these sequences.208 Presence of a potentially electrophilic ester group as a substituent in the conjugate enolate permits very efficient Dieckmann cycliza-tion to take place as the second step of a MIRC sequence (e.g. equations 5118 and 52).24 Ortho esters are far more reactive, giving p-keto esters as adducts when used in sequences that employ enones as substrates.230... 

---