Fluorination esters

Fluorinated esters have synthetic utility m Claisen condensations [24, 25] (equation 21) and Dieckmann cyclizfitions [26]. 

Ketene acetals prepared from fluorinated esters by trimethylsilylation undergo Lewis acid-promoted aldol condensations giving satisfactory yields but low diastereoselectivity [27] (equation 22). 

Fluorinated esters may also act as electrophiles in reactions with nonfluori-nated ketones [28] (equation 23) or malononitrile [29] (equation 24). Unfortunately, the yields of -diketones may be modest, but those of p-keto nitnles are excellent (Table 9)... 

The reactions of fluorinated esters and amides to form, respectively, enol ethers [47] and enamines [4S] give high yields and are interesting synthetic transformations (equations 34 and 35) (Table 12)... 

Table 12. Fluorinated Esters and Amides in the Wittig Reaction to Form Enol Ethers [47 and Enamines 48 ...

In addition to the successful hydrogenation of the two fluorinated esters, this report describes the hydrogenation of dimethyl oxalate. Using the reactive anionic ruthenium catalyst, a 70% conversion to methyl glycolate could be achieved (TON = 235 TOF 12 h-1) (Scheme 15.19, Table 15.14, final entry). 

Aromatic and a-substituted aliphatic aldehydes react with oxiranes under neutral conditions to yield 1,3-dioxoIanes [35] whereas a-unsubstituted aldehydes undergo a simple aldol condensation under such conditions. In a somewhat similar manner, perfluorocarboxylic esters react with oxiranes to produce cyclic orthoesters (-50%) [36]. The corresponding reaction with non-fluorinated esters fails. 

Linear Solvents. For novel solvents that are intended to replace linear carbonates, the emphasis was placed on such properties as melting temperature, viscosity, and inflammability of the solvents. As Table 12 shows, these solvents can be roughly classified into four groups (1) linear esters or carbonates,(2) fluorinated esters,(3) fluorinated carbonates," and (4) fluorinated ethers." " ... 

Table 5. Lipase-catalyzed Asymmetric Hydrolysis of Fluorinated Esters...

For reviews of triflates. nonaflates. and other fluorinated ester leaving groups, sec Stang I lanack Subramanian Synthesis 1982, 85-126 Howells Me Cown Chem. Rev. 1977, 77, 69-92, pp. 85-87 wCrossland Wells Shiner J. Am. Chem. Soc. 1971, 93, 4217. 

The type of products formed in reactions of /(-hydroxy acids with sulfur tetrafluoride depends on the substituents on the carbon alom adjacent to the hydroxy group. Acids such as 3-hy-droxypropanoic acid (21) and 3-hydroxybutanoic acid (22) give, in addition to 1,1,1,3-tetra-fluoroalkanes, mainly dimeric products, i.e. fluorinated esters or ethers, together with small amounts of unsaturated acid fluorides. In contrast, n7C -4,4,4-trifluoro-3-hydroxybutanoic acid (23), depending on the reaction conditions, affords 3,3,3-trifluoro-l-(lrifluoromethyl)propyl fluorosulfite or a mixture of 1,1,1.2,4,4,4-hcptafluorobutanc and l,l,1.4.4,4-hcxafluorobut-2-ene.116... 

The only known method enabling a selective replacement of a-fluorine substituents in aliphatic polyfluorinated esters without affecting the carbonyl group is pholoinduced reduction (e. g., 1 - 2) the reaction yield is dependent on the solvent (hydrogen donor) used, e. g. propan-2-ol > tetrahydrofuran > 1,3-dioxolane.106 The reactivity of fluorinated esters is also dependent on the group attached to the /(-carbon, e. g. 3 > 4 > 5.106... 

The reaction of a citronellic enolate—a potential precursor to a 5-hexenyl-type radical clock—with various fluorinating agents leads exclusively to the corresponding a-fluorinated ester derivatives. The complete absence of cyclic fluorinated products is proof that the fluorination does not occur via radical intermediates, while the formation of a cyclic product in the reaction with xenon difluoride is an indication that electron transfer is a competitive process which does not give a fluorinated product31 (Scheme 9). 

Fluorinated esters have synthetic utility in Claisen condensations, c.g. formation of 26, - and Dieekmann cyclizations. ... 

Acid chlorides- react with alcohols to form fluorinated esters. Acid bromides " are less regularly used. If a base is necessary, tricthylamine - or pyridine [with... 

Using these approaches, the successful further fluorination of partly fluorinated esters has been cleverly developed into a process for the synthesis of the important copolymer component perfluoro(propyl vinyl ether), PPVE [131] (Figure 2.25). 

Acid chlorides230 256 react with alcohols to form fluorinated esters. Acid bromides257 - 259 are less regularly used. If a base is necessary, triethylamine252 255 or pyridine240-250 [with 4-(dimethylamino)pyridine251] are commonly utilized to trap the acid. Addition of a stabilizer is advantageous for the synthesis of a,/i-unsaturated esters, e.g. 10 from 9.234-255... 

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