Trifluoroethanol as solvent

The use of trifluoroethanol as solvent or absorption of the dienone on silica gel promotes the photoconversion of dienones into bicyclic ketenes.<47) For the photolysis<48 60) of (63) it has been shown by low-temperature infrared and ultraviolet spectroscopy that the initial photolysis gives a ketene which can be efficiently trapped by cyclohexylamine or, in the absence of a good nucleophile, thermally rearranges by a OA, + 20) allowed process to a bicyclic ketone (64) ... 

The bromination of /razw-stilbenes in methanol, trifluoroethanol and in acetic acid leads almost exclusively to the erythro adducts via a 100% anti addition, regardless the substituents and the solvent. In contrast, the stereochemistry of the reaction of c/s-stilbenes exhibits a considerable dependence on the substituents and on the solvents the reaction of p-methoxystilbenes is always stereo-convergent, that of p,p -bis(trifluoromethyl)stilbenes is stereo-specific in all investigated solvents, whereas unsubstituted stilbenes can produce variable stereochemical outcomes ranging from stereo-specific to stereo-convergent in going from methanol to trifluoroethanol as solvent. 

In addition, the use of trifluoroethanol as solvent has been shown to provide very high yields of epoxides, both teiminal and internal, as exempUfied by the oxidation of vinylcyclohexane (24) <99CC821>. 

Higher alcohol formate esters do not appear here because an equivalent amount of methanol is consumed in the homologation process which produced them.) In reactions using trifluoroethanol as solvent, the observed... 

Protection of phenols by the foregoing methods is complicated by the rapid Friedel-Crafts rearrangement of the nascent rm-butyl ether. By using trifluoro-methanesulfonic add at -78 PC, the rate of /erf-butyl ether formation is fast and the Friedel-Crafts alkylation does not compete [Scheme 4.126].226 Similarly, attempts to deprotect phenol ferf-butyl ethers with trifluoroacetic acid or titanium tetrachloride may give complex mixtures, again as a result of Friedel-Crafts alkylation of the phenol but this side reaction can be suppressed by using a catalytic amount of trifluoromethanesulfonic acid in 2.2,2-trifluoroethanol as solvent at -5 DC. 

Enantiomeric distinction can also be achieved using chiral solvents without shift reagents. The determination of enantiomeric purity and the assignment of absolute configuration of cyclic and acyclic sulphinate esters has recently been achieved (573) using chiral l-aryl-2,2,2-trifluoroethanols as solvents. Enhanced enantiomeric distinction has been demonstrated using a combination of both a chiral solvent and an achiral LSR. (514)... 

For a few combinations of less reactive dienes and dienophiles, transition metal catalyzed variants of the Diels Alder reaction have been developed. An example is the cycloaddition of an unpolar diene and an unactivated alkyne however, except when the reaction is catalyzed with iron, nickel, cobalt, or rho-dium(I) complexes, the temperature required often causes competing decomposition, even for the intramolecular version. [2] Wilkinson s catalyst [3] - tris(triphenylphosphane)rho-dium(I) chloride - frequently used for hydrogenations and for decarbonylations, permits the cyclization of 4 to the annelated cyclo-hexadiene 5 in excellent yield in only 15 minutes at 55 °C in trifluoroethanol as solvent (Scheme 2). [2c]... 

In addition to the ahove-quoted methods, tertiary 3-lactams have heen reduced to the corresponding cyclic amines hy dropwise addition of MeOH to the refluxing mixture of NaBHa and substrate in t-BuOH, or hy using trifluoroethanol as solvent. This reaction was applied during a synthesis of indolizidine alkaloid swainso-nine for the reduction of lactam (1) to amine (2) (eq 5). ... 

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