Perfluorinated enolates

Although nitrosation of (l-dicarbonyl compounds becomes increasingly more facile upon successive replacement of the a alkyl groups with perfluoroalkyl groups because of the increased ionization of the perfluorinated enolate (equation 7), the stability of the nitrosodiketone tautomers decreases Thus, 1,1,1-trifluoro pentane-2,4-dione and 1,1,1,5,5,5-hexafluoropentane 2,4 dione mtrosate much faster than penta-2,4 dione but yield ketoximes, which decompose upon workup... 

In sharp contrast to the prominent position of metal enolates in synthetic organic chemistry, the chemistry of perfluorinated enolates (F-enolates) remains relatively unexplored, mainly because of the lack of practical methods for their generation. Only a few examples of metal F-enolates have been recorded in the literature. (7, 2). We believe that perfluoro-enolate chemistry can play an equally important role in organofluorine synthesis (Figure 1). In this paper we wish to describe the recent advances in ketone F-enolate chemistry which have been made in our laboratory. 

An investigation of keto-enol tautomerism for perfluorinated keto-enol systems was undertaken. N-methylpyrrolidone (NMP) catalyzes equilibration of the keto and enol forms, but if used in more than trace amounts, it drives the equilibrium strongly toward enol because of hydrogen bonding to the amide. The enol is much more thermodynamically stable than its ketone, and it was found that in mildly Lewis basic solvents, such as ether, THE, acetonitrile, and NMP, the enohzation equilibrium lies too far right to allow detection of ketone (Correa et al., 1994). 

The nucleophilic reactivities of silyl enol ethers (58, R1 = alkyl) and silyl ketene acetals (58, R1 = 0-alkyl) have been measured for the triphenylsilyl (R2 = H5) substrate, and its perfluoro analogue (R2 = F5), using benzhydrylium cations as reference electrophiles.224 The triphenyl compound is 10 times less reactive than its trimethyl equivalent, but the perfluorination causes the C=C nucleophilicity to drop by 3-4 orders of magnitude. The new compounds have been placed on scales of nucleophilicity taken from the literature. 

In this chapter, we focus on the effect of fluorine as a substituent in a simple polymeric system, polyacetylene. Polyacetylene, of course, has several potentially practical uses because of its conducting and optoelectronic properties (15) and we are interested in studying how F substitution might influence these properties. Our model systems are butadiene and hexatriene, and we discuss both partially fluorinated and perfluorinated materials. Because we discovered that CF - HC hydrogen bonding is important in these systems, we also present results on the nature of the intramolecular hydrogen bond between the CF and OH groups in alcohols and enols. Related results on intramolecular coordination of alkali metals to C-F bonds in fluoroenolates are briefly described. 

The Mannich and related reactions provide one of the most fundamental and useful methods for the synthesis of P-amino ketones or P-amino esters. Three-component Mannich-type reactions of aldehydes, amines, and silyl enol ethers have been developed. With the development of green solvent systems, this reaction was also examined in a fluorous phase using perfluorinated rare earth metal salts including Sc(0S02C8Fi7)3 [5]. A characteristic point of this system is that it can be reused many times without reloading a new catalyst. There are also many reports on other scandium-catalyzed Mannich reactions. For example, Sc(OTf)3 was found to be an efficient catalyst for the three-component Mannich-type reactions of aromatic aldehydes, ketones, and nitriles in the presence of acetyl chloride (Scheme 12.3) [6]. 

The best yield was achieved by the following method. Following the reaction of diethyl methylphosphonate and perfluorinated carboxylic ester, a 69 mmol solution of the resultant ketone-enol mixture was added to 100 mL of anhydrous methylene chloride. It was then cooled to -40°C (nitrogen), diisopropylmethylamine 32.1 g (248 mmol) was quickly added, and after 10 minutes stirring, 23.4 g (83 mmol) tri-fluoromethanesulfonic anhydride was added dropwise at -30°C. The suspension was kept for 3 hours at —30°C and 3 hours at 5—7°C, then... 

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