Fluorinated ylides

Fluonnated ylides have also been prepared in such a way that fluonne is incorporated at the carhon P to the carbamonic carbon Vanous fluoroalkyl iodides were heated with tnphenylphosphine in the absence of solvent to form the necessary phosphonium salts Direct deprotonation with butyUithium or hthium dusopropy-lamide did not lead to yhde formation, rather, deprotonation was accomparued by loss of fluonde ion Flowever deprotonation with hydrated potassium carbonate in thoxane was successful and resulted in fluoroolefin yields of45-S0% [59] (equation 54) P-Fluorinated ylides may also be prepared by the reaction of an isopropyli-denetnphenylphosphine yhde with a perfluoroalkanoyl anhydnde The intermediate acyl phosphonium salt can undergo further reaction with methylene tnphenylphosphorane and phenyUithium to form a new yhde, which can then be used in a Wittig olefination procedure [60] (equation 55) or can react with a nucleophile [6/j such as an acetyhde to form a fluonnated enyne [62] (equation 56)... 

Carbomethoxymethylene or benzoylmethylene triphenylarsorane reaction with perfluoropropylene oxide to give the corresponding fluorinated ylides (27). The following mechanism was suggested (44) ... 

Fluorinated ylides also undergo addition to fluoroalkyl ketones however, this olefination procedure may be accompanied by considerable side reactions resulting from deprotonation of the fluoroalkyl ketones. In most cases the yields arc not better than 30 / . ... 

Whereas access to fluoromethylene ylides is easy, higher homologues are more difficult to prepare because ot the facile (1-elimination of fluorine However hexafluoroisoprapylidene ylides may be prepared in situ from tetrakisftnfluoro-methyl)-l,3-dithietane and tnphenylphosphme [54 (equation 51) or 2,2-dichloro-hexafluoropropane and tnphenylphosphme [55, 56, 57 (equation 52) (Table 21) Pcrfluoro-2-buteiie reacts with tnbutylphosphine to cleanly form a solution of a fluonnated ylide [5/ ] (equation 53)... 

Fluorinated alkynes are readily prepared by treatment of the mtermediate formed by acylation with a second equivalent of ylide followed by pyrolysis [63. 64, 65] (equation 57)... 

Reactions offluorinated dipoles. In recent years, much effort has been devoted to the preparation of tnfluoromethyl-substituted 1,3-dipoles with the goal of using them to introduce trifluoromethyl groups into five-membered nng heterocycles Fluorinated diazoalkanes were the first such 1,3-dipoles to be prepared and used in synthesis A number of reports of cycloadditions of mono- and bis(tnfluo-romethyl)diazomethane appeared prior to 1972 [9] Other types of fluonne-substi-tuted 1,3-dipoles were virtually unknown until only recently However, largely because of the efforts of Tanaka s group, a broad knowledge of the chemistry of tnfluoromethyl-substituted nitrile oxides, nitnle imines, nitnle ylides, and nitrones has been accumulated recently... 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

Possibly, the most common protocols used in the generation of azomethine ylides are those based on the in situ, fluorine-mediated desilyation of cyanoami-nosilanes developed by Padwa et al. (2). Typically, treatment of precursor 1 with AgF, in the presence of dimethyl acetylenedicarboxylate (DMAD), led to the formation of the intermediate cycloadduct 2, which was subjected to immediate DDQ oxidation to give pyrrole 3. The mechanistic rationale invokes fluoride-mediated desilyation to form the intermediate anion 4, which then undergoes loss of cyanide furnishing the corresponding azomethine yhde (Scheme 3.1). 

A more recent report has outlined the use of a-silylimidates for the construction of aromatic pyrroles (7). Treatment of the precursor 29, with trifluorophenylsilane and DMAD furnished the adduct 30 in 97% yield after purification. The reaction was rationalized via quaternization of the imidate and subsequent intramolecular desilylation by fluorine to develop the ylide, which underwent in situ cycloaddition and subsequent aromatization delivering 30 (Scheme 3.7). 

Hashimoto and co-workers (139) further looked at an intermolecular carbonyl ylide cycloaddition screening several different chiral rhodium catalysts. The Hashimoto group chose to study phthaloyl amino acid derivatives for enantiocon-trol of the cycloaddition reactions (Fig. 4.8). Using fluorinated or ethereal solvents with the phthaloyl catalysts gave ee ratios of 20-69%. 

Fluorinated dicarbonyl triphenylphosphoranes, easy to prepare (by trifluoro-acetylation of the stabilised ylides), have been transformed stereoselectively into / -substituted jS-trifluoromethyl alkenoates by treatment with organolithium reagents (Eq. 115) [306]. The stereoselectivity could be reversed by O-methyla-tion of the initial adduct followed by protonation [307]. 

Tris(diethyl)amino]phosphoniuni 2,2,3,3,4,4-hexafluorocyclobutane ylide (2) is a very hydroscopic solid, which is stable for long periods of time when stored under anhydrous conditions.13 It is unreactive towards most functional groups such as C = C bonds, ketones, aldehydes, and esters but reacts smoothly under neutral conditions with alcohols or carboxylic acids with replacement of the hydroxy group by a fluorine atom.44,45 Other tris(dialkylammo-nium)phosphonium perfluorocyclobutanes have been synthesized however, they are less reactive fluorinating reagents.44,45... 

Triethylammonium 2,2,3,3,4,4-hexafluorocyclobutane ylide (5) is a mild fluorinating agent that has identical properties to [tris(diethyl)amino]phosphonium 2,2.3,3,4,4-hexafluorocyclobutane ylide (2) and gives comparable yields for fluorination of alcohols and carboxylic acids.44... 

Dixon and Smart71 examined a series of fluorine-substituted phosphonium ylides. Fluorine stabilizes carbanions via an inductive effect, favouring pyramidal carbanions. Dixon and Smart argued that the first substitution of F on the ylidic carbon (H3P=CHF) causes the carbon to become very pyramidal (the sum of the angles at C is 338.5°), which reduces the potential overlap of the anionic orbital with any P orbitals. The leads to the long P—C distance of 1.723 A. The second F substitution (H3P=CF2) actually breaks the P—C bond and the system is best described as a weak interaction of phosphine with CF2. Trifluoromethyl groups act to stabilize the anions via hyperconjugation. This leads to a planar ylidic carbon in H3P=C(CF3)2. 

These ylides, e.g. 6, can also be described as carbenoids stabilized by coordination to phosphorus.143 Fluorinated carbenes without 7t-electron-donating substituents are typically ground-state triplet species however, perfluoroalkylcarbenes, due to their kinetic stability, can be characterized spectroscopically.13S-139-144... 

Chemistry of Fluorine-Containing Phosphine and Arsenic Ylides and Their Salts ... 

Fluoroolefins may be prepared by the reaction of Wittig reagents and other pho sphorus-contaimng ylides with fluorinated carbonyl compounds. (A discussion of the fluorinated Wittig reagents or other fluorinated phosphorus reagents with nonfluonnated carbonyl compounds is on page 581.) Tnphenylphosphoranes, derived from alkyltriphenyl phosphonium salts, react with 1,1,1-trifluoroacetone [3/] or other trifluoromethyl ketones [32, 33] (equation 26) (Table 10). 

For arsonium ylides 1 and 2 the molecular ion peak and (M - 1)+ peak were characteristic, whereas for the fluorinated arsonium ylides (3 to 9) they were not discernible at all. 

---