Pyrroles fluoride

When information was available, calix[4]pyrrole-fluoride interactions were discussed on the basis of 1H NMR, conductometry, titration calorimetry, UV-Visible and fluorescence spectroscopy. 

Sessler. J.L. Tetrabutylammonium me. o-octamethylca-lix[4]pyrrole fluoride dichloromethane solvate. Acta Cryst. 33. 2001. E57 (9), 08I6-08I8. 

In the area of ion sensing, cation recognition by electrodes containing functionalized redox-active polymers has been an area of considerable interest. Fabre and co-workers have reported the development of a boronate-functionalized polypyrrole as a fluoride anion-responsive electroactive polymer film. The electropolymerizable polypyrrole precursor (11) (Fig. 11) was synthesized by the hydroboration reaction of l-(phenylsulfonyl)-3-vinylpyrrole with diisopinocampheylborane followed by treatment with pinacol and the deprotection of the pyrrole ring.33 The same methodology was utilized for the production of several electropolymerizable aromatic compounds (of pyrrole (12) (Fig. 11), thiophene (13 and 14) (Fig. 11), and aniline) bearing boronic acid and boronate substituents as precursors of fluoride- and/or chloride-responsive conjugated polymer.34... 

The pyrrole-3-boronic acid 67 has been prepared from the 3-iodopyrrole by Muchowski and subjected to a range of Suzuki couplings to afford 2-arylpyrroles 68 [59]. Subsequent fluoride deblocking to give 69 occurs in excellent yield. 

Muchowski has utilized Af-TIPS-3-iodopyrrole to prepare a series of 3-alkynyl pyrroles 119 using standard Sonogashira conditions [59]. Pyrroles 120 were obtained after fluoride cleavage of the TIPS group. Similarly, 3,4-bis(alkynyl)pyrroles were prepared from N-TIPS-3,4-diiodopyrrole [59]. 

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). 

Similarly l-chloro-2,3,4,5-tetrakis(trifluoromethylsulfanyl)pyrrole (14) is converted to the 1-fluoro derivative 15 by reaction with sodium fluoride in the presence of antimony(III) oxide in 76% yield.16... 

A mixture of silver(I) fluoride and 2,2 -bipyridine replaces chlorine with fluorine in 1-chloro-octane in 86% yield after 45 minutes at 130°C.28 Some heterocyclic polychloro derivatives are also fluorinatcd by silver(I) fluoride. Thus, perfluoro(2,3,4,5-tetrahydropyridine) (1) and per-fluoro(3,4-dihydro-2//-pyrrole) are prepared from 2,2,6-trichloro-3,3,4,4,5,5-hexafluoro-2,3.4,5-tetrahydropyridine and 2,2,5-trichloro-3,3,4.4-tetrafluoro-3,4-dihydro-2//-pyrrole by reaction with silver(l) fluoride at room temperature.31 Since the fluorination of 2,2,5-trichloro-3,3,4,4-tetrafluoro-3,4-dihydro-2//-pyrrole with silver(l) fluoride is slow, recycling is necessary.31... 

Methyl-1 //-pyrrole gives93 a very similar product mix Over cobalt(III) fluoride at 140 C to 1-methylpyrrolidine,92 suggesting that the latter is converted to the former by some sort of desaturation reaction. Fluorination over potassium tetrafluorocobaltate(III) at 220 C gives a similar result.93... 

However, of all the salts tested (RbCl, CsF, CS2CO3 0.2 mol per 1 mol KOH), only cesium fluoride shows a perceptible catalytic effect (yield gain up to 8%) at the stage of pyrrole formation for only the first three hours (Fig. 2, curve 3). The observed effect can be explained in terms of poor solubility of potassium fluoride in DMSO (86MI1). 

Crystals of both compounds were obtained by slow evaporation of acetonitrile solutions in the presence of excess tetrabutylammonium fluoride. It was determined that both compounds crystallized as the tetrabutylammonium salts of the doubly deprotonated pyrrole anions Figure 12). Each individual dianion essentially possesses a planar conformation and forms the interlocking arrangement previously seen Figure 10). 

In order to overcome this oxidation problem the synthesis of 21 and 22 was achieved in which two methyl groups occupy the positions bridging the two pyrrolic groups (Figure 20).16 Although compounds 21 and 22 were found to be stable, less stable complexes with anions were formed than with the first generation dipyrrolylmethane systems. However, despite this reduction in affinity, the stability constants for these receptors could only be calculated in DMSO-c/6/5% H20 for fluoride (124 and 89 M 1), dihydrogen phosphate (1092 and 81 M"1), benzoate (1092 and 81 M"1) calculated for receptors 21 and 22 respectively. 

Other early work in this field included the use of tetrakis(p-aminophenyl)-porphyrin (Fig. 7a), which was electrodeposited onto glassy carbon and showed a near-Nernstian response to iodide [76]. Electrodeposited methylthiophene-methylpyrrole copolymer was deposited and shown to give a near-Nernstian response to bromide [77]. Pyrrole-3-boronate (Fig. 7b) could be deposited to give films with a good response and marked selectivity to fluoride [78]. A cobalt aminophthalocyanine could also be electropolymerised to give a good sensor for nitrite [79] or sulphide ion [80]. 

The next stage of synthesis was the replacement of pyrrole hydrogen atoms by fluorine atoms. The zinc complex was fluorinated by cobalt fluoride according to the appropriate technique [44, 46], The target product was extracted from reaction products by chromatographic separation in a column filled with neutral aluminum oxide. 

Triazines are trimers of unstable imines and may serve as imine precursors. Treatment of trimer of A1-pyrrol ine with a trimethylsilylmethyl triflate gives trimethylsilylmethyl-imonium triflate which may be desilylated by cesium fluoride, providing an ylid suitable for 1,3-dipolar cycloaddition reactions and constmction of the hexahydro-pyrrolizine framework.386 This strategy has been applied to prepare trachelanthamidine, supinidine and isoretronecanol alkaloids.387... 

Quinones have also been used as dipolarophiles, giving pyrrole derivatives after oxidation with excess silver fluoride. Reineta isoindole, a naturally occurring antimicrobial compound, was synthesized by this route in good yield.444... 

IV-silylmethyl a-cyanoimines have been studied as a precursor of nitrile ylids. They undergo facile cycloaddition with dipolarophiles. Thus reaction with DM AD in the presence of silver fluoride leads to the formation of the corresponding pyrrole derivative in good yield, but no reaction occurs with the non-activated n-bond of styrene.453... 

Condensation of an MSMA isonitrile with an aroylfluoride gives a salt which undergoes loss of trimethylsilyl fluoride to form a nitrile ylid. This transient species reacts with DMAD to form 2-aroyl pyrrole in high yield.162 Substitution of acyl chloride for acyl fluoride in the reaction affords only poor yields of adducts.465... 

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