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Phase Transfer Catalyst Pdf 15 __LINK__

The first few reports on successful synthesis of fullerenol dated back in 1992 [3, 4]. To date, there are several different methods to produce fullerenol from C60 fullerene, which vary in terms of reaction conditions, complexity, duration and operational safety [5,6,7,8,9]. One of the most frequently selected methods for the synthesis of fullerenol is the hydroxylation of fullerene through phase-transfer catalysis using tetrabutylammonium hydroxide (TBAH) as phase-transfer catalyst and sodium hydroxide (NaOH) solution as the source of hydroxide ions, reported by Li et al. [5]. The process appears rather simple and safe for operation, compared to several other alternatives. Nevertheless, the process still has not been thoroughly studied or understood.

phase transfer catalyst pdf 15

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A good understanding of the production of any type of material is crucial for its further research and development. This study therefore conducted preliminary experiments to gain further understanding of the process. The study focused on the process response (in terms of the achieved level of hydroxylation, i.e. number of hydroxyl groups successfully added onto the fullerene molecule) to the amount of phase-transfer catalyst TBAH and the amount of the source of hydroxide ions. Problems associated with contamination from unwanted reactions with carbon dioxide and ozone (previously published as an abstract for a conference [10]) arose during the time when the experiments were conducted and are also discussed in this article. The discovered phenomena formed important bases for the establishment of precautions for the process.

Ideally, variation of concentration should be applied in order to avoid errors on the total volume of the reaction mixture. Nevertheless, such concept encountered some practical limitations which prevented effective operation and could lead to more severe errors. Considering the facts that the volume of NaOH solution (100% W/V) used in the original synthesis is 20 ml [5] and that solubility of NaOH at 20 C is 109 g per 100 ml [35], concentration beyond 100% W/V leads to viscosity and solubility issues. The highly concentrated solution would be very viscous hence difficult transfer and could lead to greater errors from the total amount of NaOH in deficit. Consequently, variation of volume was used instead.

Phase-transfer catalysis with TBAH and NaOH solutions is often chosen by several researchers as the method to produce fullerenol. However, the process itself is still not very much understood. This study therefore aimed to gain a better understanding of the reaction and phenomena associated with the process through preliminary experiments on two selected process parameters (amount of TBAH and NaOH used for synthesis) and the response from the process in terms of the achieved level of fullerene hydroxylation.

It is worth to note that besides chiral cation-based phase-transfer catalysts (like the aforementioned onium catalysts, which are by far the most commonly employed chiral PTCs), also the use of chiral anion-based PTCs has recently attracted a lot of attention and lead to a variety of highly versatile methods for asymmetric catalysis [24,55-63]. In contrast to chiral cation-based PTCs, which mainly operate through coordination and control of the nucleophile, these anionic PTCs usually coordinate cationic (and often hardly soluble) electrophilic reagents and this complementary strategy has as well been rather impressively used for different asymmetric heterofunctionalization reactions. Accordingly, also some selected very recent examples for the use of chiral anion-based PTCs for asymmetric α-heterofunctionalization reactions of prochiral nucleophiles will be discussed herein.

The stereoselective electrophilic α-fluorination of carbonyl compounds became a thoroughly investigated field over the course of the last 15 years [68-72]. A variety of different catalytic approaches, either relying on the use of chiral metal complexes, or chiral organocatalysts have been reported, and the use of chiral PTCs became a powerful strategy herein too [44,56,57,73-79].

The groups of Ma and Cahard have intensively investigated the use of chiral spirocyclic phosphonium salts F as phase-transfer catalysts for asymmetric α-heterofunctionalization reactions [79,80]. Hereby they also reported the fluorination of 3-substituted benzofuranones 4 by using NFSI (2) as the electrophilic F-transfer reagent [80]. The reaction could be carried out in excellent yields and with modest enantioselectivities for a rather broad substrate scope when using just 2 mol % of phosphonium salt F1 under liquid/liquid biphasic conditions (Scheme 3).

The stereoselective synthesis of chiral α-chlorinated carbonyl compounds is an important topic, since these targets can be valuable intermediates for the synthesis of a variety of different biologically active molecules [85,86]. The main interest in these compounds comes from their versatility for further functionalizations by stereospecific nucleophilic SN2-type displacement reactions with different nucleophiles [66,85-87]. Accordingly, it comes as no surprise that their asymmetric synthesis has been intensively investigated in the past, either relying on asymmetric (transition)-metal- or organo-catalysis [76,85-94]. In sharp contrast to the numerous reports describing the use of asymmetric phase-transfer catalysis for α-fluorination reactions (as stated above), the use of chiral PTCs for enantioselective α-chlorinations of prochiral nucleophiles has been much less systematically investigated so far [76,90,92].

In 2013, Maruoka and co-workers introduced a new family of bifunctional phosphonium salts J which turned out to be highly active catalysts for different α-heterofunctionalization reactions of ketoesters 1 under base-free conditions [90]. Key to success in this spectacular report was the use of the systematically optimized sulfonamide-containing phosphonium salt J1. With this catalyst, they were able to achieve high selectivities in the α-chlorination of 1 with N-chlorophthalimide (13) as the Cl-source under H2O-rich conditions, as outlined in Scheme 6. Around the same time, the groups of Meng and Lu also demonstrated that the cinchona alkaloid-based catalyst A2 not only allows them to carry out the above described asymmetric α-fluorination reactions (Scheme 2), but also holds promise for the highly enantioselective α-chlorination of β-ketoesters 1, by using N-chlorosuccinimide (14) as the Cl-transfer agent [76]. In continuation of our efforts to develop bifunctional ammonium salts D for asymmetric catalysis we have also recently investigated the asymmetric synthesis of α-chloroketoesters 12 [92]. We found that catalyst derivative D2 performs best for this purpose, even with rather low catalyst loadings, but it has to be admitted that in general the enantioselectivities are lower than in the two other case studies shown in Scheme 6 [92].

Besides those methods that make use of already α-functionalized carbonyl compounds, the direct stereoselective α-oxygenation or α-hydroxylation of simple prochiral nucleophiles with either oxygen as such, or an electrophilic oxygen species became by far the most important and most thoroughly investigated strategy. Hereby both, approaches relying on either asymmetric metal- or organocatalysis, have been well-investigated already [105-122]. In the field of non-covalent asymmetric organocatalysis, chiral H-bonding catalysis [37,121,122] and chiral phase-transfer ion-pairing catalysis [110-119] turned out to be extremely powerful.

Besides making use of oxygen or air together with a stoichiometric activator/reductant as described above (Scheme 7 and Scheme 8), the photooxygenation of prochiral substrates like β-ketoesters 1 with O2 or air in the presence of a chiral PTC and TPP (tetraphenylporphyrin) as a photosensitizer has recently been reported to proceed with satisfying selectivities by the groups of Meng and Gao [114,115]. In their first report [114], they made use of the classical cinchona catalyst A5 together with catalytic amounts of TPP under irradiation with a 100 W halogen lamp with air as the oxygen source. Very recently, they then introduced the N-oxide-containing PTC A6, which gave even higher selectivities and was successfully used under yellow LED (3 W) irradiation [115] (Scheme 9).

All the approaches described so far made use of O2 or air as the oxygen source, which is of course the most economical way of carrying out oxidations. However, there have also been several rather successful and highly enantioselective reports that describe analogous α-hydroxylation reactions by using alternative oxygen-transfer reagents (Scheme 10). A few years ago, Meng et al. carried out the α-hydroxylation of substrates 1 by using cumyl hydroperoxide (22) as an easily available oxidizing agent, which worked well to access a series of differently substituted products 21 in high yields and with high enantioselectivities when using the carefully optimized sterically demanding PTC A2 [112,113]. Our group has very recently investigated the use of bifunctional catalysts D for such α-hydroxylations [116]. Hereby we realized that oxaziridines like compound 23a are versatile reagents for this reaction, giving access to products 21 with excellent enantioselectivities under base-free conditions (for selected other uses of oxaziridines in asymmetric α-hydroxylation reactions please see [123,124]). Interestingly, this transformation is accompanied by a kinetic resolution of the employed oxaziridine with s-factors up to 45 [116].

One potentially useful simple reagent to carry out oxygen-transfer reactions is hydrogen peroxide (H2O2). Unfortunately, the direct use of this base-chemical under asymmetric organocatalysis turned out to be rather tricky for α-hydroxylation reactions. One recent report by the Ooi group overcame some of the limitations by using H2O2 in combination with trichloroacetonitrile (Cl3CCN) [118]. This combination leads to the in situ formation of peroxy imidic acid 24, which then serves as the O-transfer reagent for the asymmetric α-hydroxylation of oxindoles 17 in the presence of the chiral triazolium-based ion pairing catalyst L1 (Scheme 11).


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