Additives HFiP

Best results were obtained when HFIP (42) was added. Furthermore, this conversion is very clean and 2,4-dimethylphenol (37) is transformed to 38 in 47% yield. Application of abundant electric current renders a lower yield and product quality of 38. A trifluoromethyl group of the additive can be substituted by a phenyl moiety in order to stabilize the oxyl spin center. Therefore, alcohol 43 provides results similar to the additive HFIP. In conclusion, the chemical yield reached about 50%, a maximum when a current of 1—1.3 F was applied per mole 37 and represents a reasonable compromise between yield and current efficiency [27] (Scheme 19) (Table 2). 

Controlling for these forces requires variation in the amount of salt, organic solvent, and the pFI of the mobile phase. It is impractical to perform such experiments with 50 mM formic acid an alternative additive must be used that maintains its chaotropic properties independent of salt content or pFI. Fortunately, mobile phases containing 50 mM hexafluoro-2-propanol (HFIP) afford a fractionation range comparable to that of the formic acid (Fig. 8.6), permitting the effects of these variables to be studied systematically. 

FIGURE 8.7 Effect of pH on retention of amino acids. Column and flow rate Same as Fig. 8.1. Mobile phase 10 mA1 potassium phosphate with SO mM HFIP pH as indicated (adjusted prior to the addition of HFIP). 

The use of hexafluoroisopropanol (HFIP) as an SEC eluent has become popular for the analysis of polyesters and polyamides. Conventional PS/DVB-based SEC columns have been widely used for HFIP applications, although the relatively high polarity of HFIP has led to some practical difficulties (1) the SEC calibration curve can exhibit excessive curvature, (2) polydisperse samples can exhibit dislocations or shoulders on the peaks, and (3) low molecular weight resolution can be lost, causing additive/system peaks to coelute with the low molecular weight tail of the polymer distribution... 

The difference in structure between 16 and 17 is readily understood in terms of the addition of strongly electron-donating substituents, but the contrast between 16 and 20 is less easily rationalized. Photolysis of 19 was carried out in HFIP (dielectric constant (e) = 16.75), while TRIR experiments with diphenyl diazomethane (22) were carried out in dichloromethane (e = 9.08), suggesting that a-lactone structure may be dependent on solvent polarity. 

As may be seen from Table 3.22, MTBE does not extract additives from PA6, as opposed to dissolution in the expensive solvent HFIP. It is also evident that in these conditions intact Ultranox 626 is not observed the hydrolysis product 2,4-di-f-butylphenol (2,4-DTBP) is observed instead. 31P NMR confirms hydrolysis of Ultranox 626. The results do not discriminate between hydrolysis during mixing or analysis. As also SFE does not detect Ultranox 626 hydrolysis is likely to occur in the mixing step. Dissolution with HFIP and SFE (after optimisation) give identical results. In this case the added value of SFE extraction consists in a considerable cost reduction. 

While additive analysis of polyamides is usually carried out by dissolution in HFIP and hydrolysis in 6N HC1, polyphthalamides (PPAs) are quite insoluble in many solvents and very resistant to hydrolysis. The highly thermally stable PPAs can be adequately hydrolysed by means of high pressure microwave acid digestion (at 140-180 °C) in 10 mL Teflon vessels. This procedure allows simultaneous analysis of polymer composition and additives [643]. Also the polymer, oligomer and additive composition of polycarbonates can be examined after hydrolysis. However, it is necessary to optimise the reaction conditions in order to avoid degradation of bisphenol A. In the procedures for the analysis of dialkyltin stabilisers in PVC, described by Udris [644], in some instances the methods can be put on a quantitative basis, e.g. the GC determination of alcohols produced by hydrolysis of ester groups. 

Figure 17 Reaction scheme for conversion of acid ends to esters by addition of 1,1,1,3,3,3-hexaflouro-2-propanol (HFIP).

Although the selectivities are excellent, prolonged reaction times (2 1 days) are noted under these conditions. The addition of alcohols, particularly 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), was found to decrease reaction times (4 days to 36 h under identical conditions). In the presence of HFIP, Michael adducts are generated in comparable yields and selectivities suggesting that the principal role of the alcohol is catalyst turnover. 

Mukaiyama Michael reactions of alkylidene malonates and enolsilanes have also been examined (244). The stoichiometric reaction between enolsilane (342a) and alkylidene malonate (383) proceeds in high selectivity however, catalyst turnover is not observed under these conditions. The addition of HFIP effectively promotes catalyst turnover, presumably by protonation and silyl transfer from the putative copper malonyl enolate generated in this reaction. The reaction proved general for bulky P-substituents (aryl, branched alkyl), Eq. 209. 

Figure 16 Abrupt solvatochromism of 49 on addition of HFIP.243 Reprinted with permission from Oka, K. Fujiue, N. Dohmaru, T. Yuan, C.-H. West, R. J. Am. Chem. Soc. 1997, 119, 4074-4075, 1997 American Chemical Society.

Finally, concerning dialkylpolysilanes, it is interesting to note that the solid-state (film) UV spectral profile of the thermochromism exhibited by 49,157 shown in Figure 28, almost exactly matches that of the solvatochromism (see Figure 16 above). This indicates that the before and after conformations are essentially the same and that the reduction of temperature or addition of HFIP are responsible for similar conformational changes in the polymer an abrupt straightening of the polysilane backbone. 

Reaction of 4a with furan also proceeds in HF1P containing hydrogen chloride at 0 °C to yield a cyclopentenone product 26 as well as the products obtained in the reaction with Lewis acid, 23 and 24 (entries 7-9). The formation of cyclopentenone product 26 did not occur when less than one equivalent of furan was employed a simple cyclopentenone product 25 was obtained instead (entry 7). Addition of THF also retarded the formation of 26 in the reaction in HFIP as solvent with an increasing yield of 25 (entry 10). Preferential formation of the [4 + 3] cycloadduct 23 was observed without formation of 26 in the reaction of 4b in HFIP (entries 11 and 12) as was observed with Lewis acid in dichloromethane (entries 3-5). Reaction of 4c in HFIP gave 26 (entry 13), with a product distribution similar to that of 4a in HFIP. 

A general procedure has been reported by Killian et al. for the incorporation of hydro-phobic peptides into micelles [75]. Therein, the peptides were first dissolved in TFA for deaggregation, dried under nitrogen and redissolved as 5 mM (clear) solutions in TFE or HFIP. The solution is subsequently diluted 1 1 by addition of an aqueous solution containing a varying SDS concentration (typically 500 mM, depending on the protein concentration). Water is then added to yield a 16 1 ratio of water to TFE (HFIP) by volume. Upon addition of excess water the peptide loses its solubility, but at the same time the... 

Furthermore, the hydrogen bond donor ability of an HFIP hydroxyl group is greatly enhanced upon coordination of a second or even third molecule of HFIP (Figs. 12 and 13). Aggregation beyond the trimer has no significant additional effect [49, 52]. 

Various alkylamino-artemisinins 155 were synthesized by the nucleophilic displacement of bromoartemisin 149, which was prepared in situ from 148 (Scheme 19), followed by reaction with alkylamines <2004AGE1381>. A more direct route involved treatment of 29a firstly with a mixture of NaBr and TMSCl and then with the amine <2006AGE2082>. HFIP can be used as an additive in the amination step to increase the yield <2005AGE2060>. Similar aminoartemisinins 155 (where NRR = NHAr) were prepared by reacting 29a with anilines, in the presence of a catalytic amount of pyridinium sulfate in pyridine, in good yields (e.g., Ar = Ph, 93% 72% ... 

Scanning force microscopy imaging provided further evidence for the successful conversion of the supramolecular polymers into covalent, conjugated polymers with retention of their hierarchical structure. First of all, SFM images obtained from any of the polymerizable macromonomers A-E looked virtually identical before and after polymerization. However, while the addition of a small amount of a deaggregating cosolvent such as hexafluoroisopropanol (HFIP) to the sample... 

Figure 4.10 UV-induced topochemical polymerization, (a) UV spectra of compounds F-l did not show any indication of poly(diacetylene) formation, (b, c) Macromonomers A-D were successfully converted into poly(diacetylene)s. (d) The reactivity toward topochemical polymerization was related to the total number of hydrogen bonds, (e) Upon addition of HFIP to a sample solution in DCM after UV irradiation, helical fibrillar...

The observed chemoselectivity is unique. Anodic treatment on BDD of 3,4,5-trimethoxy toluene results in the exclusive formation of the mixed biaryl 56. This method can be further performed with benzo[l,3]dioxole-containing arenes as reaction partners, giving biaryls 57 and 58 in acceptable yields. Furthermore, naphthalene moieties can be directly located onto 4-methyl guaiacol as the products 59 and 60 reveal. This novel cross-coupling can be expanded to other phenolic reaction partners as well [28]. The displayed selection of mixed biaryls 53-60 is accessible in a single step. In the workup protocol, HFIP is almost quantitatively recovered since it represents the most volatile component in the electrolyte. In addition, nonconverted starting materials can be recycled by short path distillation with approximately 80% efficiency (Scheme 23). 

In two reports Hanzawa and co-workers described a mild and facile Pictet-Spengler reaction catalyzed by perfluorooctane sulfonic acid (PFOSA) in water <07T4039 07TL835>. Many of their examples outline the reaction of 3,4-dimethoxyphenethylamines 124 with a variety of aldehydes catalyzed by PFOSA in water using l,l,l,3,3,3-hexafluoro-2-propanol (HFIP) as an additive resulted in the formation of 1,2-disubstituted tetrahydroisoquinoline derivatives 125 in high yield. 

In early negative-ion ESI-MS studies, halogenated solvent additives were applied to reduce the risk of discharge formation (Ch. 6.3.2). The use of 1,1,1,3,3,3,-hexafluoro-2-propanol (HFIP) to the mobile phase was proposed for oligonucleotides [22]. This is for instance applied in the rapid characterization of synthetic oligonucleotides by microcapillary LC-MS on a quadrupole-time-of-flight hybrid (Q-TOF) instrument [20]. 

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