Yield drop

TrialkyIboranes (p. 9), which can be synthesized from olefins and diborane, undergo alkyl coupling on oxidation with alkaline silver nitrate via short-lived silver organyls. Two out of three alkyl substituents are coupled in this reaction. Terminal olefins may be coupled by this reaction sequence in 40 - 80% yield. With non-terminal olefins yields drop to 30 - 50% (H.C. Brown, 1972C, 1975). 

A report on the continuous flash pyrolysis of biomass at atmospheric pressure to produce Hquids iadicates that pyrolysis temperatures must be optimized to maximize Hquid yields (36). It has been found that a sharp maximum ia the Hquid yields vs temperature curves exist and that the yields drop off sharply on both sides of this maximum. Pure ceUulose has been found to have an optimum temperature for Hquids at 500°C, while the wheat straw and wood species tested have optimum temperatures at 600°C and 500°C, respectively. Organic Hquid yields were of the order of 65 wt % of the dry biomass fed, but contained relatively large quantities of organic acids. 

Sulfonation. The main sulfonation product of quinoline at 220°C is 8-quinoHnesulfonic acid [85-48-3]-, at 300°C it rearranges to 6-quinolinesulfonic acid [65433-95-6] (10). Optimum conditions for sulfonation, 2 h at 140°C and a 1 4 quinoline/40% (wt) oleum ratio, produces 80% yield. The yield drops to 64% at 130°C with a 1 3 reactant ratio (11). Somewhat higher, but variable, yields of 8-quinoHnesulfonic acid hydrochloride [85-48-3] have been reported with chlorosulfonic acid (12). 

Table 6 shows the effect of varying coil oudet pressure and steam-to-oil ratio for a typical naphtha feed on the product distribution. Although in these tables, the severity is defined as maximum, in a reaUstic sense they are not maximum. It is theoretically possible that one can further increase the severity and thus increase the ethylene yield. Based on experience, however, increasing the severity above these practical values produces significantly more fuel oil and methane with a severe reduction in propylene yield. The mn length of the heater is also significantly reduced. Therefore, this is an arbitrary maximum, and if economic conditions justify, one can operate the commercial coils above the so-called maximum severity. However, after a certain severity level, the ethylene yield drops further, and it is not advisable to operate near or beyond this point because of extremely severe coking. 

In other words, by the nitric acid oxidation it is difficult to obtain a product completely free from benzoin. The yields by the nitric acid method are generally about 95-96 per cent, whereas with the copper sulfate-pyridine method the yield drops to approximately 86 per cent. 

The ammonium chloride solution is prepared by dissolving 50 g. of ammonium chloride in 150 cc. of water at 25-30°. The literature recommends the use of 30 per cent sulfuric acid for decomposing the magnesium compound when this is used, the yield drops to 50 -55 per cent, probably owing to the ready polymerization of the hydrocarbon in the presence of mineral acids. 

It is advisable to start the reduction as soon as the reactants are mixed. The yield dropped to 87% when the reaction mixture was allowed to stand for 3 hours before hydrogenating. 

The use of sufficient sodium acetate is essential. If the acidic reaction products are not neutralized, the yield drops to as low as 15-20% of badly discolored product which cannot be readily purified. 

With a primary 20-hydroxy-21-norpregnane a 58% yield" of the 18,20-ether was obtained, whereas with the tertiary 20-methyl-20-hydroxypregnane the yield drops to 10% due to extensive 17,20-fragmentation." ... 

Because diacetylene is unstable, a stable diacetylene derivative, 1-methoxybut-l-en-3-yne (65CB98), is often employed in the synthesis of pyrroles. The reaction with ammonia proceeds under conditions of heterogeneous catalysis (a mixture of reagent vapors is passed through a catalyst-containing reactor heated to 150°C), approaching a yield of 50-70% but with primary aromatic amines, the yield drops to 20%. 

The chiral (V-camphanoyl iminium ion 7, prepared by hydride abstraction from 2-camphanoyl-l,2,3,4-tetrahydro-6,7-dimethoxyisoquinoline 6 (see Appendix) with triphenylcarbenium te-trafluoroborate, reacts with silyl enol ethers to give 1-substituted tetrahydroisoquinoline derivatives with reasonable diastereoselectivity, 0°. On addition of titanium(IV) chloride, prior to the addition of the silyl enol ether, the diastereoselectivity gradually rises to an optimum at 2.5 equivalents of the Lewis acid, but the yield drops by 20%. 

When smaller amounts of the selenide are prepared, the yield drops to 70-75 per cent. The method may be applied equally well to the preparation of the selenides from the three toluidines, giving yields of 50-70 per cent. 

The effect of temperature is similar in these systems. Thus, yield was little affected or was reduced with decrease in temperature. At a certain temperature level, depending on the nature and concentration of initiator and solvent, yields dropped to zero. This floor temperature is -60 °C for f-BuCl/MeCl and f-BuBr/MeCl and -40 °C for f-BuCl/MeBr and t-BuBr/MeBr. 

The effect of temperature on yields was insignificant in the range from -25 °C to -55 °C. At and below —60 °C, yields dropped slightly while polymerization did not occur at -75 °C. The floor temperature for these systems is about —65 °C. 

Lastly, the effect of temperature on PIB yield for the r-BuX/Et2 AlX/MeX systems can be explained. In general, the yield is unaffected or decreases somewhat with decrease in temperature. At a certain temperature, yield drops to zero. Such an effect of temperature indicates, in agreement with our assumption, that the yield is dependent on the rate of initiation. 

An IL solvent system is applicable to not only lipase but also other enzymes, though examples are still limited for hpase-catalyzed reaction in a pure IL solvent. But several types of enzymatic reaction or microhe-mediated reaction have been reported in a mixed solvent of IL with water. Howarth reported Baker s yeast reduction of a ketone in a mixed solvent of [hmim] [PFg] with water (10 1) (Fig. 16). Enhanced enantioselectivity was obtained compared to the reaction in a buffer solution, while the chemical yield dropped. 

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

Later, Araki et al. found that the allylation of aldehydes and ketones can be carried out by using catalytic amounts of indium(III) chloride in combination with aluminum or zinc metal.109 This reaction was typically performed in a THF-water (5 2) mixture at room temperature, although the conversion was much slower compared to the same reaction mediated by use of a stoichiometric amount of indium and it required days to complete. When the reaction was carried out in anhydrous THF alone, the yield dropped considerably and side-reactions such as reduction to alcohol increased. The combinations of Al-InCL or Zn-InCl3 gave comparable results. 

Our initial studies (23) were performed in toluene, and Table I shows the results from the polymerization of a number of representative monomers. The data reported in Table I are for direct addition of the monomer to the sodium dispersion. Inverse addition often leads to higher molecular weights, although the overall polymer yields are usually lower (15,23). The results in Table I show that, under these reaction conditions, a bimodal molecular molecular weight distribution is normally obtained. Furthermore, it is obvious that the crude polymer yields drop precipitously as the steric hindrance in the monomer increases. 

Amino 4-oxo 4//-pyrimido[l,2-4]pyridazin-3-diazonium tetrafluoroborates 73 underwent ring transformation into l-(pyridazin-3-yl)-l//-l,2,3-triazole-4-carboxylates 74 on heating in dry MeOH. Yields dropped drastically when EtOH, instead of MeOH, was used as solvent (Equation 6) <2002ARK143>. 

Symmetrical piperazines 364 have been obtained from the corresponding 4,5-dihydrazinofurazano[3,4- ]pyraz ne 363 in good yield on reaction with acetic anhydride in the presence of a Lewis acid (Equation 98) <1999CHE499>. When formaldehyde was used, the yield was slightly reduced at 76%. Acid chlorides can also be used in this reaction although the yield drops to 23% when trichloroacetyl chloride is used. 

The reaction was performed in a Synthewave 402 apparatus under controlled temperature conditions to avoid decomposition of alkyl glycosides. Comparison with classical heating under strictly the same conditions revealed that yields dropped dramatically. 

Interestingly, after reaching the maximum at the 6-membered cycle, the yields drop again. This decrease in efficiency occurs despite the appreciable reduction in the distance between the terminal acetylenic carbons relative to the 6-membered analogue. Here, the efficiency may simply be a function of how photochemical excitation is distributed in the reactive excited state. Calculated enediyne geometries suggest the cyclization is more efficient for those enediynes where the terminal phenyl groups are rotated outside of the enediyne plane (Table 3). 

In contrast, an excellent yield was obtained for the reaction between 5-iodoimidazole and propargyl alcohol in the absence of Cul to install the 5-alkynyl derivative [10, 41], The adduct was then deprotected to 5-alkynyl- l-P-D-ribofuranosylimidazole-4-carboxamide 58, an antileukemic agent. It is noteworthy that if the Sonogashira reaction was conducted in the presence of Cul, the yield dropped to 19%. 

In 2000, Benaglia and coworkers reported preparation of MeO-PEG supported quaternary ammonium salt (10) and examined the catalytic efficiency in a series of phase-transfer reactions (Fig. 5.3) [69]. The reactions occurred at lower temperatures and with shorter reaction times than with comparable insoluble 2% cross-linked polystyrene-supported quaternary ammonium salts, although yields varied with respect to classical solution phase quaternary ammonium salt catalyzed reactions. It was observed that yields dropped with a shorter linker, and that PEG alone was not responsible for the extent of phase-transfer catalysis. While the catalyst was recovered in good yield by precipitation, it contained an undetermined amount of sodium hydroxide, although the presence of this byproduct was found to have no effect on the recyclability of the catalyst... 

The above synthesis has a few noteworthy points. The nitrolysis of bicyclic amides like (67) are frequently problematic in terms of inertness towards nitrolysis and the ease with which ring decomposition occurs. This synthesis is an interesting balancing act. Ring decomposition results when the bicycle (67) is treated with absolute nitric acid, mixed acid or nitronium salts. When the diacetyl equivalent of the bicycle (67) is treated with dinitrogen pentoxide-absolute nitric acid-TFAA reagent, the yield drops to 10 %. 

Mandelic acid with a 13 - 20 % enantiomeric excess of the (R)-isomer has been obtained from reduction of phenylglycolic acid in aqueous alcohol buffers containing strychnine in low concentration. The optical yield depends upon pH and is highest (20 - 24 %) at pH 0 or 9.2 and at low current density. In the pH range 2-4, the optical yield drops to 2-8% [42]. The higher result compares well with the maximum value of 20% excess R-isomer found from reduction of (-)-menthyl phenylglycolate in aqueous buffers where the (-)-menthyl ester is the only chiral reagent present [43 ]. 

These final steps are not included in excerpt 4G, but if you are interested, the full article is included at the end of chapter 2.) The text describes these efforts, following the order of the entries in Table f. First, different solvents were tried beginning with methanol (entry f). Next, various alcohol mixtures were tried, but yields dropped (entries 2—4). The yield improved slightly when water was mixed with methanol (entry 5) hence, water alone was tried at different temperatures (entries 6—9). This truly is a story of scientific discovery The readers learn both what did not work and what did. This approach is quite common in papers describing organic synthesis. 

Yield reduction in the mid-portion of the yield drop curve... 

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