Slow addition procedure

On the other hand, the method of Mukaiyama can be succesfully applied to silyl enol ethers of acetic and propionic acid derivatives. For example, perfect stereochemical control is attained in the reaction of silyl enol ether of 5-ethyl propanethioate with several aldehydes including aromatic, aliphatic and a,j5-unsaturated aldehydes, with syir.anti ratios of 100 0 and an ee >98%, provided that a polar solvent, such as propionitrile, and the "slow addition procedure " are used. Thus, a typical experimental procedure is as follows [32e] to a solution of tin(II) triflate (0.08 mmol, 20 mol%) in propionitrile (1 ml) was added (5)-l-methyl-2-[(iV-l-naphthylamino)methyl]pyrrolidine (97b. 0.088 mmol) in propionitrile (1 ml). The mixture was cooled at -78 °C, then a mixture of silyl enol ether of 5-ethyl propanethioate (99, 0.44 mmol) and an aldehyde (0.4 mmol) was slowly added to this solution over a period of 3 h, and the mixture stirred for a further 2 h. After work-up the aldol adduct was isolated as the corresponding trimethylsilyl ether. Most probably the catalytic cycle is that shown in Scheme 9.30. 

Stilbene is only sparingly soluble in aqueous acetone. As the reaction proceeds it gradually dissolves in the reaction mixture thereby approximating slow addition conditions. In the case of soluble unsaturated substrates the following slow addition procedure can be performed 4c. 

Procedure B. This method is identical to procedure A in all respects except for the fact that the TEOS/methanol solution, rather than being added to the flask contents all at once, was added at 4 ml/min (from 37.5 ml TEOS, 25 ml methanol) for 16 minutes. This slow addition procedure was only applied for methanol at this single immersion time. 

One paradoxical characteristic with the slow addition procedure is observed for the most hydrolytically resistant alkenes, where the reaction proceeds faster the slower the reagent is added. As can be deduced from the discussion above, the slow addition and the acetate effect are not mutually exclusive, and can be used in combination for the most arduous alkenes (Table 3, entries 2 and 6). With the mechanistic insight into the details of the asymmetric dihydroxylation, and the remedies to maintain the enantioselectivity in the addition step, the enanti-oselectivity in the chiral selectors (DHQD-CLB and DHQ-CLB) became limiting factors in the AD process. 

The main example of a category I indole synthesis is the Hemetsberger procedure for preparation of indole-2-carboxylate esters from ot-azidocinna-mates[l]. The procedure involves condensation of an aromatic aldehyde with an azidoacetate ester, followed by thermolysis of the resulting a-azidocinna-mate. The conditions used for the base-catalysed condensation are critical since the azidoacetate enolate can decompose by elimination of nitrogen. Conditions developed by Moody usually give good yields[2]. This involves slow addition of the aldehyde and 3-5 equiv. of the azide to a cold solution of sodium ethoxide. While the thermolysis might be viewed as a nitrene insertion reaction, it has been demonstrated that azirine intermediates can be isolated at intermediate temperatures[3]. 

Nitration. It is difficult to control nitration of thiophene, which yields 2-nitrothiophene [609-40-9]. The strongly electropbilic nitronium ion leads to significant yields (12—15%) of 3-isomer. A preferred procedure is the slow addition of thiophene to an anhydrous mixture of nitric acid, acetic acid, and acetic anhydride. 

Manufacture. Cinnamaldehyde is routinely produced by the base-cataly2ed aldol addition of ben2aldehyde /7(9(9-with acetaldehyde [75-07-0], a procedure which was first estabUshed in the nineteenth century (31). Formation of the (H)-isomer is favored by the transition-state geometry associated with the elimination of water from the intermediate. The commercial process is carried out in the presence of a dilute sodium hydroxide solution (ca 0.5—2.0%) with at least two equivalents of ben2aldehyde and slow addition of the acetaldehyde over the reaction period (32). 

The procedure which had originally been used by Lehn et al. involved slow addition (over a period of ca. 8 h) of ca. 0.1 M solutions of diamine and diacyl halide in benzene. Dye et al. found that the reactions could be conducted more rapidly as long as stirring was kept efficient. This observation suggested the use of a mixing chamber of the type normally used for stopped-flow kinetic studies. Utilizing this type of set-up, the latter authors were able to obtain a 70% yield for 1, slightly inferior to the yield reported by Lehn, but a similar yield of 3 which is better than that previously ob-tained. Note that the chemical features of this synthetic method are essentially identical to the approach shown in Eq. (8.1) and differ primarily in the mechanics. 

This isoxazolium salt (10 g.) (obtained from the Aldrich Chemical Company, Inc.) was dissolved in 45 ml. of aqueous 1 M hydrochloric acid and reprecipitated by the slow addition with swirling of 400 ml. of acetone. The salt was collected, washed with 300 ml. of acetone, and dried overnight at 25° under reduced pressure (< 1 min.) to give a fluffy product, m.p. 206-208° (decomp.). An isomeric salt, A-ethyl-5-phenylisoxazolium-4 -sulfonate, which may be obtained by the usual synthetic procedure,2 is also useful in peptide synthesis. 

D. 2(S)-(fl-tert-Butoxycarbonyl-a-(R)-hydroxyethyl)-4-(R)-hydroxy-pyrrolidine- 1-carboxylic acid, tert-butyl ester. The identical procedure was followed, in this case using the (,S)-BINAP catalyst (5)-l. Hydrogenation is conducted for 64 hr, and the reaction mixture is then transferred to a 250-mL, round-bottomed flask and concentrated to dryness. The residue is dissolved in 17 mL of methanol and cooled to 15°C. After the slow addition of 7 mL of DI water, the solution is aged for 15 min gradually forming a thin slurry. More DI water (75 mL) is added over 1 hr and the mixture is allowed to stand for an additional 1 hr at 15°C. The resulting crystals (Note 19) are filtered at 15°C, washed with 10 mL of 1 4-MeOH water, and then dried overnight in a vacuum oven (35°C, 686 mm) to yield 7.0 g (70%) of (R)-hydroxy ester 4b (Note 20). 

Reaching the equilibrium where the amino acid zwitterion predominates is a slow process. After acidifying to pH 6.5, the solution is allowed to stir at 0°C for 25 min during which time the pH of the solution slowly increases. The pH is readjusted to pH 6.5 by slow addition of 2.0N HC1 at 0°C. Repetition of this procedure as many as ten times may be necessary to insure the pH value of the aqueous solution remains at 6.5. 

The synthetic approach is very simple and does not require any special set up. In a typical room temperature reaction, 1.0 mL aqueous solution of cadmium chloride was added to 20 mL aqueous solution of soluble starch in a 50 mL one-necked round-bottom flask with constant stirring at room temperature. The pH of the solution was adjusted from 6 to 11 using 0.1 M ammonia solution. This was followed by a slow addition of 1.0 mL colourless selenide ion stock solution. The mixture was further stirred for 2 h and aged for 18 h. The resultant solution was filtered and extracted with acetone to obtain a red precipitate of CdSe nanoaprticles. The precipitate was washed several times and dried at room temperature to give a material which readily dispersed in water. The same procedure was repeated for the synthesis of PVA and PVP - capped CdSe nanoparticles by replacing the starch solution with the PVA and PVP polymers while the synthesis of elongated nanoparticles was achieved by changing the Cd Se precursor ratio from 1 1 to 1 2. The synthesis of polymer capped ZnSe nanoparticles also follows the same procedure except that ZnCb solution was used instead of CdCb solution. 

In a similar procedure for the preparation of iodotrimethyl-silane, aluminum, iodine, and hexamethyldisiloxane are combined, and the mixture is heated to reflux. When this procedure was attempted by the submitters, violent exothermic reactions occurred at ca. 50-60°. The slow addition of iodine to the warm mixture described in the present procedure leads to a controlled, reproducible reaction. 

The use of excess lithium LiDBB in reductive lithiations is a drawback for preparative-scale reactions. A modification of Yus procedure [72, 73] allowed for the generation of a-alkoxylithium reagents under catalytic conditions [45] (Scheme 35). Slow addition of the phenyl sulfide 185 to a suspension of lithium... 

General procedure for the production of Boc-protected a,a-diaryl-L-prolinols (6). To 100 mL of a 0.42 M solution of ArMgBr in THF (0.042 mol) at 0°C was added N- t- BOC)-L-proline methyl ester (5) (3.0 mL, 0.014 mol) dropwise via syringe over a five-minute period. The solution was stirred for at least 4 h at 25°C and then cooled to 0°C. After slow addition of 3 mL of water, the solution was slowly warmed to ft with stirring. The mixture was decanted and the solid was washed with 100 mL of ethyl ether. The organics were pooled and washed with brine, dried with sodium sulfate and evaporated to yield the final compound. 

During disposal of the tribromide by a recommended procedure involving slow addition to a mixture of soda ash and dry slaked lime, a violent reaction, accompanied by flame, occurred a few seconds after the first drop. Cautious addition of the bromide to a large volume of ice water is suggested for disposal. 

We now consider the reverse addition procedure used by us, involving a fast addition of monomer to the initiator solution, and contrast if both with the direct addition just described and with the very slow reverse addition described earlier which produces the quiescent mixtures. When the monomer is added fast to an A1X3 solution, it encounters both A1X2+ ions and A1X3 molecules. In contrast to the slow addition experiments, the monomer is consumed by the very fast polymerisation before it can complex with the... 

A runaway reaction and reactor explosion occurred in a resins production facility that killed one worker and injured four others. To control the reaction rate, an operating procedure called for the slow addition of one of the raw materials to the reactor. The runaway was triggered when the raw materials and catalysts were improperly charged to the reactor simultaneously, followed by heat addition. 

Tetrahydrofuran serves equally well as a solvent. However the quantity of sodium borohydride should be reduced to 1.0 g. and the isolation procedure modified in the following way. After the solution has been refluxed for 8 hours, the reaction mixture is cooled and the excess sodium borohydride is decomposed by the slow addition of dilute aqueous hydrochloric acid. The resulting mixture is extracted with ether and the ethereal solution is washed as described. 

A modified procedure suitable for intramolecular reductive coupling is achieved using low-valence titanium prepared by reduction of titanium trichloride with a zinc-copper couple followed by the extremely slow addition of ketone to the refluxing reaction mixture (0.0003 mol over a 9-hour period by use of a motor-driven syringe pump) [S60. ... 

The present procedure is based on the method published by Fu, Birnbaum and Greenstein. The yields are increased by the very slow addition of an aqueous solution of sodium nitrite to the reaction mixture as well as by a nwdified work-up procedure, i.e, careful removal of nitrogen oxides and the final decomposition of their adducts with carboxylic acids by buffering with sodium carbonate. 

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