Reactions at low temperatures

Besides ice baths and solid carbon dioxide-acetone baths, convenient cooling baths contain solvents cooled to a slush by stirring with liquid nitrogen. A selection is listed in Table 2.3 (and see also Table 2.2 and Ref [4]). It should be noted that published experimental details do not always 

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This is a stepwise process in which chloramine [10599-90-3] is first formed from ammonia and hypochlorite in a rapid reaction at low temperature ... 

When the temperature increases the second term predominates. However, due to the very low rates of non-catalyzed reactions at low temperatures only very few experiments have been reported under these experimental conditions. 

Chloride is the predominant residual product from reactions at low temperature and in contact with the gaseous products, whereas in vacuum and at high temperature, MgO is formed. It has also been suggested that MgO Mg(C104)2 is a decomposition intermediate [855]. 

In a recent total synthesis of the novel neurotrophic agent merrilactone A (22, Scheme 4) by Inoue and Hirama [24], key intermediate 21 with the cis-bicyclo[3.3.0] octane framework embedded within the caged pentacycle 22 was elaborated from cyclobutane 18 by a sequence of RCM and immediate cleavage of the resulting bicyclic vicinal diol 19 to raeso-diketone 20. Cyclooctenedione 20 then underwent regioselective transannular aldol reaction at low temperature (LHMDS, THF, -100 °C) to produce a 3 1 mixture of isomers in 85% combined yield. The major isomer 21 with the required stereochemistry was then converted into the racemic natural compound ( )-22 in 19 steps. 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

Recently, this concept could be transferred to the homologous silylium ions 3-Si (Si in cx-position to the ferrocene core) [18, 19], which were found to be potent catalysts for Diels-Alder reactions at low temperature [19]. The electron-rich ferrocene core buffers the Lewis acidity, thus avoiding the irreversible formation of Lewis pairs. 

As the final choice, we examined the reaction at low temperatures because of its high reaction rate and found that lowering the temperature to —40°C increased the E value of 17 (at 30°C) to a practically acceptable level of 99 however, further lowering the temperature rather decreased the enantioselectivity (Scheme 1). 

Tg=—2 Amorphous, rubbery if molecular weight is high. When prepared by controlled reaction at low temperatures, polymer is semicrystal-line ... 

The formation of the pentacoordinated intermediate was confirmed later by performing the reaction at low temperature and by using a poly-... 

Ketones can also be prepared from acyl chlorides by reaction at low temperature using an excess of acyl chloride. Tetrahydrofuran is the preferred solvent.91 The reaction conditions must be carefully controlled to prevent formation of tertiary alcohol by addition of a Grignard reagent to the ketone as it is formed. 

The cationic polymerization of polystyrene occurs very fast. We perform this type of reaction at low temperature in order to obtain small scale samples with very high molecular weights. Cationic polymerization is not widely practiced outside the laboratory. 

We can employ coordination polymerization to produce stereoregular polystyrene. By performing this type of reaction at low temperatures, using Ziegler-Natta or single-site catalysts, we can prepare isotactic and syndiotactic versions of polystyrene. 

A vigorous explosion dining chlorination of 3-chloropropyne in benzene at 0°C over 4 h was attributed to presence of excess chlorine arising from the slow rate of reaction at low temperature. 

Force-field methods, calculation of molecular structure and energy by, 13,1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271 Free radicals, and their reactions at low temperature using a rotating cryostat, study of, 8. I Free radicals, identification by electron spin resonance, 1, 284... 

Iida, H., Kondo, K., and Igarashi, A. 2006. Effect of Pt precursors on catalytic activity of Pt/Ti02 (rutile) for water gas shift reaction at low-temperature. Catal. Commun. 7 240-44. 

Smith I. W. M. et al. (2004). Rapid neutral-neutral reactions at low temperatures a new network and first results for TMC-1 Monthly Notices of the Royal Astronomical Society 350 323. 

If n = 2, one set of ligands can be accommodated. If n = 0, two sets lead to (olefin)2-PtH2(—Si=)2. Both cases have Pt(IV) in the catalytic species. Perhaps this is not a necessary condition for catalysis, but it seems likely as the condition that leads to very high rates of reaction at low temperatures. 

The radiation yield depends on the temperature of oxidation and the initiation rate, i.e., the intensity of radiation IT [233], Radoxidation occurs as an initiated chain reaction at an elevated temperature when peroxyl radicals react more rapidly with hydrocarbon RH than disproportionate, kp(2kt) [RH]2 > (see Chapter 2)]. Radoxidation proceeds as a nonchain reaction at low temperatures when peroxyl radicals disproportionate more rapidly than react with hydrocarbon. The temperature boundary Tv between these two regimes of oxidation depends on the value of radiation intensity 7r. The values of Tv for irradiated heptane oxidation is as follows [233] ... 

There are several examples of the concerted mechanism. However, no report of an insertion of a carbon—carbon triple bond into a metallacyclopentadiene had appeared prior to discovery of this reaction. At low temperatures, during the reaction of zirconacyclopentadienes with DMAD, the formation of trienes (79) is observed upon hydrolysis. This clearly indicates that the benzene formation involves the insertion (addition) reaction of DMAD. As shown in Eq. 2.50, the alkenyl copper moiety adds to the carbon—carbon triple bond of DMAD and elimination of Cu metal leads to the benzene derivatives 72. Indeed, a copper mirror is observed on the wall of the reaction vessel. 

Ionic Diels-Alder reactions (12, 531-532). The allyl cations derived from allyl alcohols or ethers are reactive dienophiles that undergo Diels-Alder reactions at low temperature with high stereoselectivity.1 Example ... 

The results of adsorption and desorption of CO mentioned above suggest that for the reaction at low temperature, the sites for relatively weakly chemisorbed CO are covered by the deposited carbon and the reaction occurs between molecularly adsorbed CO and oxygen on the carbon-free sites which are the sites for relatively strongly chemisorbed CO. Therefore, the definition of the turnover rate at 445 K remains as given in Equation 1. For the reaction at 518 K, however, this definition becomes inappropriate for the smaller particles. Indeed, to obtain the total number of Pd sites available for reaction, we now need to take into consideration the number Trp of CO molecules under the desorption peak. Furthermore, let us assume that disproportionation of CO takes place through reaction between two CO molecules adsorbed on two adjacent sites, and let us also assume that the coverage is unity for the CO molecules responsible for the LT desorption peak, since this was found to be approximately correct on 1.5 nm Pd on 1012 a-A O (1). Then, the number Np of palladium sites available for reaction at 518 K is given by HT/0 + NC0 LT s nce t ie co molecules under the LT desorption peak count only half of the available sites. Consequently, the turnover rate at 518 K should be defined as ... 

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