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Solvent-promoted Reactions

When MeNOa [233a] and DMSO [81] are used as solvents, Michael addition of KSA proceeds smoothly at room temperature without additional catalyst. Coordination of the solvent molecule to the silicon atom would enhance the nucleophilicity of KSA to effect the uncatalyzed reaction. [Pg.471]


Lee et al. investigated the photoisomerism of fran.y-stilbene in supercritical ethane to observe the so-called Kramer s turnover region where the solvent effects are in transition from collisional activation (solvent-promoting reaction) to viscosity-induced friction (solvent-hindering reaction) (76). In the experiments the Kramer s turnover was observed at the pressure of about 120 atm at 350 K. (See Scheme 2.)... [Pg.28]

Aryl, heteroaryl, and alkenyl cyanides are prepared by the reaction of halides[656-658] or triflates[659,660] with KCN or LiCN in DMF, HMPA, and THF. Addition of crown ethers[661] and alumina[662] promotes efficient aryl and alkenyl cyanation. lodobenzene is converted into benzonitrile (794) by the reaction of trimethylsiiyl cyanide in EtiN as a solvent. No reaction takes place with aryl bromides and chlorides[663]. The reaction was employed in an estradiol synthesis. The 3-hydroxy group in 796 was derived from the iodide 795 by converting it into a cyano group[664]. [Pg.246]

This process has many similarities to the Phillips process and is based on the use of a supported transition metal oxide in combination with a promoter. Reaction temperatures are of the order of 230-270°C and pressures are 40-80 atm. Molybdenum oxide is a catalyst that figures in the literature and promoters include sodium and calcium as either metals or as hydrides. The reaction is carried out in a hydrocarbon solvent. [Pg.211]

Shibasald et al. reported that lithium-containing, multifunctional, heterobimetallic catalysts such as LaLi3tris((l )-6,6 -dibromobinaphthoxide) 35, with moderate Lewis acidity in non-polar solvents, promote the asymmetric Diels-Alder reaction to give cycloadducts in high optical purity (86% ee) [53] (Scheme 1.67). The lithium... [Pg.42]

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. [Pg.366]

The results of reactions with and without MW irradiation are reported in Table 4.11. The reaction yields are comparable, but the reaction times of the irradiated reactions are considerably reduced. The alumina does not give acceptable results. The same reactions were carried out in nitrobenzene as solvent and under free-solvent conditions with and without MW irradiation. The results are reported in Table 4.12. In this case too, the only significant difference is the reaction time, so that the authors [41] concluded that MW-promoted reactions proceed like the thermal reactions except for a much higher reaction rate. [Pg.162]

The classical Pechmann approach for the synthesis of coumarins via the micro-wave-promoted reaction [68] has been extended to solvent-free system wherein salicy-... [Pg.191]

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... [Pg.109]

Lewis add catalysis has been and continues to be of great interest in organic synthesis.111 While various kinds of Lewis add-promoted reactions have been developed and many have been applied in industry, these reactions must generally be carried out under stridly anhydrous conditions. The presence of even a small amount of water stops the reaction because most conventional Lewis adds read immediately with water, rather than with the substrates, and decompose. This destrudive reaction has restrided the use of Lewis acids in organic synthesis. From a viewpoint of today s environmental consciousness, however, it is desirable to use water instead of organic solvents as a reaction solvent.1231... [Pg.4]

The chemistry of the reactions involved in coupling peptides is the same as that for coupling TV-alkoxycarbonylamino acids. However, the oxazolone that is formed by the activated peptide is chirally unstable, it is formed more readily, and there is an added impetus for it to form because the rate of bond formation between segments is lower. In addition, segments usually have to be coupled in polar solvents because they are insoluble in nonpolar solvents, and polar solvents promote the undesirable side reaction. The result is that the number of procedures actually used for coupling peptides is rather small. The methods in question are addressed below. [Pg.57]

As the supported glycol catalysts worked better in promoting reactions in a single solvent system, we explored the direct carbonylation of benzyl halides using an alcohol solvent, base, and cobalt carbonyl. Our initial experiments concentrated on the reaction of benzyl bromide at room temperature and one atmosphere carbon monoxide. We chose sodium hydroxide as the base, methanol as the solvent, and looked at the product distribution. We were interested in the selectivity to ester and the reactivity of this system. The results are given in Table III. [Pg.146]

Fig. 7.11 Reaction profiles for L-catalyzed isomerization of cis-to-trans ML2X2. In (A) an ionic intermediate is favored by a polar solvent. In (B) ion-pair formation arises with a less polar solvent. In (C) a non-polar solvent promotes a 5-coordinated intermediate. In (C), pseudo-rotation occurs. Based on D. G. Cooper and J. Powell, J. Amer. Chem. Soc. 95, 1102 (1973) see also Ref. 90. Reproduced with permission from D. G. Cooper and J. Powell, J. Amer. Chem. Soc. 95, 1102 (1973). (1973) American Chemical Society. Fig. 7.11 Reaction profiles for L-catalyzed isomerization of cis-to-trans ML2X2. In (A) an ionic intermediate is favored by a polar solvent. In (B) ion-pair formation arises with a less polar solvent. In (C) a non-polar solvent promotes a 5-coordinated intermediate. In (C), pseudo-rotation occurs. Based on D. G. Cooper and J. Powell, J. Amer. Chem. Soc. 95, 1102 (1973) see also Ref. 90. Reproduced with permission from D. G. Cooper and J. Powell, J. Amer. Chem. Soc. 95, 1102 (1973). (1973) American Chemical Society.
Polar aprotic solvents promote this type of reaction. [Pg.26]

Water has also been shown to be essential for the liquid phase polymerization of isobutylene with stannic chloride as catalyst (Norrish and Russell, 87). The rates of reaction were measured by a dilatometric method using ethyl chloride as common solvent at —78.5°. With a mixture consisting of 1.15% stannic chloride, 20 % isobutylene, and 78.8% ethyl chloride, the rate of polymerization was directly proportional to the amount of added water (up to 0.43% of which was added). A rapid increase in the rate of polymerization occurred as the stannic chloride concentration was increased from 0.1 to 1.25% with higher concentrations the rate increased only gradually. It was concluded that a soluble hydrate is formed and functions as the active catalyst. The minimum concentration of stannic chloride below which no polymerization occurred was somewhat less than half the percentage of added water. When the concentration of the metal chloride was less than about one-fifth that of the added water, a light solid precipitated formation of this insoluble hydrate which had no catalytic activity probably explains the minimum catalyst concentration. The addition of 0.3% each of ethyl alcohol, butyl alcohol, diethyl ether, or acetone in the presence of 0.18% water reduced the rate to less than one-fifth of its normal value. On the other hand, no polymerization occurred on the addition of 0.3 % of these substances in the absence of added water. The water-promoted reaction was halved when 1- and 2-butene were present in concentrations of 2 and 6%, respectively. [Pg.75]

Oppolzer sultam-like chiral auxiliary (e.g., Xc in 304) has been studied in Diels-Alder cycloaddition reactions (Scheme 43) <2003JP0700>. The TiCU-promoted reaction of dienophile 304 and 1,3-cyclopentadiene 305 in DCM is complete within 18h and excellent diastereoselectivity of product 306 is observed. The same reaction in the absence of Lewis acid provides product 306 in very low yield. However, switching to trifluoroethanol as the solvent, the cycloaddition reaction proceeds to completion, albeit with slightly diminished levels of diastereoselectivity for Diels-Alder adduct 306. Surprisingly, the use of hexane as the solvent affords the opposite (23, J 31-diastereomer of 306 as the major product. [Pg.561]

Iodide-promoted reactions in phosphine oxide solvents have been observed under some conditions to produce ethanol from H2/CO with good rates and high selectivities (193-195) (Table XVI, Expts. 1-3). Experimental evidence suggests that the ethanol is a secondary product, although its selectivity is high even after very short reaction times (193). An acid component is believed to be involved in alcohol homologation by this system, which will be described in more detail below. [Pg.389]

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). [Pg.389]


See other pages where Solvent-promoted Reactions is mentioned: [Pg.471]    [Pg.111]    [Pg.115]    [Pg.471]    [Pg.111]    [Pg.115]    [Pg.105]    [Pg.397]    [Pg.35]    [Pg.1322]    [Pg.160]    [Pg.153]    [Pg.115]    [Pg.174]    [Pg.381]    [Pg.271]    [Pg.274]    [Pg.5]    [Pg.263]    [Pg.135]    [Pg.465]    [Pg.391]    [Pg.104]    [Pg.40]    [Pg.268]    [Pg.209]    [Pg.2]    [Pg.3]    [Pg.63]    [Pg.783]    [Pg.427]    [Pg.80]    [Pg.355]   


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Achiral Lewis Acid-promoted Reactions in Anhydrous Solvent

Elimination reactions solvent-promoted

Promoters reaction

Protic solvent-promoted reactions

Solvent-promoted

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