Solvent effects bromides

An analogous solvent effect was observed upon treatment of the chiral a-alkoxy aldehyde 11 with 2-lithio-4-methylfuran in the presence of zinc bromide. This highly diastereoselective addition reaction was the key step in a synthesis of the enantiomcrically pure C-10-C-20 fragment of the immunosuppressant KK 506139. 

As mentioned briefly in Chapter 5, the photodimerization of acenaphthylene is subject to a very interesting heavy-atom solvent effect. The results of the photolysis of acenaphthylene in some heavy-atom solvents are given in Table 10.6.<4a) The data in Table 10.6 show that the heavy-atom solvents n-propyl bromide and ethyl iodide yield product ratios similar to that obtained in the sensitized photolysis, indicating a greater role of the triplet state in... 

In fact, the analogy between the mechanisms of heterolytic nucleophilic substitutions and electrophilic bromine additions, shown by the similarity of kinetic substituent and solvent effects (Ruasse and Motallebi, 1991), tends to support Brown s conclusion. If cationic intermediates are formed reversibly in solvolysis, analogous bromocations obtained from bromine and an ethylenic compound could also be formed reversibly. Nevertheless, return is a priori less favourable in bromination than in solvolysis because of the charge distribution in the bromocations. Return in bromination implies that the counter-ion, a bromide ion in protic solvents, attacks the bromine atom of the bromonium ion rather than a carbon atom (see [27]). Now, it is known (Galland et al, 1990) that the charge on this bromine atom is very small in bridged intermediates and obviously nil in /f-bromocarbocations [28]. 

A full report has now appeared of solvent effects on the rates of hydrolysis of benzyltriphenylphosphonium bromide. The remarkable increase in rate in media of low polarity is largely attributable to a shift of the pre-equilibrium between phos-phonium and hydroxide ions in favour of the intermediate hydroxyphosphorane.129 In a similar vein, a study of medium and deuterium isotope effects on the rate of hydrolysis of tetraphenylphosphonium chloride in acetone-water mixtures has been reported.130... 

In a similar manner, Jt-allyl complexes of manganese, iron, and molybdenum carbonyls have been obtained from the corresponding metal carbonyl halides [5], In the case of the reaction of dicarbonyl(r 5-cyclopentadienyl)molybdenum bromide with allyl bromide, the c-allyl derivative is obtained in 75% yield in dichloromethane, but the Jt-allyl complex is the sole product (95%), when the reaction is conducted in a watenbenzene two-phase system. Similar solvent effects are observed in the corresponding reaction of the iron compound. As with the cobalt tetracarbonyl anion, it is... 

The chemical shift differences of the diastereotopic hydrogens are listed in Table 17 they depend strongly on solvent effects, as expected for an ionic product. They are in the range of 8 = 0.01 -0.1, well suited for measurement of the enantiomeric purity of the phosphanes. An alternative method for the measurement of Horner phosphanes is by 13C-NMR spectroscopy of diastereomeric complexes formed with [>/3-( + )-0 7 ,57 )-pinenyl]nickel bromide dimer73. 

When the alcohol adduct from the allenylzinc reagent and diisopropyl ketone was treated with 80 mol% of allenylzinc bromide in HMPA, a mixture containing 12% of diisopropyl ketone and 88% of recovered alcohol was obtained after 7 days at ambient temperatures (equation 1). Thus, it may be deduced that the allenylzinc additions are reversible. Presumably, the propargyl adducts are intrinsically favored, but steric interactions between the R1 and R2 substituents in the propargyl product favors an increased proportion of allenyl adducts in a reversible process (see Table 1). HMPA would expectedly facilitate reversal of the addition by decreasing the ion pairing between the alkoxide anion and ZnBr cation of the adducts. This expectation was subsequently confirmed by a study of solvent effects. 

TABLE 2. Solvent effects in aflenylzinc bromide additions... 

The above crystal solvent effect on the ethanolate and the methanolate of [Fe(2-pic)3] CI2 Sol has later been confirmed by magnetic susceptibility measurements181 . Sinn et al. 181) have also investigated the unsolvated bromide [Fe(2-pic)3] Br2 as well as its solvates with Sol = C2HsOH and CH3OH, respectively these systems all show temperature dependent spin transition with pronounced differences in the transition behaviour. Differences in the spin transition behaviour of corresponding solvates [Fe(2-pic)3] X2 Sol (X = Cl, Br) with different anions are also noticeable 181). 

The solvent effect on the stereoselectivity of the replacement of the anomeric hydroxyl to form glycosyl fluorides was thoroughly investigated. These derivatives are more stable than the corresponding bromides but can still be used for glycosylation reactions142. 

Falciola, L., Gennaro, A., Isse, A.A., Mussini, P.R. and Rossi M. (2006) The solvent effect in the electrocatalytic reduction of organic bromides on silver. J. Electroanal. Chem. 593,47-56. 

The solvent effect has long been recognized as an important factor in that it affects the lithium-oxygen bond polarization but also the electrophilic reagent380,398. The effect on aggregation was evaluated by measurement and comparison of the reactivities of monomeric, dimeric and tetrameric forms of LiPhIBP and LiPhAT or LiPhIBP in various ethers252. In the less polar solvent methyl-tert-butyl ether, lithium enolates are tetrameric and do not react with benzyl bromide. On the contrary, with added HMPA the dissociation of the tetrameric LiPhIBP is accompanied by solvation of each monomer by 1 -2... 

The unexpected downfield shift in 66a,b, in connection with strong temperature and solvent effects on the 29Si chemical shift (Table XXI), indicates that the dibromo complexes 66, unlike the difluoro- (64) and dichloro- (65) analogs, undergo ionization to form the bromide salts (68) already at room temperature (Eq. 27).20,52... 

Initial studies of solvent effects, on the reactions of triarylarsonium benzoylylides with p-nitrobenzaldehyde in N, A-dimethylformamide, dimethyl sulphoxide or methanol, indicated little solvent effect in these cases" ", but later studies of the more finely balanced reactions of semi-stabilized ylides have provided examples of strong influences due to the effect of different base and solvent when the ylide is generated in the presence of a carbonyl compound ". Thus, when benzyltriphenylarsonium bromide or p-chloroben-zyltriphenylarsonium bromide were treated with sodium hydride in benzene in the presence of a variety of p-substituted benzaldehydes the products were alkenes, but if sodium ethoxide in ethanol was used the isolated products were epoxides ". Likewise, when triphenylarsonium benzylylide was generated by phenyllithium in the presence of either benzaldehyde or acetaldehyde, the preponderant product was the epoxide whereas use of sodium amide as base provided mostly the alkene . Similar results were obtained when an allyltriphenylarsonium salt was deprotonated using different hexamethyldisilaz-... 

Solvent effects were studied in the fate 1950s for reactions of alkylmagnesium compounds with 1-hexyne in diethyl ether. These reactions have already been discussed several times in this and in Chapter 11 (see Scheme 10) [22a], When 1.0 molEq triethylamine was added to the reaction mixture, the relative reactivity of methylmagnesium iodide toward 1-hexyne, which is 7 (the reactivity of ethylmagnesium bromide was arbitrarily set... 

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