Sulfonates displacement, halogenation

Scheme 5.7 illustrates these and other applications of the hydride donors. Entries 1 and 2 are examples of reduction of alkyl halides, whereas Entry 3 shows removal of an aromatic halogen. Entries 4 to 6 are sulfonate displacements, with the last example using a copper hydride reagent. Entry 7 is an epoxide ring opening. Entries 8 and 9 illustrate the difference in ease of reduction of alkynes with and without hydroxy participation. 

Cesium fluoride has been used to displace halogens, sulfonate groups and nitro groups among others, with fluorine. 1-Bromooctane (1), ethyl bromoacetate (2), benzyl bromide (3) and octyl tosylate (4) have been converted to the respective fluorides 5-7 by treatment with cesium fluoride in the presence of 10% tetraalkylammonium salt (Bu4NBr or Aliquat 336) in the absence of solvent.166... 

Sulfonate displacement has been well established as a common method for introducing halogen atoms into carbohydrates. The high reactivity and easy preparation of sulfonic esters contribute to their use in carbohydrate chemistry. Triflate displacements are popular for the preparation of secondary halogenated carbohydrates and often give satisfactory results when mesylates and tosylates fail to give products. 

Bordwell and Cooper211 drew attention to the inertness of a-halosulfones and related compounds towards nucleophilic displacements of the halogen. Thus chloromethyl p-tolyl sulfone reacts with potassium iodide in acetone at less than one-fiftieth of the rate for n-butyl chloride. On the other hand, l-(p-toluenesulfonyl)-3-chloro-l-propene reacts about 14 times faster than allyl chloride. This contrast (and other comparisons) led the authors to attribute the inertness of a-halosulfones to steric hindrance, which was eliminated when the sulfonyl group was more remote from the reaction center. 

Sn2 and SN2 Reactions with Halides and Sulfonates. Corey and Posner discovered that lithium dimethylcuprate can replace iodine or bromine by methyl in a wide variety of compounds, including aryl, alkenyl, and alkyl derivatives. This halogen displacement reaction is more general and gives higher yields than displacements with... 

Copper iodide acts as an efficient reagent for the nucleophilic displacement of 1-haloalkynes. It transforms 1-bromoalkynes (72) into 1-iodoalkynes (73) which, on further treatment with copper(II) bis(arenesulfinate), are converted into the corresponding alkynyl aryl sulfones (74). An electron transfer between 1-haloalkynes and copper(I) salts is believed to take place for the copper-assisted halogen-exchange reaction at the acetylenic carbon atom. 

Oxidation of 2-methyl-3-(methylthio)quinoxaline 1,4-dioxide with 1 equivalent of m-chloroperbenzoic acid yields the corresponding sulfoxide, and 2 equivalents of oxidizing agent gives the sulfone, both products being isolated in good yield. Treatment of the substituted sulfoxide- or sulfone-quinoxaline dioxides with aqueous halogen acids leads to nucleophilic displacement and the formation of 2-haloquinoxaline 1,4-dioxides.199... 

TH EMES Displacements of Sulfonates Halogenation of Alcohols Deoxygenation C-Branching Chain Extension... 

Since the pioneering work of Karl Freudenberg on displacements of carbohydrate p-toluene-sulfonates [1-5], bimolecular nucleophilic substitutions became one of the most employed and useful reactions in carbohydrate chemistry. Indeed SN2-type reactions have allowed the introduction of a variety of heteroatoms (halogens, N-, O-, S-) into carbohydrates, and the resulting compounds have been used in many synthetic and biological contexts [6],... 

Dibromothiochromone 1-oxide and its 1,1-dioxide are both well known species. The 2-halogen is easily displaced, while the 3-substituent is much less easily replaced in the sulfoxide than in the sulfone in the sulfone, addition/elimination mechanisms are available... 

This route requires a hydrogen a to the carbonyl function and may give rearrangement in the dehydration step (Sections 8-9B and 15-5E). Alternatively, the hydroxyl can be converted to a better leaving group (halogen or sulfonate ester), which then may be displaced by H e (as LiAlH4 see Table 16-6) ... 

Bordwell, F. G. Cooper, G. D. The effect of the sulfonyl group on the nucleophilic displacement of halogen in a-halo sulfones and related substances./. Am. Chem. Soc. 1951, 73, 5184-5186. 

Halogeno saccharides can be prepared by the displacement of a sulfonate ester with a halogen ion. Primary fluorides, bromides, chlorides and iodides can be prepared from mesylates (R0S02CH3) and tosylates (R0S02PhMe) (Scheme 3.3a).12 The latter derivatives can be obtained cheaply by treatment of alcohols with a sulfonyl chloride in pyridine or triethylamine. 

The use of nickel-aluminum alloy in aqueous alkali for the reduction of organic compounds has been observed (60c) to bring about the displacement by hydrogen of methoxy, halogen, and sulfonic acid groups from several types of benzenoid compounds. Table III summarizes the... 

Displacement of Halogen and Sulfonic Groups by Nickel-Aluminum Alloy in Aqueous... 

Because of the presence of nitrogen in the aromatic ring, electrons in pyridine are distributed in such a way that their density is higher in positions 3 and 5 (the P-positions). In these positions, electrophilic substitutions such as halogenation, nitration, and sulfonation take place. On the contrary, positions 2, 4, and 6 (a- and y-positions, respectively) have lower electron density and are therefore centers for nucleophilic displacements such as hydrolysis or Chichibabin reaction. In the case of 3,5-dichlorotrifluoropyridine, hydroxide anion of potassium hydroxide attacks the a- and y-positions because, in addition to the effect of the pyridine nitrogen, fluorine atoms in these position facilitate nucleophilic reaction by decreasing the electron density at the carbon atoms to which they are bonded. In a rate-determining step, hydroxyl becomes attached to the carbon atoms linked to fluorine and converts the aromatic compound into a nonaromatic Meisenheimer complex (see Surprise 67). To restore the aromaticity, fluoride ion is ejected in a fast step, and hydroxy pyridines I and J are obtained as the products [58],... 

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