Acyl hypohalites, reactions

The acyl hypohalit reaction with iodine ob a halogen component is the well-known Prevost reaction,18i0 whioh has found some application during recent years in the field of natural prodwtB.. u. . w. In thiB ease the acyl hypoholite is acetyl hypoiodite, generated by reaction of iodine with silver nitrate in glacial acetic arid (Eq. 130). 

The first step is not a free-radical process, and its actual mechanism is not known. Compound 30 is an acyl hypohalite and is presumed to be an intermediate, though it has never been isolated from the reaction mixture. Among the evidence for the mechanism is that optical activity at R is lost (except when a neighboring bromine atom is present, see p. 899) if R is neopentyl, there is no rearrangement, which would certainly happen with a carbocation and the side products, notably RR, are consistent with a free-radical mechanism. There is evidence that the Simonini... 

Acyl hypohalites are usually prepared in situ by reaction of a metal salt of die carboxylic acid widi a halogen (equation 3). Classically the silver salt is used, but problems associated widi die preparation of dry silver carboxylates, as well as the more obvious economic factor, have led to the development of methods using mercury and thallium salts. Evidently, those functional groups which react readily widi halogens are not compatible with this approach. A major limitation of the acyl hypohalites is the readiness with which they transfer halogen atoms to alkyl radicals this property essend ly limits their use to decarboxylative halogenation reactions. 

This is a good method of synthesising symmetrical alkanes. The Kolbe reaction is usually performed using the potassium or sodium salt of the carboxylic acid. If the silver salt is reacted with bromine, then decarboxylation occurs again, but this time the alkyl bromide is formed. This reaction is called the Hunsdiecker reaction. The first step involves the formation of an acyl hypohalite, RC02X. 

Halogen cations afford hypohalogen acids or X2 + H20, or 7V-halo compounds. The HO- is obtained from water, which must be present in sufficient amount.261 The corresponding halohydrin ethers are formed when the reaction is carried out in an alcohol. Halogens and the silver salts of carboxylic acids in CC14 give acyl hypohalites RCOOX, which are added as X+ and RCOO- to C=C bonds at low temperatures.262-265... 

The Birckenbach-Goubeau-Waters reaction373-375 is another method of producing halogen cations acyl hypohalites are produced as intermediates in the reaction mixture by treating iodine (or bromine) with a molar equivalent amount of mercury acetate or silver acetate, trichloroacetate, perchlorate, sulfate, or trifluoroacetate (reactions b and c) ... 

The primary products of the Hunsdiecker reaction are probably the hypothetical acyl hypohalites that later undergo radical decomposition ... 

The route to 4-2 outlined here is unlikely to be profitable, because oxidation of Itydrazine leads to diimide, HNNH, rather than the dihydroxy hydrazine 4-4 [49], Precursor 4-3 is appeahng because it might be possible to photochemically extrade the very stable molecule benzene from it, providing 4. However, the bicyclic precursor to 4-5 suffers from the same kind of synthetic problem as 4-4. Indeed, organic compounds with N-O bonds are uncommon, and the synthesis of 4-2 or 4-3 may require some audacity. One unorthodox approach would be the reaction of a hydrazine metal salt ( rather inaccessible species [50]) with an acyl hypohalite [51], e.g. (surprisingly, 4-6 seems to be unknown) ... 

The latter is an extremely reactive species. The inductive effect of the trifluoroacetyl group makes it a good leaving group, and facilitates cleavage of the O-Br bond during reaction with an aromatic compound. The acyl hypohalites are also the active... 

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