Chloroenamine

Rearrangement of an enamine to a Sehiff s base through N- to C-alkyl migration was reported 729). These authors also found that enamines, rather than aminals, were formed from butyraldehyde and seeondary amines (730). Chloramines and aeetylenes reacted to give chloroenamine intermediates, which hydrolyzed on work-up of the reactions (731). 

In the Meth-Cohn quinoline synthesis, the acetanilide becomes a nucleophile and provides the framework of the quinoline (nitrogen and the 2,3-carbons) and the 4-carbon is derived from the Vilsmeier reagent. The reaction mechanism involves the initial conversion of an acylanilide 1 into an a-iminochloride 11 by the action of POCI3. The a-chloroenamine tautomer 12 is subsequently C-formylated by the Vilsmeier reagent 13 derived from POCI3 and DMF. In examples where acetanilides 1 (r = H) are employed, a second C-formylation of 14 occurs to afford 15 subsequent cyclisation and... 

The nucleophilic attack of f-butyllithium on lithium vinylidene carbenoids has also been used for synthetic purposes in as far as the reaction permits to generate sterically hindered alkenes. Thus, treatment of the dibromoalkene 78 generated from adamantanone with an excess of f-butyllithium results in the formation of the alkene 79 that contains three bulky substituents at the double bond (equation 43) . In an analogous way, a f-butyl residue is introduced into chloroenamine 80 (equation 44) . 

Both 2-aminoethanethiol and 2-aminothiophenol can be reacted with the same chloroenamine 282 to give 121 and 122 (Scheme 60) <2005RJ0508>. 

Reaction of chloramines and acetylenes to give chloroenamine intermediates [196]. 

A variety of substituted 3-aminobenzo[6]thiophenes have been obtained by ring closure reactions of nitriles (equation 57). Various o -cyanophenylthioacetyl derivatives, when treated with base, yield the 3-amino derivatives (Section 3.15.2.2.3). The benzonitriles may be obtained by displacement of the o-nitro group from o -nitrobenzonitriles (74JOC3440). 2-Arylthio-l-chloroenamines are cyclized to 3 -dialkylaminobenzo[6]thiophenes in the presence of Lewis acid catalysts (equation 58). The 1 -chloroenamines may be prepared from t-amides or ynamines <8lH(15)l 179>. 

Keteniminium salts are more electrophilic than ketenes and are thus able to react with less nucleophilic olefins. Ketoketeniminium salts can be conveniently prepared from the corresponding a-chloroenamines and Lewis acids.3 However, the method cannot be applied well to the preparation of the less stable aldoketeniminium salts. 

CHLOROCYCLOPROPYLIDENE METHYL ESTER (82979-45-1) 69, 148 (Chlorodimethylsilyl)methyl chloride, 69. 98 a-Chloroenamines, 66, 119... 

Figure 6.12 shows that carboxylic acids can also be converted into acid chlorides without releasing HC1. This is possible when carboxylic acids are treated with the chloroenamine A. First the carboxylic acid adds to the C=C double bond of this reagent electrophilically (see Figures 3.51 and 3.53). Then, the addition product B dissociates completely to give the ion pair C. This constitutes the isopropyl analog of the Vilsmeier-Haack intermediate B of the DMF-catalyzed carboxylic chloride synthesis of Figure 6.11. The new Vilsmeier-Haack intermediate reacts exactly like the old one (cf. previous discussion) The chloride ion undertakes an SN reaction at the carboxyl carbon. This produces the desired acid chloride and isobutyric N, IV-dimethylamide.

Obviously, the elimination goes through a-chloroenamines, which have themselves become useful synthones 70,71). Using two or more equivalents of a base, ynamines are formed in a straightforward manner (38, 39) 66,72). 

Amide should be first transformed to its hydrochloride in order to cut down the formation of a-chloro-jf(-chlorocarbonyl enamines 72,73). The method is very general but the choice of experimental conditions and of the base is critical and yields may vary strongly depending on the case (39). There is also one side-reaction which can hardly be avoided the ynamine already formed reacts with the yet unreacted amide chloride, the a-chloroenamine 70) or even with the tert-amine hydrochloride to form the very stable cyclobutene cyanines and/or allenamidinium salts. These by-products have an interest of their own and will be discussed later. Formation of these salts may strikingly lower the yield of ynamines, but because of their salt character they are not harmfull during the work-up72). 

Secondary amides react with two equivalents of phosgene or PI to N-acylated chloroenamines 18, 19 respectively (42) 81-82). 

Chloroenamines 85) under the same conditions are deprotonated in a to the chlorine atom. Chlorine is then substituted by alkyl from alkyllithium which leads again to /1-lithiated enamines 84). 

Grignard reagents are not enough basic to abstract a vinylic hydrogen, nor do they participate in halogen metal exchange. As shown, they instead substitute the reactive chlorine atom in a-chloroenamines. More recent work has shown that 87 a, R = Me can be obtained from co-chloroacetophenone and tris-dimethylamino) arsine in 86 % yield I61). 

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