A-Halo ketone effect

In addition to solvolysis and nitrenium ion formation, Af-aLkoxy-A-chloroamides (2) also undergo bimolecular reactions with nucleophiles at nitrogen. Not only is the configuration destabilized by the anomeric effect, it also parallels that of a-halo ketones, where halogen on an sp carbon is activated towards reactions by the adjacent carbonyl. This rate-enhancing effect on 8 /2 processes at carbon is well-known, and has been attributed to conjugation of the p-orbital on carbon with the carbonyl jr-bond in the S 2 transition state stabilization of ionic character at the central carbon as outlined by Pross as weU as electrostatic attraction to the carbonyl carbon. The transition states are also affected by the nature of the nucleophile. ... 

Dekalogenation of a-haio ketones. This reaction can be effected with Nal in T1II- 11,0 (1 1) containing 5% H, S04 at 25". In the case of aliphatic a-halo ketones... 

The previous discussion of the halogenation of ketones is incomplete in one important respect concerning base-induced halogenation. That is, once an a-halo ketone is formed, the other hydrogens on the same carbon are rendered more acidic by the electron-attracting effect of the halogen and are replaced much more rapidly than the first hydrogen ... 

The halogen of an a-halo aldehyde or an a-halo ketone is exceptionally unreactive in SN1-displacement reactions, but is exceptionally reactive in Sn2 displacements, compared with the halogen of alkyl halides having comparable potential steric effects. Similar behavior is observed with a-halo carboxylic acids and is discussed further in Chapter 18. 

For Sn2 reactions the effect of electron-withdrawing groups is less easy to predict than for SnI reactions. a-Halo ketones, a-halo nitriles, or haloacetic acid derivatives undergo bimolecular substitutions at much higher rates than unfunctionalized alkyl halides (Scheme 4.42). 

Trichloro(methyl)silane-Sodium iodide, 11, 553-554. This in situ equivalent of io-dotrimethylsilane is also effective for cleavage of esters and lactones, selective conversion of tertiary and benzylic aleohols into iodides, dehalogenation of a-halo ketones, deoxygenation of sulfoxides, and conversion of dimethyl acetals to carbonyl compounds. ... 

Dehalogenation. Cerium(III) sulfate in combination with sodium iodide reduces a-halo ketones at room temperature in 30 minutes to the parent ketone in 80-90% yield. Presumably Cels is the effective reagent. 

Condensations. Alumina promotes the formation of a-hydroxyphosphonate esters from aromatic aldehydes and dialkyl phosphonates, and the adducts are converted to a-aminophosphonate esters on reaction with ammonia. A solvent-free synthesis of a-nitro ketones comprises mixing nitroalkanes, aldehydes, and neutral alumina and oxidizing the adducts with wet, alumina-supported CrOj (15 examples, 68-86%). The Knoevenagel reaction, the Michael addition of nitromethane to gcm-diactivated alkenes, and the formation of iminothiazolines from thioureas and a-halo ketones are readily effected with alumina under microwave irradiation. 

Alkylation (R" = alkyl or substituted alkyl) is most satisfactorily accomplished with active halogen compounds such as allyl, benzyl, and propargyl halides, but a-halo ketones, esters, and nitriles can also be used.467,474 Treating l-(l-cyclohexenyl)pyrrolidine with allyl bromide in the presence of /V-ethyl-dicyclohexylamine as base leads to introduction of two allyl groups, i.e., formation of 2,6-diallylcyclohexanone.475 Arylation can be effected by, for example, l-chloro-2,4-dinitrobenzene (R" = 2,4-dinitrophenyl).476 A-Isobutyl-fl-butylamine477 and pyrrolidine478 have been recommended as amine components for C-alkylation by simple alkyl halides such as ethyl and methyl iodides. The following two examples are illustrative ... 

Reformatsky-type reactions. In the condensation of a-halo ketones with aldehydes to form enones, SnCh and Na2S03 can be used as promoters, whereas SnCl2 alone is used to effect synthesis of )3,/3-dichloro /3-nitroalcohols from trichloroni-tromethane and aldehydes. ... 

Scheme 5 illustrates Tilak thiannulation by means of an acyclic a-halo ketone. This variation leads to an alkyl-substituted, terminally condensed thiophene ring. The cyclization of 194 to 195 (m.p. 271.5°C UV similar to that of chrysene) was effected in only 14% yield.63 In the only other use of... 

A variety of approaches have been employed to effect the preparation of a-alkenyl ketones and carblnols, including reactions of metallo alkenes with epoxides, a-halo ketones, or enolonium ion equivalents and the reactions of... 

Optically active halohydrins or styrene oxide derivatives obtained by the reduction of a-halo ketones followed by treatment with base have been widely used as key intermediates in the synthesis of many chiral drugs containing the p-amino alcohol moiety. Examples of such drugs include (R)-denopamine (13) [46a], (R)-isoproterenol (14) [46b], d-solatol (15) [47], (R)-fluoro(nor)epinephrine (16) [48], (R)-salmeterol (17) [49], and (R,R)-formoterol (18) [50] (Scheme 11.4). Most OABs were effective for the reduction of 2-bromo- or 2-chloroacetophenone derivatives (22), providing the corresponding halohydrins with high enantioselectivities (Table 11.4). 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

In the absence of steric factors e.g. 5 ), the attack is antiparallel (A) (to the adjacent axial bond) and gives the axially substituted chair form (12). In the presence of steric hindrance to attack in the preferred fashion, approach is parallel (P), from the opposite side, and the true kinetic product is the axially substituted boat form (13). This normally undergoes an immediate conformational flip to the equatorial chair form (14) which is isolated as the kinetic product. The effect of such factors is exemplified in the behavior of 3-ketones. Thus, kinetically controlled bromination of 5a-cholestan-3-one (enol acetate) yields the 2a-epimer, (15), which is also the stable form. The presence of a 5a-substituent counteracts the steric effect of the 10-methyl group and results in the formation of the unstable 2l5-(axial)halo ketone... 

A new approach to the formation of the 2-aryl thietane derivatives 124 includes Friedel-Crafts acylation to give a )9-halo ketone. Following conversion of the ketone to the chloride, cyclization is effected with thiourea ... 

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