Ethylene ketones

Owing to their particular interest two individual reactions will now be discussed separately. The reaction of methoxycarbonylhydrazine and 3-bromo-2,4-pentanedione affords, in addition to the expected pyrazole (608), a pyrazolium salt (609), the structure of which was established by X-ray crystallography (74TL1987). Aryldiazonium salts have been used instead of arylhydrazines in the synthesis of pyrazolines (610) and pyrazoles (611) (82JOC81). These compounds are formed by free radical decomposition of diazonium salts by titanium(n) chloride in the presence of a,/3-ethylenic ketones. 

Under the conditions of the Corey reaction quarternary Mannich bases (10) would be expected to undergo /9-elimination to the unsaturated ketones (12) which could then react in situ with dimethylsulfoxonium methylide (11) to give the a,of-ethylene ketones (13). 

This suggestion has been utilized to prepare the 2,2-ethylene (14), 16,16-ethylene (15) and 21,21-ethylene ketones (16). ... 

The trifluorinated a, J-ethylenic ketone shown in equation 36 was prepared by two different methods and behaves like a Michael addition acceptor in reactions... 

Similarly, fluorinated ketones are prepared and react with enamines [50], This reaction involves the intermediacy of an a,P-ethylenic ketone and leads to annelation-aromatization products [5tJ] (Table 13) (equation 37). 

In contrast to a, -ethylenic ketones or even a, -ethylenic sulfones, a, ) -ethylenic sulfoxides generally are not sufficiently electrophilic to undergo successful nucleophilic j8-addition . a-Carbonyl-a, j8-ethylenic sulfoxides, however, are potent, doubly activated alkenes which undergo rapid and complete -addition of various types of nucleophiles even at — 78 °C. A brief account summarizing this area is available . The stereochemical outcome of such asymmetric conjugate additions to enantiomerically pure 2-sulfmyl 2-cycloalkenones and 2-sulfinyl-2-alkenolides has been rationalized in terms of a metal-chelated intermediate in which a metal ion locks the -carbonyl sulfoxide into a rigid conformation (36 cf. 33). In this fixed conformation, one diastereoface of the cyclic n... 

Cristau, H.J., Vors, J.P, Beziat, Y., Niangoran, C., and Cristol, H., Umpolung of a,p-ethylenic ketones and aldehydes by phosphorus groups, in Phosphorus Chemistry, Proceedings of the 1981 International Conference, Quin, L.D. and Verkade, J.G., Eds., American Chemical Society, Washington, D.C., 1981, p. 59. 

This manganese-copper-catalyzed conjugate addition reaction compares favorably with the classical copper-catalyzed reaction. The two reactions are easily and similarly carried out under mild conditions, but the first one gives higher yields. This difference, already observed in the case of p-monosubstituted o,p-ethylenic ketones, is especially noticeable with p,p-disubstituted or [Pg.70]

MANGANESE-COPPER-CATALYZED CONJUGATE ADDITION OF ORGANOMAGNESIUM CHLORIDES TO a,p-ETHYLENIC KETONES IN THF AT 0°C... 

G. Cahiez, S. Marquais, and M. Alami 135 MANGANESE-COPPER-CATALYZED CONJUGATE ADDITION OF ORGANO-MAGNESIUM REAGENTS TO a,(J-ETHYLENIC KETONES PREPARATION OF 2-(1,1 -DIMETHYL-PENTYL)-5-METHYLCYCLO-HEXANONE FROM PULEGONE... 

Similarly, aliphatic thiones are also readily reduced at microcathodes in DMF. Thus, thiocamphor exhibits a one-electron reversible step at —1.88 V versus SCE, which is about 0.9 V more positive than the potential of the reduction of camphor under the same experimental conditions. q , 8-unsaturated thiones such as (18) were also investigated polarographically in pro-tic media. Similarly [179-182], reduction of Q , 8-ethylenic ketones and aldehydes in acidic media yielded dimers. 

Q , 8-Ethylenic sulfones exhibit a behavior that could be considered as specific and totally different from that of Q , 8-ethylenic ketones or nitriles. Thus, with the present series, there is practically no case of dimerization or double-bond saturation. This seems to be because of the fact that reduction of compounds of this series cannot be completed neither in acidic nor in aqueous solutions because a fast cleavage occurs at the level of the anion radical. 

TABLE 8. Yields of a, / -ethylenic ketones. Reprinted with permission from Reference 41. Copyright (1983) American Chemical Society... 

These vinyl selenones are of interest because they undergo a 1,4-fragmentation at room temperature in the presence of bases to give an ethylenic ketone (equation I). If 1 he double bond is tetrasubstituted, the fragmentation results in an acetylenic ketone (equation II). 

Umpolung of cr,/3-Ethylenic Ketones and Aldehydes by Phosphorus Groups... 

We present here a reversible umpolung of a,p-ethylenic ketones and aldehydes, in which the ylides3are synthetic equivalents of P-acylvinylanions in two unlike pathways (i) and (ii) (Scheme T, see next page). 

The extension of this reaction to carbanions of B-keto-phosphorothiolates JM which could have led, by analogy, to a new a,g-ethylenic ketone synthesis, has brought about a different reaction path. 

The stannylation of conjugated ethylenic ketones coupled with an oxidative destanny-lation provided the synthetic means to various new carbonyl compounds (equations 207 and 208)106. 

The reaction of 1,4-bis-zinc derivatives with CuCN-2LiCl allows the preparation of a range of new polyfunctional zinc-copper reagents.309 They undergo selectively 1,4-additions on a,/3-ethylenic ketones in the presence of TMSC1 providing a new zinc-copper reagent, which can react with another electrophile (Equation (175)). 

When trifluoromethyl substituents are attached to the /i-carbon oF a./J-ethylenic ketones, they lower the LUMO level and allow [4 + 2] hetero-Diels-Alder reactions. Despite the enhanced coefficient of the a-carbon, a rcgiospecific /+attack occurs. Considering possible competitive reactions, such as [2 + 2] cycloadditions and Michael additions, the reaction is fairly chemoselec-tive. e.g. formation of 4. ... 

Cycloadditions have also been used to form germanium-carbon bonds at germanium surfaces. For example, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene react with the dimers present on Ge(100)-2xl to form a six-membered ring and two new Ge-C bonds. Reactions with other unsaturated species such as ethylene, ketones, and nitriles have also been explored. ... 

Trimethylsiloxycyclopropanes, readily available from silyl enol ethers and carbene or carbenoid species, constitute, now, very useful synthetic intermediates . Thus, they undergo acid-, base-inducedor thermal ring-opening into a-methylated aldehydes and ketones in very good yields. The reaction is highly regiospecific in the case of an unsymmetrical ketone, monomethylation can be oriented either to the a- or a -position (equation 58) °°. This sequence has been applied to ethylenic ketones and in the steroid field . ... 

Vinylcyclopropanols can be prepared either from the readily available cyclopro-panone hemiacetaP, from 1-hydroxycyclopropanecarboxaldehyde derivativesfrom a,a -dichloroacetone, from the silver Simmons-Smith cyclopropanation of a-ethylenic ketone silyl enol ethers from the dye-sensitized photo-oxygenation of alkylidene-cyclopropanes or from the ring-opening of oxaspiropentanes (cf. Section III.C). Consequently, they become participants of choice in a number of useful chemical transformations (see also Section IV.A). 

However, in a recently published, detailed study on the stereochemistry of the addition reaction of allylic Grignard reagents (mainly crotylmagnesium chloride) with a,/ -ethylenic ketones [15], addition to aryl-substituted enones occurred without any selectivity at all. The authors suggested that this was due to a single-electron transfer, since poor diastereoselectivity is observed when a carbon-carbon bond is formed by a radical- radical combination [64]. No further mechanistic details on this difference in reactivity were given. 

Aromatic a,/ -ethylenic ketones give more 1,4-addition products than aliphatic x,j -ethylenic ketones [1-4]. 

On the other hand, 1,4-addition of saturated, vinylic and phenylic Grignard reagents may be catalyzed by Cu salts [for examples, see Refs. 1-4,10,11]. When the a,j -ethylenic ketones possess a j -electron-donor group (e.g., an ether or amine group), the reaction proceeds through a 1,4-addition-elimination process [3 Eq. (3)] ... 

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