Nickel ketone enolates

NaBHj/NiC or Raney nickel, the menthyloxy group is removed with NaBH /KOH to give 3,4-disubstituted butyrolactones with a high diastereo- and enantioselectivity (Figure 7.69). Corey and Houpis [1458] have described asymmetric Michael reactions of ketone enolates with a 2-thiophenyl crotonate of 8-phenmenthol. Chirality has also been introduced on the amino group of 2-ami-nomethyiacrylates to perform the asymmetric addition of the anion of the tert-Bu ester of cyclopentanecarboxylate [1459], More important developments have been reported with chiral a,p-unsaturated sulfoxides and nitro compounds as Michael acceptors (see below). 

The scope of catalytic ring-opening [3+2] cycloadditions was expanded to include simple cyclopropyl ketones, which combine with vinyl ketones to provide substituted cyclopentane derivatives (Scheme 3-32). Cyclopropyl imines were also found to be effective cycloaddition substrates under similar conditions, as illustrated in the sample procedure below. " Oxidative additions of nickel(O) to cyclopropyl ketones were found to produce six-membered metallacyclic nickel (9-enolates, which were competent species in the [3+2] cycloaddition process. " ... 

Scheme 8.75 Nickel-P-Phos-catalyzed a-arylation of ketone enolates, as described by Kwong and Chan [127],...

Bis-thioesters undergo smooth Dieckmann cyclizations [NaH EtSH(cat.), (MeOCH2)2, 2—6 h, 20 °C] to give cyclic j8-keto-thioesters. As with the oxygen analogues, only five- or six-membered rings can be efficiently prepared in this way. Decarboxylation occurs on desulphurization with W2 Raney nickel (EtOH, 20 °C). j8-Keto-thioesters are also available by treatment of ketone enolates with excess S,5 -dimethyl dithiocarbonate " they can also be alkylated in much the same way as jS-keto-esters. ... 

Dimethyl ketals and enol ethers are stable to the conditions of oxime formation (hydroxylamine acetate or hydroxylamine hydrochloride-pyridine). Thioketals and hemithioketals are cleaved to the parent ketones by cadmium carbonate and mercuric chloride. Desulfurization of thioketals with Raney nickel leads to the corresponding methylene compounds, while thioenol ethers give the corresponding olefin. In contrast, desulfurization of hemithioketals regenerates the parent ketone. ... 

In neutral medium A16-, A17(20)-, and A20-olefins are hydrogenated over palladium in preference to the double bonds of A4-3-ketones.67 The double bonds of A4-3-ketones and A16-20-ketones are reduced in preference to A5-,93 A7-48 155 and A9(I -olefins.2 169 The double bond of a A14-16-ketone is saturated before a 5-ene.70,163 In basic medium the carbonyl conjugated double bond is preferentially saturated in all cases92 presumably because of the strong adsorption of the enolate anion (see section II-C). Debromination (9a-bromo 11-ketone) occurs before saturation of the A4-3-keto double bond14 over Raney nickel but hydrogenation of benzyl ethers takes place concurrently with the hydrogenation of this double bond over palladium in neutral medium.96... 

Allyl cyanides can be added across alkynes in the presence of a nickel catalyst prepared from (COD)2Ni and (4-CF3CeH4)3P in situ to give functionalized di- or tri-substituted acrylonitriles in a highly stereoselective manner, presumably via n-allylnickel intermediates. a-Siloxyallyl cyanides also react at the y -position of a cyano group with both internal and terminal alkynes to give silyl enol ethers, which can be converted into the corresponding aldehydes or ketones upon hydrolysis.70... 

Oxygenation of silyl enol ethers. Oxygenation of a silyl enol ether under the conditions cited above results in a silyloxy epoxide, which rearranges spontaneously to an a-silyloxy ketone. The preferred Ni catalyst for this epoxidation is bis(3-methyl-2,4-pentanedionato)nickel(II), Ni(mac)2. The a-silyloxy ketone is converted... 

The sulfide (SR) can be removed from the product with Raney nickel to give a simple ketone. This ketone has apparently been made by the alkylation of a silyl enol ether with a primary alkyl group (R2CH2). This would be impossible without stabilization of the cation by the sulfur atom. 

Phenylthioalkylation of silyl enol ethers. Silyl enol ethers of ketones, aldehydes, esters, and lactones can be alkylated regiospecifically by a -chloroalkyl phenyl sulfides in fhe presence of a Lewis acid. Zinc bromide and titanium(IV) chloride are the most effective catalysts. The former is more satisfactory for enol ethers derived from esters and lactongs. ZnBr2 and TiCL are about equally satisfactory for enol ethers of ketones. The combination of TiCL and Ti(0-f-Pr)4 is more satisfactory for enol ethers of aldehydes. Since the products can be desulfurized by Raney nickel, this reaction also provides a method for alkylation of carbonyl compounds. Of more interest, sulfoxide elimination provides a useful route to a,B-unsaturated carbonyl compounds. 

The main product in hydrosilation of a,P-unsaturated ketones and aldehydes catalyzed by chloro-platinic acid, platinum on alumina, or metallic nickel is the corresponding silyl enol ether. With nickel catalyst, product distribution is highly dependent on the enone structure. Hydridosilanes add to a, -unsaturated esters, producing the corresponding silyl enolate as well as carbon silylated products. The course of addition depends on substrate structure and the hydridosilane utilized. Thus, triethylsilane undergoes 1,4-addition to methyl acrylate in the presence of chloroplatinic acid, while trichlorosilane with either chloroplatinic acid or Pt/C gives the -silyl ester (Scheme 65). ... 

Methyl and methylene groups adjacent to carbonyl groups are easily oxidized to carbonyls to yield a-keto aldehydes or a-diketones. The reagent of choice is selenium dioxide or selenious acid. The reaction is catalyzed by acids and by acetate ion and proceeds through transition states involving enols of the carbonyl compounds [518]. The oxidation is carried out by refluxing the ketone with about 1.1 mol of selenium dioxide in water, dilute acetic acid, dioxane, or aqueous dioxane [517]. The byproduct, black selenium, is filtered off, but small amounts of red selenium sometimes remain in a colloidal form and cannot be removed even by distillation of the product. Shaking the product with mercury [523] or Raney nickel [524] takes care of the residual selenium. The a-dicarbonyl compounds are yellow oils that avidly react with water to form white crystalline hydrates (equations 407 and 408). 

Cobalt catalysts were used for substitution reactions of organic halides [442] and a small-ring enol lactone [Eq. (201) 443]. The cobalt salt seems to be the most effective one among a few transition metal salts involving copper, nickel, [see Eq. (130)], and iron. Without the catalyst, Grignard reagent attacked the carbonyl carbon of the enol lactone to give a mixture of a few ketonic compounds in a low yield. 

Many other metal ions have been reported as catalysts for oxidations of paraffins or intermediates. Some of the more frequently mentioned ones include cerium, vanadium, molybdenum, nickel, titanium, and ruthenium [21, 77, 105, 106]. These are employed singly or in various combinations, including combinations with cobalt and/or manganese. Activators such as aldehydes or ketones are frequently used. The oxo forms of vanadium and molybdenum may very well have the heterolytic oxidation capability to catalyze the conversion of alcohols or hydroperoxides to carbonyl compounds (see the discussion of chromium, above). There is reported evidence that Ce can oxidize carbonyl compounds via an enol mechanism [107] (see discussion of manganese, above). Although little is reported about the effectiveness of these other catalysts for oxidation of paraffins to acetic acid, tests conducted by Hoechst Celanese have indicated that cerium salts are usable catalysts in liquid-phase oxidation of butane [108]. 

One solution is a [2,3] sigmatropic rearrangement of a sulfonium ylid. A stable allylic thiol reacts cleanly with an a-haloketone 64 to give a sulfide and hence the sulfonium salt after ethylation. Only a weak base is need to make the ylid 65 as this is also an enolate. Notice the regioselectivity here the alternative ylid would also be stabilised but only by an alkene. The [2,3] shift joins the allylic fragment to the enolate of the ketone and at the same time turns it inside out. This automatically produces the more difficult product 63 after desulfurisation with Raney nickel. We shall see alternative solutions to this problem later in the chapter.9... 

Synthesis ofp-epoxy ketones.20-21 Nickel carbonyl reacts with an a-bromo ketone (DMF, argon, 30°, 5 hrs.) to give a/3-epoxy ketone, for example (2). Yields are in the range 50-80%. The reaction is considered to proceed via an aldol-type condensation between the ketone and the nickel enolate followed by elimination of NiBr2. 

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