Of unsaturated ketones

Metal-ammonia solutions reduce conjugated enones to saturated ketones and reductively cleave a-acetoxy ketones i.e. ketol acetates) to the unsubstituted ketones. In both cases the actual reduction product is the enolate salt of a saturated ketone this salt resists further reduction. If an alcohol is present in the reaction mixture, the enolate salt protonates and the resulting ketone is reduced further to a saturated alcohol. Linearly or cross-conjugated dienones are reduced to enones in the absence of a proton donor other than ammonia. The Birch reduction of unsaturated ketones to saturated alcohols was first reported by Wilds and Nelson using lithium as the reducing agent. This metal has been used almost exclusively by subsequent workers for the reduction of both unsaturated and saturated ketones. Calcium has been preferred for the reductive cleavage of ketol acetates. 

Reductions of unsaturated ketones and a-acetoxy ketones usually are effected with an excess of reducing agent. For optimum yields of saturated ketones, the intermediate enolate salt obviously must not become protonated while... 

The halogenation-dehydrohalogenation of ketones and their derivatives is the most widely applied method for the preparation of unsaturated ketones, and the different combinations of alternatives which exist for both steps extend the scope of this approach. Consequently, this route will be discussed in considerable detail. 

Halogenation of the double bond usually prevents the satisfactory preparation of vinylogous a-halo ketones by direct reaction of unsaturated ketones with... 

The dehydrogenative condensation of unsaturated ketones with methyl ketones was used for preparing various series of 2,4,6-tri-arylpyrylium salts not only by Dilthey, but also by Wizinger and co-workers (for combinations of phenyl, p-anisyl, and p-dimethyl-aminophenyl substituents), by Amoros-Marin and Carlin (combinations of phenyl and p-chlorophenyl), by Le Fevre and Le Fevre (for combinations of pbenyl and m- or p-nitrophenyl), and by others. ... 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

The interconversioir of unsaturated ketones described in Problem 22.36 is also catalyzed by base. Explain. 

The titaniated (25)-2,5-dihydro-2-isopropyl-3,6-dimethoxypyrazines derived from cyclo(L-Val, Gly) or cyclo(L-Val, Ala) (1, R1 = H, CH3) react with a,/I-unsaturatcd aldehydes exclusively by 1.2-addition (cf. nearly exclusive 1,4-addition of ,//-unsaturated ketones with cuprate complexes of 2,5-dialkoxy-3,6-dihydropyrazines, see Section D. 1.5.2.3.1.4.) in a highly diastereoselective mode to give virtually only the (l S,2R)-diastereoniers 2 ". In reactions with the corresponding lithiated pyrazines both regioselectivity and diastereofacial differentiation at C-2 are also remarkably high (dc 95 %), but the diastereomeric excess at C-l is substantially smaller (30 50%) ... 

In contrast, the acid-catalyzed hydration of arylbenzoylacetylenes differs markedly from the hydration of a-unsaturated ketones. Hydration of unsaturated ketones has been shown to proceed via a 1,4-addition mechanism where protonation occurs on oxygen to give an oxonium salt, followed by attack of water at the 0-carbon to give a hydroxy enol. The rate-limiting step has been shown to be the protonation of the hydroxy enol (27) ... 

Schuster, D. I. Krull, I. S. Photochemistry of unsaturated ketones in solution. XIX. Photochemistry of spiro[2.5]octa-4,7-dien-6-one. 2. Mechanistic aspects and the relationship to the photochemistry of quinone methides. Mol. Photochem. 1969, 1, 107-133. 

If the solution becomes too alkaline, the nitrile formed will add to a second molecule of unsaturated ketone so readily that the product will consist almost entirely of a high-melting (284-286°) substance. For this reason it is essential to measure the acetic acid accurately if too much is used, addition of hydrocyanic acid will not take place. 

Table 15.10 Hydrogenation of unsaturated ketone at 28°C and 4 bar H2 (keto-ne RuCI2(PPh3)3 H2N(CH2)2NH2 KOH = 500 1 1 2).

Reduction of a., -unsaturated carbonyl compounds. Hydrosilanes, particularly (QH,)2SiH2, in the presence of Pd(0), and a Lewis acid, particularly ZnCl2, can effect selective conjugate reduction of unsaturated ketones, aldehydes, and carboxylic acid derivatives. Chloroform is the solvent of choice. In addition, 1 equiv. of water is required. Experiments with D,0 and (C6H,),SiD2 indicate that... 

One of the most active and selective catalysts in this kind of reaction is undoubtedly Ir/support, as recently demonstrated by Jacobs and coworkers [276], Therefore, by combining the carbonyl affinity of metallic iridium with the promotion effect of the H-fl zeolite, which is a strong Bronsted acid, one can reduce a large variety of unsaturated ketones and aldehydes to allylic alcohols with high conversions, selectivities, and diasteieoselectivities. 

Earlier reviews on the photochemistry of unsatured ketones and amines are available39,40. The photoreactions of a,/i-unsaturated carbonyl compounds in the presence of amines have been reported to yield 1 1 amine adducts32,33 as well as photoreduction... 

Scheme 34 Cathodic cyclization of unsaturated ketones R alkyl, yields 75-98%.

The Stereochemistry of Hydrogenation of , -Unsaturated Ketones Robert L. Augustine Asymmetric Homogeneous Hydrogenation J. D. Morrison, W. F. Masler, and M. K. Neuberg... 

Semmelhack et al. chose CuBr, together with either Red-Al or LiAl(OMe)3H in a 1 2 ratio, to afford presumed hydrido cuprates, albeit of unknown composition [llj. In THF, both the former Na complex and the latter Li complex are heterogeneous (and of differing reactivities), yet each is capable of 1,4-reductions of unsaturated ketones and methyl esters (Eq. 5.4). Commins has used a modified version, prepared from lithium tri-t-butoxy-aluminium hydride and CuBr (in a 3 4.4 ratio), to reduce a 3-substituted-N-acylated pyridine regioselectively at the a-site [12]. 

Reduction of unsaturated ketones to unsaturated alcohols is best carried out Nit v complex hydrides. a,/3-Unsaturated ketones may suifer reduction even at the conjugated double bond [764, 879]. Usually only the carbonyl group is reduced, especially if the inverse technique is applied. Such reductions are accomplished in high yields with lithium aluminum hydride [879, 880, 881, 882], with lithium trimethoxyaluminum hydride [764], with alane [879], with diisobutylalane [883], with lithium butylborohydride [884], with sodium boro-hydride [75/], with sodium cyanoborohydride [780, 885] with 9-borabicyclo [3.3.1]nonane (9-BBN) [764] and with isopropyl alcohol and aluminum isopro-... 

Reduction of unsaturated ketones to saturated alcohols is achieved by catalytic hydrogenation using a nickel catalyst [49], a copper chromite catalyst [50, 887] or by treatment with a nickel-aluminum alloy in sodium hydroxide [555]. If the double bond is conjugated, complete reduction can also be obtained with some hydrides. 2-Cyclopentenone was reduced to cyclopentanol in 83.5% yield with lithium aluminum hydride in tetrahydrofuran [764], with lithium tris tert-butoxy)aluminium hydride (88.8% yield) [764], and with sodium borohydride in ethanol at 78° (yield 100%) [764], Most frequently, however, only the carbonyl is reduced, especially with application of the inverse technique (p. 21). 

There are some examples of pyran synthesis by Method 2, which involves the reactions of unsaturated ketones with methylene-active nitriles. In a typical case of Method 2, the Michael reaction of a,j5-unsaturated carbonyl compounds 26 (77TL1835, 78JHC57,... 

Intermolecular Addition with Rearrangement Formation of Unsaturated Ketones... 

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