Unsaturated ketones metal hydride

One pervasive mechanistic feature of many of the hydrogenations described in other chapters of this handbook concerns the bonding of the unsaturated substrate to a metal center. As illustrated in generalized form in Eq. (1) for the hydrogenation of a ketone, a key step in the traditional mechanism of hydrogenation is migratory insertion of the bound substrate into a metal hydride bond (M-H). 

Next to the cyclopropane formation, elimination represents the simplest type of a carbon-carbon bond formation in the homoenolates. Transition metal homoenolates readily eliminate a metal hydride unit to give a,p-unsaturated carbonyl compounds. Treatment of a mercurio ketone with palladium (II) chloride results in the formation of the enone presumably via a 3-palladio ketone (Eq. (24), Table 3) [8], The reaction can be carried out with catalytic amounts of palladium (II) by using CuCl2 as an oxidant. Isomerization of the initial exomethylene derivative to the more stable endo-olefin can efficiently be retarded by addition of triethylamine to the reaction mixture. 

The double bonds that are generally not affected by metallic hydrides may be isolated or conjugated, but double bonds that are conjugated with the C=0 group may or may not be reduced, depending on the substrate, reagent, and reaction conditions.252 Some reagents that reduce only the C=0 bonds of a,(3-unsaturated aldehydes and ketones are... 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

Allyl ketones can be prepared by trapping acylpalladium intermediates with main group metal compounds. The allyl ketone 251 can be prepared from the allyl ester 248 by trapping acylpalladium 249 with the alkylzinc compound 250 at room temperature and 1 atm [115]. Reaction of geranyl chloride (252) with the furylstannane 253 under CO pressure afforded the ketone 254, which was converted to dendrolasin (255) [93]. Aldehydes are prepared by trapping with a metal hydride. The /l,y-unsaturated aldehyde 257 is prepared by the carbonylation of the allyl chloride 256 in the presence of Bu3 SnH [116,117],... 

In the second part of the reaction of Figure 12.24, the a-palladated cyclohexanone E decomposes (since it is an alkylpalladium(II) compound with a syn-H atom in the position /3 to the metal) to an alkene, namely the previously mentioned unsaturated ketone B, and to H-Pd-Cl. In Chapter 16 this type of reaction will be repeatedly encountered in connection with the keyword /3-hydride elimination as A —> B in the Figures 16.13 and 16.14 and as step 7 of Figure 16.35 (part II). H-Pd-Cl decomposes according to the equation H-Pd-C1 —> Pd(0) + HC1, which describes a reductive elimination. Reductive eliminations will be... 

Fig. (2). The cyclization of enone (9), gives origin of two Cyclized products (10) and (11). Ketone (10), Ketone (10) is converted to the saturated ketone (14)under standard organic reactions.Bromination and dehydrobromination of ketone (14) yields the a,P-unsaturated ketone (IS), which on subjection to catalytic hydrogenation affords (16) and this on reduction, produces alcohol (17). The compound (13) yields (18) by standard reactions that are used for the transformation of (12) to (16). Reduction with metal hydride followed by oxidation affords ketone (11), which is converted to alcohol (17)...

The reduction of unsaturated carbonyl compounds by metal hydrides, and the reaction of organometallic nucleophiles with them, is a complicated story.87 It is more common than not, in each case, to get direct attack at the carbonyl group, but reaction in the conjugate position is well known. Conjugate reduction of a/i-unsaturated ketones by metal hydrides increases88 in the sequences Bu2iAlH < LiAlH4 < LiAlH(OMe)3 < LiAlH(OBu )3 and... 

The olefinic bond of a,g-unsaturated carbonyl compounds can also be reduced by two transition metal hydrides, NaHFe2(CO)881 and NaHCr2(CO)10 82 Neither reagent reduces nitriles, ketones, aldehydes or non-conjugated carbon-carbon double bonds. 

These trends agree with the frontier orbital analysis. In particular, the delivery of hydride from carbon breaks a relatively unpolarised bond, making the hydride notably soft, as we saw earlier in its capacity to attack pyridinium salts preferentially at the 4-position. The metal hydrogen bond will be more polarised, and metal hydrides should therefore be harder. Similarly, the delivery of hydride from boron will make it softer than when it is delivered from the more electropositive metal, aluminium. It also seems that, among a,(3-unsaturated carbonyl compounds, the susceptibility to conjugate reduction increases in the sequence ketones < esters < acids < amides but there are too few examples to be sure. 

Organopalladium complexes that are stabilized by chelation with amine groups undergo insertions with styrene or a,)S-unsaturated ketones , and the adducts quickly eliminate metal hydride to form olefinic products ... 

Aldol condensation between campholenic aldehyde (6.72) and an aldehyde or a ketone (6.73) produces an unsaturated derivative (6.74). It should be noted that this material contains an -double bond. The reason for this lies in the mechanism of the aldol condensation as described in Figure 6.4 and associated text. The carbonyl group of (6.74) can be reduced to the corresponding alcohol by, for example, a complex metal hydride such as lithium aluminium hydride or sodium boro-hydride. This produces the sandalwood material (6.75). Three typical examples are shown at the bottom of Figure 6.18. The first, (6.76), is known under various trade names such as Bangalol (Quest) and is... 

As an alternative to enolization and addition of a silyl halide or trillate, silyl enol ethers may be prepared by the 1,4-hydrosilylation of an a,3-unsaturated ketone. This can be done by using a silyl hydride reagent in the presence of a metal catalyst. Metal catalysts based on rhodium or platinum are most effective and provide a regiospecific approach to silyl enol ethers (1.25). 

P-hydride elimination to produce Morita-Baylis-Hilman type products [74]. In addition, Ru - H species also were found to work as the same catalyst [75]. For example, the coupling of vinyl methyl ketone and propanal (200 mol %) was catalyzed with RhH(PPh3)4 (lmol%) and RuH2(PPh3)4 (lmol%) at 40 °C for 40 h without solvent to form the unsaturated ketone 169 in good yields, 78% and 82%, respectively (Scheme 43). It was proposed that P-hydride elimination from metal-aldolates could release the a, -imsaturated P -hydroxy ketones, which were Morita-Baylis-Hilman type products. 

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