Unsaturated carbonyl compounds rhodium

Rhodium-catalysed addition (10) of hydridosilanes (Chapter 17) to a/3-unsaturated carbonyl compounds can be performed regioselectively, to afford either the product of 1,2-addition, or, perhaps more usefully, that of 1,4-addition, i.e. the corresponding silyl enol ether this latter process is an excellent method for the regiospecific generation of silyl enol ethers. Of all catalyst systems investigated, tris(triphenylphosphine)rhodium(l) chloride proved to be the best. 

In addition, Wu and Li recently have developed an efficient rhodium-catalyzed cascade hydrostannation/conjugate addition of terminal alkynes and unsaturated carbonyl compounds in water stereoselectively (Scheme 4.5).88... 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

Besides rhodium catalysts, palladium complex also can catalyze the addition of aryltrialkoxysilanes to a,(3-unsaturated carbonyl compounds (ketones, aldehydes) and nitroalkenes (Scheme 60).146 The addition of equimolar amounts of SbCl3 and tetrabutylammonium fluoride (TBAF) was necessary for this reaction to proceed smoothly. The arylpalladium complex, generated by the transmetallation from a putative hypercoordinate silicon compound, was considered to be the catalytically active species. 

The use of organosilanes for rhodium-catalyzed conjugate addition is also feasible. In 2001, Mori reported that the catalytic conjugate addition of aryl(ethyl)silanediols to a,f -unsaturated carbonyl compounds takes place in the presence of [Rh(OH)(COD)]2 in a... 

Analysis of the Mukaiyama-type aldol coupling (Eq. 2) and the well-known hydrosilyla-tion of a,/l-unsaturated carbonyl compounds 11 in the presence of a rhodium catalyst, indicate that both can be explained by the intervention of the rhodium enolate 13. This line of reasoning provided the impetus to develop a new crossed aldol coupling using a hydrosilane, an a,yS-unsaturated ketone 11, and an aldehyde to form 15 (Scheme 6.4). 

The 1,4-addition reaction of unsaturated carbonyl compounds with Ph3Bi smoothly proceeded in the presence of rhodium catalysts (Scheme 54) [67,68]. Interestingly, the reaction can be conducted in a H20/THF mixture under air. Methyl cinnamate, having an OH group on the aromatic ring, was efficiently phenylated without the protection of the OH group. 

Palladium(II) compounds have unique characteristics suitable for efficient catalysed cyclopropanation of electron-deficient alkenes using diazoalkanes. Neither copper nor rhodium(II) catalysts have shown comparable reactivity with diazoalkanes, although these catalysts are superior to palladium(II) catalysts for cyclopropanation with diazocarbonyl compounds. A few examples of palladium(II) catalysed cyclopropanation of a,fl-unsaturated carbonyl compounds with diazoalkanes are shown in equations 20-242 °. 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Diarylindium(III) hydroxides, Ar2InOH, have been shown to react with a,p-unsaturated carbonyl compounds R1CH=CHCOR2 in the presence of a rhodium catalyst to afford the 1,4-addition products R1CH(Ar)CH2R2 in high yield.252... 

A rhodium(l)-catalyzed system in THF is also effective in the Mizoroki-Heck-type reaction of arylsilanediols with acrylates (Scheme 4).53 Interestingly, the use of aqueous THF switches the reaction to 1,4-addition forming /3-arylated esters. The proposed catalytic cycles for these reactions involve 1,4-addition of an arylrhodium species to an acrylate. The change of the reaction pathway is probably because, in aqueous THF, the resultant Rh enolate 6 undergoes protonolysis rather than /3-elimination. Similar Rh-catalyzed 1,4-additions to a,/3-unsaturated carbonyl compounds have been achieved with arylsilicones,54 arylchlorosilanes,55 and aryltrialkoxysilanes.56,57 The use of a cationic Rh-binap complex leads to highly enantioselective 1,4-additions of alkenyl- and arylsilanes.58 583... 

A combination of rtiodium(III) chloride with silver acetate, and treatment of rhodium(II) acetate in acetic acid solution with ozone, are two methods for generation of the (is-oxotrimetal-acetato complex of rhodium [Rhs0(0Ac)6 2O)3]0Ac. This RhsO complex was found to effect catalytic allylic oxidation of alkenes efficiently to give the corresponding a -unsaturated carbonyl compounds in the presence of a reoxidant such as r-butyl hydroperoxide, although in disappointing yield (equation 44). 

Selective reduction of n,P-unsatnrated carbonyl compounds. Japanese chemists have reported selective reduction of a,/i-unsaturated carbonyl compounds in the terpene field by use of triethylsilane and the rhodium(l) eomplex as catalyst. Thus... 

Vedejs has iso discussed the inefficiency of copper catalysts in ylidic 3,2-sigmatropic processes. Optimization of ring expansion reactions using diazomalonates were found to be unsatisfactory and analogous reactions using diazoketones were totally unsuccessful. Takano has taken advantage of a rhodium-promoted ylide formation followed by a 3,2-rearrangement, in a useful synthesis of y,8-unsaturated carbonyl compounds (Scheme 45). ... 

The copper-, rhodium-, or ruthenium-mediated cyclopropanation of electron-deficient alkenes, such as a,[S-unsaturated carbonyl compounds, acrylonitriles or vinylboronates, fails in most cases and is very inefficient in others. One of the exceptions is the copper-catalyzed cyclopropanation of but-l-en-3-one with ethyl diazoacetate. ... 

Takano, S., Tomita, S.i., Takahashi, M., and Ogasawara, K., Efficient route to Y,8-unsaturated carbonyl compounds from allyl sulfides and a-diazocarbonyls using a rhodium catalyst, Chem. Lett.. 1569, 1987. 

Selective reduction of both double bonds in a,jS-unsaturated carbonyl compounds that also contain an isolated olefin is possible with appropriate catalyst and reaction conditions . Chlorotris(biphenylphosphine)rhodium hydrogenates the isolated double bond. Thus, carvone, 13, is reduced to dihydrocarvone, 14, with careful measure of the uptake of Hj ... 

Azran, J., 0. Buchman, I. Amer, and J. Blum, Selective Hydrogenation of a, 8-Unsaturated Carbonyl Compounds by Rhodium Trichloride and Aliquat 336 in a Two-Phase Liquid System,7. MoL CataL, 34, 229 (1986). 

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