Rhodium enolate complex

The four most common methods for the synthesis of late transition metal enolates are oxidative addition to halocarbonyl compoxmds, ligand metathesis with main group enolates or silyl enol ethers, nucleophilic addition of anionic metal complexes to halocarbonyl electrophiles, and insertion of an a,3-imsaturated carbonyl compoimd into a metal hydride. Examples of these synthetic routes are shown in Equation 3.47-Equation 3.50. Equation 3.47 shows the synthesis of a palladium enolate complex by oxidative addition of ClCHjC(0)CHj to Pd(PPh3), Equation 3.48 shows the synthesis of a palladium enolate complex by the addition of a potassium enolate to an aryl Pd(II) halide complex, and Equation 3.49 shows the synthesis of the C-bound W(II) enolate complex in Figure 3.7 by the addition of Na[( n -C5R5)(CO)jW] to the a-halocarbonyl compound. Finally, Equation 3.50 shows the synthesis of a rhodium enolate complex by insertion of but-l-en-3-one into a rhodium hydride. This last route has also been used to prepare enolates as intermediates in reductive aldol processes. - ... 

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... 

This catalytic reaction was believed to proceed analogously to those with phenylboronic acids (Scheme 49) 137 137a Transmetallation of the arylstannane with the cationic rhodium complex generated the rhodium aryl species a and trimethyltin tetrafluoroborate. Conjugate addition generated rhodium enolate b, which subsequently reacted with... 

This strategy also gives access to a variety of non-natural a-amino acids. Furthermore, rhodium-DuPHOS complexes catalyse the asymmetric reduction of enol esters of the type PhCH = CH — C(OCOCH3) = CH2 to give (R)-2-acetoxy-4-phenylbut-3-ene (94% ee)[64]. 

Although the pattern of this reaction is identical to that of the Morita-BayUs-Hilbnan reaction [6], we concluded on the basis of control experiments that the presence of a rhodium or ruthenium complex is crucial for smooth coupling. The possible intermediacy of the enolate complex in the above examples suggests a transition metal-cata-... 

The decomposition of the peroxometallacycle (72a) or (72b) occurs in a way different from that previously shown to occur in the epoxidation of alkenes by molybdenum-peroxo complexes (equation 26). The three possibilities are shown in equations (58)-(60) and involve (a) a [C-/6, C-a] hydride shift which directly produces the methyl ketone and the rhodium-oxo complex, or the hydroxo species from (72b equation 58) (b) a [C-/3,0-/3] hydride shift which gives enol (equation... 

Platinacyclobutane complex 118 undergoes equilibrium heterolytic scission of the exocyclic carbon-carbon bond to form a cationic allyl complex and the organic enolate ion (Equation 35) <1993OM3019>. Similar dissociative ionization was previously reported for rearrangements of iridium and rhodium metallacyclobutane complexes formed by nucleophilic alkylation < 1990JA6420>. This carbon-carbon bond activation is generally associated with reversible central carbon alkylation of Jt-allyl complexes (Section 2.12.9.3.3), but the homolytic equivalent has recently been... 

Enolates can also be prepared by rhodinm-catalyzed isomerization of allylic Uthinm alcoholates, such as 14 (equation 5)". Subsequent treatment of the intermediately formed rhodium hydride complexes (15 and 16) with an electrophile led to the formation of various products. For example, alkyl halides gave a-alkylated ketones (17) in good yields, as shown in Table 4. Interestingly, addition of benzaldehyde under kinetically controlled... 

The rhodium/11 complex is also an effective catalyst for the hydrogenation of enol phosphinates giving optically active secondary alkyl alcohols with up to 78% ee (Scheme 2-45) [64], although the role of the hydroxyl group on the ligand remains to be clarified. 

Figure 2. Absorption spectra of rhodium(I) complexes containing olefins and other ligands in hexane acac, enolate anion of acetylacetone coe, cyclooctene dbmr, enolate anion of dibenzoylmethane.

If rhodium enolates are used in a catalytic cycle they can promote aldol reactions under reasonably mild conditions. For example, the aldol reactions of trimethylsilyl enol ethers and ketene silyl acetals (37) with aldehydes can be catalyzed by various rhodium(I) complexes, under essentially neutral conditions, to give p-trimethylsiloxy ketones and esters (38 equation 14 and Table 6). The study of Matsuda and coworkers suggests that use of the rhodium complex Rlu(CO)i2 (39 at 2 mol %) in benzene at 100 C gives best results for the formation of adduct (38 Table 6, entries 1-7). There is negligible diastereoselectivity in most cases. Various cationic ihodium complexes such as (40) also catalyze the reaction. Reetz and Vougioukas have found that this aldol reaction proceeds well with the more reactive ketene silyl acetals, (37) for R = OMe or OEt, in CH2CI2 at room temperature (Table 6, entries 8-13). The intermediacy of an ti -O-bound rhodium enolate, such as (41), in the catalytic cycle is like-... 

Crossed aldol reactions of enol silyl ethers with aldehydes have been successfuly performed with the aid of catalytic amounts of the rhodium complex ((COD)Rh(DPPB)]+X (X = PFs or CIO4) or Rh4(CO)i2. Although the intermediacy of rhodium enolate has been suggested for these reactions, the fact that the same rhodium catalysts can promote the condensations of acetals as well (Scheme 38) tends to indicate that the reactive species may not be a metal enolate. 

In addition to rhodium phosphane complexes, ruthenium phosphane complexes have also been successfully applied as catalysis for enantioseleetive hydrogenation of 2-acylamino-2-alkenoic acids and esters1 71,72b 3, enol acetates 18 (R = i-Pr E = COOEt X = OCOCH3 98% ee with BINAP)137, and itaeonic acid138. The absolute configuration of the products from the ruthenium-catalyzed reactions shown below is opposite to that obtained with the corresponding rhodium catalysts. 

The authors proposed an electrophilic rhodium carbene complex to account for formation of 129. Thus reaction of 128 with Rh2(OAc)4 generates the carbene complex 130, which is trapped by benzonitrile to afford the nitrilium species 131. Cyclization of 131 through the enolate oxygen then yields 129 (Scheme 1.36). 

A hydrophobic polymer-supported scandium(III) catalyst was also successfully used in the Michael reaction of unsaturated ketones with silyl enol ethers. Recently, an amphiphilic resin-supported rhodium/phosphine complex was used as catalyst in the 1,4-addition of various boronic acids to enones in water at 25°C. High yields were obtained and the catalyst was easily separated and subjected to a second and third round of reactions with no decrease in activity. ... 

The methyl hydrogens in the tri-p-phenoxo complex [ Rh(C5Mes) 2(OPh)3][(PhO)5H4] exchange moderately quickly ti/2 — 40 min at 20°C) with deuterium in [ He] acetone solution. A mechanism was proposed involving formation of the enolate complex 4, followed by transfer of a methyl hydrogen to the metal to form a fulvene-rhodium hydride species 5 from which the H can be eliminated by transfer to enolate to give coordinated acetone. 

If rhodium enolates are used in a catalytic cycle they can promote aldol reactions under reasonably mild conditions. For example, the aldol reactions of trimethylsilyl enol ethers and ketene silyl acetaU (37) with aldehydes can be catalyzed by various rhodium(I) complexes, under essentially neutral conditions, to give p-trimethylsiloxy ketones and esters (38 equation 14 and Table The study of... 

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