Hydroxorhodium complex-catalyzed carbon-carbon bond-forming reactions of silanediols with alpha,beta-unsaturated carbonyl compounds. Mizoroki-Heck-type reaction vs conjugate addition

1,4-Additions of arylboron, -silicon, and -bismuth compounds to α,β-unsaturated carbonyl compounds catalyzed by dicationic palladium(II) complexes

An enantioselective synthesis of cyclic and acyclic β-aryl ketone and aldehydes via Pd(II)-catalyzed 1,4-addition of Ar-m [m = B(OH)2, BF3K, Si(OMe)3, SiF3, BiAr2] to α,β-unsaturated ketones or aldehydes is described. The catalytic cycle involves transmetallation between Ar-m and Pd complexes as a key process, the mechanism of which is discussed on the basis of characterization of the transmetallation intermediate and electronic effect of the substituents. The enantioselection mechanism and efficiency of a chiraphos ligand for structurally planar α,β-unsaturated ketones are discussed on the basis of the X-ray structure of the catalyst and results of density functional theory (DFT) computational studies on the model of coordination of the substrates to the phenylpalladium(II)/(S,S)-chiraphos intermediate.

Nickel-Catalyzed Mizoroki–Heck- versus Michael-Type Addition of Organoboronic Acids to α,β-Unsaturated Alkenes through Fine-Tuning of Ligands

Various arylboronic acids reacted with activated alkenes in the presence of [Ni(dppe)Br2], ZnCl2, and H2O in CH3CN at 80 degrees C to give the corresponding Mizoroki-Heck-type addition products in good to excellent yields. Furthermore, 1 equivalent of the hydrogenation product of the activated alkene was also produced. By tuning the ligands of the nickel complexes and the reaction conditions, Michael-type addition was achieved in a very selective manner. Thus, various p- and o-substituted arylboronic acids or alkenylboronic acid reacted smoothly with activated alkenes in CH3CN at 80 degrees C for 12 h catalyzed by Ni(acac)2, P(o-anisyl)3, and K2CO3 to give the corresponding Michael-type addition products in excellent yields. However, for m-substituted arylboronic acids, the yields of Michael-type addition products are very low. The cause of this unusual meta-substitution effect is not clear. By altering the solvent or phosphine ligand, the product yields for m-substituted arylboronic acids were greatly improved. In contrast to previous results in the literature, the present catalytic reactions required water for Mizoroki-Heck-type products and dry reaction conditions for Michael-type addition products. Possible mechanistic pathways for both addition reactions are proposed.

Base-Free Mizoroki−Heck Reaction Catalyzed by Rhodium Complexes

A base-free rhodium-catalyzed Mizoroki-Heck (M-H) reaction using potassium aryltrifluoroborates as the arylating agent of alkenes and acetone as a green "oxidant" is described. Thanks to the ready availability of organoboranes, this reaction should constitute an interesting alternative to conventional M-H reactions using aryl halides.

Palladium(II)-Catalyzed Conjugate Addition of Arylsiloxanes in Water

Palladium-phosphinous acids catalyze the conjugate addition of arylsiloxanes to a wide range of alpha,beta-unsaturated substrates in water. A microwave-assisted procedure is described that uses 5 mol % of POPd1 to afford beta-substituted ketones, aldehydes, esters, nitriles, and nitroalkanes in 83% to 96% yield within 4 h. This method eliminates the need for stoichiometric additives and an excess of arylsiloxane, and does not require an inert atmosphere.

Heck-type coupling vs. conjugate addition in phosphine-rhodium catalyzed reactions of aryl boronic acids with alpha,beta-unsaturated carbonyl compounds: a systematic investigation

The competition between Heck-type coupling and conjugate addition in phosphine-rhodium catalyzed reactions of aryl boronic acids with alpha,beta-unsaturated carbonyls has been systematically investigated in a toluene-H(2)O biphasic system. Aside from the intrinsic nature of rhodium and the enolization of carbonyls, the phosphine supporting ligand on rhodium, the ratio of aryl boronic acid to alpha,beta-unsaturated carbonyl and the pH value of the aqueous phase were found to affect the competition significantly. Highly selective rhodium-based catalyst systems have therefore been developed for both Heck-type coupling and conjugate addition by synergistically tuning the supporting ligand, the boronic acid to olefin ratio and other reaction conditions. Conjugate addition with selectivity >99% and Heck-type coupling with selectivity of up to 100%, 98% and 84% for acrylates, acrylamides and methyl vinyl ketone, respectively, could be achieved in the rhodium-catalyzed reactions of aryl boronic acids with alpha,beta-unsaturated carbonyls using the corresponding optimized rhodium-based catalyst systems.

Enantioselective palladium-catalysed conjugate addition of arylsiloxanes

The complex formed from Pd(CH(3)CN)(4)(BF(4))(2) and (R,R)-MeDUPHOS is a highly enantioselective catalyst for the asymmetric conjugate addition of aryltriethylsiloxanes to a variety of unsaturated ketones, lactones and lactams.

Cascade Reactions in Total Synthesis

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Enantioselective rhodium enolate protonations. A new methodology for the synthesis of beta2-amino acids

[reaction: see text] Rhodium-catalyzed conjugate addition of an aryl boronic acid to alpha-methylamino acrylates followed by enantioselective protonation of the oxa-pi-allylrhodium intermediate provides access to aryl-substituted beta(2)-amino acids. The impact of the different variables of the reaction on the levels of enantioselectivity has been assessed.

A combined stopped-flow, electrospray ionization mass spectrometry and31P NMR study on the acetic acid-mediated fragmentation of the hydroxo-chalcogenide cluster [W3Se4(OH)3(dmpe)3]+(dmpe = 1,2-bis(dimethylphosphanyl)ethane) to yield the dinuclear [W2Se2(µ-Se)2(µ-CH3CO2)(dmpe)2]+complex

The reaction of the incomplete-cuboidal [W(3)Se(4)(OH)(3)(dmpe)(3)](+) ([1](+)) cluster with acetic acid in acetonitrile solution leads to cluster fragmentation with formation of the dinuclear [W(2)Se(2)(mu-Se)(2)(mu-CH(3)CO(2))(dmpe)(2)](+) ([2](+)) complex. The X-ray structure of [2]PF(6) presents two equivalent metal centres bridged by one acetate ligand. Each W atom is additionally coordinated by one terminal selenium atom, two bridging selenido and two diphosphane phosphorus atoms in an essentially octahedral environment. Stopped-flow and conventional UV-vis studies indicate that fragmentation of [1](+) into [2](+) occurs through a complex mechanism. Three steps can be distinguished in the stopped-flow time scale, all of them showing a first order dependence with respect to the acetic acid concentration, followed by very slow spectral changes that lead to the formation of [2](+). Phosphorus NMR, electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS) have been used to identify the nature of the reaction intermediates formed in the different steps. These studies indicate that the first two steps correspond to the formal substitutions of the hydroxo ligands at two metal centres by terminal acetate ligands. The third step involves bridging of one of the terminal acetate ligands, which actually prepares the trinuclear cluster to afford the acetate-bridged [W(2)Se(2)(mu-Se)(2)(mu-CH(3)CO(2))(dmpe)(2)](+) ([2](+)) complex. Although the precise details of the final conversion to [2](+) have not been established, the results obtained by combination of the different experimental techniques provide a complete picture of the speciation of the cluster [1](+) in acetonitrile solutions containing acetic acid.

Copper Salt Catalyzed Addition of Arylboronic Acids to Azodicarboxylates

The addition of arylboronic acids 1 to azodicarboxylates 2 in the presence of a catalytic amount of a copper salt under mild reaction conditions gives aryl-substituted hydrazines 3 in high yields. The reaction is tolerant of a wide variety of functional groups.

Organosilanols as Catalysts in Asymmetric Aryl Transfer Reactions

[reaction: see text] Various ferrocene-based organosilanols have been synthesized in four steps starting from achiral ferrocene carboxylic acid. Applying these novel planar-chiral ferrocenes as catalysts in asymmetric phenyl transfer reactions to substituted benzaldehydes afforded products with high enantiomeric excesses. The best result (91% ee) was achieved in the addition to p-chlorobenzaldehyde with organosilanol 2b, which has a tert-butyl substituent on the oxazoline ring and an isopropyl group on the silanol fragment.

Rhodium/diene-catalyzed asymmetric 1,4-addition of arylboronic acids to α,β-unsaturated Weinreb amides

Rhodium/chiral diene (S,S)- complex has been found to effectively catalyze the 1,4-addition of arylboronic acids to alpha,beta-unsaturated Weinreb amides, furnishing useful beta-chiral Weinreb amides in high enantioselectivity.

Mechanistic studies on the catalytic cycle of rhodium-catalyzed asymmetric 1,4-addition of aryltitanate reagents to alpha,beta-unsaturated ketones

Addition of lithium aryl(tetraisopropoxy)titanates [ArTi(OPr-i)(4)(-)Li(+)] to alpha,beta-unsaturated ketones proceeded with high enantioselectivity (up to 99% ee) in the presence of an excess amount of chlorotrimethylsilane and a rhodium catalyst (3 mol % Rh), generated from [RhCl(C(2)H(4))(2)](2) and (S)-binap, in tetrahydrofuran at 20 degrees C to give high yields of the corresponding silyl enolates as 1,4-addition products. The presence of chlorotrimethylsilane is essential for the 1,4-addition to take place. (31)P NMR spectroscopic studies revealed that the catalytic cycle consists of three transformations, that is, (i) insertion of an enone into arylrhodium species forming (oxa-pi-allyl)rhodium intermediate, (ii) silylation of the (oxa-pi-allyl)rhodium with chlorotrimethylsilane giving silyl enolate and a chloro-rhodium complex, and (iii) transmetalation of aryl group from aryltitanate to the chloro-rhodium regenerating the aryl-rhodium.

Ruthenium-Catalyzed Heck-Type Olefination and Suzuki Coupling Reactions: Studies on the Nature of Catalytic Species

Ruthenium-catalyzed Heck olefination and Suzuki cross coupling reactions have been developed. When starting with a ruthenium complex [RuCl(2)(p-cymene)](2) as a homogeneous catalyst precursor, induction periods were observed and ruthenium colloids of zero oxidation state were generated under catalytic conditions. Isolated ruthenium colloids carried out the olefination, implying that active catalytic species are ruthenium nanoclusters. To support this hypothesis, ruthenium nanoparticles stabilized with dodecylamine were independently prepared via a hydride reduction procedure, and their catalytic activity was subsequently examined. Olefination of iodobenzene with ethyl acrylate was efficiently catalyzed by the ruthenium nanoparticles under the same conditions, which could be also reused for the next runs. In poisoning experiments, the conversion of the olefination was completely inhibited in the presence of mercury, thus supporting our assumption on the nature of catalytic species. No residual ruthenium was detected from the filtrate at the end of the reaction. On the basis of the postulation, a heterogeneous catalyst system of ruthenium supported on alumina was consequently developed for the Heck olefination and Suzuki cross coupling reactions for the first time. It turned out that substrate scope and selectivity were significantly improved with the external ligand-free catalyst even under milder reaction conditions when compared to results with the homogeneous precatalyst. It was also observed that the immobilized ruthenium catalyst was recovered and reused up to several runs with consistent efficiency. Especially in the Suzuki couplings, the reactions could be efficiently carried out with as low as 1 mol % of the supported catalyst over a wide range of substrates and were scaled up to a few grams without any practical problems, giving coupled products with high purity by a simple workup procedure.

Triethylaluminum- or triethylborane-induced free radical reaction of alkyl iodides and alpha,beta-unsaturated compounds

A series of alpha,beta-unsaturated compounds, 1a-c, 9, 13, and 17, were used as reactants in free radical conjugate addition reactions with different radicals generated from alkyl iodides such as 3, 4, or 5 in the presence of triethylborane-oxygen in air or via the use of triethylaluminum-benzoyl peroxide as a free radical initiator. When the reactions were carried out using triethylborane-air, the products, in most cases, were clean and were easily purified. However, higher yields of the 1,4-adducts and less side reactions occurred when less reactive substrates were used as Michael acceptors in reactions with triethylaluminum-benzoyl peroxide and alkyl iodide under similar conditions. A mechanism for this is proposed in Scheme 1.

Oxidative heck-type reaction involving cleavage of a carbon-phosphorus bond of arylphosphonic acids

Cleavage of a carbon-phosphorus bond is achieved under palladium catalysis in an oxidative Heck-type reaction which exploits arylphosphonic acids. The reaction of arylphosphonic acids with alkenes provides arylation products in good yields in the presence of TBAF with trimethylamine oxide as an oxidant.

Asymmetric 1,4-addition of organosiloxanes to alpha,beta-unsaturated carbonyl compounds catalyzed by a chiral rhodium complex

Highly enantioselective 1,4-addition of organosiloxanes to alpha,beta-unsaturated carbonyl compounds was found to be catalyzed by a chiral rhodium complex generated from [Rh(cod)(MeCN)(2)]BF(4) and (S)-BINAP. Both (E)- and (Z)-1-alkenyl groups as well as aryl groups can be introduced enantioselectively into the beta-position of a variety of ketones, esters, and amides. [reaction--see text]

Rhodium-catalyzed Heck-type reaction of arylboronic acids with α,β-unsaturated esters: tuning β-hydrogen elimination vs. hydrolysis of alkylrhodium species

The elusive rhodium-catalyzed Heck-type coupling of arylboronic acids with alpha,beta-unsaturated esters is effected using rhodium chloride as catalyst, indicating the hydrolysis of Rh-C bond could be finely tuned by the steric environment around rhodium.

Rhodium-catalyzed asymmetric 1,4-addition of aryltitanium reagents generating chiral titanium enolates: isolation as silyl enol ethers

The addition of aryltitanium triisopropoxide (ArTi(OPr-i )3) to alpha,beta-unsaturated ketones proceeded with high enantioselectivity (94-99.8% ee) in the presence of 3 mol % of [Rh(OH)((S )-binap)]2 in THF at 20 degrees C to give high yields of the titanium enolates as 1,4-addition products. The titanium enolates were converted into silyl enol ethers by treatment with chlorotrimethylsilane and lithium isopropoxide.

Diastereoselective formation of indanes from arylboronate esters catalyzed by rhodium(I) in aqueous media

[reaction: see text] Arylboronate esters bearing a pendant Michael-acceptor alkene can add to norbornene and cyclize to give indane systems in yields ranging from 62% to 95% with high diastereomeric excess (>20:1). The reaction is performed in an organic/aqueous emulsion and catalyzed using [Rh(COD)Cl]2 with t-Bu-amphos chloride, a sterically bulky, electron-rich, water-soluble phosphine ligand.