Conservation of Absolute Configuration in the Acyclic Rhodium-Catalyzed Allylic Alkylation Reaction: Evidence for an Enyl (σ + π) Organorhodium Intermediate

Retention of regiochemistry of monosubstituted allyl acetates in the ruthenium catalysed allylic alkylation with malonate anion

In the RuCl2(p-cymene)/PPh3 catalysed regioselective allylic alkylation of monosubstituted allyl acetates with malonate anion, the selective substitution at the position originally substituted with acetate was observed.

Asymmetric synthesis: From transition metals to organocatalysis

Umpolung in the allylation reaction is discussed with examples drawn from transition-metal-catalyzed allylic substitution (with the allylic unit acting as an electrophile) and Lewis base-catalyzed allylation of aldehydes with allyltrichlorosilane (with the allyl acting as a nucleophile). Iridium-catalyzed electrophilic allylation of O-nucleophiles has been employed in our new approach to C-nucleoside analogs, where the C-O bond (rather than C-C) was constructed stereospecifically. Variation of the absolute configuration in the starting segments allowed the synthesis of all four combinations of D/L-α/β-ribosides. In the nucleophilic allylation of aldehydes, chiral pyridine-type N-oxide catalysts are presented, in particular QUINOX and METHOX, and the intriguing behavior of QUINOX is discussed. Here, the π-π interactions between the substrate aldehyde and the catalyst are suggested to rationalize the experimental observations. Good correlation between the calculated energies for the transition states and the experimentally observed enantioselectivities has been obtained.

An efficient protocol of iridium-catalyzed allylic substitution reaction and its application to polymer synthesis: complementary regio- and stereoselective allylation polycondensation via Ir and Pd catalyses

An efficient protocol of the Ir-catalyzed allylic substitution reaction is reported using N,O-bis(trimethylsilyl)acetamide as a base in the presence of nBu4NF as a cocatalyst. The reaction completely proceeded under very mild conditions, and a branched allylated compound that is not easy to access via the Tsuji-Trost reaction can be synthesized. The reaction system is practical enough to be applicable for polymer syntheses. The Ir- and Pd-catalyzed allylation polycondensations generally show complementary regio- and stereoselectivities. The Ir-catalyzed reaction is versatile, and a mixed dual regioselectivity such as a branched-linear selectivity on each electrophile can also be achieved.

Rhodium-catalyzed asymmetric allylic substitution with boronic acid nucleophiles

An enantio-, regio-, and diastereoselective rhodium(I)-catalyzed desymmetrization of a meso-cyclic allylic dicarbonate with organoboronic acid nucleophiles is described. The rhodium(I) catalyst formed in situ from [Rh(cod)OH]2 and Xyl-P-PHOS allowed the S(N)2' allylic substitution product to be obtained with a range of arylboronic acids in enantiomeric excesses of up to 92% with regioselectivities of up to >20:1.

Stereospecific and stereodivergent construction of tertiary and quaternary carbon centers through switchable directed/nondirected allylic substitution

This study introduces the ortho-diphenylphosphanylbenzoate (o-DPPB)/o-DPPB oxide system as a switchable directing/nondirecting leaving group in a copper-mediated allylic substitution with Grignard and organozinc reagents. With this system, the regioselective, stereospecific, and stereodivergent construction of quaternary as well as tertiary carbon centers is possible in a reliable and predictable fashion. Starting from one substrate enantiomer, both optical antipodes of the substitution products are readily available. Hence, this methodology features reversed polarity in comparison to established enolate alkylation chemistry and may be an interesting alternative, particularly for the construction of quaternary stereogenic carbon centers.

Iridium-catalysed asymmetricallylic substitutions

Ir-catalysed allylic substitution is supplementing the traditional Pd-catalysed variant. With simple, easily available monosubstituted allylic acetates and carbonates as substrates, Ir catalysts generally favour chiral, branched products, while Pd catalysts typically give rise to linear, achiral products. With phosphorus amidites as ligands, regioselectivities >10 : 1 and enantiomeric excess in the range 95-99 %ee are currently routinely achieved. A broad range of nucleophiles can be employed: for example stabilised carbanions, amines including their sulfonyl- and diacyl-derivatives, phenolates and alkoxides. A few applications, based on combinations of the allylic substitution and ring closing metathesis, indicate considerable potential of the method for the synthesis of biologically active compounds.

Hydrogen-mediated aldol reductive coupling of vinyl ketones catalyzed by rhodium: high syn-selectivity through the effect of tri-2-furylphosphine

[reaction: see text]. Catalytic hydrogenation of methyl vinyl ketone (MVK) and ethyl vinyl ketone (EVK) in the presence of diverse aldehydes at ambient temperature and pressure using tri-2-furylphosphine-ligated rhodium catalysts enables formation of aldol products with high levels of syn-diastereoselectivity. A progressive increase in diastereoselectivity is observed upon sequential replacement of phenyl residues for 2-furyl residues (Ph3P, FurPh2P, Fur2PhP, Fur3P). Hydrogen-labile functional groups, including alkynes, alkenes, benzylic ethers, and nitroarenes, remain intact under the coupling conditions.

Asymmetric Allylic Substitution Catalyzed byC1-Symmetrical Complexes of Molybdenum: Structural Requirements of the Ligand and the Stereochemical Course of the Reaction

Application of new chiral ligands (R)-(-)-12 a and (S)-(+)-12 c (VALDY), derived from amino acids, to the title reaction, involving cinnamyl (linear) and isocinnamyl (branched) type substrates (4 and 5 --> 6), led to excellent regio- and enantioselectivities (>30:1, < or =98 % ee), showing that ligands with a single chiral center are capable of high asymmetric induction. The structural requirements of the ligand and the mechanism are discussed. The application of single enantiomers of deuterium-labeled substrates (both linear 38 c and branched 37 c) and analysis of the products (41-43) by (2)H{(1)H} NMR spectroscopy in a chiral liquid crystal matrix allowed the stereochemical pathways of the reaction to be distinguished. With ligand (S)-(+)-12 c, the matched enantiomer of branched substrate was found to be (S)-5, which was converted into (R)-6 with very high regio- and stereoselectivity via a process that involves net retention of stereochemistry. The mismatched enantiomer of the branched substrate was found to be (R)-5, which was also converted into (R)-6, that is, with apparent net inversion, but at a lower rate and with lower overall enantioselectivity. This latter feature, which may be termed a "memory effect", reduced the global enantioselectivity in the reaction of the racemic substrate (+/-)-5. The stereochemical pathway of the mismatched manifold has been shown also to be one of net retention, the apparent inversion occurring through equilibration via an Mo-allyl intermediate prior to nucleophilic attack. Incomplete equilibration leads to the memory effect and thus to lower enantioselectivity. Analysis of the mismatched manifold over the course of the reaction revealed that the memory effect is progressively attenuated with the nascent global selectivity increasing substantially as the reaction proceeds. The origin of this effect is suggested to be the depletion of CO sources in the reaction mixture, which attenuates turnover rate and thus facilitates greater equilibrium. The linear substrate was also converted into the branched product with net syn stereochemistry, as shown by isotopic labeling. An analogous process operates in the generation of small quantities of linear product from branched substrate.

Enantioselective syntheses of tremulenediol A and tremulenolide A

[reaction: see text] An enantioselective entry to the skeleton of the tremulane sesquiterpenes is described. The approach features a series of efficient transition metal-catalyzed reactions commencing with an enantioselective rhodium(II)-catalyzed intramolecular cyclopropanation followed by a regioselective allylic alkylation and a diastereoselective rhodium(I)-catalyzed [5 + 2] cycloaddition. This strategy was applied to the first enantioselective syntheses of tremulenediol A and tremulenolide A.

Allylic etherification via Ir(I)/Zn(II) bimetallic catalysis

[reaction: see text] An efficient allylic etherification of aliphatic alcohols with allylic carbonates has been achieved by an iridium catalysis using stoichiometric zinc alkoxides or a two-component bimetallic catalytic system where the Ir(I) catalyst acts on allylic carbonates to generate electrophiles while aliphatic alcohols are separately activated by Zn(II) coordination to function as nucleophilies. This reaction occurs with complete regiospecificity and tolerates a wide range of functional groups.

Mechanistic insight into copper-catalysed allylic substitutions with bis(triorganosilyl) zincs. Enantiospecific preparation of α-chiral silanes

Isotopic desymmetrisation, as well as (stereo)chemical correlation, has illuminated significant aspects of the sigma-pi-sigma mechanism of copper-catalysed allylic substitution reactions: an enantiospecific and regioselective access to alpha-chiral silanes is presented.

A Simple Iridium Catalyst with a Single Resolved Stereocenter for Enantioselective Allylic Amination. Catalyst Selection from Mechanistic Analysis

A study of the relationship between the stereochemical elements of a phosphoramidite ligand and the stereoselectivity of iridium-catalyzed amination of allylic carbonates is reported. During catalyst activation, a complex of a phosphoramidite ligand possessing one axial chiral binaphtholate group and two resolved phenethyl substituents converts to a more reactive cyclometalated complex containing one distal chiral substituent at nitrogen, one substituent that becomes part of the metalacycle, and one unperturbed binaphtholate group. Systematic changes were made to the different stereochemical elements. Replacement of the distal chiral phenethyl substituent with a large achiral cycloalkyl group led to a catalyst that reacts with rates and enantioselectivities that are similar to those of the original catalyst with the phenethyl group. Studies of the reactions of diastereomeric ligands containing (R) or (S) binaphtholate groups on phosphorus, along with one (R)-phenethyl and one achiral cyclododecyl group on nitrogen, show that the complexes of the two diastereomeric ligands undergo cyclometalation at much different rates. To access both diastereomeric catalysts and to determine if the reaction can occur selectively with an even simpler ligand containing a phenethyl substituent at nitrogen as the only resolved stereochemical element, the catalyst derived from a phosphoramidite containing a biphenolate group was studied. Catalysts generated from this ligand were shown to react in all cases examined with nearly the same rates, regioselectivities, and enantioselectivities as catalysts derived from the original more elaborate ligand. The absolute stereochemistry of the product implies that the major enantiomer is formed from the (R(a),R(c))-atropisomer of the catalyst containing the biphenolate group.

Enantioselective Synthesis of 2-Aryl-4-piperidones via Rhodium/Phosphoramidite-Catalyzed Conjugate Addition of Arylboroxines

[reaction: see text] The highly enantioselective synthesis of 2-aryl-4-piperidones by rhodium/phosphoramidite-catalyzed conjugate addition of arylboroxines to 2,3-dihydro-4-pyridones is described. Both enantiomers of a variety of products with sterically and electronically different R substituents were obtained in high isolated yield and with excellent enantioselectivity up to 99%.

Ruthenium-catalyzed stereospecific decarboxylative allylation of non-stabilized ketone enolates

The ruthenium-catalyzed stereospecific decarboxylative allylation of ketone enolates provides access to gamma,delta-unsaturated ketones with good yields and enantio-enrichments.

Enantioselective Allylic Substitution of Cinnamyl Esters Catalyzed by Iridium−Chiral Aryl Phosphite Complex: Conspicuous Change in the Mechanistic Spectrum by a Countercation and Solvent

Iridium-catalyzed asymmetric allylic alkylation of monoaryl substrates 4-6 with chiral phosphites 1-3 has been investigated. Although branched isomers were formed with high regioselectivities, the enantioselectivities of these products were remarkably influenced by solvents, countercations, and additives (ZnCl(2) and LiCl).

Regio- and Diastereoselective Rhodium-Catalyzed Allylic Substitution with Acyclic α-Alkoxy-Substituted Copper(I) Enolates: Stereodivergent Approach to 2,3,6-Trisubstituted Dihydropyrans

The regio- and diastereoselective transition metal-catalyzed allylic alkylation using alpha-heteroatom-substituted ketones and carboxylic acid derivatives represents a challenging and important synthetic transformation. We have developed a regio- and diastereoselective rhodium-catalyzed allylic alkylation reaction utilizing the copper(I) enolates derived from alkyl protected acyclic alpha-alkoxy aryl ketones. This study suggests that the ability to form a chelated enolate intermediate is crucial for obtaining high diastereoselectivity, whereas excellent regioselectivity is obtained regardless of the substituent. Hence, the excellent selectivity coupled with the synthetic utility of alpha-alkoxy aryl ketone enolates makes this an important new method for the construction of acyclic adjacent ternary stereogenic centers. Finally, the synthetic utility of this protocol was highlighted through the conversion of the aryl ketone to the ester and the development of a stereodivergent approach to 2,3,6-trisubstituted dihydropyrans relevant to target-directed synthesis.

Mechanistic studies of the molybdenum-catalyzed asymmetric alkylation reaction

Enantiomerically enriched, deuterated branched carbonates (Z)-(S)-PhCH(OCO(2)Me)-CH = CHD (1-D), (Z)-(R)-PhCH(OCO(2)Me)CH = CHD (2-D), and linear carbonate (E)-(S)-PhCH = CHCHD(OCO(2)Me) (3-D) were used as probes in the Mo-catalyzed asymmetric allylic alkylation with sodium dimethyl malonate, catalyzed by ligand-complex 11 derived from the mixed benzamide/picolinamide of (S,S)-transdiaminocyclohexane and (norbornadiene)Mo(CO)(4). The results of these studies, along with x-ray crystallography and solution NMR structural analysis of the pi-allyl intermediate, conclusively established the reaction proceeded by a retention-retention pathway. This mechanism contrasts with that defined for Pd-catalyzed allylic alkylations, which proceed by an inversion-inversion pathway. A proposed rationale for the retention pathway for nucleophilic substitution involves CO-coordination to form a tri-CO intermediate, followed by complexation with the anion of dimethyl malonate to produce a seven-coordinate intermediate, which reductively eliminates to afford product with retention of configuration.

Rhodium-catalyzed asymmetric ring opening reactions of oxabicyclic alkenes: catalyst and substrate studies leading to a mechanistic working model

Catalyst and substrate studies have been performed on the rhodium-catalyzed asymmetric ring opening reaction. A working model is advanced that involves oxidative insertion with retention to form an organorhodium intermediate that then undergoes nucleophilic attack with inversion. Kinetic and competition experiments have uncovered evidence for a proton transfer step in the catalytic cycle that may activate both the allylrhodium intermediate and the nucleophile. We have also conducted experiments designed to understand which properties of the PPF-P(t)Bu(2) ligand contribute to the high reactivities and enantioselectivities.

Direct, Stereoselective Substitution in [Rh(CO)2Cl]2-Catalyzed Allylic Alkylations of Unsymmetrical Substrates

[Rh(CO)(2)Cl](2) has been found to possess the unusual property of catalyzing allylic alkylations of unsymmetrical allylic carbonates with high levels of regioselectivity to provide products arising from substitution at the carbon atom bearing the leaving group, irrespective of the structure of the starting carbonate. The substitution reaction occurs with retention of stereochemistry at the reacting center, and the carbon-carbon double-bond stereochemistry of primary (Z)-allylic carbonates is maintained. [reaction: see text]

Regioselective and Enantiospecific Rhodium-Catalyzed Allylic Alkylation Reactions Using Copper(I) Enolates: Synthesis of (−)-Sugiresinol Dimethyl Ether

The transition metal-catalyzed allylic alkylation represents a fundamentally important cross-coupling reaction for the construction of ternary carbon stereogenic centers. We have developed a regioselective and enantiospecific rhodium-catalyzed allylic alkylation of acyclic unsymmetrical allylic alcohol derivatives using copper(I) enolates to prepare beta-substituted ketones. This protocol represents a convenient asymmetric Claisen rearrangement surrogate in which alpha-substituted enolates permit the introduction of an additional stereogenic center. The synthetic utility of this transformation was highlighted in the construction of a trans-1,2-disubstituted cyclohexene and the total synthesis of (-)-sugiresinol dimethyl ether. Finally, we anticipate that copper(I) enolates may prove useful nucleophiles in related metal-catalyzed reactions.

Regio- and enantiospecific rhodium-catalyzed arylation of unsymmetrical fluorinated acyclic allylic carbonates: inversion of absolute configuration

The transition metal-catalyzed allylic substitution with unstabilized carbon nucleophiles represents an important cross-coupling reaction for the construction of ternary carbon stereogenic centers. We have developed a new regio- and enantiospecific rhodium-catalyzed allylic alkylation of acyclic unsymmetrical chiral nonracemic allylic alcohol derivatives with aryl zinc bromides. This study demonstrates that the hydrotris(pyrazolyl)borate rhodium catalyst and zinc(II) halide salt are crucial for efficiency, while the addition of lithium bromide to the catalyst is necessary for obtaining optimal regiospecificity. The stereochemical course of this reaction was established through the synthesis of (S)-ibuprofen, which demonstrated that the alkylation proceeds with net inversion of absolute configuration consistent with direct addition of the nucleophile to the metal center followed by reductive elimination.

Novel rhodium-catalyzed cycloisomerization of 1,6-enynes with an intramolecular halogen shift

A new Rh-catalyzed 1,6-enyne cycloisomerization process with a pi-allyl rhodium species as the key intermediate is investigated. A halogen shift happened in this novel process. The synthesis of stereodefined alpha-halomethylene gamma-butyrolactones has been achieved using the readily accessible Rh catalysts.

High enantioselectivity in rhodium-catalyzed allylic alkylation of 1-substituted 2-propenyl acetates

[reaction: see text] Rhodium-catalyzed asymmetric allylic alkylation of 1-substituted 2-propenyl acetates with dimethyl malonate proceeded with high enantioselectivity in the presence of cesium carbonate as a base and a rhodium catalyst generated from Rh(dpm)(C(2)H(4))(2) (dpm = dipivaloylmethanato) and a chiral phosphino-oxazoline whose basic skeleton is axially chiral binaphthyl to give branch alkylation products in greater than 90% ee.

Hydrogen bond directed highly regioselective palladium-catalyzed allylic substitution

The palladium-catalyzed allylic substitution of 5-vinyloxazolidinones and derivatives was investigated. Unusual and high regioselectivity for the branched product was observed. The regioselectivity was influenced by the type of substrate, the solvents, and the nucleophile. Imide-type nucleophiles were found to be directed to the internal carbon (branched:linear, 75:25 to >98:2), whereas sulfonamides, amines, and malonates added only to the terminal carbon of the allyl complex. Relatively nonpolar solvents such as toluene and THF favored the branched product (97:3 and 95:5, respectively). Acetonitrile and dichloromethane afforded lower regioselectivity (50:50 and 67:33, respectively), and the use of the protic solvent ethanol resulted in reversal of the regioselectivity (12:88). The directing group on the substrate was important. Amides afforded almost complete formation of the branched product, and carbamates gave a 50:50 mixture of regioisomers with phthalimide as the nucleophile. Evidence for direction of the nucleophile via hydrogen bonding was obtained by replacing the hydrogen of the amide with a methyl, resulting in the production of only the normal linear product.