Regiocontrol in the palladium-catalyzed hydrophosphinylation of terminal alkynes

The regioselectivity of the palladium-catalyzed hydrophosphinylation of terminal alkynes was investigated. Complementary conditions to achieve the predominant formation of either the linear or the branched alkenyl-H-phosphinate products were identified. With Pd/xantphos in acetonitrile, the linear isomer is generally obtained with good to excellent selectivity, and E-stereospecificity. On the other hand, by using Pd/dppf in the non-polar solvent toluene, good selectivity for the branched alkenyl-H-phosphinate is typically observed. The role of various reaction parameters is studied.     

Allyl stannanes as electrophiles or nucleophiles in the palladium-catalyzed reactions with alkynes

The reactivity of the allyl stannanes can be inverted by changing the oxidation state of the catalyst from Pd(II) to Pd(0). Whereas with Pd(II) an anti nucleophilic attack of the allyl stannane on the alkyne takes place, the reaction with Pd(0) proceeds by oxidative addition to form (η³-allyl)palladium complexes leading to a formal syn addition to the alkyne. This mechanistic proposal is supported by DFT calculations.     

Cobalt- and nickel-catalyzed regio- and stereoselective reductive coupling of alkynes, allenes, and alkenes with alkenes

Transition-metal-catalyzed coupling of two different C-C pi components through a metallacycle intermediate is a highly atom economical method to construct C-C bonds in organic synthesis. The metal-catalyzed coupling of an alkene and alkyne generally gives an Alder-ene or reductive coupling product. In this article, we focus on the cobalt- and nickel-catalyzed reductive coupling of alkynes, allenes, and alkenes with alkenes. These reductive coupling reactions provide convenient methods for the synthesis of various alkenes, dienes, functionalized alkanes, lactones, lactams, and cyclic alcohols in a highly regio- and stereoselective manner. A chemselective formation of metallacyclopentene intermediate from the two different C-C pi components and a low-valence metal species plays a key role for the high regio- and stereoselectivity of the catalytic reaction.     

A theoretical investigation of the regio- and stereoselectivities of the 1,3-dipolar cycloaddition of C-diethoxyphosphoryl-N-methylnitrone with substituted alkenes

A theoretical study of the regio- and stereoselectivities of the 1,3-dipolar cycloaddition of C-diethoxyphosphoryl-N-methylnitrone with substituted alkenes (allyl alcohol and methyl acrylate) is carried out using DFT at the B3LYP/6-31G(d,p) level of theory. The FMO analysis and DFT-based reactivity indices confirmed the experimental ortho regioisomeric pathway. Potential energy surface analysis shows that these 1,3-dipolar cycloaddition reactions favor the formation of the ortho-trans cycloadduct in both cases. The obtained results are in agreement with experimental data.     

A simple catalyst system for the palladium-catalyzed coupling of aryl halides with terminal alkynes

A convenient catalyst system consisting of Pd(OAc)₂, PPh₃, K₃PO₄ and DMSO was found to be effective for the coupling reaction of aryl halides with terminal alkynes as well as the deacetonative coupling reaction using a 4-aryl-2-methylbut-3-yn-2-ol as a terminal alkyne precursor. An iminophosphine as a ligand worked more effectively for some combination of substrates than triphenylphosphine.     

Origin of the regio- and stereoselectivity in palladium-catalyzed electrophilic substitution via bis(allyl)palladium complexes

Palladium-catalyzed allylic substitution of aryl allyl chlorides with aromatic and heteroaromatic aldehydes was performed in the presence of hexamethylditin. This procedure involves palladium-catalyzed formation of transient allylstannanes followed by generation of a bis(allyl)palladium intermediate, which subsequently reacts with the aldehyde electrophile. The catalytic substitution reaction proceeds with high regio- and stereoselectivity. The stereoselectivity is affected by the steric and electronic properties of the allylic substituents. Various functionalities including NO(2), COCH(3), Br, and F groups are tolerated under the applied catalytic conditions. Density functional calculations at the B3PW91/DZ+P level of theory were applied to study the steric and electronic effects controlling the regio- and stereoselectivity of the electrophilic addition. The development of the selectivity was studied by modeling the various bis(allyl)palladium species occurring in the palladium-catalyzed substitution of cinnamyl chloride with benzaldehyde. It was found that the electrophilic attack proceeds via a six-membered cyclic transition state, which has a pronounced chair conformation. The regioselectivity of the reaction is controlled by the location of the phenyl group on the eta(1)-allyl moiety of the complex. The stereoselectivity of the addition process is determined by the relative configuration of the phenyl substituents across the developing carbon-carbon bond. The lowest energy path corresponds to the formation of the branched allylic isomer with the phenyl groups in anti configuration, which is in excellent agreement with the experimental findings.     

An efficient palladium-catalyzed Heck coupling of aryl chlorides with alkenes

Catalyst system PdCl₂(PCy₃)₂/Cs₂CO₃ in dioxane was found to be the efficient catalyst system for Heck cross-coupling reactions of deactivated, neutral, and activated aryl chlorides with a variety of alkenes under mild conditions to afford selectively E-arylated alkenes in good to excellent yields.     

The role of ancillary ligands and of electron poor alkenes and alkynes in stabilizing Pd(0) derivatives: A comparative study

The peculiar characteristics of the ligand neocuproine (2,9-dimethylphenanthroline) allow a number of exchange equilibrium studies between the low valence complex [Pd(η²-nq)(Neocup)] (nq = naphthoquinone; Neocup = neocuproine) and several alkenes and alkynes. A new order of stability which compares differently unsaturated molecules was established. An overview of the factors governing the stability of palladium(0) alkene and alkyne derivatives as a function of the steric and electronic characteristics of both the unsaturated molecule and the ligand becomes accessible and a comparison with the previously determined order was therefore feasible. Such a comparison enlightens the importance of the substituent methyl groups in ortho position of the hetero-aromatic ring which represents the molecular fragment common to all the ligands considered. Taking advantage of the steric requirements of the alkene tmetc (tmetc = tetramethylethylenetetracarboxylate) a kinetic investigation of the reaction between the olefin itself and the complexes [Pd(η²-dmfu)(L-L′)] (dmfu = dimethylfumarate; L-L′ = 8-diphenylphosphanyl-2-methyl-quinoline, neocuproine, phenanthroline) was carried out. The structures of the complexes [Pd(η²-dmfu)(DPPQ)] and [Pd(η²-deta)(Neocup)] (deta = but-2-ynedioc acid diethyl ester) were also reported in the present paper. The structure of the latter represents the first example of a palladium(0) complex in which the N₂C₂ donor set around the metal centre is supported by a chelating η²-alkyne.     

Rhodium-catalyzed regio- and stereoselective codimerization of alkenes and electron-deficient internal alkynes leading to 1,3-dienes

A cationic rhodium(I)/H(8)-BINAP complex catalyzes codimerization of alkenes bearing no alpha-hydrogen and electron-deficient internal alkynes, leading to 1,3-dienes in good yields with moderate to excellent regio- and stereoselectivity. The same complex also catalyzes codimerization of an acrylate and phenyl-substituted electron-rich internal alkynes, leading to 1,3-dienes.     

A theoretical study of the mechanism and stereoselectivity of the Diels-Alder cycloaddition between difluoro-2-methylencyclopropane and furan

A theoretical study of the molecular mechanism and stereoselectivity of the Diels-Alder cycloaddition reaction between difluoro-2-methylencyclopropane and furan has been carried out at the B3LYP/6-31G+^∗∗ level of theory. The calculation of activation and reaction energies indicates that the 3-endo cycloadduct is favored both kinetically and thermodynamically, which is in agreement with the experimental data. Analysis of the bond order and charge transfer in the transition states shows that this reaction takes place via a synchronous-concerted mechanism.     

Use of acetate as a leaving group in palladium-catalyzed nucleophilic substitution of benzylic esters

The palladium complex prepared in situ from [Pd(η³-C₃H₅)(cod)]BF₄ and bidentate phosphine DPPF was a good catalyst for the nucleophilic substitution of benzyl acetate. Significant acceleration of the palladium-catalyzed substitution was observed when an alcohol was employed as a reaction solvent. The palladium catalyst was effective for the benzylation of various stabilized carbanions, amines, and benzenesulfinate with benzylic acetates.     

Ligand and base additive effects on the reversibility of nucleophilic addition in palladium-catalyzed allylic aminations monitored by nucleophile crossover

A nucleophile crossover experiment was used to monitor the reversibility of nucleophilic addition of benzylamine to π-allylpalladium complexes. Dppe, dppp, dppb, and PHOX showed more crossover than PPh₃ and dppm in both DMF and dichloromethane. Crossover was inhibited by the addition of DBU or Cs₂CO₃, but much less elimination to diene side products was observed with Cs₂CO₃. Analysis of percent crossover vs. percent reaction completion using the PHOX ligand revealed that with added DBU or Cs₂CO₃ crossover only began occurring after 100% completion had been reached.     

Highly regio- and stereoselective hydrometallation reactions of fluorine-containing internal alkynes: Novel approaches to fluoroalkylated alkenes

Hydroalumination, hydrocupration, and hydroboration reactions of various fluorine-containing alkynes were investigated. The alkyne reacted smoothly with 2.0 equiv. of Red-Al at −78 °C to give the hydroaluminated adduct in a highly regio- and stereoselective manner, which was treated with iodine, the corresponding vinyliodide being produced in moderate yield. Hydrocupration of the alkynes also took place, but the resulting vinylmetal reacted with various electrophiles sluggishly. In sharp contrast, the reaction with dicyclohexylborane proceeded smoothly to afford the cis-addition products preferentially, which were subjected to Suzuki-Miyaura cross-coupling reaction, leading to trisubstituted alkenes in high yields.     

A recombinant ketoreductase tool-box. Assessing the substrate selectivity and stereoselectivity toward the reduction of β-ketoesters

The substrate selectivity and stereoselectivity of a series of ketoreductases were evaluated toward the reduction of two sets of β-ketoesters. Both the structural variety at β-position and the substituent at α-position greatly affected the activity and stereoselectivity of these ketoreductases. For the first set of β-ketoesters, at least one ketoreductase was found that catalyzed the formation of either (d) or (l) enantiomer of β-hydroxyesters from each substrate with high optical purity, with the only exception of ethyl (d)-3-hydroxy-3-phenylpropionate. For the second set of β-ketoesters with α-substituents, the situation is more complex. More commonly, a ketoreductase was found that formed one of the four diastereomers in optically pure form, with only a few cases in which enzymes could be found that formed two or more of the diastereomers in high optical purity. The continued development of new, more diverse ketoreductases will create the capability to produce a wider range of single diastereomers of 2-substituted-3-hydroxy acids and their derivatives.     

Origins of stereoselectivity in the oxido-alkylidenation of alkynes

A mild, convenient oxido-alkylidenation of alkynes is described. The three-step sequence involves the 1,3-dipolar cycloaddition of a nitrone and an alkynoate, oxidation of the resulting isoxazoline, and stereoselective extrusion of nitrosomethane. Quantum mechanical calculations identified the interactions of R3 with the oxidant and the preferred conformation of a diradical intermediate as major factors controlling the stereoselectivity of the oxidation and torquoselectivity of the extrusion.     

Preparation, structural characterization of 1,2-digermacyclobut-3-enes, and their palladium-catalyzed insertion of alkynes

1,2-Digermacyclobut-3-enes were prepared by the treatment of Z-α,β-bis(chlorodialkylgermyl)ethenes with Na metal in boiling toluene and their structures were fully established by spectroscopic methods coupled with X-ray crystallography. In the presence of an appropriate catalyst, 1,2-digermacyclobut-3-enes reacted smoothly with alkynes to give the corresponding insertion products, 1,4-digermacyclohexa-2,5-dienes in moderate to good yields. Conventional complexes, such as [Pd(PPh₃)₄] and [Pt(PPh₃)₄], serve as efficient catalysts. The mechanism of the insertion reaction of alkynes into the germanium-germanium bond of 1,2-digermacyclobut-3-enes is discussed in terms of a key intermediate, a 1,4-digerma-2-buten-1,4-diylpalladium.     

Synthesis of internal alkynes via one-pot palladium-catalyzed and dehydrobromination reactions of 1,1-dibromo-1-alkenes

Tandem palladium-catalyzed cross-coupling of alkyl, alkenyl, alkynyl, aryl, and heteroaryl 2-substituted 1,1-dibromo-1-alkenes with aryl or heteroaryl boronic acids or borate esters and dehydrobromination of the intermediate coupled products afforded internal alkynes in moderate to good yields (up to 89%). The synthesis has been carried out in a one-pot process and in a two-step sequence according to the nature of the starting 1,1-dibromoalkenes. The reported protocol is compatible with the presence in the 1,1-dibromo-1-alkene molecule of additional reactive halogen–carbon bonds, thus allowing to build up more complex alkyne derivatives.     

A new regio- and stereoselective intermolecular Friedel-Crafts alkylation of phenolic substrates with aryl epoxides

A conceptually new regioselective and highly syn-stereoselective intermolecular Friedel-Crafts-type O-alkylation of phenols with aryl epoxides by the use of appropriately substituted aryl borates is reported. The carbon-carbon bond formation occurs in neutral and mild conditions without the need for external Lewis acids or transition metal catalysts.     

Kinetic aspects of stereoselectivity in hydrogenation of fatty acids

Kinetic peculiarities of stereoselectivity in hydrogenation of fatty acids are discussed. The reaction mechanism should be able to account for independence of the reaction rate on hydrogen pressure and conversion, stereoselectivity on conversion as well as an increase in cis/trans ratio with pressure increase. Various mechanisms advanced in the literature for formation of cis- and trans mono-unsaturated acids and their derivatives are considered, showing behavior contradicting with experiments. A reaction mechanism is proposed, which is consistent with experimental observations.