Development of Chiral Terminal-Alkene−Phosphine Hybrid Ligands for Palladium-Catalyzed Asymmetric Allylic Substitutions

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Copper(I)-Catalyzed Asymmetric Hydrophosphination of 3,3-Disubstituted Cyclopropenes

Herein, a copper(I)-catalyzed asymmetric hydrophosphination of 3,3-disubstituted cyclopropenes is reported. It provides a series of phosphine derivatives in high to excellent diastereo- and enantioselectivities. The methodology enjoys broad substrate scope on both 3,3-disubstituted cyclopropenes and diarylphosphines. The high stereoselectivity is attributed to both the high stability of the Cu(I)-(R,R)-QUINOXP* complex in the presence of stoichiometric HPPh2 and the produced phosphines, and the high-performance asymmetric induction of the Cu(I)-(R,R)-QUINOXP* complex. Finally, the method is used for the synthesis of new chiral phosphine-olefin compounds built on a cyclopropane skeleton, one of which serves as a wonderful ligand in Rh-catalyzed asymmetric conjugate addition of phenylboronic acid to various α,β-unsaturated compounds.

Axially Chiral N,N-Ligand-Promoted Pd-Catalyzed Enantioselective Allylic Amination and Alkylation

An axially chiral N,N-ligand developed from a [1,1'-binaphthalene]-2,2'-diol (BINOL)-based skeleton and phenanthroline is found to be capable of promoting Pd-catalyzed asymmetric allylic amination and alkylation of allyl acetates. The reaction is compatible with cyclic and acyclic secondary amines, primary aliphatic amines, malononitrile, and dialkyl malonates, affording the corresponding chiral products in up to 99% yield and with up to >99% enantiomeric excess.

Atroposelective Synthesis of Triaryl α-Pyranones with 1,2-Diaxes by N-Heterocyclic Carbene Organocatalysis

Atropisomers bearing multiple stereogenic axes are of increasing importance to the field of material science, pharmaceuticals, and catalysis. However, the atroposelective construction of multi-axis atropisomers remains rare and challenging, due to the intrinsical difficulties in the stereo-control of the multiple stereogenic axes. Herein, we demonstrate a single-step construction of a new class of 1,2-diaxially chiral triaryl α-pyranones by an N-heterocyclic carbene organocatalytic asymmetric [3+3] annulation of well-designed alkynyl acylazolium precursors and enolizable sterically hindered 2-aryl ketones. The protocol features broad substrate scope (>50 examples), excellent stereo-control (most cases >20 : 1 dr, up to 99.5 : 0.5 er), and potentially useful synthetic applications. The success of this reaction relies on the rational design of structurally matched reaction partners and the careful selection of the asymmetric catalytic system. DFT calculations have also been performed to discover and rationalize the origin of the high stereoselectivity of this reaction.

Catalytic Synthesis of Atropisomeric o-Terphenyls with 1,2-Diaxes via Axial-to-Axial Diastereoinduction

Herein, we report a modular and convergent strategy for the assembly of atropisomeric o-terphenyls with 1,2-diaxes via palladium/chiral norbornene cooperative catalysis and axial-to-axial diastereoinduction. Readily available aryl iodides, 2,6-substituted aryl bromides, and potassium aryl trifluoroborates are used as the building blocks, laying the foundation for diversity-oriented synthesis of these scaffolds (46 examples). Other features include the unique axial-to-axial diastereoinduction mode, construction of two axes in a single operation, and step economy. DFT calculations are performed to rationalize the axial-to-axial diastereoinduction process. Synthetic utilities of this method in preparation of atropisomeric oligophenyls, chiral catalysts, and ligands are demonstrated.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

Synthesis, Structure, and Reactivity of Binaphthyl Supported Dihydro[1,6]diazecines

A short approach to chiral diaza-olefines from protected 2,2'-diamino-1,1'-binaphthyl is presented. Cis- and trans-olefines can be selectively obtained by twofold N-allylation followed by RCM or by bridging a 2,2'-diamino-1,1'-binaphthyl precursor with trans-1,4-dibromo-2-butene. Deprotection afforded cis- and trans-dihydro[1,6]diazecines 1 in 58 and 64% overall yield. The reactivity of the but-2-ene-1,4-diyl fragment was investigated yielding corresponding epoxides, diols, and mono- and dibromo products. In several cases rearrangements and participation of the proximate N-Boc group was observed. In no case could allylic substitution be accomplished. From 13 compounds X-ray structure analyses could be obtained.

A readily accessible and modular carbohydrate-derived thioether/selenoether-phosphite ligand library for Pd-catalyzed asymmetric allylic substitutions

A large library of thioether/selenoether-phosphite ligands have been tested in the Pd-catalyzed asymmetric allylic substitution reaction. The presented ligands are derived from cheap and available carbohydrates and they are air-stable solids and easy to handle. Their highly modular nature has made it possible to achieve excellent enantioselectivities in the substitution of a range of hindered and unhindered substrates (ees up to 99% and 91%, respectively). In addition, twelve C-, N- and O-nucleophiles can be efficiently introduced, independently of their nature. Among the whole library, ligands that contain an additional chiral centre in the alkyl backbone chain next to the phosphite group and an enantiopure biaryl phosphite group provided the best enantioselectivities. In general, there is a cooperative effect between these two chiral elements, and therefore, a matched combination between them is necessary to achieve the highest enantioselectivities. However, in the case of cyclic substrates, this cooperative effect is less pronounced and advantageously, both enantiomers of the product can be obtained by setting up the desired configuration of the biaryl phosphite group. Studies of the key Pd-π-allyl intermediates allowed us to better understand the enantioselectivities obtained experimentally.

Ligand-Free, Quinoline N-Assisted Copper-Catalyzed Nitrene Transfer Reaction To Synthesize 8-Quinolylsulfimides

An efficient copper-catalyzed, quinolyl N-directed nitrene transfer reaction to 8-quinolylsulfides was described. A variety of 8-quinolylsulfimides with different functional groups were synthesized in moderate to high yields. The obtained 8-quinolylsulfimides were proved to be promising novel type of bidentate ligands in Pd(II)-catalyzed allylic alkylation.

Synthesis and application of P,olefin type axially chiral ligands with sec-alkyl groups

Palladium-catalyzed asymmetric allylic substitution with indoles using a chiral ligand 3b, which have fewer hindered substituents such as secondary alkyl groups than 1-adamantyl group, gave desired products (R)-9 with high enantioselectivities (up to 96% ee).

N,N-Disubstituted Allylic Amine Type Aminophosphines with C(aryl)-N(amine) Bond Axial Chirality: Synthesis and Application to Palladium-Catalyzed Asymmetric Allylic Alkylation with Malonates

We designed and synthesized a series of N,N-disubstituted allylic amine type aminophosphines 2, 3 and 4, which are derivatives of chiral ligands 1. Aminophosphines 2-4 (except 2a) exist in C(aryl)-N(amine) bond axial chirality by chiral HPLC analysis. Both enantiomeric isomers of 4b were successfully obtained in an enantiomerically pure form. We demonstrated that 1a, 1b, and 4b can be used as effective chiral ligands for the palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with malonates in high enantioselectivities (up to 90% ee).

Rhodium-Catalyzed Asymmetric Addition of Organoboronic Acids to Aldimines Using Chiral Spiro Monophosphite-Olefin Ligands: Method Development and Mechanistic Studies

The synthesis of a novel type of chiral spiro monophosphite-olefin (SMPO) ligands based on a hexamethyl-1,1'-spirobiindane scaffold was accomplished starting from Bisphenol C. The optimal ligand could serve as an elegant chiral bidentate ligand in the Rh-catalyzed asymmetric 1,2-addition of organoboronic acids to various acyclic/cyclic aldimines, leading to chiral amines with high yields and excellent enantioselectivities. Detailed stereochemical models for enantioselective induction were elucidated through DFT calculations and postulated the origins of the higher enantioselectivity of phosphite-olefin ligands.

Palladium-catalyzed tandem allylic substitution/cyclization and cascade hydrosilylated reduction: the influence of reaction parameters and hydrosilanes on the stereoselectivity

Pd-catalyzed AAA of 1,2-bifunctional nucleophiles and one-pot tandem allylic cyclization/reduction gave corresponding heterocycle products with promising enantioselectivity in good yields.

[Development of New Chiral Phosphine Ligands with Helical Environments and Their Application in Asymmetric Catalytic Reactions]

 The design and development of new chiral ligands to enable precise stereocontrol in a wide variety of reactions is one of the most important branches of organic synthesis. To date, the development of hybrid ligands containing both σ-donating and π-donating groups has attracted considerable attention, with unprecedented reactivities and stereoselectivities being observed. Therefore to develop efficient hybrid chiral ligands with novel structural motifs, we envisage that helicene would be a suitable π-donor efficiently to construct a helical environment around a metal center. In this context, we herein describe our recent efforts to develop a series of novel chiral [5]helicene-derived phosphine ligands (L1, with a 7,8-dihydro[5]helicene core structure, and L2, with a fully aromatic [5]helicene core structure). The prepared ligands, and in particular L1, were found highly effective in the asymmetric allylation of 1,3-diphenylallyl acetate with indoles and etherification with alcohols. Furthermore, in the asymmetric Suzuki-Miyaura coupling reaction, L2 exhibited excellent enantioselectivities. Finally, density functional theory studies were employed to propose a model that accounts for the origin of such high enantioselectivity in these reactions.

Chiral ligands designed in China

Asymmetric catalysis has become an indispensable and productive field within the Chinese organic chemistry society. The design of chiral ligands is one of the most prominent research areas in this field. Since the late 1990s, Chinese organic chemists have developed numerous chiral ligands possessing novel chiral skeletons and design concepts. Some of these ligands have been widely adopted and can be regarded as ‘privileged ligand’, which have shown excellent performance in many asymmetric catalytic reactions. In this review, we provide an overview of the chiral ligands designed by Chinese scientists with the aim of promoting the development of this area in China and with the hope of encouraging more scientists across the world to use these ligands when designing asymmetric reactions.

P(O)R2-Directed Enantioselective C-H Olefination toward Chiral Atropoisomeric Phosphine-Olefin Compounds

An effective synthesis of chiral atropoisomeric biaryl phosphine-olefin compounds via palladium-catalyzed enantioselective C-H olefination has been developed for the first time. The reactions are operationally simple, tolerate wide functional groups, and have a good ee value. Notably, P(O)R₂ not only acts as the directing group to direct C-H activation in order to make a useful ligand but also serves to facilitate composition of the product in a useful manner in this transformation.

Stereocontrolled C(sp3)–P bond formation with non-activated alkyl halides and tosylates

The current work provides a general approach for making C–P bonds with stereocontrol from chiral secondary alcohols.

Rational Design and Synthesis of [5]Helicene-Derived Phosphine Ligands and Their Application in Pd-Catalyzed Asymmetric Reactions

A series of novel optically active [5]helicene-derived phosphine ligands (L1, with a 7,8-dihydro[5]helicene core structure- and L2, with a fully aromatic [5]helicene core structure) were synthesized. Despite their structural similarities, L1 and L2 exhibit particularly different characteristics in their use as chiral ligands. L1 was highly effective in the asymmetric allylation of indoles with 1,3-diphenylallyl acetate (up to 99% ee), and in the etherification of alcohols (up to 96% ee). In contrast, L2 was highly effective in the stereocontrol of helical chirality in Suzuki–Miyaura coupling (SMC) reaction (up to 99% ee). Density functional theory analysis was employed to propose a model that accounts for the origin of the enantioselectivity in these reactions.

Two phenyls are better than one or three: synthesis and application of terminal olefin-oxazoline (TOlefOx) ligands

A novel terminal olefin-oxazoline ligand was introduced into rhodium-catalyzed asymmetric conjugate addition of arylboronic acids to enones and gave excellent enantioselectivities. The two phenyls proved better than one or three in ligand evaluations.

Fei-Phos ligand-controlled asymmetric palladium-catalyzed allylic substitutions with structurally diverse nucleophiles: scope and limitations

The asymmetric palladium-catalysed alkylation of structurally diverse nucleophiles, including alcohols, activated methylene compounds, indoles, and aromatic amines, led to the formation of corresponding products in good yields and high enantioselectivities (up to 99% ee).

Chiral N-1-adamantyl-N-trans-cinnamylaniline type ligands: synthesis and application to palladium-catalyzed asymmetric allylic alkylation of indoles

Chiral P/olefin ligands with C–N bond axial chirality were synthesized and utilized for palladium-catalyzed asymmetric allylic alkylation of indoles.

Chirality Transfer in Gold(I)-Catalysed Direct Allylic Etherifications of Unactivated Alcohols: Experimental and Computational Study

Gold(I)-catalysed direct allylic etherifications have been successfully carried out with chirality transfer to yield enantioenriched, γ-substituted secondary allylic ethers. Our investigations include a full substrate-scope screen to ascertain substituent effects on the regioselectivity, stereoselectivity and efficiency of chirality transfer, as well as control experiments to elucidate the mechanistic subtleties of the chirality-transfer process. Crucially, addition of molecular sieves was found to be necessary to ensure efficient and general chirality transfer. Computational studies suggest that the efficiency of chirality transfer is linked to the aggregation of the alcohol nucleophile around the reactive π-bound Au-allylic ether complex. With a single alcohol nucleophile, a high degree of chirality transfer is predicted. However, if three alcohols are present, alternative proton transfer chain mechanisms that erode the efficiency of chirality transfer become competitive.

Pd-catalyzed asymmetric allylic amination using easily accessible metallocenyl P,N-ligands

Compared to their C1-symmetric counterparts, planar chiral C2-symmetric metallocenyl P,N-ligands are efficient chiral ligands for Pd-catalyzed asymmetric allylic aminations, providing a number of amination products with high enantioselectivities. A non-C2-symmetric ferrocenyl P,N-ligand (a by-product obtained during the synthesis of the above C2-symmetric species) was also found to be an efficient ligand for asymmetric allylic aminations. A mixed ligand system consisting of both C2- and non-C2-symmetric ferrocene complexes was examined and showed high catalytic activity with the amination products being obtained with excellent enantioselectivities.

Dehydrative thiolation of allenols: indium vs gold catalysis

Intermolecular additions of thiols to allenols via formal S(N)2' selectivity to produce functionalized dienes are described. Although this dehydrative reaction was initially developed using gold(I) catalysis, indium(III) proves to be a far superior catalyst in terms of selectivity and substrate scope.

Base-promoted [1,4]-Wittig rearrangement of chalcone-derived allylic ethers leading to aromatic β-benzyl ketones

Strong base promoted [1,4]-Wittig rearrangement of allylic ethers was developed in this work, in which the reaction provided a facile approach to the synthesis of aromatic β-benzyl ketones under controllable radical reaction conditions.

Palladium-catalyzed asymmetric allylic amination of racemic butadiene monoxide with isatin derivatives

Pd-catalyzed asymmetric aminations of butadiene monoxide with isatin derivatives using P/alkene ligands gave high yields, ees and regioselectivity ratios.

Asymmetric synthesis of axially chiral anilides by Pd-catalyzed allylic substitutions with P/olefin ligands

Palladium-catalyzed allylic substitutions of ortho-substituted anilides using P/olefin ligands were achieved to afford chiral anilides with up to 84% ee.