New Approaches for Biaryl-Based Phosphine Ligand Synthesis via P

Article pubs.acs.org/accounts

New Approaches for Biaryl-Based Phosphine Ligand Synthesis via PO Directed C−H Functionalizations Yan-Na Ma,† Shi-Xia Li,† and Shang-Dong Yang*,†,‡ †

State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China

‡

CONSPECTUS: Given the important influence of phosphine ligands in transition metal-catalyzed reactions, chemists have searched for straightforward and efficient methodologies for the synthesis of diverse phosphine ligands. Although significant progress has been made in this aspect over the past decades, the development of new phosphorus-containing ligands with properties superior to their predecessors remains a central task for chemists. Recently, researchers have demonstrated that biphenyl monophosphine ligands function as highly efficient ligands for transition-metal-catalyzed organic transformations, especially for reactions where chelating bisphosphine ligands cannot be used. In 1998, Buchwald introduced a new class of air-stable phosphine ligands based on the dialkylbiaryl phosphine backbone. These ligands have been successfully used for a wide variety of palladium-catalyzed carbon−carbon, carbon−nitrogen, and carbon−oxygen construction processes as well as serving as supporting ligands for a number of other reactions. At the same time, the use of the biphenyl monophosphine ligands often allows reactions to proceed with short reaction times and low catalyst loadings and under mild reaction conditions. However, the synthesis of chiral biphenyl monophosphine ligands, especially those the chirality of which is due to biaryl axial chirality, is very limited. In this Account, we summarize our methodologies for the synthesis of this kind of biphenyl monophosphine ligands including the PO directed C−H functionalization, PO directed diastereoselective C−H functionalization, PO directed enantioselective C−H functionalization, and metal-free diastereoselective radical oxidative C−H amination under mild reaction conditions. With these methods, a series of biphenyl phosphine ligand precursors containing achiral or axially chiral centers and precursors possessing both axial chirality and a chirogenic phosphorus center with different electronic properties and steric effect have been obtained under different reaction conditions. For the preparation of chiral biphenyl monophosphine ligands, which not only possess axial chirality but in many cases also possess chirality at phosphorus, the primary means of introducing chirality is through the use of the menthyl phenylphosphinate. As a chiral auxiliary group, the menthyl phenylphosphinate has some unique features: (i) it is easy to prepare; (ii) the products contain both axial chirality and central chirality on the phosphorus atom; (iii) the menthyl group could easily be transformed into other functional groups, which is crucial for the diversity of the corresponding biphenyl ligands. In our reaction, the PO group not only acts as the directing group but also facilitates the construction of the phosphine ligands. In addition, the application of these products in asymmetric catalysis has also been studied with good results obtained in some reactions. The further application of these ligands, especially the chiral biphenyl monophosphine ligands in catalysis reactions is underway in our laboratory, and we hope different kinds of reactions will be achieved with these ligands. determines selectivity. This permits the fine-tuning of the coordinated species and enables enhancing of the complex’s desired properties at different steps of a catalytic cycle. Phosphinebased ligands have attracted much attention because of their high efficiency in transition-metal-catalyzed reactions.3 The development of new phosphine ligands with suitable properties to efficiently effect a reaction is an important and challenging aspect in catalyst development. Although significant progress

1. INTRODUCTION Transition metal catalysis has offered powerful and straightforward methods for carrying out selective and effective chemical transformations in organic synthesis over the last few decades.1 Generally, the ligand plays a key role in these catalytic transformations and directly influences the reactivity and selectivity of the metal catalyst.2 The ligand modifies the electronic properties of the catalytic metal, leading to changes in the reactivity. Meanwhile, the steric properties of the ligand also shape the space around the catalytic center, which directs the substrate approach to the metal center and as a consequence © 2017 American Chemical Society

Received: April 5, 2017 Published: May 9, 2017 1480

DOI: 10.1021/acs.accounts.7b00167 Acc. Chem. Res. 2017, 50, 1480−1492

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Accounts of Chemical Research Scheme 1. Methods for the Synthesis of Biphenyl-Based Phosphine Ligands

Scheme 2. PO Directed C−H Functionalization

Scheme 3. PO Directed C−H Functionalization through the Seven-Membered Cyclopalladium Intermediate

has been made in this aspect over the past decades, the development of new phosphorus-containing ligands with properties superior to their predecessors remains a central task for chemists. In 1998, Buchwald developed a new class of airstable phosphine ligands based on the dialkylbiaryl phosphine backbone.4 These ligands can be used for a wide variety of palladium-catalyzed carbon−carbon,5 carbon−nitrogen,6 carbon−oxygen,7 and carbon−fluorine8 construction processes. At the same time, the use of this type of phosphine ligand often allows reactions to proceed along short reaction times and low catalyst loadings and under mild reaction conditions. Generally, these ligands can be prepared in a direct one-pot protocol by the addition of an aryl Grignard or an aryllithium reagent to a benzyne intermediate generated in situ, followed by trapping the intermediate with an appropriate chlorophosphine reagent (A, Scheme 1). This method provides a simple and efficient pathway for the synthesis of biphenyl-based phosphine ligands that can be achieved on a large scale. The method is limited, however, by the sensitivity of these conditions to air and moisture. Chiral phosphine ligands in particular are difficult

to synthesize with this method. Therefore, efficient access to novel and especially chiral ligands with mild and reliable reaction conditions is highly desirable.9 Over the past several years, we have focused on the development of simple and efficient protocols for the synthesis of new type of phosphine compounds.10 Herein, we wish to report our new approaches for the synthesis of new biphenyl phosphine ligands, which in some cases possess both axial chirality and a chirogenic phosphorus center (B, Scheme 1). We have relied on a series of different strategies, including the PO directed C−H functionalization, PO directed diastereoselective C−H functionalization, PO directed enantioselective C−H functionalization, and metal-free C−H amination. 1481


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Accounts of Chemical Research Table 1. Evaluation of Different Directing Groups

Table 2. Scope of PO Directed C−H Olefination

Table 3. Scope of PO Directed C−H Hydroxylation

2. Pd(II)-CATALYZED PO DIRECTED C−H FUNCTIONALIZATION Transition metal-catalyzed C−H bond activation and functionalization has become a powerful tool in organic synthesis because of its broad substrate scope and high atom economy.11 Chelation-assisted C−H cleavage has figured importantly in large part due to its efficiency and good site selectivity. Over the past several decades, many efforts have been devoted to chelation-assisted C−H functionalization, and researchers have identified various transition metals and directing groups. In recent years, PO directed C−H functionalization has also attracted much attention; indeed, different compounds containing the PO group have been studied by Glorius,12 Lee,13 Kim,14 Loh,15 Zhao,16 Miura,17 Daugulis,18 Han,19 Shi,20 and Chang,21 as well as others22 (Scheme 2). For our own focus, we selected the 2-diphenylphosphine oxide biphenyl compounds as starting material to operate Pd-catalyzed C−H functionalizations and achieved a series of C−H olefination,23 hydroxylation,24 acetoxylation,25 arylation,26 acylation,27 and iodination (Scheme 3). Unlike previously reported syntheses involving C−H functionalizations directed by phosphine oxides, phosphoric acids or phosphate esters, which had to pass through a five- or six-membered cyclometal transition state, our reactions feature a sevenmembered cyclopalladium intermediate (Notably, Colobert has also reported a series of sulfoxide-directed C−H functionalization through seven-membered cyclopalladium intermediate28). Meanwhile, these Pd(II)-catalyzed C−H functionalizations

a

10 mol % Pd(TFA)2 at 80 °C.

Scheme 4. PO Directed C−H Acetoxylation, Arylation, and Iodination

provide further concise and efficient new strategies for the synthesis of biphenyl ligands. 2.1. Pd(II)-Catalyzed PO Directed C−H Olefination

In the initial study, we first examined the PO directed C−H olefination with different types of substrates (Table 1). We chose this reaction for two reasons: (1) Heck-type coupling reactions have become an indispensable strategic tool for C−C 1482


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Accounts of Chemical Research Scheme 5. PO Directed C−H Acylation

Scheme 6. Transformations of the Acylation Products

Scheme 7. Proposed Mechanism of R2(O)P-Directed Pd(II)Catalyzed C−H Olefination

bond formation and play a pivotal role in modern organic synthesis. (2) The expected products are important precursors of alkene−phosphine hybrid ligands.29 When triphenylphosphine oxide and naphthalen-1-yldiphenylphosphine oxide were used as substrates, no C−H olefination products were obtained. On the other hand, -P(O)Ph2, -P(O)(OEt)2, and -P(O)(i-Pr)2 substituted biphenyls can react smoothly, and the desired products can be obtained in good yields. Notably, these results demonstrated that the seven-membered cyclopalladium pretransition state was necessary to this reaction. As diverse phosphine oxides and phosphinates can be employed as the directing group, this chemistry allows access to biphenyl-based phosphines with differing electronic and steric properties. Based on the 2-diphenylphosphine oxide biphenyl backbone, a series of substituted diphenylphosphine oxide derivatives and various acrylates were examined and obtained in good results (Table 2). We also discovered that not only were the electrondeficient olefins such as methyl, butyl, and benzyl acrylates, alkenylphosphite, and vinyl sulfone compatible with this transformation, but also far fewer electrophilic styrenes had to be used in this reaction.

Table 4. Preparation of Chiral Biphenyl Phosphine Ligand Precursors

2.2. Pd(II)-Catalyzed PO Directed C−H Hydroxylation

After the initial discovery of Pd(II)-catalyzed C−H olefination of 2-substituted diphenylphosphine oxide biphenyl compounds 1483


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Accounts of Chemical Research

using different olefins, we needed to determine whether this Pd(II)-catalyzed PO directed C−H functionalization could be extended to other types of reactions. Aiming to construct synthetically useful scaffolds, we wished to expand this methodology to hydroxylation as the products are important precursors of various biphenyl P,O-ligands. After systematic studies, we found that the Pd(TFA)2/PhI(OTFA)2 catalytic system was

Scheme 8. Synthesis of Phosphine−Alkene Ligands

Scheme 9. Synthesis of Axially Chiral Biaryls through DKR

Scheme 10. Asymmetric Synthesis of Axially Chiral Biaryl Phosphinates with Menthyl Phenylphosphate as the Auxiliary Group

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Accounts of Chemical Research the best choice in CH3NO2 at 60 °C under an air atmosphere. With these reaction conditions, a series of biphenyl P,O-ligand precursors with different electronic and steric properties were obtained with high yields (Table 3). Interestingly, when diethyl biphenyl-2-ylphosphonate was used, we obtained the phosphoryl lactone. When electron-withdrawing groups such as F, Cl, and CF3 lay on the 4′-position, the corresponding products were afforded in excellent yields with an increasing catalyst loading at a higher temperature.

P(O)Ph2 or P(O)R2, P(O)tBuPh proved an efficient directing group for the appropriate electronic properties. Then, we turned our attention to other acyl sources and wished to expand the application of this reaction. To our delight, benzyl alcohol, benzaldehyde, and toluene could also been used as acylation reagents, which offer the possibility to select conditions according to the properties of the products and substrates (Scheme 5). These products can be further transformed into other functional groups under appropriate conditions (Scheme 6), which is important for the diversity of the desired biphenyl ligand precursors. Through these transformations, a series of biphenyl ligand precursors with different electronic and steric properties were obtained. On the basis of the observed experimental results and pioneering reports, we proposed a plausible mechanistic pathway outlined in Scheme 7 with the olefination reaction as the example. First, Pd(OAc)2 coordinates with Ac-Gly-OH to form the active palladium catalyst, which reacts with substrate 1a to produce the seven-membered cyclopalladium intermediate 1A. Then, the active species 1A coordinates with alkenes to form complex 1B, which then undergoes insertion to afford the complex 1C. Finally, the product 2a is afforded through β-hydride elimination, and the active palladium catalyst is regenerated in the presence of AgOAc.

2.3. Pd(II)-Catalyzed PO Directed C−H Acetoxylation, Arylation, and Iodination

In light of the above results of PO directed C−H hydroxylation, we hypothesized that using less reactive PhI(OAc)2 as the oxidant might lead to acetoxylation products under a similar catalytic system, because the hydroxylation products might derive from the labile corresponding trifluoroacetoxylated compounds. As we expected, the acetoxylation product was gained with good yield after a simple screening of solvents and catalysts (Scheme 4a). Owing to the lower reactivity of PhI(OAc)2, some useful and sensitive functional groups also proceeded smoothly, and the corresponding products could be obtained in good yields. In the meantime, similar hypervalent iodine reagents were investigated in order to synthesize polyaromatic monophosphorus compounds via PO directed Pd(II)-catalyzed C−H arylation (Scheme 4b). Through this reaction, a series of terphenyl electron-rich monophosphine oxide compounds were prepared with good yields. Next, the PO directed C−H functionalization was further extended to the iodination reaction, and the desired product, an important coupling reagent in many cross-coupling reactions, was obtained in good yield (Scheme 4c).

3. PREPARATION OF CHIRAL 2′-PHOSPHORYLBIPHENYL COMPOUNDS THROUGH Pd(II)-CATALYZED C−H FUNCTIONALIZATION Catalytic transformations that result in the enantioselective formation of C−C bonds have found great use in the syntheses of natural products and active pharmaceutical ingredients. In these reactions, chiral phosphine ligands play a crucial role in the process of controlling enantioselectivities. As we know, the asymmetric synthesis of axially chiral biphenyl ligands is very limited. So, next we turned our attention to the synthesis of axially chiral biphenyl phosphine ligands. Initially, we synthesized axially chiral biaryl phosphine oxides and subjected them to the Pd(II)-catalyzed C−H functionalization reaction conditions (Table 4).30 As expected, the functionalized biaryl products were obtained in moderate yields with minimal loss of % ee. No new chiral center was introduced in this procedure, however. In order to showcase the ability to synthesize corresponding trivalent phosphine ligands, we selected the axially chiral olefination product 13 as the model to carry out the reduction with HSiCl3 in toluene and obtained the enantiopure binaphthylbased alkene−phosphine hybrid ligands in good yield and with no decrease in the ee value (Scheme 8).31

2.4. Pd(II)-Catalyzed PO Directed C−H Acylation

As the carbonyl moiety is central to many broadly used synthetic modifications and many aryl ketones are also key functionalities found in natural products, medicinally relevant molecules, and functional materials, we finally turned our attention to the PO directed C−H acylation. While only moderate yields were observed when the phosphine oxide directing group was Scheme 11. Tranformations of the Coupled Product

4. Pd(II)-CATALYZED ASYMMETRIC C−H ACTIVATION AND DYNAMIC KINETIC RESOLUTION The dynamic kinetic resolution (DKR) of biaryl atropoisomers, which occur in tandem with in situ racemization and resolution, provides one of the most convenient and efficient approaches for the synthesis of a wide range of enantiomerically enriched biaryl compounds.32 In 2000, Murai reported the first atropselective alkylation of biaryl compounds through transition metal-catalyzed C−H activation and DKR (Scheme 9, eq 1).33 Yields and enantioselectivities were moderate, however. Ten years later, Miller’s group developed the DKR of biaryl atropisomers via peptide-catalyzed asymmetric bromination (Scheme 9, eq 2).34 1485


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Accounts of Chemical Research Table 5. Scope of Asymmetric C−H Olefination/DKR

asymmetric transformation of the racemic prefunctionalized (naphthyl) quinoline derivatives (Scheme 9, eqs 3 and 4).35 Recently, Colobert reported the asymmetric synthesis of a series of axially chiral biaryls through chiral sulfoxide-directed diastereoselective C−H activation and DKR (Scheme 9, eq 5).28b In 2015, we reported the asymmetric Suzuki−Miyaura crosscoupling reaction36 with menthyl phenylphosphinate as the directing group. With this method, we obtained the axially chiral biaryl menthyl phosphinates with three ortho substituents at the axis when chiral Rp-(+)-menthyl 1-bromonaphthalen-2yl(phenyl)phosphinate (18)37 and 2′ substituted phenyl boronic acids (19) were used as the Suzuki−Miyaura crosscoupling reagents (Scheme 10, eq 1).38 The chiral menthyl phenylphosphinate group successfully enabled a diastereoselective cross-coupling between the bromonaphthalenephosphinate (18) and the 2-susbstituted phenyl boronic acids (19) in order to afford chiral trisubstituted biaryl products (20) in high yields with excellent dr values. Analogous to the work by Colobert, we hypothesized that the menthyl phenylphosphinate group could be used to direct the asymmetric C−H functionalization through DKR. With this design principle in mind, we synthesized the axially achiral substrates under Suzuki− Miyaura cross-coupling reaction conditions. The functionalized axially chiral biphenyl monophosphorus ligand precursors were then obtained under the PO directed C−H bond functionalization reaction conditions (Scheme 10, eq 2).39 As a chiral auxiliary group, the menthyl phenylphosphinate has some unique features: (i) it is easy to prepare; (ii) the products with axial chirality can be influenced to favor a single diastereomer from the central chirality on the phosphorus atom; (iii) the menthyl group could easily be transformed into other functional groups, which is crucial for the diversity of the corresponding biphenyl ligands. For example, the menthyl

Scheme 12. Asymmetric C−H Acetoxylation and Iodination/ DKR

Scheme 13. Asymmetric C−H Acylation/DKR

In 2013, the groups of Stoltz, Vergil, Fernández, and Lassaletta simultaneously but independently developed the asymmetric synthesis of axially chiral heterobiaryls via dynamic kinetic Scheme 14. Asymmetric C−H Acylation/KR

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Accounts of Chemical Research Table 6. Chiral Ligands of L-Amino Acid Screening

group could easily be replaced by alkyl, phenyl, and amino with alkyllithium, phenyllithium, and aminolithium or aminosodium under mild conditions in order to give corresponding phosphine oxides and phosphinic amides with no decrease in both the diastereomeric ratios and enantiomeric ratios (Scheme 11).40 The chiral phosphinic amide can be used as an important organocatalyst in palladium-catalyzed asymmetric C−H bond arylation,41 and the chiral phosphine oxide is an efficient catalyst in the asymmetric conjugate reduction of α,β-unsaturated ketones.42 Initially, the racemic substrate S-menthyl-(2′-methyl-[1,1′biphenyl]-2-yl)(phenyl)phosphinate (25a) was evaluated in the asymmetric C−H olefination under the above PO directed C−H olefination reaction conditions. To our delight, the desired product was obtained in 58% yield with excellent diastereomeric ratio (>95:5). Through further screening of oxidants and catalysts, the best reaction conditions were obtained. Then, different substituted achiral menthyl-biarylphosphinate derivatives (25) and various acrylates were examined. The corresponding products (26) were obtained via C−H activation in moderate to good yields with excellent diastereoselectivities (Table 5). Finally, the menthyl naphthylphosphinate (25m) could also go through the diastereoselective process successfully via desymmetrization. For this case, we observed the formation of di-ortho-alkenylated products. Next, we turned our attention to diastereoselective C−H functionalization with other reagents. When (Sp)-(−)-menthyl

Table 7. Scope of Substrates

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Accounts of Chemical Research Scheme 15. Metal-Free Diastereoselective Synthesis of MAP-Type Ligands

hydroxylation, and acylation, but only a small amount of the desired products were obtained. The menthyl group could be replaced with a tert-butyl group in a stereospecific manner using tert-butyl lithium in order to give compound 29, which is still a rapidly interconverting atropisomer. With 29 as the substrate, the product 30 was obtained in moderate yield with excellent diastereoselectivity under previous reaction conditions when phenylglyoxylic acid and benzyl alcohol were used as the acylation reagent (Scheme 13). That the utility of DKR process tends toward substrates with relatively low rotational barriers in order to allow for a dynamic process is well-known.43 For the substrates with higher rotational barriers, the kinetic resolution (KR) process may be necessary. Therefore, we synthesized the axially racemic substrate (Rp)-[1,1′-binaphthalen]-2-yl(tert-butyl)(phenyl)phosphine oxide (31) and subjected it to the asymmetric C−H acylation reaction. As expected, the reaction proceeded smoothly, and both product and substrate were obtained with good yields and excellent diastereoselectivities (Scheme 14). The absolute configuration of product 32 was confirmed through X-ray crystal analysis and other compounds were determined by analogy. The atroposelective mild C−H functionalization occurred with chiral menthyl phenyl phosphinate or tert-butyl phenyl phosphine

Scheme 16. Stability of Bridging Lactone A and Bridging Lactphosphamide B

(2′-methyl-[1,1′-biphenyl]-2-yl)(phenyl)phosphinate (25a) was subjected to the C−H acetoxylation under the above PO directed C−H acetoxylation reaction conditions, the desired acetoxylation product 27 was obtained in 46% yield with excellent diastereomeric ratio (Scheme 12). We also recovered the substrate (25a), which is axially achiral, again demonstrating the power of this diastereoselective C−H activation. Meanwhile, subsequent investigations on the Pd(II)-catalyzed asymmetric C−H activation was further extended to iodination. The desired product 28 was obtained in 54% yield with excellent diastereomeric ratio under previous iodination reaction conditions. Then the (Sp)-(−)-menthyl (2′-menthyl-[1,1′-biphenyl]-2yl)(phenyl)phosphinate (25a) was subjected to C−H arylation, 1488


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Accounts of Chemical Research Table 8. Scope of the Metal-Free C−H Amination

(Table 6). The best result was obtained with the Boc-L-Val-OH (L9) as a chiral ligand.45 Next, a series of substituted diphenylphosphine oxide derivatives (33) and various olefins were examined and obtained in good results (Table 7). The reactions are operationally simple, tolerate a wide array of functional-groups, and have good ee values. This is the first example for the effective synthesis of chiral atropoisomeric biaryl phosphine−olefin compounds via palladium-catalyzed enantioselective C−H olefination.

Scheme 17. Tandem Oxidative C−H Amination and Iodization/Bromination

6. METAL-FREE C−H AMINATION Chiral aminophosphine compounds with biaryl backbones (MAP-type ligand) have been widely used in many asymmetric reactions because of their high reactivities and selectivities.46 Moreover, chiral aminophosphine ligands are also easier to modify and convert into a variety of other chiral ligands or organocatalysts (Scheme 15A).47 So we next turned our attention to the synthesis of axially chiral aminophosphine ligands through diastereoselective radical oxidative C−H amination and further reduction of the products (Scheme 15B).48 As early as 1990s, Bringmann’s group studied the stability of bridging lactone A. As a consequence of the steric repulsion between the naphthalene moiety and the ortho substituent R in the phenolic part, the lactones A are helically distorted and thus exist as a racemic mixture of their two enantiomeric forms (Scheme 16).49 For lactphosphamide B, the atropisomeric axis probably can be controlled by the chiral phosphamide, which makes possible the asymmetric synthesis of B through asymmetric C−H bond amination by dynamic kinetic resolution or desymmetrization (Scheme 16).

oxide as auxiliary through DKR or KR toward the synthesis of olefination, acetoxylation, iodination, and acylation atropisomeric biaryl phosphorus compounds represents a rare example of C−H activation-based asymmetric strategy enabling axial stereocontrol.

5. P(O)R2-DIRECTED ENANTIOSELECTIVE C−H OLEFINATION Encouraged by the above results, we next turned our attention to the palladium-catalyzed enantioselective C−H olefination44 with chiral monoprotected amino acids as ligands for the synthesis of chiral atropoisomeric biaryl phosphine−olefin compounds. We selected the potential axially chiral biaryls of 2-diphenylphosphino-2′-methylbiphenyl (33a) and ethyl acrylate as model substrates to screen different chiral amino acids 1489


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Accounts of Chemical Research Scheme 18. Utility of (Ra,Sp)-36n and 38 in Asymmetric Catalysis

catalysis reactions, especially asymmetric catalysis, will be achieved with these compounds.

Initially, we synthesized the chiral phosphinic amide 35a through our previous Pd-catalyzed cascade asymmetric crosscoupling reaction38 and further reacted it with lithium methylamide in high yield with excellent enantioselectivity. Then the substrate was put into the diastereoselective radical oxidative C−H amination. After a series screening of the reaction conditions, the desired product 36a was obtained in 93% yield with excellent diastereoselectivity. Next, different substituted phosphamides were examined. The corresponding products were obtained through NIS-promoted C−H amination and DKR or desymmetrization process in good yields with excellent diastereoselectivities (Table 8). The absolute configuration of the products was confirmed by single-crystal X-ray crystallography ((Ra,Sp)-36k), which is axially chiral. Obviously, this method provides a concise and highly valuable pathway for the synthesis of enantiopure aminophosphine ligands in large scale. During the reaction conditions screening process, we discovered the tandem oxidative C−H amination and iodination product 39 when PhI(OAc)2/I2 was used in CH3CN (Scheme 17, eq 1). In light of the above results, we next changed the iodine sources to KBr and the desired bromination product 40 was obtained in 62% yield with excellent selectivity when toluene/H2O (4:1) was used as the solvent. A higher yield of 82% was gained if the PhI(OAc)2 was changed to oxone (Scheme 17, eq 2).50 In order to display the utility of the products, we first used the chiral aminophosphine (Ra,Sp)-36 as the organocatalyst to prompt the asymmetric reduction of 41 and obtained 42 in 93% yield with 87% ee value when (Ra,Sp)-36n was used (Scheme 18, eq 1).42 Furthermore, we also chose the corresponding trivalent phosphine compound 38 as the chiral ligand to the asymmetric Suzuki−Miyaura cross-coupling and synthesized axial chirality biaryl compounds 43 in 87% yield with 79% ee value (Scheme 18, eq 2).

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AUTHOR INFORMATION

Corresponding Author

*Shang-Dong Yang. E-mail: [email protected]. ORCID

Shang-Dong Yang: 0000-0002-4486-800X Notes

The authors declare no competing financial interest. Biographies Yan-Na Ma was born in Hebei (China) in 1988. She obtained her B.S. degree in chemistry at Lanzhou University in 2012. She is currently working toward her Ph.D. in organic chemistry in the college of chemistry and chemical engineering of Lanzhou University under the direction of Prof. Shang-Dong Yang. Her research focuses on the asymmetric synthesis of chiral phosphine ligands. Shi-Xia Li was born in Henan (China) in 1991. She obtained her B.S. degree in chemistry at Lanzhou University in 2014. She is currently working toward her master’s degree in organic chemistry in the college of chemistry and chemical engineering of Lanzhou University under the direction of Prof. Shang-Dong Yang. Her research focuses on the asymmetric C−H activation. Shang-Dong Yang was born in Gansu (China) in 1973. He studied chemistry in Lanzhou University, where he received his bachelor’s degree in 1997 and his Ph.D. in 2006 under the direction of Prof. Yong-Ming Liang. He worked as a Postdoctoral Fellow with Prof. Zhang-Jie Shi at Peking University from 2006 to 2007 and with Chuan He at the University of Chicago from 2007 to 2009. In 2009, he joined Lanzhou University as a Professor of State Key Laboratory of Applied Organic Chemistry. His group’s current researches focused on the organophosphorous compounds synthesis and transition-metal catalyzed asymmetric C−H activation.

7. SUMMARY AND OUTLOOK In summary, we have developed a series of simple and efficient protocols for the synthesis of biphenyl based phosphine ligands. We achieved this through the phosphine oxide or phosphinatedirected C−H functionalization, diastereoselective C−H functionalization through DKR or KR, enantioselective C−H functionalization, and metal-free diastereoselective radical oxidative C−H amination under mild reaction conditions. As a result, a series of biphenyl-phosphine ligand precursors containing racemic or axially chiral centers and those possessing both axial chirality and a chirogenic phosphorus center with different electronic properties and steric effect have been obtained under different reaction conditions. Further application of these compounds is underway in our laboratory. We hope that different

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ACKNOWLEDGMENTS We are grateful for the NSFC (Nos. 21472076 and 21532001) and PCSIRT (IRT_15R28 and lzujbky-2016-ct02) financial support.

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REFERENCES

(1) (a) Applied Homogeneous Catalysis with Organometallic Compounds: A Comprehensive Hand-book in Three Volumes; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, Germany, 2002. (b) Transition Metals for Organic Synthesis: Building Blocks and Fine Chemicals. Beller, M.; Bolm, C., Eds.; Wiley-VCH: Weinheim, Germany, 2004.

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