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BINAP

BINAP, or 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl, is an axially chiral C₂-symmetric diphosphine ligand renowned for its role in transition-metal-catalyzed asymmetric synthesis.^[1] First synthesized in 1974 by Ryoji Noyori and Hidemasa Takaya at Nagoya University, with optically active forms resolved by 1976 and the initial publication appearing in 1980, BINAP features a rigid binaphthyl backbone that imparts steric control for high enantioselectivity in catalytic reactions.^[2]^[1] The ligand's versatility stems from its ability to form stable complexes with metals such as rhodium, ruthenium, and palladium, enabling efficient asymmetric transformations.^[3] In asymmetric hydrogenation, BINAP-rhodium complexes achieve up to 100% enantiomeric excess (ee) in the synthesis of amino acids from enamides, while BINAP-ruthenium catalysts hydrogenate β-keto esters and allylic alcohols with >99% ee, facilitating industrial production of pharmaceuticals like naproxen.^[2]^[3] Beyond hydrogenation, BINAP promotes enantioselective isomerization of allylic amines for (-)-menthol synthesis (96–99% ee, >1500 tons annually) and carbon-carbon bond formations such as the Heck reaction.^[2]^[3] Noyori's work with BINAP contributed to his 2001 Nobel Prize in Chemistry, shared with William S. Knowles and K. Barry Sharpless, for pioneering chiral catalysis that has transformed the efficient, scalable production of enantiomerically pure compounds essential in drug development and fine chemicals.^[4] Its enduring impact is evident in derivatives like SEGPHOS and numerous commercial applications, underscoring BINAP's status as a cornerstone of modern stereoselective synthesis.^[3]

Structure and properties

Molecular structure

BINAP, or 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl, exists as stable (R)- and (S)-enantiomers due to its axial chirality arising from atropisomerism, a consequence of the restricted rotation around the central C(1)–C(1') bond in the binaphthyl core. This stereogenic axis is maintained by the bulky naphthalene rings fused to the biphenyl framework, creating a high rotational energy barrier that prevents racemization at room temperature.^[5] The chemical formula of BINAP is C₄₄H₃₂P₂, reflecting the binaphthyl backbone substituted with two diphenylphosphino (PPh₂) groups at the peri positions (2 and 2'). The molecular architecture features two naphthalene units linked at their 1 and 1' positions, with the PPh₂ moieties projecting outward from the 2 and 2' sites, forming a C₂-symmetric structure that is ideal for bidentate coordination to metals. The dihedral angle between the two naphthyl planes is approximately 90°, which contributes to the ligand's twisted conformation and influences its steric environment in complexes.^[6] This angle arises from the balance of steric repulsion between the ortho substituents on the naphthyl rings and the conjugative stabilization of the biaryl system. In metal complexes, the diphosphine adopts a bite angle of about 93°, defined as the P–M–P angle in chelates, which is relatively wide compared to smaller diphosphines and promotes specific geometries favorable for catalysis.^[6] This value, measured in structures like [PdCl₂(BINAP)], underscores how the ligand's backbone rigidity and phosphine separation optimize trans-spanning coordination.^[6] Compared to achiral analogs like BIPHEP (2,2'-bis(diphenylphosphino)-1,1'-biphenyl), BINAP's extended naphthalene rings impose greater steric bulk, elevating the atropisomerization barrier to ensure stable enantiopurity, whereas BIPHEP racemizes rapidly due to its more flexible biphenyl core.^[5]

Physical and chemical properties

BINAP appears as a white to off-white crystalline solid.^[7] Its molecular formula is C₄₄H₃₂P₂, with a molar mass of 622.67 g/mol.^[8] The compound exhibits enantiomer-specific melting points: 239–241 °C for the (R)-enantiomer and 238–240 °C for the (S)-enantiomer.^[8]^[9] BINAP demonstrates high solubility in common organic solvents such as tetrahydrofuran, benzene, and dichloromethane, with modest solubility in diethyl ether, methanol, and ethanol; it is insoluble in water.^[10] BINAP is air-stable under normal laboratory conditions, allowing for straightforward handling without the need for inert atmospheres.^[8] However, the phosphine groups are susceptible to oxidation to phosphine oxides upon prolonged exposure to oxygen, particularly in solution.^[6] Chemically, BINAP acts as a bidentate ligand, readily forming stable coordination complexes with transition metals through σ-donation from its phosphorus atoms, but it exhibits no catalytic activity on its own.^[10] The enantiomers display characteristic optical rotations, such as [α]²⁰_D +222° (c = 0.5 in benzene) for (R)-BINAP and the corresponding negative value for the (S)-enantiomer.^[8]

History and development

Discovery

BINAP, or 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl, was first synthesized in 1974 by Ryōji Noyori and Hidemasa Takaya at Nagoya University in Japan, with optically active forms resolved by 1976; its first report appeared in 1980.^[2]^[1] The development stemmed from efforts to create more effective chiral bidentate phosphine ligands for transition-metal catalysis, particularly to enhance enantioselectivity in rhodium-catalyzed asymmetric hydrogenation reactions. Earlier ligands, such as DIOP introduced by Henri Kagan in 1971, had achieved moderate optical yields (typically up to 80% enantiomeric excess) but suffered from limitations in chiral recognition and catalytic efficiency for certain substrates. Noyori's team sought a ligand with axial chirality and a rigid framework to provide superior steric control and stability in catalytic complexes.^[11]^[12] The initial synthesis of enantiomerically pure BINAP began with optically active 1,1'-bi-2-naphthol (BINOL), which was deprotonated via dilithiation using n-butyllithium to form the dianion, followed by reaction with chlorodiphenylphosphine to introduce the phosphino groups at the 2,2'-positions. This method yielded the atropisomeric ligand with high stereochemical integrity due to the restricted rotation in the binaphthyl backbone. The first report of this synthesis and its application appeared in a 1980 communication in the Journal of the American Chemical Society, where the authors detailed the preparation of (R)-BINAP and its coordination to rhodium(I) to form cationic complexes. These BINAP-Rh(I) catalysts demonstrated exceptional performance in the asymmetric hydrogenation of α-(acylamino)acrylic acids and esters, such as α-acetamidocinnamic acid derivatives, achieving enantiomeric excesses exceeding 95% under mild conditions (1 atm H₂, room temperature).^[1]^[1] This breakthrough laid the foundation for BINAP's role as a versatile chiral auxiliary in asymmetric catalysis. The ligand's effectiveness in promoting highly enantioselective reductions marked a significant advancement over prior diphosphine systems, enabling practical synthesis of chiral amino acids and related compounds. Noyori's contributions to this field, including the invention and application of BINAP, were recognized with the 2001 Nobel Prize in Chemistry, shared with William S. Knowles for their pioneering work on chirally catalyzed hydrogenation reactions.^[13]^[12]

Key developments

In 1986, H. Takaya and colleagues at Nagoya University reported the first practical large-scale synthesis of enantiomerically pure BINAP, starting from binaphthol and involving a key Ullmann coupling followed by phosphination and resolution via diastereomeric salt formation.^[14] This breakthrough overcame earlier challenges in producing gram-scale quantities of the chiral ligand, enabling its commercial availability through suppliers like Takasago International Corporation and facilitating broader adoption in asymmetric catalysis.^[15] During the 1990s, R. Noyori extended BINAP's utility by developing ruthenium complexes for the asymmetric hydrogenation of ketones, particularly β-keto esters, achieving enantioselectivities exceeding 99% ee under mild conditions (e.g., 1 atm H₂, room temperature). These Ru-BINAP-dihalide or dicarboxylate catalysts marked a significant advancement over rhodium-based systems, expanding BINAP's scope to unfunctionalized and functionalized carbonyls while maintaining high turnover numbers (up to 10,000).^[2] BINAP's industrial adoption accelerated in the late 1990s, exemplified by its use in the large-scale production of (S)-naproxen, a nonsteroidal anti-inflammatory drug, via Ru-BINAP-catalyzed asymmetric hydrogenation at Sumitomo Chemical.^[15] This process replaced classical resolution methods, reducing waste and costs while yielding >99% ee on multi-ton scales, demonstrating BINAP's economic viability in pharmaceutical manufacturing.^[2] The versatility of BINAP across transition metals—rhodium for olefin hydrogenations, ruthenium for ketone reductions, and palladium for cross-couplings—was a cornerstone of Noyori's 2001 Nobel Prize in Chemistry, shared with W. S. Knowles for pioneering chirally catalyzed hydrogenations. This recognition underscored BINAP's role in enabling stereoselective syntheses with minimal catalyst loadings (0.01–1 mol%).^[3] In recent years, BINAP has been integrated into continuous-flow processes for enhanced scalability and safety, such as Ru-BINAP-catalyzed hydrogenations in microreactors, achieving yields >95% with reduced reaction times compared to batch methods.^[16] These developments reflect ongoing refinements rather than paradigm shifts, maintaining BINAP's status as a benchmark ligand.

Synthesis

Preparation methods

BINAP is prepared from 1,1'-bi-2-naphthol (BINOL), which is commercially available in both racemic and enantiopure forms.^[1] The standard synthetic route to both racemic and enantiopure BINAP involves initial conversion of BINOL to 2,2'-bis(trifluoromethanesulfonyloxy)-1,1'-binaphthyl (bis-triflate) by treatment with triflic anhydride in the presence of a base such as pyridine or triethylamine. This electrophilic activation of the phenolic hydroxy groups facilitates subsequent carbon-phosphorus bond formation. The bis-triflate then undergoes metal-catalyzed cross-coupling with diphenylphosphine (HPPh₂) or chlorodiphenylphosphine (ClPPh₂). For instance, palladium-catalyzed coupling of the bis-triflate with HPPh₂, using Pd(OAc)₂ or Pd₂(dba)₃ as the precatalyst along with a phosphine ligand like dppp, proceeds under mild conditions (typically in DMF or toluene at 80–100 °C) to afford BINAP directly after workup.^[17]^[6] Racemic BINAP is synthesized from achiral (racemic) BINOL via this bis-triflate route, often achieving isolated yields up to 90% for the coupling step, though protection of the naphthol hydroxy groups may be incorporated in earlier stages if needed to prevent side reactions during scale-up. The multi-step process from BINOL typically delivers overall yields of 70–80%, making it suitable for industrial production.^[6]^[15] Enantiopure BINAP retains the stereochemistry of the starting enantiopure BINOL throughout, with no racemization observed under these conditions. An alternative coupling variant employs nickel catalysis, such as NiCl₂(dppe) with HPPh₂ and DABCO as base, yielding 77–87% for the chiral ligand.^[18] The original 1980 method reported by Noyori and coworkers provides an alternative direct route without triflate activation: BINOL is treated with chlorodiphenylphosphine oxide (Ph₂P(O)Cl) in pyridine to form the bis(phosphine oxide) intermediate, which is isolated and then reduced with trichlorosilane (HSiCl₃) in benzene to yield BINAP in 85% from the intermediate (overall ~50–60% from BINOL due to earlier steps). This approach, while effective for laboratory scale, has been largely superseded by the higher-yielding triflate methods for production.^[1] Purification of BINAP commonly involves silica gel chromatography to remove phosphine oxide byproducts formed during coupling or reduction, followed by recrystallization from solvents like ethanol or toluene to achieve high purity (>99%).^[18]

Enantioselective synthesis

Enantiopure BINAP is essential for asymmetric catalysis, and its preparation typically involves resolution of racemic mixtures or direct synthesis from chiral precursors. Resolution of racemic BINAP or its precursors, such as the bis-phosphine oxide, is commonly achieved through diastereomeric salt formation using chiral resolving agents like dibenzoyl tartaric acid. Salt formation remains the prevalent method for BINAP.^[19] A seminal approach, reported by Takaya and coworkers in 1986, involves synthesizing the racemic bis-phosphine oxide from BINOL, followed by resolution via diastereomeric salt formation with (R,R)-dibenzoyl tartaric acid to isolate the enantiopure (S)-phosphine oxide (yield ~45%). The resolved phosphine oxide is then reduced using phenylsilane in the presence of a palladium catalyst, affording enantiopure (S)-BINAP with >99% ee. This method established a practical route for both (R)- and (S)-BINAP, with the (R)-enantiomer obtained similarly using (S,S)-dibenzoyl tartaric acid after upgrading lower-purity fractions.^[19] Asymmetric synthesis from enantiopure BINOL represents a preferred modern route, bypassing resolution steps and directly yielding high enantiopurity. Enantiopure BINOL, obtained from chiral pool sources or asymmetric catalysis, is converted to the bis-triflate, which undergoes nickel-catalyzed cross-coupling with diphenylphosphine (Ph₂PH) to form BINAP in 77% yield and >99% ee. This approach maintains chiral integrity throughout, with overall efficiencies exceeding 95% ee.^[20] Commercial production of enantiopure BINAP, primarily by companies like Takasago International, scales these methods to yield grams to kilograms, supporting industrial asymmetric processes such as menthol synthesis.^[6] Due to BINAP's atropisomeric nature, where the axial chirality arises from restricted rotation about the biaryl bond, the ligand exhibits high configurational stability under typical reaction conditions.^[6]^[21]

Applications in catalysis

Asymmetric hydrogenation

BINAP serves as a key chiral ligand in rhodium and ruthenium complexes for enantioselective hydrogenation reactions, particularly targeting prochiral alkenes and ketones to produce valuable chiral building blocks with high enantiomeric excess (ee). These systems leverage BINAP's axial chirality to create a sterically biased environment around the metal center, enabling selective hydrogen delivery to one face of the substrate.^[2] In the rhodium-BINAP system, cationic Rh(I) complexes effectively hydrogenate α,β-unsaturated carboxylic acids and esters, such as in the conversion of methyl (Z)-acetamidocinnamate to the (S)-phenylalanine precursor with >95% ee under mild conditions. This application, first demonstrated in 1980, marked a significant advancement in synthesizing optically active amino acids. Ruthenium-BINAP complexes extend this capability to the hydrogenation of β-ketoesters and imines, with notable efficiency in reducing simple ketones. For instance, Noyori's 1995 development using Ru(II) complexes with BINAP and chiral diamine co-ligands enables the production of (R)-1,2-propanediol from hydroxyacetone with turnover numbers (TON) up to 10,000 and ee >99%. A representative example is the hydrogenation of acetophenone:

\ce{PhC(O)CH3 + H2 ->[Ru-BINAP/diamine][H+] PhCH(OH)CH3}

yielding (R)-1-phenylethanol with ee >99%. The mechanism in the Ru-BINAP system involves an outer-sphere pathway, where the chiral environment from BINAP's axial chirality, combined with the metal-ligand bifunctional activation of H₂ (as hydridic Ru-H and protic N-H), induces substrate facial selectivity without direct coordination of the ketone to the ruthenium center. This contrasts with inner-sphere mechanisms in some Rh systems. Compared to earlier ligands like DIPAMP, BINAP-based catalysts offer superior turnover numbers and broader substrate scope, particularly for unfunctionalized ketones, enabling industrial-scale applications.^[2]^[12]

Other asymmetric transformations

BINAP has been employed in palladium-catalyzed asymmetric Heck reactions, enabling the enantioselective arylation of alkenes to form chiral cyclic structures. For instance, intramolecular Heck cyclizations using Pd-BINAP complexes achieve high enantioselectivities, such as >90% ee in the synthesis of 2,3-dihydrobenzofurans from aryl iodides tethered to alkenes.^[22]^[23] Silver-BINAP complexes facilitate the enantioselective protonation of silyl enol ethers, providing access to α-chiral ketones under mild conditions. The AgF-BINAP system in dichloromethane-methanol at low temperatures protonates trimethylsilyl enol ethers derived from ketones like propiophenone, yielding products with up to 95% ee.^[24] Ruthenium-BINAP catalysts promote the asymmetric isomerization of allylic alcohols to enones, converting secondary allylic alcohols into chiral ketones with excellent enantiocontrol. This transformation, often using RuCl2(BINAP) complexes in basic media, routinely delivers >98% ee for a range of substrates.^[25] Notable applications include the Pd-BINAP-catalyzed asymmetric cyclization in the 1990s synthesis route to (-)-menthol developed by Hayashi, which constructs key chiral carbon centers via intramolecular coupling.^[25] BINAP also features in the preparation of taxol intermediates through enantioselective C-C bond formations, such as allylic substitutions that install stereocenters in the taxane core.^[26] Beyond these, BINAP excels in other C-C bond-forming reactions, including palladium-catalyzed asymmetric allylic alkylations of enolates or amines, which proceed with 85-99% ee depending on the substrate.^[27] Rhodium-BINAP systems enable conjugate additions of arylboronic acids to α,β-unsaturated carbonyls, affording β-aryl products in >90% ee.^[28] However, BINAP shows reduced efficacy in certain allylic alkylations involving sterically hindered nucleophiles compared to more modern ligands like SEGPHOS derivatives.^[29]

Derivatives and analogs

Modified BINAP ligands

Modified BINAP ligands have been developed to address limitations of the parent compound, such as steric constraints, solubility issues, and substrate specificity in catalytic applications. These modifications typically involve alterations to the binaphthyl backbone or the phosphine substituents, enabling fine-tuning of electronic and steric properties for improved enantioselectivity and reaction scope.^[6] Axial modifications on the naphthyl rings, particularly at the 3,3'-positions, introduce substituents to adjust the ligand's steric environment. For instance, 3,3'-disubstituted BINAP derivatives with alkoxy or acetoxy groups have been synthesized through convergent routes involving resolution by supercritical fluid chromatography, achieving high enantiopurity. These ligands enhance enantioselectivity in ruthenium-catalyzed asymmetric hydrogenations, with the 3,3'-dimethoxy variant delivering up to 99% ee in the hydrogenation of 1,1-diarylalkenes, outperforming unmodified BINAP under identical conditions. Variations in the phosphine moieties replace the phenyl groups on phosphorus with sulfonate groups to improve water solubility, facilitating biphasic catalysis. Sulfonated BINAP (BINAS) is prepared by selective sulfonation of the phenyl rings, yielding highly water-soluble derivatives in 43% overall yield from BINAP. This modification enables rhodium-catalyzed asymmetric hydrogenations in aqueous media, achieving enantioselectivities comparable to the parent ligand while allowing easy catalyst recovery through phase separation.^[6]^[30] Dendritic BINAP ligands incorporate multiple BINAP units into multi-generation dendrimer structures, synthesized via allylation reactions of dendritic polyaryl ethers with BINAP precursors. These ligands form ruthenium complexes that catalyze asymmetric hydrogenations of β-ketoesters with high efficiency and recyclability, recovering up to 90% of the catalyst after several runs without loss of enantioselectivity.^[31]^[32] The octahydro derivative, H₈-BINAP, features saturated naphthyl rings, increasing backbone flexibility and lipophilicity compared to BINAP. Developed by Hidemasa Takaya and coworkers in the mid-1990s, it is synthesized from H₈-BINOL analogs and excels in palladium-catalyzed allylation reactions, providing high enantioselectivities.^[6]^[33] Synthesis of these modified BINAP ligands generally proceeds through analogs of BINOL, the binaphthol precursor, followed by phosphination steps, with overall yields typically ranging from 60-80%. Certain derivatives achieve enantiomeric excesses exceeding 99.9% in targeted asymmetric transformations, effectively overcoming substrate limitations of unmodified BINAP in ruthenium and palladium catalysis.^[6] Related bidentate phosphine ligands, such as SEGPHOS and its variants, BIPHEP, P-Phos, and analogs like MOP, have been developed to complement BINAP by offering alternative structural features that enhance performance in specific asymmetric catalyses while addressing challenges like high cost and steric bulk associated with the naphthyl backbone. These ligands typically retain the C₂-symmetric diphosphine motif central to BINAP's success but incorporate modified biaryl backbones or hybrid designs for improved solubility, reactivity, or selectivity. Evolved primarily during the 1990s and 2000s, they enable broader applications in transition-metal catalysis, particularly for ruthenium and palladium systems. The SEGPHOS family, exemplified by the bulky derivative 4,4'-bis(1,1-dimethylethyl)-4,4',6,6'-tetramethyl-2,2'-bisphosphino-1,1'-biphenyl (DTBM-SEGPHOS), features a biaryl backbone with 1,3-benzodioxole units that confer a smaller dihedral angle of approximately 65° compared to BINAP's 73.5°, enhancing orbital overlap and enantiocontrol in metal complexes. Developed by Takasago researchers building on earlier work by Sawamura and Noyori, this ligand excels in ruthenium-catalyzed asymmetric hydrogenations, achieving enantiomeric excesses up to 99.5% for β-ketoesters due to its electron-rich and sterically demanding environment. In industrial applications, such as the synthesis of naproxen, SEGPHOS variants surpass BINAP by delivering higher enantioselectivities (often >99%) and better catalyst efficiency, facilitating large-scale production of the anti-inflammatory drug.^[34]^[15] BIPHEP, or 2,2'-bis(diphenylphosphino)-1,1'-biphenyl, serves as an achiral analog of BINAP, lacking the fused naphthyl rings that enforce axial chirality, and is frequently employed as a benchmark to evaluate the contributions of axial versus point chirality in catalytic performance. Its flexible biphenyl backbone allows easier rotation, making it less selective but useful for comparing reactivity profiles in achiral versus chiral environments, particularly in rhodium- and palladium-catalyzed processes. Unlike BINAP, BIPHEP's simpler structure reduces synthesis costs, highlighting trade-offs in steric control. P-Phos ligands, such as (S)-2,2',6,6'-tetramethoxy-4,4'-bis(diphenylphosphino)-3,3'-bipyridine, incorporate methoxy-substituted pyridine rings in an atropisomeric biaryl framework, providing bidentate coordination with tunable electronics for palladium-catalyzed cross-couplings and allylations. Inspired by BINAP's axial chirality, these ligands offer superior activity in C-C bond formations, often achieving higher turnover numbers than BINAP analogs due to the nitrogen donors' influence on metal acidity. Meanwhile, the related monodentate MOP ligand, 2-(diphenylphosphino)-2'-methoxy-1,1'-binaphthyl, adapts BINAP's phosphine motif for single-point binding in palladium systems, enabling high enantioselectivities (up to 98%) in allylic alkylations and hydrosilylations without the need for bidentate chelation.^[35]^[36] In comparisons across these ligands, SEGPHOS variants demonstrate superior enantioselectivity in ruthenium hydrogenations for certain substrates but exhibit reduced versatility compared to BINAP's broad substrate scope, while P-Phos and MOP excel in palladium-mediated transformations where BINAP underperforms due to steric constraints. All share the core C₂-symmetric diphosphine architecture, yet their optimizations—such as narrower angles in SEGPHOS or hybrid heteroatoms in P-Phos—address BINAP's limitations in cost and specificity, expanding the toolkit for asymmetric synthesis.

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