BINAP is the axially chiral bisphosphine that made asymmetric hydrogenation a production technology rather than a laboratory curiosity. It is sold as three separate products under three separate CAS numbers – racemic, (R) and (S) – and the three are physically indistinguishable on paper: same molecular formula, same molecular weight, same database identifier.
That last point is the reason this page exists. A specification that quotes formula, molecular weight and assay tells you nothing about which BINAP you are buying, and BINAP is the product in this catalogue where that confusion does the most damage.
The three grades
| Product | CAS | Formula / MW | PubChem CID | What it is |
|---|---|---|---|---|
| Racemic BINAP | 98327-87-8 | C44H32P2 / 622.7 | 634876 | 1:1 mixture of the two enantiomers – the screening baseline |
| (R)-BINAP | 76189-55-4 | C44H32P2 / 622.7 | 634876 | The (R) enantiomer, the (+)-rotating form |
| (S)-BINAP | 76189-56-5 | C44H32P2 / 622.7 | 634876 | The (S) enantiomer, the (-)-rotating form |
All three are 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl. The only difference is which of two mirror-image arrangements of the two naphthyl rings the molecule adopts.
Why the molecule is chiral at all
BINAP has no stereogenic carbon, no stereocentre in the conventional sense. Its chirality is axial: the two naphthalene rings are joined by a single bond, and the four substituents at the 2 and 2′ positions (the diphenylphosphino groups) are bulky enough that rotation about that bond is blocked. The molecule is locked into one of two conformations that are mirror images of each other and cannot interconvert at room temperature.
This is the phenomenon called atropisomerism. Two consequences follow that matter commercially:
- The two enantiomers are stable products, not a transient mixture. (R)-BINAP does not spontaneously racemise on the shelf, which is why it can be sold as a defined enantiopure reagent.
- The naphthyl backbone holds the two phosphorus atoms at a fixed geometry, wider and more rigid than a flexible bisphosphine like dppe. That rigidity is what makes the chiral environment predictable at the metal.
The identifier trap: why CID and formula cannot tell you the grade
Query a chemical database for CAS 98327-87-8, 76189-55-4 or 76189-56-5 and you get the same record: CID 634876, formula C44H32P2, molecular weight 622.7. The record describes the constitution of the molecule – which atoms are bonded to which – and says nothing about the three-dimensional arrangement.
The practical implication is direct: a certificate that quotes only formula, molecular weight and assay cannot distinguish racemic BINAP from (R)-BINAP from (S)-BINAP. They all match those fields. The only two things that identify the grade are the CAS number and a measurement of optical purity.
| Field on a certificate | Distinguishes the three grades? | Why |
|---|---|---|
| Molecular formula | No | Identical for all three |
| Molecular weight | No | Identical for all three |
| Assay / chemical purity (HPLC) | No | A racemate can be 99% chemically pure |
| Melting point | Weakly – often similar | Not a reliable discriminator; use rotation |
| Specific rotation | Yes | Opposite sign for (R) and (S); near zero for the racemate |
| Chiral HPLC or enantiomeric excess | Yes | Directly measures the enantiomer ratio |
| CAS number | Yes | The contractual identifier for the grade ordered |
Chemical purity versus optical purity
The two numbers answer different questions and both are needed for a chiral ligand.
Chemical purity (98% on our sheet, measured by HPLC) says how much of the sample is BINAP at all – the balance being phosphine oxide, unreacted starting material, or the mono-phosphine from an incomplete coupling.
Optical purity says how much of that BINAP has the right handedness, usually expressed as enantiomeric excess. A sample at 98% chemical purity and 99% ee contains almost no racemate; a sample at 99% chemical purity and 85% ee has a better assay figure and will give a worse result in an asymmetric reaction, because the minor enantiomer of the ligand produces the minor enantiomer of the product.
This is why optical purity is not a premium extra on a chiral ligand specification – it is the specification. A supplier who quotes chemical purity alone for (R)-BINAP has quoted the less important of the two numbers.
What BINAP is used for
| Application | Why BINAP’s geometry suits it |
|---|---|
| Asymmetric hydrogenation of beta-keto esters to beta-hydroxy esters | The BINAP-Ru system hydrogenates the ketone with high enantioselectivity, and the products are intermediates to chiral building blocks and pharmaceuticals |
| Asymmetric hydrogenation of enamides and dehydroamino acids | Gives the corresponding N-acyl amino acid derivatives; a textbook route to non-natural amino acids |
| Industrial synthesis of naproxen-class profens | BINAP-Ru hydrogenation replaced a resolution-based route – the demonstration that asymmetric hydrogenation can be a production process |
| Asymmetric isomerisation of allylic alcohols and amines | BINAP-Rh converts a racemic allylic alcohol into one that is enriched – the basis of routes to chiral citronellal and related terpenes |
| Axially chiral ligand for C-C and C-N coupling | Where a chiral environment is wanted without a change of donor strength |
Note that these are descriptions of established applications, not of conditions. The ligand alone does not determine the outcome: the metal (Ru, Rh, Pd, or others), the counterion, the solvent and the substrate’s own structure all matter, and the (R)/(S) choice must be established for the specific substrate.
What racemic BINAP is for
Buying the racemic grade is not a compromise – it is a deliberate screening decision. Running a reaction with racemic BINAP tells you whether the BINAP skeleton and bite angle suit the substrate at all, without spending enantiopure material on a substrate that may not work. Once the skeleton is confirmed, switch to (R) or (S) to find the matching handedness.
Racemic BINAP is also the starting material for classical resolution and for the preparation of diastereomeric derivatives, and it is used where chirality is not the point – as a bulky, electron-rich, rigid bisphosphine for ordinary catalysis.
How it differs from the other bisphosphines in the catalogue
| Ligand | CAS | Architecture | Chirality | Typical role |
|---|---|---|---|---|
| BINAP | 76189-55-4 / -56-5 / 98327-87-8 | Axially chiral binaphthyl, two diphenylphosphino groups | Axial – (R), (S) or racemic | Asymmetric hydrogenation and isomerisation |
| dppf | 12150-46-8 | Ferrocene, two diphenylphosphino groups | Achiral as the parent | General cross-coupling, wide bite angle |
| Xantphos | 161265-03-8 | Xanthene backbone, very wide bite angle | Achiral | Amination, carbonylation, copper photoredox |
| dppb | 1179-06-2 | para-Phenylene bridge, rigid | Achiral | Bridging two metals; MOF and polymer building |
| dppf-type chiral variants | see selection guide | Planar-chiral ferrocene | Planar + central | Asymmetric hydrogenation with a different chirality shape |
Xantphos and dppf are wide-bite-angle ligands chosen for reactivity; BINAP is a chiral ligand chosen for selectivity. They are not alternatives to one another even though they are grouped as bisphosphines.
Handling
BINAP is a triarylphosphine, so it is considerably less air-sensitive than the electron-rich alkylphosphines such as XPhos or tri-tert-butylphosphine. That is a real convenience – it does not require pyrophoric handling or a solution supply form. It is still stored under inert gas and protected from light, because slow oxidation to BINAP dioxide is the failure mode that quietly reduces selectivity.
- Store cold and under argon, away from strong light and from oxidising agents.
- Watch for the dioxide – 31P NMR shows it directly, and a slightly yellow tint on an old sample is the visible clue.
- Protect optical purity in storage. The (R) and (S) grades do not racemise at room temperature under normal conditions, but repeated heating in solution with a base, or contact with strongly acidic media, will erode ee.
- Do not assume the racemate is what you need. The cheapest grade is the wrong grade if the project needs selectivity.
Buying checklist
- Order by CAS, not by name. “BINAP” alone does not specify a product.
- Require optical purity (specific rotation and/or enantiomeric excess) on the certificate for (R) and (S).
- Accept that formula and molecular weight are identical across the three grades – do not use them to verify a delivery.
- Ask for the metal compatibility you need – BINAP is most often used with ruthenium or rhodium, and the counterion matters.
- Decide whether you need the ligand or the pre-formed catalyst. Pre-formed BINAP-Ru and BINAP-Rh catalysts are available from precious-metal sources; we can supply them alongside the ligand.
Product pages and further reading
Current specifications are on the (R)-BINAP (CAS 76189-55-4), (S)-BINAP (CAS 76189-56-5) and racemic BINAP (CAS 98327-87-8) pages. For where BINAP fits among the achiral bisphosphines, see the phosphine ligand selection guide, and for the Buchwald-type ligands in the same catalogue, our comparison of XPhos and SPhos.
FAQ
What is the difference between (R)-BINAP and (S)-BINAP?
They are mirror images. They have identical molecular formula, molecular weight and database identifiers, and they give opposite enantiomers in an asymmetric reaction. Only the CAS number and an optical purity measurement distinguish them.
Which one gives me the product I want?
It depends on the substrate, not on a general rule. The two enantiomers of the ligand give opposite absolute configurations of product, but which one gives the desired configuration has to be established for each substrate – usually by running both.
Is 98% purity enough for a chiral ligand?
98% chemical purity is a normal figure, but it is not the number that decides performance. Ask for enantiomeric excess as well. A ligand at 98% chemical purity and 90% ee will give a worse selectivity than the same material at 99% ee.
Why does PubChem give the same CID for all three grades?
Because that record describes which atoms are connected, not how they are arranged in space. Stereo is a property of the specific CAS number, not of the constitution.
Does racemic BINAP work in asymmetric catalysis?
It works as a catalyst but gives racemic product – which is exactly what makes it the right reagent for checking whether the BINAP skeleton suits a substrate before spending enantiopure material.
Is BINAP pyrophoric like tri-tert-butylphosphine?
No. It is a triarylphosphine and is far less air-sensitive than the electron-rich alkylphosphines. It still benefits from inert storage to prevent slow oxidation to the dioxide.
References and notes
- CAS numbers, molecular formula, molecular weight and PubChem CID 634876 verified against PubChem records; the single CID covering racemic, (R) and (S) BINAP is a property of constitution-only indexing and is used here as a specification warning.
- Optical rotation signs follow the produced names: the (R) grade is the (+)-rotating enantiomer, the (S) grade the (-)-rotating one.
- Applications listed are established uses of the BINAP-metal systems; no performance figures are quoted because they are substrate-specific.
- Purity figures are chemical purity from our current specification sheets.
Disclaimer. This page describes the product, its chirality and the specification fields that identify it. It is not a route recommendation, and it quotes no enantioselectivity or yield figures because those depend on substrate, metal, counterion and conditions. Confirm the grade and the optical purity against the certificate before use.
For a certificate showing optical purity, a sample of any of the three grades, or a quotation that pairs BINAP with a ruthenium or rhodium precatalyst, contact PolyBlueChem.