Hydrophosphination is the insertion of a carbon-carbon multiple bond into a phosphorus-hydrogen bond forming a new phosphorus-carbon bond. Like other hydrofunctionalizations, the rate and regiochemistry of the insertion reaction is influenced by the catalyst. Catalysts take many forms, but most prevalent are bases and free-radical initiators. [1] Most hydrophosphinations involve reactions of phosphine (PH3). [2]
The usual application of hydrophosphination involves reactions of phosphine (PH3). Typically base-catalysis allows addition of Michael acceptors such as acrylonitrile to give tris(cyanoethyl)phosphine: [2]
Acid catalysis is applicable to hydrophosphination with alkenes that form stable carbocations. These alkenes include isobutylene and many analogues: [2]
Bases catalyze the addition of secondary phosphines to vinyldiphenylphosphine: [3]
Many hydrophosphination reactions are initiated by free-radicals. AIBN and peroxides are typical initiators, as well as Ultraviolet irradiation. In this way, the commercially important tributylphosphine and trioctylphosphine are prepared in good yields from 1-butene and 1-octene, respectively. [2]
The reactions proceed by abstraction of an H atom the phosphine precursor, producing the phosphino radical, a seven electron species. This radical then adds to the alkene, and subsequent H-atom transfer completes the cycle. [4] Some highly efficient hydrophosphinations appear not to proceed via radicals, but alternative explanations are lacking. [5]
Metal-catalyzed hydrophosphinations are not widely used, although they have been extensively researched. Studies mainly focus on secondary and primary organophosphines (R2PH and RPH2, respectively). These substrates bind to metals, and the resulting adducts insert alkenes and alkynes into the P-H bonds via diverse mechanisms. [6] [7] [8]
Metal complexes of d0 configurations are effective catalysts for hydrophosphinations of simple alkenes and alkynes. [9] [10] Intramolecular reactions are facile, e.g. starting with α,ω-pentenylphosphine. The primary phosphine undergoes a σ-bond metathesis with the bis(trimethylsilyl)methylene ligand forming the lanthanide-phosphido complex. Subsequently, the pendant terminal alkene or alkyne inserts into the Ln-P bond. Finally, protonolysis of the Ln-C bond with the starting primary phosphine releases the new phosphine and regenerates the catalyst. Given that the metal is electron-poor, the M-C bond is sufficiently enough to be protonolyzed by the substrate primary phosphine.
Most metal catalyzed hydrophosphinations proceed via metal phosphido intermediates. Some however proceed by metal-phosphinidene intermediates, i.e. species with M=PR double bonds. One such example is the Ti-catalyzed hydrophosphination of diphenylacetylene with phenylphosphine. [11] This system involves a cationic catalyst precursor that is stabilized by the bulky 2,4,6-tri(isopropyl)phenyl- substituent on the phosphinidene and the close ionic association of methyltris(pentafluorophenyl)borate. This precursor undergoes exchange with phenylphosphine to give the titanium-phenylphosphinidene complex, which is the catalyst. The Ti=PPh species undergoes a [2+2] cycloaddition with diphenylacetylene to make the corresponding metallacyclobutene. The substrate, phenylphosphine, protonolyzes the Ti-C bond and after a proton shift regenerates the catalyst and releases the new phosphine.
Titanium-catalyzed 1,4-hydrophosphination of 1,3-dienes with diphenylphosphine has been demonstrated. [12] It is a rare example of a d2 catalyst. In the first step, the Ti(II) precursor inserted in the P-H bond of diphenylphosphine (Ph2PH).
Late transition metal hydrophosphination catalysts, i.e. those reliant on the nickel-triad and neighboring elements, generally require alkenes and alkynes with electron withdrawing substituents. A strong base is required as a cocatalyst. [7]
Some late metal hydrophosphination catalysts proceed via oxidative addition of a P-H bond. For example, a Pt(0) catalyst undergoes oxidative addition of a secondary phosphine to form the corresponding hydrido Pt(II) phosphido complex. These systems catalyze hydrophosphination of acrylonitrile, although this reaction can be achieved without metal catalysts. The key P-C bond-forming step occurs through an outer-sphere, Michael-type addition. [7]
The usual mechanism for hydrophosphination for late metal catalysts involves insertion of the alkene into the metal-phosphorus bond. Insertion into the metal-hydrogen bond is also possible. The product phosphine is produced through reductive elimination of a P-C bond rather than a P-H bond in Glueck's system. [13] [14] The Ni(0) catalyst involves oxidation addition of a P-H bond to the metal, followed by insertion of the alkene into the M-H bond.
Utilizing phosphorus(V) precursors hydrophosphorylation entails the insertion of alkenes and alkynes into the P-H bonds of secondary phosphine oxides: [15]
The reaction can be effected both using metal catalysts or free-radical initiators.
Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces double and triple bonds in hydrocarbons.
In organic chemistry, hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes from alkenes. This chemical reaction entails the net addition of a formyl group and a hydrogen atom to a carbon-carbon double bond. This process has undergone continuous growth since its invention: production capacity reached 6.6×106 tons in 1995. It is important because aldehydes are easily converted into many secondary products. For example, the resultant aldehydes are hydrogenated to alcohols that are converted to detergents. Hydroformylation is also used in speciality chemicals, relevant to the organic synthesis of fragrances and pharmaceuticals. The development of hydroformylation is one of the premier achievements of 20th-century industrial chemistry.
Phosphine oxides are phosphorus compounds with the formula OPX3. When X = alkyl or aryl, these are organophosphine oxides. Triphenylphosphine oxide is an example. An inorganic phosphine oxide is phosphoryl chloride (POCl3).
A carbometallation is any reaction where a carbon-metal bond reacts with a carbon-carbon π-bond to produce a new carbon-carbon σ-bond and a carbon-metal σ-bond. The resulting carbon-metal bond can undergo further carbometallation reactions or it can be reacted with a variety of electrophiles including halogenating reagents, carbonyls, oxygen, and inorganic salts to produce different organometallic reagents. Carbometallations can be performed on alkynes and alkenes to form products with high geometric purity or enantioselectivity, respectively. Some metals prefer to give the anti-addition product with high selectivity and some yield the syn-addition product. The outcome of syn and anti- addition products is determined by the mechanism of the carbometallation.
Organophosphines are organophosphorus compounds with the formula PRnH3−n, where R is an organic substituent. These compounds can be classified according to the value of n: primary phosphines (n = 1), secondary phosphines (n = 2), tertiary phosphines (n = 3). All adopt pyramidal structures. Organophosphines are generally colorless, lipophilic liquids or solids. The parent of the organophosphines is phosphine (PH3).
In organic chemistry, hydroamination is the addition of an N−H bond of an amine across a carbon-carbon multiple bond of an alkene, alkyne, diene, or allene. In the ideal case, hydroamination is atom economical and green. Amines are common in fine-chemical, pharmaceutical, and agricultural industries. Hydroamination can be used intramolecularly to create heterocycles or intermolecularly with a separate amine and unsaturated compound. The development of catalysts for hydroamination remains an active area, especially for alkenes. Although practical hydroamination reactions can be effected for dienes and electrophilic alkenes, the term hydroamination often implies reactions metal-catalyzed processes.
In coordination chemistry, the bite angle is the angle on a central atom between two bonds to a bidentate ligand. This ligand–metal–ligand geometric parameter is used to classify chelating ligands, including those in organometallic complexes. It is most often discussed in terms of catalysis, as changes in bite angle can affect not just the activity and selectivity of a catalytic reaction but even allow alternative reaction pathways to become accessible.
In organometallic chemistry, a migratory insertion is a type of reaction wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products. However, often the two are used interchangeably because the mechanism is sometimes unknown. Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:
Asymmetric hydrogenation is a chemical reaction that adds two atoms of hydrogen to a target (substrate) molecule with three-dimensional spatial selectivity. Critically, this selectivity does not come from the target molecule itself, but from other reagents or catalysts present in the reaction. This allows spatial information to transfer from one molecule to the target, forming the product as a single enantiomer. The chiral information is most commonly contained in a catalyst and, in this case, the information in a single molecule of catalyst may be transferred to many substrate molecules, amplifying the amount of chiral information present. Similar processes occur in nature, where a chiral molecule like an enzyme can catalyse the introduction of a chiral centre to give a product as a single enantiomer, such as amino acids, that a cell needs to function. By imitating this process, chemists can generate many novel synthetic molecules that interact with biological systems in specific ways, leading to new pharmaceutical agents and agrochemicals. The importance of asymmetric hydrogenation in both academia and industry contributed to two of its pioneers — William Standish Knowles and Ryōji Noyori — being collectively awarded one half of the 2001 Nobel Prize in Chemistry.
Organocobalt chemistry is the chemistry of organometallic compounds containing a carbon to cobalt chemical bond. Organocobalt compounds are involved in several organic reactions and the important biomolecule vitamin B12 has a cobalt-carbon bond. Many organocobalt compounds exhibit useful catalytic properties, the preeminent example being dicobalt octacarbonyl.
Hydroacylation is a type of organic reaction in which an alkene is inserted into the a formyl C-H bond. The product is a ketone. The reaction requires a metal catalyst. It is almost invariably practiced as an intramolecular reaction using homogeneous catalysts, often based on rhodium phosphines.
Organogold chemistry is the study of compounds containing gold–carbon bonds. They are studied in academic research, but have not received widespread use otherwise. The dominant oxidation states for organogold compounds are I with coordination number 2 and a linear molecular geometry and III with CN = 4 and a square planar molecular geometry.
Organoplatinum chemistry is the chemistry of organometallic compounds containing a carbon to platinum chemical bond, and the study of platinum as a catalyst in organic reactions. Organoplatinum compounds exist in oxidation state 0 to IV, with oxidation state II most abundant. The general order in bond strength is Pt-C (sp) > Pt-O > Pt-N > Pt-C (sp3). Organoplatinum and organopalladium chemistry are similar, but organoplatinum compounds are more stable and therefore less useful as catalysts.
A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).
In organometallic chemistry, the activation of cyclopropanes by transition metals is a research theme with implications for organic synthesis and homogeneous catalysis. Being highly strained, cyclopropanes are prone to oxidative addition to transition metal complexes. The resulting metallacycles are susceptible to a variety of reactions. These reactions are rare examples of C-C bond activation. The rarity of C-C activation processes has been attributed to Steric effects that protect C-C bonds. Furthermore, the directionality of C-C bonds as compared to C-H bonds makes orbital interaction with transition metals less favorable. Thermodynamically, C-C bond activation is more favored than C-H bond activation as the strength of a typical C-C bond is around 90 kcal per mole while the strength of a typical unactivated C-H bond is around 104 kcal per mole.
In organic chemistry, hydrovinylation is the formal insertion of an alkene into the C-H bond of ethylene. The more general reaction, hydroalkenylation, is the formal insertion of an alkene into the C-H bond of any terminal alkene. The reaction is catalyzed by metal complexes. A representative reaction is the conversion of styrene and ethylene to 3-phenybutene:
A phosphetane is a 4-membered organophosphorus heterocycle. The parent phosphetane molecule, which has the formula C3H7P, is one atom larger than phosphiranes, one smaller than phospholes, and is the heavy-atom analogue of azetidines. The first known phosphetane synthesis was reported in 1957 by Kosolapoff and Struck, but the method was both inefficient and hard to reproduce, with yields rarely exceeding 1%. A far more efficient method was reported in 1962 by McBride, whose method allowed for the first studies into the physical and chemical properties of phosphetanes. Phosphetanes are a well understood class of molecules that have found broad applications as chemical building blocks, reagents for organic/inorganic synthesis, and ligands in coordination chemistry.
A lanthanocene is a type of metallocene compound that contains an element from the lanthanide series. The most common lanthanocene complexes contain two cyclopentadienyl anions and an X type ligand, usually hydride or alkyl ligand.
A transition metal phosphido complex is a coordination complex containing a phosphido ligand (R2P, where R = H, organic substituent). With two lone pairs on phosphorus, the phosphido anion (R2P−) is comparable to an amido anion (R2N−), except that the M-P distances are longer and the phosphorus atom is more sterically accessible. For these reasons, phosphido is often a bridging ligand. The -PH2 ion or ligand is also called phosphanide or phosphido ligand.
In organic chemistry, carboboration describes an addition of both a carbon and a boron moiety to certain carbon-containing double and triple bonds, such as alkenes, alkynes, and allenes.