Organophosphorus

Overview
Organophosphorus compounds are chemical compounds containing carbon-phosphorus bonds. Organophosphorus chemistry is the corresponding science exploring the properties and reactivity of organophosphorus compounds. Phosphorus shares group 15 in the periodic table with nitrogen and phosphorus compounds and nitrogen compounds have much in common.

Phosphorus can adopt oxidation states &minus;3, &minus;1, 1, 3 and 5. In chemical literature very often compounds with +3 or &minus;3 oxidation state are grouped together as having a (III) oxidation state regardless of sign. In an official and more descriptive nomenclature phosphorus compounds are identified by their coordination number δ and their valency λ. In this system a phosphine is a δ3λ3 compound.

Phosphanes & phosphines
The parent compound of the phosphanes is PH3, called phosphine in the US and UK and phosphane elsewhere. Replacement of one or more protons by an organic residue, gives PH3-xRx, an organophosphine or organophosphane, again depending on the country. The phosphorus atom in phosphanes/phosphines has a formal oxidation state &minus;3 (δ3λ3) and are the phosphorus analogues of simple amines.

An often used organic phosphine is triphenylphosphine. Like amines, phosphines have a trigonal pyramidal molecular geometry although with larger angles. The C-P-C bond angle is 98.6° for trimethylphosphine increasing to 109.7° when the methyl groups are replaced by tert-butyl groups. Commonly the size of these ligands are described by the parameter called cone angle.

The barrier to inversion is also much higher than in amines for a process like nitrogen inversion to occur and therefore phosphines with three different substituents can display optical isomerism.

The basicity of phospines is less than that of corresponding amines, for instance phosphonium ion itself has a pKa of -14 compared to 9.21 for ammonium ion; trimethylphosphonium has a pKa of 8.65 compared to that of 9.76 of trimethylamine; and triphenylphosphonium (pKa 11.2) is less basic than triphenylammonium (pKa 19).

Amines and phosphines both have a lone pair of electrons but with a difference. Whereas the lone pair in an amine shares its electrons at every opportunity for delocalization for instance in pyridine, the phosphorus atom in a similar configuration will not. For this reason, the phosphorus equivalent of pyrrole called phosphole is not at all aromatic.

The reactivity of phosphines match that of amines with regard to nucleophilicity in the formation of Phosphonium salts with the general structure PR4+X&minus;. This property is used in the Appel reaction converting alcohols to alkyl halides. A difference in reactivity with amines is the ease of oxidation of phosphines to phosphine oxides.

Synthesis
Synthetic produces for phosphines are:


 * Nucleophilic displacement of phosphorous halides with organometallic reagents such as Grignard reagents.
 * Nucleophilic displacement of metal phosphides, generated by reaction of potassium metal with phosphine as in sodium amide with alkyl halides.
 * Nucleophilic addition of phosphine with alkenes in presence of a strong base (often KOH in DMSO), Markovnikov's rules apply. Phosphine can be prepared in situ from red phosphorus and potassium hydroxide. Primary (RPH2) and secondary phospines (RRPH) do not require a base with electron-deficient alkenes such as acrylonitriles.


 * Radical addition of phosphines to alkenes with AIBN or organic peroxides to give anti-Markovnikov adducts.
 * Nucleophilic addition of phosphine and phosphines to alkynes in presence of base. Secondary phosphines react with electron-deficient alkynes such as phenylcyanoacetylene without base.
 * Organic reduction of phosphine oxides for instance with chlorosilane.

Reactions
The main reaction types of phosphines are:
 * as nucleophiles for instance with alkyl halides to phosphonium salts. Phospines are key nucleophilic catalysts in the dimerization of enones in the Rauhut-Currier reaction.
 * as reducing agents:
 * Phosphines are reducing agents in the Staudinger reduction converting azides to amines and in the Mitsunobu reaction converting alcohols into esters. In these processes the phosphine is oxidized to the phosphine oxide. Phosphines have also been found to reduce activated carbonyl groups for instance the reduction of an α-keto ester to an α-hydroxy ester in scheme 2. In the proposed reaction mechanism the first proton is on loan from the methyl group in trimethylphosphine (triphenylphosphine does not react).


 * When modified with suitable substituents as in certain diazaphospholenes (scheme 3) the polarity of the P-H bond can actually be inverted (see: umpolung) and the resulting phosphine hydride can reduce a carbonyl group as in the example of benzophenone in yet another way.


 * Multidentate phosphines such as BINAP are important ligands in organometallic chemistry.

Phosphanes
Primary phosphanes are under-used in chemistry due to their general lack of stability towards oxygen. One study reports on several novel air-stable aromatic primary phosphanes prepared by organic reduction of the corresponding phosphonate:

The stability is attributed to conjugation between the aromatic ring and the phosphorus lone pair.

Phosphine oxides
Phosphine oxides (designation δ3λ3) have the general structure R3P=O with formal oxidation state &minus;1. Phosphines form hydrogen bonds and many phosphines are therefore soluble in water. The P=O bond is very polar with a dipole moment of 4.51 D for triphenylphosphine.

The nature of the phosphorus to oxygen double bond is a matter of debate. Pentavalent phosphorus like nitrogen is not compatible with the octet rule. In older literature the bond is represented as a dative bond just like an amine oxide. The prevailing view is that of a full double bond with back bonding taking place between a filled oxygen electron pair and an empty phosphorus d-orbital (lacking in nitrogen). problem is: the P=O bond does not react as any C=C double bond as addition reactions are absent and involvement of a phosphorus d-orbital in bonding is not supported in silico. Alternative theories favor an ionic bond P+&minus;O&minus; which on its own strength should explain the short bond length. Molecular Orbital Theory proposes that the short bond length is attributed to the donation of the lone pair electrons from oxygen to the antibonding phosphorus-carbon bonds. This proposal is supported by ab initio calculations and has gained consensus in the chemistry community.

Phosphines are easily oxidized to phosphine oxides as examplified by the directed synthesis of a phospha crown, the phosphorus analogue of an aza crown where it is not possible to isolate the phosphine itself.

Phosphonates
Phosphonates have the general structure R&minus;P(=O)(OR)2. In the Horner-Wadsworth-Emmons reaction and the Seyferth-Gilbert homologation phosphonates are used as stabilized carbanions in reactions with carbonyl compounds. Phosphonates have many technical applications and bisphosphonates are a class of drugs.

Phosphite and phosphate esters
Phosphite esters or phosphites have the general structure P(OR)3 with oxidation state +3. Phosphites are employed in the Perkow reaction and the Arbusov reaction. Phosphate esters with the general structure P(=O)(OR)3 and oxidation state +5 are of great technological importance as flame retardant agents and plasticizers. Lacking a P&minus;C bond, these compounds are technically not organophosphorus compounds.

Phosphoranes
Phosphoranes have a &minus;5 oxidation state (δ5λ5) with parent compound the non-stable phosphoran PH5 or λ5-phosphane (lambda 5 phosphane). Phosphorus ylides are unsaturated phosphoranes used in the Wittig reaction.

Phosphorus multiple bonds
Many compounds exist with carbon phosphorus multiple bonds (P=C) as phosphaalkenes (R2C::PR) and phosphaalkynes (RC:::PR). In the compound phosphorine one carbon atom in benzene is replaced by phosphorus. The reactivity of phosphaalkenes is often compared to that of alkenes and not to that of imines. The reason is that the HOMO of phosphaalkenes is not the phosphorus lone pair (as in imines the amine lone pair) but the double bond. Therefore like alkenes, phosphaalkenes engage in Wittig reactions, Peterson reactions, Cope rearrangements and Diels-Alder reactions.

The first phosphaalkene was synthesised in 1974 by Becker as a keto-enol tautomerism akin a Brook rearrangement:

with R = methyl or phenyl and tms representing trimethylsilyl.

In the same year Harold Kroto established spectroscopically that thermolysis of Me2PH yielded CH2=PMe.

A general method for the synthesis of phosphaalkenes is by 1,2-elimination of suitable precursors, initiated thermally or by base such as DBU, DABCO or triethylamine:

The Becker method is used in the synthesis of the phosphorus pendant of Poly(p-phenylene vinylene):

Diphosphenes are compounds containing P=P phosphorus double bonds. Phosphazenes have a P=N double bond.