Phosphonous and Phosphinous Halides

The lower alkyl halophosphines (6.138) are colourless liquids, heavier than water, which can be distilled without decomposition. They have a sharp disagreeable smell and can cause headaches and nausea. They are soluble in organic solvents and are very sensitive to moisture and air. Dimethylfluorophosphine, Me2PF, is particularly easily oxidised and is spontaneously inflammable in air. The halophosphines are generally very reactive, but their reactivity decreases with increasing molecular weight. 

Phosphines react vigorously with halogens to give halophosphines (6.139, 6.140). Tertiary phosphines give rather unstable phosphoranes which themselves can be decomposed by heat to give trivalent halophosphines (6.132). 

RPHz -P 2CI2 RPCI2 + 2HC1 R2PH + CI2 -P R2PCI + HCl R3P + CI2 R3PCI2 R2PCI + RCl 

A general method of preparation of alkyl phosphonous dihalides is the reaction of phosphorus trihalide with a dialkyl mercury, a dialkyl cadmium or a trialkyl aluminium. 

Good yields of dialkyl phosphinous chlorides are obtained by the reaction of dry hydrogen chloride with a phosphinous amide (6.143), while alkylphosphonous dichlorides can be obtained from phosphonous diamides (6.144). 

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Despite statements made in Chapter 2 about the unpopularity of this procedure for the preparation of phosphonic and phosphinic esters from those of phosphonous and phos-phinous acids by the addition of oxygen, partly because of the lack of availability of the phosphorus(III) esters, but also because of the high reactivity exhibited by those esters towards oxidizing agents, the reduced reactivity shown towards the higher chalcogens by phosphonous and phosphinous halides (dihalo- and monohalo-phosphines), and even by some of the more reactive phosphorus(III) species, e.g. amides, makes such addition reactions feasible propositions. 

Studies on the alkaline hydrolysis of various phosphonic and phosphinic esters have provided information on the electronic or steric effects of substituents and the effects of changes in reaction conditions amongst the substrates so extensively examined are the O-aryl esters of dimethylphosphinothioic acid (552) esters of diphenylphosphinic acid and O-aryfand S -aryf esters of diphenylphosphinothioic acid (553 Z, Y = O or S). Other studies have concentrated on aryl esters of diaryIphosphinic acids (554) , and the effects of the stepwise replacement of P-Me by P-Ph in esters of dimethyl-, methylphenyl- and diphenyl-phosphinic acids The esters 555 (R = EtO, R = Me or Ph, R = R = Ph X = SEt or hydrolyse under alkaline conditions faster than do the comparable 5 -(4-substituted-butyl) esters. Comparable steric and electronic influences on the hydrolyses of phosphonic and phosphinic fluorides phosphinic chlo-rides the phosphonothioic chlorides 556 and other phosphonic and phosphinic esters have been noted. Phosphonic and phosphinic halides are prone to undergo halo-gen-exchange reactions, a process which, in general, is faster for derivatives of phosphonic than for those of phosphonothioic and phosphonoselenoic acids, and to be particularly important for acid fluorides . ... 

Phosphonic and Phosphinic Halides and related compounds - A mixture of 2.5 eq. 

Phosphines may be obtained by reaction with Grignard reagents (6.45), phosphites from reactions with alcohols (6.296,6.310) and phosphonic and phosphinic halides from reactions with metal alkyls (6.142,6.153) or other organic compounds (6.145-6.148,6.151,6.153). 

Fluorophosphoranes can be obtained from phosphonous and phosphinous halides by reactions with arsenic or antimony obtained with KHF2 (6.518). Some phosphinous halides react with sulphur compounds to give phosphinothioites (9.420), with phenyl azide to give monophosphazenes (7.447), and with ylids fluorides (6.504, 6.505). Hydrofluorophosphoranes produces phosphonium salts (6.377). Phosphonous and phosphinous halides can be condensed to form polyphosphines or cyclic derivatives (6.660, 6.666,6.680,6.684), or reacted to give P-P linkages (6.737). 

Moderately stable phosphonous and phosphinous cyanides, RP (CN)2 and R2PCN are obtainable by reacting silver cyanide with phosphonous and phosphinous halides, respectively (Figures 6.4 and 6.5). 

Examples of the intermolecular C-P bond formation by means of radical phosphonation and phosphination have been achieved by reaction of aryl halides with trialkyl phosphites and chlorodiphenylphosphine, respectively, in the presence of (TMSlsSiH under standard radical conditions. The phosphonation reaction (Reaction 71) worked well either under UV irradiation at room temperature or in refluxing toluene. The radical phosphina-tion (Reaction 72) required pyridine in boiling benzene for 20 h. Phosphinated products were handled as phosphine sulfides. Scheme 15 shows the reaction mechanism for the phosphination procedure that involves in situ formation of tetraphenylbiphosphine. This approach has also been extended to the phosphination of alkyl halides and sequential radical cyclization/phosphination reaction. ... 

The pyrolysis of organoarsenic compounds containing the arsenyl moiety has some limited preparative applications [arsenyl (As=0) by analogy with phosphoryl (P=0)]. The compounds are based on the arsonic acid RAs(0)(0H)2, the arsinic acid R2As(0)0H and the arsine oxide R3As=0 structures. The acids are in interesting contrast with the phosphorus series. The phosphonic and phosphinic esters are prepared from the phosphorus(III) precursors via the Arbuzov synthesis. This synthetic route fails with the arsenic analogues, and further, if an alkyl halide or a salt is added in the pyrolysis of arsonic or arsinic acid esters a retro-Arbuzov reaction takes place . ... 

Methyleneaminophosphines (10), on treatment with methyl iodide, give the expected phosphonium salts (11), which on thermolysis undergo a reaction analogous to the Arbusov reaction to give imidophosphoranes. Various phosphonates and phosphine oxides have been prepared by the reaction of the anions (RO)aP—0 Na+ and R2P—0 Na+ with alkyl halides. The analogous reactions of the chlorohydrin (12) with sodium diethyl phosphonate give the cyclic phosphonate (13) as a mixture of geometrical isomers of approximately equal stability. ... 

The Michaelis-Arbuzov-Kaehne reaction is probably the most widely used reaction in organophosphorus chemistry for the preparation not only of phosphonic and phosphinic acids (as their esters), but also of tertiary phosphine oxides. The reaction, discovered by Michaelis and Kaehne in 1898, and extensively developed by A. Arbuzov in the early years of this century, consists essentially in the interaction of phosphorus(III) acid ester with a reactive carbon-based species, generally an alkyl halide, and can be represented in very general terms as in equation 1 the reaction has been extensively reviewed... 

A rapid survey of the contents of the previous four chapters, which dealt primarily with the synthesis of various types of phosphonic and phosphinic acids, is all that is necessary to realize that both classes of acids are synthesized by the direct formation of a limited selection of types of derivatives. Most often these are either esters as, for example, in the Michaelis-Arbuzov reaction, or acid halides, almost invariably the chloride as in the phosphorylation of alkenes with PCI5. In any multi-step synthesis, the interconversions of acids, acid halides and esters are consequently amongst the most important of translocations of ligands attached to phosphorus, and their success may even become of critical importance. 

Further kinetic information on hydrolysis of sulphonyl chlorides (R SOgCl) is contained in a paper which correlates kinetic data for hydrolysis of these and other halides, for example of phosphonic and phosphinic acids, with i.r. and n.q.r, spectroscopic data. Hydrolysis of the adduct McaEtN.SOa has been studied over a wide pH range. The rate-law determined in basic solution indicates 5 n2 attack by hydroxide at the sulphur atom, as earlier proposed for the parent adduct EtaN.SOg,... 

Ar. phosphonic and phosphinic acid esters from halides... 

Phosphonic (phosphonyl) halides and phosphinic (phosphinyl) halides (6.177) may be obtained from the corresponding phosphonic and phosphinic acids by the action of PCI5 or SOCI2. 

The lower molecular weight phosphonyl and phosphinyl halides are colourless liquids which fume in air. They hydrolyse easily to phosphonic and phosphinic acids, and react with alcohols to give the corresponding esters (6.199, 6.200). Their reactivity decreases as the size of the group R is increased. 

Phosphonous and phosphinous amides (amino organophosphines) (7.165), can be obtained from organophosphine halides by treatment with a primary or secondary amine in ether or benzene at or below room temperature. 

Thiohalides of the type RP(S)X2 and R2P(S)X can be made by heating sulphur with the corresponding phosphonous or phosphinous halides. Phosphonothioic (thiophosphonic) halides can be obtained by the action of hydrogen sulphide on tetrachlorophosphoranes (9.425), or phosphonous halide-aluminium trichloride complex (Chapter 6), or P4SJ0 on the corresponding phosphonic dihalide (9.426), or by reaction (9.427) in which PSCI3 acts as a sulphur donor. Monophenyl phosphine and thionyl chloride produce phenyl phosphonothionic dichloride, which can also be obtained by thermal isomerisation (9.428). 

In addition, aryl triflates have proven to be viable substrates for the Pd-catalyzed Csp2—P bond formation reactions [87-90], Intriguingly, phosphorylation can be achieved from the Pd-catalyzed coupling of alkenyl triflate with not only dialkylphosphites, but also with hypophosphorous acid [88]. Thus, phosphinic acid 87 was obtained when triflate 86 was treated with hypophosphorus acid in the presence of Pd(Ph3P)4. Due to the abundance of alkenyl triflates and milder reaction conditions, alkenyl triflates have certain advantages over the corresponding alkenyl halides as substrates for Pd-catalyzed phosphorylations to make alkenyl phosphonates or phosphinates. 

Type I (fast homodimerization) Terminal olefins, allylsilanes" Terminal olefins, allylsilanes," 1° allylic alcohols, ethers, and esters, " allyl boronate esters, allyl halides, alkyl-substituted allenes Terminal olefms, allyl boronate esters, 1° allylic alcohols, ethers, and esters,styrenes (no large ortho substit.), " " allyl allylsilanes, allyl sulfides, allyl phosphonates, " allyl phosphine oxides, protected allylamines ... 

In contrast to stabilizers, fire retardants must be added in much higher concentrations, which affect thermal and mechanical properties as well as cost. Sherr and co-workers report that novel derivatives of phosphine oxides, phosphonic acids, phosphinic acid, and phosphonium halides may be used generally in concentrations as low as 2.5-5 p.p.h. to be effective fire retardants in polyethylene and poly (methyl methacrylate). 

Mono- and diphosphonium halides have been found to be flame retardants for plastic materials. Their effectiveness can be related to the formation of various active phosphorus compounds, as well as to many of the postulated mechanisms for flame retardant action. The compounds are postulated to be effective because they decompose on ignition to thermally stable phosphine oxides or phosphonic acids which, in turn, are decomposed to continuous films of phosphate glass. In addition, the phosphonium halides form alkyl halides which cool the flame and/or form halogen acids which are fame retardants. 

Tn the preceding chapter (19) we described the use of phosphine A oxides, phosphonic acids, and phosphinic acids as flame retardants for thermoplastic materials. We also have found phosphonium halides to be effective flame retardants for plastics (5, 6). These compounds were either the monophosphonium halides,... 

The Michaelis-Arbuzov reaction is the most used and well-known method for the synthesis of phosphonates and their derivatives and may also be used to synthesize phosphinates and tertiary phosphine oxides. The simplest form of the Michaelis-Arbuzov reaction is the reaction of a trialkyl phosphite, 3, with an alkyl halide, 4, to yield a dialkyl alkylphosphonate, 6, and new alkyl halide, 7 (Scheme 2). During this transformation the phosphorus atom of a ter-valent phosphorus(III) species (3) acts as a nucleophile resulting in the formation of an intermediate alkoxy phosphonium salt 5, containing a new [P—C] bond. The precise structure of the intermediates 5 is a subject of debate—as reflected by common reference to them as pseudophosphonium salts —with a penta-coordinate species (containing a [P—X] bond) being proposed and detected in some cases.18 Decomposition (usually rapid under the reaction conditions) of the intermediate 5 by nucleophilic attack of X- on one of the alkyl groups R1, with concomitant formation of a [1 =0] bond yields the product pentavalent phosphorus(V) compound (6) and the new alkyl halide, 7. 

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