Synthesis of
ORGANOPHOSPHORUS COMPOUNDS For the chemist who would engage in synthesis or study theory, the organic chemistry of phosphorus is a fertile area
DR. MARTIN GRAYSON, American Cyanamid Co., Stamford, Conn. Organophosphorus chemistry is a field which has experienced a sharp upsurge of interest during the past decade. Contributions to our knowledge of organophosphorus compounds have come from many sources, and both academic and industrial laboratories have been prominent in making those contributions. Much activity has been concerned with new methods for preparing the compounds—in some cases for the sake of their unique physical and chemical properties, and in others for theoretical studies related to a variety of new reactions. Since the field is so broad, we must necessarily limit our discussion. Therefore, my intention is to offer a brief outline of methods for forming the carbon-phosphorus bond, with emphasis on those reactions where the phosphorus atom functions as a nucleophilic reagent. It is probably no coincidence that a renaissance in this field began some 12 years ago, at about the same time that the encyclopedic compendium, "Organophosphorus Compounds," by Prof. Gennedy M. Kosolapoff, appeared. Certainly, no discussion of synthetic organophosphorus chemistry should proceed before at least mentioning this and acknowledging our indebtedness to the book for its role in sparking current interest. Much of this interest is due to some very special proper90
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ties of phosphorus which contrast sharply with those of related organonitrogen compounds. Phosphorus forms stronger bonds than nitrogen does with oxygen, carbon, and halogens, but forms a weaker bond with hydrogen. The phosphorus-carbon bond is as strong as the carbon-carbon bond. The phosphorus-oxygen single bond is stronger than the carbon-oxygen single bond by some 10 kcal., and the phosphorus-oxygen "double bond" is some 55 kcal. stronger still, with a dissociation energy of about 135 to 140 kcal. This affinity for oxygen provides much of the driving force for many of the reactions of phosphorus compounds, such as the Wittig olefin synthesis in which a new P = 0 group is formed: R 3 P±=CHR'+ R"CHO—>R3P = 0 + R'CH=CHR" The bond dissociation energies in phosphine, ammonia, and methane increase in the order given (76, 84, and 91 kcal.), and this in part accounts for the ability of phosphines to participate in free radical and ionic additions to multiple bond systems. Furthermore, phosphorus is a larger atom than nitrogen, with more readily lost or polarized nonbonded electrons. This, coupled with nearly equal basicity in analogous compounds, produces greater nucleophilicity— greater reactivity toward centers of electron deficiency.
C&EN A point of major difference, compared to the first row elements, which phosphorus has in common with a number of other second-row metalloids, is its ability to accommo date up to 10, and sometimes 12, rather than only eight, valence electrons. This outer shell expansion results from vacant 3d orbitals of low energy, in addition to the s and ρ orbitals to which nitrogen and other first-row elements are limited. This availability of vacant bonding orbitals, coupled with the nucleophilicity of phosphines, permits the formation of strong phosphorus-metal bonds. Trivalent or triply connected phosphorus compounds function as power ful ligands in many inorganic complexes. When we write multiple bonds between phosphorus and metals, carbon, oxygen, or nitrogen, we are actually describing a valence shell expansion with interaction between the vacant d or bitals of phosphorus and the p-electrons of the other atom (άπ-ρ-π bonding). This type of interaction is sometimes called back bonding. Along these lines, phosphorus also forms stable, fivecovalent compounds such as pentaphenylphosphorane— (C 6 H 5 ) 5 P—in which d, s, and ρ orbitals have rehybridized to give the dsps system, which can be geometrically a bipyramid or a square pyramid. Recent x-ray work indicates the square pyramid structure to be correct for ( C 6 H 5 ) 5 P , although the geometry for most reactive phosphorane in termediates and transition states remains in doubt. Studies of the stereochemistry and reaction mechanisms involving pentacovalent intermediates are therefore of great theo retical interest. This is the area in which much of the most exciting fundamental work in phosphorus chemistry is being carried out today. Many of these characteristics of phosphorus confer unusual and desirable properties on its compounds. Ther mal stability and fire resistance are often associated with organophosphorus compounds. They can be used in gaso line to prevent preignition. In mining, they can be used for flotation and extraction of metals, including uranium. A host of other applications could be mentioned, including adhesives, lubricants, chelating agents, textile finishes, plasticizers, and so forth. But all in all, the industrial ap plication of organophosphorus chemistry still seems to be in its earliest stages, with much yet to come. It is both convenient and instructive to group methods of forming phosphorus-carbon bonds into those in which the phosphorus atom functions as a nucleophilic, or electrondonating, reagent, those in which it acts as an electrophilic, or electron-accepting, reagent, and those wherein the reac tions involve free radicals. Unfortunately, space does not permit a complete discussion of the electrophilic and free radical reactions, and in these areas I shall merely indicate the newer reactions of major, preparative value. Nucleophilic
chemical science series
as a carbonium ion, from an alkyl halide, olefin, or carbonyl compound. This is then attacked by the nonbonded pair of electrons of the nucleophilic phosphorus atom. Base-Catalyzed Nucleophilic Reactions. Compounds con taining a phosphorus-hydrogen bond are common starting materials for nucleophilic reactions. *Much of our own early work at the Cyanamid Laboratories made use of phosphine (PH 3 ) generated from metal phosphides and water. The use of aluminum or magnesium-aluminum phosphides, rather than calcium phosphide, eliminates by-product generation of biphosphine (H 2 P-PH 2 ), which apparently has been largely responsible for the reported hazards of this gas. Of course, even pure phosphine is toxic and flam mable, but handling in conventional equipment presents few difficulties. The discovery of the base-catalyzed nucleophilic addi tion of phosphine to acrylonitrile by I. Hechenbleikner and M. M. Rauhut at Cyanamid made the interesting series of mono-, bis-, and tris-2-cyanoethylphosphines available:
In this reaction, product distribution can be altered by appropriate changes in reactant ratios, to give high yields of any one of the tNree phosphines. These materials served as useful, easily prepared models for much of our early exploratory work. Uncatalyzed addition of acrylonitrile to phenylphosphine, as previously reported in the literature, required tempera tures of at least 130° C. Repetition of this reaction with the ION KOH catalyst at 2C° C. gave 82% bis(2-cyanoethyl) phenylphosphine in a t t w hours. This reaction is probably a Michael addition process, involving the nucleo philic phosphide ion:
The absence of acrylonitrile polymers or hydrolyzed prod ucts clearly demonstrates the greater nucleophilicity of the phosphide intermediates, compared to hydroxide ion. Similar base-catalyzed addition reactions of dialkyl phosphonates, probably involving the ion [ ( R O ) 2 P = 0 ] - , were reported by a number of workers in the early 1950's. The enhanced acidity of the phosphonates was probably a major factor in the early recognition of their nucleophilic properties. This highly nucleophilic ion is also the basis for the well-known and useful Michaelis-Becker-Nylen re action with alkyl halides to give dialkyl alkylphosphonates:
Reactions
This class includes most of the reactions having widest application and the greatest utility for forming a phos phorus-carbon bond. Many of these reactions are cata lyzed by bases, or they require formation of a negatively charged phosphide ion or metal salt of a phosphine, phosphine oxide, or dialkyl phosphonate. On the other hand, some of the most useful reactions involve the acidcatalyzed formation of an electron-deficient center, such
Secondary phosphine oxides also contain a relatively acidic phosphorus-hydrogen bond, and they readily form the nucleophilic anion with bases. At Cyanamid, for ex ample, the author and Patricia T. Keough found that ethylene carbonate is attacked by secondary phosphine oxides in the presence of bases such as heptamethylbiguanide ( H M B G ) , producing reactive 2-hydroxyethyl phos phine oxide intermediates. These are comparable to the DEC.
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BASE-CATALYZED NUCLEOPHILIC REACTIONS Secondary phosphine oxides, having a relatively acidic phosphorus-hydrogen bond, readily form a nucleophilic anion with bases. They attack ethylene carbonate in the presence of heptamethylbiguanide,
for example, to give 2-hydroxyethyl phosphine oxide intermediates. These react, through dehydration and Michael addition of another secondary phosphine oxide, to give ethylenebis-disubstitutedphosphine oxides:
Interestingly, ethylene carbonate can be used to oxidize secondary phosphines as a starting material in
the above reactions. Secondary phosphine oxide, then, is rapidly formed concurrently with the olefin:
Ethylene carbonate and other cyclic carbonates will also oxidize tertiary phosphines by way of a four-
centered mechanism, as in the Wittig olefin synthesis from aldehydes and methylene phosphoranes:
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2-hydroxyethyl phosphines obtained by others from metal phosphides and ethylene oxide. These materials can be isolated in some cases, but they generally react further to give ethylenebis-disubstituted phosphine oxides. This takes place through a dehydration to vinylphosphine oxide, followed by Michael addition of a second mole of starting material. The dehydration and Michael addition processes are quite rapid, and this again emphasizes the phosphinyl anion's great nucleophilicity in adding to activated olefins, as well as in displacement on saturated carbon. The ethylene carbonate reaction is an interesting one, since this reagent can also serve as an oxidizing agent and permits the use of secondary phosphines as starting materials in the reaction. The phosphine is rapidly oxidized to the secondary phosphine oxide, with concurrent olefin formation. The mechanism of this oxidation involves a phosphorus-carbon bond cleavage in an intermediate in a way that is quite analogous to the cleavage involved in the Wittig olefin synthesis. With a tertiary phosphine, the oxidation reaction can be isolated. The oxidation reaction may have synthesis applications in converting 1,2-glycols to olefins via the cyclic carbonate. Nucleophilic addition of phosphinyl anions, derived from secondary phosphine oxides, to active olefins, such as acrylonitrile, can also be carried out. Such reactions have been reported by R. C. Miller, J. S. Bradley, and L. A. Hamilton of Socony-Mobil and Temple University, and also by M. M. Rauhut and H. A. Currier at Cyanamid. In this connection, I should point out that, contrary to earlier reports, secondary phosphine oxides can be prepared in high yield by air oxidation of the phosphines in neutral organic solvents; phosphinic acids are formed only when strong oxidizing agents or alkaline conditions are used. Therefore, secondary phosphine oxides are readily available as starting materials, obtainable from olefins and phosphine by nucleophilic or free radical addition reactions. Furthermore, work by S. A. Buckler and M. Epstein at Cyanamid has led to the discovery of hitherto unknown primary phosphine oxides, R P ( 0 ) H 2 , and these materials also act as nucleophilic reagents in base-catalyzed addition reactions. With the added driving force of ring formation, base catalysis becomes unnecessary. This was shown by R. P. Welcher and Nancy E. Day, of Cyanamid, in their diaddition of primary phosphines to conjugated dienones. The product 4-phosphorinones are six-membered rings, containing carbonyl and tertiary phosphine functions. In terms of commercial potential, nucleophilic addition of phosphine to active olefins, such as acrylonitrile, offers perhaps the cheapest route to trivalent organophosphorus compounds containing functional groups. A considerable body of knowledge concerning the reactions of these materials has been developed, but discussion of that chemistry is beyond the scope of this article. Acid-Catalyzed Nucleophilic Reactions. In general, nucleophilic attack by phosphorus compounds on unsaturated centers, such as olefins and carbonyl groups, is promoted by acid-catalyzed formation of an electron-deficient center, such as a carbonium ion. Generation of a carbonium ion is essential in reactions involving the weakly nucleophilic phosphorus trihalides. For example, alkyl halides are converted to carbonium ions by aluminum chloride in a FriedelCrafts type of reaction with phosphorus trichloride, giving complex salts containing a new phosphorus-carbon bond:
Organic phosphorus compounds have valuable flameretardant properties. Helen C. Gillham, American Cyanamid, compares organophosphorus-treated plastic strip with a burning untreated strip (right)
These salts can be converted to alkyl phosphonic dichlorides [RP(0)C1 2 ] or the corresponding acids or esters, depending on work-up procedures. Reducing agents such as metals or other phosphorus compounds, including elemental phosphorus, can also be used to convert the complexes to alkyl phosphonous dichlorides (RPG1 2 ). In 1951, G. O. Doak and L. D. Freedman, at the University of North Carolina, reported an equally general and useful method for preparing aromatic phosphonic or phosphinic acids. Although not acid-catalyzed, this method is formally a nucleophilic substitution by phosphorus trichloride or RPC1 2 on an aryl cation, formed by coppercatalyzed loss of nitrogen from a diazonium salt. The reaction may well involve radical intermediates, however, as in the related conversion of tertiary phosphines to quaternary phosphonium compounds by diazonium salts. In 1952, H. C. Brown of Purdue, in a patent to Standard Oil (Ind.), disclosed a clear-cut case of acid-catalyzed nucleophilic addition of phosphine to olefins. This was amplified by M. C. Hoff and P. Hill of the same company in 1959. Presumably, the olefin is protonated on the least substituted carbon to give a secondary or tertiary carbonium ion, and carbon-phosphorus bond formation follows. Primary phosphines are the chief products, as the result of salt formation with the acid. Furthermore, at the University of Mainz, L. Horner and H. Hoffmann have shown that stable quaternary phosphonium compounds can be formed by protonation of the zwitterion produced by adding tertiary phosphines to multiple bond systems. In the absence of protonation, anionic polymerization can be initiated with active olefins; and with highly electronegative groups on the olefin, such as fluorine, acid catalysis may not be required. D. C. England and G. W. Parshall of Du Pont, for example, disclosed the noncatalyzed addition of phosphine to tetrafluoroethylene in 1959. In addition, compounds that ionize to stable carbonium ions do not require catalysis either. Triphenylmethylcarbinol DEC. 3, 196 2 C&EN
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falls into this class, and with phosphorus trichloride it forms triphenylmethylphosphonic dichloride. One of the most interesting acid-catalyzed nucleophilic reactions involves adding phosphines to carbonyl com pounds. Early work with phosphine or phosphonium io dide and aldehydes in organic solvents was carried out in the past century. In the 1920's, acid was used to prepare tetr akis ( hydroxy methyl ) phosphonium chloride ( THPC ) from formaldehyde and phosphine. In the general reac tion, R can be either an alkyl group or hydrogen:
Recently, the utility of metal salt catalysts in the absence of acid, as well as the uncatalyzed reaction of aqueous formalin and phosphine under slight pressure, was reported for this reaction. At Cyanamid, this area has been reopened by S. A. Buckler and M. Epstein, with interesting results. They found, for example, that ketones and aromatic aldehydes differ from the simple aliphatic compounds. Apparently, transfer of oxygen from carbon to phosphorus, in addition to phosphorus-carbon bond formation, takes place in strong mineral acid solutions. The primary phosphine oxides formed with ketones can predominate, if large groups block the carbonyl group. Cyclotetraphosphines (RP) 4 , are also formed in small amounts in a few cases. A carbonium ion intermediate following normal carbonyl addition has been proposed as the mechanism, since strong acid and the presence of groups that can stabilize the positive charge are required. The reaction is completed by water addition and suitable proton shifts. A hydride shift from phos phorus to carbon was excluded as a possibility, since S. Trippett of Leeds University has shown that 1-hydroxyalkyl tertiary phosphines give a related reaction in strong acids. In addition to demonstrating that the reaction of unbranched aldehydes to give tetr akis 1-hydroxy alky lphosphonium salts is quite general, Buckler and Epstein, along with V. P. Wystrach, reported formation of a number of interesting cyclic, spirocyclic, and polycyclic products when branched aldehydes, dialdehydes, β-diketones, and pyruvic acid were used. For example, isobutyraldehyde reacted with phosphine in aqueous hydrochloric acid to give 2,4,6triisopropyl-l,3-dioxa-5-phosphacyclohexane; glutaraldehyde gave 1,5,7,1 l-tetrahydroxy-6-phosphoniaspiro [5,5] undecane chloride; 2,5-pentanedione gave the polycyclic acetal 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phosphaadamantane; and pyruvic acid reacted to give 2,2',2"-phosphinidyne trilactic acid trilactone. Reactions of this type are not limited to phosphine; primary, secondary, and tertiary phosphines can be reacted with simple aldehydes to give quaternary phosphonium compounds, and reactions lead ing to more complex products, such as those described above, are possible with some primary and secondary phosphines. Oxidized phosphorus compounds containing a phos phorus-hydrogen bond can also be used. For example, hypophosphorous acid and pyruvic acid react to give bis ( 1carboxy-1 -hydroxyethyl ) phosphinic acid ( CHEPA ),
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which is also obtained by the nitric acid oxidation of the pyruvic acid—phosphine adduct. CHEPA is a potent séquestrant for ferric ions at high basicity, a property which very few commercial chelating agents have. In addition to these methods, aldehydes—especially formaldehyde—and ketones react thermally with phosphoruschlorine and phosphorus-hydrogen compounds to give products containing phosphorus-carbon bonds. Chloromethylphosphonic dichloride is prepared commercially this way. It is clear that carbonyl addition reactions of phosphine offer broad, general techniques for synthesizing a host of new phosphorus-carbon compounds. Compounds having both oil and water solubility can be made. The reactivity of the phosphorus-carbon-hydroxyl group in esterification, ether formation, dehydration, and so on, has been largely unexplored. This appears to be a potentially fruitful area for development activity and theoretical study. Simple aldehydes and ketones are low-cost commercial materials, and their acid-catalyzed nucleophilic reactions with phosphorus-hydrogen compounds may become major building blocks for a new family of industrial chemicals.
Nucleophilic Quaternization Reactions. Some of the best known methods for forming carbon-phosphorus bonds involve alkylation of trivalent phosphorus compounds by a bimolecular, nucleophilic substitution (SN2) reaction to give quaternary phosphonium compounds. Tertiary phosphines and alkyl halides provide perhaps the simplest example of this, and recent studies of the reaction provide new insight into the effect of structure on the nucleophilicity of phosphorus. Aside from the alkylation of phosphines, which goes back over 100 years to A. W. Hofmann's pioneering work, the most important example of the quaternization process for phosphorus-carbon bond formation is the classical Michaelis-Arbusov reaction. This is a two-step process. Quaternization of a tertiary phosphite is followed by dealkylation by halide ion, to give a dialkyl alkylphosphonate ester. The quaternary intermediates can be isolated with triarylphosphites, and the dealkylation step is greatly aided by simultaneous formation of the stable phosphoryl group. This type of driving force is the key to many important reactions of phosphonium compounds. For example, at Cyanamid we found that tertiary phosphine oxides, ethers, and hydrocarbons can be obtained by reacting benzylphosphonium salts with alkoxides. The slow step in this reaction again is a dealkylation, this time by alkoxide ion, to give P = 0 and ROR. In much the same way, the rate of the interesting hydroxide cleavage of phosphonium salts is governed by the phosphine oxide formation step. This reaction has been shown by W. E. McEwen of the University of Massachusetts, by H. Hoffmann at Mainz, and by recent work at Cyanamid with THPC, to be effectively second order in hydroxide ion. The basic mechanism involving pentacovalent phosphorus was proposed in 1929 by G. W. Fenton and C. K. Ingold of University College, London. Arylation of phosphines is, of course, much more difficult to achieve than alkylation. However, a number of useful free radical procedures, involving diazonium salts and Grignard reagents, have been developed in recent years. The use of aluminum or nickel salts to prepare tetraarylphosphonium salts from aryl halides has also been reported. But the simple nucleophilic displacement reaction provides the greatest insight into the effect of structure on the reac-
ACID-CATALYZED NUCLEOPHILIG REACTIONS The acid-catalyzed reaction of phosphine with ketones or aromatic aldehydes differs from that with the simple
aliphatics—besides formation of the Ρ—C bond, oxygen apparently transfers from carbon to phosphorus:
Ketones give predominantly primary phosphine oxides, if large groups block the carbonyl group. Forma-
tion of a carbonium ion intermediate, after normal carbonyl addition, has been proposed as the mechanism:
2,4,6-triisopropyl-l ,3dioxa-5-phosphacyclohexane
l,3,5,7-tetramethyl-2,4,8trioxa-6-phosphaadamantane
1,5,7,11 -tetrahydroxy6-phosphoniaspiro [5,5] undecane chloride
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NUCLEOPHILIC QUATERNIZATION The classic Michaelis-Arbusov reaction is the most important example of phosphorus-carbon bond formation by quaternization. The process involves
quaternization of a tertiary phosphite, which is followed by dealkylation by halide ion to give a dialkyl alkylphosphonate ester:
The dealkylation step is helped by simultaneous formation of a stable phosphoryl group, and this
type of driving force is the key to many of the reactions of the phosphonium compounds:
The basic mechanism for this type of reaction, which had been proposed by G. W. Fenton and C. K.
Ingold of London in 1929, involves pentacovalent phosphorus:
tivity of trivalent phosphorus, and this is the one that most merits our attention. W. A. Henderson and S. A. Buckler, at the Cyanamid laboratories, found that the reaction rate of a variety of tertiary phosphines with ethyl iodide could be correlated with the inductive effects of the substituents on phosphorus. This means that resonance interactions between aromatic groups and phosphorus are weak. Here again, phosphorus and nitrogen differ considerably, and this is an example of the general notion that second-row elements form weak ρπ-ρπ double bonds. Deviations from the inductive effect
correlation were observed with methylphosphines, which reacted faster or were more nucleophilic than predicted. Likewise, such deviations were also found with sterically hindered phosphines, such as triisobutyl, which was less reactive than predicted on the basis of inductive effects alone. The explanation for the enhanced nucleophilicity of methylphosphines is complex and tentative. Certainly, more work is needed to understand the factors governing this case. Geometry and bond hybridization are both im portant in this respect. Understandably, electron-supply-
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ing groups generally enhance the nucleophilic reactivity of phosphines by increasing the negative charge or density of the free electron pair on phosphorus. Further, the reaction goes faster in more polar solvents, as might be expected from the increase in charge separation in the transition state. Greater solvation, of course, reduces the activation energy in reactions producing charged species, and this is primarily responsible for the rate increase. In a related study, W. A. Henderson and C. A. Streuli found that the phosphine basicity is also correlated primarily by inductive effects with deviations, again principally the result of steric factors. Since we are dealing in this case with protonation, rather than interaction with a more or less bulky alkylating agent, the steric effect is probably related to solvation energy factors. This is apparent from the operation of such an effect in the secondary and primary phosphines, as well as in the tertiary series. It is also interesting that, in contrast to the nitrogen bases, arylphosphines fall on the same straight-line correlation of basicity with inductive effect as do the alkylphosphines. Finally, it is noteworthy that, although the amines are generally stronger bases than the phosphines, the order with respect to the degree of substitution is reversed with equivalent substituents ( R N H 2 > R 2 H N > R 3 N > R 3 P > R 2 P H > RPH 2 ). Quaternary phosphonium salts themselves may serve as useful synthetic intermediates for the formation of new carbon-phosphorus bonds. Our work has shown that 2-cyanoethylphosphonium salts undergo elimination with base (E2); and the resultant tertiary phosphine, less a 2-cyanoethyl group, can be requaternized with a new alkyl halide to replace these groups successively, in effect, with alkyl substituents. Similar work has been done by several groups with hydroxymethylphosphonium salts. In this case, bases reverse the aldehyde addition reaction, and stepwise quaternization and base treatment procedures to give new alkylphosphines can be carried out, starting with commercially available THPC. Stoichiometry is important with THPC and some alkyl tris (hydroxymethyl) phosphonium salts, since excess aqueous caustic gives hydrogen gas and a tertiary phosphine oxide instead of a phosphine. In terms of intrinsic properties, quaternary phosphonium compounds show interesting utility in a number of areas where the unique properties of the phosphorus atom, coupled with an ionic character, seem to play a key role. Incorporation of these properties in plastics and other polymers may have far-reaching commercial significance. As far as the chemistry of organophosphorus compounds is concerned, quaternary compound reactions offer some of the most fertile subjects of study concerning the stereochemistry and mechanisms of fundamental organic processes in this field. Not the least of these is the "ylid" chemistry of the conjugated bases (methylene phosphoranes) that take part in the Wittig and similar reactions. Elemental
Phosphorus
Because of its importance as the lowest-cost source of the element in a reactive form, the reactions of white phosphorus merit a special place in any survey of general methods for forming phosphorus-carbon bonds. Most of the interesting new reactions involve nucleophilic attack on the P 4 tetrahedron by some anionic reagent, followed by reaction of the resulting phosphide nucleophile with an unsaturated or potentially electron-deficient center. The
Monsanto scientist Dr. Gail Birum {center) discusses an organopolyphosphorus molecule with G. A. Richardson {left) and J. L. Dever. Organophosphorus compounds show promise as additives and plasticizers, as well as for many other uses
new phosphorus-carbon bond may be formed at either stage of the process. Fittingly, then, this class of reactions comes between the nucleophilic and electrophilic reactions of phosphorus. An interesting old reaction between olefins, white phosphorus, and oxygen—which may involve free radicals—was re-examined in 1958 by C. Walling of Columbia University. In this process, intermediate products (which may be polymers) containing carbon-phosphorus and carbon-oxygenphosphorus bonds are formed in organic solvents, but hydrolysis or oxidation with nitric acid gives a 2-hydroxyalkylphosphonous or -phosphonic acid. A number of even older reactions of limited value are in the literature. These include high temperature, sealed-tube reactions of alcohols and alkyl halides. In all of these reactions, mixtures of products containing from one to four carbon-phosphorus bonds are formed. More recently, the reaction of phosphorus with trifluoromethyliodide was reported to give a mixture consisting of (CF 3 ) 3 P, (CF 3 ) 2 PI, and CF 3 PI 2 with the tertiary phosphine predominating. In 1959, L. Maier of Monsanto found that, in general, lower alkyl halides react with red phosphorus and copper powder in a hot tube to give predominantly the phosphonous dihalide, RPC12, derivative. Perhaps the only other useful method available in the past for preparing organophosphorus compounds from the element involves the use of sodium metal. This method has not been applied extensively, and the possibilities of forming carbon-phosphorus bonds this way remain to be fully explored. It appears likely that metal-phosphorus systems may be significantly useful in future synthetic methods. As a part of our over-all phosphorus research effort at Cyanamid Central Research Laboratory, M. M. Rauhut and A. M. Semsel, in their study of the chemistry of elemental phosphorus, discovered several new reactions for preparing organophosphorus compounds. These reactions are remarkable for their mild conditions and simple products. The first type of reaction involves strong aqueous basepromoted addition of phosphorus to an active olefin at temperatures of 30° to 35° C. An organic solvent, such as acetonitrile or ethanol, is used; and tertiary phosphine DEC. 3, 196 2 C&EN
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SUGGESTED ADDITIONAL READING Berlin, K. D., and Butler, G. B., "The Preparation and Properties of Tertiary and Secondary Phosphine Oxides," Chem. Rev., 60, 243 (1960). Crofts, P. C , "Compounds Containing Carbon-Phos phorus Bonds," Quart. Rev. (London), 12, 341 (1958). Frank, A. W., "The Phosphorus Acids and Their Deriva tives," Chem. Rev., 6 1 , 389 (1961). Freedman, L. D., and Doak, G. O., "Preparation and Properties of Phosphonic Acids," Chem. Rev., 57, 479 (1957). Kosolapoff, G. M., "Organophosphorus Compounds," John Wiley & Sons, New York, N.Y., 1950. Schollkopp, U., "The Wittig Reaction," Angew. Chem., 7 1 , 2 6 0 (1959). Trippett, S., "The Wittig Reaction," Advan. Org. Chem., Vol. I, 83, Interscience (John Wiley & Sons), New York, N.Y., 1960. VanWazer, J. R., "Phosphorus and Its Compounds," Interscience (John Wiley & Sons), New York, N.Y., 1958.
oxide, the sole organic product, separates as a solid from the reaction mixture. Acrylonitrile is converted to tris (2cyanoethyl)phosphine oxide (TPO) in 72% yield: P4 + CH2=CHCN + KOHaq ^ ? , N > (NCCH 2 CH 2 ),P=0 + KH2P02 + K2HP03
Hypophosphite and phosphite salts are by-products, since about 50% of the phosphorus is converted to the organic product. Acrylamide also reacts in this way in ethanol as solvent to give tris(2-carbamoylethyl)phosphine oxide (CARPO) in high yield. The reaction probably occurs by a series of cleavages of phosphorus-phosphorus bonds by hydroxide ion, followed by nucleophilic addition of the phosphide intermediates to the olefinic group. CARPO and TPO, or their derivatives, are of considerable interest as components in certain resin formulations, where fire resistance and heat stability are imparted. The second type of reaction involves formation of the carbon-phosphorus bond in the initial stage of attack on the P 4 tetrahedron. Organometallic reagents are the nucleophiles in this case, and the type of product is deter mined by the nature of the electrophilic reagent used to quench the first organophosphides produced. For example, reaction with aryl Grignard or lithium reagents in ether or tetrahydrofuran, followed by hydrolysis with water gives primary aryl phosphines in 20 to 40% conversions of phos phorus, with occasional trace amounts of secondary and tertiary phosphine as coproducts. If alkyl halides instead of water are added to the reaction mixture, the phosphorus is converted in 50 to 80% yields into about equal amounts of two unsymmetrical tertiary phosphines. These can be derived from 2 moles of organometallic and 1 mole of the alkyl halide, or vice versa:
Other electrophilie quenching reagents can be used. For example, propylene oxide leads to 2-hydroxypropylphosphines and phosphonium salts. 98
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With alkyl organometallics, such as butyl lithium or sodium and butyl halides, secondary and tertiary phos phines are formed. However, butylmagnesium bromide reacts with phosphorus and butyl bromide in tetrahydro furan to give tetrabutylcyclotetraphosphine as the major product. Because of their high phosphorus content, cyclotetraphosphines may become a future source of valuable gasoline additives:
From studies made of the reaction and the forma tion of amorphous organopolyphosphides, especially with hydrolytic quenching, it is evident that the reaction in volves complex intermediates that contain phosphorusphosphorus bonds with varying numbers of organic groups per phosphorus atom. These reactions provide access to organophosphorus compounds that would be difficult to prepare by conventional methods. Also, they illustrate the concept that nucleophilic cleavage of phosphorusphosphorus bonds provides a general approach to the chemistry of elemental phosphorus. Electrophilic Reactions of Phosphorus Reactions in which nucleophilic reagents such as carbanions, olefins, and aromatic compounds attack phos phorus, with the formation of a new phosphorus-carbon bond, provide many useful synthetic methods. Generally, the new bond results from halogen replacement by an organometallic reagent, since the electrophilic reactivity of phosphorus is enhanced by electronegative groups. Alkoxy groups can also be replaced. In addition, other general methods include the addition of phosphorus pentachloride to olefins and acetylenes, to give unsaturated phosphonic acids, as well as phosphonation of aromatic hydrocarbons and olefins by phosphorus trichloride and aluminum chlo ride. Aromatic substitution may also be achieved with P 4 S 1 0 and A1C13, or with P4S 10 or P0O5 alone at higher temperatures. Aryl phosphonic acids are the ultimate products. More recently, phosphorus trichloride has been added to olefins in the presence of aluminum chloride to obtain products of yet undetermined structure containing a phosphorus-carbon bond. In all of these Friedel-Craftslike reactions, a PCL^AIC^ complex containing P C l 2 e A1C1 4 ^ is probably the active reagent. Free Radical Phosphorus-Carbon Bond Formation Free radical methods for making organophosphorus com pounds have considerable preparative value, and they also serve to illustrate the chemistry that prevails in many oxidation and sulfur abstraction reactions of trivalent phosphorus. One fundamental underlies the free radical chemistry of phosphorus. This is the ready homolysis of the phos phorus-hydrogen and phosphorus-chlorine bonds. This is probably a dual function of their relatively low bond strengths in trivalent, as well as tetravalent, or quadruply connected compounds, and of the ability of phosphorus to sustain an odd electron. The latter is especially significant with trivalent compounds such as PC1 3 , P ( O R ) 3 , and PR 8 , where the phosphoranyl radical, Ξ Ρ - Ζ , forms readily by addition of radical Z*, which may be alkyl, aryl, alkoxy, thiyl, halo, or some other group. Presumably, empty d
CONVERSION OF ELEMENTAL PHOSPHORUS Most interesting new reactions for forming phosphorus-carbon bonds, starting with elemental phosphorus, involve a nucleophilic attack on the P 4 tetrahedron by an anionic reagent, followed by reaction of
the new phosphide nucleophile with an unsaturated or potentially electron-deficient center. Acrylonitrile is converted to tris(2-cyanoethyl) phosphine oxide, with hypophosphite and phosphite salts as by-products:
orbitals of phosphorus play an important part in giving some stability to this reactive, nine-electron configuration. Perhaps the most useful reactions in this class are those involving the peroxide, ultraviolet, or x-ray catalyzed addition of phosphorus-hydrogen and phosphorus-chlorine compounds to olefins. The peroxide-induced addition of phosphorus trichloride to an olefin to give 2-chloroalkylphosphonous dichlorides was reported in 1945 by M. S.
Kharasch, E. V. Jensen, and W. H. Urry at the University of Chicago. Homolysis of the phosphorus-chlorine bond was proposed as the initiation and chain-carrying steps. Addition of the dichlorophosphinyl radical to the olefin provides the carbon-phosphorus bond formation. The use of phosphorus-hydrogen compounds was disclosed 10 years ago by A. R. Stiles, F. F. Rust, and W. E. Vaughn of the Shell Development Co., who reported addiDEC. 3, 1962 C&EN
99
DR. MARTIN GRAYSON is a group leader in the chemical research department of American Cyanamid's Central Research Division, where he has been doing organophosphorus research for the past six years. Previously, he had been with Allied Chemical's Nitrogen Division. He completed his undergraduate work with honors at New York University and received his Ph.D. from Purdue University in 1952, where he had been an AEC predoctoral fellow. As an undergraduate he was elected to Sigma Xi and Phi Beta Kappa and was awarded the AIC medal for chemistry. Dr. Grayson is coeditor of "Progress in Phosphorus Chemistry,>y with Dr. E. J. Griffith of Monsanto Chemical, a series of critical reviews to be published by Interscience (John Wiley b- Sons).
tion to olefins in the presence of free radical initiators, such as di-f-butylperoxide. Primary, secondary, and tertiary phosphines were produced with phosphine. The availability of low-cost olefins makes this one of the most attractive routes to phosphines for commercial exploitation. A study of the scope and utility of the phosphine-olefin reaction was carried out at Cyanamid by M. M. Rauhut, H. A. Currier, A. M. Semsel, and V. P. Wystrach. These workers found that a,a'-azobisisobutyronitrile (AIBN) initiated the reaction more effectively than peroxides. Excellent yields of primary or tertiary phosphines could be obtained with appropriate PH 3 /olefin reaction ratios. Further, they noted a steric effect with shielded double bonds and, with cyclohexene, no tertiary phosphine was formed. Unsymmetrical products were obtained with primary or secondary phosphines as reactants, and reactive olefins such as acrylonitrile, styrene, and ethyl acrylate were used successfully. Reaction with acetylenes gave olefinic phosphines in moderate yields. In 1960, E. K. Fields and R. J. Rolih of Standard Oil (Ind.) reported an interesting free radical phosphonation of aromatic compounds involving addition of a phosphonyl radical, generated by di-f-butyl peroxide and a dialkyl phosphonate, to the aromatic ring. Radical reactions involving addition to phosphorus, to give the phosphoranyl radical as an intermediate, include the formation of phosphonium salts from triarylphosphines and phenyl or halomethyl radicals, and the unusual oxidative chlorophosphonation reaction. The latter was first reported by J. O. Clayton and W. L. Jensen of California Research Corp. in 1948, and it entails the formation of alkylphosphonyl chlorides, RPOCl 2 , from phosphorus trichloride, oxygen, and saturated hydrocarbons. Excess phosphorus trichloride is used, since much of this reagent is oxidized directly to POCl 3 . Yields are generally poor, and isomeric mixtures similar to those obtained in chlorination are produced. The reaction works with many organic materials as well as substituted phosphorus halides. Mechanistic studies have been reported by C. E. Boozer and R. L. Flurry of Emory University and by F. R. Mayo of Stanford Research Institute. The reaction is obviously complex; but formation of the carbon-phosphorus bond probably proceeds by direct addition of a hydrocarbon radical to phosphorus trichloride, which can compete effec100
C&EN
DEC. 3f 196 2
tively with oxygen as a scavenger. The hydrocarbon radical is probably generated by chlorine atoms, which in effect serve as chain carriers. Oxidation is no doubt partly accomplished by peroxy radicals, although largely so by oxygen, with a phosphorus-peroxy structure, Ph3RP—O—O, as a possible intermediate. The radical chemistry of trivalent phosphorus is a fascinating subject for the theoretical investigator, as well as for the practical polymer chemist. Polymers with phosphorus in the backbone have been made by radical reactions—the reactions with oxygen, disulfides, and many others not involving carbon-phosphorus bond formation are radical in nature. Much remains to be done in elucidating mechanisms and stereochemistry of these reactions, and the potential in this area of organophosphorus chemistry appears to be considerable. It is apparent that organophosphorus chemistry has come through a period of vigorous activity during the past few years, with many general principles being uncovered and many novel and convenient synthesis methods being found. Clearly, this broad base of fundamental knowledge places us in a position to advance farther in several technical and theoretical directions. Applying nucleophilic addition reactions of phosphine to unsaturated systems in an industrial way should bring on a variety of new commercial materials. This kind of development requires little more than a good commercial process for phosphine production. Such a process might be based on elemental phosphorus, or better still, on phosphate minerals. In commercial terms, direct synthesis of organophosphorus compounds from elemental phosphorus has many advantages. But fundamental work still needs to be done to develop selective, low cost procedures for trivalent compounds. On the other hand, results obtained so faf clearly indicate that such reactions are attainable. For the investigator in synthesis and theory, the organic chemistry of phosphorus is represented by a map of predominantly unexplored territory. With respect to new phosphorus-carbon bond formation, for example, nucleophilic addition to heterocyclic systems seems wide open for study. In the entire area of radical addition and pentavalent stereochemistry much remains to be done. We know something about diphosphine and cyclotetraphosphine chemistry, but little about other linear and cyclic organic poly phosphines. Then, of course, there are the carbon-like metalloid elements—silicon, germanium, and others—whose phosphorus chemistry is still in its infancy. In all of these studies, nuclear magnetic resonance will provide clear guideposts to the structural chemist who incorporates phosphorus-31 into new molecules.
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