Diphosphines, reactions

Many compounds of the types in Table 9.6 can be made by adding sulphur to the appropriate phosphine. Diphosphine disulphides are thus derived from biphosphines, bis(phosphinodthioic) acids from di-secondary biphosphines (9.584), alkylene bis(alkylphosphine sulphides) and aUcylene bis(phosphinodithioic)acids (Table 9.6) from secondary alkylene biphosphines (9.585, 9.586), and alkylene bis(dialkylphosphine sulphides) from alkylene diphosphines. Reaction (9.583) can also be noted. 

Asymmetric dimerisation of 3,4-dimethyl-1-phenylphosphole in the presence of an organoplatinum(II) complex derived from (R)-Ar,AT-dimethyl-l-(l-naph-thyl)ethylamine affords an optically pure P-chiral diphosphine. " " Reaction of racemic Sb-chiral (+ )-l-phenyl-2-trimethylsilylstibindole with di-p-chloro-bis (iS)-2-[l-(dimethylamino)ethyl]phenyl-C,Ar dipalladium(II) gives a 1 1 mixture of the diastereomeric complexes 51a and 51b. Separation of the complexes by chromatography followed by treatment with triphenylphosphine affords the optically pure l-phenyl-2-trimethylsilylstibinoles in quantitative yields. " The... 

All phosphoms oxides are obtained by direct oxidation of phosphoms, but only phosphoms(V) oxide is produced commercially. This is in part because of the stabiUty of phosphoms pentoxide and the tendency for the intermediate oxidation states to undergo disproportionation to mixtures. Besides the oxides mentioned above, other lower oxides of phosphoms can be formed but which are poorly understood. These are commonly termed lower oxides of phosphoms (LOOPs) and are mixtures of usually water-insoluble, yeUow-to-orange, and poorly characteri2ed polymers (58). LOOPs are often formed as a disproportionation by-product in a number of reactions, eg, in combustion of phosphoms with an inadequate air supply, in hydrolysis of a phosphoms trihahde with less than a stoichiometric amount of water, and in various reactions of phosphoms haUdes or phosphonic acid. LOOPs appear to have a backbone of phosphoms atoms having —OH, =0, and —H pendent groups and is often represented by an approximate formula, (P OH). LOOPs may either hydroly2e slowly, be pyrophoric, or pyroly2e rapidly and yield diphosphine-contaminated phosphine. LOOP can also decompose explosively in the presence of moisture and air near 150° C. 

Commercially, phosphinic acid and its salts are manufactured by treatment of white phosphoms with a boiling slurry of lime. The desired product, calcium phosphinite [7789-79-9], remains ia solution andiasoluble calcium phosphite [21056-98-4] is precipitated. Hydrogen and phosphine are also formed, the latter containing sufficient diphosphine to make it spontaneously flammable. The details of this compHcated reaction, however, are imperfectly understood. Under some conditions, equal amounts of phosphoms appear as phosphine and phosphite, and the volume of the hydrogen Hberated is nearly proportional to the hypophosphite that forms. 

The alkah metal phosphides of formula M P and the alkaline-earth phosphides of formula M2P2 contain the P anion. Calcium diphosphide [81103-86-8] CaP2, contains P reaction with water Hberates diphosphine and maintains the P—P linkage. 

Reaction of the diphosphine ligand R2P(CH2)2PR2 (R = benzothiazolyl) (L) with [RhCl(PPh3)3] gives the exclusively P-coordinated product [RhCl(PPh3)(L)] (88JOM(338)C31, 92JCS(D)241), which is perhaps a common feature of the P-substituted derivatives of oxazole and thiazole. 

Reaction of the diphosphines Ph2P(CH2) PPh2 (n = 1-3) with MCl2(PhCN)2 affords 1 1 m-complexes (Figure 3.46) [102]. (Note the use of the labile PhCN adducts if the MCl salts are used, Magnus type compounds M(P-P)2+MCl4- are formed.) Similar complexes are formed with other halides for the thiocyanates see section 3.8.6. The structures of the palladium complexes have been determined (Table 3.10) with square coordination only achieved for n = 3 with the formation of a six-membered metal-chelate ring. 

With bulky diphosphines Bu2P(CH2) PBu2 (n = 8-12), similar reactions of the diphosphines with MCl2(PhCN)2 give separable mixtures of monomer, dimer and trimer. With small phosphines (n = 5-7) dimers predominate (Figure 3.48). 

Elimination reactions have been particularly studied in the case of dialkyls. They depend on the alkyl groups being cis trans-complexes have to isomerize before they can eliminate, and a complex with a trans-spanning diphosphine ligand is stable to 100°C (Figure 3.56). 

As mentioned in Sect. 2.2, phosphine oxides are air-stable compounds, making their use in the field of asymmetric catalysis convenient. Moreover, they present electronic properties very different from the corresponding free phosphines and thus may be employed in different types of enantioselective reactions, m-Chloroperbenzoic acid (m-CPBA) has been showed to be a powerful reagent for the stereospecific oxidation of enantiomerically pure P-chirogenic phos-phine-boranes [98], affording R,R)-97 from Ad-BisP 6 (Scheme 18) [99]. The synthesis of R,R)-98 and (S,S)-99, which possess a f-Bu substituent, differs from the precedent in that deboranation precedes oxidation with hydrogen peroxide to yield the corresponding enantiomerically pure diphosphine oxides (Scheme 18) [99]. 

The borane group of MiniPHOS was removed using the same protocol (Scheme 21). In contrast, however, reaction of free diphosphines 13 with [Rh(nbd)2]+X (X=Bp4, PFg) afforded in all cases, and independently of the reaction conditions, bischelate complexes 106. James and Mahajan demonstrated... 

Although sulfur is unHkely to chelate the metal in this case, it is worth mentioning the axially chiral diphosphine Hgands, based on hz-thienyl systems which increase the electronic density at phosphorus such as 159 (used in Ru-catalyzed reduction of /1-keto esters with 99% ee) [llla],BITIANP 160,andTMBTP 161 (in a Pd-catalyzed Heck reaction, the regio- and enantioselectivity are high with 160 but low with 161) [mb]. 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

Micellar effects can play an important part in aqueous organometallic reactions. Surface active diphosphines have been synthesized and sparingly soluble solutes like decene may well benefit through miceller effects. 

Even bidentate diphosphines do not act as catalyst poisons. For example, Pt(0) catalyzed smooth addition of ethyl acrylate to 1,2-diphosphinobenzene (Scheme 5-12, Eq. 1) no reaction took place in the absence of Pt(0), and radical initiators gave a mixture of products. 

A P NMR study of stoichiometric reactions using the di-primary phosphine H2PCH2CH2CH2PH2 provided more information on the reaction mechanism (Scheme 5-12, Eq. 2). Norbornene was displaced from Pt(diphosphine)(norbornene) by ethyl acrylate. Reaction with the diphosphinopropane was very fast this gave the hydrophosphination product, which, remarkably, did not bind Pt to give Pt(diphos-phine), instead, Pt(diphosphine)(norbornene) was observed [12]. 

Similar catalytic reactions allowed stereocontrol at either of the olefin carbons (Scheme 5-13, Eqs. 2 and 3). As in related catalysis with achiral diphosphine ligands (Scheme 5-7), these reactions proceeded more quickly for smaller phosphine substrates. These processes are not yet synthetically useful, since the enantiomeric excesses (ee s) were low (0-27%) and selectivity for the illustrated phosphine products ranged from 60 to 100%. However, this work demonstrated that asymmetric hydrophosphination can produce non-racemic chiral phosphines [13]. 

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