Hydrophosphination

Because organophosphorus compounds are important in the chemical industry and in biology, many methods have been developed for their synthesis [1]. This chapter reviews the formation of phosphorus-carbon (P-C) bonds by the metal-catalyzed addition of phosphorus-hydrogen (P-H) bonds to unsaturated substrates, such as alkenes, alkynes, aldehydes, and imines. Section 5.2 covers reactions of P(lll) substrates (hydrophosphination), and Section 5.3 describes P(V) chemistry (hydrophosphorylation, hydrophosphinylation, hydrophosphonylation). Scheme 5-1 shows some examples of these catalytic reactions. 

Although a wide variety of metals were claimed as active catalysts for formaldehyde hydrophosphination, platinum salts were preferred. Similarly, Group 10 metal salts were used to catalyze acrylonitrile hydrophosphination. Russian workers showed that Ni(II) or Co(II) salts in the presence of ammonia or amines would also catalyze the addition of phosphine to formaldehyde [6]. More recently, academic and industrial interest in these reactions was sparked by a series of papers by Pringle, who investigated late metal phosphine complexes as hydrophosphination catalysts. These and related studies are arranged below by substrate. 

Scheme 5-2 Metal-catalyzed hydrophosphination of formaldehyde and acrylonitrile 5.2.1...

Scheme 5-3 Proposed mechanism for metal-catalyzed hydrophosphination of formaldehyde...

Scheme 5-4 Platinum-catalyzed hydrophosphination of acrylonitrile using PH3 (Eq. I) and PH (CH2CH2CN)2 (Eq. 2). The proposed structure of a telomeric by-product of this reaction (I) Is also shown...

Scheme 5-5 Proposed mononuclear mechanism of Pt-catalyzed acrylonitrile hydrophosphination at high [P(CH2CH2CN)3]...

Scheme 5-6 Proposed dinuclear mechanism of ates 3 and 4), but one-at-a-time intermediates Pt-catalyzed acrylonitrile hydrophosphination at 5 and 6 were also proposed. Several mononu-low [P(CH2CH2CN)3. The main pathway illus- clear intermediates (see Scheme 5-5) have been trates pairwise P-C bond formation (intermedi- removed for clarity...

The mononuclear mechanism is similar to the one proposed for platinum-catalyzed hydrophosphination of formaldehyde (Scheme 5-3), but also includes a second P-C bond-forming pathway nucleophilic (Michael) attack of the phosphido ligand on coordinated acrylonitrile. The binuclear mechanism is similar, but P-C bond formation is proposed to occur by cooperative action of two Pt centers, with complexes 4 - 6 as possible intermediates [8]. 

To avoid the complications posed by formation of dinuclear complexes, Glueck and coworkers studied the hydrophosphination of acrylonitrile using the Pt(0) catalysts Pt(diphos)(CH2CHCN) (diphos = dppe or dcpe. Scheme 5-7). 

Scheme 5-7 Pt(diphos)-catalyzed hydrophosphination of acrylonitrile with primary and secondary phosphines...

In most cases, the dppe complex decomposed rapidly, but the more robust dcpe derivative was observed to be the catalyst resting state by NMR. Smaller phosphines reacted more quickly but less selectively in these systems, and selectivity for the hydrophosphination products ranged from 29 to >95%. It is likely that the phosphine byproducts included telomers such as 1 (Scheme 54), but they were not characterized. [9]... 

Hydrophosphination catalyzed by a palladium(II) precursor has been reported. The P-H bonds of a protected bis(phenylphosphino)pyrrolidine added to acrylonitrile in the presence of catalytic amounts of PdCl2 and K2CO3 (Scheme 5-9). Without palladium, using KOH or K2CO3 as base gave only 30-40% yield. Similar catalytic chemistry was reported briefly for methyl acrylate [10]. 

Scheme 5-9 Palladium-catalyzed hydrophosphination of aery lonitrile with a disecondary phosphine...

AIBN-promoted addition of PH3 to ethyl acrylate (70 °C, 25 atm of PH3) was reported to give a mixture of hydrophosphination products, including primary, secondary, and tertiary phosphines in ca. 1 1 1 ratio (Scheme 5-10, Eq. 1) [4c]. 

The acrylate complex 10 was suggested to be the major solution species during catalysis, since the equilibrium in Scheme 5-11, Eq. (2) lies to the right (fQq > 100)-Phosphine exchange at Pt was observed by NMR, but no evidence for four-coordinate PtL, was obtained. These observations help to explain why the excess of phosphine present (both products and starting materials) does not poison the catalyst. Pringle proposed a mechanism similar to that for formaldehyde and acrylonitrile hydrophosphination, involving P-H oxidative addition, insertion of olefin into the M-H bond, and P-C reductive elimination (as in Schemes 5-3 and 5-5) [11,12]. 

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]. 

Scheme 5-12 Platinum-catalyzed hydrophosphination of ethyl acrylate as a route to biden-tate diphosphines (Eq. 1). Stoichiometric reac-...

Recently, the chiral Pt(0) precatalyst Pt[(R, R)-Me-Duphos](trows-stilbene) (11) has been used to prepare enantiomerically enriched chiral phosphines via hydrophosphination of acrylonitrile, t-butyl acrylate and related substrates. This chemistry is summarized in Scheme 5-13. 

Scheme 5-13 Platinum(Me-Duphos)-catalyzed asymmetric hydrophosphination...

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]. 

Scheme 5-14 Stoichiometric reactions of Pt(Me-Duphos) complexes relevant to the proposed catalytic cycle for asymmetric hydrophosphination...

Scheme 5-16 Proposed mechanism of organolanthanide-cat-alyzed hydrophosphination/cyclization...

Metal-catalyzed hydrophosphination has been explored with only a few metals and with a limited array of substrates. Although these reactions usually proceed more quickly and with improved selectivity than their uncatalyzed counterparts, their potential for organic synthesis has not yet been exploited fully because of some drawbacks to the known reactions. The selectivity of Pt-catalyzed reactions is not sufficiently high in many cases, and only activated substrates can be used. Lanthanide-catalyzed reactions have been reported only for intramolecular cases and also sulfer from the formation of by-products. Recent studies of the mechanisms of these reactions may lead to improved selectivity and rate profiles. Further work on asymmetric hydrophosphination can be expected, since it is unlikely that good stereocontrol can be obtained in radical or acid/base-catalyzed processes. 

---