Phosphination, hydrophosphination

If the alkene is connected to the phosphine, hydrophosphination results in cyclization, as observed for a series of lanthanocene catalysts. DPT and experimental studies were consistent with the mechanism of Scheme 19, where P-C bond formation occurs by the approximately thermoneutral insertion of an alkene into a... 

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 5-7 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]... 

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]. 

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. 

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]. 

Building on from these results, catalytic hydrophosphination of activated alkenes was developed. Addition of PH3 to acrylonitrile (R = CN, Equation (17)) at room temperature affords tris cya-noethyl)phosphine in the presence of three-coordinate [Pt(P CH2CH2CN 3)3].188... 

Asymmetric hydrophosphination has been utilized as a route for preparing chiral phosphines. The Pt° complex [(Me-DUPHOS)Pt(t/ tf/ ,s-PhCII ClIPh)] (73) brings about the catalytic P-H addition of bulky secondary phosphines to activated alkenes with modest enantioselectivity. The most promising substrate combinations for further development appear to be bulky alkenes and less bulky phosphines (Scheme 46).195... 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

Hoye, P.A.T., Pringle, P.G., Smith, M.B., and Worboys, K., Hydrophosphination of formaldehyde catalyzed by tris(hydroxymethyl)phosphine complexes of platinum, /. Chem. Soc., Dalton Trans., 269, 1993. 

Pringle, P.G., Brewin, D., and Smith, M.B., Metal-catalyzed hydrophosphination as a route to water-soluble phosphines, in Aqueous Organometallic Chemistry and Catalysis, Vol. 5, Horvath, I.T. and Joo, F., Eds., Kluwer, Dordrecht, the Netherlands, 1995, p. 111. 

Kovacik, I., Wicht, D.K., Grewal, N.S., Glueck, D.S., Incarvito, C.D., Guzei, I.A., and Rheingold, A.L., Pt(Me-Duphos)-catalyzed asymmetric hydrophosphination of activated olefins enantioselective synthesis of chiral phosphines,... 

Regiospeciflc, uncatalysed hydrophosphination of typical Michael acceptors, such as methyl acrylate, has been reported to proceed readily with alkenyl- an alkynyl-phosphine oxides, e.g. R(l )P(H)0. Good stereoselectivity was observed when a chiral electrophile was used. The reaction is believed to proceed owing to the strong... 

Hydrophosphination is the addition of a P-H unit onto a double bond which can be catalyzed by transition metal phosphine complexes. In fact this reaction has been known for long [22,43] addition of PH3 onto formaldehyde serves as a basis for production of P(CH20H)3, a flame resisiting agent for wood and textiles. The details of this reaction have been recently scmtinized [38, 40], besides that the first hydrophosphination of an... 

In view of excess phosphine being inhibitory in many catalytic reactions, it is surprising that the hydrophosphination reaction is not suppressed by the phosphines formed. In agreement with this, the product phosphine P(CH2CH2CN)3 is reluctant to form tetrakis(phosphine)platinum species, allowing the metal complex to be coordinatively unsaturated. Likewise, the... 

The mechanistic study on the hydrophosphination of activated olefins, in conjunction with rapid inversion of the configuration at the phosphorus center, was elaborated to develop asymmetric hydrophosphination catalyzed by a chiral phosphine platinum complex although the % ee is not excitingly high yet (Scheme 9) [15]. 

Simultaneous publication of the iminium ion catalysed hydrophosphination of a,p-unsaturated aldehydes by Melchiorre and Cordova showed diarylprolinol silyl ether 55 was effective in the conjugate addition of diphenylphosphine 74 [117, 118], Direct transformation of the products allowed for one-pot methods for the preparation of P-phosphine alcohols 75 (72-85% yield 90-98% ee), P-phosphine oxide acids 76 (65% yield 92% ee) and 3-amino phosphines 77 (71% yield 87% ee) (Scheme 34). These reports represent the first examples of the addition of P-centred nucleophiles and the resulting highly functionalised products may well have further use in asymmetric catalysis. 

The addition of secondary phosphines H PR2 to prop-2-ynols in the presence of RuCl (C5Me5)(cod) or RuCl(C5Me5)(PPh3)2 provides the first regio- and stereo-selective direct hydrophosphination of propargylic alcohols and leads to bifunctional... 

Most of the hydrophosphinic acids are unstable in air. They easily autoxidize to hydroxy-phosphinic acids. There are other methods leading to the hydro-phos-phinic acids. The known compounds of this type are summarized in Table 13. 

To date, the hydrofunctionalization route has mainly been used to prepare PBs. Both conceivable strategies, namely the hydroboration of unsaturated phosphines and the hydrophosphination of unsaturated boranes have been reported (Scheme 25). With dialkyl and diarylboranes, the reactions proceed spontaneously under mild conditions, while the addition of boronates HB(OR)2 is catalyzed by a titanium complex, and the... 

The inteimolecular hydrophosphination of phenylacetylene (76, equation 20) with di-phenylphosphine using Ni(acac)2 (47) and (Et0)2P(0)H has been reported to give aUcenyl-phosphine (77) in high yields . Alkenylphosphines are an important class of ligands whose applications have been limited by the lack of a simple and convenient synthetic procedure. This reaction gave the corresponding iyn-addition products with a mixture of a- and / -adducts. Selectivities could be fine-tuned by judicious choice of catalyst precursor used to facilitate this reaction. 

Metal complex chemistry, homogeneous catalysis and phosphane chemistry have always been strongly connected, since phosphanes constitute one of the most important families of ligands. The catalytic addition of P(III)-H or P(IV)-H to unsaturated compounds (alkene, alkyne) offers an access to new phosphines with a good control of the regio- and stereoselectivity [98]. Hydrophosphination of terminal nonfunctional alkynes has already been reported with lanthanides [99, 100], or palladium and nickel catalysts [101]. Ruthenium catalysts have made possible the hydrophosphination of functional alkynes, thereby opening the way to the direct synthesis of bidentate ligands (Scheme 8.35) [102]. 

From a commercial standpoint, the preparation of phosphorus species consumes the largest volume of Grignard reagents in this set of elements. Although there are alternative reactions for the preparation of phosphines, such as hydrophosphination [Eq. (48)], the Grignard reaction is still preferred when sterically hindered phosphines are desired (Scheme 4). 

Other routes include using readily prepared (hydrolysis of aluminum phosphide) or commercially available PH3 and the appropriate alkene, under either free-radical or metal-catalyzed hydrophosphination conditions (see Section 1.12.2.4.6).39 40 However, separation problems are often encountered, and hence this reaction has so far received limited attention as a convenient route to primary phosphines compared with other, more traditional methods. 

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