Catalytic Hydrophosphination

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... 

A mechanism that involves ytterbium phosphide species has been proposed, similarly to the foregoing intramolecular hydrophosphination. Generation of the phosphide species is supported by the formation of Ph2CDNHPh (after aqueous quench) upon treatment of the imine complex with Ph2PD (Scheme 15). Lanthanide phosphide is known to react with THF, forming a 4-diphenylphosphino-l-butoxyl species [21], which was indeed found as a side product in the catalytic hydrophosphination of disubstituted aliphatic alkynes run in THF, supporting further the ytterbium-phosphide intermediate (Scheme 16). 

Significant progress in organolanthanide-catalyzedhydroamination over the last two decades has sparked the interest in the development of analogous catalytic hydrofunctionalization protocols. Catalytic hydrophosphination of unsaturated carbon-carbon linkages offers an attractive atom-economic route to phosphines [87,177], It should be noted that inter- and intramolecular hydrophosphinations are also... 

Scheme 6.3 Catalytic hydrophosphination of alkenes (X = lone pair, BH3 or oxygen).

These results suggest that the reaction proceeds via insertion of acrylonitrile into the Pt-P bond and subsequent coupling of the hydrido and 2-phosphide-l-cyanoethyl ligands for the catalytic hydrophosphination, rather than an alternative pathway involving coupling of the alkyl and phosphide ligands bonded to Pt. 

Reaction mechanism Based uprm a series of stoichiometric reactions, and in contrast to the previously discussed hydrophosphinatiOTi reactions (vide supra), it was postulated that unsaturated homoleptic heavier alkaline-earth phosphides are unlikely to be long-lived intermediates in the catalytic hydrophosphination of carbodiimides conducted in non-coordinating solvents. Rather, it was proposed that the phosphaguanidine product was acting as a ligand for the Lewis acidic metal center [114]. 

Table 1 Palladacycle promoted catalytic hydrophosphination of enones ...

Shulyupin MO, Kazankova MA, Beletskaya IP (2002) Catalytic hydrophosphination of styrenes. Org Lett 4 761-763... 

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. 

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]. 

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...

In contrast to the other X-H additions surveyed in this chapter, there have been only a limited number of studies on the addition of P H linkages across C C (and C—O) multiple bonds. At the same time, there has been significant progress made in the development of catalytic systems for performing hydrophosphination and hydrophosphorylation compared to what was known pre-1982. 

Hydrophosphination of ethyl acrylate using PH3 (R = C02Me, Equation (17)) is catalyzed by a mixture of the zero-valent platinum complexes (72a c), which are formed upon addition of P CH2CH2C02Et 3 to Pt(norbornene)3] (Scheme 44). Failure of these complexes to bring about P H addition to CH2 = CHCF3 indicates that Michael activation of the alkene through I and R effects of the substituents is crucial for catalytic activity in this class of metal complexes.190... 

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... 

Abstract In the first part of this mini review a variety of efficient asymmetric catalysis using heterobime-tallic complexes is discussed. Since these complexes function at the same time as both a Lewis acid and a Bronsted base, similar to enzymes, they make possible many catalytic asymmetric reactions such as nitroal-dol, aldol, Michael, Michael-aldol, hydrophosphonyla-tion, hydrophosphination, protonation, epoxide opening, Diels-Alder and epoxi-dation reaction of a, 3-unsaturated ketones. In the second part catalytic asymmetric reactions such as cya-nosilylations of aldehydes... 

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... 

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]. 

For example, an effective procedure for the synthesis of LLB (where LL = lanthanum and lithium) is treatment of LaCls 7H2O with 2.7 mol equiv. BINOL dilithium salt, and NaO-t-Bu (0.3 mol equiv.) in THF at 50 °C for 50 h. Another efficient procedure for the preparation of LLB starts from La(0-/-Pr)3 [54], the exposure of which to 3 mol equiv. BINOL in THF is followed by addition of butyllithium (3 mol equiv.) at 0 C. It is worthy of note that heterobimetallic asymmetric complexes which include LLB are stable in organic solvents such as THF, CH2CI2 and toluene which contain small amounts of water, and are also insensitive to oxygen. These heterobimetallic complexes can, by choice of suitable rare earth and alkali metals, be used to promote a variety of efficient asymmetric reactions, for example nitroaldol, aldol, Michael, nitro-Mannich-type, hydrophosphonylation, hydrophosphination, protonation and Diels-Alder reactions. A catalytic asymmetric nitroaldol reaction, a direct catalytic asymmetric aldol reaction, and a catalytic asymmetric nitro-Mannich-type reaction are discussed in detail below. 

Homoleptic lanthanide alkyls of the form Ln[GH(SiMe3)2]3 (Ln = Y, La, Nd, Sm, Lu) serve as efficient precatalysts for intramolecular homogeneous hydrophosphination. Both phosphinoalkenes and phosphinoalkynes undergo cyclization to the corresponding heterocyclic structures.1055 The catalytic intramolecular hydrophosphination/cycli-zation of phosphinoalkenes and phosphinoalkynes using organolanthanide precatalysts of the type... 

The mechanism and scope of rare-earth metal-catalyzed intramolecular hydrophosphination has been studied in detail by Marks and coworkers [147,178-181]. The hydrophosphination of phosphinoalkenes is believed to proceed through a mechanism analogous to that of hydroamination. The rate-determining alkene insertion into the Ln-P bond is nearly thermoneutral, while the faster protolytic o-bond metathesis step is exothermic (Fig. 22) [179,181]. The experimental observation of a first-order rate dependence on catalyst concentration and zero-order rate dependence on substrate concentration are supportive of this mechanism. A notable feature is a significant product inhibition observed after the first half-life of the reaction. This is apparently caused by a competitive binding of a cyclic phosphine to the metal center that impedes coordination of the phosphinoalkene substrate and, therefore, diminishes catalytic performance [179]. 

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