Hydrophosphinations alkenes

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. 

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... 

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... 

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]. 

The amide [Sm(45a) N(SiMe3)2 ] was a catalyst for an aUene-based hydroamination/ cyclisation. As an illustration, one such product upon hydrogenation yielded a naturally occurring alkaloid. Scheme 4.8. " " The same samarium(lll) amide was also active for the intramolecular hydrophosphination/cyclisation of phosphino-alkenes or -alkynes e.g., H2P(CH2)3C=CPh was transformed into 76. " ... 

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]. 

The group VB hydrides show trends in reactivity similar to those of group IVB. The N-H bond can be reacted with alkenes only under the influence of catalysts or under forcing conditions. The P-H bond can be added to alkenes (hydrophosphination) in a free radical chain process, or under photolytic conditions. Such reactions proceed in good yield and in an anti-Markovnikov manner. Some typical free radical P-H additions are listed in Table 1 . The addition of phosphinyl radicals is reversible and can lead to... 

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. 

It is also interesting to note that hydrophosphination reactions can be used to generate fluorous-modified trialkylphosphines in high yields. Hence convenient multigram syntheses of the symmetrically substituted fluorous trialkylphosphines P(CH2CH2Rf)3 (138) by free-radical chain reaction of PH3 to the corresponding alkenes H2C=CH(CH2)m 2Rf have been described.289,290... 

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]. 

Organolanthanide-catalyzed intermolecular hydrophosphination is a more facile process than intermolecular hydroamination. The reaction of alkynes, dienes, and activated alkenes with diphenylphosphine was achieved utilizing the ytterbium imine complex 9 (Fig. 8) as catalyst [185-188]. Unsymmetric internal alkynes react regioselectively, presumably due to an aryl-directing effect (48) [186]. 

The hydrophosphination of 1,3-butadiene with PH3 catalyzed by Cp2EuH should proceed predominantly via a 1,4-addition and to a lesser extend through a 1,2-addition pathway based on a computational study [189]. The reaction of isoprene with diphenylphosphine indeed forms both regioisomers (49) [186], Isolated double bonds are also reactive, as styrene derivatives are almost as reactive as alkynes (50) however, simple unactivated alkenes, such as 1-decene, are unreactive even at elevated temperatures [186]. 

The initial studies of Horvdth and Rabai were concentrated on the synthesis of the fluorinated trialkylphosphane 12 which is a suitable ligand for many transition metals [4,17]. This phosphane was prepared by a hydrophosphination of the corresponding fluorinated alkene 13 Eq. (7). 

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... 

Scheme 18 Proposed mechanism for calcium-catalyzed alkene hydrophosphination (for simplicity, coordinated THF is not shown in 12 or other intermediates)...

Scheme 26 Proposed nucleophilic mechanism for Pt-catalyzed hydrophosphination of Michael acceptor alkenes (X = CN or CO2R) formation of byproducts and the effect of protic additives (HY = t-BuOH or H2O)...

Scheme 34 Proposed mechanism for nickel- or palladium-catalyzed hydrophosphination via attack on a complex of an electron-rich alkene...

Hydrophosphorylation of alkenes has been regarded as a rather difficult reaction. Tanaka discovered that efficient hydrophosphorylation of simple alkenes and cyclic alkenes is possible by using the five-membered cyclic phosphonate 52. The reaction of 1-octene with 52 proceeded at 100 "C to afford the linear phosphonate 53 in 89 % yield. Regioselectivity depends on the nature of the alkenes, and the branched phosphonate 54 was formed by the reaction of styrene. DPPB was found to be a suitable ligand [29]. Hydrophosphination of styrene with diphenylphos-phine (55) proceeded regioselectively by using phosphine-free Pd catalyst to afford 2-phenylethyl(diphenyl)phosphine (56) [30]. 

Examples of electrophilic addition of secondary phosphines to alkenes or alkynes were described. [114, 124, 125, 135]. Glueck [124-126] reported enantioselective tandem reaction of alkylated/arylation of primary phosphines catalyzed by platinum complex, proceeding with formation of chiral phosphaace-naphthenes. Palladium-catalyzed hydrophosphination of alkynes 219 tmder kinetic resolution conditions gave access to 1,1-disubstituted vinylphosphine boranes 220. However, despite screening several chiral ligands, temperatures, and solvents, the... 

The three reactions are likely to have phosphido complexes as intermediates. Pt-, Pd- and Ln-catalysed hydrophosphination of activated alkenes and Pd-catalysed phosphination of aryl halides (a cross-coupling reaction) have been known for some time whereas Pt and Ru-catalysed alkylation of secondary phosphines are more recent. 

Hydrophosphination reactions, i.e. reactions where a P H bond is added across a multiple bond (usually of an activated alkene) can proceed uncata-lysed or catalysed by transition metal complexes. With suitable substrates. 

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

Mechanistically, it is accepted that hydrophosphination involves oxidative addition of the P-H bond to a low-valent metal centre, often a Pt(0) complex, followed by alkene insertion and reductive elimination to yield the product. Reports on metal-catalysed addition reactions to alkenes show that two different pathways are possible (Scheme 6.4). Path A involves alkene insertion into the M-P bond to form complexes 5 followed by C H reductive elimination whereas in path B the alkene inserts into the M H bond to form complexes 6 followed by P-C reductive elimination. 

The previously accepted pathway consisted of P-H oxidative addition to Pt(0) to form 19 followed by coordination and insertion of the alkene in the Pt-P bond to form 20 and a final reductive elimination to furnish the product and regenerate the catalyst. Another possibility is the nucleophilic attack of phosphido complex 19 to the alkene ( Michael addition mechanism, as in anionic polymerisation) to generate the zwitterionic intermediate 21. This complex can yield the hydrophosphination product 11 via one of two complementary pathways. Carbanion attack at the cationic platinum hydride i.e. intramolecular hydrogen transfer) would yield the final phosphine complexed to Pt(0) that would be displaced by an equivalent of PHR R to furnish, after oxidative addition, starting complex 19. Alternatively, the anionic carbon atom in 21 could attack the platinum centre directly, forming the cyclic intermediate 22. From here Pt-P bond dissociation would generate 20, which would furnish the product after reductive elimination. 

To test the validity of this mechanism, it was reasoned that a weak add (t-BuOH or water) should quench the zwitterion 21 and suppress or at least decrease the formation of by-products. This is indeed the case, although the addition of more alkene increases the quantity of by-products, even in the presence of t-BuOH. It should be noted that the presence of these protic additives is not innocent, since it also increases the reaction rates and affects the enantioselectivity. For example adding 20 equivalents of t-BuOH to the reaction of PH(Is)Me with tert-butyl acrylate halves the time for the completion of the reaction (from 5 to 2 days) and doubles the enantiomeric excess from 28% to 56%. The latter enantioselectivity is the best obtained to date with the systems discussed in this section. More evidence for the Michael addition mechanism came from trapping intermediate 21 with electrophiles other than a proton. Scheme 6.12 shows that performing the hydrophosphination reaction in the presence of benzaldehyde produced some of the three-component coupling product 25. 

Scheme 6.11 Mechanism for by-product formation in Pt-catalysed hydrophosphination of activated alkenes.

Platinum catalysts of the preceding section activate only the nucleophile (a secondary phosphine), which then reacts with the electrophile (an activated alkene). There are, however, interesting examples of lanthanide complexes activating both the nucleophile and the electrophile towards intramolecular diastereoselective hydrophosphination/cyclisations. In 2000 Marks and co-workers found that certain lanthanide complexes catalyse the intramolecular hydrophosphination of alkenes and alkynes (Scheme 6.13). 

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