Hydrophosphination of alkenes

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

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). 

Diastereomerically pure catalysts 33 were obtained after recrystallisation and tested in intramolecular hydrophosphination of alkenes to yield phospholanes 35 (Scheme 6.14). 

Example 4.20 Hydrophosphination of Alkenes Using an Iron Catalyst [66]. 

Pringle et al. have reported Pt-mediated hydrophosphination of acrylonitrile as the first example of transition-metal-catalyzed hydrophosphination of alkenes [111]. Tris(cyanoethyl)phosphine complex of Pt catalyzed addition of H-P(CH2CH2CN)2 with formation of the P(CH2CH2CN)3. The reaction in acetonitrile was carried out at RT. Kinetic measurements and NMR studies suggested two parallel reaction pathways including mononuclear and dinuclear Pt intermediates (Scheme 8.43) [111-113],... 

Scheme 8.45 Proposed mechanism of Ni-catalyzed hydrophosphination of alkenes...

Rather unusual hydrophosphination of alkenes was reported using calcium-catalyzed transformation [131]. The reaction was carried out in benzene at 25-75 °C for 13-36 h and resulted in 78-95% conversion. Anti-Markovnikov addition products were formed with high selectivity. Proposed reaction mechanism involves insertion of the C=C bond into the Ca-P bond followed by metathesis stage (Scheme 8.48). The metathesis stage was found as rate determining for the catalytic cycle. Thus, if an activated alkene is utilized in the reaction insertion a more rapid insertion stage compared to slow metathesis stage results in formation phosphine-terminated polymers [131]. 

Mimeau D, Delacroix O, Gaumont A-C (2003) Regioselective uncatalysed hydrophosphination of alkenes a facile route to P-alkylated phosphine derivatives. Chem Commun 2928-2929... 

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... 

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 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]. 

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). 

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)...

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. 

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

The same reaction was performed in the presence of a source of Pd and a bidentate phosphine in toluene at 50 °C. Methylphenylphosphine borane (41a) was reacted with 1-octyne in the presence of palladium acetate (5%) and dppp (10%) for 35 minutes. Interestingly, the Markovnikov product 43a is obtained in 85% yield, with no trace of 42, meaning that in this case the phosphorus attacks the internal carbon of 1-octyne. The same reaction using bdpp (2,4-dw(diphenylphosphino)pentane) and phenylacetylene (39, R = Ph) also yields regioisomer of 43 with 53% yield. Several experiments with diphenylphosphine borane under different reaction conditions (palladium source, temperature, alkyne) showed that the reaction does not occur in the absence of palladium (0) and that hydrophosphination of 1-ethynylcyclohexene (42) undergoes selective hydrophosphination at the triple bond, with the alkeneic bond untouched. 

Other catalytic hydroelementations, E-H (E = P, B, Si, H) addition to C C multiple bonds, are efficiently mediated by organolanthanide complexes, and many are highly diastere-oselective. Hydrophosphination/cyclization appears to follow the catalytic patiiway similar to hydroamination/cyclization, whereas hydroboration, hydrosilylation, and hydrogenation proceed via different catalytic cycles, in which a lanthanide-hydride is the intermediate that is generated from protonolysis of the precatalyst. The proposed catalytic cycle of hydrosilylation of alkene using Cp LnCH(TMS)2 is presented in Scheme 5. 

Intermolecular Hydrophosphination of Activated Alkenes, Alkynes, and Diynes... 

In 2007, Hill and coworkers reported the intermolecular hydrophosphination of activated alkenes with diphenylphosphine catalyzed by [ (ArNCMe)2CH Ca N(SiMe3)2 (THF)] [106]. Reactions were conducted in benzene solution and... 

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