Secondary phosphine ligands

Several low-spin cobalt(II) cyanide complexes containing tertiary or secondary phosphine ligands, and corresponding to the general formula [Co(CN)2P3] have been prepared by halide-cyanide exchange (CN -form anion resin) from tetrahedral [C0X2P2] complexes (X = Cl, Br) and an excess of phosphine (Table With ditertiary phosphines the reaction gives either diphosphine-... 

In the IR spectra of the complexes, containing primary and secondary phosphine ligands the bands of P-H stretching modes are observed, besides the bands of Cp ring. The v(P-H) frequencies (2305 cm ) in the complexes are 50 cm higher than those for free phosphines (2253 cm ). The increase of frequencies is a consequence of the formation of the P — Ln bond. 

One report details the unusual transformation of an t] -bound dithiophosphinate moiety in [W(CO)5 Ti -P(Ph)2CS2 ] into dithiocarbamate and secondary phosphine ligands upon addition of c/r-[W(CO)4(pip)2] (pip = piperidine) (Eq. 43) (229). It is not obvious how this transformation proceeds, but it may involve a binuclear intermediate. 

P-Chirogenic diphosphine 19, which rhodium-chelate complex forms a seven-membered ring (rare case for P-stereogenic ligand), was also prepared in reasonable yield (68%) using the wide chemistry of secondary phosphine borane [37]. Deprotonation of the enantiomerically enriched ferf-butylmethylphos-phine-borane 88 (Scheme 15) followed by quenching with a,a -dichloro-o-xylene and recrystallization afforded optically active diphosphine-borane 89 (precursor of free phosphine 19). 

Very few reports concerning transformations of ligands with other donor atoms exist (Table V). P—H activation at secondary phosphines is the most common motif, with the metal-metal bonds at the heterometallic faces stabilizing the resulting fragments in each case (Figs. 31. 32, 33).In the formation of both... 

In the framework of this concept a series of new mixed P/N ligands was prepared, employing the phosphorus-analogous Mannich reaction (Scheme 9). This transformation permits the substitution of primary or secondary amines by methylenephos-phine residues -CH2PRR (R, R = alkyl, aryl) through reaction with a secondary phosphine and formaldehyde (46). Based on the... 

Bismuth phosphine complexes represent a substantial component of the established phosphine complexes of heavier p-block elements, and an excellent overview has presented an important bonding model for these systems (7). The observed structures are considered as trigonal-pyramidal BiX3 units with three secondary trans bonds. If the acceptor orbitals are the Bi-X trans arrangement is expected, as the relationship between the trans X-Bi-P bond distances. The shortest Bi-P distance [2.7614(2) vs 2.866(3) A] is trans to the longer Bi-Br distance [3.403(1) vs 2.9916(1) A], as the only arrangement that will allow the phosphine ligands to occupy trans... 

Secondary phosphine oxides are known to be excellent ligands in palladium-catalyzed coupling reactions and platinum-catalyzed nitrile hydrolysis. A series of chiral enantiopure secondary phosphine oxides 49 and 50 has been prepared and studied in the iridium-catalyzed enantioselective hydrogenation of imines [48] and in the rhodium- and iridium-catalyzed hydrogenation functionalized olefins [86]. Especially in benzyl substituted imine-hydrogenation, 49a ranks among the best ligands available in terms of ex. 

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

By Addition ofP—H to Unsaturated Compounds. This route continues to be exploited for the synthesis of polydentate tertiary phosphine ligands. Thus base-catalysed addition of diphenylvinylphosphine to the secondary phosphine (25) affords (26).23 Neopentylpolytertiaryphosphines, e.g. (27), have been similarly prepared24 by addition of primary or secondary phosphines to vinylphosphines (or the related phosphine sulphides, followed by a desulphurization step). 

Similarly, secondary phosphines and a,a -dibromo-m-xylene usually react to give diprotonated ditertiary diphosphines [44-48]. However, in the case of the fluorous secondary phosphine 12-Rfs (Scheme 2, bottom), dialkylation occurred to give the metacyclophane 13-Rfs. As detailed elsewhere [40], extensive efforts to adjust the stoichiometry to favor noncyclized products failed. Fortunately, the reduction of 13-Rfs with IiAlH4 gave some of the target ligand lO-Rfs. 

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