Monodentate phosphine ligands

Substitution of monodentate phosphine ligands by other monodentate phosphines is slow ... 

Following the general trend of this account, monodentate phosphinous amide ligands and bidentate AT-phosphino phosphinous amides or bis(amino-phosphanes) are included in the following discussion, but not other bidentate ligands bearing additional, different phosphorus functionalities, as for instance phosphinous amide-phosphane bidentate ligands. 

The change in the group trans to the silyl ligand is thought to account for the reversal of substitution pattern in the case of the monodentate phosphine 63). Six-coordinate Pt(IV) intermediates have been postulated for these Pt(II) reactions (27, 71). Such an intermediate has recently been isolated [Eq. (62)] (27). 

Other dichloro(ditertiary phosphine)nickel(II) complexes (see Table VI) catalyze both hydrosilylation and H/Cl exchange, but analogous complexes containing monodentate phosphine ligands or bidentate amine groups are essentially inactive (173). 

Experimental studies show that chelating spectator ligands impart a degree of stability to complexes of type 23 (Scheme 13.10) [42]. If monodentate phosphine ligands are used decomposition is rapid at 20°C, however, using dppp no decomposition is detected after 24 h [19]. It was found that the rate of decomposition could be linked to the chelate ring size at 65°C, with dppp decomposition was complete after 6 h, with dppe only a small amount of decomposition occurred after this time [42]. 

Use of the above conditions in conjunction with the enol tosylate 32, provided only low yields of 22, prompting an extensive screening of structurally diverse phosphine ligands/solvents and palladium sources to attempt to define suitable conditions. Quite quickly a number of conditions were found to be effective, with chelating diphosphines being superior to monodentate phosphines (Table 9.7). In... 

It should be noted that the Grob fragmentation reaction and the reductive cyclization (homoallylation) discussed in this section involve the same oxanickellacyclopentane 66 as a common intermediate (Scheme 17). The reversibility of these C - C bond cleavage reaction and C - C bond formation reaction is also supported by the isolation and characterization (by X-ray analysis) of an oxanickellacyclopentane-like 66 (without a tether), which is prepared from a stoichiometric amount of Ni(cod)2, a diene, an aldehyde, and a monodentate phosphine ligand [41]. 

The bridging PBu 2I I molecule is not as tightly bonded as other bridging bidentate ligands. This can easily be confirmed by comparing the reactivities of monodentate phosphines.832 The strength of the overall Pd2(/ -PBu 2[ I) interaction is greater than that of a terminal Pd—Bu 2H bond.832... 

The hemilabile phosphino-ester and phosphino-thiophene ligands (139)-(142) behave like monodentate phosphine under catalytic conditions.476... 

The Co system is more reactive as well as much more selective than the Ni and Rh catalyst systems (Table XVII). The best systems allow almost 100% conversion with almost 100% yield of c -l,4-hexadiene. The best of the Ni and Rh systems known so far are still far from such amazing selectivity. The tremendous difference between the Ni system and the Co or Fe system must be linked to the difference in the nature of the coordination structures of the complexes, i.e., hexacoordinated (octahedral complexes) in the case of Co and Fe and tetra- or penta-coordinated (square planar or square pyramidal) complexes in the case of Ni. The larger number of coordination sites allows the Co and Fe complex to utilize chelating phosphines which are more effective than monodentate phosphines for controlling the selectivity discussed here. These same ligands are poison for the Ni (and Rh) catalyst system, as shown earlier. 

This section essentially catalogs some of the newer catalyst systems that have not been considered in the previous sections. A number of the catalysts are certainly derived from more established ones (e.g., use of chelated aminophosphine ligand instead of two monodentate phosphines... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

In 1968, Knowles et al. [1] and Horner et al. [2] independently reported the use of a chiral, enantiomerically enriched, monodentate phosphine ligand in the rhodium-catalyzed homogeneous hydrogenation of a prochiral alkene (Scheme 28.1). Although enantioselectivities were low, this demonstrated the transformation of Wilkinson s catalyst, Rh(PPh3)3Cl [3] into an enantioselective homogeneous hydrogenation catalyst [4]. 

Scheme 28.3 Chiral monodentate phosphine ligands (men = menthyl, see 2).

Recently, two new P- and C-chiral monodentate phosphines 13 were reported. The ligands were applied in a number of transition metal-catalyzed reactions, though ee-values in the rhodium-catalyzed hydrogenation of N-acyl dehydrophenylalanine were only moderate [37]. 

Table28.8 Enantioselective hydrogenation of/l-keto esters using monodentate phosphine ligands.

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