Phosphinates, monodentate

The addition of nitriles to 3,3-dihexylcyclopropene 11 produces the corresponding allylated nitriles (Scheme 2, Eq. 5) [10]. The diallylation product 12b is isolated in the reaction with the active methylene lb (Eq. 12). The non-ring-opened products 15 are produced in the Pd-catalyzed hydrocarbonation of methyleneaziridines 14 (Scheme 2, Eq. 6) [11]. Although this reaction can proceed in the presence of Pd catalyst only, the use of phosphine monodentate ligands provides better results. 

Attention should be paid to the fact that the ratio of Pd and phosphine ligand in active catalysts is crucial for determining the reaction paths. It is believed that dba is displaced completely with phosphines when Pd2(dba)3 is mixed with phosphines in solution. However the displacement is not eom-plcte[16]. Also, it should be considered that dba itself is a monodentate alkene ligand, and it may inhibit the coordination of a sterically hindered olefinic bond in substrates. In such a case, no reaction takes place, and it is recommended to prepare Pd(0) catalysts by the reaction of Pd(OAc)2 with a definite amount of phosphinesflO]. In this way a coordinatively unsaturated Pd(0) catalyst can be generated. Preparation of Pd3(tbaa)3 tbaa == tribenzylidene-acetylacetone) was reported[17], but the complex actually obtained was Pd(dba)2[l8],... 

The reactions of monodentate phosphines have been studied in most depth [58b]. The products of the reaction of a tertiary phosphine with RuC13 depends on both the phosphine concerned (size and reducing power) and upon the reaction conditions (Figure 1.22). 

It has 6-coordination with a chelating acetate [106] and may be converted (reversibly) into Ru(OAc)2(PPh3)3, which has the/ac-configuration with one monodentate and one bidentate acetate. It is fluxional at room temperature but at —70°C the phosphines are non-equivalent on the NMR timescale [107],... 

This will add halide ions or tertiary phosphines to give octahedral Ru(NO)Xj- or Ru(NO)X3(PR3)2, respectively, all of these having the linear Ru-N-O geometries characteristic of (Ru(NO) 6 systems. The preference for octahedral coordination is such that in Ru(NO)(S2CNEt2)3, one dithio-carbamate ligand is monodentate (Figure 1.43) [117]. 

The complex reacts with CO reversibly via a series of redox reactions. Rh(TMPP)2+ forms adducts with bulky isocyanides RNC (R = Bu , Pr ), retaining the +2 state but changing to a trans-geometry (Figure 2.34) with monodentate phosphines (and uncoordinated ethers) (R = Bu , Mefr = 2.04 B gj. = 2.45, g = 1.96). 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

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). 

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