Ligand hydroxyphosphines

On the way to water-soluble polyether phosphines Mayer and co-workers synthesized a tripodal polyhydroxyalkylphosphine ligand in a three-step synthesis starting from the relevant hydroxyphosphine (Eq. 2) [13]. The most interesting feature of this approach is the protection of the phosphine groups as a Mo(CO)3 complex during the synthesis. 

The desired acids 31 were obtained by simply bubbling CO2 through a solution of the deprotonated phosphine boranes 4 and could be quantitatively reduced to (3-hydroxyphosphines 32. The hydroxyl group of 32 was then mesylated or tosylated in good yields to alford compounds 33, which were employed to prepare other compounds like P/N and P/S bidentate ligands (see later in this section) and unsymmetric BisP ligands (Section 5.2.1.3). The yields of compounds 31-33 are listed in Table 5.7. 

A closely related cooperative effect has been described to explain the remarkable activity of hydroxyphosphine ligands such as 56 for the cross-coupling of aryl halides such as fluorides, chlorides, polyfluorides, polychlorides, and carbamates and phosphates [45] (Scheme 5.9). 

Mechanistic investigations have shown that reduction of Ni(II) to Ni(0) and deprotonation of the hydroxyphosphine ligand leads to a nickel-magnesium bimetallic species. Oxidative addition occurs through transition state TS2 wherein the bimetallic species performs a push-pull cooperative activation of the electrophile (Figure 5.2). This activation differs from the one observed when using... 

Scheme 5.9 Ni-catalyzed cross-coupling reaction of arylmagnesium reagents with aryl fluorides in the presence of hydroxyphosphine ligands [45].

Figure 5.2 Proposed transition state for the insertion of Ni(0) into the carbon-halogen bond in the presence of hydroxyphosphine and diphosphine ligands [45].

Probably the presence of a second ligating group in the ligand enhances the stability of phosphine oxide-rhodium complexes [177]. Thus, the group of Alper utilized successfully a-hydroxyphosphine oxides, which work, for example, in the reaction of HPPh2 with benzaldehyde (Scheme 2.53) [178], as promoting ligands in the rhodium-catalyzed hydroformylation [179]. 

Fora review of the synthesis of hydroxyphosphines, see Holz, J., Quirmbach, M. and Borner, A., Strategies for the synthesis of chiral hydroxyphosphines - a class of versatile Ugands and ligand precursors for asymmetric catalysis. Synthesis, 1997,983. 

Borner A (2004) Other concepts hydroxyphosphines as ligands. In COTnils B, Herrmann WA (eds) Aqueous-phase organometalhc catalysis, 2nd edn. Wiley-VCH, Weinheim, pp 187-193... 

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