Tetraphosphines

The coordination behavior of tetraphosphines with rhodium depends on their geometric structure. Ligands without any steric restrictions may form catalytically inactive rhodium complexes where four ligating atoms block the coordination of substrates and/or reagents. Only by dissociation of two phosphine groups from the metal center, this inhibition can be suspended. The situation is similar to that described with triphosphines previously. 

Et2PCH=CH2, AIBN, cyclohexane, about 13 Pa, reflux to 170 °C Et2P-v / p-  

In the hydroformylation of terminal olefins, superior results in comparison to BISBI at higher temperature were noted l/b = 54.2 in comparison to BISBI Hb = 2.4) [66]. Moreover, with these tetraphosphines more than 95% linear selectivity and up to 94% yield of total aldehydes starting from 2-alkenes (2-pentene, 2-hexene, 2-octene) were observed in isomerizing hydroformylation [67]. The M-regioselectivity increased in relation to the nature of Ar in the following order, indicating a remarkable electronic effect  

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Valence Isomerism. Restricted Rotation and Permutational Isomer i sm. - The variable temperature spectra of bicyclic tetraphosphines (29) has been analysed in terms of valence isomerism involving the corresponding two coordinate open-chain isomers.62 A report on ab initio MO calculations of the inversion barriers for a series of methyl phosphines includes a discussion of the electronic consequences of steric effects.63 Inversion barriers of 1,2-diphosphinobenzenes are in the range 100 - 110 kJ mol". 66 The ring inversion barrier for a dibenzophosphorin has also been measured.63... 

The complex consists of a bis(diphosphinomethanide) ligand binding a lithium atom at each end in an t/2-PP coordination mode [Li-P = 2.542(6) and 2.559(6) A]. The lithium atoms achieve four-coordination through the coordination of two molecules of THF each. Hydrolysis of this complex is an excellent route to the otherwise inaccessible tetraphosphine (Ph2P)2CHCH2 2, a potentially useful polyphosphine ligand analogous to two linked bis(diphenylphosphino)-methane (dppm) moieties. 

Later, this study was extended to other tripodal tetraphosphines such as PP3 [PP3 = P(CH2CH2PPh2)3] and other transition metals (Fe, Ru, Os), as Ir was observed to be inactive in the presence of such ligands [46]. In the case of the latter system, a vacant site for substrate coordination was easily generated by the loss of a weakly bound ligand from the metal precursor, that is [(PP3)MH(H2)]L... 

A new family of chiral ligands for asymmetric homogeneous hydrogenation has been developed. The performance of mono- and bis-rhodium complexes of these chiral ferrocene tetraphosphine ligands in the hydrogenation of model substrates was surveyed in comparison to their ferrocene bis-phosphine analogs. 

Figure 1. Structures of chiral ferrocene bisphosphine and tetraphosphine ligands. Results and Discussion...

Figure 2. Binding modes for mono-Rh and bis-Rh complexes of ferrocene tetraphosphine ligands.

Thus, while the behaviour of the mono-rhodium tetraphosphine complexes in homogeneous hydrogenation may be expected to correspond to that previously observed for Josiphos or Mandyphos if the additional phosphine act purely in a spectator mode, new behaviour may be observed if either of the two new cross-... 

The six ligands studied in these investigations are shown in Figure 3. For each new ferrocene tetraphosphine prepared, we evaluated the hydrogenation behavior relative to the corresponding diphosphine of the Josiphos or Bophoz type. 

For our initial studies we chose to evaluate the hydrogenation of two unsaturated carbonyl model prochiral substrates with rhodium complexes of chiral ferrocene diphosphine and tetraphosphine ligands using a standard set of conditions. The substrates screened were methyl a-acetamido cinnamate (MAC) and dimethyl iticonate (DIMI). The substrates, catalysts, conditions, and experimental results are shown in Table 1. 

The hydrogenation reactions were all performed at the same substrate-to-rhodium ratio and, since the tetraphosphine ligands 2, 4, and 6 can form bidentate... 

As seen in Table 1, the mono- and bis-rhodium complexes of tetraphosphine 2 provide similar enantioselectivities in the chiral hydrogenation of both substrates as the rhodium complex of the diphosphine (Josiphos) ligand 1 does. The bis-rhodium complex of 6 provides higher conversion but similar enantioselectivity as the rhodium complex of the diphosphine (Bophoz) ligand 5 in the chiral hydrogenation of MAC. 

From these initial survey results, we postulate that the dominant species formed by the rhodium complexes of tetraphosphine ligand 2 are the same as the rhodium complex formed by Josiphos ligand 1 and the two catalytic sites in ligand 2 act essentially independently. 

However, because the rhodium complexes of tetraphosphine ligand 4 behave differently than the rhodium complex of Josiphos ligand 3 for both substrates, we postulate that either a) different rhodium complexes are being formed (different binding modes as discussed above, see Fig. 2) by 3 and 4, or b) the substituents on the second cyclopentadiene (cp) ring influence the catalytically active site in the rhodium complexes of 4. 

The major resonances suggest that complexation of two Rhodium ions by the tetraphosphine ligand 4 occurs in the same manner as binding of one rhodium by the diphosphine Josiphos ligand 3 (see Fig. 2). While the minor resonances observed... 

Related cations based on three- or four-membered polyphosphorus rings are obtained when sterically strained alkyl-substituted cyclo-tri- or -tetraphosphines are treated with methyl trifluoromethanesulfonate (MeOTf) or an in situ-generated phosphenium cation R2P (Scheme 11.2). ... 

Actually a square pyramidal structure has been found in the complex [NiBrLJBr (168) which has been prepared with the tetraphosphine ligand Me4p41377 and in the complexes [NiXL]BPh4 (169) with the mixed-donor ligand 2,3,2-p2S2.1382,1383... 

Meek s free radical-catalyzed reaction of PhHPCH2CH2CH2PHPh with Ph2PCH=CH2 yields a tetraphosphine with a central trimethylene backbone.65 As before, dialkylphosphino groups may be incorporated (equation 25).64... 

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