Phosphinines electron-acceptors

The electronic properties of phosphinines are located between those of phosphines and phosphites, with somewhat more similarity to phosphites. In general, phosphinines are characterized by considerable it-acceptor properties and thus beneficial for several catalytic applications [32]. A theoretical study on Rh-phosphinine complexes provided evidence that the directionality of the ir-backdonation rather than the overall acceptor ability was responsible for the high catalytic activity [33]. In the rhodium-catalyzed hydroformylation of styrene with phosphinines as ligands, superior results in terms of conversion and TOP (turnover frequency) were noted in comparison to the use of PPhj or standard mono-triarylphosphites [34]. Atropisomeric 2-arylphosphmines have been prepared by Muller s group [35]. In the hydroformylation of trans-2-octene, a clear preference for the formation of 2-methyloctanal was noted. 

Table 6.1 shows typical values for phosphinines in comparison with phosphines and phosphites, which suggest that phosphinines are electron withdrawing ligands (7t-acceptors) with properties similar to phosphites as already anticipated from the corresponding MO scheme (Fig. 6.2). 

The / -coordination mode is typically observed in early transition metal complexes in order to compensate for the electron deficiency of the metal center (Fig. 6.9). This has been reported, for example, for Ti(0) and V(0) complexes (18), or in combination with additional yr-acceptor ligands as in LM(CO)3 complexes (M = Cr, Mo, W) (19) [44 7]. Moreover, the y/ -coordination mode can be imposed by steric effects as it has been found in complexes containing phosphinines with sterically demanding substituents, such as -butyl- or Me3Si- groups in ortho-position (20), which prevents cr-coordination via the lone-pair [48, 49]. 

Due to the strong Ti-acceptor properties of phosphinines and their preference to stabilize late-transition metals in low oxidation states, successful applications of these ligands might be anticipated in reactions in which the rate determining step leads to an increase in the electron density at the metal center. This has been explored in the Ni-catalyzed intramolecular [4 + 2] cycloaddition of dieneynes, in which the reductive elimination of the product from the metal center is most likely the rate determining step (Scheme 6.13) [24]. 

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