Phosphinine-phosphites

The first successful asymmetric reaction using a chiral phosphinine-based ligand was reported by our group [32]. The chiral BINOL-substituted phosphinine-phosphite 11 (Fig. 6.5) was obtained quantitatively from the hydroxy-substituted phosphinine 36 by reaction with the corresponding phosphorchloridite in the presence of an amine (Scheme 6.15). 

Rhodium complexes of phosphorus based ligands are of considerable importance as (pre-)catalysts in hydroformylation which has developed into one of the most important homogeneous catalytic processes [57]. A recent advantage in this field involved the use of phosphinines as ligands whose low-lying r -orbitals provide for similar r-acceptor qualities as for phosphites [13, 58]. 

Ethoxycarbonylpyridinium cations are attacked by phosphites (Scheme 31). The intermediates can be further reacted to give 4-alkylpyridines as shown. Pyrylium salts with nucleophilic phosphines yield phosphinines 274. 

Phosphole sulfides reacted with diazoalkanes to give homophospholes which were transformed to phosphinines. Thus, 3,4-dimethyl-l-(methylsulfanyl)phosphole 1-sulfide on heating with ethyl diazoacetate reacted to give the homophosphole which rearranged on treatment with triphenyl phosphite to afford ethyl 4,5-dimethylphosphinine-2-carboxylate in good yield (Table 4). ... 

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). 

Using a 1/Ni catalyst, a maximum conversion of 92% within 20 h in cyclohexane could indeed be observed (T = 80 °C, 10% Ni(COD)2). Under the same reaction conditions, lower conversions (70-89%) were achieved with the bulky phosphite P(0-o-C6H4Ph)3. The reaction is strongly solvent dependent and a significant increase in rate was observed for the phosphinine/Ni system in passing from a coordinating solvent (THF, 7% conversion) to the non-coordinating solvent cyclohexane (63-92% conversion). 

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