Cationic phosphenium complexes formation

IV. Competitive Reactions with Cationic Phosphenium Complex Formation. . 120... 

COMPETITIVE REACTIONS WITH CATIONIC PHOSPHENIUM COMPLEX FORMATION... 

When E is Si or Ge, the cationic phosphenium complex (36) is stable and Fe-Si and Fe-Ge bonds remain unreacted.22,23 The formation of the silyl phosphenium complex, [Cp(CO)(SiMe3)Fe P(NMeCH2)2 ]BPh4 was confirmed by X-ray analysis.23... 

Eq. (22)].38 Although there is no evidence supporting the formation of a cationic phosphenium complex, it may be generated and serve as a catalyst for hydroformylation of styrene. 

Alkoxide anion abstraction by a Lewis acid from phosphorus ligand on a transition metal is one of the most useful methods to prepare cationic phosphenium complexes. It has been reported that OR anion abstraction from an alkoxyalkyl ligand, C(OR)R2, on a transition metal complex leads to the formation of a cationic carbene complex.39-42 It is similarly expected that OR anion abstraction from an alkoxysilyl ligand, Si(OR)R2, would cause the formation of a cationic silylene complex. Therefore, increased attention has been focused on the reaction with a Lewis acid of transition metal complexes bearing both alkoxyalkyl and phosphite ligands and of complexes bearing both alkoxysilyl and phosphite ligands. 

The donor-acceptor formation can be considered by transfer of electrons from the donor to the acceptor. In principle one can assume donor-acceptor interaction from A (donor) to B (acceptor) or alternatively, since B (A) has also occupied (unoccupied) orbitals, the opposite charge transfer, from B to A. Such a view refers to mutual electron transfer and has been commonly estabUshed for the analysis of charge transfer spectra of n-complexes [12]. A classical example for a donor-acceptor complex, 2, involving a cationic phosphorus species has been reported by Parry et al. [13]. It is considered that the triaminophosphines act as donor as well as an acceptor towards the phosphenium cation. While 2 refers to a P-donor, M-donors are in general more common, as for example amines, 3a, pyridines, 3b, or the very nucleophilic dimethylaminopyridine (DMAP) [ 14], 3c. It is even a strong donor towards phosphorus trichloride [15]. 

The opening of oxazaphospholidine rings (/ p)-122 with tert-butyllithium occurred diastereoselectively with retention of absolute configuration on the phosphorus atom, affording the borane complex of aminophosphine (Rp)-124 [67, 68]. The reaction possibly proceeds via formation of chiral o X -phosphenium cation 126, which was obtained from (Sc)-chlorophosphine 125 and then isolated as borane complex 127 (Scheme 38). 

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