Phosphenium phosphorus

Although electrophilic character is expected for cationic phosphenium complexes, only a few examples have been reported. Mer-10 readily reacts with Nu- (Me-, OEt-) to generate 29 [Eq. (20)].20 This shows that mer-10 is susceptible to nucleophilic attack at the phosphenium phosphorus. Complex 29 is not obtained in the reaction of the corresponding phosphite complex (30) with Me- or OEt-. 

When E is carbon, the produced phosphenium complex is too reactive to be detected. The alkyl ligand immediately migrates to the phosphenium phosphorus to give 37. The 16 electron species can be converted into an isolable complex 38 by the addition of PPh3.22,23... 

With a cationic phosphenium complex of a transition metal, 7c-donation from the transition metal to phosphenium phosphorus is also conceivable. A cationic phosphenium complex can be described in the resonance forms shown in Scheme 6. 58 corresponds to a complex where a plus charge is located on the phosphorus and a phosphenium cation coordinates to a transition metal through its lone pair. Thus, the bond between M and P in 58 can be seen as a dative bond. If a sufficient electron density flows from the filled d orbital of a transition metal into the vacant p orbital on the phosphorus, the plus charge would be located on a transition metal and the M-P bond would become a double bond (57). The Ti-electron donation... 

An amino substituent on the phosphenium phosphorus makes cationic phosphenium complexes stable, but the ji-donation from an amino substituent is only a minor contribution. So what is the role of an amino substituent The role may be, as is proposed for Arduengo carbene, to protect the approach of a nucleophile to phosphenium phosphorus by high p7i lone pair density flanking the phosphenium center.32... 

Synthesis and reaction chemistry of stable two-coordinate phosphorus cations (phosphenium ions). A. H. Cowley and R. A. Kemp, Chem. Rev., 1985,85, 367 (100). 

The symmetrical bis(ylidyl)phosphenium chlorides 103, obtained from the reaction of trimethylsilyl ylides 102 with PCI3 are the first phosphenium salts which do not need counterions of low basicity such as AICI4 to be isolated (Scheme 30) [119]. The explanation of their stability lies in the delocalisation of the phosphenium charge in the two phosphonium parts. The reactivity study of these species is reported and for example the phosphenium 103 (R=Ph) adds ortho quinones to the central phosphorus to give the corresponding dioxaphospholenium salts 104 via a [4-1-1] cycloaddition. 

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]. 

It is informative to begin the further discussion with a consideration of the various cations which will be the aim of the following sections. The most prominent examples of the low-coordinated cations in phosphorus chemistry are the phosphenium (phosphanyhum) cations. They were scrutinized in recent reviews [32-35], and hence will not be discussed in details in the present review. 

Heterocyclic phosphines 32 were prepared from base-induced condensation of a secondary 1,8-diamino-naphthalene with phosphorus trichloride (the corresponding As- and Sb-analogues were obtained analogously) and converted into cyclic phosphenium cations 33 by Lewis acid promoted halide abstraction using GaCl3 or trimethylsilyl triflate as reagents (Scheme 18) [15, 92],... 

Detailed investigations of the chemical reactivity of the diketiminato-stabilized phosphenium cations like 28 (Scheme 17) are to date rare and include only two reports dealing with the substitution and reduction of P-halogen-derivatives. Thus, reaction of 28 (X=Br) with sodium hydroxide in toluene was reported to proceed with displacement of the halide substituent at phosphorus and conservation of the heterocyclic ring to give a mixture of bromide and triflate salts containing a P-hydroxy-substituted cation, both of which were isolated in small yields [89], The products are remarkable as they represent one of very few examples of a stable phosphinous acid which does not rearrange to the tautomeric secondary phosphine oxide. Potassium reduction of the P-chloro-substituted derivative 34 produced the... 

This extremely air-sensitive compound, which is valence isoelectronic to an olefin, has been structurally characterized by X-ray diffraction. It has a short carbon-phosphorus double bond (1.62 A) the phosphorus and carbon atoms adopt a trigonal planar geometry with a dihedral angle of 60° (Fig. 3). This value is significantly larger than that reported for the most crowded olefin.61 Formally, this compound can be viewed as the product of a car-bene-carbenoid coupling between bis(trimethylsilyl)carbene and bis(diiso-propylamino)phosphenium triflate. Note that another route to methylene-phosphonium salt has been reported by Griitzmacher et al.62... 

The phosphorus ring member has phosphenium character. It adds tetrachloro or/Ao-quinone and 2,3-butanedione and it also reacts with dimethylaniline and anisole in a pura-substitution... 

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