Phosphenium

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. 

Electron density is transferred from a Lewis donor to a Lewis acceptor, here shown for an amine (donor) interacting with a phosphenium cation (acceptor). The lone pair orbital at the amine dives into the emptyp-orbital of the phosphenium cation, hollowing the ideas of Mulliken the donation towards an acceptor can be viewed as sketched in Scheme 2. 

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. 

According to these consideration the diamino-substituted phosphenium (an alternative suggestion for its nomenclature is phosphanylium) cation, 5, and the phosphanetriylammonium (iminophosphenium) cation, 6, possess the largest intrinsic (gas phase) stabihties. Since in the X-ray structures the molecules are to a first-order isolated, this theoretical stability scale determined for the gas phase should also mimic the various trends of the stabilities of the cations and their chelation behaviour. The methylenephosphenium, 7, and the PjH cations, 8, suffer from poor stabihties. On the other hand the phosphirenium cation, 11, is considered to be fairly well stabilized. It is due to n-electron delocalisation of the positive charge in the phosphirenium cation. Intermediate cases in stabihty are the PO+ (9) and PS+ cations (10). Of further interest are the frontier orbital considerations, as shown in Fig. 2. 

Another approach to a donor adduct of the methylene phosphenium cation is the addition of a phosphonium cation to the phosphaalkyne. The reaction of the protic cation [HPPhal + lCFaSOa] with CjoHuCP produced a white powder which was identified as the P-phosphonio-substituted phosphaalkene [74]. Alternatively to the elimination reaction the phosphaalkynes were protonated. C-protonation of adamantylphosphaacetylene and ferf-butylphosphaacetylene occurred in superacid media under formation of phosphavinyl cations. From these spirocyclic betaines by reaction of l-Ad-C=P (Ad = adamantyl) withB(OTf)3 a phosphavinyl cation could be detected [75]. 

In this progress report we have reviewed the latest developments in the large area of cationic low-coordinated species and their coordination with Lewis donors. It is clear that these species are of a broad interest, in particular for catalysis. In some cases, e. g. the methylene phosphenium cation, the donor adducts also open new routes for synthesis. Regarding the mechanism for the diverse donor-addition reactions, the structural details are only poorly understood and need a better classification. In particular the variation of the Lewis-donor has to be established. Hitherto in most cases iV-donation is studied. It includes amines or pyridines. Obviously the effect of other donors, such as phosphines, thioethers needs to be studied as well. The siliconium cation for which these effects are better known could provide an understanding for further investigations within this field. 

Useful reviews on areas of relevance for this chapter are one by Cowley and Kemp on the synthesis and reactions of phosphenium ions,... 

Phosphenium ion synthesis and reactions have been reviewed. A... 

Keywords 1,3,2-diazaphospholes 1,3,2-diazaphospholenes 1,3,2-diazaphosphinines 2,3-dihydro-l,3,2-diazaphosplinines diketiminto-phosphenium ions N-heterocyclic phosphines N-heterocyclc phosphenium ions 1,2,3-diazaphospholenium ions... 

Reactions of P-halogen-NHPs with Lewis acids leading to halide abstraction, or metathetic replacement of the halide by a nucleofugic, noncoordinating anion form by far the most important routes to access phosphenium ions in general [51, 52] and also 1,3,2-diazaphospholenium cations in particular. As Lewis acid assisted P-X... 

Quantitative assessment of the electrophilic character of various types of phosphenium ions has been attempted using computational studies on hydride and halide exchange reactions, and the results attribute to 1,3,2-diazaphospholenium ions a lower electrophilicity (and thus higher stability) than other types of phosphenium ions [20, 66], The gain in stability due to aromatic -delocalization is predicted to be somewhat larger than inductive stabilization resulting from exhaustive A-alkylation of the parent diaminophosphenium ion, [P(NH2)2]+. 

The direct synthesis of neutral phosphenium complexes was reported from ionic... 

Synthesis of 3-diketiminate stabilized phosphenium salts (R Dipp, C6F3 X=Br, Cl)... 

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],... 

Scheme 18 Synthesis of naphtho-annulated six-membered phosphenium heterocycles...

Apart from characterization of the individual types of six-membered heterocycles by routine spectroscopic methods, several cationic cyclic diketiminato-phosphenium ions and 1,8-diamidonaphthalene-derived P-chlorophosphines and phosphenium ions, respectively, were characterized by single-crystal X-ray diffraction studies. The P-halogen- and P-hydroxy-substituted cyclic diketiminato-phosphenium ions 28... 

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