Phosphenium cation

Phosphenium cations PHJ and PCIJ (3.3d) are unstable species which have only been observed in the mass spectrometer (Chapter 14.2). Like phosphinidenes they may also be based on six-electron valence shells. Some derivatives R2P are known and cations such as (Me2N)-P+-(NMe2) form quite stable salts (7.123). 

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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]. 

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. 

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... 

Only limited number of neutral monocyclic 11,3,2 diazaphosphole representatives have been reported, which have mostly been prepared by [4+1] cyclocondensation of diaminomaleodinitrile (DAMN) with P(III) reagent and the alkylation of the initially formed 1,3,2-diazaphospholide [2, 4, 7], During recent times, 67t-aromatic [l,3,2]diazaphospholium ions of type 46 [45], more often represented as cyclic phosphenium cation 47 [46,47], have attracted more attention due to their isoelec-tronic nature with Arduengo carbenes . Nature of bonding and aromaticity of these cations have been the subject of several experimental and theoretical studies (Structure 2) [48-52],... 

Electrophilic addition to spiro phosphenium cations 258 provides a route to spirophosphoranes. However, only very active electrophiles are suitable, for example, benzenesulfonyl azide which on reflux in acetonitrile with 258 yields 259 (Equation 70) <1998RJC530>. 

Related cations based on three- or four-membered polyphosphorus rings are obtained when sterically strained alkyl-substituted cyclo-tri- or -tetraphosphines are treated with methyl trifluoromethanesulfonate (MeOTf) or an in situ-generated phosphenium cation R2P (Scheme 11.2). ... 

The phosphenium cation [(Cy3N)2P]+ (Gy = cyclohexyl) adds to the silicon atom in silylene 83 to give the transient cation 100. The cation 100 is trapped by chloride anion and the chlorosilane 101 was observed as the final product (Scheme 7) <2004CC546>. 

Phosphide anions are well documented and are used widely as reagents. However, much less is known concerning phosphinyl radicals and phosphenium cations. 

Interesting work by Lappert, Goldwhite, and co-workers (1,2) has established that phosphinyl radicals can be stabilized by means of bulky substituents such as (Me3Si)2CH. We have found that these radicals constitute a new class of ligand intermediate in character between phosphenium cations and phosphide anions. 

Phosphines are classical Lewis bases or ligands in transition-metal complexes, but the cationic species shown in Fig. 15.4.l(i) are likely to exhibit Lewis acidity by virtue of the positive charge. Despite their electron-rich nature, an extensive coordination chemistry has been developed for Lewis acidic phosphorus. For example, the compound shown below has a coordi-natively unsaturated Ga(I) ligand bonded to a phosphenium cation it can be considered as a counter-example of the traditional coordinate bond since the metal center (Ga) behaves as a Lewis donor (ligand) and the non-metal center (P) behaves as a Lewis acceptor. 

Phosphines and their derivatives are known to be very useful ligands toward transition metals and a variety of complexes with phosphine as a ligand have been prepared for all kinds of transition metals. If one of the substituents on a coordinating tertiary phosphorus compound is abstracted as an anion, it would form a cationic phosphenium complex. Actually this strategy has been widely used, and halide, hydride, and alkoxide have been abstracted as an anion by an appropriate Lewis acid. An alternative method to prepare cationic phosphenium complexes is a direct reaction of a phosphenium cation with a transition metal complex having appropriate... 

A phosphenium cation reacts with a transition metal complex having an appropriate leaving ligand such as carbonyl, phosphine, and acetonitrile to generate a cationic phosphenium complex [Eqs. (10)-( 14)].2,26-29 This reaction seems to be the simplest method for the preparation of a cationic phosphenium complex. The drawback of this method is that known stable phosphenium cations are limited in number. 

As shown in Eq. (14), W(bpy)(CO)3(MeCN) reacts with the phosphenium cation (17) to give a cationic phosphenium complex (18). The same phosphenium complex (though the counter anion is different) (19) is also prepared in the reaction of the same starting W complex with the corresponding chlorophosphine [Eq. (15)].29 In this reaction, P-Cl heterolytic cleavage spontaneously occurs to form the phosphenium complex. Complex 19 is unstable and is gradually converted into 20 by CP/CO exchange. 

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... 

P increments (a -P), phosphenium cations, phosphaalkenee and heteroatom analogues, X3-phosphinines, phospholes with further heteroatoms in the ring (cr2-P) and, finally, the large field of phosphoranes with two, doubly bonded other elements as well as the X.5-phosphaalkynes (cx3-P).8... 

Phosphenium cations of the general formula [R-P-RJ+ with an electron-sextet at the phosphorus atom can formally be generated by heterolytic cleavage of the P-X bond in precursor compounds R2PX, and in many cases the synthesis follows this route. The [R2P] species are analogs of the neutral carbenes [R2C] and hence of great current interest (74a,b). 

Because of the high electron deficit at the two-coordinate phosphorus atom, phosphenium cations are only stable with electron-donating substituents R, predominantly with nitrogen donor atoms. This is an obvious parallel to the situation with stable carbenes of the Wanzlick-Arduengo type. Therefore most complexes in this series are derived from diazaphospholes (75a,... 

The first aliphatic phosphenium cations involving a phosphorus-sulfur bond, e.g. (330) have been prepared and characterised. The stereochemistry, regioselectivity and mechanism of the insertion of the diisopropylamino (chloro) phosphenium cation into the phenylcyclopropane ring, to form phosphetanes, has been studied,Ligand-exchange processes involving mono- and bis-adducts of phosphenium ions have received a theoretical treatment. The coordination chemistry of the carbene adduct (331) of the... 

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