Phosphirene

Regitz noted the importance of the resonance (Scheme 9) in pentacoordinated phosphirenes for the short C-P bonds and the long C=C bond [39] as compared to the corresponding bonds in 1/f-phosphirene. 

Kawashima reported the structure of 1-phenyl-l//-phosphirene 102 which has a distorted square-pyramidal structure (Scheme 35) [82, 83], The observed elongation of the C=C bond has been interpreted in terms of the tt-o interaction in similar ways by Regitz [39] for cyclic phosphirene and by Clark [14] for phosphirenium ions 17. 

In contrast to the cyclopropenylium ion, which has been known for a long time, its phosphorus analogue was first synthesized just recently. The phosphirenyHum cation, 32, can be obtained by addition of the strong Lewis acid B(OTf)3 to a solution of the IH-phosphirene, 31, in liquid SO2 [77] (Scheme 19). 

It is the only example of a free, persistent phosphirenylium ion, and also, only one stable transition-metal complex of this species was published [78,79]. Quantum chemical calculations [80,81] indicated that in the halogeno-phosphirenes the P-X bonds already possesses a high ionic character and can be described as interactions between phosphirenylium and halide ions. The aromatic character of the phosphirenylium ion was shown to be based on a three-centre two-electron bond of 7i-type and the resonance energy was assessed by calculation to 38 kcal/mol. Before the generation of 32, substituted phosphirenylium ions were... 

The most widely employed electrophilic phosphinidene complexes are generated from M(C0)5 (M=W, Mo, Cr) complexed 7-phosphanorbornadienes 7, which were reported by Marinetti et al. in 1982 [46]. They trapped the transient phosphinidene complex 8 at 110 °C (or at 55 °C if CuCl was used as catalyst) with olefins and alkynes to obtain three-membered phosphiranes and phosphirenes, respectively. 

Recently, the group of Lammertsma developed an alternative route, using Collman s reagent and dichlorophosphine 17, to generate Fe(CO)4 complexed phosphinidene 18 in situ [61]. This reactive amino substituted species is trappable at about 0 °C with alkynes and terminal alkenes to give stable phosphirene... 

When 1,3-dienes remain in an anti orientation, inhibiting a [l,3]-shift of the first formed vinylphosphirane, a second phosphinidene addition can occur to give bisphosphiranes 35 [80]. Bis- and trisphosphirenes (36-38) result from addition to alkyne substituted phosphirenes (39) [81,82]. 

The chemistry of the phosphirene system is attracting increasing attention, with the eventual goal of preparing the potentially aromatic Huckel system (214). A simple preparation of tervalent phosphirenes (215) is afforded by the reactions of dichloro-phosphines with alkynes in the presence of aluminium chloride,... 

Borst et al. <2005CEJ3631> conducted a study on the synthesis of strained bicyclic phosphirane and phosphirene iron-tetracarbonyl complexes (Scheme 11). It was shown that, depending on the ring size of the resulting heterocycle, electrophilic phosphinidene [Ri-PrNP=Fe(CO)4] could be trapped intramolecularly with both double and triple bonds (compounds 146-150). The phosphinidene addition was found to be reversible at room temperature and when using phenylacetylene as solvent, exchange between phenylacetylene and the phosphinidene group took place. Compound 151 was isolated as the dimer, compound 152. 

Similarly, the (phosphino)(silyl)carbene 2a reacts at -30°C with a slight excess of the tert-butylphosphaalkyne cleanly affording the 2-phosphino-2//-phosphirene 34.53 The reaction leading to 34 is strictly analogous to that observed on reacting the transient dichlorocarbene with the tert-butyl-phosphaalkyne, in which the 2//-phosphirene 36 was obtained.54 The three-membered heterocycle 34 appeared to be rather unstable and rearranged, quantitatively, to afford the lA5,2A3-diphosphete 35 after 3 h at room temperature.55 Once again, these results as a whole indicate that a concerted [1 + 2]-cycloaddition process is involved in the formation of the 2//-phosph-irene 34. 

The phosphirene cation (5) (with two resonance structures 5a and 5b) has been predicted to possess a Zji aromatic system for quite some time." The aromaticity of the corresponding cyclopropenylium cation (C3H3+) is well-known.Substituted derivatives of 5, however, have only recently been observed in solution and in complexed form. 5 is the global... 

The potential usefulness of 3//-l,2,4-diazaphospholes for the synthesis of other phosphorus heterocycles was demonstrated using 234 (Scheme 8.55) which, after UV irradiation at —40°C, gave a 5 1 mixture of 4,5-dihydro-3//-phospholes 235 and 236. The latter compound is the hrst example of the previously unknown 2H-phosphirene system (288). This reaction probably proceeds via a photochemical ring opening of 234 and formation of a phosphavinyl carbene. 

The higher reactivity of ring-strained olefins has been exploited by several workers in 1,3-dipolar cycloaddition reactions of milnchnones. Thus, Kato and co-workers (112) reported that miinchnone 38 reacts with 1,2,3-triphenyl-1//-phosphirene (202) to give l-methyl-2,3,4,5-tetraphenylpyrrole (203) (45% yield). Control experiments demonstrated that phosphirene 202 does not decompose to diphenylacetylene appreciably under the reaction conditions. Moreover, the reaction of diphenylacetylene and miinchnone 38 afforded only a 21% yield of pyrrole 203. 

Silyl substituted lf/-phosphirene 40 undergoes photochemical ring expansion to the 1,2-dihydro-1,2-phosphasilete 41 with cleavage of a silicon-silicon bond <99MI1581>. 

Reactions related to cyclopropanation can also be carried out with (phosphino) (silyl)carbenes (I). For example, benzaldehyde reacts with la at 0 °C leading to the corresponding epoxide, again as only one diastereomer. Even more striking are the reactions with benzonitrile and tert-butylphosphaalkyne that lead initially to azirine and phosphirene. Both three-membered heterocycles subsequently undergo ring expansion reactions affording azaphosphete and diphosphete, respectively (Scheme 8.18). This reaction is a new route for the synthesis of heterocyclobutadienes, and this demontrates the usefulness of (phosphino)(silyl)carbenes (I) for the synthesis of novel species. 

Phosphiranes, with iron carbonyls, 6, 40 Phosphirenes with iron carbonyls, 6, 44 with tungsten carbonyls, 5, 623 Phosphites... 

The unsaturated three-membered ring phosphirene (5) is nonaromatic in the n-system (similarly to cyclopropene), since the lone pair of the non-planar tricoordinate phosphorus is not available for 71-interaction [96-98] (Scheme 8). 

Unlike 5, the phosphirene cation (7), in analogy to the corresponding cyclopro-penylium cation [101], has been predicted [102] and shown [103] to have aromatic... 

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