Diphosphirenes

An intramolecular [4+2] addition between a diphosphirene and a P-cyclopentadiene substituent is believed to result in the formation of a tetracyclic P-C cage compound with a 1,2-diphosphetane moiety <1997PS545>. 

Reaction of the azaphosphirene complex 8b with i-Pr(Me3Si)NC=P leads to the displacement of benzonitrile with incorporation of the phosphaalkyne to give 96 as a representative of the rare class of diphosphirene complexes, as shown in Eq. (20).52... 

The l,2-dihydro-l,2,3-triphosphete complex 117 is isolated from the reaction of 8b with f-BuC=P in toluene at 80°C. In contrast with the formation of 96, a second phosphinidene unit is incorporated into the respective 1-H-diphosphirene complex see Eq. (25).52... 

It is conceivable that this transformation is initiated by a [2+1] cycloaddition of a transient phosphinidene complex with the phosphaalkyne to give the 2//-diphosphirene derivative 163. Ring opening and the subsequent... 

The scope of three-membered rings with two heteroatoms of heavier group 15 elements includes essentially heterocyclopropanes and heterocyclopropenes with two phosphorus atoms. This chapter also covers bicyclic and polycyclic molecules containing the structural motif of the rings under discussion. Such species frequently result from transformations of phosphaalkenes or oligomers. Monocyclic diphosphirane and diphosphirene derivatives are synthesized from acyclic precursors with a P-C-P linkage or alternatively via [2-1-1] cycloadditions. A few examples... 

Co-thermolysis of a P-pentamethylcyclopentadienyl-substituted 277-azaphosphirene complex 86 with alkyl-phos-phaalkynes furnished cage compound 88a and 88b featuring the structural motif of a diphosphirane complex. The key step of this reaction was the formation of transient diphosphirene complexes 87a and 87b which underwent a spontaneous intramolecular Diels-Alder reaction to the final product (Scheme 31) <1997PS545>. 

In this section synthetic aspects of free lf/-diphosphirenes A, diphosphirenium salts B, and of salts G containing two... 

The tautomeric 3//-diphosphirenes A are still unknown, however, postulated as intermediates in the generation of a trans-l,2,4,5-tetraphosphatricyclo[3.1.0.0]hexane < 1991 AGE90>. The first stable l//-diphosphirene 89 was prepared from a functionalized aminophosphane <1990CB1245> by reaction with an amino-substituted phosphaalkyne (Equation 8) <1989AGE1673>. 

From a formal point of view this process may be regarded as the [2-f 1] cycloaddition of a transient phosphinidene to the PC triple bond of the electron-rich phosphaalkyne. A little later a different approach to diphosphirenes was devised by Bertrand. When a toluene solution of phosphaalkene Pr NP=C(NPr2 )P(NPt2 )2 90 was treated with 2 equiv of boron trifluoride-diethylamine complex at room temperature, a clean reaction took place to afford the pale yellow crystalline diphosphirenium salt 91 (60% yield) <1991JA8160> <1994JA6146>. 

The reduction of 91 with LiAlH4 afforded the phosphaalkene EjZ H P=C(NPr2 )P(NPt2 )2 92, which upon treatment with boron trifluoride-diethylether complex was converted to the l//-diphosphirene 93 (Scheme 32) <1997CC2399> <1999EJI1479>. 

This section covers only transition-metal complexes which are directly formed during the synthesis of the heterocycles, or such which result from ligand displacement reactions with suitable metal complexes. Chemical transformations at the metal-bound three-membered ring with metal complexes whereby the structural integrity of the diphosphirene moiety is destroyed is considered in Section 1.16.5.2 in more detail. 

The first 177-diphosphirene complex 99 resulted from the [2-1-1 ]cycloaddition of an in situ generated phosphini-dene complex to P=C-Bu (Scheme 35) <1991CC1305>. 

As briefly indicated in Scheme 31, 377-azaphosphirene complexes also serve as sources for reactive W(CO)s-phosphinidene complexes under mild conditions, and may be intercepted by phosphaalkynes. Isolable diphosphirene complexes 87c and 87d were formed when the thermolysis of the precursors 86 [R = CsMes, CH(TMS)2] was performed in the presence of P=CN(Pr )TMS (Scheme 36) <1995CC2113, 1997PS545>. 

Diphosphirene 93 as well as the diphosphirenium salt 91 were converted into their respective pentacarbonyl tungsten complexes 104 and 105 by [W(CO)sTHF] in THF solution <1999EJI1479>. The 7 -iron tetracarbonyl complex 106 was readily obtained in 85% yield by reaction of 93 with 1 equiv of [Fe2(CO)9] <1999CC1535> (Scheme 38). 

The dinuclear complex 107 was isolated as yellow crystals in near quantitative yield, when a twofold excess of [W(CO)sTHF] was employed in the reaction with 93. The free l//-diphosphirene 93 and 1 equiv of [W(C0)4(THF)2] underwent reaction to give the dinuclear complex 108, which was obtained in 60% yield as yellow crystals <1999EJI1479> (Scheme 39). 

Based on spectroscopic and X-ray structural evidence, diphosphirenes such as 89 show the features of a phos-phaalkene with an inverse distribution of rt-electrons. Similarly, for diphosphirenium salts, structure 91 underlines the characteristics of a phosphinidene phosphorane <1989AGE1673, 1991JA8160, 2001AGE2471>. 

Table 2 Selected NMR data for some diphosphirenes and diphosphirene complexes...

The chemical properties of diphosphirenes, diphosphirenium, and diphosphirenylium salts were investigated in the last decade by Bertrand. 

As already described as a crucial step in the synthesis of diphosphirene 93 from diphosphirenium salt 91 ring opening of the latter species by LiAlH4 gave phosphaalkene H-P=C(NR2)P(NR2)2 (R = Pr ) in 80% yield <1997CC2399, 1999EJI1479>. 

Similarly, diphosphirenium complex 104 underwent reaction with LiAlH4 to afford phosphaalkene complex 155, and subsequently diphosphirene complex 105 (Equation 19) <1999EJI1479>. 

The singlet biradical 129, which is an analogue of the postulated /4tricyclohexylene has been isolated as a red crystalline solid in 45% yield from the reaction of diphosphirene 93 with catalytic amounts of BF3 (5%) and NEts (5%) in THE at 50°C (Equation 21). Radical 160 was postulated as the initially formed species <1998SCI2080>. 

In this section reactions of diphosphirene derivatives with transition metal carbonyl complexes, where also the ring skeleton was subject to transformations, are discussed. 

Interestingly, treatment of the mononuclear 1//-diphosphirene complex 106 with 2 equiv of trifluoromethanesul-fonic acid at -78 °C led to the diphosphirenylium binuclear complex 162. This species was not formed by the protolysis of 161. Instead, protonation of an iron center occurred leading to the hydridocarbonyl complex 163. Compound 162 was quantitatively converted into the neutral binuclear complex 161 by exposure to diisopropylamine. The tricoordinate phosphorus atom in 161 imparts ligand properties to the cage. This was substantiated by the synthesis of red trinuclear complex 164 when 161 was combined with an excess of [Fe2(CO)9] in THF at room temperature (70% yield) (Scheme 52) <1999CC1535>. 

This idea was supported by the reaction of diphosphirenylium salt 110a with Z equiv of diisopropylamine with water and bis(triphenylphosphoranylidene)ammonium chloride, or with tetrabutylammonium fluoride, whereby diphosphirenes 105, 127, 109, and 128 were formed quantitatively (Scheme 55) <1999EJI1479>. 

Methanolysis of the N-Si bond in diphosphirene complex 87d gave a 48% yield of complex 130 as a yellow solid (Equation 22) <2001AGE2471>. 

The first 1,3,4-triphosphole 172 was produced by the thermally induced regiospecific insertion of phosphaalkyne P=C-N(Pr )TMS into the P-P bond of the l//-diphosphirene complex 130 (35% yield). Interestingly, a similar ring expansion was not observed with the sterically more bulky diphosphirene complex 87d, which is the precursor of 130... 

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