Reactivity of phosphaalkynes

Phosphaalkynes without sterically demanding substituents can also be manipulated either in solution or by freezing out at low temperatures. They are often generated by thermal P-elimination of hydrogen halide under vacuum pyrolysis conditions [10]. 

halocarbenes undergo smooth addition to the P/C triple bonds of compounds 9 to furnish 1-phosphirenes (for intramolecular ring closures phosphavinylcarbenes to give the same products, see [16]), which rearrange to the thermodynamically more stable 2-isomers 11 by [l,3]-halogen shifts [17]. 

The cycloaddition behavior of phosphaalkynes toward 1,3-dipoles is particularly pronounced. Thus, reactions with diazo compounds give rise to the 37/-l,2,4-diazaphospholes 12 or their l//-isomers, respectively [16,18]. Azides [18a, 19], nitrilium betaines [18a, 20] mesoionic species [21], and sextet dipoles such as selenoxocarbenes [22] react analogously to form heteroatom-substituted phospholes. 

Phosphaalkynes also play a prominent role as dienophiles in Diels-Alder reactions hence the X -phosphinines 13 [23] are formed from cyclic 1,3-dienes such as a-pyrone or cyclopen-tadienones by way of extrusion of CO2 or CO, respectively reactions with anthracene provide an access to the phosphabarrelene series [24]. This type of reaction is also of significance for the construction of phosphorus-carbon cage compounds. The same is true for homo-Diels-Alder reactions which, with 2-phosphabicyclo[2.2.2]octa-2,5-diene as reaction partner, lead to the diphosphatetracyclodecenes 14 [25]. Last but not least, ene reactions with phosphaalkynes as enophiles [25] are also valuable for the construction of polycyclic phosphorus-carbon compounds. The reactions of 9 with 2,3-dimethyl-2-butene (- 15) [26, 27] emphasize this behavior. 

Finally, the cyclooligomerization of phosphaalkynes (e.g., - 16) [10, 28] in the coordination sphere of a metal should be mentioned. Zirconium complexes, which will be discussed in detail below, dominate the chemistry of phosphaalkyne cyclotetramers. 

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Methylidynephosphine (HC=P), the parent member of this class of compounds, is isoelectronic with acetylene. It stands at the very beginning of the history of phosphaalkyne chemistry 5 two decades passed before the successful synthesis of the kinetically stabilized compound 9 (R = i-Bu, Scheme 2)19,20 that is employed most frequently for studies on this class of compounds. Figure 8.2 shows a survey of the general reactions of this highly reactive triple bond system. 

A high-yielding synthesis was required before systematic investigations of the reactivity of the phosphacubane system could be realized. This was achieved by splitting the cyclotetrameriza-tion of the phosphaalkyne into two cyclodimerization steps the first step is the synthesis of the ziiconocene complex 59 and the second is the removal of its Cp2Zr fragment with subsequent renewed dimerization to furnish the tetraphosphapentacyclic system 53. 

In contrast to their aU-carbon analogues, phosphaalkyne cyclooligomers only became accessible a few years ago. A milestone in the chemistry of the cyclotrimers was the synthesis and structm-al characterization of the 1,3,5-triphosphinines 11, obtained by the trimerization of phosphaalkynes in the presence of a vanadimn catalyst. This review is focused on the reactivity of these new phosphorus heterocycles. 

Trialkoxy complexes of tungsten with terminal phosphido ligands could not yet be isolated. They were postulated to be very reactive intermediates in different transformation reactions, e.g., during the metathesis reaction of [W2(0R)6] with phosphaalkynes [6, 14]. However, we were able to characterize the complex [(t-BuO)3W=P] (3c) by P-NMR spectroscopy by monitoring the metathesis reaction of [W2(Ot-Bu)6] with MesC=P in the tempera-... 

In the first structurally characterized complexes of type A the metal-phosphorus triple bonds are kinetically stabilized by bulky substituents at the amido ligands. Therefore, these compounds reveal exclusively end-on reactivity via the phosphorus lone pair. This reactivity pattern seems also valid for the solution stable alkoxide derivative [(C/0)3Mo=P], for which the reaction potential is under investigation [13]. In contrast, due to their lesser degree of kinetic stabilization by bulky substituents the short-lived alkoxide containing complexes [(R 0)3W=Pj (R =t-Bu (3c), Ph (3d)), generated by the metathesis reaction between the alkoxide-dimer and the phosphaalkyne (cf. Eq. 8), show additionally a high side-on reactivity towards the phos-phaalkynes of the reaction mixture. Thus, there occurs a formal cycloaddition reaction with the phosphaalkynes, and a subsequent 1,3-OR shift yields the formation of four-membered diphospha-metallo-cyclobutane derivatives 6(Eq. 8) [15,31, 37]. 

Turning from iminophosphanes to alkylidenophosphanes (phospha-alkenes), the orientation of the [2 + 2]-cycloaddition is inverted, as far as phosphorus is concerned only one example has been worked out (product VIII) 19). The phosphaalkyne iBuC=P does not react with the iminoborane BuB=NtBu, which instead trimerizes (IS). An exotic [2 + 2]-cycloaddition is observed when the very reactive titanaethene... 

Under harsher conditions (120 °C in toluene) phosphaalkynes 209 exhibit an analogous reactivity toward elemental tellurium. The previously unknown 1,2,4-telluradiphospholes 74, 304, and 305 were obtained in 15-20% yield along with oligomers of the phosphaalkynes (Equation 42). 1,2,4-Telluradiphospholes 74, 304, and 305 are thermally labile and decompose on exposure to light with deposition of elemental tellurium. 

In contrast to phosphaalkynes, nitriles show quite a different chemical reactivity towards lithium trimethylsilylphosphanides. Whereas with benzonitrile and one equivalent of the lithium phenyltrimethylsilyl compound l-[(l,2-dimethoxyethane-0,0 )hthium-trimethyl-silylamido]benzylidenephosphane is formed, l-(l,2-dimethoxyethane-0,0 )lithium bis(trimethylsilyliminobenzoyl)phosphanide has been isolated from a similar reaction with lithium bis(trimethylsilyl)phosphanide in a molar ratio of 2 1. Solvent coordinate lithium is not bound to phosphorus, but to both the nitrogen atoms. Protonation gives the related bis(trimethylsllyliminobenzoyl)phosphane, which exists only as imino-enamine tautomer in the solid as well as in even very polar solvents. 

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>. 

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