Reactivities phosphaalkynes

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. 

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. 

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. 

Since alkylidene- and alkylidynephosphanes show an alkene- or alkyne-like chemical reactivity, they are often called phosphaalkenes and phosphaalkynes. 

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>. 

With regard to their reactivity, the phosphaalkynes 9 reveal more parallels with the alkynes (as is already indicated by the name) than with the nitriles [15] (Scheme 6-3). 

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. 

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