Phosphasilenes synthesis

An alternative approach was achieved by the one-pot reaction of dichlo-rosilane with RPH2 in the presence of two equivalents of nBuLi. However, neither synthetic method yields crystalline phosphasilenes. The first synthesis of the crystalline phosphasilene 13 was achieved by a two-step reaction of fBuSiCl3 with 4 equivalents of RPHLi (R = 2,4,6-/Bu3C6H2) and Ph2PCl by Niecke et al. in 1993 (Scheme 3).10c This reaction proceeds... 

Scheme 3. Synthesis of the phosphasilene 13, proceeding via the 1,3-diphospha-2-silaal-lylide derivative 13. ...

Scheme 4. Synthesis of the stable P-silylated phosphasilenes 15a-i, proceeding via 16-18.

The synthesis of compounds 44 containing Si=As bonds was achieved by a route analogous to that used for the phosphasilenes 15 (Eq. 13).12,14 26 The thermolysis of colorless solutions of 45a-d in hexane at 60°C yielded the desired products 44a-d as orange-red oils or orange solids. The starting... 

The first synthesis of a crystalline phosphasilene 832 was achieved by a two-step reaction of t-Bu SiCI with four equivalents of SupPHLi (Sup = 2,4,6-t-Bu3CgH2) and Ph2PCl384 385 via the diphosphasila-allyl salt 833386, a remarkable conjugated sila-zr system (equation 288). The Si=P bond length in 832 is 2.094 A and the silicon atom is pyramidalized (E <= 356.7°). The 1 Jp=si of 832 is remarkably high with 203 Hz. 

Bickelhaupt published an improved route for the synthesis of phosphasilenes 836 in a one-pot reaction via the dilithiated phosphine 835 obtained from the primary phosphine 834 (equation 289). The drawback is that the substituent at phosphorous may not be smaller than the supermesityl (Sup) group387. 

The concept of the stabilization of unsaturated silicon centres by coordination has been applied in the synthesis of phosphasilene 837388. 

The attempt to stabilize transients in the formation reaction of phosphasilenes by transition metal centres directly bound to the precursors was not successful392. The metal substituent was either cleaved or behaved like a normal substituent, as is shown for the synthesis of phosphasilene 842 from 841 (equation 291). The NMR parameters for 842... 

As mentioned in this chapter, in recent years much progress has been made in the chemistry of silicon-chalcogen multiple bonds. For silicon-sulfur doubly-bonded compounds, we have now several isolated examples, both kinetically stabilized and thermodynamically stabilized. Furthermore, there have been reports of the synthesis and characterization of stable compounds with silicon-nitrogen double bonds (i.e. silanimines or iminosilanes) as well as their heavier group 15 element analogues such as phosphasilenes and arsasilenes. 

Preparations of phosphasilenes have been mentioned in Sections II and V. Driess recently reviewed the synthesis and properties of phosphasilenes137. In most of the reported reactions63,130,132,137 of phosphasilenes, cleavage of phosphorus-silicon double bonds occur to give compounds with a P—Si single bond, reflecting the high reactivity of the —P=Si< bond thus they lie beyond the scope of this Chapter. 

In 1991 we reported on the synthesis of P-silyl and -phosphanyl substituted silylidenephosphanes (phosphasilenes) 2, i.e., compounds of type IIA and III, which are accessible by thermally induced elimination of LiF from corresponding / -lithium-(fluorosilyl)phosphanides IV [5, 6] and possess a remarkable thermal stability (up to 110°C). In 1993 the first crystalline phosphasilene 5 [7], a compound of type IV, had been prepared and its structure was established by X-ray diffraction [8], whereas the Si=P compounds of type V were merely characterized by means of NMR spectroscopy [9]. Compound 5 possesses a relatively long Si=P bond length and a non-planar geometry around the A, , a -Si atom, which can be explained in terms of steric hindrance or a second-order Jahn-Teller distortion [10, 11]. In comparison, the Si=P bond in 6 [12] is significantly shorter and the Si atom is trigonal planar coordinated. 

The synthesis of the phosphasilenes 2a-h and of the arsasilenes 4a-c was achieved via the corresponding lithium-(fluorosilyl)pnictides la-h and lla-c, respectively (Scheme 2). It has been shown that perfect steric protection of the highly reactive Si=E bonds (E = P, As) in 2 and 4 is provided by the 2,4,6-triisopropylphenyl substituent (Is = isityl) attached to the low-coordinated silicon center. The appropriate precursors 1 and 3 were prepared in a multiple-step procedure, starting from 8 and 10, respectively. 

The synthesis of arsasUenes followed the same route as that anployed for phosphasilenes . Heating solutions of the precursors 843 to 60 °C gave the arsasilenes 844 (equation 292). 

The wastes from the synthesis of phosphasilene are hydrolyzed in a well-ventilated hood and discarded as hazardous wastes. 

The phosphasilene 2 has demonstrated the capability of transferring the elusive PH species to an imsaturated organic substrate. This behavior of the phosphasilene enables facile access to PH-functionalized organic molecules, which have been shown to be useful tools for ligand synthesis in coordination chemistry. 

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