Silylphosphanes

For group 14, C-tin-substituted phosphorus ylides have been studied in the past but less than the corresponding C-silyl ylides which, owing to their stability and reactivity, are of considerable interest. Indeed a recent review concerning the silylphosphanes with one part concerning the synthesis and applications of silylated phosphorus ylides has been published [112]. 

The formation of oligomeric silicon chlorides SixCly from 1 with Si2Cl6 [1] might be due to base -catalysed decomposition of intermediate Me3Si-SiCl3 (a known compound), which would be the product of an anionic trichlorosilylation of the trimethylsilyl group of silylphosphane 1. 

Dilithiated primary phosphanes could only be isolated in a reproducible crystalline form if the lithiation of the phosphane with BuLi (ratio of 1 2) in toluene was carried out in the presence of a trace amount of LiOH (3). This is evident by the respective transformation of the silylphosphanes 1, which furnished, depending on the steric demand of the silyl group, a dodecameric or a octameric associate (Scheme 1). Thus, reaction of la with BuLi in the molar ratio of 1 2 led to the globular, Li20-filled cluster 5a, whereas metalation of the bulkier substituted phosphane lb gave the Li20-filled octamer 5b. 

Scheme 1. Synthesis of the lithium-rich clusters 3a-6a, 5b, and 7a, starting from the primary silylphosphanes la-lc, and 2a-2e.

The first Sn4P4 cube 22f has been prepared in an analogous procedure, starting from the very bulky silylphosphane and Lappert s stan-nanediyl (Eq. 15) (49), which has been isolated in the form of dark red crystals in 95% yield. 

A convenient route to andiides, where Li and A1 centers are, at the same time, involved is represented by the reaction of LiAlH4 with primary silylphosphanes and silylarsanes (Scheme 4) (67). However, the outcome of such reactions is dependent on the stoichiometry. The silylarsane 2c reacts with LiAlH4 in the molar ratio of 4 1 in 1,2-dimethoxyethane, under evolution of H2, resulting in the corresponding tetrakis(arsaneyl)-substituted lithium alanate 36c in quantitative yield. The similar transformation of the arsane 2a with LiAlH4 in the... 

Scheme 3.6-10. Mg Em cluster formation by magnesiation reactions of primary silylphosphanes and silylarsanes with Bu2 Mg.

The capacity to form phosphonium salts decreases when negatively charged substituents are introduced at the silicon atom. Thus, with F3Si—PH2, only a slight tendency to form adducts and to split the Si—P bond is observed (9). Silylphosphanes with a PH2 group like... 

Whereas SiH-containing silylphosphanes such as H3Si—PEt2 react with LiPEtj by substituting the SiH group, lithium alkyls on the other hand cleave the Si—P bond (ii) as shown in the following case. 

This finding led to the question, how far in multiply silylated silylphosphanes can Si—P bonds be formed from the cleavage reactions. This was investigated on P(SiMc3)3 (12). At — 40°C in THE the reaction with LiBu proceeds practically completely according to ... 

Our present knowledge of the chemistry of the phosphorus compounds and in particular of the chemistry of the silylphosphanes is not sufficient for a full understanding and explanation of these complicated reactions in every detail. The investigations reviewed in this article were all undertaken to broaden the basic knowledge in this field with the final goal of permitting a full understanding of these reactions. 

At first the fact that the formation of the bicyclic compound P(SiMe2)3P could not be observed seemed incomprehensible. An explanation can be that this compound, under the given reaction conditions, reacts further to form compound 10. But compound 10 cannot be formed only in this way, as is demonstrated by the isolation of its precursor 9 and of the lithiated compound 11, which should react with Me2SiCl2 to form 10. Compound 10 is doubtlessly a favored molecule in the series of these cyclic silylphosphanes. 

To investigate such reactions further, various substituted, phosphorus-rich silylphosphanes are needed as starting compounds. The following investigations aim at the preparation of phosphorus functional tri- and tetraphosphanes, in which particular phosphorus atoms with reactive substituents (H, Li, halogen, SiMe3) are built in, while other phosphorus atoms with equally defined positions remain blocked by alkyl groups. 

The lithiation shown in Eq. (10) takes places at 20°C in ra-pentane. Under these conditions, the reactivity of the lithimn alkyl is reduced to such an extent that only the most reactive bond in 53 (and in 59, respectively) is acted upon and therefore the selective exchange of the phosphorus hydrogen with lithium takes place. The phosphides formed can easily be separated and purified because they dissolve with difficulty in nonpolar solvents. Precipitation of these phosphides is, however, incomplete and can fail to occur if the reaction solutions contain significant concentrations of partially alkylated silylphosphanes such as (Me3C)P(SiMej)2 and MeP(SiMe3)2. 

It is known from the preceding research that phosphorus-rich silylphosphanes or their related lithium phosphides undergo, with LiBu in ether, reactions in which a structural transformation occurs, as shown in Scheme 16 20). The reactions of the partially silylated tri- and cyclotetraphosphanes were explored in order to come closer to understanding the above reactions. It can be taken for granted that the P—C bond is not affected in such reactions. 

---