Unsaturated silicon compounds

The chemistry of unsaturated silicon compounds, i.e. silylenes and molecules having (p-p)ic-sili-con element multiple bonds >Si=E (E = C, Si, Ge, Sn, N, P, As, O, S), is an interesting field of research for the theoretician as well as for the preparative chemist because of the unexpected and fascinating results which can be obtained. Yet 30 years ago, such compounds were considered "non existent" because of the classical "double bond rule", established by Pitzer and Mulliken in the early fifties. Since then, the chemistry of unsaturated silicon compounds proceeded from the investigation of small" species in the gas phase to the synthesis and isolation of stable species with bulky substituents at the > Si =E moiety, and to the determination of their structural features. 

Today two directions of research are of interest On the one hand, investigations on the reactivity of basic systems are important to elucidate the typical" Si=E-multiple bond properties, in particular with respect to their use as synthons in organo silicon chemistry without being hampered in their synthetical potential by bulky substituents in this context, a comparison on their reactivity with the carbon analogues is still attractive. On the other hand, the isolation of new stable unsaturated silicon compounds and their structure determination continues to be of interest for quite a number of research groups worldwide. 

Unsaturated Silicon Compounds - Matrix IR Investigations and Quantum Chemical Calculations... 

Similarly to vinyltrichlorosilane, vinylmethyldichlorosilane and methylthyenildichlorosilane, HTC is used to produce allyltrichlorosilane, thyenil-trichlorosilane, vinylethyldichlorosilane, allylmethyldichlorosilane, vinyl-phenyldichlorosilane and other chlorine-containing silicone monomers with organic radicals attached to the silicon atom. Table 7 lists the physicochemical properties of some chlorine derivatives of unsaturated silicone compounds. 

Much attention has been focused on possible 1,2-silyl migrations in unsaturated silicon compounds like disilenes, silaethenes and silylenes, especially since their stable derivatives had been isolated. 

In 1984, the first very effective example of metathesis (disproportionation) of vinylsubstituted silicon compounds catalyzed by ruthenium complexes was reported [33]. It opened a new route of great synthetic importance and has allowed synthesis of a series of unsaturated silicon compounds according to the following equations, with the yield predominantly higher than 70%. Numerous reports on vinylsilane disproportionation (Eq. 21) [5, 33-38] and its co-dis-proportionation with olefins (Eq. 22) [37-44] have been published. 

The above ab initio and related calculations clearly reveal that unsaturated silicon compounds are less stable than their carbon analogs (156,170), but as gas-phase studies and the chemistry of sterically crowded silicon derivatives have proved (153,154,156,158,162,172-175) such compounds can no longer be labeled as nonexistent (176,177). A number of isolable double-bonded silicon derivatives have recently been synthesized and characterized in agreement with theoretical predictions (Table III) (169,173,178-195). The data illustrate well that the kinetic stability of multiply bonded silicon species can be greatly improved by substitution of sufficiently bulky groups across the p -pw bond. The ease of addition reactions is then severely restricted. The new target for organosilicon chemists appears to be silicon triple-bonded compounds (196,197). 

Elimination processes of the type depicted in Eq. (36) constitute an important pathway for generating unsaturated silicon compounds. 

The C vs P reactivity of phosphinomethanides towards silicon centers may be turned by the substitution pattern of both, the phosphinomethanide and the chlorosilane. Novel heterocycles, skeleton rearrangements and high coordinations numbers may be achieved. 9 and 14 potentially be precursors for a specific type of unsaturated silicon compounds, adding to the growing class of compoimds at the interface between organophosphorus and organosilicon chemistry. 

Upon irradiation (X > 395 nm) compound 6 is transformed into 3 (therefore, both compounds are indeed isomers). Simultaneously, C2Si (7), which is formed concurrently with 6, reacts with eliminated hydrogen that is trapped in the same matrix cage to also give 3. Trapping reactions of this kind have also been observed for various other unsaturated silicon compounds [7]. 

In this connection I intend to discuss three themes concerning unsaturated silicon compounds. But before I enter into the essential points, I will offer some comments about the imsaturated silicon compounds in question, as well as about the homologous unsaturated germanium and tin compoimds. These unsaturated systems have the general composition >E=Y, shown in the middle of Scheme 2. 

Investigations during the past years show that unsaturated silicon compounds are stabilized by protection of the p -p bond with very bulky substituents against consecutive reactions. Thus, metastable Si=Y-7t-compounds (Y = Si, C, N, P, As, S, Se) could be fiilly charaterized by NMR and/or X-Ray crystallography under normal conditions in condensed phase or in the solid state. Some examples are given below. 

The mesityl substituent 1 and the 2-(dimethylaminomethyl)phenyl substituent la have shown their ability to stabilize unsaturated silicon centers in a kinetic and thermodynamic manner, respectively [1, 2]. The 2-dimethylaminomethyl-4,6-dimethylphenyl substituent 2 combines the steric demand of 1 and the chelating amino side-chain of la (Fig. 1). Along the synthetic pathway to potential precursors of unsaturated silicon compounds, 2-dimethylaminomethyl-4,6-dimethylphenyl substituted silanes have been synthesized and structurally characterized. 

In conclusion, in spite of its steric demand, the 2-dimethylaminomethyl-4,6-dimethylphenyl substituent is able to form highly coordinated silicon compounds. Investigations currently in progress are directed towards the synthesis of unsaturated silicon compounds stabilized by this substituent. 

In addition to the case studies presented in detail (Sections II.B-II.D), the following PES investigations on other unsaturated silicon compounds—listed in order of increasing molecular size and already partly mentioned—have been reported. 

For reviews covering predominantly those unsaturated silicon compounds, which are kin-etically stabilized by bulky inert substituents, cf. e.g. G. Raabe and J. Michl, Chem. Rev., 85,419... 

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