Multiply bonded silicon compounds

Second, chapters on topics which were covered incompletely or partially in Vol. 2 were extended here by including new sub-topics related to the same themes. These include 29Si NMR, ion-molecule reactions of silicon ions and the reactivity of multiply-bonded silicon compounds. 

Preparative methods leading to multiply bonded silicon compounds and the reaction modes of such compounds have been summarized in a series of charts. In these charts, labels at the reaction arrows refer to the subsection or subsections in which the reactions are discussed. 

The first stable silaallene, 56, was synthesized in 1993 " " by the intramolecular attack of an organolithium reagent at the /f-carbon of a fluoroalkynylsilane (Scheme 16). Addition of two equivalents of r-butyllithium in toluene at O C to compound 54 gave intermediate 55. The a-lithiofluorosilane then eliminated lithium fluoride at room temperature to form the 1-silaallene 56, which was so sterically hindered that it did not react with ethanol even at reflux temperatures. 1-Silaallene 56 was the first, and so far the only, multiply bonded silicon species to be unreactive toward air and water. The X-ray crystal structure and NMR spectra of 56 is discussed in Sect. IVA. 

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). 

Structure and Bonding in the Parent Hydrides and Multiply Bonded Silicon and Germanium Compounds from MHn to RJM = M R2 and RM=M R... 

On the theoretical side, Schaefer reviewed the early studies on silene that contributed greatly to the theoretical interest in these compounds (35). The review by Raabe and Michl (79) on multiply bonded silicon included a comprehensive examination of the early theoretical work. Gordon (85,86) has summarized multiply bonded silicon, and the studies of Luke et al. deserve mention as a comprehensive study of multiply bonded silicon at a uniform level of theory (87,88). Nagase et al. (89) have reviewed their studies on doubly bonded silicon and germanium, with particular emphasis on reactivity. Most recently, Apeloig (90) has provided encyclopedic coverage of theoretical studies of silicon compounds through the middle of 1987. 

We now proceed to review the methods of generation and the reactivity of compounds containing multiply bonded silicon. We do so in a way that is fairly comprehensive but not exhaustive. We emphasize those developments that appear most significant to us and those that are too recent to have been included in the reviews quoted above. The interested reader is referred there for a more encyclopaedic coverage of previous work. Our literature search extends to the end of 1986. 

The multiply bonded silicon structures under consideration have been collected in Table 1, which also defines the nomenclature we have adopted. We use the common term silylenes for organic compounds of divalent silicon, analogous to carbenes. The existence of the more awkward but IUPAC-sanctioned term silanediyl for this class of compounds needs to be mentioned. 

This class of compounds represents a very recent addition to the list of directly observed species with a multiply bonded silicon. Only a little is known about their chemistry at this time. The formation of a silanephosphimine was postulated in the thermal decomposition of 2,2-dimethyl-2-sila-l-phenylphosphetane which occurs during distillation at 100 °C under reduced pressure. The silaphosphetane was obtained from a reaction of / -chlorosilylphosphine with butyllithium355. The P-phenyldimethylsilanephosphimine was presumably responsible for the formation of an addition product to the 2-silaphosphetane and of a head-to-tail dimer (equation 179)355. 

While very little is known about compounds of these two types, simple representatives have been known for a long time. The very first observed compound of multiply bonded silicon was the diatomic radical Si=N, detected in the gas phase in 1913399 and identified in 1925400. It has been investigated in considerable detail more recently401,402. The isonitrile HNSi was studied in matrix isolation as early as 1966403. It was prepared by UV irradiation of silyl azide. 

Silicon-carbon and silicon-nitrogen multiply bonded compounds... 

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