Stable silylenes

Investigations of the reactivity of stable silylene complexes are still at an early stage of development. The reactions known so far, however, are of interest, since most of them are model cases which have important mechanistic implications. 

Stable silylene complexes with halogen substituents are a useful starting material for further displacement reactions. Starting from 7 and 11, the Si complexes 22-24 are accessible in high yield. However, the intermediate dichlorosilylene complex... 

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

Tokitoh et al. have synthesized a sterically hindered disilene TbtMesSiSiMesTbt 210 that is kinetically stable but thermally labile, thus providing the corresponding silylene TbtMesSi 211, whose reaction with isocyanides bearing bulky substituents has resulted in the formation of the first stable silylene isocyanide complexes TbtMesSiCNR (212, R = Tbt 213, R = Mes ) (Scheme 30).327... 

Synthesis, Structures, Reactions, and Dimerizations of Stable Silylenes. 684... 

SYNTHESIS, STRUCTURES, REACTIONS, AND DIMERIZATIONS OF STABLE SILYLENES... 

Recently, introduction of amino substituents on the silicon atom resulted in an epoch-making breakthrough in this field, that is, the synthesis of stable silylenes such as 122-125 (Scheme 14.56). ° Although these silylenes are all well characterized by either X-ray crystallographic analysis or electron-diffraction analysis,... 

Stable silylenes all react with alcohols via the expected insertion into the O—H bond to afford the corresponding alkoxyhydrosilanes (126). Reaction of... 

Cyclic compounds containing Si2E2 rings (10.56, E = S, Se) are also formed from the reaction of a stable silylene with sulfur or selenium [eqn (10.37)]. The reaction of the divalent germanium compound Ge[N(SiMe3)2]2 with... 

The first bis(amino)silylene 1 was described in 1992, but was stable only up to 77 The transient Si(NPr 2)2 was identified by trapping reactions and by electronic spectra at low temperatures. The major breakthrough in the chemistry of stable silylenes was the report... 

A new type of six-membered A-heterocyclic stable silylene has been reported recently. The yellow crystalline [SiCL )] (L = N(R)C(Me)CHC(=CH2)NR, R = C6H3P 2-... 

As discussed in Section 9.2.4, the stable silylenes 2 and 4 have also been studied by cyclic voltammetry (CV) similar procedures were earned out on the isoleptic germanium(II) amides The latter were less readily oxidised or reduced than 2 and 4 the oxidation... 

The stable silylene 756 was reacted with triphenylmethyl azide to give a silanimine (757) (equation 250)335. 

A major advance in silylene chemistry over the past five years has been the synthesis of persistent dicoordinate silicon compounds stable silylenes . 

The field of stable silylenes commenced with the synthesis of compound 59, isostrac-tural with carbene 58b, in 1994367. This development was promptly followed by reports of four additional examples 6098, the saturated analog to 59 and the benzene-368 and pyridine-369 fused bicyclic silylenes 61, 62 and 63. 

Silylene 59 was obtained starting from glyoxal and t-butylamine, according to Scheme 15367. The final step, dehalogenation of the dichloride, was carried out using molten potassium metal in refluxing THF, and these rather vigorous conditions have become standard for the synthesis of stable silylenes of this type. 

The properties of the stable silylenes raise interesting questions of chemical bonding, many of which apply also to the carbenes, 58a-f. First, why are these compounds so stable compared to ordinary silylenes (carbenes) Steric hindrance may be a factor, but it is unlikely to be a major one silylenes far more hindered than 59-63 dimerize at low temperatures, as was described in Section III.C181. 

The stable silylenes react, however, with water and alcohols, by straightforward insertion into O—H bonds. Silylenes 59, 61 and 62 all react with methyl iodide, inserting into the C—I bond. Examples of these reactions are shown for 59 in Scheme 18. Compounds 59 and 61 also react with elemental chalcogens. For 61 with Ss, Ses or Te 37°, and for 59 with... 

In a number of ways the reactions of stable silylenes resemble those of phosphines, R3P, to which they are isolobal analogs. Examples are provided by the reactions of 59 with covalent azides. Phosphines are known to react with azides to give phosphineimines, Ph3P=NR. In similar fashion, 59 reacted with triphenylmethyl azide in THF to give the silanimine 72 as its THF complex (equation 109)148. This reaction provides a new method for synthesizing compounds containing Si=N double bonds, which have previously been made by salt elimination reactions375. 

In all three complexes the Si-metal bond is short (Si—Fe = 219, Si—Ni = 221, Si—Cr = 228 pm). These distances are about 10 pm less than for typical Si-metal single bonds, consistent with partial back-bonding from the metal d-orbitals to the p-orbital on silicon. Reactions of 59 to form silylene-metal complexes appear to be general, and many more examples are likely to be prepared in the future, for this and other stable silylenes. 

We see that although the stable silylenes lack some reaction pathways common for transient silylenes, they participate in many reactions previously unknown for silylenes. A number of these lead to products with structures of considerable interest. Further developments in the chemistry of stable silylenes are to be expected. 

Robert West was bom in New Jersey and educated at Cornell University (B.A.) and Harvard University (A.M., Ph.D.). For the past 45 years he has been a faculty member in the chemistry department at the University of Wisconsin, where he is now E. G. Rochow Professor and Director of the Organosilicon Research Center. His many awards include the Frederick Stanley Kipping Award, the Wacker silicone prize, the Alexander von Humboldt Award, and the main group chemistry medal. He has published more than 600 scientific papers, mostly in the area of silicon chemistry. Major discoveries in his laboratories include the first soluble polysilanes (1978), the silicon-silicon double bond (1981), the first stable silylenes (1994), and electrically conducting organosilanes for high energy density batteries (2000). He is an airplane pilot and a mountaineer, with numerous first ascents in Canada and Alaska. 

Stable diaminosilylene 132 (R = /- Bu) does not dimerize to the corresponding tetraaminodisilene but undergoes an insertion reaction giving 131, which further dimerizes to diaminodisilyldisilene 62. Disilene 62 is stable in the solid state but equilibrates with stable silylene 132 (R = /-Bu) via 131 in solution [Eq. (52)].33,34 Intermediacy of 131 is evidenced by the reaction of crystals of 62 with methanol giving a methanol adduct of 131, 133. 

Since the first report on the synthesis of cyclic bisaminosilylenes in 1994, their reactions with various reagents have been studied in much detail. However, the investigations were restricted to stable silylenes 83-85 and nearly nothing... 

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