Silylene stable

M. Denk, R. West, R. Hayashi, Y Apeloig, R. Pauncz, M. Kami, "Silylenes, Stable and Unstable , in Organosilicon Chemistry II From Molecules to Materials (Eds. N. Auner, J. Weis), VCH, Weinheim, 1996, p. 251. 

In the vast majority of its compounds Si is tetrahedrally coordinated but sixfold coordination also occurs, and occasional examples of other coordination geometries are known as indicated in Table 9.2 (p. 335). Unstable 2-coordinate Si has been known for many years but in 1994 the stable, colourless, crystalline silylene [ SiNBu CH=CHNBu j, structure (1), p. 336, was... 

Very recently, the coordination chemistry of low valent silicon ligands has been established as an independent, rapidly expanding research area. With the discovery of stable coordination compounds of silylenes [35-38], a major breakthrough was achieved. Within a short time a variety of stable complexes with a surprising diversity of structural elements was realized. Besides neutral coordination compounds (A, B) [35, 36, 38], and cationic compounds (C) [37], also cyclic bissilylene complexes (D) [39,40] exist. A common feature of the above-mentioned compounds is the coordination of an additional stabilizing base (solvent) to the silicon. However, base-free silylene complexes (A) are also accessible as reactive intermediates at low temperatures. 

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

Does a Silylene-Complex exist This rhetorical question is the title of a theoretical paper published in 1983 [84], As a result of an ab-initio calculation, the authors came to the conclusion that a moderately positive answer can be given. However, silylene complexes are thermodynamically less stable than carbene complexes (the MSi bond energy for the hypothetical complex (OC)5Cr = Si(OH)H is 29.6 kcal/mol, the bond energy of the MC bond in (OC)5Cr = C(OH)H is 44.4 kcal/mol) [85], and therefore silylene complexes should be difficult to isolate. 

In 1987 a major breakthrough was achieved when two research groups independently succeeded in the synthesis of monomeric silylene complexes in the form of stable base adducts [35-38]. 

Silylene complexes are not only stable with donor substituents but also with simple alkyl residues at silicon. These alkyl complexes still have a sufficient thermodynamic stability, but otherwise are reactive enough to allow a rich and diverse chemistry. Particularly the chlorocompounds 7 and 11 are valuable starting materials for further functionalization reactions the details of these reactions will be discussed in the forthcoming sections. The data for the known compounds are summarized in Table 1. 

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. 

Recently, a variety of silylenes were generated and characterized by matrix isolation techniques. The observed loose donor adducts between silylenes and the matrix molecules (THF, CO) are only stable at very low temperatures. Melting of the matrix induces polymerization of the silylenes which proceeds through disilenes. However, 0->Si transfer reactions do not occur only in the case of 1-methyl-THF has an insertion of the silylene into the C —O bond been observed [155-158],... 

At this stage of the discussion it is obvious that stable donor adducts of silylene complexes show a modified silylene reactivity and can thus be considered as model compounds for otherwise inaccessible reactive intermediates. 

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... 

Also, reactive silylene complexes of iron and chromium can be generated at low temperatures and subsequently derivatized by trapping reagents. In THF as solvent, first labile THF adducts are formed, which are converted to the more stable HMPA adducts. The THF complexes dimerize above —40 °C with loss of THF. The silylene complexes can be utilized for reactions if they are generated in the presence of reagents like dimethylcarbonate. The resulting reaction products... 

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. ... 

Phosphinidenes (R-P) differ from other low-coordinate organophosphorus compounds, such as phosphaalkynes (R-C=P), phosphaalkenes (R2C=PR), and phosphaaromatics, in that the phosphorus atom carries only a single a-bonded substituent [7-9]. They relate to carbenes, nitrenes, and silylenes and likewise can exist as singlet and triplet species. The advances that led to stable carbenes [10, 11] and silylenes [12] stimulated an exploration of the chemistry of phosphinidenes. 

Ab initio (3-21G( )//STO-3G) calculations by Chandrasekhar and Schleyer163 on 1,4-disilabenzene 58, its Dewar benzene isomer 59, and a silylene isomer 60 showed that all three species exhibited approximately similar stabilities, the silylene 60 being 9.9 kcal mol-1 more stable than the planar aromatic form 58, which was 5.9 kcal mol-1 more stable than the Dewar benzene form 59. 

A fourth preparation for a stable iminosilane as the THF adduct is the reaction of a silylene with trityl azide.23... 

To a large extent the chemical shifts of carbon and silicon run parallel, but the chemistry of the two elements is somewhat different. Thus silicon can have extend its valence shell beyond the coordination number of 4. A few stable or-ganosilicon compounds in which silicon is divalent are known (the silylenes), and compounds with a silicon-silicon double bond also exist (the disilenes). 

More recently, Belzner et al. reported a new type of oxygen transfer reaction from isocyanates to bis[2-(dimethylaminomethyl)phenyl]silylene (8)18 which was thermally generated from the corresponding cyclotrisilane 7, and they obtained some convincing results of the involvement of silanone 9 (Scheme 3). However, they found that silanone 9 is not stable enough to be isolated. Only cyclic di- and trisiloxanes 10 and 11 (i.e., the cyclic dimer and trimer of the silanone 9) were obtained together with the corresponding isonitrile as other main products when... 

When a C6D6 solution of the silylene-isocyanide complex 21 was treated with an equimolar amount of mesitonitrile oxide at room temperature, the blue color of 21 immediately disappeared, and the 29Si and 13C NMR spectra of the reaction mixture exhibited characteristic strong signals (8Si = 26.9, 8C = 184.3) assignable to those of compound 22a having a novel 1,2,4-oxazasilete ring system (Scheme 5). The oxazasilete 22a was not stable at ambient temperature, and... 

The formation of silaneselone 57 was evidenced by the trapping reaction with mesitonitrile oxide leading to the corresponding cycloadduct 58 and was also supported by the observation of a remarkably downfield 29Si chemical shift (8Si = 174) indicative of the Si=Se double bond of 57. Although this direct selenation of silylene 55 with an equimolar amount of selenium was not reproducible, the use of excess amount of elemental selenium resulted in the formation of a new cyclic diselenide, diselenasilirane 59, as a stable compound (8Si = -44 and... 

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