Silylenes aromatic compounds

In this section we will review the computational studies on reactions of silylenes, mainly addition and insertion reactions. Some reactions, in particular the following isomeriz-ations of silylenes to the corresponding multiply bonded species, were discussed above (a) to silaethylene in Section V.A.l.a.v, (b) to substituted silenes in Section V.A.l.b.iv, (c) to disilenes in Sections V.A.2.d and f, (d) to silanimines in Section Y.A.3, (e) to silanephos-phimines in Section V.A.4, (f) to silanones in Section V.A.5, (g) to silanethiones in Section V.A.6, (h) to silynes in Section V.B.l, (i) to disilynes in Section V.B.2, (j) to aromatic compounds in Section VI.A and to antiaromatic compounds in Section VI.D. 

In Table 7, the studies on silylene addition to c-bond systems are summarized. They are classified primarily according to the types of silylenes. The addition reactions of monomeric SiF2 formed by the high-temperature Si-SiF4 reactions are omitted in this table because they have already been summarized in Table 3. In the following sections, the addition reactions are discussed according to the type of molecules the silylenes are adding to, namely, olefins, dienes, alkynes, and aromatic compounds. 

This silylene formation from 27 under mild conditions permits the synthesis of a variety of interesting carbo- and heterocycles, most of which are new types of compounds. The results are summarized in Schemes 5 and 6. The reactions with benzene and naphthalene represent the first examples of [2+1] cycloadditions of a silylene with aromatic C=C double bonds.59 623 The reactions with carbon disulfide and isocyanide (Scheme 6) are also of great interest because of their unusual reaction patterns.62b... 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

West and coworkers38 isolated a silylene (13) stable enough to be distilled at 85°C/0.1 Torr and reported 29Si (and other nuclei) NMR chemical shift (S + 78.3). The structure was also confirmed by X-ray crystallographic analysis (Figure 5) and quantum chemical calculations. It was suggested that the compound has an aromatic ground state. 

The much studied photochemistry of aryldisilanes carried out in earlier years has been reviewed51,52. Cleavage of the silicon-silicon bond of the disilyl moiety is always involved, but various other reactions have been observed depending on the structure of the disilane and the conditions employed. Thus cleavage to a pair of silyl radicals, path a of Scheme 15, is normally observed, and their subsequent disproportionation to a silene and silane, path b, is often observed. There is evidence that the formation of this latter pair of compounds may also occur by a concerted process directly from the photoex-cited aryldisilane (path c). Probably the most common photoreaction is a 1,3-silyl shift onto the aromatic ring to form a silatriene, 105, path d, which may proceed via radical recombination52. A very minor process, observed occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

The h NMR spectrum provides some evidence for aromatic stabilization in 5. The important number is the chemical shift for the ring C-H protons, which falls at 6.75 ppm. This is significantly deshielded compared with the same protons in the precursor 4, 5.73 ppm, or the corresponding dihydride (LSiH2), 6.00 ppm. Theoretical calculations also support the idea that 5 is an aromatic molecule, with aromatic resonance energy of 12 s kcal mol . Particularly convincing, however, is a comparison of the properties of 5 with those of its saturated analog [22]. Silylene 13 was made by a reaction sequence similar to that for 5 in Eq. 4 and Eq. 5, except that for the final step it was necessary to use the dibromo rather than the dichloro compound (Eq. 13). For 13 an X-ray crystal structure could be determined it is shown in Fig. 5. 

Silane N2 can be used as a silylene-type protecting group for diol compounds. The introduction can be easily achieved by using 1-hydroxybenzotriazole (HOBt) as catalyst and triethylamine as base in acetonitrile at ambient temperature [7]. Protection can be achieved under milder conditions compared to the di-tert-butyl silylene group (heating is necessary). The additional preparation of a highly reactive (and sensitive) ditriflate, which is commonly used for the di-tert-butylsilyl derivative, is not necessary. This method is suitable for aliphatic and aromatic 1,2-diols as well as for 1,3-diols. 

In 2013 Roesky et al. have summarized a number of base stabilized silylenes which were used as versatile synthon in orgonosi-licon chemistry (1). Herein, we used the silylene [CH (C= CH2) CMe) 2,6-iPr2C6H3N)2 ]Si (1) with two-coordinate (2) and silylene [PhC Nffiu)2]SiCl (2) (3) with three-coordinate silicon for aromatic C—H and C—F bond activation to get access to compounds with silicon—hydrogen and silicon—fluorine bonds, respectively (4). 

The chemistry of 50 and other fiillerenes has developed rapidly since the first preparative-scale Isolation of this molecule in 1990 (7). The new field of fullerene chemistry is leading to many novel derivatives some of which may have extraordinary optical and electronic properties. The many Ceo derivatives reported so far include transition metal complexes, (2--, complexes with aromatic rings (4), Diels-Alder products (5), addition compounds with carbenes (d), silylene (7) or oxygen (8) and other organic derivatives (P). Several polymers of C60 have also been reported (JO-75). 

---