Silylenes aromaticity

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. 

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. 

Surprising thermal and chemical stability of the cyclic silylene (29) may be explained by an enhanced aromaticity. This can be seen by comparison with its percursor (30) the difference in length between the single N—C (1.415 A) and double C=C (1.322 A) bonds in (30) decreases in (29) (N—C 1.400 A, =C 1.347 A) to imply an increased delocalization. Also the Si—N bond length increases in (29) only by 0.056 A (from 1.697 A to 1.753 A) instead of the 0.080-0.100 A typical for transition from tetra- to dicoordinate silicon this may indicate developing partial Si= N double bond character in (29) <94JA269l>. 

In conclusion, all the criteria discussed above point to the existence of jr-electron delocalization and thus to some degree of aromaticity in the unsaturated carbenes, silylenes and germylenes of type 75. However, the degree of conjugation and aromaticity depends... 

The relative energies at HF/6-31G //HF/6-31G of various isomers of monosilacyclobu-tadiene are given in Figure 22161. The global minimum on the C3SiFl4 PES is silylene 127, which is stabilized by the interaction of the vacant p-orbital on silicon with the C=C jr-bond to form a 27T-aromatic system. Four other silylenes 128-131 follow 127. These silylenes are all lower in energy than the isomeric structures which possess a C=Si double bond or strained rings, such as 132-137. This stability order contrasts with... 

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 1,3-silyl shift in aryl disilanes is suppressed when the aromatic ring is ortho-substituted144. An attempted silylene synthesis from 1,3-dimesitylhexamethyltrisilane 259, however, led to low yields of silylene trapping products (ca 30% generation of Me2S ). The major pathway is the homolytic cleavage of the trisilane, followed by disproportionation of the radicals 260 and 261 to the silene 262 and the disilane 263 (equation 65). 

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. 

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