7,8-Disilabicyclo octa-2,5-dienes

As a typical example, Fig. 1 shows the H NMR NOE difference spectra of the monomer 3, 1-phenyl-7,7-dihexyl-8,8-dimethyl-7,8-disilabicyclo[2.2.2]octa-2,5-diene. Irradiation of the methyl peak on silicon of the major isomer enhanced the peak intensities of the methyne (H and two vinylic hydrogens (Hb). The fact clearly indicates that the major isomer has the structure 3a. The isomer ration (7.8/1) was calculated from the relative area of methyl peaks. In a similar way, structures and isomer ratios were determined for all the monomers. Table 1 lists the results. Apparently, the ratio of the isomers a/b increased with increasing steric bulkiness of the alkyl substituents on the dichlorodisilanes, reaching to 96/4 for the di(2-methylpropyl)dimethyl-substituted derivative 5. 

Disilabicyclo[2.2.2]octa-2,5-dienes, prepared by the interaction of the radical anion of anthracene, naphthalene or biphenyl with 1,2-dichlorotetra-methyldisilane were found to undergo thermal decomposition472). 

Only four transient disilenes have been studied to date by fast time-resolved spectroscopic techniques l,l,2-trimethyl-2-phenyldisilene (103), ( )- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (104) and tetrakis(trimethylsilyl)disilene (35). The first three compounds were generated by photolysis of the 7,8-disilabicyclo[2.2.2]octa-2,5-diene derivatives 101 and 102 (equation 76)148 while 35 was generated, together with 106, by photolysis of the 1,2-disilacyclobutane derivative 33 (equation 77)68. 

Photolysis of phenanthraquinone (PQ) in the presence of disilane precursors such as 7,8-disilabicyclo[2.2.2]octa-2,5-diene using two 500 W tungsten-halogen lamps led to the formation of 227 and 228 as silylene-transfer products (Equation 46) <2001JOM63>. 

Low-temperature photolysis of 3,3-benzo-7,7,8,8-tetra-t-butyl-7,8-disilabicyclo[2.2.2]octa-2,5-diene produces a number of compounds among which is tetra-t-butyldisilene. This compound is thermally unstable having a... 

Anionic polymerization of masked disilenes has opened up a novel route to polysilanes (95). I-Phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes can be used as masked disilenes. n-BuLi works as an initiator. The polymerization may involve the attack of the polysilanyl anions on a silicon atom of the monomer, resulting in the formation of the new propagating polymer anion and biphenyl. This method is applicable to aminopolysilane synthesis (Scheme 28). 

Irradiation of silacyclopropene (77) in the presence of excess nitrile gave aza-2,8-disilabicyclo-[3.2.l]octa-3,6-diene (78) (four examples, 15-43% yield) (Equation (31)). This reaction is thought to proceed via sequential [2 + 2] and [4 + 2] cycloadditions, consuming two equivalents of silacyclopropene and one equivalent of the nitrile <78CC80>. 

A new and potentially valuable photochemical route to tetra-methyldisilene (175) has been reported and involves irradiation of 7,7,8,8-tetramethyl-7,8-disilabicyclo[2.2.0]octa-2,5-diene(176)in an argon matrix at 10 the disilene readily undergoes [ 4 + 2] cycloaddition to benzene to regenerate the precursor. The silane-selenones (177), reactive intermediates with a silicon-selenium double bond, can be photochemically generated and trapped with hexamethylcyclotrisiloxane as shown in Scheme 9. Irradiation of hexamesitylcyclotrisilane (178) in the presence of azobenzene... 

Additional evidence for the disilene-to-silylsilylene rearrangement was obtained when 2,3-benzo-l,4-diphenyl-7,7,8-trimethyl-8-(trimethylsilyl)-7,8-disilabicyclo[2.2.2]octa-2,5-diene (7) was pyrolyzed at 300 °C in the presence of 2,3-dimethylbutadiene to yield, among others, a trapping product, 11, expected for the trapping of the silylene 9 produced from the primary disilene 8 by a 2,1-trimethylsilyl shift52. Trapping products due to the silylene which would result from a 2,1-methyl shift were not detected (see equation 13 and Section II.A.3.b). 

The 2,3-disilabicyclo[2.2.2]octa-5-ene ring can be prepared by the co-condensation reaction between difluorosilylene and cyclohexa-1,3-diene (via initial formation of F2SiSiF2% which adds to the ring) <82JOM(226)2l> and 2,3-disilabicyclo[2.2.2]octa-5,7-dienes can be made from the Diels-Alder reaction of hexafluorobut-2-yne and stereoisomeric mixtures of 1,2-methyl-1,2-phenyl-1,2-disilacyclohexa-2,4-dienes, the products from which may both be used as disilene precursors (Scheme 6) <93JA11460>. 

Irradiation of tris(trimethylsilyl)phenylsilane in the presence of hex-l-yne and other alkynes affords the corresponding silacyclopropenes. Those silacyclopropenes formed from monosubstituted acetylenes undergo further photoisomerization to disilanylacetylenes via a 1,2-hydrogen shift. Two competing pathways have been observed on irradiation of the disilabicyclo[2.2.2]octa-2,5-diene... 

Sakurai et al. [91] have developed a synthetic route towards polysilanes via masked disilenes (7,8-disilabicyclo[2.2.2]octa-2,5-dienes). The starting materials are accessible from dichlorodisilanes and lithium biphenylide [92] (RS R R R = Me, Pr, Bu, Hex) ... 

Polysilanes (or polysilylenes) are usually prepared by the Wurtz-type couphng reaction of dichlorodialkylsilanes or, alternatively, via a transition metal-catalyzed dehydrogenation of dialkylsilanes both approaches often exhibit difficulties in terms of controlling the molecular weight and chain-structure, however. Nonetheless, by using masked silylene monomers (l-phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes), a series of novel, well-defined linear poly(silylene)s (M /Mn 1.3) was successfully obtained via an anionic ROP (Scheme 5.11) [147-150]. 

Scheme 5.11 Anionic ring-opening polymerization of 1-phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-diene.

Several methods are available for generating disilenes but photolysis of masked disilenes is most convenient for mechanistic studies. 7,8-Disilabicyclo[2.2.2]octa-2,5-dienes, the formal adducts of the addition of disilenes to benzene, naphthalene, anthracene and biphenyl, are well established to generate the corresponding reactive disilenes by either thermolysis or photolysis. The parent 7,7,8,8-tetramethyl-7,8-disilabicyclo[2.2.2]octa-2,5-diene (10) generates tetramethyldisilene (11) in an argon matrix by photolysis at 10 Tetramethyldisilene (11) was also observed at 344 nm in UV spectra by 3-methylpentane (3-MP) and EPA (ether isopentane ethanol = 5 5 2) matrices (equations 2 and 3, respectively). On annealing the EPA matrix, a product of addition of ethanol to 11,... 

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