Radical reactions, disilenes

The masked disilene monomers 1-5 were prepared by reaction of dichlorodisilanes of the type ClSiR1R2SiMe2Cl with the biphenyl anion radical as described before. These are composed of two regio isomers, a and b, the predominant isomer being a, as determined by H NMR NOE difference spectra. 

As a result of several decades of research it is now known that a polysilane of three or more contiguous silicon atoms is susceptible to reaction by one or more of several pathways when photolyzed, each associated with cleavage of a silicon-silicon bond. The two most common processes observed are the homolysis of a silicon-silicon bond to yield a pair of silyl radicals, and the elimination of a silicon atom from the chain in the form of a silylene. As discussed in Section VII, the use of trisilanes, particularly where the central silicon atom bears aryl groups, has become an important route for the preparation of a wide variety of diarylsilylenes, A Si , many of which have been captured in glasses at low temperature, or have been allowed to dimerize to disilenes by warming. 

The reactions of cyclic disilenes with haloalkanes proceed in a similar way. Cyclotrisilene 48 reacts with carbon tetrachloride without cleavage of endocyclic Si-Si bonds to give trans-1,2-dichlorocyclotrisilane 179 stereo specifically [Eq. (84)].24 The a h -stereochemistry is in good accord with the radical mechanism of the reactions as stated above. Similarly, cyclic disilenes 49 and 50 react with carbon tetrachloride and 1,2-dibromoethane giving the corresponding trau.v-1,2-dihalodi-silanes 180 [Eq. (85)].129 Methyl iodide adds to cyclic disilene 50 in an a tz-addition manner to give 181.104... 

Stepwise radical mechanisms have been proposed for the apparent [2 + 2] cycloaddition of disilenes with ketones by Baines et a/.140,141 They have found that the reactions of tetramesityldisilene 1 with trara-2-phenylcyclopropane carbaldehyde (208a) and fra/M,fra/M-2-methoxy-3-phcnylcyclopropane carbaldehyde (208b), a mechanistic probe developed by Newcomb et al.,142 undergo characteristic cyclopropane ring-opening as shown in Eq. (99). 

There are some indications, but no unequivocal evidence, that disilenes undergo thermal 2 + 2 cyclodimerization to cyclotetrasilanes48,49,62. In the best documented cases, the reaction might well have been photochemical48,49 (see Section II.B.4.f) or proceeded via a radical anion49. The last-named results were obtained upon reduction of... 

Reactions of disilenes with elemental sulfur92,93 probably belong to the miscellaneous rather than radical attack category. The products from the reactions of elemental sulfur with tetramesityldisilene and ( )-l,2-di-fert-butyl-l,2-dimesityldisilene have been characterized as disilathiiranes (equation 32), and X-ray structure analysis of the former (38) has been performed93. A single isomer of the latter is formed in the reaction, presumably trans. 

The interaction of alkali metals with sterically congested 1,2-dihalo-l,1,2,2-tetraorganodisilanes leads via the disilenes to the corresponding tetraorganodisilenyl radical anions [518], which have been characterised by their ESR spectra (see Chapter 5). fer/-Butyl and wo-propyl groups are suitable bulky groups to have on the sihcon for this reaction (Eq. 3.276). 

Tetrakis di-fbutylmethylsilyl)disilene has proved to be a versatile precursor for the isolation of remarkable radical species (6,7). The disilene bearing four fBu2MeSi groups can be successfully synthesized by two routes (7). The treatment of tetrakis di-fbutyl-methylsilyl)disilene with 2.2 equiv of lithium naphthalenide in THF at — 78°C afforded the silylene anion radical in 56% yield. The reaction rnkture was first slowly warmed to room temperature. The dark blue color of disilene completely disappeared and a red solution was produced during the reaction. The eventual addition of 4.3 equiv of 12-crown-4 to the resulting reaction mixture led to the isolation of the silylene anion radical as air-and moisture-sensitive red crystals (8). 

A lithium naphthalenide solution can be prepared by stirring equimolar quantities of naphthalene and lithium in THF. This solution of lithium naphthalenide (0.34 mmol) in THF was added to tetrakis di-fbutylmethylsilyl)disilene (112 mg, 0.16 mmol) in THF 2.5 mL) at -78°C with the help of a syringe. Afterward the reaction mixture was allowed to warm up to room temperature over 2 h. There was a rapid color change resulting in the formation of the 1,2-dianion of the disilene (6). This dianion was further treated with 12-crown-4 (110 irL, 0.69 mmol, 4.3 equiv) at room temperature. Afterward THF was removed in vacuo and hexane was added to the reaction mixture. The hexane solution was filtered and overnight standing at -30°C led to the isolation of silylene anion radical as air- and moisture-sensitive red crystals (128 mg, 56%). Mp 138-139°C (Scheme 6.1.1.1). 

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