Disilenes bridged

Subsequent studies by Sakurai and coworkers of the photolysis of disilyl-bridged aromatic compound 78, indicated that two interesting (not necessarily independent) processes were occurring, namely elimination of the disilyl group as a disilene, 79, another reactive intermediate of silicon chemistry, and secondly, Si-C homolysis to a diradical 80, which overall underwent a 1,2-silyl shift yielding the isomeric disilyl-bridged compound 8141-42 (Scheme 11). 

The photolysis of silicon-bridged dihydroaromatic compounds has been utilized to prepare reactive organosilicon species such as disilenes. Thus, photolysis of the... 

Of the more specialized methods for the formation of acyclic disilenes, two new methods deserve mention. Kira and coworkers reported that photolysis of the silirene 1 afforded the diaminosilylene 223. In contrast to theoretical predictions24-27, the silylene 2 apparently does not undergo dimerization to afford the bridged dimer 3 as is usually preferred in the presence of electronegative substituents but rather, according to the results of variable-temperature electronic spectroscopy, furnishes the disilene 4 (equation 4)23. 

Disilenes and their Novel Doubly Bridged Isomers... 

How is the bridged 11b formed Calculations show that two silylenes (e.g., HFSi ) can dimerize in two different modes, as shown in Scheme 3. Approach of two silylenes according to path a would lead to the disilene while coupling along path b would give the fluorine-bridged isomer. Approach of type b is driven... 

Can one find systems, in which the bridged non conventional isomer is more stable than the conventional disilene structure Examination of the two dimerization paths in Scheme 3 suggests that a higher donor ability of the lone-pair on the substituent should favor the bridged structure. 

We have therefore studied computationally the diamino- and dihydroxy-substituted disilenes and their isomeric bridged structures 12-15. The calculated relative energies of the isomeric pairs are given in Scheme 4. 

A major question which we still caimot answer is, how to direct the dimerization reactions of silylenes along path b in Scheme 2 to yield the bridged isomers in preference to the corresponding disilenes. For... 

Anionic polymerization of masked disilenes has been used for the synthesis of polysilanes. The monomers are bridged disilabicyclooctadienes such as (8). Treatment of these with an organohthium compound converts them to living, anionically terminated polysilanes (equation 37). The... 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

Bis(diisopropylamino)silylene has been generated by photolysis of the precursor (76), and trapped chemically by reaction with triethylvinylsilane and 2,3-dimethylbuta-l,3-diene.Contrary to theoretical prediction, its dimer seems to have a disilene structure and not a bridged structure. 

In 37a the calculated (3-21G ) Si-Si distance is 2.114 A, less than the Si-Si double-bond length of 2.12 A in disilene This contrasts with the predicted C-C bridge bond length of 37b which is over 0.2 A longer than the C-C bond in the ethane. Compound 37c also has... 

The proton affinity of disilene was calculated to be 207192b kcal mol"1 and 223 kcal mol"1193 according to two different estimations. Both values are substantially higher than that of ethylene. The hydrogen bridged and the classical structures of the protonated species are very close in energy192b 193. 

---