Silylenoid

Silylenoids R2Si(Li)Cl are unstable and easily undergo a-elimination of LiCl to form R2Si or self-condensation to form R2Si=SiR2, then oligomeric substances. 

It is interesting to note that (alkoxy)phenylsilanides, such as /-BuOPh2SiLi 141, show a more pronounced silylenoide character than comparable (amino)phenylsilanides, and undergo self-condensation at 0 °C.2 1 An unstable... 

In recent years, several attempts have been made to prepare the first disilynes, compounds having Si-Si triple bonds,791 792 until Sekiguchi et al. in 2004 succeeded in the isolation and full characterization of the first member of this compound class.793 West et al. have studied the dehalogenation of 2,6-Mes2C6H3SiF3 761 with sodium, which has afforded the bis(silafluorene) derivative 762, whose formation may be rationalized in terms of two intramolecular insertion reactions of a bis(silylenoid) 763 into two C(.r/>z)-C(.r/>3) bonds (Scheme 99). 794... 

FIGURE 21. Optimized geometries for reactive intermediate (Int), transition state (TS) and product (P) at MP2//HF/6-31G for the self-condensation reaction of Uthium(alkoxy)silylenoid. Bond lengths are in angstroms, and bond angles in degrees. Reprinted with permission from Reference 26. Copyright 1998 American Chemical Society... 

Clark and Schleyer20 were the first to report theoretical evidence for the complexation of H2Si by halogen donors when investigating the structural isomers of the silylenoid H2SiLiF. A silylene-lithium fluoride complex 14 was found to be only 5.4 kJ moL1 less stable at the MP2/6-21G//3-21G... 

There is a difficulty with the mechanism of Scheme 3 in that the fragmentation of a triplet diradical should conserve spin, yet neither triplet Me2Si nor triplet tetraphenyl-naphthalene have been detected. The diradical pathway for the photofragmentation of silanorbornadienes confirms an earlier proposal by Barton and coworkers52. There is always the possibility that diradical intermediates such as the singlet and triplet 13 S and 13 T could function as silylenoids. Thus, the assumption that products from pyrolysis of 7-silanorbornadienes are formed from free silylenes must be treated with caution. 

The inversion in selectivity between the insertion into an H—Si bond favored by the intermediate in the dehalogenation and the addition to a triple bond favored by the intermediate formed upon silirane pyrolysis suggests that different reactive intermediates are formed. Since free silylene is implicated in the silirane pyrolysis, the lithium-induced deiodination probably involves a silylenoid such as a complex of the silylene with Lil or with THF. For a further discussion of this possibility, see Section II.E. 

These silylenoid species have been made by lithium-tin exchange from a Si—Sn precursor96, or by reduction of chlorosilanes with lithium l-(dimethylamino)naphthalenide (LDMAN) at —78°C (equations 42 and 43)97. The resulting silyllithium species react as nucleophiles with Mc SiCI to give the corresponding disilanes (equation 44). An... 

We will make a tentative suggestion as to how the results of Boudjouk s experiment might be explained. Possibly the halogen-containing silylenoids are in equilibrium with a... 

Another conceivable, but probably less likely, explanation of these experimental results56 is that the triethylsilane somehow catalyzes the decomposition of silylenoid into free silylene, which can then insert into the Si—H bond. In any case, the intermediacy of silylenes in a-dehalogenations remains an open question, worthy of further study. 

A careful study of the direct reaction was carried out by Clarke and Davidson354, who interpreted their results in terms of surface-bound silylenoid species, MeCISi (surf) and Cl2Si (surf), which were thought to react with methyl chloride to give the principal products, Me2SiCl2 and MeSiCl3. Consistent with this model, addition of butadiene to the gas stream led to formation of silacyclopent-3-enes (equation 106). Small amounts of gaseous MeCISi and CL Si were also believed to be present, but were probably not the key intermediates in the direct process. 

Also obscure is the nature of the reaction of dihalosilanes with active metals. Further experiments should show under what circumstances, if any, such dehalogenation reactions lead to silylenes, or to silylenoids, another area of likely future activity. In a related area, isolable silylenoids are quite new, and further development of their chemistry can be anticipated. 

Reactivity and Attributes of Metal Silylenoids and Silylmetal Complexes... 

Transition Metal Silylenoid Complex-Catalyzed Hydrosilation Reactions... 

Transition Metal Silylenoid-Catalyzed Atom Transfer Reactions... 

This chapter examines the synthesis and reactivity of transition metal silylenoids and silylmetal complexes to provide context for the focus of this chapter silver-mediated silylene transfer reactions. 

REACTIVITY AND ATTRIBUTES OF METAL SILYLENOIDS AND SILYLMETAL COMPLEXES... 

Late transition metal silylenoid complexes were first synthesized by exploiting the lability of silicon-triflate bonds (Scheme 7.2).34 37 Tilley and coworkers reported that... 

Scheme 7.1. Light-induced 1,2-silyl shift to synthesize early transition metal silylenoid complexes.

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