Silylene insertion reactions

The reaction of H2Si with H20 was actually the first silylene insertion reaction to be investigated by theoretical methods. Raghavachari et a/.14 calcu-... 

Kinetic isotope effects (KIE) of silylene insertion reactions were determined in solution as well as in the gas phase. Steele and Weber39 found KIEs between 1.8 and 2.3 for the insertion of silylene 13 into various alcohols no significant variation of these values was observed with a change of solvent from cyclohexane to THF. These results are consistent with two mechanisms (Scheme 8), both of them proceeding via a nonlinear or triangular transition state. Mechanism B, which is favored over mechanism A by the authors, is in agreement with theoretical results, which, in general,... 

Mechanisms of silylene insertion reactions are known for the methoxy-methylpolysilanes. Detailed investigations are reported in Chapter 7 Silylenes. The factors important in ... 

Scheme 1. Silylene insertion reaction leading to the formation of methylchlorosilanes.

Si—H and Si—Si bonds. Arrhenius parameters for the silylene-insertion reactions (102)—(104) have been calculated. The gas-phase reaction of... 

These values indicate that the A factors for SiHj insertions are close to collision frequencies, while those for larger silylenes are still better than 0.1 times the collision frequencies. The various quantitative values quoted in this paragraph indicate that on both energy and entropy groimds, silylene insertion reactions such as those shown in equations (80)-(85) are extremely efficient processes. 

Besides the ones discussed in the previous sections, the only other silylene insertion reaction involving silicon-containing bonds is the reaction of SiClj with halo-substituted polysilanes as shown in equation (93). ... 

Transition Structures Using Nonlocal Corrections. Silylene Insertion Reactions into the Hydrogen Molecule. 

The synthetic techniques used to make Si-Si bonds have changed little since the earliest developments. By far the most widely used methods involve variations on the Wurtz-type coupling where chlorosUanes are reductively coupled using an alkali metal (Eq. (14.4)) [35,40). Various metals have been employed under a variety of conditions with Na and Li remaining the most popular. Other techniques include reaction of metallosilanes with halosilanes (e.g., Eq. (14.5)) [41], photolysis of hydrosilanes [42], and silylene insertion reactions (e.g., Eq. (14.6)). 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

The insertion of 1 into element-element bonds is a crucial step in an extensive series of remarkable transformations. From the perspective of the particular substrate, such reactions are oxidative additions to a silylene-like center. The primary insertion products all possess reactive pentamethylcy-clopentadienyl-silicon cr-bonds. In this chapter only those insertion reactions... 

The important chemical behaviour of silylene is the insertion reaction like carbenes. Some insertion reactions which have come to light are ... 

Insertion reactions of silylene into a number of single bonds have been observed. The bonds include Si—O, Si N, Si H, Si halogen, strained C O, Si Si, O—H, N—H, and C—H (intramolecular only). Insertion into an X H bond can be initiated by the formation of silaylide (50) with the donation of a pair of electrons from a heteroatom X to form a bond to a divalent sihcon atom (Scheme 14.26). 

Disilanyl enol ethers (22) are the main product of the reaction of alkyl ketones with photochemically generated phenyltrimethylsilylsilylene (Scheme 30) (77JOM(135)C45). They could result from silylene insertion into the enol O—H bond, particularly as many have been isolated from dodecamethylcyclohexasilane and alkyl ketones or aldehydes on photolysis (Scheme 31) (78CL609). 

The first 1,2-disilacyclobutene (82) was prepared in 1973 by the gas phase reaction of dimethylsilylene and 2-butyne (73JOM(52)C21). It probably results through silylene insertion into the intermediate silacyclopropene (Section 1.20.3.4), but silylene dimerization followed by addition to the alkyne is also suggested (76JA7746), since (82) is formed in good yield if the disilene is generated directly (Scheme 127) (78JOM(162)C43). 

Only few theoretical studies have been devoted exclusively to the coordination of Lewis bases to silylenes. Most calculational evidence for the formation of silylene-Lewis base complexes was obtained from the computational investigation of the insertion reaction of HiSi into various H-X a bonds, where X is an heteroatom center possessing one or more free... 

According to the reactivity-selectivity principle, reduced reactivity of a compound is expected to be accompanied by increased selectivity of its reactions. Exactly this was observed in early experiments by Steele and Weber37 when the insertion reaction of dimethylsilylene (13) into the O-H bond of alcohols was performed in different solvents In a more polar solvent such as THF the reaction of the silylene with the... 

Insertion of SiH2 into H—H and H—C bonds, respectively, has been assumed in reports of rate constants from laser flash photolysis experiments for reactions of SiH2 with H2108 109, D2109 and CH4108. Since the rate of silylene insertion into the H—H bond was more rapid than previous estimates, interest was reawakened in the activation barrier112-114. The barrier for insertion of SiH2 into H2 was revised downward to ca 1 kcalmol-1. 

---