Silylenes addition reactions

The reversibility of most silylene addition reactions allows the cycloadduct of a silylene to 1,3-diene to be employed as a sUylene source. Extrusion of a silylene from l-silacyclopent-3-ene (3) has been achieved by thermolysis in the gas phase and also by photolysis in solution (Scheme 14.4). 

Table 7. Summary of Silylene Addition Reactions to 7r-Bond Systems... 

The stability of molecules depends in the first place on limiting conditions. Small, mostly triatomic silylenes and germylenes have been synthesized successfully at high temperatures and low pressures, 718). Their reactions can be studied by warming up the frozen cocondensates with an appropriate reactant, whereas their structures are determined by matrix techniques 17,18). In addition, reactions in the gas phase or electron diffraction are valuable tools for elucidating the structures and properties of these compounds. In synthetic chemistry, adequate precursors are often used to produce intermediates which spontaneously react with trapping reagents 7). The analysis of the products is then utilized to define more accurately the structure of the intermediate. 

Silylated triphosphanes and triphosphides, synthesis, 31 188-194 yields, 31 194 Silylenes, 29 2-6 addition reactions, 29 4-6 to butadiene, 29 4 to ethylene, 29 4 to hexadienes, 29 5 mechanism, 29 4 nitric oxide scavenging, 29 4 complexes, 25 37, 51, 116, 118 as catalyst intermediates, 25 118 extrusion from disilanes, 25 114, 118 halides, 3 225 from hydridosilanes, 25 14 insertion into element-hydrogen bonds, 29 3-4... 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

The first 1,2,4-thiadisiletane results from the reaction of carbon disulphide and the hindered silylene [2,4,6- (Me3Si)2CH 3C6H2]MesSi (TbtMesSi ) formed from the Z-disilene precursor. The mechanism is thought to involve a skeletal rearrangement of the 3,3 -spirobi(l,2-thiasilirane) intermediate formed by silylene addition to each carbon-sulphur double bond (equation 31)64. 

Extrusion of a silylene from a silirane or silirene is of course the inverse of silylene addition to alkenes or alkynes, respectively. The reversibility of most silylene reactions allows the inverse of addition to 1,3-dienes to also be employed as a silylene source. The first such reaction was reported by Chernyshev and coworkers, who found that transfer of SiCl2 units from l,l-dichlorosilacyclopent-3-enes was a unimolecular process and hence was likely to consist of silylene extrusion and readdition (equation 38)82. Dimethylsilylene extrusion has been found in the pyrolysis of silacyclopentenes and other products of... 

Silylene transfer to a -unsaturated esters produces oxasilacyclopentenes and provides a new method for regio- and stereo-selective formation of enolate that can undergo facile and selective Ireland-Claisen rearrangements and aldol addition reactions to provide products with multiple contiguous stereocenters and quaternary carbon centers (Scheme 37). 

Many of the addition reactions of the silylenes 83-85 with molecules having multiple bonds and also many of their insertion reactions into activated (T-bonds are thought to proceed via cyclic transition states. For that reason these reactions are summarized in this section. It should be emphasized, however, that in only a few cases mechanistic investigations either by theoretical means or by experimental methods have been undertaken to substantiate the claim for a one-step cycloaddition. 

The available experimental data to estimate the heat of the addition reaction between the oxygen molecule and the silylene center (see also reaction 10 in Table 7.4) is used here ... 

The best studied addition reactions of silylene have been those with conjugated dienes. Earlier studies using the nuclear recoil system have shown that 31SiH2 added to 1,3-butadiene to give silacyclopent-3-ene (44, 99, 100). 

Silver can mediate oxidation reactions and has shown unique reactivity. In a few cases, namely, nitrene-, carbene-, and silylene-transfer reactions, novel reactivity was found with homogeneous silver catalysts. Some of these reactions are uniquely facilitated by silver, never having been reported with other metals. While ligand-supported silver catalysts were extensively utilized in enantioselective syntheses as Lewis acids, disappointingly few cases were reported with oxidation reactions. Silver-catalyzed oxidation reactions are still underrepresented. Silver-based catalysts are cheaper and less toxic versus other precious metal catalysts. With the input of additional effort, this field will undoubtedly give more promising results. 

Summary The thermally stable silylene Si[ N(CH2tBu) 2C6H4-l,2] 1 readily undergoes addition reactions with compounds containing CO and CN multiple bonds (e g., ketones, imines, nitriles or isonitriles). The products, most of them characterized by their X-ray structures, contain novel skeletal frameworks SiCOSi, SiOSiCCC, SiCNSi, CSi(CN),... 

An interesting organolanthanide-catalyzed reaction which has been studied in recent years is the addition of terminal alkynes to carbodiimides leading to the novel class of ,A -disubstituted propiolamidines. It was found that half-sandwich rare earth metal complexes bearing silylene-linked cyclopentadienyl-amido ligands can act as excellent catalysts in this addition reaction. As illustrated in Scheme 58, a rare earth amidinate species has been confirmed to be a true catalytic species [68]. 

---