Silicon reactions with butadiene

We were faced with a dilemma six years ago when the major product from reactions of recoiling silicon atoms with butadiene turned out to be one for which no authentic sample could be synthesized (4,39). 

Figure 5.13. Cycloaddition products at the silicon dimer of the Si(100)-2 x 1 surface, (a) shows the [2 + 2] cycloaddition product formed in the reaction with ethylene, and (b) shows the [4 + 2], or Diels-Alder, cycloaddition product formed in the reaction with 1,3-butadiene.

A key report investigated a variety of substrates in their reaction with silicon in an effort to find evidence for silylene intermediates during the silicon direct process reaction. When silicon, copper and methanol were reacted as described above but in the presence of alkenes, alkyldimethoxysilanes and (MeO SiH were formed95-97. The use of allyl propyl ether instead of alkenes gave allyldimethoxysilane, with 38% selectivity. These results and the reaction of silicon with MeCl in the presence of butadiene to give silacyclopent-3-enes indicates intermediate formation of silylenes. 

Photolysis of silacyclobutenes allows a straightforward approach to the generation of l-sila-1,3-butadienes. Furthermore the wide variability of substituents at the silicon atom enables a study of their influence on the ring opening reactions. First results of the photolysis of a dimethylated silacyclobutene 2 demonstrate the synthetic potential of this class of compounds. On the one hand the intermediate silabutadiene reacts like a silaethene with methoxytrimethylsilane, on the other hand it shows "butadiene behaviour" in a [4+2] cycloaddition reaction with acetone [10-13]. 

The recent development of dienes incorporating latent has greatly expanded the utility of Diels Alder approaches of complex molecules. Butadiene substituted by silicon at is available from l,4-dichloro-2-butyne by H2PtCl2catalyzed reaction with Et3SiH followed by dehydrohalogenation (Zn). The product reacts readily with dlenophiles. ... 

Fumed silicas (Si02). Fumed silicas are common fillers in polychloroprene [40], natural rubber and styrene-butadiene rubber base adhesives. Fumed silicas are widely used as filler in several polymeric systems to which it confers thixotropy, sag resistance, particle suspension, reinforcement, gloss reduction and flow enhancement. Fumed silica is obtained by gas reaction between metallic silicon and dry HCl to rend silica tetrachloride (SiCU). SiC is mixed with hydrogen and air in a burner (1800°C) where fumed silica is formed ... 

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

Reactions favoring [2 + 2] cycloaddition tended to be those that had strongly electronegative groups on the sp2-hybridized silicon but only H and the neopentyl group on the sp2-hybridized carbon atom. Butadiene and cyclohexadiene generally favored [2 + 2] cycloaddition with these silenes. The [2 + 2] adducts with cyclohexadiene appear to be kinetic products, since they cleanly isomerized to the Diels-Alder adducts over time.182... 

Corriu et al. have reported that the coupling reaction of 2-(iV,iV-dimethylaminomethyl)phenyllithium with (McvSi)vSiCI 53 affords 2-(iV,iV-dimethylaminomethyl)-l-[tris(trimethylsilyl)silyl]benzene 894. No evidence has been found that the intramolecular iV-ligand coordinates to the silicon atom of 894. Upon UV irradiation, the trisilane forms a transient silyene 895, which has been trapped with 2,3-dimethyl-2,3-butadiene and triethylsilane to give the oligosilanes 896 and 897 as well as 898-900, (Scheme 126).859 Apparently, the bulk on the two ligands is insufficient to provide kinetic stabilization of the silylene intermediate 895. 

Figure 5.15. Density functional theory study of the reaction of 1,3-butadiene with the Si(100)—2 x 1 surface, modeled using a nine silicon atom cluster and examining two possible products the product of the Diels-Alder cycloaddition, and the product of the [2 + 2] cycloaddition [237,238].

It has been reported that by heating in the presence of water vapor, silicon can be incorporated into the lattice of y-Fe203 (55,64). It is unlikely that this effect occurs under the butene oxidation conditions since a physical mixture of large size iron oxide and silica gel retains its selectivity for butadiene after a prolonged reaction time without any indication of changes in the catalyst. If silicon substitution takes place, the water from the oxidation reaction could catalyze the substitution. In fact, deliberate pretreatment of such a physical mixture with water vapor before reaction does not change the activity or selectivity (54). 

Most importantly, the scope of the Diels-Alder reaction is very high - not only allowing the synthesis of cyclohexenes and 1,4-cyclohexadienes using 1,3-butadienes and alkenes and alkynes, respectively, but also giving access to a multitude of different heterocycles by exchanging the atoms a-d in the butadiene as well as the atoms e and f in the alkene by hetero atoms such as oxygen, nitrogen and sulfur. However, also dienes and dienophiles with several other atoms as phosphorous, boron, silicone, and selenium have been described. Thus, many different heterodienes and heterodienophiles have been developed over the years (Tables 1-1 and 1-2). 

---