Allylic disilane

Disilanes and trisilanes with olefinic substituents (such as vinyl- and allyl-disilanes) react in the presence of H2PtCl6 6 H20 undergoing a Si-Si cleavage692. A second reaction sometimes occurs so that simple cleavage products are not obtained. For example, vinylpentamethyldisilane reacts with the cleavage product trimethylsilane, giving an ethane derivative ... 

While investigating trisubstituted allylic ethers, Woerpel and coworkers observed insertion of silylene into the allylic C-O bond (Scheme 7.56).136 The product formation was dependent on the identity of the catalyst. Silver trifluor-oacetate promoted the formation of allylic disilane 201, whereas copper(I) triflate... 

Insight into the mechanism for the formation of allylic disilane 201 and silane 202 was obtained from a series of control experiments (Scheme 7.57). Submission of allylic silane 202 to reaction conditions did not produce disilane 201. Woerpel and coworkers interpreted this result to indicate that disilane formation does not occur through subsequent silylene insertions. Crossover experiments established that no dissociation of the alkoxy group occurs during the reaction to suggest that silylene insertion is intramolecular. 

Whereas ethylene oxide gives with 17 at ambient temperature a quantitative yield of l-trimethylsilyloxy-2-iodoethane [5, 31], substituted epoxides such as 846b react with 17 to give 848 as the main product [32]. Excess 17, however, leads to the bis-iodo compounds 849 and HMDSO 7 [4, 5]. In the presence of DBU the epoxides 850 are converted by 17, which is generated in situ from hexamethyl-disilane 857 and I2, into the allyl alcohols 851 [4, 32] (Scheme 6.14). Cycloctene epoxide 852 is opened by SiCl4 at -78 °C in the presence of catalytic amounts of the asymmetric catalyst 853 to give 61% of the chlorohydrin 854 in 98% ee [33]. 

Indeed, allyl-type dienes were more efficient than divinyl olefins, even for the branched RF group. Various heptafluoroalkyl disilanes were thus obtained with Tg varying from 12 to -59°C.25... 

Addition of disilanes to ir-allylpalladium complexes proceeds selectively to yield the silyl group appended to the less substituted allyl terminus (equations 335 and 336).338>339... 

Reductive cleavage of disilanes by alkali metals is the most versatile method to prepare metalated arylsilanes, which carry, in addition to the aromatic group, also an aliphatic, benzylic or allylic substituent. Table 1 reports all the metalated arylsilanes which have been prepared by reductive cleavage of disilanes with alkali metals. 

As an example of carbometallation, the 1,4-carbosilylation product 218 is obtained by the reaction of dienes, disilanes and acid chlorides of aromatic and a,/i-unsaturatcd acids at 80 °C. The phenylpalladium 216 is formed by the oxidative addition of benzoyl chloride, followed by facile decarbonylation at 80 °C, and reacts with butadiene to generate the benzyl-7i-allylic complex 217. Then, transmetallation with the disilane and reductive elimination afford 4-silyl-2-butenylbenzene 218 [92], Regioselective carbomagnesation of isoprene with allylic magnesium bromide 219 catalysed by Cp2TiCl2 gives 220, which is useful for terpene synthesis [93,94],... 

In the proposed mechanism, the RPdCl generated by decarbonylation reacts with the diene to give a (7r-allyl)palladium intermediate. Reaction of this intermediate with the disilane replaces the chloride by trimethylsilyl. Subsequent reductive elimination gives the product. In mechanistic studies, the chlorodimer 31 corresponding to the (5r-allyl)palladium chloride intermediate in Scheme 8-5 was prepared and allowed to react with Me3SiSiMe3. This led to Me3SiCl (characterized by Si NMR) and l-phenyl-4-(trimethylsilyl)but-2-ene [Eq.(31)]. 

Reactions of 1,3-dienes with various disilanes were reported by means of group 10 transition-metal catalyst, giving synthetically useful allylic silane derivatives. Two major pathways, i.e., formation of 1 1 and 1 2 adducts, are known and controlled by the choice of disilanes and catalyst. 

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