1,2-Silyl migrations cationic

Conjugated ketones and esters react with allenylsilanes to yield acylcyclopentenes (Eq. 9.60) [63]. These products are formed by initial 1,4-addition to the conjugated double bond to afford a silyl-stabilized vinyl cation intermediate. 1,2-Silyl migration gives rise to a second silyl-stabilized vinyl cation which cyclizes to the acyl cyclopen-tene (Scheme 9.14). 

The reaction of acylsilanes with acid chlorides in the presence of A1C13 leads to furans (Table 9.41) [45]. In these reactions an acyl cation initiates the addition with ensuing silyl migration yielding an intermediate vinyl cation. Attack of the carbonyl oxygen followed by proton loss affords the observed products (Scheme 9.16). An analogous reaction with nitrosyl fluoroborate provides a route to oxazoles (Table 9.42) [65]. The nitrosyl cation serves as the electrophile in this application. 

A regioselective [3 + 2]-cycloaddition approach to substituted 5-membered carbo-cycles was made available by the use of allenylsilanes [188]. The reaction involves regioselective attack of an unsaturated ketone by (trimethylsilyl)allene at the 3-position. The resulting vinyl cation undergoes a 1,2-silyl migration. The isomeric vinyl cation is intercepted intramolecularly by the titanium enolate to produce a highly substituted (trimethylsilyl)cyclopentene derivative. 

The reaction of allenylsilanes with a,/8-unsaturated acylsilanes presents a new [3 + 3]-cycloaddition approach to a six-membered carbocycle [189]. Lewis acid-promoted ring expansion of the [3 + 2]-annulation product 260 is followed by a second cationic 1,2-silyl migration to produce the cyclohexenone 261 after desilylation. 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

The ethoxycarbocation intermediate (363) produced by the action of acid on the cyclobutenedione monoacetal (362) has been found to react with bis(trimethylsilyl)-acetylene to afford a 2-methylenecyclopent-4-ene-l,3-dione derivative (365). The authors426 proposed that the rearrangement results from an unprecedented cationic 1,2-silyl migration on the alkynylsilane, subsequent ring expansion via a vinyl cation intermediate (364), and re-closure by intramolecular addition of an acyl cation to a silylallene in a 5-exo-trig mode (see Scheme 90). 

Cationic 1,2-silyl migration was proposed to be involved during the reactions of carbon monooxide and isocyanides with transition metal-carbon bonds (ligand distribution) (equation 18)55-65. Typically, the reaction of l-sila-3-zirconacyclobutane 29 with carbon monooxide afforded a dioxasilazirconacyclohexane derivative 30. The reaction was considered to proceed via a CO insertion into a Zr—C bond followed by a 1,2-silyl migration as shown in equation 1960. This type of reactions are well-documented in a review of Durfee and Roth well66. 

Cationic 1,2-silyl migrations from Si to Si were reported by Ishikawa, Kumada and coworkers67 and by Hengge and coworkers68 (equation 20). The ring contraction of dodecamethylcyclohexasilane 31 to 1-trimethylsilylnonamethylcyclopentasilane 32 in the presence of AICI3 (equation 20) was proposed to proceed via a 1,2-silyl migration to a cationic silicon center as shown in equation 2167,69. 

Silyl migration in chloromethylpentamethyldisilane 33 X=Y=Si has been known to occur thermally or more easily in the presence of aluminum chloride70. Similar rearrangements promoted by a Lewis acid were observed in (chloromethyl)pentamethylgermasilanes71. Presumably, the reaction does not involve a carbenium or a silicenium cation as a discrete reactive intermediate but proceeds via the cyclic transition state 34 involving aluminum chloride participation (equation 22). 

The following reactions are examples of cationic 1,2-silyl migrations from silicon to carbon during the reactions of tris(trialkylsilyl)silyl-substituted ketones72,73 (equations 23 and 24), alcohols74 (equation 25) and ethers75 (equation 26) in the presence of Lewis acids. 

The [3 + 2] reaction of an allylsilane with an enone was proposed to proceed via a regiospecific electrophilic substitution (cf 80) of the enone at C-3 of the allylsilane followed by a cationic 1,2-silyl migration (equation 57). At the yyw-clinal transition state (81), the carbonyl group of the unsaturated ketone was assumed to occupy an endo orientation in relation to the allylsilane142. The transition state was thought to be favorable due to minimization of the charge separation and to possible secondary orbital stabilization. 

The cationic 1,2-silyl migrations affording [3 + 2] cycloadducts (85) were suppressed in the reactions of triisopropylallylsilane 82 with a,/i-unsaturated bicyclic lactams 83... 

A cationic 1,2-silyl migration was observed during the deaminosilylation through the diazotization of /i-aminosilanes 112, which gave 113 and 114 (equations 77 and 78)185. 

In the presence of A1C13 and Me3SiCl, 1,3-dienes react with 10 to afford trans-[3 + 2]-cycloadducts (Equation (46)). A trimethylsilyl cation-mediated mechanism involving 1,2-silyl migration has been proposed.183... 

Similar reactions, but not accompanied by the silyl migration, have also been reported. When a bulky allylsilane is reacted with benzoylformates, the product obtained is an oxetane, as shown in Eq. (100) [270]. If this reaction is executed with allyltrimethylsilane, the expected homoallyl alcohol is produced in good yield. When, moreover, interception of the /3-silyl cation with a neighboring nucleophile is faster than desilylation, even the trimethylsilyl group is preserved in the product (Eq. 101) [271]. 

Transition metal carbonyls such as Co2(CO)8 and CoH(CO)4, formed in the reaction of R3SiH with dimer (but also Fe(CO)5 and M3(CO)i2 (M = Fe, Ru, Os)) have been found to be active catalysts for the hydrosilylation of olefins, dienes, unsaturated nitriles, and esters as well as for hydrosilylation C=0 and C=N bonds [56]. Hydrosilylation of phenylthioacetylenes in the presence of this catalyst is extremely regioselective [57]. Cobalt(I) complexes, e. g., CoH(X)2L3 (X = H, N), could be prospective candidates for investigation of the effectiveness of alkene hydrosilylation by trialkoxysilanes as well as dehydro-genative silylation [58]. Direct evidence for the silyl migration mechanism operative in a catalytic hydrosilylation pathway was presented by Brookhart and Grant [59] using the electrophilic Co cationic complex. 

Experimentally, 1,2-silyl migrations with formation of double bonds between the migration origin and the cationic or neutral target have been found to take place intramolecularly, e.g., [1], o X -C-+a I -C [2], [3], o X -Si-W, -Si [4], and... 

In the expected course of the reaction, the electrophile combines with the silylallene resulting in the intermediate silylvinyl cation 21, which either undergoes desilylation to produce the expected product alkyne 22 or traps chloride ion to produce vinylchloride 23. In the case of the annulation reaction, the intermediate silylvinyl cation 21 underoes an apparent 1,2-sp -silyl migration process. The resulting isomeric silylvinyl cation 24 is then able to react with the pendent titanium enolate leading to the observed annulation product 25. 

The generally accepted mechanism of the classic Danheiser annulation involves three basic steps the Lewis acid-catalyzed electrophilic combination of the a, 0-unsaturated ketone with the silylallene, a 1,2-sp-silyl migration, and a final cyclization step. This mechanism was first proposed by Danheiser in the original publication of the annulation and has been generally accepted but has never been formally investigated. A more detailed account of the reaction pathway is shown below. Treatment of the a,y5-unsaturated ketone 1 with TiCU produces a titanium complex existing as two resonance-stablized cations 26 and 27. Attack of the 2,3-7c-bond of the... 

Lewis Acid-catalyzed Addition to Epoxides. The Lewis acid-catalyzed addition of l,3-bis(silyl)propenes to epoxides has been studied for both inter- and intramolecular reactions. In the inter-molecular reaction, monosubstituted epoxides give higher yields (66-53%) than bis-substituted epoxides (33-19%) (eq 5). It is proposed that the epoxide opens in a Sn 1 fashion, followed by nucleophilic attack of the bis(silyl)propene on the resultant cation. This leads to a silicon-stabilized carbocation, which undergoes C O silyl migration 3uelding the final alkene. 

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