Benkeser reaction

Alkyl or acyl halides can be transformed into trichlorosilanes by treatment with HSiCl3 and NEt3 (Benkeser reaction)30,31 (equation 19). 

Benkeser reaction. Another general route is the Benkeser reaction, which allows efficient preparation of precursors with an allylic group. Via this reaction the benzylic precursor 45134 is also prepared by silylation and methanolysis of the Si—Cl bond (equation 8)93 100. 

The Birch and Benkeser reactions of some unsaturated organic compounds [318 and references therein], which consist of a reduction by sodium or lithium in amines, can be mimicked electrochemically in the presence of an alkali salt electrolyte. The cathodic reaction is not the deposition of alkali metal on the solid electrode but the formation of solvated electrons. Most of the reactions described were performed in ethylenediamine [319] or methylamine [308,320]. A feature of these studies is variety introduced by the use of a divided or undivided cell. In a divided cell, the product distribution appears to be the same as that in the classic reduction by metal under similar conditions. In contrast, in an undivided cell the corresponding ammonium salt is formed at the anode it plays the role of an in situ generated proton donor. Under such conditions, the proton concentration... 

Most of the kinetic studies have been carried out by Eaborn and by Benkeser and their coworkers, and the first study was that of Eaborn656, who measured, spectrophotometrically, the rates of cleavage of 4-methoxyphenyltrimethylsilane by hydrochloric and perchloric acids in methanol, and by hydrochloric acid in aqueous dioxan, both at 47.9 °C (Table 226). The reaction was first-order in silane... 

Further mechanistic evidence was provided by Benkeser and Krysiak658, who determined the effects of added salts and water on the rates of cleavage of xylyltrimethylsilanes by p-toluenesulphonic acid in acetic acid at 25 °C, the progress of the reaction being followed by dilatometry the first-order rate coefficients are given in Table 227. Clearly the addition of water retards the reaction, as... 

Another very important ring substitution reaction of ferrocene is its ability to undergo metalation with organolithium and organosodium compounds. Metalation with n-butyllithium in ethyl ether, first reported by Nesmeyanov and coworkers (63) and independently by Benkeser, Goggin, and Schroll (3),"leads to rather low yields of ferrocenyllithium (XXXI, M = Li) and l,l -ferrocenylenedi-lithium (XXXII, M = Li). It was subsequently shown by Mayo, Shaw, and... 

By proper choice of reaction conditions (metal, solvent, the order of addition of reagent and reactant), fused polycyclic aromatics can be converted to different partially reduced derivatives with high selectivity. When the red complex of naphthalene, for example, formed in the Na-NH3 solution, is quenched with aqueous ammonium chloride solution, 1,4-dihydronaphthalene is formed209 [Eq. (11.58)]. Reaction in the presence of an alcohol yields 1,4,5,8-tetrahydronaphthalene210 [Eq. (11.59)]. The Benkeser reduction affords further saturation to the isomeric octalins211 [Eq. (11.60)] or even to decalin212 [Eq. (11.61)] ... 

As Margerison and Nyss noted 289) this reaction path is similar to one given by Benkeser et al.290) to explain the formation of l,l,3-trimethyl-3-phenylindane in the course of the metallation of isopropylbenzene by w-pentylsodium ... 

In addition to these two series of reactions based on the substitution of hydrogen, a third set of data may be derived from reactions which involve the replacement of groups other than hydrogen.1 Kuivila, Benkeser, and Eaborn and their associates employed this approach to study the effects of many substituents. For example, Kuivila and Hendrickson (1952) examined the bromodeboronation of substituted phenylboronic acids. 

The displacement of metalloid groups from naphthalene, however, is unusually slow. The rate differential between the 1- and 2-positions is little more than a factor of three for protodesilylation or protodegermyla-tion. Even more important is the failure to observe a steric acceleration for these reactions. Benkeser and Krysiak (1954) showed that the rate of protodetrimethylsilylation reaction was increased by o-methyl substituents to an extent greater than the anticipated electronic contribution. Presumably, steric strains are relieved in the transformation from trigonal to tetrahedral geometry in the transition state (de la Mare, 1958). The failure to observe this acceleration and a greater lf-N/2f-N ratio for these reactions is puzzling. 

A remarkably specific Me/Cl exchange was discovered by Beck and Benkeser (29). The reaction is typified by Eq. (46) the catalyst is chloro-platinic acid and typical conditions are 140°C/24 h. The data in Table I... 

Relatively little work has appeared on the higher silanes. West (61) reports that the application of Beck and Benkeser s (29) reaction [see Eq. (46)] to dodecamethylcyclohexasilanes gives the products shown in Eqs. (96) and (97) (61). It thus appears that the first chlorine introduced exerts a... 

Trimelhylsiloxy esters. The reaction of iodotrimethylsilane with esters to form trimethylsiloxy esters is markedly catalyzed by iodine, possibly by formation of triiodotrimethylsilane, (CHylaSils. This observation explains the fact that tri-methylphenylsilane in combination with excess iodine can be more effective than iodotrimethylsilane. Aryl trimethylsiloxy esters, unlike ordinary esters, can be reduced to methyl groups by the method of Benkeser (4, 525-526). The reaction can be conducted without isolation of intermediates (equation I). 

The practice of selective reduction is stilt an art - . Note the different products for EtC CEt and PhC=CPh in Table 9 produced under different conditions. Now, Scheme 5 was devised to provide a broad rationale for diverse results in both named (Birch, Benkeser, Normant) and unnamed reductions. One learns that the presence of acids, i.e. NHJ in Na-NHj or /-BuOH in Na-HMPT, and low reaction temperatures favour anti reduction - Proton donors usually preclude alkyne-allene... 

Extensive investigations have been made into further methods for the reduction of aromatic rings based on the use of dissolving metals in other solvents, especially the lower molecular weight amines (the Benkeser reduction), electrochemical methods (cathodic reductions), photochemical methods and the reaction of radical anions with silylating reagents rather than proton sources. The aim of much of this work has been to produce the normal Birch products more conveniently or cheaply, but very often the outcome has been quite distinct. The alternative method may then provide access to products which are not so easily obtained by the standard metal-liquid ammonia methodology. 

R. A. Benkeser Silane addition reactions— their synthetic utility and mechanism, pp. 133-140 (20). 

It is well known that the reaction of trichlorosilane in the presence of ter/.-amines, provides the anion SiCU (Benkeser type system) [1]. 

However, the similar silylation of the P-chlorophosphaalkene 6a with two equivalents of the trichlorosilane/triethylamine reagent does not proceed with formation of a P=P double bond. The main product of this reaction is the stable bis(trimethylsilyl)methylbis(trichlorosilyl)phosphane 2d, which indicates a hydrosilylation of the P=C double bond by trichlorosilane, like the known Benkeser additions at unsaturated carbon-carbon bond systems [8]. 

Early reports stated that the course of reaction is strongly dependent on the reaction conditions (L e. the employed catalyst) Benkeser " and his co-wor-kers intensively investigated hydrosilylation of monosubstituted acetylenes 1 [R = i-prop- la) and t-But- lb). ... 

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