Silyl-metal compounds

The double silylation of unsaturated organic compounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic compounds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes2,3 have been implicated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit 4 (ii) the generation of a new class of heterocyclic compounds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile 4 and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions.5... 

Metal-lithium exchange reaction of a silyl metal compound (especially Group 12 metals) with alkyllithium or lithium metal. 

Summary 1,2-Dichlorodisilanes are prepared by reaction of silyl metal compounds with dichlorosilanes and subsequent chlorination. Alternatively, 1,2-dichlorodisilanes are obtained by insertion of a silylene in the Si-Cl bond of dichlorosilanes. Dehalogenation of 1,2-dichlorodisilanes with magnesium leads to cyclic oligosilanes or vicinally dimetallated species. Ab initio calculations show that coordinated disilenes are plausible intermediates in some of these reactions. 

The growing interest in volatile silyl-metal complexes for chemical vapor deposition reactions should also be mentioned. This technique is extremely useful for the preparation of silicide films in microelectronic devices. Further examples of applications of silicon-metal compounds are given in the appropriate sections. 

The first silyl-metal complex was synthesized in 1956 with the compound CpFe(CO)2SiMe3 [93, 94]. Since then, methods for the formation of M-Si bonds have been investigated systematically and developed to an effective set of tools. Numerous articles covering this subject give evidence for the continuous development of this area [64, 95],... 

These compounds form an interesting contrast to the Lewis base adducts of silyl-metal carbonyls (Section II,C,4), where silicon is thought to be present in a cation. 

Judging from these findings, the mechanism of Lewis acid catalysis in water (for example, aldol reactions of aldehydes with silyl enol ethers) can be assumed to be as follows. When metal compounds are added to water, the metals dissodate and hydration occurs immediatdy. At this stage, the intramolecular and intermolecular exchange reactions of water molecules frequently occur. If an aldehyde exists in the system, there is a chance that it will coordinate to the metal cations instead of the water molecules and the aldehyde is then activated. A silyl enol ether attacks this adivated aldehyde to produce the aldol adduct. According to this mechanism, it is expected that many Lewis acid-catalyzed reactions should be successful in aqueous solutions. Although the precise activity as Lewis acids in aqueous media cannot be predicted quantitatively... 

Hydrosilylation can be applied to alkenes, alkynes, and aldehydes or ketones. A wide range of metal compounds can be used as a catalyst. The most common and active ones for alkenes and alkynes are undoubtedly based on platinum. Hydrosilylation of C-0 double bonds gives silyl ethers, which are subsequently hydrolysed to their alcohols. The reaction is of interest in its enantioselective version in organic synthesis for making chiral alcohols, as the achiral hydrogenation of aldehydes or ketones does not justify the use of expensive silanes as a reagent. 

Organosilyl substituted 7)5-cyclopentadienyl complexes may be prepared by two main procedures (a) reactions of silylated cyclopenta-dienes or their alkali metal derivatives, with an appropriate transition metal compound (carbonyl or halide) with formation of ir-bonds (b) metalation of a preformed 7j5-cyclopentadienyl complex, followed by treatment with an organohalosilane. 

Direct silylation of ferrocene, without the intermediacy of an organo-metallic compound, has been achieved under Friedel-Crafts conditions, with chloro- and aminosilanes (154) ... 

In an indirect method of achieving this conversion, the silyl enol ether of a simple ketone is treated with DDQ3 or with triphenylmethyl cation37 (for another indirect method, see 7-12). Simple linear alkanes have been converted to alkenesby treatment with certain transition metal compounds.38... 

A less explored area of transition metal catalysis involves bond formation between Group 14 elements and nitrogen. In direct analogy to previously discussed areas of research, silicon-nitrogen bonds can be formed by dehydrocoupling, hydrosilylation, and dehydrogenative silylation. The compounds produced are valuable for use in organic synthesis or as polymer precursors to silicon nitride ceramics. 

Alternatively, the reaction may proceed via 1,6-ring closure giving the isolated 2,3-disilanaphthalene 372109. It should be mentioned, however, that under the reaction conditions it is possible that silyl lithium compound 375 formed from the precursor of the silene 376 (obtained from metalation of 377) by a 1,3-trimethylsilyl shift adds to the intermediate silene 371 and that subsequent ring closure with elimination of lithium silanolate gives rise to the observed products 372 and 373 (equation 105). 

Certain reactions of silyl—alkali metal compounds, such as coupling of (CH3) (C6H5)3 BSiLi with appropriate chlorosilanes (51) and hydrolysis of [(CH3)3Si]3SiLi (63), lead to the formation of organopolysilanes with the silicon-hydrogen bond. There have been few reports of such synthesis. 

Various types of partially arylated alkylpolysilanes are most conveniently obtainable by use of the silyl metallic reagents. Since a number of examples of the synthesis of such a class of compounds have been given in recent reviews (48, 51, 73, 76a, 212), only a few are illustrated below. 

Analogous products were obtained from the reaction of silylene 85 with silyl lithium compounds, with alkali metal silylamides and alkali metal alkylamides, and sodium methoxide <2000CC1427, 2004JCD3288, 2005JCD2720, 2005CC5112>. In the case of the reaction with metallated silylamides a thermally initiated rearrangement (114 — 115) to give the new silylamide 115 took place (Scheme 11). 

C. Reaction of a Silyl Alkali-Metal Compound with a Metal Halide. .. 11... 

The method is of limited applicability, since although silyl alkali metal compounds RaSiM are known when RaSi is H2n+iSin (n = 1-5), Me2n+iSi in = 1, 3, 4), and Me Ph3-nSi (n = 0-2), and several dilithium derivatives Li(SiPh2) Li (n = 4-6) have also been described, all are rather difficult to prepare and handle (24). It will be seen that only some of them have yet been used to prepare silicon-transition-metal compounds. 

Reactions of Silyl Alkali Metal Compounds with Metal Halides... 

The silanolysis of alcohols to form silyl ethers is an attractive method since the only by-product, H2, is easily removed making workup procedures trivial. Many transition metal catalysts for this exothermic process have been introduced, but most suffer from low activities and inconvenient preparations. The silane activation mechanism has parallels to the mechanisms by which transition metal compounds bring about this transformation and the B(C6F5)3/silane catalytic system turns out to be highly active for this transformation.255 In contrast to the normal procedures involving silyl chlorides... 

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